MODIFIED FACTOR VIII

Abstract: Specific amino acid loci of human factor VIII interact with inhibitory antibodies of hemophilia patients who have developed such antibodies after being treated with factor VIII. Modified factor VIII is disclosed in which the amino acid sequence is changed by a substitution at one or more amino acids of positions 484-508 of the A2 domain. The modified factor VIII is useful as a clotting factor supplement for hemophiliacs.

Description: MODIFIED FACTOR VIII CROSS REFERENCE TO RELATED APPLICATIONS This application claims priority from United States Patent Application No. 09/315,179 filed May 20, 1999. ACKNOWLEDGEMENT OF FEDERAL RESEARCH SUPPORT The government has rights in this invention arising from National Institutes of Health Grant No. HL46215 that partially funded the research leading to this invention. BACKGROUND OF THE INVENTION This invention relates generally to a hybrid factor VIII having human and animal factor VIII amino acid sequence or having human factor VIII and non-factor VIII amino acid sequence and methods of preparation and use thereof. Blood clotting begins when platelets adhere to the cut wall of an injured blood vessel at a lesion site. Subsequently, in a cascade of enzymatically regulated reactions, soluble fibrinogen molecules are converted by the enzyme thrombin to insoluble strands of fibrin that hold the platelets together in a thrombus. At each step in the cascade, a protein precursor is converted to a protease that cleaves the next protein precursor in the series. Cofactors are required at most of the steps. Factor VIII circulates as an inactive precursor in blood, bound tightly and non- covalently to von Willebrand factor. Factor VIII is proteolytically activated by thrombin or factor Xa, which dissociates it from von Willebrand factor and activates its procoagulant T function in the cascade. In its active form, the protein factor Villa is a cofactor that increases the catalytic efficiency of factor IXa toward factor X activation by several orders of magnitude. People with deficiencies in factor VIII or antibodies against factor VIII who are not treated with factor VIII suffer uncontrolled internal bleeding that may cause a range of serious symptoms, from inflammatory reactions in joints to early death. Severe hemophiliacs, who number about 10,000 in the United States, can be treated with infusion of human factor VIII, which will restore the blood's normal clotting ability if administered with sufficient frequency and concentration. The classic definition of factor VIII, in fact, is that substance present in normal blood plasma that corrects the clotting defect in plasma derived from individuals with hemophilia A. The development of antibodies ("inhibitors" or "inhibitory antibodies") that inhibit the activity of factor VIII is a serious complication in the management of patients with hemophilia. Autoantibodies develop in approximately 20% of patients with hemophilia A in response to therapeutic infusions of factor VIII. In previously untreated patients with hemophilia A who develop inhibitors, the inhibitor usually develops within one year of treatment. Additionally, autoantibodies that inactivate factor VIII occasionally develop in individuals with previously normal factor VIII levels. If the inhibitor titer is low enough, patients can be managed by increasing the dose of factor VIII. However, often the inhibitor titer is so high that it cannot be overwhelmed by factor VIII. An alternative strategy is to bypass the need for factor VIII during normal hemostasis using factor IX complex preparations (for example, KONYNE * , Proplex * ) or recombinant human factor Vila. Additionally, since porcine factor VIII usually has substantially less reactivity with inhibitors than human factor VIII, a partially purified porcine factor VIII preparation (HYATE:C * ) is used. Many patients who have developed inhibitory antibodies to human factor VIII have been successfully treated with porcine factor VIII and have tolerated such treatment for long periods of time. However, administration of porcine factor VIII is not a complete solution because inhibitors may develop to porcine factor VIII after one or more infusions. Several preparations of human plasma-derived factor VIII of varying degrees of purity are available commercially for the treatment of hemophilia A. These include a partially- purified factor VIII derived from the pooled blood of many donors that is heat- and detergent- treated for viruses but contain a significant level of antigenic proteins; a monoclonal antibody- purified factor VIII that has lower levels of antigenic impurities and viral contamination; and recombinant human factor VIII, clinical trials for which are underway. Unfortunately, human factor VIII is unstable at physiologic concentrations and pH, is present in blood at an extremely low concentration (0.2 μg/ml plasma), and has low specific clotting activity. Hemophiliacs require daily replacement of factor VIII to prevent bleeding and the resulting deforming hemophilic arthropathy. However, supplies have been inadequate and problems in therapeutic use occur due to difficulty in isolation and purification, immunogenicity, and the necessity of removing the AIDS and hepatitis infectivity risk. The use of recombinant human factor VIII or partially -purified porcine factor VIII will not resolve all the problems. The problems associated with the commonly used, commercially available, plasma- derived factor VIII have stimulated significant interest in the development of a better factor VIII product. There is a need for a more potent factor VIII molecule so that more units of clotting activity can be delivered per molecule; a factor VIII molecule that is stable at a selected pH and physiologic concentration; a factor VIII molecule that is less apt to cause production of inhibitory antibodies; and a factor VIII molecule that evades immune detection in patients who have already acquired antibodies to human factor VIII. It is therefore an object of the present invention to provide a factor VIII that corrects hemophilia in a patient deficient in factor VIII or having inhibitors to factor VIII. It is a further object of the present invention to provide methods for treatment of hemophiliacs. It is still another object of the present invention to provide a factor VIII that is stable at a selected pH and physiologic concentration. It is yet another object of the present invention to provide a factor VIII that has greater coagulant activity than human factor VIII. It is an additional object of the present invention to provide a factor VIII against which less antibody is produced. SUMMARY OF THE INVENTION The present invention provides isolated, purified, hybrid factor VIII molecules and fragments thereof with coagulant activity including hybrid factor VIII having factor VIII amino acid sequence derived from human and pig or other non-human mammal (together referred to herein as "animal"); or in a second embodiment including a hybrid equivalent factor VIII having factor VIII amino acid sequence derived from human or animal or both and amino acid sequence having no known sequence identity to factor VIII ("non-factor VIII amino acid sequence"), preferably substituted in an antigenic and/or immunogenic region of the factor VIII, is described. One skilled in the art will realize that numerous hybrid factor VIII constructs can be prepared including, but not limited to, human/animal factor VIII having greater coagulant activity than human factor VIII ("superior coagulant activity"); non- immunogenic human/equivalent factor VIII; non- antigenic human/equivalent or human/ animal factor VIII; non-immunogenic human/ animal or human/equivalent factor VIII having superior coagulant activity; non-antigenic human/animal or human/animal/equivalent factor VIII having superior coagulant activity; non-immunogenic, non-antigenic human/equivalent or human/equivalent/animal factor VIII; and non-immunogenic, non-antigenic human/animal/equivalent factor VIII having superior coagulant activity. The hybrid factor VIII molecule is produced by isolation and recombination of human and animal factor VIII subunits or domains; or by genetic engineering of the human and animal factor Vm genes. In a preferred embodiment, recombinant DNA methods are used to substitute elements of animal factor VIII for the corresponding elements of human factor VIII, resulting in hybrid human animal factor VIII molecules. In a second preferred embodiment, recombinant DNA methods are used to replace one or more amino acids in the human or animal factor VIII or in a hybrid human/animal factor VIII with amino acids that have no known sequence identity to factor VIII, preferably a sequence of amino acids that has less immunoreactivity with naturally occurring inhibitory antibodies to factor VIII ("nonantigenic amino acid sequence") and/or is less apt to elicit the production of antibodies to factor VIII ("non-immunogenic amino acid sequence") than human factor VIII. An example of an amino acid sequence that can be used to replace immunogenic or antigenic sequence is a sequence of alanine residues. In another embodiment, subunits of factor VIII are isolated and purified from human or animal plasma, and hybrid human/animal factor VIII is produced either by mixture of animal heavy chain subunits with human light chain subunits or by mixture of human heavy chain subunits with animal light chain subunits, thereby producing human light chain/animal heavy chain and human heavy chain/animal light chain hybrid molecules. These hybrid molecules are isolated by ion exchange chromatography. Alternatively, one or more domains or partial domains of factor VIII are isolated and purified from human or animal plasma, and hybrid human/animal factor VIII is produced by mixture of domains or partial domains from one species with domains or partial domains of the second species. Hybrid molecules can be isolated by ion exchange chromatography. Methods for preparing highly purified hybrid factor VIII are described having the steps of: (a) isolation of subunits of plasma-derived human factor VIII and subunits of plasma- derived animal factor VIII, followed by reconstitution of coagulant activity by mixture of human and animal subumts, followed by isolation of hybrid human/animal factor VIII by ion exchange chromatography; (b) isolation of domains or partial domains of plasma-derived human factor VIII and domains or partial domains of plasma-derived animal factor VIII, followed by reconstitution of coagulant activity by mixture of human and animal domains, followed by isolation of hybrid human/ animal factor VIII by ion exchange chromatography; (c) construction of domains or partial domains of animal factor VIII by recombinant DNA technology, and recombinant exchange of domains of animal and human factor VIII to produce hybrid human animal factor VIII with coagulant activity; (d) creation of hybrid human animal factor VIII by replacement of specific amino acid residues of the factor VIII of one species with the corresponding unique amino acid residues of the factor VIII of the other species; or (e) creation of a hybrid equivalent factor VIII molecule having human or animal amino acid sequence or both, in which specific amino acid residues of the factor VIII are replaced with amino acid residues having no known sequence identity to factor VIII by site-directed mutagenesis. The determination of the entire DNA sequence encoding porcine factor VIII set forth herein has enabled, for the first time, the synthesis of full-length porcine factor VIII by expressing the DNA encoding porcine factor VIII in a suitable host cell. Purified recombinant porcine factor VIII is therefore an aspect of the present invention. The DNA encoding each domain of porcine factor VIII as well as any specified fragment thereof, can be similarly expressed, either by itself or in combination with DNA encoding human factor VIII to make the hybrid human/porcine factor VIII described herein. Furthermore, porcine fVIII having all or part of the B domain deleted (B-domainless porcine fVIII) is made available as part of the present invention, by expression DNA encoding porcine fVIII having a deletion of one or more codons of the B-domain. Some embodiments of hybrid or hybrid equivalent factor VIII have specific activity greater than that of human factor VIII and equal to or greater than that of porcine factor VIII. Some embodiments of hybrid or hybrid equivalent factor VIII have equal or less immunoreactivity with inhibitory antibodies to factor VIII and/or less immunogenicity in humans or animals, compared to human or porcine factor VIII. Also provided are pharmaceutical compositions and methods for treating patients having factor VIII deficiency comprising administering the hybrid or hybrid equivalent factor VIII. BRIEF DESCRIPTION OF THE DRAWINGS Figs.1A-1H taken together provide an aligned sequence comparison of the human, pig and mouse factor VIII acid sequences. DETAILED DESCRIPTION OF THE INVENTION Unless otherwise specified or indicated, as used herein, "factor VIII" denotes any functional factor VIII protein molecule from any animal, any hybrid factor VIII or modified factor VIII, "hybrid factor VIII" or "hybrid protein" denotes any functional factor VIII protein molecule or fragment thereof comprising factor VIII amino acid sequence from human, porcine, and/or non-human, non-porcine mammalian species. Such combinations include, but are not limited to, any or all of the following hybrid factor VIII molecules or fragments thereof: (1) human/porcine; (2) human/non-human, non-porcine mammalian, such as human/mouse; (3) porcine/non-human, non-porcine mammalian, such as mouse/dog. Such combinations also include hybrid factor VIII equivalent molecules or fragments thereof, as further defined below, comprising factor VIII amino acid sequence of hybrid, human, porcine, or non-human, non-porcine mammalian origin in which amino acid sequence having no known sequence identity to factor VIII is substituted. Such hybrid combinations also include hybrid factor VIII amino sequence derived from more than two species, such as human/pig/mouse, or from two or more species in which amino acid sequence having no known sequence identity to factor VIII is substituted. Unless otherwise indicated, "hybrid factor VIII" includes fragments of the hybrid factor VIII, which can be used, as described below in one exemplary embodiment, as probes for research purposes or as diagnostic reagents. As used herein, "mammalian factor VIII" includes factor VIII with amino acid sequence derived from any non-human mammal, unless otherwise specified. "Animal" , as used herein, refers to pig and other non-human mammals. A "fusion protein" or "fusion factor VIII or fragment thereof" , as used herein, is the product of a hybrid gene in which the coding sequence for one protein is extensively altered, for example, by fusing part of it to the coding sequence for a second protein from a different gene to produce a hybrid gene that encodes the fusion protein. As used herein, a fusion protein is a subset of the hybrid factor VIII protein described in this application. A "corresponding" nucleic acid or amino acid or sequence of either, as used herein, is one present at a site in a factor VIII or hybrid factor VIII molecule or fragment thereof that has the same structure and/or function as a site in the factor VIII molecule of another species, although the nucleic acid or amino acid number may not be identical. A sequence "corresponding to" another factor VIII sequence substantially corresponds to such sequence, and hybridizes to the sequence of the designated SEQ ID NO. under stringent conditions. A sequence "corresponding to" another factor VIII sequence also includes a sequence that results in the expression of a factor VIII or claimed procoagulant hybrid factor VIII or fragment thereof and would hybridize to the designated SEQ ID NO. but for the redundancy of the genetic code. A "unique" amino acid residue or sequence, as used herein, refers to an amino acid sequence or residue in the factor VIII molecule of one species that is different from the homologous residue or sequence in the factor VIII molecule of another species. "Specific activity, " as used herein, refers to the activity that will correct the coagulation defect of human factor VIII deficient plasma. Specific activity is measured in units of clotting activity per milligram total factor VIII protein in a standard assay in which the clotting time of human factor VIII deficient plasma is compared to that of normal human plasma. One unit of factor VIII activity is the activity present in one milliliter of normal human plasma. In the assay, the shorter the time for clot formation, the greater the activity of the factor VIII being assayed. Hybrid human/porcine factor VIII has coagulation activity in a human factor VIII assay. This activity, as well as that of other hybrid or hybrid equivalent factor VIII molecules or fragments thereof, may be less than, equal to, or greater than that of either plasma-derived or recombinant human factor VIII. The human factor VIII cDNA nucleotide and predicted amino acid sequences are shown in SEQ ID NOs: 1 and 2, respectively. Factor VIII is synthesized as an approximately 300 kDa single chain protein with internal sequence homology that defines the "domain" sequence NH ^ - A1-A2-B-A3-C1-C2-COOH. In a factor VIII molecule, a "domain", as used herein, is a continuous sequence of amino acids that is defined by internal amino acid sequence identity and sites of proteolytic cleavage by thrombin. Unless otherwise specified, factor VIII domains include the following amino acid residues, when the sequences are aligned with the human amino acid sequence (SEQ ID NO:2): Al, residues Alal-Arg372; A2, residues Ser373- Arg740; B, residues Ser741 -Arg 1648; A3, residues Serl690-Ile2032; CI , residues Arg2033- Asn2172; C2, residues Ser2173-Tyr2332. The A3-C1-C2 sequence includes residues Serl690- Tyr2332. The remaining sequence, residues Glu 1649-Arg 1689, is usually referred to as the factor VIII light chain activation peptide. Factor VIII is proteolytically activated by thrombin or factor Xa, which dissociates it from von Willebrand factor, forming factor Villa, which has procoagulant function. The biological function of factor Villa is to increase the catalytic efficiency of factor IXa toward factor X activation by several orders of magnitude. Thromb inactivated factor Villa is a 160 kDa A1/A2/A3-C1-C2 heterotrimer that forms a complex with factor IXa and factor X on the surface of platelets or monocytes. A "partial domain" as used herein is a continuous sequence of amino acids forming part of a domain. "Subunits" of human or animal factor VIII, as used herein, are the heavy and light chains of the protein. The heavy chain of factor VIII contains three domains, Al , A2, and B. The light chain of factor VIII also contains three domains, A3, CI, and C2. The hybrid factor VIII or fragment thereof can be made (1) by substitution of isolated, plasma-derived animal subunits or human subunits (heavy or light chains) for corresponding human subunits or animal subunits; (2) by substitution of human domains or animal domains (Al, A2, A3, B, CI, and C2) for corresponding animal domains or human domains; (3) by substitution of parts of human domains or animal domains for parts of animal domains or human domains; (4) by substitution of at least one specific sequence including one or more unique human or animal amino acid(s) for the corresponding animal or human amino acid(s); or (5) by substitution of amino acid sequence that has no known sequence identity to factor VIII for at least one sequence including one or more specific amino acid residue(s) in human, animal, or hybrid factor VIII or fragments thereof. A "B-domainless" hybrid factor VIII, hybrid equivalent factor VIII, or fragment of either, as used herein, refers to any one of the hybrid factor VIII constructs described herein that lacks the B domain, or a portion thereof. The terms "epitope" , "antigenic site", and "antigenic determinant" , as used herein, are used synonymously and are defined as a portion of the human, animal, hybrid, or hybrid equivalent factor VIII or fragment thereof that is specifically recognized by an antibody. It can consist of any number of amino acid residues, and it can be dependent upon the primary, secondary, or tertiary structure of the protein. In accordance with this disclosure, a hybrid factor VIII, hybrid factor VIII equivalent, or fragment of either that includes at least one epitope may be used as a reagent in the diagnostic assays described below. In some embodiments, the hybrid or hybrid equivalent factor VIII or fragment thereof is not cross- reactive or is less cross-reactive with all naturally occurring inhibitory factor VIII antibodies than human or porcine factor VIII. The term "immunogenic site" , as used herein, is defined as a region of the human or animal factor VIII, hybrid or hybrid equivalent factor VIII, or fragment thereof that specifically elicits the production of antibody to the factor VIII, hybrid, hybrid equivalent, or fragment in a human or animal, as measured by routine protocols, such as immunoassay, e.g. ELISA, or the Bethesda assay, described herein. It can consist of any number of amino acid residues, and it can be dependent upon the primary, secondary, or tertiary structure of the protein. In some embodiments, the hybrid or hybrid equivalent factor VIII or fragment thereof is nonimmunogenic or less immunogenic in an animal or human than human or porcine factor VIII. As used herein, a "hybrid factor VIII equivalent molecule or fragment thereof" or "hybrid equivalent factor VIII or fragment thereof" is an active factor VIII or hybrid factor VIII molecule or fragment thereof comprising at least one sequence including one or more amino acid residues that have no known identity to human or animal factor VIII sequence substimted for at least one sequence including one or more specific amino acid residues in the human, animal, or hybrid factor VIII or fragment thereof. The sequence of one or more amino acid residues that have no known identity to human or animal factor VIII sequence is also referred to herein as "non-factor VIII amino acid sequence" . In a preferred embodiment, the amino acid(s) having no known sequence identity to factor VIII sequence are alanine residues. In another preferred embodiment, the specific factor VIII sequence for which the amino acid(s) having no known sequence identity to factor VIII sequence are substituted includes an antigenic site that is immunoreactive with naturally occurring factor VIII inhibitory antibodies, such that the resulting hybrid factor VIII equivalent molecule or fragment thereof is less immunoreactive or not immunoreactive with factor VIII inhibitory antibodies. In yet another preferred embodiment, the specific hybrid factor VIII sequence for which the amino acid(s) having no known sequence identity to factor VIII sequence are substituted includes an immunogenic site that elicits the formation of factor VIII inhibitory antibodies in an animal or human, such that the resulting hybrid factor VIII equivalent molecule or fragment thereof is less immunogenic. "Factor VIII deficiency, " as used herein, includes deficiency in clotting activity caused by production of defective factor VIII, by inadequate or no production of factor VIII, or by partial or total inhibition of factor VIII by inhibitors. Hemophilia A is a type of factor VIII deficiency resulting from a defect in an X-linked gene and the absence or deficiency of the factor VIII protein it encodes . As used herein, "diagnostic assays" include assays that in some manner utilize the antigen-antibody interaction to detect and/or quantify the amount of a particular antibody that is present in a test sample to assist in the selection of medical therapies. There are many such assays known to those of skill in the art. As used herein, however, the hybrid or hybrid equivalent factor VIII DNA or fragment thereof and protein expressed therefrom, in whole or in part, can be substituted for the corresponding reagents in the otherwise known assays, whereby the modified assays may be used to detect and/or quantify antibodies to factor VIII. It is the use of these reagents, the hybrid or hybrid equivalent factor VIII DNA or fragment thereof or protein expressed therefrom, that permits modification of known assays for detection of antibodies to human or animal factor VIII or to hybrid human/animal factor VIII. Such assays include, but are not limited to ELISAs, immunodiffusion assays, and immunoblots. Suitable methods for practicing any of these assays are known to those of skill in the art. As used herein, the hybrid or hybrid equivalent factor VIII or fragment thereof that includes at least one epitope of the protein can be used as the diagnostic reagent. Examples of other assays in which the hybrid or hybrid equivalent factor VIII or fragment thereof can be used include the Bethesda assay and anticoagulation assays. The "expression product" of a DNA encoding a human or animal factor VIII or a human/animal hybrid factor VIII or a modified factor VIII is the product obtained from expression of the referenced DNA in a suitable host cell, including such features of pre- or post-translational modification of protein encoded by the referenced DNA, including but not limited to glycosylation, proteolytic cleavage and the like. It is known in the art that such modifications can occur and can differ somewhat depending upon host cell type and other factors, and can result in molecular isoforms of the product, with retention of procoagulant activity. See, e.g. Lind, P. et al. , Eur. J. Biochem. 232: 1927 (1995) incorporated herein by reference. "Immunoreactivity reducing" amino acids are defined herein as those amino acids that are minor contributors, if at all, to the binding energy of an antibody-antigen pair. Non- limiting examples of some amino acids known to be immunoreactivity-reducing include alanine, methionine, leucine, serine and glycine. It will be understood that the reduction of immunoreactivity achievable by a given amino acid substitution in a given antigen-antibody pair will also depend on any effects the substitution may have on protein conformation, epitope accessibility and the like. GENERAL DESCRIPTION OF METHODS U.S. Serial No. 07/864,004 described the discovery of hybrid human/porcine factor VII! molecules having coagulant activity, in which elements of the factor VIII molecule of human or pig are substituted for corresponding elements of the factor VIII molecule of the other species. U.S. Serial No. 08/212,133 and PCT/US94/ 13200 describe procoagulant hybrid human/animal and hybrid equivalent factor VIII molecules, in which elements of the factor VITI molecule of one species are substituted for corresponding elements of the factor VIII molecule of the other species. The present invention provides hybrid human/ animal, animal/animal, and equivalent factor VIII molecules, modified factor VIII molecules and fragments thereof, and the nucleic acid sequences encoding such hybrids and modified factor VIII molecules, some of which have greater coagulant activity in a standard clotting assay when compared to highly-purified human factor VIII; and/or are less immunoreactive to inhibitory antibodies to human or porcine factor VIII than human or porcine factor VIII; and/or are less immunogenic in a human or animal than human or porcine factor VIII; and/or have other thereapeutically useful properties . These hybrid and/or modified factor VIII molecules can be constructed as follows. At least five types of active hybrid human/porcine or hybrid equivalent factor VIII molecules or fragments thereof, the nucleic acid sequences encoding these hybrid factor VITI molecules, and the methods for preparing them are disclosed herein: those obtained (1) by substituting a human or porcine subunit (i.e. , heavy chain or light chain) for the corresponding porcine or human subunit; (2) by substituting one or more human or porcine domain(s) (i.e. , Al, A2, A3, B, CI, and C2) for the corresponding porcine or human domain(s); (3) by substituting a continuous part of one or more human or porcine domain(s) for the corresponding part of one or more porcine or human domain(s); (4) by substimting at least one specific sequence including one or more unique amino acid residue(s) in human or porcine factor VIII for the corresponding porcine or human sequence; and (5) by substituting at least one sequence including one or more amino acid residue(s) having no known sequence identity to factor VIII ("non-factor VIII amino acid sequence") for at least one specific sequence of one or more amino acids in human, porcine, or hybrid human/porcine factor VIII. Modified factor VII! molecules have one or more amino acid replacements at specified positions. At least five types of active hybrid human/non-human, non-porcine mammalian or hybrid equivalent factor VIII molecules or fragments thereof, and the nucleic acid sequences encoding them, can also be prepared by the same methods: those obtained (1) by substituting a human or non-human, non-porcine mammalian subunit (i.e. , heavy chain or light chain) for the corresponding non-human, non-porcine mammalian or human subunit; (2) by substituting one or more human or non-human, non-porcine mammalian domain(s) (i.e. , Al , A2, A3, B, CI and C2) for the corresponding non-human, non-porcine mammalian or human domain(s); (3) by substituting a continuous part of one or more human or non-human, non-porcine mammalian domain(s) for the corresponding part of one or more non-human, non-porcine mammalian or human domain(s); (4) by substimting at least one specific sequence including one or more unique amino acid residue(s) in human or non-human, non-porcine mammalian factor VIII for the corresponding non-human, non-porcine mammalian or human sequence; and (5) by substituting at least one sequence including one or more amino acid residue(s) having no known sequence identity to factor VIII ("non-factor VIII amino acid sequence") for at least one specific sequence of one or more amino acids in human, non-human, non-porcine mammalian, or hybrid human/non-human, non-porcine mammalian factor VIII. Individual amino acid replacements can be obtained by site-directed mutagenesis of the corresponding segment of coding DNA. Further, one skilled in the art will readily recognize that the same methods can be used to prepare at least five types of active hybrid factor VIII molecules or fragments thereof, corresponding to types (l)-(5) in the previous two paragraphs, comprising factor VIII amino acid sequence from two or more non-human mammals, such as porcine/mouse, and further comprising non-factor VIII amino acid sequence. Hybrid human/animal, animal/animal, and equivalent factor VIII proteins or fragments thereof listed above under groups (l)-(3) are made by isolation of subunits, domains, or continuous parts of domains of plasma-derived factor VIII, followed by reconstitution and purification. Hybrid human/animal, animal/animal, and equivalent factor VIII proteins or fragments thereof described under groups (3)-(5) above are made by recombinant DNA methods. The hybrid molecule may contain a greater or lesser percentage of human than animal sequence, depending on the origin of the various regions, as described in more detail below. Since current information indicates that the B domain has no inhibitory epitope and has no known effect on factor VIII function, in some embodiments the B domain is deleted in the active hybrid or hybrid equivalent factor VIII molecules or fragments thereof ("B(-) factor VIII") prepared by any of the methods described herein. It is shown in Example 4 that hybrid human/porcine factor VIII comprising porcine heavy chain and human light chain and corresponding to the first type of hybrid listed above has greater specific coagulant activity in a standard clotting assay compared to human factor VIII. The hybrid human/animal or equivalent factor VIII with coagulant activity, whether the activity is higher, equal to, or lower than that of human factor VIII, can be useful in treating patients with inhibitors, since these inhibitors can react less with hybrid human/ animal or equivalent factor VIII than with either human or porcine factor VIII. Preparation of hybrid factor VIII molecules from isolated human and animal factor VIII subunits by reconstitution: The present invention provides hybrid human/ animal factor VIII molecules or fragments thereof, with subunit substitutions, the nucleic acid sequences encoding these hybrids, methods for preparing and isolating them, and methods for characterizing their procoagulant activity. One method, modified from procedures reported by Fay, P.J. et al. (1990) J. Biol. Chem. 265:6197; and Lollar, J.S. et al. (1988) J. Biol. Chem. 263: 10451 , involves the isolation of subunits (heavy and light chains) of human and animal factor VIII, followed by recombination of human heavy chain and animal light chain or by recombination of human light chain and animal heavy chain. Isolation of both human and animal individual subunits involves dissociation of the light chain heavy chain dimer. This is accomplished, for example, by chelation of calcium with ethylenediaminetetraacetic acid (EDTA), followed by monoS™ HPLC (Pharmacia-LKB, Piscataway, NJ). Hybrid human/ animal factor VIII molecules are reconstituted from isolated subunits in the presence of calcium. Hybrid human light chain/animal heavy chain or animal light chain/human heavy chain factor VIII is isolated from unreacted heavy chains by monoS™ HPLC by procedures for the isolation of porcine factor VIII, such as described by Lollar, J.S. et al. (1988) Blood 71 : 137-143. These methods, used in one embodiment to prepare active hybrid human/porcine factor VIII, described in detail in the examples below, result in hybrid human light chain/porcine heavy chain molecules with greater than six times the procoagulant activity of human factor VIII. Other hybrid human/non-human, non-porcine mammalian factor VIII molecules can be prepared, isolated, and characterized for activity by the same methods. One skilled in the art will readily recognize that these methods can also be used to prepare, isolate, and characterize for activity hybrid animal/animal factor VIII, such as porcine/mouse, comprising the light or heavy chain or one species is combined with the heavy or light chain of the other species. Preparation of hybrid factor VIII molecules from isolated human and animal factor VIII domains by reconstitution: The present invention provides hybrid human/animal factor VIII molecules or fragments thereof with domain substitutions, the nucleic acid sequences encoding them, methods for preparing and isolating them, and methods for characterizing their procoagulant activity. One method involves the isolation of one or more domains of human and one or more domains of animal factor VIII, followed by recombination of human and animal domains to form hybrid human/animal factor VIII with coagulant activity, as described by Lollar, P. et al. (Nov.25, 1992) J. Biol. Chem. 267(33): 23652-23657, for hybrid human/porcine factor VIII. Specifically provided is a hybrid human/porcine factor VIII with substitution of the porcine A2 domain for the human A2 domain, which embodiment illustrates a method by which domain-substituted hybrid human/non-human, non-porcine mammalian factor VIII can be constructed. Plasma-derived non-human, non-porcine mammalian and human A1/A3-C1- C2 dimers are isolated by dissociation of the A2 domain from factor Villa. This is accomplished, for example, in the presence of NaOH, after which the mixmre is diluted and the dimer is eluted using monoS™ HPLC (Pharmacia-LKB, Piscataway, NJ). The A2 domain is isolated from factor Villa as a minor component in the monoS™ HPLC. Hybrid human/animal factor VIII molecules are reconstituted by mixing equal volumes of the A2 domain of one species and the A1/A3-C1-C2 dimer of the other species. Hybrid human/animal factor VIII or fragments thereof with one or more domain substitutions is isolated from the mixmre of unreacted dimers and A2 by monoS™ HPLC by procedures for the isolation of porcine factor VIII, as described by Lollar, J.S. et al. (1988) Blood 71 : 137-143. Routine methods can also be used to prepare and isolate the Al , A3, CI , C2, and B domains of the factor VIII of one species, any one or more of which can be substimted for the corresponding domain in the factor VIII of the other species. One skilled in the art will readily recognize that these methods can also be used to prepare, isolate, and characterize for activity domain-substituted hybrid animal/animal factor VIII, such as porcine/mouse. These methods, described in detail in the examples below, result in hybrid factor VIII molecules with procoagulant activity. Preparation of hybrid factor VIII molecules by recombinant engineering of the sequences encoding human, animal, and hybrid factor VIII subunits, domains, or parts of domains: Substitution of subunits. domains, continuous parts of domains: The present invention provides active, recombinant hybrid human/animal and hybrid equivalent factor VIII molecules and fragments thereof with subunit, domain, and amino acid sequence substitutions, the nucleic acid sequences encoding these hybrids, methods for preparing and isolating them, and methods for characterizing their coagulant, immunoreactive, and immunogenic properties. The human factor VIII gene was isolated and expressed in mammalian cells, as reported by Toole, J.J. et al. (1984) Nature 312:342-347 (Genetics Institute); Gitschier, J. et al.(1984) Nature 312:326-330 (Genentech); Wood, W.I. et al. (1984) Nature 312:330-337 (Genentech); Vehar, G.A. et al. (1984) Nature 312:337-342 (Genentech); WO 87/04187; WO 88/08035; WO 88/03558; U.S. Patent No. 4,757,006, and the amino acid sequence was deduced from cDNA. U.S. Patent No. 4,965, 199 to Capon et al. discloses a recombinant DNA method for producing factor VIII in mammalian host cells and purification of human factor VIII. Human factor VIII expression on CHO (Chinese hamster ovary) cells and BHKC (baby hamster kidney cells) has been reported. Human factor VIII has been modified to delete part or all of the B domain (U.S. Patent No. 4,868,112), and replacement of the human factor VIII B domain with the human factor V B domain has been attempted (U.S. Patent No. 5,004.803). The cDNA sequence encoding human factor VIII and predicted amino acid sequence are shown in SEQ ID NOs: l and 2, respectively. In SEQ ID NO: l , the coding region begins at nucleotide position 208, the triplet GCC being the codon for amino acid number 1 (Ala) as given in SEQ ID NO:2. Porcine factor VIII has been isolated and purified from plasma [Fass, D.N. et al. (1982) Blood 59:594]. Partial amino acid sequence of porcine factor VIII corresponding to portions of the N-terminal light chain sequence having homology to ceruloplasmin and coagulation factor V and largely incorrectly located were described by Church et al. (1984) Proc. Natl. Acad. Sci. USA 81 :6934. Toole, J.J. et al. (1984) Nature 312:342-347 described the partial sequencing of the Ν-terminal end of four amino acid fragments of porcine factor VIII but did not characterize the fragments as to their positions in the factor VIII molecule. The amino acid sequence of the B and part of the A2 domains of porcine factor VIII were reported by Toole, J.J. et al. (1986) Proc. Natl Acad. Sci, USA 83:5939-5942. The cDΝA sequence encoding the complete A2 domain of porcine factor VIII and predicted amino acid sequence and hybrid human/porcine factor VIII having substitutions of all domains, all subunits, and specific amino acid sequences were disclosed in U.S. Serial No. 07/864.004 entitled "Hybrid Human/Porcine factor VIII" filed April 7, 1992 by John S. Lollar and Marschall S. Runge, which issued as U.S. Patent No. 5,364,771 on November 15, 1994, and in WO 93/20093. The cDNA sequence encoding the A2 domain of porcine factor VIII having sequence identity to residues 373-740 in mature human factor VIII, as shown in SEQ ID NO: 1 , and the predicted amino acid sequence are shown in SEQ ID NOs: 3 and 4, respectively. More recently, the nucleotide and corresponding amino acid sequences of the Al and A2 domains of porcine factor VIII and a chimeric factor VIII with porcine Al and/or A2 domains substimted for the corresponding human domains were reported in WO 94/11503. The entire nucleotide sequence encoding porcine factor VIII, including the complete Al domain, activation peptide, A3, CI and C2 domains, as well as the encoded amino acid sequence, is disclosed in U.S. Patent 5,859,204, issued January 12, 1999. Both porcine and human factor VIII are isolated from plasma as a two subunit protein. The subunits, known as the heavy chain and light chain, are held together by a non-covalent bond that requires calcium or other divalent metal ions. The heavy chain of factor VIII contains three domains, Al , A2, and B, which are linked covalently. The light chain of factor VIII also contains three domains, designated A3, CI , and C2. The B domain has no known biological function and can be removed, or partially removed from the molecule proteolytically or by recombinant DNA technology methods without significant alteration in any measurable parameter of factor VIII. Human recombinant factor VIII has a similar strucmre and function to plasma-derived factor VIII, though it is not glycosylated unless expressed in mammalian cells. Both human and porcine activated factor VIII ("factor Villa") have three subunits due to cleavage of the heavy chain between the Al and A2 domains. This strucmre is designated A1/A2/A3-C1-C2. Human factor Villa is not stable under the conditions that stabilize porcine factor Villa, presumably because of the weaker association of the A2 subunit of human factor Villa. Dissociation of the A2 subunit of human and porcine factor Villa is associated with loss of activity in the factor Villa molecule. Yakhyεev, A. et al. (1997) Blood 90:Suppl. 1 , Abstract #126, reported binding of A2 domain by low density lipoprotein receptor-related protein, suggesting that cellular uptake of A2 mediated by such binding acts to down-regulate factor VIII activity. Specifically provided as an exemplary embodiment is active recombinant hybrid human porcine factor VIII having substimted A2 domain, the nucleic acid sequence encoding it, and the methods for preparing, isolating, and characterizing its activity. The methods by which this hybrid construct is prepared can also be used to prepare active recombinant hybrid human/porcine factor VIII or fragments thereof having substitution of subunits, continuous parts of domains, or domains other than A2. One skilled in the art will recognize that these methods also demonstrate how other recombinant hybrid human/non-human, non-porcine mammalian or animal/animal hybrid factor VIII molecules or fragments thereof can be prepared in which subunits, domains, or continuous parts of domains are substimted. Recombinant hybrid human porcine factor VIII is prepared starting with human cDNA (Biogen, Inc.) or porcine cDNA (described herein) encoding the relevant factor VIII sequence. In a preferred embodiment, the factor VIII encoded by the cDNA includes domains A1-A2- A3-C1-C2, lacking the entire B domain, and corresponds to amino acid residues 1-740 and 1649-2332 of single chain human factor VIII (see SEQ ID NO: 2), according to the numbering system of Wood et al. (1984) Nature 312:330-337. Individual subunits, domains, or continuous parts of domains of porcine or human factor VIII cDNA can be and have been cloned and substimted for the corresponding human or porcine subunits, domains, or parts of domains by established mutagenesis techniques. For example, Lubin, I.M. et al. (1994) J. Biol. Chem. 269(12): 8639-8641 describes techniques for substimting the porcine A2 domain for the human domain using convenient restriction sites. Other methods for substimting any arbitrary region of the factor VIII cDNA of one species for the factor VIII cDNA of another species include splicing by overlap extension ("SOE"), as described by Horton, R.M. et al. (1993) Meth. Enzymol 217:270-279. The hybrid factor VIII cDNA encoding subunits, domains, or parts of domains or the entire hybrid cDNA molecules are cloned into expression vectors for ultimate expression of active hybrid human/porcine factor VIII protein molecules in cultured cells by established techniques, as described by Selden, R.F., "Introduction of DNA into mammalian cells, " in Current Protocols in Molecular Biology. F.M. Ausubel et al. , eds (1991). In a preferred embodiment, a hybrid human/porcine cDNA encoding factor VIII, in which the porcine sequence encodes a domain or part domain, such as the A2 domain or part domain, is inserted in a mammalian expression vector, such as ReNeo. to form a hybrid factor VIII construct. Preliminary characterization of the hybrid factor VIII is accomplished by insertion of the hybrid cDNA into the ReNeo mammalian expression vector and transient expression of the hybrid protein in COS-7 cells. A determination of whether active hybrid protein is expressed can then be made. The expression vector construct is used further to stably transfect cells in culture, such as baby hamster kidney cells, using methods that are routine in the art, such as liposome-mediated transfection (Lipofectin™ , Life Technologies, Inc.). Expression of recombinant hybrid factor VIII protein can be confirmed, for example, by sequencing, Northern and Western blotting, or polymerase chain reaction (PCR). Hybrid factor VIII protein in the culture media in which the transfected cells stably expressing the protein are maintained can be precipitated, pelleted, washed, and resuspended in an appropriate buffer, and the recombinant hybrid factor VIII protein purified by standard techniques, including immunoaffinity chromatography using, for example, monoclonal anti-A2- Sepharose™. In a further embodiment, the hybrid factor VIII comprising subunit, domain, or amino acid sequence substimtions is expressed as a fusion protein from a recombinant molecule in which sequence encoding a protein or peptide that enhances, for example, stability, secretion, detection, isolation, or the like is inserted in place adjacent to the factor VIII encoding sequence. Established protocols for use of homologous or heterologous species expression control sequences including, for example, promoters, operators, and regulators, in the preparation of fusion proteins are known and routinely used in the art. See Current Protocols in Molecular Biology (Ausubel, F.M. , et al., eds), Wiley Interscience, N.Y. Expression is enhanced by including portions of the B-domain. In particular, the inclusion of those parts of the B domain designated "SQ" [Lind, P. et al. (1995) supra] results in favorable expression. "SQ" constructs lack all of the human B domain except for 5 amino acids of the B domain N- terminus and 9 amino acids of the B domain C-terminus. The purified hybrid factor VIII or fragment thereof can be assayed for immunoreactivity and coagulation activity by standard assays including, for example, the plasma-free factor VIII assay, the one-stage clotting assay, and the enzyme-linked immunosorbent assay using purified recombinant human factor VIII as a standard. Other vectors, including both plasmid and eukaryotic viral vectors, may be used to express a recombinant gene construct in eukaryotic cells depending on the preference and judgment of the skilled practitioner (see, for example, Sambrook et al. , Chapter 16). Other vectors and expression systems, including bacterial, yeast, and insect cell systems, can be used but are not preferred due to differences in, or lack of, glycosylation. Recombinant hybrid factor VIII protein can be expressed in a variety of cells commonly used for culmre and recombinant mammalian protein expression. In particular, a number of rodent cell lines have been found to be especially useful hosts for expression of large proteins. Preferred cell lines, available from the American Type Culmre Collection, Rockville, MD, include baby hamster kidney cells, and Chinese hamster ovary (CHO) cells which are cultured using routine procedures and media. The same methods employed for preparing hybrid human/porcine factor VIII having subunit, domain, or amino acid sequence substitution can be used to prepare other recombinant hybrid factor VIII protein and fragments thereof and the nucleic acid sequences encoding these hybrids, such as human/non-human, non-porcine mammalian or animal/animal. Starting with primers from the known human DNA sequence, the murine and part of the porcine factor VIII cDNA have been cloned. Factor VIII sequences of other species for use in preparing a hybrid human/animal or animal/animal factor VIII molecule can be obtained using the known human and porcine DNA sequences as a starting point. Other techniques that can be employed include PCR amplification methods with animal tissue DNA, and use of a cDNA library from the animal to clone out the factor VIII sequence. As an exemplary embodiment, hybrid human/mouse factor VIII protein can be made as follows. DNA clones corresponding to the mouse homolog of the human factor VIII gene have been isolated and sequenced and the amino acid sequence of mouse factor VIII protein predicted, as described in Elder, G. , et al. (1993) Genomics 16(2):374-379. which also includes a comparison of the predicted amino acid sequences of mouse, human, and part of porcine factor VIII molecules. The mouse factor VIII cDNA sequence and predicted amino acid sequence are shown in SEQ ID NO: 5 and SEQ ID NO: 8, respectively. In a preferred embodiment, the RNA amplification with transcript sequencing (RAWTS) methods described in Sarkar, G. et al. (1989) Science 244:331-334. can be used. Briefly, the steps are (1) cDNA synthesis with oligo(dT) or an mRNA-specific oligonucleotide primer; (2) polymerase chain reaction (PCR) in which one or both oligonucleotides contains a phage promoter attached to a sequence complementary to the region to be amplified; (3) transcription with a phage promoter; and (4) reverse transcriptase-mediated dideoxy sequencing of the transcript, which is primed with a nested (internal) oligonucleotide. In addition to revealing sequence information, this method can generate an in vitro translation product by incorporating a translation initiation signal into the appropriate PCR primer: and can be used to obtain novel mRNA sequence information from other species. Substitution of amino acid(s): The present invention provides active recombinant hybrid human animal and animal/animal factor VIII molecules or fragments thereof comprising at least one sequence including one or more unique amino acids of one species substimted for the corresponding amino acid sequence of the other species or fragments thereof, nucleic acid sequences encoding these hybrids, methods for preparing and isolating them, and methods for characterizing their coagulant, immunogenic and immunoreactive properties. The A2 domain is necessary for the procoagulant activity of the factor VIII molecule. Studies show that porcine factor VIII has six-fold greater procoagulant activity than human factor VIII (Lollar, P. et al. (1991) /. Biol. Chem. 266: 12481-12486, and that the difference in coagulant activity between human and porcine factor VIII appears to be based on a difference in amino acid sequence between one or more residues in the human and porcine A2 domains (Lollar, P. et al. (1992) /. Biol. Chem. 267:23652-23657. Further, the A2 and C2 domains and possibly a third light chain region in the human factor VIII molecule are thought to harbor the epitopes to which most, if not all, inhibitory antibodies react, according to Hoyer (1994) Semin. Hewatol 31 : 1-5. Recombinant hybrid human/animal, animal/animal, or equivalent factor VIII molecules or fragments thereof can be made by substitution of at least one specific sequence including one or more unique amino acids from the A2, C2, and/or other domains of the factor VIII of one species for the corresponding sequence of the other species, wherein the amino acid sequences differ, as illustrated in more detail below, between the molecules of the two species. In an exemplary preferred embodiment described herein, the present invention provides active recombinant hybrid human/porcine factor VIII comprising porcine amino acid sequence substimted for corresponding human amino acid sequence that includes an epitope, wherein the hybrid factor VIII has decreased or no immunoreactivity with inhibitory antibodies to factor VIII. In a further embodiment, active recombinant hybrid factor VIII molecules can also be made comprising amino acid sequence from more than one species substituted for the corresponding sequence in a third species. Recombinant hybrid equivalent molecules can also be made, comprising human, animal, or hybrid factor VIII including at least one sequence including one or more amino acids that have no known sequence identity to factor VIII, as further described below. Any hybrid factor VIII construct having specific amino acid substitution as described can be assayed by standard procedures for coagulant activity and for reactivity with inhibitory antibodies to factor VIII for identification of hybrid factor VIII molecules with enhanced coagulant activity and/or decreased antibody immunoreactivity. Hybrid molecules may also be identified that have reduced coagulant activity compared to human or porcine factor VIII but also have decreased antibody reactivity. One skilled in the art will recognize that hybrid factor VIII molecules or fragments thereof having less, equal, or greater coagulant activity, compared to human or porcine factor VIII, are useful for treating patients who have a factor VIII deficiency. The methods described herein to prepare active recombinant hybrid human porcine factor VIII with substimtion of specific amino acids can be used to prepare active recombinant hybrid human/non-human, non-porcine mammalian factor VIII protein, hybrid animal- l/animal-2 factor VIII, and hybrid equivalent factor VIII or fragments thereof. Hybrid factor VIII molecules with altered coagulant activity: The present invention provides procoagulant recombinant hybrid human/ animal, animal/animal, or equivalent factor VIII molecules or fragments thereof comprising at least one specific sequence including one or more unique amino acids having procoagulant activity in the factor VIII of one species substimted for the corresponding amino acid sequence of the factor VIII of the other species, using established site-directed mutagenesis techniques as described herein. The specific sequences to be used in the substimtion are selected and the hybrid constructs are prepared and assayed for coagulant activity, as follows. Specifically provided as a preferred and exemplary embodiment is a hybrid human/porcine factor VIII comprising amino acid substimtions in the A2 domain. It is understood that one skilled in the art can use these methods to prepare other hybrid human/ animal , animal/animal , and equivalent factor VIII molecules or fragments thereof having altered coagulant activity, preferably increased coagulant activity compared to human factor VIII. The basis for the greater coagulant activity in porcine factor VIII appears to be the more rapid spontaneous dissociation of the A2 subunit of human factor Villa than porcine factor Villa, which leads to loss of activity, according to Lollar, P. et al. (1990) J. Biol. Chem. 265: 1688-1692; Lollar, P. et al. (1992) J. Biol Chem. 267:23652-23657; Fay, P.J. et al. (1992) 7. Biol. Chem. 267: 13246-13250. A comparison of the alignment of the amino acid sequences of the human and porcine factor VIII A2 domains (residue numbering starts at position 373 with respect to the full length amino acid sequence of human factor VIII, SEQ ID NO:2) is shown in Fig. lC. For preparation of a hybrid human/porcine factor VIII molecule with altered coagulant activity, the initial target candidates for mutagenesis, which were revealed upon comparison of the human and porcine A2 amino acid sequences (SEQ ID NOs: 2 and 6, respectively) within the human A2 domain, are shown in Table I. TABLE I HUMAN AMINO ACID SEQUENCE TARGET CANDIDATES FOR MUTAGENESIS (SEQ ID NO:2) Sequence Residues Mismatches Charge Changes 398-403 6 4 1 434-444 10 4 3 484-496 13 7 3 598-603 6 4 2 536-541 6 4 0 713-722 10 6 2 727-737 11 6 2 Table I and Figs. lA-lB illustrate seven sequences in the human and pig A2 domain amino acid sequences (SEQ ID NOs: 2 and 6, respectively) that constitute only 17% of the A2 domain but include 70% of the sequence differences between human and porcine A2 domains. A recombinant hybrid human porcine construct is described in which amino acids Ser373-Glu604 in the A2 domain (Ser373-Arg740) of human factor VIII have been replaced with the homologous porcine sequence. This construct does not react with A2 inhibitors and has the same coagulant activity as human B(-) factor VIII. A plasma-derived hybrid molecule is described that comprises a complete porcine A2 domain substimtion in the human factor VIII that has increased coagulant activity compared to human factor VIII. Comparison of these constructs indicates that a region between residues Asp605 and Arg740 is responsible for the difference in activity between human and porcine factor VIII. This region can be defined more specifically by systematically making recombinant hybrid human/porcine factor VIII molecules with porcine substimtions in the region between Asp605 and Arg740 by using established site- directed mutagenesis techniques, for example, the "splicing by overlap extension" (SOE) method that has been used extensively to make hybrid factor VIII molecules containing porcine substimtions in the NH 2 -terminal region of A2. These molecules can be expressed in COS-7 cells and baby hamster kidney cells as described above. They can be purified to homogeneity using methods known in the art, such as heparin-Sepharose™ and immunoaffinity chromatography. Protein concentration can be estimated by absorption of ultraviolet light at A 280 , and the specific activity of the constructs can be determined by dividing coagulant activity (measured in units per ml by single stage clotting assay) by A 280 . Human factor VIII has a specific activity of approximately 3000-4000 U/A 28ϋ , whereas porcine factor VIII has a specific activity of approximately 20,000 U/A 28ϋ . In a preferred embodiment, the procoagulant recombinant hybrid human/porcine factor VIII has a specific activity of 20,000 U/A 280 and contains a minimal amount of porcine substimtion in the A2 domain. As described herein, site-directed mutagenesis techniques are used to identify hybrid protein with coagulant activity that can be enhanced, equal to, or reduced, compared to human factor VIII, but preferably is enhanced. In the hybrid human porcine embodiment, specific human sequences are replaced with porcine sequences, preferably using the splicing by overlap extension method (SOE), as described by Ho, S.N., et al. , 77 Gene 51-59 (1994), and in Examples 7 and 8. Oligonucleotide-directed mutagenesis can also be used, as was done to loop out the amino acid sequence for part of the human A2 domain (see Example 7). As functional analysis of the hybrids reveals coagulant activity, the sequence can be further dissected and mapped for procoagulant sequence by standard point mutation analysis techniques. The present invention contemplates that hybrid factor VIII cDNA and protein can be characterized by methods that are established and routine, such as DNA sequencing, coagulant activity assays, mass by ELISA and by UV absorbance at 280 nm of purified hybrid factor VIII, specific coagulant activity (U/mg), SDS-PAGE of purified hybrid factor VIII, and the like. Other known methods of testing for clinical effectiveness may be required, such as amino acid, carbohydrate, sulfate, or metal ion analysis. A recombinant hybrid factor VIII having superior coagulant activity, compared to human factor VIII, may be less expensive to make than plasma-derived factor VIII and may decrease the amount of factor VIII required for effective treatment of factor VIII deficiency. Hybrid factor VIII molecules with reduced immunoreactivity: Epitopes that are immunoreactive with antibodies that inhibit the coagulant activity of factor VIII ("inhibitors" or "inhibitory antibodies") have been characterized based on known structure-function relationships in factor VIII. Presumably, inhibitors could act by disrupting any of the macromolecular interactions associated with the domain strucmre of factor VIII or its associations with von Willebrand factor, thrombin, factor Xa, factor IXa, or factor X. However, over 90% of inhibitory antibodies to human factor VIII act by binding to epitopes located in the 40 kDa A2 domain or 20 kDa C2 domain of factor VIII, disrupting specific functions associated with these domains, as described by Fulcher et al. (1985) Proc. Natl. Acad. Sci USA 82:7728-7732; and Scandella et al. (1988) Proc. Natl. Acad. Sci. USA 85:6152- 6156. In addition to the A2 and C2 epitopes, there may be a third epitope in the A3 or CI domain of the light chain of factor VIII, according to Scandella et al. (1993) Blood 82: 1767- 1775. The significance of this putative third epitope is unknown, but it appears to account for a minor fraction of the epitope reactivity in factor VIII. Anti-A2 antibodies block factor X activation, as shown by Lollar et al. (1994) J. Clin. Invest. 93:2497-2504. Previous mapping smdies by deletion mutagenesis described by Ware et al. (1992) Blood Coagul. Fibrinolysis 3:703-716, located the A2 epitope to within a 20 kDa region of the NH 2 -terminal end of the 40 kDa A2 domain. Competition immunoradiometric assays have indicated that A2 inhibitors recognize either a common epitope or narrowly clustered epitopes, as described by Scandella et al. (1992) Throm. Haemostas 67:665-671. and as demonstrated in Example 8. The present invention provides active recombinant hybrid and hybrid equivalent factor VIII molecules or fragments thereof, the nucleic acid sequences encoding these hybrids, methods of preparing and isolating them, and methods for characterizing them. These hybrids comprise human/ animal, animal/animal, or equivalent hybrid factor VIII molecules, further comprising at least one specific amino acid sequence including one or more unique amino acids of the factor VIII of one species substimted for the corresponding amino acid sequence of the factor VIII of the other species; or comprises at least one sequence including one or more amino acids having no known sequence identity to factor VIII substimted for specific amino acid sequence in human, animal, or hybrid factor VIII. The resulting hybrid factor VIII has reduced or no immunoreactivity to factor VIII inhibitory antibodies, compared to human or porcine factor VIII. Using the approach described in the previous section for substimtion of amino acids in the factor VIII molecule, mutational analysis is employed to select corresponding factor VIII amino acid sequence of one species, preferably porcine, which is substimted for at least one sequence including one or more amino acids in the factor VIII of another species, preferably human, or for amino acid sequence of a hybrid equivalent factor VIII molecule, that includes one or more critical region(s) in the A2, C2, or any other domain to which inhibitory antibodies are directed. The methods are described in more detail below. The resulting procoagulant recombinant hybrid construct has reduced or no immunoreactivity to inhibitory antibodies, compared to human factor VIII, using standard assays. Through systematic substimtion of increasingly smaller amino acid sequences followed by assay of the hybrid construct for immunoreactivity, as described below, the epitope in any domain of a factor VIII molecule is mapped, substimted by amino acid sequence having less or no immunoreactivity, and a hybrid factor VIII is prepared. It is understood that one skilled in the art can use this approach combining epitope mapping, construction of hybrid factor VIII molecules, and mutational analysis of the constructs to identify and replace at least one sequence including one or more amino acids comprising an epitope in the A2, C2, and/or other domains to which inhibitory antibodies are directed and to construct procoagulant recombinant hybrid human/ animal, animal/animal, or equivalent factor VIII or fragments thereof having decreased or no immunoreactivity compared to human or porcine factor VIII. This approach is used, as described in Example 8, to prepare a recombinant procoagulant hybrid human/porcine factor VIII having porcine amino acid substimtions in the human A2 domain and no antigenicity to anti-factor VIII antibodies as an exemplary embodiment. Usually, porcine factor VIII has limited or no reaction with inhibitory antibodies to human factor VIII. The recombinant hybrid human/porcine factor VIII molecules having decreased or no reactivity with inhibitory antibodies based on amino acid substimtion in the A2 domain are prepared, as an example of how hybrid factor VIII can be prepared using the factor VIII of other species and substimtions in domains other than A2, as follows. The porcine A2 domain is cloned by standard cloning techniques, such as those described above and in Examples 6, 7, and 8, and then cut and spliced within the A2 domain using routine procedures, such as using restriction sites to cut the cDNA or splicing by overlap extension (SOE). The resulting porcine amino acid sequence is substimted into the human A2 domain to form a hybrid factor VIII construct, which is inserted into a mammalian expression vector, preferably ReNeo, stably transfected into cultured cells, preferably baby hamster kidney cells, and expressed, as described above. The hybrid factor VIII is assayed for immunoreactivity, for example with anti-A2 antibodies by the routine Bethesda assay or by plasma-free chromogenic substrate assay. The Bethesda unit (BU) is the standard method for measuring inhibitor titers. If the Bethesda titer is not measurable ( < 0.7 BU/mg IgG) in the hybrid, then a human A2 epitope was eliminated in the region of substimted corresponding porcine sequence. The epitope is progressively narrowed, and the specific A2 epitope can thus be determined to produce a hybrid human/porcine molecule with as little porcine sequence as possible. As described herein, a 25-residue sequence corresponding to amino acids Arg484- Ile508 that is critical for inhibitory immunoreactivity has been identified and substimted in the human A2 domain. Within this sequence are only nine differences between human and porcine factor VIII. This region can be further analyzed and substimted. Hybrid human/porcine factor VIII molecules having decreased or no reactivity with inhibitory antibodies based on substimtion of amino acid sequence in the C 1 , C2 or other domain, with or without substimtion in the A2 domain, can also be prepared. The C2 epitope, for example can be mapped using the homolog scanning approach combined with site-directed mutagensesis. More specifically, the procedures can be the same or similar to those described herein for amino acids substimtion in the A2 domain, including cloning the porcine C2 or other domain, for example by using RT-PCR or by probing a porcine liver cDNA library with human C2 or other domain DNA; restriction site techniques and/or successive SOE to map and simultaneously replace epitopes in the C2 or other domain; substimtion for the human C2 or other domain in B(-) factor VIII; insertion into an expression vector, such as pBluescript; expression in cultured cells; and routine assay for immunoreactivity. For the assays, the reactivity of C2 hybrid factor VIII with a C2-specific inhibitor, MR [Scandella et al. (1992) Thomb. Haemostasis 67:665-671 and Lubin et al. (1994)], and/or other C2 specific antibodies prepared by affinity chromatography can be performed. The C2 domain consists of amino acid residues 2173-2332 (SEQ ID NO:2). Within this 154 amino acid region, inhibitor activity appears to be directed to a 65 amino acid region between residues 2248 and 2312, according to Shima, M. et al. (1993) Thromb. Haemostas 69:240-246. If the C2 sequence of human and porcine factor VIII is approximately 85% identical in this region, as it is elsewhere in the functionally active regions of factor VIII, there will be approximately ten differences between human and porcine factor VIII C2 amino acid sequence, which can be used as initial targets to construct hybrids with substimted C2 sequence. It is likely that clinically significant factor VIII epitopes are confined to the A2 and C2 domains. However, if antibodies to other regions (Al, A3, B, or CI domains) of factor VIII are identified, the epitopes can be mapped and eliminated by using the approach described herein for the nonantigenic hybrid human/porcine factor VIII molecules. More specifically, mapping of the putative second light chain epitope and/or any other epitope in any other animal or human factor VIII domain can also be accomplished. Initially, determination of the presence of a third inhibitor epitope in the A3 or CI domains can be made as follows. Using human ("H") and porcine ("p") factor VIII amino acid sequences as a model, Al p -A2 p -A3 p -C1 H -C2 p and Al p -A2 p -A3 H -Cl p -C2 p B-domainless hybrids will be constructed. Inhibitor IgG from approximately 20 patient plasmas (from Dr. Dorothea Scandella, American Red Cross) who have low or undetectable titers against porcine factor VIII will be tested against the hybrids. If the third epitope is in the A3 domain, inhibitory IgG is expected to react with Al p -A2 p -A3 H -Cl p -C2 p but not Al p -A2 p -A3 p -C1 H -C2 p . Conversely, if the third epitope is in the CI domain, then inhibitory IgG is expected to react with Al p -A2 p - A3 p -C1 H -C2 p but not Al p -A2 p -A3 H -Cl p -C2 p . If a third epitope is identified it will be characterized by the procedures described herein for the A2 and C2 epitopes . For example, antibodies specific for the CI or A3 domain epitope can be isolated from total patient IgG by affinity chromatography using the Al p -A2 p -A3 H -Cl p -C2 p and Al p -A2 p -A3 p - C1 H -C2 p hybrids, and by elimination of C2 specific antibodies by passage over recombinant factor VIII C2-Sepharaose™. The putative third epitope will be identified by SOE constructs in which, in a preferred embodiment, portions of the human factor VIII A3 or CI domain are systematically replaced with porcine sequence. Hybrid factor VIII molecules with reduced immunogenicity: A molecule is immunogenic when it can induce the production of antibodies in human or animal. The present invention provides a procoagulant recombinant hybrid human animal or animal/animal factor VIII molecule, hybrid factor VIII equivalent molecule, or fragment of either that is less immunogenic than wild-type human porcine factor VIII in human or animal, comprising at least one specific amino acid sequence including one or more unique amino acids of the factor VIII of one species substimted for the corresponding amino acid sequence that has immunogenic activity of the factor VIII of the other species; or at least one amino acid sequence including one or more amino acids having no known identity to factor VIII substituted for amino acid sequence of the human, animal, or hybrid factor. This hybrid can be used to lower the incidence of inhibitor development in an animal or human and to treat factor VIII deficiency, and would be preferred in treating previously untreated patients with hemophilia. In a preferred embodiment, a modified factor VIII comprises human factor VIII amino acid sequence, further comprising one or more alanine residues substimted for human amino acid sequence having immunogenic activity, resulting in a procoagulant recombinant hybrid equivalent molecule or fragment thereof having reduced or no immunogenicity in human or animal. The process described herein of epitope mapping and mutational analysis combined with substimtion of non-antigenic amino acid sequence in a factor VIII molecule, using hybrid human/porcine factor VIII, produces hybrid molecules with low antigenicity . Using this model and the associated methods, any of the hybrid constructs described herein can be altered by site-directed mutagenesis techniques to remove as much of any functional epitope as possible to minimize the ability of the immune system to recognize the hybrid factor VIII, thereby decreasing its immunogenicity. One method that can be used to further reduce the antigenicity and to construct a less immunogenic hybrid factor VIII is alanine scanning mutagenesis, described by Cunningham, B.C. et al. (1989) Science 244: 1081-1085. of selected specific amino acid sequences in human, animal, or hybrid equivalent factor VIII. In alanine scanning mutagenesis, amino acid side chains that are putatively involved in an epitope are replaced by alanine residues by using site- directed mutagenesis. By comparing antibody binding of alanine mutants to wild-type protein, the relative contribution of individual side chains to the binding interaction can be determined. Alanine substimtions are likely to be especially useful, since side chain contributions to antibody binding are eliminated beyond the β carbon, but, unlike glycine substimtion, main chain conformation is not usually altered. Alanine substimtion does not impose major steric, hydrophobic or electrostatic effects that dominate protein-protein interactions. In protein antigen-antibody interactions, there usually are about 15-20 antigen side chains in contact with the antibody. Side chain interactions, as opposed to main chain interactions, dominate protein-protein interactions. Recent smdies have suggested that only a few (approximately 3 to 5) of these side chain interactions contribute most of the binding energy. See Clackson, T. et al. (1995) Science 267:383-386. An extensive analysis of growth hormone epitopes for several murine monoclonal antibodies revealed the following hierarchy for side chain contributions to the binding energy: Arg > Pro > Glu - Asp - Phe -He, with Trp, Ala, Gly, and Cys not tested [Jin, L. et al. (1992) J. Mol. Biol. 226:851-865]. Results with the A2 epitope described herein are consistent with this, since twelve of the 25 residues in the 484-508 A2 segment contain these side chains (Fig. lC). The finding that certain amino acid residues are particularly well recognized by antibodies, indicates that elimination of these residues from a known epitope can decrease the ability of the immune system to recognize these epitopes, i.e. , can make a molecule less immunogenic. In the case of the A2 epitope, immunogenic residues can be replaced without loss of factor VIII coagulant activity. For example, in HP9, Arg484 is replaced by Ser, Pro485 is replaced by Ala, Arg489 is replaced by Gly, Pro492 is replaced by Leu, and Phe501 is replaced by Met. Further, results from the patient plasmas used to test immunoreactivity in hybrid human/porcine factor VIII constructs, described in Example 8, indicate that antibodies from different patients recognize the same or a very similar strucmral region in the A2 domain and that the residues in the A2 domain that participate in binding A2 inhibitors appear to show little variation. Thus, the A2 epitope included in human factor VIII residues 484-508 is an immunodominant epitope in that it is recognized by the human immune system better than other strucmral regions of factor VIII. Replacing this strucmre by nonantigenic factor VIII sequence from another species or by non-factor VIII amino acid sequence, while retaining full procoagulant activity, is expected to alter recognition of hybrid or hybrid equivalent factor VIII by the immune system. It is anticipated that site-directed mutagenesis to replace bulky and/or charged residues that tend to dominate epitopes with small, neutral side chains (e.g. , alanine) may produce a less immunogenic region. It is expected that a molecule containing a few of these substimtions at each significant inhibitor epitope will be difficult for the immune system to fit by the lock-and- key mechanism that is typical of antigen-antibody interactions. Because of its low antigenicity, such a hybrid molecule could be useful in treating factor VIII deficiency patients with inhibitors, and because of its low immunogenicity, it could be useful in treating previously untreated patients with hemophilia A. A general result is that mutation of one of a few key residues is sufficient to decrease the binding constant for a given protein-protein interaction by several orders of magnitude. Thus, it appears likely that all factor VIII epitopes contain a limited number of amino acids that are critical for inhibitor development. For each epitope in factor VIII, alanine substimtions for at least one sequence including one or more specific amino acids having immunogenic activity, may produce an active molecule that is less immunogenic than wild-type factor VIII. In a preferred embodiment, the hybrid factor VIII is B-domainless. The methods for preparing active recombinant hybrid or hybrid equivalent factor VIII with substimtion of amino acid sequence having little or no immunogenic activity for amino acid sequence in the factor VIII having immunogenic activity are as follows, using hybrid human/porcine factor VIII with amino acid substimtions in the A2 domain as an exemplary embodiment. There are 25 residues in the human factor VIII region 484-508. Site-directed mutagenesis can be used to make single mutants in which any of these residues is replaced by any of the other 19 amino acids for a total of 475 mutants. Furthermore, hybrid molecules having more than one mutation can be constructed. The hybrid constructs can be assayed for antigenicity by measuring the binding constant for inhibitor antibodies, as described by Friguet, B. et al. (1985) J. Immunol. Methods 77:305- 319 (1985). In a preferred embodiment, the binding constant will be reduced by at least three orders of magnitude, which would lower the Bethesda titer to a level that is clinically insignificant. For example, the IC 50 (a crude measure of the binding constant) of inhibition by A2 antibodies was reduced in hybrid human/porcine factor VIII constructs HP2, HP4, HP5, HP7, and HP9, described in Example 8, and this was associated with a reduction in Bethesda titer to an unmeasurable level. It is anticipated, for example, that a double or triple alanine mutant of human factor VIII (e. g. , a human factor VIII Arg484- > Ala. Arg489- > Ala, Phe501- > Ala triple mutant) will produce a molecule with sufficiently low antigenicity for therapeutic use. Similar mutations can be made in the C2 epitope and the putative third epitope. A preferred embodiment comprises two or three alanine substimtions into two or three factor VIII epitopes. Other substimtions into these regions can also be done. In a preferred embodiment, hybrid equivalent factor VIII molecules will be identified that are less antigenic and/or immunogenic in human and animal than either human or porcine factor VIII. Such hybrid equivalent constructs can be tested in animals for their reduced antigenicity and/or immunogenicity. For example, control and factor VIII deficient rabbits, pigs, dogs, mice, primates, and other mammals can be used as animal models. In one experimental protocol, the hybrid or hybrid equivalent factor VIII can be administered systematically over a period of six months to one year to the animal, preferably by intravenous infusion, and in a dosage range between 5 and 50 Units/kg body weight, preferably 10-50 Units/kg, and most preferably 40 Units/kg body weight. Antibodies can be measured in plasma samples taken at intervals after the infusions over the duration of the testing period by routine methods, including immunoassay and the Bethesda assay. Coagulant activity can also be measured in samples with routine procedures, including a one-stage coagulation assay. The hybrid equivalent factor VIII molecules can be tested in humans for their reduced antigenicity and/or immunogenicity in at least two types of clinical trials. In one type of trial, designed to determine whether the hybrid or hybrid equivalent factor VIII is immunoreactive with inhibitory antibodies, hybrid or hybrid equivalent factor VIII is administered, preferably by intravenous infusion, to approximately 25 patients having factor VIII deficiency who have antibodies to factor VIII that inhibit the coagulant activity of therapeutic human or porcine factor VIII. The dosage of the hybrid or hybrid equivalent factor VIII is in a range between 5 and 50 Units/kg body weight, preferably 10-50 Units/kg, and most preferably 40 Units/kg body weight. Approximately 1 hour after each administration, the recovery of factor VIII from blood samples is measured in a one-stage coagulation assay. Samples are taken again approximately 5 hours after infusion, and recovery is measured. Total recovery and the rate of disappearance of factor VIII from the samples is predictive of the antibody titer and inhibitory activity. If the antibody titer is high, factor VIII recovery usually cannot be measured. The recovery results are compared to the recovery of recovery results in patients treated with plasma-derived human factor VIII, recombinant human factor VIII, porcine factor VIII, and other commonly used therapeutic forms of factor VIII or factor VIII substitutes. In a second type of clinical trial, designed to determine whether the hybrid or hybrid equivalent factor VIII is immunogenic, i.e. , whether patients will develop inhibitory antibodies, hybrid or hybrid equivalent factor VIII is administered, as described in the preceding paragraph, to approximately 100 previously untreated hemophiliac patients who have not developed antibodies to factor VIII. Treatments are given approximately every 2 weeks over a period of 6 months to 1 year. At 1 to 3 month intervals during this period, blood samples are drawn and Bethesda assays or other antibody assays are performed to determine the presence of inhibitory antibodies. Recovery assays can also be done, as described above, after each infusion. Results are compared to hemophiliac patients who receive plasma-derived human factor VIII, recombinant human factor VIII, porcine factor VIII, or other commonly used therapeutic forms of factor VIII or factor VIII substitutes. Preparation of hybrid factor VIII molecules using human and non-porcine, non-human mammalian factor VIII amino acid sequence: The methods used to prepare hybrid human/porcine factor VIII with substimtion of specific amino acids can be used to prepare recombinant hybrid human/non-human, non- porcine mammalian or animal/animal factor VIII protein that has, compared to human or porcine factor VIII, altered or the same coagulant activity and/or equal or reduced immunoreactivity and/or immunogenicity, based on substimtion of one or more amino acids in the A2, C2, and/or other domains. Similar comparisons of amino acid sequence identity can be made between human and non-human, non-porcine mammalian factor VIII proteins to determine the amino acid sequences in which procoagulant activity, anti-A2 and anti-C2 immunoreactivity, and or immunogenicity, or immunoreactivity and/or immunogenicity in other domains reside. Similar methods can then be used to prepare hybrid human/non-human, non-porcine mammalian factor VIII molecules. As described above, functional analysis of each hybrid will reveal those with decreased reactivity to inhibitory antibodies, and/or reduced immunogenicity, and/or increased coagulant activity, and the sequence can be further dissected by point mutation analysis. For example, hybrid human/ mouse factor VIII molecules can be prepared as described above. The amino acid sequence alignment of the A2 domain of human (SEQ ID NO: 2) and mouse (SEQ ID NO: 6) is shown in Fig. IC. As reported by Elder et al. , the factor VIII protein encoded by the mouse cDNA (SEQ ID NO: 5) has 2319 amino acids, with 74% sequence identity overall to the human sequence (SEQ ID NO: 2) (87% identity when the B domain is excluded from the comparison), and is 32 amino acids shorter than human factor VIII. The amino acid sequences in the mouse A and C domains (SEQ ID NO: 6) are highly conserved, with 84-93 % sequence identity to the human sequence (SEQ ID NO: 2), while the B and the two short acidic domains have 42-70% sequence identity. Specifically, the Al , A2, and A3 mouse amino acid sequences (SEQ ID NO: 6) are 85, 85, and 90% identical to the corresponding human amino acid sequences (SEQ ID NO: 2). The CI and C2 mouse amino acid sequences are 93 and 84% identical to the corresponding human amino acid sequences. In the predicted mouse factor VIII amino acid sequence (SEQ ID NO: 6), the Al , A2, and A3 domains are homologous to human factor VIII amino acids 1-372, 373-740, and 1690-2032, respectively, using amino acid sequence identity for numbering purposes. The thrombin/factor Xa and all but one activated protein C cleavage sites are conserved in mouse factor VIII. The tyrosine residue for von Willebrand factor binding is also conserved. According to Elder et al. , the nucleotide sequence (SEQ ID NO: 5) of mouse factor VIII contains 7519 bases and has 67% identity overall with the human nucleotide sequence (SEQ ID NO: 1). The 6957 base pairs of murine coding sequence have 82% sequence identity with the 7053 base pairs of coding sequence in human factor VIII. When the B domain is not included in the comparison, there is an 88% nucleotide sequence identity. Elder et al. report that human and mouse factor VIII molecules are 74% identical overall, and that 95 % of the human residues that lead to hemophilia when altered are identical in the mouse. These data support the application of the same techniques used to identify amino acid sequence with coagulant activity and/or immunoreactivity to antibodies in the porcine factor VIII molecule to the mouse or other animal factor VIII to identify similar amino acid sequences and prepare hybrid molecules. Preparation of hybrid factor VIII molecules having reduced cross-reactivity using human and non-human, non-porcine mammalian factor VIII amino acid sequence and non-factor VIII amino acid sequence: Porcine factor VIII is used clinically to treat factor VIII deficiency patients who have inhibitory antibodies to human factor VIII. Cross-reactivity, in which human plasma reacts with porcine factor VIII, can be reduced by preparation of hybrid porcine/non-human, non- porcine mammalian or hybrid equivalent factor VIII. In a preferred embodiment, a determination of whether human A2, C2, or other domain-specific inhibitors react with non- human, non-porcine mammalian ("other mammalian") factor VIII is made, using the routine Bethesda assay and the particular other mammalian plasma as the standard. Inhibitor titers are usually measured in plasma, so purified other mammalian factor VIII is not necessary. If the inhibitors do not react with the other mammalian factor VIII, such as murine factor VIII, the sequence of which is known, then corresponding other mammalian sequence can be substimted into the porcine epitope region, as identified by using human/porcine hybrids. Once the animal sequence is known, site directed mutagenesis techniques, such as oligonucleotide-mediated mutagenesis described by Kunkel, T.A. et al. (1991) Meth. Enzymol 204: 125-139, can be used to prepare the hybrid porcine/animal factor VIII molecule. If other animal plasmas are less reactive with A2, C2, or other factor VIII inhibitors than murine or porcine factor VIII, the animal sequence corresponding to the porcine epitope can be determined by routine procedures, such as RT-PCR, and a hybrid human/animal or porcine/animal factor VIII constructed by site- directed mutagenesis. Also, hybrid human/animal or porcine/non-porcine mammalian factor VIII having reduced cross-reactivity with human plasma compared to porcine factor VIII can be prepared that has corresponding amino acid sequence substimtion from one or more other animals. In a further embodiment, cross-reactivity can be reduced by substimtion of amino acid sequence having no known identity to factor VIII amino acid sequence, preferably alanine residues using alanine scanning mutagenesis techniques, for porcine epitope sequence. After identification of clinically significant epitopes, recombinant hybrid factor VIII molecules will be expressed that have less than or equal cross-reactivity compared with porcine factor VIII when tested in vitro against a broad survey of inhibitor plasmas . Preferably these molecules will be combined A2/C2 hybrids in which immunoreactive amino acid sequence in these domains is replaced by other mammalian sequence. Additional mutagenesis in these regions may be done to reduce cross-reactivity. Reduced cross-reactivity, although desirable, is not necessary to produce a product that may have advantages over the existing porcine factor VIII concentrate, which produces side effects due to contaminant porcine proteins and may produce untoward effects due to the immunogenicity of porcine factor VIII sequences. A hybrid human/other mammalian or porcine/other mammalian factor VIII molecule will not contain foreign porcine proteins. Additionally, the extensive epitope mapping accomplished in the porcine A2 domain indicates that greater than 95 % of the therapeutic hybrid human porcine factor VIII sequence will be human. Preparation of hybrid factor VIII equivalents: The methods for amino acid substimtion in factor VIII molecules described above and in the examples can also be used to prepare procoagulant recombinant hybrid factor VIII equivalent molecules or fragments thereof comprising at least one amino acid sequence including one or more amino acids having no known amino acid sequence identity to factor VIII ("non-factor VIII sequence") substimted for at least one specific amino acid sequence that includes an antigenic and/or immunogenic site in human, animal, or hybrid factor VIII. The resulting active hybrid factor VIII equivalent molecule has equal or less reactivity with factor VIII inhibitory antibodies and/or less immunogenicity in human and animals than the unsubstimted human, animal, or hybrid factor VIII. Suitable amino acid residues that can be substimted for those sequences of amino acids critical to coagulant and/or antigenic and/or immunogenic activity in human or animal factor VIII or hybrid human/animal factor VIII to prepare a hybrid equivalent factor VIII molecule include any amino acids having no known sequence identity to animal or human factor VIII amino acid sequence that has coagulant, antigenic, or immunogenic activity. In a preferred embodiment, the amino acids that can be substimted include alanine residues using alanine scanning mutagenesis techniques. Hybrid factor VIII equivalent molecules described herein also include those molecules in which amino acid residues having no known identity to animal factor VIII sequence are substimted for amino acid residues not critical to coagulant, antigenic, or immunogenic activity. As described above, in one embodiment of a hybrid factor VIII equivalent molecule, the molecule has reduced cross-reactivity with inhibitor plasmas. One or more epitopes in the cross-reactive factor VIII are identified, as described above, and then replaced by non-factor VIII amino acid sequence, preferably alanine residues, using, for example, the alanine scanning mutagenesis method. In a preferred embodiment, a procoagulant recombinant hybrid factor VIII equivalent molecule is prepared comprising at least one sequence including one or more amino acids having no known sequence identity to factor VIII, preferably alanine residues, substimted for at least one sequence including one or more amino acids including an epitope, and/or for at least one sequence including one or more amino acids including an immunogenic site, preferably in human factor VIII. The resulting hybrid equivalent factor VIII molecule or fragment thereof has reduced or no immunoreactivity with inhibitory antibodies to factor VIII and/or reduced or no immunogenicity in human or animals. The methods for identifying specific antigenic amino acid sequence in the A2 domain of human factor VIII for substimtion by nonantigenic porcine unique amino acid sequence are described in Examples 7 and 8 and are exemplary for identifying antigenic sequence in the A2 and other domains of human and animal factor VIII and for using site-directed mutagenesis methods such as alanine scanning mutagenesis to substitute non-factor VIII amino acid sequence. Since the human A2 epitope has been narrowed to 25 or few amino acids, as described in Example 8, alanine scanning mutagenesis can be performed on a limited number of hybrid factor VIII constructs having human amino acid sequence to determine which are procoagulant, non-immunoreactive and/or nonimmunogenic hybrid factor VIII constructs based on A2 amino acid substimtions. In the A2 domain, the most likely candidates for alanine substimtions to achieve both reduced antigenicity and immunogenicity in the hybrid construct are Arg484, Pro485, Tyr487, Ser488, Arg489, Pro492, Val495, Phe501, and Ile508. The binding affinity of a hybrid construct comprising each of these mutants for mAb413 and a panel of A2 specific patient IgGs will be determined by ELISA. Any mutant that is active and has a binding affinity for A2 inhibitors that is reduced by more than 2 orders of magnitude is a candidate for the A2 substimted factor VIII molecule. Constructs having more than one mutation will be selected, based on the assumption that the more the epitope is altered, the less immunogenic it will be. It is possible that there are other candidate residues in the region between Arg484-Ile508, since there may be key residues for the epitope that are common to both human and porcine factor VIII. For example, charged residues are frequently involved in protein-protein interactions and, in fact, an alanine substitute for Arg490 produces a factor VIII procoagulated having only 0.2% of the reactivity to inhibitor of human factor VIII (Table VI). Similarly, an alanine substimtion for Lys493 is a possible candidate. This procedure will be carried out in the C2 epitope and the putative third epitope, which is thought to be in the A3 or CI domains, as well as any other epitopes identified in factor VIII, to prepare hybrid equivalent factor VIII constructs. Diagnostic Assays. The hybrid human/animal, animal/animal, or equivalent factor VIII cDNA and/or protein expressed therefrom, in whole or in part, can be used in assays as diagnostic reagents for the detection of inhibitory antibodies to human or animal factor VIII or to hybrid human/ animal factor or equivalent VIII in substrates, including, for example, samples of serum and body fluids of human patients with factor VIII deficiency. These antibody assays include assays such as ELISA assays, immunoblots, radioimmunoassays, immunodiffusion assays, and assay of factor VIII biological activity (e.g. , by coagulation assay). Techniques for preparing these reagents and methods for use thereof are known to those skilled in the art. For example, an immunoassay for detection of inhibitory antibodies in a patient serum sample can include reacting the test sample with a sufficient amount of the hybrid human/ animal factor VIII that contains at least one antigenic site, wherein the amount is sufficient to form a detectable complex with the inhibitory antibodies in the sample. Nucleic acid and amino acid probes can be prepared based on the sequence of the hybrid human porcine, human/non-human, non-porcine mammalian, animal/animal, or equivalent factor VIII cDNA or protein molecule or fragments thereof. In some embodiments, these can be labeled using dyes or enzymatic, fluorescent, chemiluminescent, or radioactive labels that are commercially available. The amino acid probes can be used, for example, to screen sera or other body fluids where the presence of inhibitors to human, animal, or hybrid human/animal factor VIII is suspected. Levels of inhibitors can be quantitated in patients and compared to healthy controls, and can be used, for example, to determine whether a patient with a factor VIII deficiency can be treated with a hybrid human/animal or hybrid equivalent factor VIII. The cDNA probes can be used, for example, for research purposes in screening DNA libraries. Pharmaceutical Compositions. Pharmaceutical compositions containing hybrid human/ animal, porcine/non-human, non-porcine mammalian, animal- l/animal-2, or equivalent factor VIII. alone or in combination with appropriate pharmaceutical stabilization compounds, delivery vehicles, and/or carrier vehicles, are prepared according to known methods, as described in Remington's Pharmaceutical Sciences by E.W. Martin. In one preferred embodiment, the preferred carriers or delivery vehicles for intravenous infusion are physiological saline or phosphate buffered saline. In another preferred embodiment, suitable stabilization compounds, delivery vehicles, and carrier vehicles include but are not limited to other human or animal proteins such as albumin. Phospholipid vesicles or liposomal suspensions are also preferred as pharmaceutically acceptable carriers or delivery vehicles. These can be prepared according to methods known to those skilled in the art and can contain, for example, phosphatidylserine/- phosphatidylcholine or other compositions of phospholipids or detergents that together impart a negative charge to the surface, since factor VIII binds to negatively charged phospholipid membranes. Liposomes may be prepared by dissolving appropriate lipid(s) (such as stearoyl phosphatidyl ethanolamine, stearoyl phosphatidyl choline, arachadoyl phosphatidyl choline, and cholesterol) in an inorganic solvent that is then evaporated, leaving behind a thin film of dried lipid on the surface of the container. An aqueous solution of the hybrid factor VIII is then introduced into the container. The container is then swirled by hand to free lipid material from the sides of the container and to disperse lipid aggregates, thereby forming the liposomal suspension. The hybrid factor or hybrid equivalent factor VIII can be combined with other suitable stabilization compounds, delivery vehicles, and/or carrier vehicles, including vitamin K dependent clotting factors, tissue factor, and von Willebrand factor (vWf) or a fragment of vWf that contains the factor VIII binding site, and polysaccharides such as sucrose. Hybrid or hybrid equivalent factor VIII can also be delivered by gene therapy in the same way that human factor VIII can be delivered, using delivery means such as retroviral vectors. This method consists of incorporation of factor VIII cDNA into human cells that are transplanted directly into a factor VIII deficient patient or that are placed in an implantable device, permeable to the factor VIII molecules but impermeable to cells, that is then transplanted. The preferred method will be retroviral-mediated gene transfer. In this method, an exogenous gene (e.g., a factor VIII cDNA) is cloned into the genome of a modified retrovirus. The gene is inserted into the genome of the host cell by viral machinery where it will be expressed by the cell. The retroviral vector is modified so that it will not produce virus, preventing viral infection of the host. The general principles for this type of therapy are known to those skilled in the art and have been reviewed in the literature [e.g. , Kohn. D.B. et al. (1989) Transufusion 29:812-820]. Hybrid factor VIII can be stored bound to vWf to increase the half-life and shelf-life of the hybrid molecule. Additionally, lyophilization of factor VIII can improve the yields of active molecules in the presence of vWf. Current methods for storage of human and animal factor VIII used by commercial suppliers can be employed for storage of hybrid factor VIII. These methods include: (1) lyophilization of factor VIII in a partially-purified state (as a factor VIII "concentrate" that is infused without further purification); (2) immunoaffmity-purification of factor VIII by the Zimmerman method and lyophilization in the presence of albumin, which stabilizes the factor VIII; (3) lyophilization of recombinant factor VIII in the presence of albumin. Additionally, hybrid factor VIII has been indefinitely stable at 4° C in 0.6 M NaCI, 20 mM MES, and 5 mM CaCl 2 at pH 6.0 and also can be stored frozen in these buffers and thawed with minimal loss of activity. Methods of Treatment. Hybrid or hybrid equivalent factor VIII is used to treat uncontrolled bleeding due to factor VIII deficiency (e.g. , intraarticular, intracranial, or gastrointestinal hemorrhage) in hemophiliacs with and without inhibitory antibodies and in patients with acquired factor VIII deficiency due to the development of inhibitory antibodies. The active materials are preferably administered intravenously. Additionally, hybrid or hybrid equivalent factor VIII can be administered by transplant of cells genetically engineered to produce the hybrid or by implantation of a device containing such cells, as described above. In a preferred embodiment, pharmaceutical compositions of hybrid or hybrid equivalent factor VIII alone or in combination with stabilizers, delivery vehicles, and/or carriers are infused into patients intravenously according to the same procedure that is used for infusion of human or animal factor VIII. The treatment dosages of hybrid or hybrid equivalent factor VIII composition that must be administered to a patient in need of such treatment will vary depending on the severity of the factor VIII deficiency. Generally, dosage level is adjusted in frequency, duration, and units in keeping with the severity and duration of each patient's bleeding episode. Accordingly, the hybrid factor VIII is included in the pharmaceutically acceptable carrier, delivery vehicle, or stabilizer in an amount sufficient to deliver to a patient a therapeutically effective amount of the hybrid to stop bleeding, as measured by standard clotting assays. Factor VIII is classically defined as that substance present in normal blood plasma that corrects the clotting defect in plasma derived from individuals with hemophilia A. The coagulant activity in vitro of purified and partially-purified forms of factor VIII is used to calculate the dose of factor VIII for infusions in human patients and is a reliable indicator of activity recovered from patient plasma and of correction of the in vivo bleeding defect. There are no reported discrepancies between standard assay of novel factor VIII molecules in vitro and their behavior in the dog infusion model or in human patients, according to Lusher, J.M. et al. 328 New Engl J. Med. 328:453-459; Pittman, D.D. et al. (1992) R/ooJ79:389-397; and Brinkhous et al. (1985) Proc. Natl Acad. Sci. 82:8752-8755. Usually, the desired plasma factor VIII level to be achieved in the patient through administration of the hybrid or hybrid equivalent factor VIII is in the range of 30-100% of normal. In a preferred mode of administration of the hybrid or hybrid equivalent factor VIII, the composition is given intravenously at a preferred dosage in the range from about 5 to 50 units/kg body weight, more preferably in a range of 10-50 units/kg body weight, and most preferably at a dosage of 20-40 units/kg body weight; the interval frequency is in the range from about 8 to 24 hours (in severely affected hemophiliacs); and the duration of treatment in days is in the range from 1 to 10 days or until the bleeding episode is resolved. See, e.g. , Roberts, H.R. , and M.R. Jones, "Hemophilia and Related Conditions - Congenital Deficiencies of Profhrombin (Factor II, Factor V, and Factors VII to XII)," Ch. 153, 1453-1474, 1460, in Hematology. Williams, W. J. , et al., ed. (1990). Patients with inhibitors may require more hybrid or hybrid equivalent factor VIII, or patients may require less hybrid or hybrid equivalent factor VIII because of its higher specific activity than human factor VIII or decreased antibody reactivity or immunogenicity. As in treatment with human or porcine factor VIII, the amount of hybrid or hybrid equivalent factor VIII infused is defined by the one- stage factor VIII coagulation assay and, in selected instances, in vivo recovery is determined by measuring the factor VIII in the patient's plasma after infusion. It is to be understood that for any particular subject, specific dosage regimens should be adjusted over time according to the individual need and the professional judgment of the person administering or supervising the administration of the compositions, and that the concentration ranges set forth herein are exemplary only and are not intended to limit the scope or practice of the claimed composition. Treatment can take the form of a single intravenous administration of the composition or periodic or continuous administration over an extended period of time, as required. Alternatively, hybrid or hybrid equivalent factor VIII can be administered subcutaneously or orally with liposomes in one or several doses at varying intervals of time. Hybrid or hybrid equivalent factor VIII can also be used to treat uncontrolled bleeding due to factor VIII deficiency in hemophiliacs who have developed antibodies to human factor VIII. In this case, coagulant activity that is superior to that of human or animal factor VIII alone is not necessary. Coagulant activity that is inferior to that of human factor VIII (i.e. , less than 3,000 units/mg) will be useful if that activity is not neutralized by antibodies in the patient's plasma. It has been demonstrated herein that hybrid factor VIII's and modified factor VIII's can differ in specific activity from human factor VIII. Hybrid, hybrid equivalent and modified factor VIII proteins having greater procoagulant activity from human factor VIII are useful in treatment of hemophilia because lower dosages will be required to correct a patient's factor VIII deficiency. Hybrid, hybrid equivalent and modified factor VIII's having lower procoagulant activity than human factor VIII are also suitable for therapeutic use provided they have at least 1 % of specific activity compared to normal human factor VIII. A hybrid, hybrid equivalent or modified factor VIII of the present invention having procoagulant activity is therefore defined as having at least 1 % of the specific activity of human factor VIII. The hybrid or hybrid equivalent factor VIII molecule and the methods for isolation, characterization, making, and using it generally described above will be further understood with reference to the following non-limiting examples. Example 1: Assay of porcine factor VIII and hybrid human/porcine factor VIII. Porcine factor VIII has more coagulant activity than human factor VIII, based on specific activity of the molecule. These results are shown in Table III in Example 4. This conclusion is based on the use of appropriate standard curves that allow human porcine factor VIII to be fairly compared. Coagulation assays are based on the ability of factor VIII to shorten the clotting time of plasma derived from a patient with hemophilia A. Two types of assays were employed: the one-stage and the two stage assay. In the one-stage assay, 0.1 ml hemophilia A plasma (George King Biomedical, Inc.) was incubated with 0.1 ml activated partial thromboplastin reagent (APTT) (Organon Teknika) and 0.01 ml sample or standard, consisting of diluted, citrated normal human plasma, for 5 min at 37°C in a water bath. Incubation was followed by addition of 0.1 ml 20 mM CaCl 2 , and the time for development of a fibrin clot was determined by visual inspection. A unit of factor VIII is defined as the amount present in 1 ml of citrated normal human plasma. With human plasma as the standard, porcine and human factor VIII activity were compared directly. Dilutions of the plasma standard or purified proteins were made into 0.15 M NaCI, 0.02 M HEPES, pH 7.4. The standard curve was constructed based on 3 or 4 dilutions of plasma, the highest dilution being 1/50, and on log 10 clotting time plotted against log 10 plasma concentration, which results in a linear plot. The units of factor VIII in an unknown sample were determined by interpolation from the standard curve. The one-stage assay relies on endogenous activation of factor VIII by activators formed in the hemophilia A plasma, whereas the two-stage assay measures the procoagulant activity of preactivated factor VIII. In the two-stage assay, samples containing factor VIII that had been reacted with thrombin were added to a mixmre of activated partial thromboplastin and human hemophilia A plasma that had been preincubated for 5 min at 37 °C. The resulting clotting times were then converted to units/ml, based on the same human standard curve described above. The relative activity in the two-stage assay was higher than in the one-stage assay because the factor VIII had been preactivated. Example 2: Characterization of the functional difference between human and porcine factor VIII. The isolation of porcine and human plasma-derived factor VIII and human recombinant factor VIII have been described in the literature in Fulcher, CA. et al. (1982) Proc. Natl. Acad. Sci. USA 79: 1648-1652; Toole et al. (1984) Nature 312:342-347 (Genetics Institute); Gitschier et al. (1984) N ture 312:326-330 (Genentech); Wood et al. (1984) Nature 312:330- 337 (Genentech); Vehar et al. 312 Nature 312:337-342 (Genentech); Fass et al. (1982) Blood 59:594; Toole et al. (1986) Proc. Natl. Acad. Sci. USA 83:5939-5942. This can be accomplished in several ways. All these preparations are similar in subunit composition, although there is a functional difference in stability between human and porcine factor VIII. For comparison of human recombinant and porcine factor VIII, preparations of highly- purified human recombinant factor VIII (Cutter Laboratories, Berkeley, CA) and porcine factor VIII [immunopurified as described in Fass et al. (1982) Blood 59:594] were subjected to high- pressure liquid chromatography (HPLC) over a Mono Q™ (Pharmacia-LKB, Piscataway, ΝJ) anion-exchange column (Pharmacia, Inc.). The purposes of the Mono Q™ HPLC step were elimination of minor impurities of exchange of human and porcine factor VIII into a common buffer for comparative purposes. Vials containing 1000-2000 units of factor VIII were reconstimted with 5 ml H 2 0. Hepes (2 M at pH 7.4) was then added to a final concentration of 0.02 M. Factor VIII was applied to a Mono Q™ HR 5/5 column equilibrated in 0.15 M NaCI, 0.02 M Hepes, 5mM CaCl 2 , at pH 7.4 (Buffer A plus 0.15 M NaCI); washed with 10 ml Buffer A + 0.15 M NaCI; and eluted with a 20 ml linear gradient, 0.15 M to 0.90 M NaCI in Buffer A at a flow rate of 1 ml/min. For comparison of human plasma-derived factor VIII (purified by Mono Q™ HPLC) and porcine factor VIII, immunoaffmity -purified, plasma-derived porcine factor VIII was diluted 1 :4 with 0.04 M Hepes, 5 mM CaCl 2 , 0.01 % Tween-80, at pH 7.4, and subjected to Mono Q™ HPLC under the same conditions described in the previous paragraph for human factor VIII. These procedures for the isolation of human and porcine factor VIII are standard for those skilled in the art. Column fractions were assayed for factor VIII activity by a one-stage coagulation assay. The average results of the assays, expressed in units of activity per A 280 of material, are given in Table II, and indicate that porcine factor VIII has at least six times greater activity than human factor VIII when the one-stage assay is used. TABLE II COMPARISON OF HUMAN AND PORCINE FACTOR VIII COAGULANT ACTIVITY Activity (U/A 280 ) Porcine 21,300 Human plasma-derived 3 , 600 Human recombinant 2,400 Example 3: Comparison of the stability of human and porcine factor VIII. The results of the one-stage assay for factor VIII reflect activation of factor VIII to factor Villa in the sample and possibly loss of formed factor Villa activity. A direct comparison of the stability of human and porcine factor VTII was made. Samples from Mono Q TM J_JPLC (Pharmacia, Inc. , Piscataway, N.J.) were diluted to the same concentration and buffer composition and reacted with thrombin. At various times, samples were removed for two-stage coagulation assay. Typically, peak activity (at 2 min) was 10-fold greater for porcine than human factor Villa, and the activities of both porcine and human factor Villa subsequently decreased, with human factor Villa activity decreasing more rapidly. Generally, attempts to isolate stable human factor Villa are not successful even when conditions that produce stable porcine factor Villa are used. To demonstrate this, Mono Q™ HPLC -purified human factor VIII was activated with thrombin and subjected to Mono S™ cation-exchange (Pharmacia, Inc.) HPLC under conditions that produce stable porcine factor Villa, as described by Lollar et al. (1989) Biochemistry 28:666. Human factor VIII, 43 μg/ml (0.2 μM) in O.2 M NaCI, 0.01 M Hepes. 2.5 mM CaCl 2 , at pH 7.4, in 10 ml total volume, was reacted with thrombin (0.036 μM) for 10 min, at which time FPR-CH 2 C1 D-phenyl-prolyl-arginyl-chloromethyl ketone was added to a concentration of 0.2 μM for irreversible inactivation of thrombin. The mixmre then was diluted 1 : 1 with 40 mM 2-(N-morpholino) ethane sulfonic acid (MES), 5 mM CaCl 2 , at pH 6.0, and loaded at 2 ml/min onto a Mono S™ HR 5/5 HPLC column (Pharmacia, Inc.) equilibrated in 5 mM MES, 5 mM CaCl 2 , at pH 6.0 (Buffer B) plus 0.1 M NaCI. Factor Villa was eluted without column washing with a 20 ml gradient from 0.1 M NaCI to 0.9 M NaCI in Buffer B at 1 ml/min. The fraction with coagulant activity in the two-stage assay eluted as a single peak under these conditions. The specific activity of the peak fraction was approximately 7,500 U/A 280 . Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of the Mono S™ factor Villa peak, followed by silver staining of the protein, revealed two bands corresponding to a heterodimeric (A3-C1-C2/A1) derivative of factor VIII. Although the A2 fragment was not identified by silver staining under these conditions because of its low concentration, it was identified as a trace constituent by I25 I-labeling. In contrast to the results with human factor VIII, porcine factor Villa isolated by Mono S™ HPLC under the same conditions had a specific activity 1.6 x 10° U/A 280 . Analysis of porcine factor Villa by SDS-PAGE revealed 3 fragments corresponding to Al , A2, and A3- C1-C2 subunits, demonstrating that porcine factor Villa possesses three subunits. The results of Mono S™ HPLC of human thrombin-activated factor VIII preparations at pH 6.0 indicate that human factor Villa is labile under conditions that yield stable porcine factor Villa. However, although trace amounts of A2 fragment were identified in the peak fraction, determination of whether the coagulant activity resulted from small amounts of heterotrimeric factor Villa or from heterodimeric factor Villa that has a low specific activity was not possible from this method alone. A way to isolate human factor Villa before it loses its A2 subunit is desirable to resolve this question. To this end, isolation was accomplished in a procedure that involves reduction of the pH of the Mono S™ buffers to pH 5. Mono Q™-purified human factor VIII (0.5 mg) was diluted with H 2 O to give a final composition of 0.25 mg/ml (1 μm) factor VIII in 0.25 M NaCI, 0.01 M Hepes, 2.5 mM CaCl 2 , 0.005% Tween-80, at pH 7.4 (total volume 7.0 ml). Thrombin was added to a final concentration of 0.072 μm and allowed to react for 3 min. Thrombin was then inactivated with FPR-CH 2 C1 (0.2 μm). The mixmre then was diluted 1 : 1 with 40 mM sodium acetate, 5 mM CaCl 2 , 0.01 % Tween-80, at pH 5.0, and loaded at 2 ml/min onto a Mono S™ HR 5/5 HPLC column equilibrated in 0.01 M sodium acetate, 5 mM CaCl 2 , 0.01 % Tween-80, at pH 5.0, plus 0.1 M NaCI. Factor Villa was eluted without column washing with a 20 ml gradient from 0.1 M NaCI to 1.0 M NaCI in the same buffer at 1 ml/min. This resulted in recovery of coagulant activity in a peak that contained detectable amounts of the A2 fragment as shown by SDS-PAGE and silver staining. The specific activity of the peak fraction was tenfold greater than that recovered at pH 6.0 (75,000 U/A 280 v. 7,500 U/A 2g0 ). However, in contrast to porcine factor Villa isolated at pH 6.0, which is indefinitely stable at 4°C, human factor Villa activity decreased steadily over a period of several hours after elution from Mono S™. Additionally, the specific activity of factor Villa purified at pH 5.0 and assayed immediately is only 5% that of porcine factor Villa, indicating that substantial dissociation occurred prior to assay. These results demonstrate that both human and porcine factor Villa are composed of three subunits (Al , A2, and A3-C1-C2). Dissociation of the A2 subunit is responsible for the loss of activity of both human and porcine factor Villa under certain conditions, such as physiological ionic strength, pH, and concentration. The relative stability of porcine factor Villa under certain conditions is because of stronger association of the A2 subunit. Example 4: Preparation of hybrid human/porcine factor VIII by reconstitution with subunits. Porcine factor VIII light chains and factor VIII heavy chains were isolated as follows. A 0.5 M solution of EDTA at pH 7.4 was added to Mono Q™-purified porcine factor VIII to a final concentration of 0.05 M and was allowed to stand at room temperamre for 18-24 h. An equal volume of 10 mM histidine-Cl, 10 mM EDTA, 0.2% v/v Tween 80. at pH 6.0 (Buffer B), was added, and the solution was applied at 1 ml/min to a Mono S™ HR 5/5 column previously equilibrated in Buffer A plus 0.25 M NaCI. Factor VIII heavy chains did not bind the resin, as judged by SDS-PAGE. Factor VIII light chain was eluted with a linear, 20 ml, 0.1-0.7 M NaCI gradient in Buffer A at 1 ml/min and was homogeneous by SDS-PAGE. Factor VIII heavy chains were isolated by mono Q™ HPLC (Pharmacia, Inc. , Piscataway, N.J.) in the following way. Factor VIII heavy chains do not adsorb to mono S™ during the purification of factor VIII light chains. The fall-through material that contained factor VIII heavy chains was adjusted to pH 7.2 by addition of 0.5 M Hepes buffer, pH 7.4, and applied to a mono Q™ HR5/5 HPLC column (Pharmacia, Inc.) equilibrated in 0.1 M NaCI, 0.02 M Hepes, 0.01 % Tween-80, pH 7.4. The column was washed with 10 ml of this buffer, and factor VIII heavy chains were eluted with a 20 ml 0.1-1.0 M NaCI gradient in this buffer. Human light chains and heavy chains were isolated in the same manner. Human and porcine light and heavy chains were reconstimted according to the following steps. Ten μl human or porcine factor VIII light chain, 100 μg/ml, was mixed in 1 M NaCI, 0.02 M Hepes, 5 mM CaCl 2 , 0.01 % Tween-80, pH 7.4, with (1) 25 μl heterologous heavy chain, 60 μg/ml, in the same buffer; (2) 10 μl 0.02 M Hepes, 0.01 % Tween-80, pH 7.4; (3) 5 μl 0.6 M CaCl 2 , for 14 hr at room temperamre. The mixmre was diluted 1/4 with 0.02 M MES, 0.01 % Tween-80, 5 mM CaCl 2 , pH 6 and applied to Mono S™ Hr5/5 equilibrated in 0.1 M NaCI, 0.02 M MES, 0.01 % Tween-80, 5mM Cacl 2 , pH 6.0. A 20 ml gradient was run from 0.1 - 1.0 M NaCI in the same buffer at 1 ml/min, and 0.5 ml fractions were collected. Absorbance was read at 280 nm of fractions, and fractions were assayed with absorbance for factor VIII activity by the one-stage clotting assay. Heavy chains were present in excess, because free light chain (not associated with heavy chain) also binds Mono S™; excess heavy chains ensure that free light chains are not part of the preparation. Reconstimtion experiments followed by Mono S™ HPLC purification were performed with all four possible combinations of chains: human light chain/human heavy chain, human light chain/porcine heavy chain, porcine light chain/porcine heavy chain, porocine light chain/human heavy chain. Table III shows that human light chain/ TABLE III COMPARISON OF HYBRID HUMAN/PORCINE FACTOR VIII COAGULANT ACTIVITY WITH HUMAN AND PORCINE FACTOR III Activity (U/A 280 ) Porcine light chain/porcine heavy chain 30,600 Human light chain/porcine heavy chain 44,100 Porcine light chain/human heavy chain 1 ,100 Human light chain/human heavy chain 1,000 porcine heavy chain factor VIII has activity comparable to native porcine factor VIII (Table II), indicating that strucmral elements in the porcine heavy chain are responsible for the increased coagulant activity of porcine factor VIII compared to human factor VIII. Example 5: Preparation of active hybrid human/porcine factor VIII by reconstitution with domains. The porcine A1/A3-C1-C2 dimer, the porcine A2 domain, the human A1/A3-C1-C2 dimer, and the human A2 domain were each isolated from porcine or human blood, according to the method described in Lollar et al. (1992) J. Biol Chem. 267(33): 23652-23657. For example, to isolate the porcine A1/A3-C1-C2 dimer, porcine factor Villa (140 μg) at pH 6.0 was raised to pH 8.0 by addition of 5 N NaOH for 30 minutes, producing dissociation of the A2 domain and 95 % inactivation by clotting assay. The mixmre was diluted 1 :8 with buffer B (20 mM HEPES, 5 mM CaCl 2 , 0.01 % Tween-80, pH 7.4) and applied to a monoS column equilibrated in buffer B. The A1/A3-C1-C2 dimer eluted as a single sharp peak at approximately 0.4 M NaCI by using a 0.1-1.0 M NaCI gradient in buffer B. To isolate the porcine A2 domain, porcine factor Villa was made according to the method of Lollar et al. (1989) Biochem 28:666-674, starting with 0.64 mg of factor VIII. Free porcine A2 domain was isolated as a minor component (50 μg) at 0.3 M NaCI in the MonoS™ chromatogram. Hybrid human/porcine factor VIII molecules were reconstimted from the dimers and domains as follows. The concentrations and buffer conditions for the purified components were as follows: porcine A2, 0.63 μM in buffer A (5 mM MES; 5 mM CaCl 2 , 0.01 % Tween 80, pH 6.0) plus 0.3 M NaCI; porcine A1/A3-C1-C2. 0.27 μM in buffer B plus 0.4 M NaCI, pH 7.4; human A2. 1 μM in 0.3 M NaCI, 10 mM histidine-HCl, 5 mM CaCl 2 , 0.01 % Tween 20, pH 6.0; human A1/A3-C1-C2. 0.18 μM in 0.5 M NaCI, 10 mM histidine-Cl, 2.5 mM CaCl 2 , 0.1 % Tween-20, pH 6.0. Reconstitution experiments were done by mixing equal volumes of A2 domain and A1/A3-C1-C2 dimer. In mixing experiments with porcine A1/A3- C1-C2 dimer, the pH was lowered to 6.0 by addition of 0.5 M MES, pH 6.0, to 70 mM. The coagulation activities of all four possible hybrid factor Villa molecules - [pA2/(hAl/A3-Cl-C2)] , [hA2/(pAl/A3-Cl-C2)] , [pA2/(pAl/pA3-Cl-C2VJ , and [hA2/(hAl/A3-Cl-C2)] - were obtained by a two-stage clotting assay at various times. The generation of activity following mixing the A2 domains and A1/A3-C1-C2 dimers was nearly complete by one hour and was stable for at least 24 hours at 37 °C. Table IV shows the activity of reconstimted hybrid factor Villa molecules when assayed at 1 hour. The two- stage assay, by which the specific activities of factor Villa molecules were obtained, differs from the one-stage assay, and the values cannot be compared to activity values of factor VIII molecules obtained by a one-stage assay. TABLE IV COMPARISON OF COAGULANT ACTIVITIES OF DOMAIN-SUBSTITUTED HYBRID HUMAN/PORCINE FACTOR Villa Hybrid fVIIIa Specific Activity (U/mg) Porcine A2 + Human 140,000 A1/A3-C1-C2 Porcine A2 + Porcine 70,000 A1/A3-C1-C2 Human A2 + Porcine 40,000 A1/A3-C1-C2 Human A2 + Human 40,000 A1/A3-C1-C2 Table IV shows that the greatest activity was exhibited by the porcine A2 domain/human A1/A3-C1-C2 dimer, followed by the porcine A2 domain/porcine A1/A3-C1- C2 dimer. Thus, when the A2 domain of porcine factor Villa was mixed with the A1/A3-C1- C2 dimer of human factor Villa, coagulant activity was obtained. Further, when the A2 domain of human factor Villa was mixed with the A1/A3-C1-C2 dimer of porcine factor Villa, coagulant activity was obtained. By themselves, the A2, Al , and A3-C1-C2 regions have no coagulant activity. Example 6: Isolation and sequencing of the A2 domain of porcine factor VIII. Only the nucleotide sequence encoding the B domain and part of the A2 domain of porcine factor VIII has been sequenced previously [Toole et al. (1986) Proc. Natl. Acad. Sci. USA 83:5939-5942]. The cDNA and predicted amino acid sequences (SEQ ID NOs: 3 and 4, respectively) for the entire porcine factor VIII A2 domain are disclosed herein. The porcine factor VIII A2 domain was cloned by reverse transcription of porcine spleen total RNA and PCR amplification; degenerate primers based on the known human factor VIII cDNA sequence and an exact porcine primer based on a part of the porcine factor VIII sequence were used. A 1 kb PCR product was isolated and amplified by insertion into a Bluescript™ (Stratagene) phagemid vector. The porcine A2 domain was completely sequenced by dideoxy sequencing. The cDNA and predicted amino acid sequences are as described in SEQ ID NOs: 3 and 4, respectively. Example 7: Preparation of recombinant hybrid human/animal factor VIII The nucleotide and predicted amino acid sequences (SEQ ID NOs: 1 and 2, respectively) of human factor VIII have been described in the literature [Toole et al. (1984) Nature 312:342-347 (Genetics Institute); Gitschier et al. Nature 312:326-330 (Genentech); Wood, et al. (1984) Nature 312:330-337 (Genentech); Vehar et al. Nature 312:337-342 (Genentech)] . Making recombinant hybrid human/animal factor VIII requires that a region of human factor VIII cDΝN (Biogen Corp.) be removed and the animal cDΝA sequence having sequence identity be inserted. Subsequently, the hybrid cDΝA is expressed in an appropriate expression system. As an example, hybrid factor VIII cDΝAs were cloned in which some or all of the porcine A2 domain was substimted for the corresponding human A2 sequences. Initially, the entire cDΝA sequence corresponding to the A2 domain of human factor VIII and then a smaller part of the A2 domain was looped out by oligonucleotide-mediated mutagenesis, a method commonly known to those skilled in the art (see, e.g. , Sambrook, J. , E.F. Fritsch. and T. Maniatis, Molecular Cloning: A Laboratory Manual, Chapter 15, Cold Spring Harbor Press, Cold Spring Harbor, 1989). The steps were as follows. Materials. Methoxycarbonyl-D-cyclohexylglycyl-glycl-arginine-p-nitroanilide (Spectrozyme™ Xa) and anti-factor VIII monoclonal antibodies ESH4 and ESH8 were purchased from American Diagnostica (Greenwich, CT). Unilamellar phosphatidylcholine/phosphatidylserine (75/25, w/w) vesicles were prepared according to the method of Barenholtz, Y. , et al. , 16 Biochemistry 2806-2810 (1977)). Recombinant desulfatohirudin was obtained from Dr. R. B. Wallis, Ciba-Geigy Pharmaceuticals (Cerritos, CA). Porcine factors IXa, X, Xa, and thrombin were isolated according to the methods of Lollar et al. (1984) Blood 63: 1303-1306, and Duffy, E.J. et al. (1992) 7. Biol Chem. 207:7621-7827. Albumin-free pure recombinant human factor VIII was obtained from Baxter-Biotech (Deerfield, IL). Cloning of the porcine factor VIII A2 domain. The cDNA encoding the porcine A2 domain was obtained following PCR of reverse- transcribed porcine spleen mRNA isolated as described by Chomczyneki et al. (1987) Anal. Biochem. 162: 156-159. cDNA was prepared using the first-strand cDNA synthesis kit with random hexamers as primers (Pharmacia, Piscataway, N.J.). PCR was carried out using a 5'- terminal degenerate primer 5' AARCAYCCNAARACNTGGG 3' (SEQ ID NO: 11), based on known limited porcine A2 amino acid sequence, and a 3 '-terminal exact primer, 5' GCTCGCACTAGGGGGTCTTGAATTC 3 ' (SEQ ID NO: 12), based on known porcine DNA sequence immediately 3 ' of the porcine A2 domain. These oligonucleotides correspond to nucleotides 1186-1203 and 2289-2313 in the human sequence (SEQ ID NO: 1). Amplification was carried out for 35 cycles (1 minute 94°C, 2 minutes 50°C, 2 minutes 72°C) using Taq DNA polymerase (Promega Corp. , Madison, Wl). The 1.1-kilobase amplified fragment was cloned into pBluescript II KS-(Stratagene) at the EcoRV site using the T-vector procedure, as described by Murchuk, D. et al. (1991) Nucl. Acids Res. 19: 1154. Escherichia coli XLl-Blue- competent cells were transformed, and plasmid DNA was isolated. Sequencing was carried out in both directions using Sequenase™ version 2.0 (U.S. Biochemical Corp. , a Division of Amersham LifeScience, Inc. , Arlington Hts, IL). This sequence was confirmed by an identical sequence that was obtained by direct sequencing of the PCR product from an independent reverse transcription of spleen RNA from the same pig (CircumVent™, New England Biolabs, Beverly, MA). The region containing the epitope for autoantibody RC was identified as 373- 536 in human factor VIII (SEQ ID NO:2). Construction and expression of a hybrid human/porcine factor VIII cDNA. B-domainless human factor VIII (HB 0 from Biogen, Inc. Cambridge, MA), which lacks sequences encoding for amino acid residues 741-1648 (SEQ ID NO:2), was used as the starting material for construction of a hybrid human/porcine factor VIII. HB " was cloned into the expression vector ReNeo. To facilitate manipulation, the cDNA for factor VIII was isolated as a Xhol/Hpal fragment from ReNeo and cloned into Xhol/EcoRW digested pBlueScript I I K S . A n o l i g o n u c l e o t i d e , 5 ' CCTTCCTTTATCCAAATACGTAGATCAAGAGGAAATTGAC 3' (SEQ ID NO: 7), was used in a site-directed mutagenesis reaction using uracil-containing phage DNA, as described by Kunkel, T.A. et al. (1991) Meth. 204: 125-139. to simultaneously loop-out the human A2 sequence (nucleotides 1169-2304 in SEQ ID NO: l) and introduce a SrcaBI restriction site. The A2-domainless human factor VIII containing plasmid was digested with SnaBI followed by addition of C/al linkers. The porcine A2 domain was then amplified by PCR using the phosphorylated 5' primer 5' GTAGCGTTGCCAAGAAGCACCCTAAGACG 3 ' ( S E Q I D N O : 8 ) a n d 3 ' p r i m e r 5 ' GAAGAGTAGTACGAGTTATTTCTCTGGGTTCAATGAC 3' (SEQ ID NO:9), respectively. Clal linkers were added to the PCR product followed by ligation into the human factor VIII- containing vector. The A1/A2 and All A3 junctions were corrected to restore the precise thrombin cleavage and flanking sequences by site -directed mutagenesis using the oligonucleotide shown in SEQ ID NO: 8 and nucleotides 1-22 (5' GAA . . . TTC in SEQ ID NO: 9) to correct the 5'- and 3'- terminal junctions, respectively. In the resulting construct, designated HP1, the human A2 domain was exactly substimted with the porcine A2 domain. A preliminary product contained an unwanted thymine at the A1-A2 junction as a result of the PCR amplification of the porcine A2 domain. This single base was looped out by use of the mutagenic oligonucleotide 5 ' CCTTTATCCAAATACGTAGCGTTTGCC AAGAAG 3 ' (SEQ ID NO: 10). The resulting hybrid nucleotide sequence encoded active factor Vffl having human Al, porcine A2 and human A3, CI and C2 domains. A region containing 63% of the porcine NH 2 -terminal A2 domain, which encompasses the putative A2 epitope, was substimted for the homologous human sequence of B-domainless cDNA by exchanging SpellBam l fragments between the pBluescript plasmids containing human factor VIII and human/porcine A2 factor VIII cDNA. The sequence was confirmed by sequencing the A2 domain and splice sites. Finally, a Spell Apal fragment, containing the entire A2 sequence, was substimted in place of the corresponding sequence in HB " , producing the HP2 construct. Preliminary expression of HB " and HP2 in COS-7 cells was tested after DEAE-dextran- mediated DNA transfection, as described by Seldon, R.F. , in Current Protocols in Molecular Biology (Ausubel, F.M., et al. , eds), pp. 9.21-9.26, Wiley Interscience, N.Y. After active factor VIII expression was confirmed and preliminary antibody inhibition smdies were done, HB " and HP2 DNA were then stably transfected into baby hamster kidney cells using liposome-mediated transfection (Lipofectin * Life Technologies, Inc. , Gaithersburg, MD). Plasmid-containing clones were selected for G418 resistance in Dulbecco's modified Eagle's medium-F12, 10% fetal calf serum (DMEM-F12/10% fetal calf serum) containing 400 μg/ml G418, followed by maintenance in DMEM-F12/10% fetal calf serum containing 100 μg/ml G418. Colonies showing maximum expression of HB " and HP2 factor VIII activity were selected by ring cloning and expanded for further characterization. HB " and HP2 factor VIII expression was compared by plasma-free factor VIII assay, one-stage clotting assay, and enzyme-linked immunosorbent assay using purified recombinant human factor VIII as a standard. Specific coagulant activities of 2600 and 2580 units/mg were obtained for HB " and HP2, respectively. HB " and HP2 produced 1.2 and 1.4 units/ml/48 hours/ IO 7 cells, respectively. This is identical to that of the wild type construct (2.600 + 200 units/mg). The specific activities of HB " and HP2 were indistinguishable in the plasma-free factor VIII assay. The biological activity of recombinant hybrid human/ animal and equivalent factor VIII with Al, A2, A3, CI, and/or C2 domain substimtions can be evaluated initially by use of a COS-cell mammalian transient expression system. Hybrid human/ animal and equivalent cDNA can be transfected into COS cells, and supernatants can be analyzed for factor VIII activity by use of one-stage and two-stage coagulation assays as described above. Additionally, factor VIII activity can be measured by use of a chromogenic substrate assay, which is more sensitive and allows analysis of larger numbers of samples. Similar assays are standard in the assay of factor VIII activity [Wood et al. (1984) Nature 112:330-337; Toole et al. (1984) Nature 312:342-347]. Expression of recombinant factor VIII in COS cells is also a standard procedure [Toole et al. (1984) Nature 312:342-347: Pittman et al. (1988) Proc. Natl. Acad. Sci. USA 85:2429-2433]. The human factor VIII cDNA used as starting materials for the recombinant molecules described herein has been expressed in COS cells yielding a product with biological activity. This material, as described above, can be used as a standard to compare hybrid human/ animal factor VIII molecules. The activity in the assays is converted to a specific activity for proper comparison of the hybrid molecules. For this, a measurement of the mass of factor VIII produced by the cells is necessary and can be done by immunoassay with purified human and/or animal factor VIII as standards. Immunoassays for factor VIII are routine for those skilled in the art [See, e.g. , Lollar et al. (1988) Blood 7! : 137-143]. Example 8. Determination of inhibitory activity in hybrid human/animal and equivalent factor VIII. Sequences of human and animal factor VIII likely to be involved as epitopes (i.e., as recognition sites for inhibitory antibodies that react with factor VIII) can be determined using routine procedures, for example through use of assay with antibodies to factor VIII combined with site directed mutagenesis techniques such as splicing by overlap extension methods (SOE), as shown below. Sequences of animal factor VIII that are not antigenic compared to corresponding antigenic human sequences can be identified, and substimtions can be made to insert animal sequences and delete human sequences according to standard recombinant DNA methods. Sequences of amino acids such as alanine residues having no known sequence identity to factor VIII can also be substimted by standard recombinant DNA methods or by alanine scanning mutagenesis. Porcine factor VIII reacts less than human factor VIII with some inhibitory antibodies; this provides a basis for current therapy for patients with inhibitors. After the recombinant hybrids are made, they can be tested in vitro for reactivity with routine assays, including the Bethesda inhibitor assay. Those constructs that are less reactive than native human factor VIII and native animal factor VIII are candidates for replacement therapy. The epitopes to which most, if not all, inhibitory antibodies reactive with human factor VIII are directed are thought to reside in two regions in the 2332 amino acid human factor VIII molecule, the A2 domain (amino acid residues 373-740) and the C2 domain (amino acid residues 2173-2332, both sequences shown in SEQ ID NO:2). The A2 epitope has been eliminated by making a recombinant hybrid human-porcine factor VIII molecule in which part of the human A2 domain is replaced by the porcine sequence having sequence identity to the replaced human amino acid sequence. This was accomplished, as described in example 7, by cloning the porcine A2 domain by standard molecular biology techniques and then cutting and splicing within the A2 domain using restriction sites. In the resulting construct, designated HP2, residues 373-604 (SEQ ID NO:4) of porcine factor VIII were substimted into the human A2 domain. HP2 was assayed for immunoreactivity with anti-human factor VIII antibodies using the following methods. Factor VIII enzyme-linked immunosorbent assay. Microtiter plate wells were coated with 0.15 ml of 6 μg/ml ESH4, a human factor VIII light-chain antibody, and incubated overnight. After the plate was washed three times with H 2 O, the wells were blocked for 1 hour with 0.15 M NaCI, 10 mM sodium phosphate, 0.05 % Tween 20, 0.05 % nonfat dry milk, 0.05 % sodium azide, pH 7.4. To increase sensitivity, samples containing factor VIII were activated with 30 nM thrombin for 15 minutes. Recombinant desulfatohirudin then was added at 100 nM to inhibit thrombin. The plate was washed again and 0.1 ml of sample or pure recombinant human factor VIII (10-600 ng/ml), used as the standard, were added. Following a 2 hour incubation, the plate was washed and 0.1 ml of biotinylated ESH8, another factor VIII light-chain antibody, was added to each well. ESH8 was biotinylated using the Pierce sulfosuccinimidyl-6-(biotinamide)hexanoate biotinylation kit. After a 1 hour incubation, the plate was washed and 0.1 ml of strepavidin alkaline phosphatase was added to each well. The plate was developed using the Bio-Rad alkaline phosphatase substrate reagent kit, and the resulting absorbance at 405 nm for each well was determined by using a Vmax microtiter plate reader (Molecular Devices, Inc. , Sunny ville, CA). Unknown factor VIII concentrations were determined from the linear portion of the factor VIII standard curve. Factor VIII assays. HB " and HP2 factor VIII were measured in a one-stage clotting assay, which was performed as described above [Bowie, E.J.W. , and CA. Owen, in Disorders of Hemostasis (Ratnoff and Forbes, eds) pp. 43-72, Grunn & Stratton, Inc. , Orlando, FL (1984)], or by a plasma-free assay as follows. HB " or HP2 factor VIII was activated by 40 nM thrombin in 0.15 M NaCI, 20 nM HEPES, 5 mM CaCl 2 , 0.01 % Tween 80, pH 7.4, in the presence of 10 nM factor IXa , 425 nM factor X , and 50 μM unilamellar phosphatidylserine/phosphatidylcholine (25/75, w/w) vesicles. After 5 minutes, the reaction was stopped with 0.05 M EDTA and 100 nM recombinant desulfatohirudin, and the resultant factor Xa was measured by chromogenic substrate assay, according to the method of Hill- Eubanks et al (1990) J. Biol. Chem. 265: 17854-17858. Under these conditions, the amount of factor Xa formed was linearly proportional to the starting factor VIII concentration as judged by using purified recombinant human factor VIII (Baxter Biotech, Deerfield, IL) as the standard. Prior to clotting assay, HB " or HP2 factor VIII were concentrated from 48 hour conditioned medium to 10-15 units/ml by heparin-Sepharose™ chromatography. HB " or HP2 factor VIII were added to hemophilia A plasma (George King Biomedical) to a final concentration of 1 unit/ml. Inhibitor titers in RC or MR plasma or a stock solution of mAb 413 IgG (4 μM) were measured by the Bethesda assay as described by Kasper, C.K. et al. (1975) Thromb. Diath. Haemorrh 34:869-872. Inhibitor IgG was prepared as described by Leyte, A. et al. (1991) 7. Biol. Chem. 266:740-746. HP2 does not react with anti-A2 antibodies. Therefore, residues 373-603 must contain an epitope for anti-A2 antibodies. Preparation of hybrid human-porcine factor VIII and assay bv splicing bv overlap extension (SOE). Several more procoagulant recombinant hybrid human/porcine factor VIII B-domainless molecules with porcine amino acid substimtions in the human A2 region have been prepared to further narrow the A2 epitope. Besides restriction site techniques, the "splicing by overlap extension" method (SOE) as described by Ho et al. (1989) Gene 77:51-59, has been used to substimte any arbitrary region of porcine factor VIII cDNA. In SOE, the splice site is defined by overlapping oligonucleotides that can be amplified to produce the desired cDNA by PCR. Ten cDNA constructs, designated HP4 through HP13, have been made. They were inserted into the ReNeo expression vector, stably transfected into baby hamster kidney cells, and expressed to high levels [0.5-1 μg (approximately 3-6 units)/ IO 7 cells/24 hours] as described in Example 7. Factor VIII coagulant activity was determined in the presence and absence of a model murine monoclonal inhibitory antibody specific for the A2 domain, mAb413. In the absence of inhibitor, all of the constructs had a specific coagulant activity that was indistinguishable from B(-) human factor VIII. The hybrid human/porcine factor VIII constructs were assayed for reactivity with the anti-A2 inhibitor mAb413 using the Bethesda assay [Kasper et al. (1975) Thromb. Diath. Haemorrh. 34:869-872] . The Bethesda unit (BU) is the standard method for measuring inhibitor titers. The results are shown in Table V, and are compared to recombinant human factor VIII. TABLE V COMPARISON OF IMMUNOREACTIVITY OF AMINO ACID-SUBSTITUTED HYBRID HUMAN/PORCINE FACTOR VIII Construct Porcine Inhibition Substimtion mAb413 (BU/mg IgG) Human B(-) fVffl None 1470 HP4 373-540 < 0.7 HP5 373-508 < 0.7 HP6 373-444 1450 HP7 445-508 <0.7 HP8 373-483 1250 HP9 484-508 < 0.7 HP10 373-403 1170 HP11 404-508 <0.7 HP12 489-508 < 0.7 HP13 484-488 <0.7 The boundaries of porcine substimtions are defined by the first amino acids that differ between human and porcine factor VIII at the NH 2 -terminal and C-terminal ends of the insertion. As shown in Table V, if the Bethesda titer is not measurable ( < 0.7 BU/mg IgG), then an A2 epitope lies in the region of substimted porcine sequence. The epitope has been progressively narrowed to residues 484-509 (SEQ ID NO: 2), consisting of only 25 residues, as exemplified by non-reactivity of mAb413 with HP9. Among constructs HP4 through HP11 , HP9 was the most "humanized" construct that did not react with the inhibitor. This indicates that a critical region in the A2 epitope is located within the sequence Arg484-Ile508. Based on a comparison between human and porcine factor VIII of the amino acid sequence in this critical region, two more constructs, HP 12 and HP13, were made, in which corresponding porcine amino acid sequence was substimted for human amino acids 489-508 and 484-488, respectively. Neither reacts with mAb413. This indicates that residues on each side of the Arg488-Ser489 bond are important for reaction with A2 inhibitors. In HP12 only 5 residues are non-human, and in HP13 only 4 residues are non-human. The 484-508, 484- 488, and 489-508 porcine substimted hybrids displayed decreased inhibition by A2 inhibitors from four patient plasmas, suggesting that there is little variation in the strucmre of the A2 epitope according to the inhibitor population response. The reactivity of the most humanized constructs, HP9, HP 12, and HP13, with two anti- A2 IgG5 preparations prepared from inhibitor plasmas was determined. Like mAb413, these antibodies did not react with HP9, HP12, and HP13, but did react with the control constructs HP(-) and HP8. The region between 484-508 can be further analyzed for final identification of the critical A2 epitope, using the same procedures. The methods described in Examples 7 and 8 can be used to prepare other hybrid human/non-porcine mammalian factor VIII with amino acid substimtion in the human A2 or other domains, hybrid human/ animal or animal/animal factor VIII with amino acid substimtion in any domain, or hybrid factor VII equivalent molecules or fragments of any of these, such hybrid factor VIII having reduced or absent immunoreactivity with anti-factor VIII antibodies. Example 9. Elimination of human factor VIII A2 inhibitor reactivity by site- directed mutagenesis Example 8 showed that substimtion of the porcine sequence bounded by residues 484 and 508 into the human factor VIII A2 domain yields a molecule that has markedly decreased reactivity with a panel of A2-specific factor VIII inhibitors [see also Healey et al. (1995) J. Biol Chem. 270: 14505-145091. In this region, there are 9 amino acid differences between human and porcine factor VIII. These nine residues in human B-domainless factor VIII, R484, P485, Y487, P488, R489, P492, V495, F501 , and 1508 (using the single letter amino code), were individually changed to alanine by site-directed mutagenesis. Additionally, Mlul and Sac2 restriction sites were placed in the factor VIII cDNA at sites 5 ' and 3 ' relative to the A2 epitope, without changing the amino acids corresponding to these sites, to facilitate cloning. The nine mutants were stably transfected into baby hamster kidney cells and expressed to high levels. All nine produced biologically active factor VIII. They were partially purified and concentrated by heparin-Sepharose chromatography as described by Healey et al. The mutants have been characterized by their reactivity with the murine monoclonal inhibitor MAb413 as in Example 7. This inhibitor recognizes the same or a very closely clustered epitope in the A2 domain as all human inhibitors studied to date. Inhibitor reactivity was measured using the Bethesda assay. Briefly, the Bethesda titer of an inhibitor is the dilution of inhibitor that inhibits factor VIII by 50% in a standard one-stage factor VIII clotting assay. For example, if solution of antibody is diluted 1/420 and it inhibits the recombinant factor VIII test sample by 50%, the Bethesda titer is 420 U. In the case of a pure monoclonal like MAb413, the mass of antibody is known, so the results are expressed in Bethesda units (BU) per mg MAb413. To find the 50% inhibition point, a range of dilutions of MAb413 was made and 50% inhibition was found by a curve fitting procedure. The results are as follows: *TabIe VI Mutation MAb413 titer (BU/mg) % Reactivity* Wild-type, B(-)fVIII 9400 — R484 → A 160 1.7 P485 → A 4000 42 Y487 → A 50 0.53 S488 → A 3500 37 R489 → A 1.6 0.015 R490 → A — < 0.5 > P492 → A 630 6.7 V495 → A 10700 113 F501 → A 11900 126 1508 → A 5620 60 * Relative to wild-type These results indicate that it is possible to reduce the antigenicity of factor VIII toward the model A2 inhibitor by over a factor of 10 by making alanine substimtions at positions 484, 487, 489, and 492. The reactivity of R489 → A is reduced by nearly 4 orders of magnitude. Any of these alanine substimtions can be therapeutically useful to reduce the antigenicity and the immunogenicity of factor VIII. The results confirm the efficacy of alanine-scanning mutagenesis and further demonstrate that biological activity is retained even though the amino acid sequence has been altered within an epitope reactive to an inhibitory antibody. Five of the nine sites where the human and porcine sequences differ are also sites where the human and murine sequences differ. The factor VIIIs having alanine substimtions at these positions are therefore examples of a hybrid factor VIII equivalent molecule having a sequence with no known sequence identify with any presently known mammalian factor VIII. Further modification, e.g. by combining two alanine substimtions, can also provide greatly reduced antigenicity for a wider range of patients, since polyclonal variant antibodies differing from patient to patient can react with variants of the factor VIII A2 epitope. In addition, immunogenicity (the capacity to induce antibodies) is further reduced by incorporation of more than one amino acid substimtion. Such substimtions can include both alanine, porcine-specific amino acids, or other amino acids known to have low immunogenic potential. The substimtions at positions 490, 495 and 501 are likely to be useful in reducing immunogenicity. In addition, these substimtions are likely to reduce reactivity to certain patient antibodies. Other effective, antigenicity-reducing amino acid substimtions, besides alanine, can be made as long as care is taken to avoid those previously noted as being major contributors to antigen-antibody binding energy, or having bulky or charged side chains. Amino acids whose substimtions within an epitope reduce the antigenic reactivity thereof are termed "immunoreactivity-reducing" amino acids herein. Besides alanine, other immunoreactivity - reducing amino acids include, without limitation, methionine, leucine, serine and glycine. It will be understood that the reduction of immunoreactivity achievable by a given amino acid will also depend on any effects the substimtion may have on protein conformation, epitope accessibility and the like. Amino acid substimtions at other sites within the A2 epitope (amino acids 484-508) besides those that differ between the human and porcine sequences, are further able to reduce reactivity toward inhibitory antibodies. Alanine scanning mutagenesis can be used to provide alanine substimtions for any amino acid within the A2 epitope. Each resulting modified factor VIII can be assayed for procoagulant activity and for inhibition of that activity by an inhibitory antibody. Other immunoreactivity reducing amino acids besides alanine can be substimted to reduce antigenicity of the resulting modified factor VIII. Amino acid replacements can be combined in a single factor VIII molecule to maximize the desired properties resulting from such substimtions. Replacement of those amino acids that contribute the most to the binding energy of an antibody-factor VIII interaction is most preferred. These include substimtion of an immunoreactivity-reducing amino acid at any of positions 493, 496, 499, 500, 502, 503, 505 and 507. The data for replacements of this type, at positions 484, 485, 499, 490, 492, 501 and 508 has demonstrated that such replacements retain procoagulant activity and decrease susceptibility to inhibition by inhibitory antibodies. (Table VI) Histidine replacements have been observed in naturally-occurring sequences. For example, at position 504 the histidine of mouse factor VIII is replaced by leucine in both porcine and human factor VIII. Both porcine and mouse factor VIII have a histidine at position 487, where human factor VIII has tyrosine. Replacing the tyrosine with alanine at position 487 results in active procoagulant with substantially reduced antigenicity (Table VI). By analogy, replacement of histidine at position 497 by an immunoreactivity-reducing amino acid can also result in retention of procoagulant activity and contribute to reduced inhibition by inhibitory antibodies. Immunoreactivity- reducing amino acids can also be substimted at positions 486, 488, 491 , 494, 498, 504 and 506. Although the existing amino acids at these positions seem less likely to contribute to antibody binding, it has been demonstrated (Table VI) that substimtion of an immunoreactivity- reducing amino acid at such sites, e.g. S488A. contributes to reducing antibody inhibition of procoagulant activity. From a comparison of the human, porcine, murine (Fig.1A-1H) and canine [Cameron, C et al. (1998) Thromb. Haemost. 79:317-322] sequences within the A2 epitope, it is evident that the region tolerates a significant amount of sequence variability. Only 12 loci are conserved among all four species. None of these can be considered to be essential for procoagulant activity. In fact, replacement by alanine of the conserved arginine at position 490 (R490→A, Table VI) results in active modified factor VIII having reduced reactivity to an inhibitory antibody. One or more amino acid replacements can be made without substantially affecting procoagulant activity. For example, the replacement of two amino acids involved in antibody binding can reduce inhibition by an antibody to a greater extent than either one alone. Also, multiple replacements can render the resulting modified factor VIII less responsive to a wider variety of patient antibodies than a single amino acid replacement. Individual amino acid replacements can be assessed for their properties of reduced antigenicity, as well as for other functional attributes of factor VIII. By evaluating the properties conferred by individual amino acid replacements, it is possible to identify desired combination replacements of two or more amino acids to provide a modified factor VIII having optimized properties, insofar as the region of amino acids 484-508 is concerned. Site directed mutagenesis can be used to modify the factor VIII DNA in the region encoding amino acids 484-508 so as to provide a sequence that encodes the modified factor VIII having a desired amino acid replacement. At the appropriate site of the human factor VIII DNA sequence, the triplet encoding an existing amino acid can be changed by site directed mutagenesis to encode the desired amino acid. The triplet encoding the desired amino acid can be any one of the known triplets specified by the genetic code. Altering the natural sequence to encode a single amino acid substimtion can often be accomplished with a single base change, occasionally more, up to a maximum of three base changes. By using site-specific mutagenesis, all necessary base substimtions can be readily carried out so as to alter the existing coding to that needed to encode the desired amino acid substimtion. Some examples of base changes leading to specified amino acid substimtions are given below. These are exemplary only, and not comprehensive: R484 → G C G T → GGT P485 → A C C T → GCT L486 → S T T G → TCG Y487 → L T A T → CTT S488 → L T C A → TTA R489 → S A G G → AGT R490 → G A G A → GGA L491 →S TTA → TCA P492 → L CCA → CTA K493 → A A A A → GCA G494 → S GGT → AGT V495 → A GT A → GCA K496 → M A A A → ATG H497 → L CAT → CTT L498→S TTG → TCG K499 → M A AG → ATG D500 → A GAT → GCT F501 → S T T T → TCT P502 → L CCA → CTA 1503 → M ATT → ATG L504 → M CTG → ATG P505 → A CCA → GCA G506 →A GGA → GCA E507 → G G A A → GGA 1508 → M A T A → ATG The foregoing examples demonstrate that many immunoreactivity-reducing amino acid substimtions can be accomplished by single nucleotide changes. Other desired substimtions can be accomplished in similar fashion, making reference to the genetic code to select a desired nucleotide triplet encoding the intended amino acid substiment, then introducing the nucleotide changes necessary to generate the intended triplet, by site-directed mutagenesis. Multiply- substituted modified factor VIII can be made by simple combinations of nucleotide changes such as those just described. For example, a modified factor VIII having two amino acids of the A2 domain replaced, e.g. R489→A and P492→L can be made by introducing AGG→GCG and CCA→CTA at the appropriate sites, a change of three nucleotides. Any other desired change or combination of changes can be designed and carried out, essentially as just described. The modified factor VIII DNA sequence resulting from the described site-directed mutagenesis then differs from the natural human sequence or from otherwise modified sequences as described elsewhere herein, only by having the defined nucleotide substitution(s) at the defined site. Procoagulant activity is assayed as previously described, (Examples 1 and 8), by either the one-stage or two-stage assay. Assay for inhibitor titer is the Bethesda assay, described above and by Kasper, C.K. et al. , supra, Example 8. Example 10. Klenow fragment, phosphorylated Clal linkers, Notl linkers, T4 ligase, and Taq DNA polymerase were purchased from Promega (Madison, Wisconsin). Polynucleotide kinase was purchased from Life Technologies, Inc. , Gaithersburg, Maryland. γ 32 P-ATP (Redivue, > 5000Ci/mmol) was purchased from Amersham. pBluescript II KS- and E. coli Epicurean XLl-Blue cells were purchased from Stratagene (La Jolla, California). Synthetic oligonucleotides were purchased from Life Technologies, Inc. or Cruachem, Inc. 5'- phosphorylated primers were used when PCR products were produced for cloning purposes. Nucleotide (nt) numbering of oligonucleotides used as primers for polymerase chain reaction (PCR) amplification of porcine fVIII cDNA or genomic DNA uses the human fVIII cDNA as reference (Wood et al. (1984) supra). Porcine spleen total RNA was isolated by acid guanidinium thiocyanate-phenol- chloroform extraction [Chomczynski et al. (1987) Anal. Biochem. 162: 156-159] , Porcine cDNA was prepared from total spleen RNA using Moloney murine leukemia virus reverse transcriptase (RT) and random hexamers to prime the reaction (First-Strand cDNA Synthesis Kit, Pharmacia Biotech) unless otherwise indicated. RT reactions contained 45 mM Tris-Cl, pH 8.3, 68 mM KCl, 15 mM DTT, 9 mM MgCl 2 , 0.08 mg/ml bovine serum albumin and 1.8 mM deoxynucleotide triphosphate (dNTP). Porcine genomic DNA was isolated from spleen using a standard procedure (Strauss, W.M. (1995) In Current Protocols in Molecular Biology. F. M. Ausubel et al. , editors, John Wiley & Sons, pp. 2.2.1-2.2.3). Isolation of DNA from agarose gels was done using Geneclean II (Bio 101) or Quiex II Gel Extraction Kit (Qiagen). PCR reactions were done using a Hybaid OmniGene thermocycler. For PCR reactions employing Taq DNA polymerase, reactions included 0.6 mM MgCl 2 , 0.2 mM dNTPs, 0.5 μM oligonucleotide primers, 50 U/ml polymerase and 0.1 volume of first strand cDNA reaction mix. Except where indicated otherwise, PCR products were gel purified, blunt-ended with Klenow fragment, precipitated with ethanol, and either ligated to the EcoRV site of dephosphorylated pBluescript II KS- or ligated with phosphorylated Clal linkers using T4 ligase, digested with Clal, purified by Sephacryl S400 chromatography, and ligated to Clal-cut, dephosphorylated pBluescript II KS-. Ligations were done using T4 DNA ligase (Rapid DNA ligation kit, Boehringer Mannheim) except where indicated otherwise. Insert-containing pBluescript II KS- plasmids were used to transform E. coli Epicurean XL 1 -Blue cells. Sequencing of plasmid DNA was done using an Applied Biosystems 373a automated DNA sequencer and the PRISM dye terminator kit or manually using Sequenase v. 2.0 sequencing kit (Amersham Corporation). Direct sequencing of PCR products, including 32 P-end labelling of oligonucleotides was done using a cycle sequencing protocol (dsDNA Cycle Sequencing System, Life Technologies). Isolation of porcine fVIII cDNA clones containing 5 ' UTR sequence, signal peptide and Al domain codons. The porcine fVIII cDNA 5' to the A2 domain was amplified by nested RT-PCR of female pig spleen total RNA using a 5' rapid amplification of cDNA ends (5 '-RACE) protocol (Marathon cDNA Amplification, Clontech, Version PR55453). This included first strand cDNA synthesis using a lock-docking oligo(dT) primer [Borson, N.D. et al. (1992) PCR Methods Appl. 2: 144-148], second strand cDNA synthesis using E. coli DNA polymerase I, and ligation with a 5' extended double stranded adaptor, SEQ ID NO: 13 5'-CTA ATA CGA CTC ACT ATA GGG CTC GAG CGG CCG CCC GGG CAG GT-3 3 ' -H 2 N-CCCGTCC A-PO 4 -5 ' whose short strand was blocked at the 3 ' end with an amino group to reduce non-specific PCR priming and which was complementary to the 8 nucleotides at the 3' end (Siebert, P.D. , et al. (1995) Nucleic. Acids. Res. 23 : 1087-1088). The first round of PCR was done using an adaptor- specific oligonucleotide, SEQ ID NO: 14 5'-CCA TCC TAA TAC GAC TCA CTA TAG GGC-3' (designated API) as sense primer, and a porcine fVIII A2 domain specific oligonucleotide SEQ ID NO: 15 5'-CCA TTG ACA TGA AGA CCG TTT CTC-3' (nt 2081- 2104) as antisense primer. The second round of PCR was done using a nested, adaptor-specific oligonucleotide, SEQ ID NO: 165'-ACT CAC TAT AGG GCT CGA GCG GC-3' (designated AP2) as sense primer, and a nested, porcine A2 domain-specific oligonucleotide SEQ ID NO: 17 5'-GGG TGC AAA GCG CTG ACA TCA GTG-3' (nt 1497-1520) as antisense primer. PCR was carried out using a commercial kit (Advantage cDNA PCR core kit) which employs an antibody-mediated hot start protocol [Kellogg, D.E. et al. (1994) BioTechniques 16: 1134-1137]. PCR conditions included denamration at 94 °C for 60 sec, followed by 30 cycles (first PCR) or 25 cycles (second PCR) of denamration for 30 sec at 94°C, annealing for 30 sec at 60°C and elongation for 4 min at 68°C using tube temperamre control. This procedure yielded a prominent ~ 1.6 kb product which was consistent with amplification of a fragment extending approximately 150 bp into the 5' UTR. The PCR product was cloned into pBluescript using Clal linkers. The inserts of four clones were sequenced in both directions. The sequence of these clones included regions corresponding to 137 bp of the 5' UTR, the signal peptide, the Al domain and part of the A2 domain. A consensus was reached in at least 3 of 4 sites. However, the clones contained an average of 4 apparent PCR-generated mutations, presumably due to the multiple rounds of PCR required to generate a clonable product. Therefore, we used sequence obtained from the signal peptide region to design a sense strand phosphorylated PCR primer, SEQ ID NO: 185'-CCT CTC GAG CCA CCA TGT CGA GCC ACC ATG CAG CTA GAG CTC TCC ACC TG-3' , designated RENEOPIGSP, for synthesis of another PCR product to confirm the sequence and for cloning into an expression vector. The sequence in bold represents the start codon. The sequence 5' to this represents sequence identical to that 5 ' of the insertion site into the mammalian expression vector ReNeo used for expression of fVIII (Lubin et al. (1994) supra). This site includes an Xhol cleavage site (underlined). RENEOPIGSP and the nt 1497-1520 oligonucleotide were used to prime a Taq DNA polymerase-mediated PCR reaction using porcine female spleen cDNA as a template. DNA polymerases from several other manufacturers failed to yield a detectable product. PCR conditions included denamration at 94 °C for four min, followed by 35 cycles of denamration for 1 min at 94 °C, annealing for 2 min at 55 °C and elongation for 2 min at 72 °C, followed by a final elongation step for 5 min at 72 °C. The PCR product was cloned into pBluescript using Clal linkers. The inserts of two of these clones were sequenced in both directions and matched the consensus sequence. Isolation of porcine fViII cDNA clones containing A3. CI and 5' half of the C2 domain codons. Initially, two porcine spleen RT-PCR products, corresponding to a B-A3 domain fragment (nt 4519-5571) and a C1-C2 domain fragment (nt 6405-6990) were cloned. The 3' end of the C2 domain that was obtained extended into the exon 26 region, which is the terminal exon in fVIII. The B-A3 product was made using the porcine-specific B domain primer, SEQ ID NO:19 5' CGC GCG GCC GCG CAT CTG GCA AAG CTG AGT T 30 where the underlined region corresponds to a region in porcine fVIII that aligns with nt 4519-4530 in human fVIII. The 5' region of the oligonucleotide includes a Notl site that was originally intended for cloning purposes. The antisense primer used in generating the B-A3 product, SEQ ID NO:20 5'-GAA ATA AGC CCA GGC TTT GCA GTC RAA-3' was based on the reverse complement of the human fVIII cDNA sequence at nt 5545-5571. The PCR reaction contained 50 mM KCl, 10 mM Tris-Cl, pH 9.0, 0.1 % Triton X-100, 1.5 mM MgCl 2 , 2.5 mM dNTPs, 20 μM primers, 25 units/ml Taq DNA polymerase and 1/20 volume of RT reaction mix. PCR conditions were denamration at 94 °C for 3 min, followed by 30 cycles of denamration for 1 min at 94° C, annealing for 2 min at 50 °C and elongation for 2 min at 72 °C. The PCR products were phosphorylated using T4 DNA kinase and Notl linkers were added. After cutting with Notl, the PCR fragments were cloned into the Notl site of BlueScript II KS- and transformed into XL 1 -Blue cells. The C1-C2 product was made using the known human cDNA sequence to synthesize sense and antisense primers, SEQ ID NO:21 5'-AGG AAA TTC CAC TGG AAC CTT N-3' (nt 6405-6426) and SEQ ID NO:22 5'-CTG GGG GTG AAT TCG AAG GTA GCG N-3' (reverse complement of nt 6966-6990), respectively. PCR conditions were identical to those used to generate the B-A2 product. The resulting fragment was ligated to the pNOT cloning vector using the Prime PCR Cloner Cloning System (5 Prime-3 Prime, Inc., Boulder, Colorado) and grown in JM109 cells. The B-A3 and C1-C2 plasmids were partially sequenced to make the porcine-specific sense and antisense oligonucleotides, SEQ ID NO:23 5'-GAG TTC ATC GGG AAG ACC TGT TG-3 ' (nt 4551-4573) and SEQ ID NO:24 5'-ACA GCC CAT CAA CTC CAT GCG AAG-3 ' (nt 6541-6564), respectively. These oligonucleotides were used as primers to generate a 2013 bp RT-PCR product using a Clontech Advantage cDNA PCR kit. This product, which corresponds to human nt 4551-6564, includes the region corresponding to the light chain activation peptide (nt 5002-5124), A3 domain (nt 5125-6114) and most of the CI domain (nt 6115-6573). The sequence of the C1-C2 clone had established that human and porcine cDNAs from nt 6565 to the 3' end of the CI domain were identical. The PCR product cloned into the EcoRV site of pBluescript II KS-. Four clones were completely sequenced in both directions. A consensus was reached in at least 3 of 4 sites. Isolation of porcine fVIII cDNA clones containing the 3' half of the C2 domain codons. The C2 domain of human fVIII (nucleotides 6574-7053) is contained within exons 24- 26 [Gitschier J. et al. (1984) Nature 312:326-330]. Human exon 26 contains 1958 bp, corresponding nucleotides 6901-8858. It includes 1478 bp of 3' untranslated sequence. Attempts to clone the exon 26 cDNA corresponding to the 3 ' end of the C2 domain and the 3 'UTR by 3' RACE [Siebert et al. (1995) supra], inverse PCR [Ochman, H. et al. (1990) Biotechnology (N. Y). 8:759-760], restriction site PCR [Sarkar, G. et al. (1993) PCR Meth. App 2:318-322], "unpredictably primed" PCR [Dominguez, O. et al. (1994) Nucleic. Acids Res. 22:3247-3248] and by screening a porcine liver cDNA library failed. 3 ' RACE was attempted using the same adaptor-ligated double stranded cDNA library that was used to successfully used to clone the 5' end of the porcine fVIII cDNA. Thus, the failure of this method was not due to the absence of cDNA corresponding to exon 26. A targeted gene walking PCR procedure [Parker, J.D. et al. (1991) Nucleic. Acids. Res. 19:3055-30601 was used to clone the 3' half of the C2 domain. A porcine-specific sense primer, SEQ ID NO: 25 5 '-TCAGGGCAATCAGGACTCC-3 ' (nt 6904-6924) was synthesized based on the initial C2 domain sequence and was used in a PCR reaction with nonspecific "walking" primers selected from oligonucleotides available in the laboratory. The PCR products were then targeted by primer extension analysis [Parker et al. (1991) BioTechniques 10:94-101] using a 32 P-end labelled porcine-specific internal primer, SEQ ID NO:26 5'- CCGTGGTGAACGCTCTGGACC-3' (nt 6932-6952). Interestingly, of the 40 nonspecific primers tested, only two yielded positive products on primer extension analysis and these two corresponded to an exact and a degenerate human sequence at the 3' end of the C2 domain: SEQ ID NO:27 5'-GTAGAGGTCCTGTGCCTCGCAGCC-3' (nt 7030-7053) and SEQ ID NO:285'-GTAGAGSTSCTGKGCCTCRCAKCCYAG-3' , (nt 7027-7053). These primers had initially been designed to yield a product by conventional RT-PCR but failed to yield sufficient product that could be visualized by ethidium bromide dye binding. However, a PCR product could be identified by the more sensitive primer extension method. This product was gel- purified and directly sequenced. This extended the sequence of porcine fVIII 3' to nt 7026. Additional sequence was obtained by primer extension analysis of a nested PCR product generated using the adaptor-ligated double-stranded cDNA library used in the 5 '-RACE protocol described previously. The first round reaction used the porcine exact primer SEQ ID NO:295'-CTTCGCATGGAGTTGATGGGCTGT-3' (nt 6541-6564) and the API primer. The second round reaction used SEQ ID NO: 30 5'-AATCAGGACTCCTCCACCCCCG-3' (nt 6913-6934) and the AP2 primer. Direct PCR sequencing extended the sequence 3' to the end of the C2 domain (nt 7053). The C2 domain sequence was unique except at nt 7045 near the 3' end of the C2 domain. Analysis of repeated PCR reactions yielded either A, G or a double read of A/G at this site. Sequencing was extended into the 3 'UTR using two additional primers, SEQ ID NO: 31 5' -GGA TCC ACC CCA CGA GCT GG-3' (nt 6977-6996) and SEQ ID NO: 32 5' -CGC CCT GAG GCT CGA GGT TCT AGG-3' (nt 7008-7031). Approximately 15 bp of 3' UTR sequence were obtained, although the sequence was unclear at several sites. Several antisense primers then were synthesized based on the best estimates of the 3' untranslated sequence. These primers included the reverse complement of the TGA stop codon at their 3' termini. PCR products were obtained from both porcine spleen genomic DNA and porcine spleen cDNA that were visualized by agarose gel electrophoresis and ethidium bromide staining using a specific sense primer SEQ ID NO:33 5'-AAT CAG GAC TCC TCC ACC CCC G-3' (nt 6913-6934) and the 3 ' UTR antisense primer, SEQ ID NO: 34 5 '- CCTTGCAGGAATTCGATTCA-3' . To obtain sufficient quantities of material for cloning purposes, a second round of PCR was done using a nested sense primer, SEQ ID NO:35 5'- CCGTGGTGAACGCTCTGGACC-3' (nt 6932-6952) and the same antisense primer. The 141 bp PCR product was cloned into EcoRV-cut pBluescript II KS-. Sequence of three clones derived from genomic DNA and three clones derived from cDNA was obtained in both directions. The sequence was unambiguous except at nt 7045, where genomic DNA was always A and cDNA was always G. Multiple DNA sequence alignments of human, porcine, and mouse fVIII (Fig. 1A-1H). Alignments of the signal peptide, Al, A2, A3, CI , and C2 regions were done using the CLUSTALW program [Thompson, J.D. et al. (1994) Nucleic. Acids. Res. 22:4673-4680]. Gap open and gap extension penalties were 10 and 0.05 respectively. The alignments of the human, mouse, and pig B domains have been described previously [Elder et al. (1993) supra]. The human A2 sequence corresponds to amino acids 373-740 in SEQ ID NO:2. The porcine A2 amino acid sequence is given in SEQ ID NO:4, and the mouse A2 domain amino acid sequence is given in SEQ ID NO:6, amino acids 392-759. Example 11. Expression of active, recombinant B-domainless porcine factor VIII (PB ] Materials Citrated hemophilia A and normal pooled human plasmas were purchased from George King Biomedical, Inc. Fetal bovine serum, geneticin, penicillin, streptomycin, DMEM/F12 medium and AIM-V medium were purchased from Life Technologies, Inc. Taq DNA polymerase was purchased from Promega. Vent DNA polymerase was purchased from New England Biolabs. Pfu DNA polymerase and the phagemid pBlueScript II KS " were purchased from Stratagene. Synthetic oligonucleotides were purchased from Life Technologies or Cruachem, Inc. Restriction enzymes were purchased from New England Biolabs or Promega. 5 '-phosphorylated primers were used when PCR products were produced for cloning purposes. Nucleotide (nt) numbering of oligonucleotides used as primers for polymerase chain reaction (PCR) amplification of porcine fVIII cDNA or genomic DNA uses the human fVIII cDNA as reference [Wood et al. (1984) Nature 312:330-3371. A iVIII expression vector, designated HB " /ReNeo, was obtained from Biogen, Inc. HBVReNeo contains ampicillin and geneticin resistance genes and a human fVIII cDNA that lacks the entire B domain, defined as the Ser741 -Arg 1648 cleavage fragment produced by thrombin. To simplify mutagenesis of fVIII C2 domain cDNA, which is at the 3' end of the fVIII insert in ReNeo, a Notl site was introduced two bases 3 ' to the stop codon of HBVReNeo by splicing -by-overlap extension (SOE) mutagenesis [Horton, R.M. et al. (1993) Methods Enzymol. 217:270-279] . This construct is designated HB ReNeo/Not/. Total RNA was isolated by acid guanidinium thiocyanate-phenol-chloroform extraction [Chomczynski, P. et al. (1987) Anal. Biochem. 162: 156-1591. cDNA was synthesized from mRNA using Moloney murine leukemia virus reverse transcriptase (RT) and random hexamers according to instructions supplied by the manufacturer (First-Strand cDNA Synthesis Kit, Pharmacia Biotech). Plasmid DNA was purified using a Qiagen Plasmid Maxi Kit (Qiagen, Inc.). PCR reactions were done using a Hybaid OmniGene thermocycler using Taq, Vent, or Pfu DNA polymerases. PCR products were gel purified, precipitated with ethanol, and ligated into plasmid DNA using T4 DNA ligase (Rapid DNA ligation kit, Boehringer Mannheim). Insert-containing plasmids were used to transform E. coli Epicurean XLl-Blue cells. All novel fVIII DNA sequences generated by PCR were confirmed by dideoxy sequencing using an Applied Biosystems 373a automated DNA sequencer and the PRISM dye terminator kit. Construction of a hybrid fVIII expression vector, HP20, containing the porcine C2 domain. A porcine fVIII cDNA corresponding to the 3 ' end of the CI domain and all of the C2 domain was cloned into pBluescript by RT-PCR from spleen total RNA using primers based on known porcine fVIII cDN A sequence [Healy , J . F . et al . ( 1996) Blood 88 : 4209-4214] . This construct and HBVReNeo were used as templates to construct a human CI -porcine C2 fusion product in pBlueScript by SOE mutagenesis. The C1-C2 fragment in this plasmid was removed with Apal and Notl and ligated into Apal/Notl-cut HBVReNeo/Not/ to produce HP20/ReΝeo/Not/. Construction of B-domain deleted hybrid human/porcine fVIII containing the porcine light chain (HP18V The human fVTII light chain consists of amino acid residues Aspl649-Tyr2332. The corresponding residues in the porcine fVIII cDNA were substimted for this region of HB " to produce a hybrid human/porcine fVIII molecule designated HP18. This was done by substimting a PCR product corresponding to porcine A2 region, the A3 domain, the CI domain, and part of the C2 domain for the corresponding region in HP20. To facilitate constructions, a synonymous Avrll site was introduced into nt 2273 at the junction of the A2 and A3 domains of HP20 by SOE mutagenesis. Construction of B-domain deleted hybrid human/porcine fVIII containing the porcine signal peptide. Al domain and A2 domain (HP22)- The human fVIII signal peptide, Al domain and A2 domains consist of amino acid residues Met(-19)-Arg740. The corresponding residues in the porcine fVIII cDNA were substituted for this region of HB " to produce a molecule designated HP22. Additionally, a synonymous Avrll site was introduced into nt 2273 at the junction of the A2 and A3 domains of HP22 by SOE mutagenesis. HP22 was constructed by fusion of a porcine signal peptide-Al- partial A2 fragment in pBlueScript [Healy et al. (1996) supra] with a B-domainless hybrid human porcine fVIII containing the porcine A2 domain, designated HP1 [Lubin et al. (1994) supra] . Construction of porcine B domainless fVIII-(PB ) A Spel/Notl fragment of HP18/BS (+ Avrll) was digested with AvrlllNotl and ligated into vr/Z/NotZ-digested HP22/BS (+ Avrll) to produce a construct PBVBS (+ Avrll), which consists of the porcine fVIII lacking the entire B domain. PB- was cloned into ReNeo by ligating an XbalNotl fragment of PBVBS (+ Avrll) into HP22/ReNeo/Not/ (+ Avrll). Expression of recombinant fVIII molecules VB IReNeol Notl (+ Avrll) and HP22/ReNeo/NotI ( + Avrll) were transiently transfected into COS cells and expressed as described previously [Lubin. I.M. et al. (1994) 7. Biol Chem. 269:8639-86411. HBVReNeo/Notl and no DNA (mock) were transfected as a control. The rVIII activity of PB " , HP22, and HB " were measured by a chromogenic assay as follows. Samples of fVIII in COS cell culmre supernatants were activated by 40 nM thrombin in a 0.15 M NaCI, 20 mM HEPES, 5Mm cAC12, 0.01 % Tween-80, pH 7.4 in the presence of 10 nM factor IXa, 425 nM factor X, and 50 μM unilamellar phosphatidylserine- [phosphatidycholine (25/75 w/w) vesicles. After 5 min, the reaction was stopped with 0.05 M EDTA and 100 nM recombinant desulfatohirudin and the resultant factor Xa was measured by chromogenic substrate assay. In the chromogenic substrate assay, 0.4 mM Spectrozyme Xa was added and the rate of para-nitroanilide release was measured by measuring the absorbance of the solution at 405 nm. Results of independently transfected duplicate cell culmre supernatants (absorbance at 405 nm per minute) HB " : 13.9 PB : 139 HP22: 100 mock: <0.2 These results indicate that porcine B-domainless fVIII and a B-domainless fVIII consisting of the porcine Al and A2 subunits are active and suggest that they have superior activity to human B-domainless fVIII. PB " was partially purified and concentrated from the growth medium by heparin- Sepharose chromatography. Heparin-Sepharose (10 ml) was equilibrated with 0.075 M NaCI, 10 mM HEPES, 2.5 mM CaCl 2 , 0.005 % Tween-80, 0.02% sodium azide, pH 7.40. Medium (100-200 ml) from expressing cells was applied to the heparin-Sepharose, which then was washed with 30 ml of equilibration buffer without sodium azide. PB " was eluted with 0.65 M NaCI, 20 mM HEPES, 5 mM CaCl 2 , 0.01 % Tween-80, pH 7.40 and was stored at -80 °C. The yield of fVIII coagulant activity was typically 50-75 % . Stable expression of porcine B-domainless fVIII (PB ) Transfected cell lines were maintained in Dulbecco's modified Eagle's medium-F12 containing 10% fetal bovine serum, 50 U/ml penicillin, 50 μg/ml streptomycin. Fetal bovine serum was heat inactivated at 50 °C for one hour before use. HBVReNeo and PB ReNeo/Nøt/ (+ Avrll) were stably transfected into BHK cells and selected for geneticin resistance using a general protocol that has been described previously [Lubin et al. (1994) Biol. Chem. 269:8639- 8641] except that expressing cells were maintained in growth medium containing 600 μg/ml geneticin. Cells from Corning T-75 flasks grown to confluence were transferred to Νunc triple flasks in medium containing 600 μg/ml geneticin and grown to confluence. The medium was removed and replaced with serum-free, AIM-V medium (Life Technologies, Inc.) without geneticin. Factor VIII expression was monitored by one-stage factor VIII coagulant activity (vide supra) and 100-150 ml of medium was collected once daily for four to five days. Maximum expression levels in medium for HB " and PB " were 102 units per ml and 10-12 units per ml of factor VIII coagulant activity, respectively. Purification of PB " PB " was precipitated from culmre supernatant using 60% samrated ammonium sulfate and then purified by W3-3 immunoaffinity chromatography and mono Q high pressure liquid chromatography as described previously for the purification of plasma-derived porcine factor Viπ [Lollar et al. (1993) Factor Vlll/factor Villa. Methods Enzymol 222: 128-143]. The specific coagulant activity of PB " was measured by a one-stage coagulation assay [Lollar et al. (1993) supra] and was similar to plasma-derived porcine factor VIII. When analyzed by SDS-polyacrylamide gel electrophoresis, the PB " preparation contained three bands of apparent molecular masses 160 kDa, 82 kDa, and 76 kDa. The 82 kDa and 76 kDa bands have been previously described as heterodimer containing the A1-A2 and ap-A3-Cl-C2 domains (where ap refers to an activation peptide) [Toole et al. (1984) Nature 312:342-347] . The 160 kDa band was transferred to a polyvinylidene fluoride membrane and subjected to NH2-terminal sequencing, which yielded Arg-Ile-Xx-Xx-Tyr (where Xx represents undermined) which is the NH2-terminal sequence of single chain factor VIII [Toole et al. (1984) supra]. Thus, PB " is partially processed by cleavage between the A2 and A3 domains, such that it consists of two forms, a single chain Al-A2-ap-A3-Cl-C2 protein and a Al-A2/ap- A3-C1-C2 heterodimer. Similar processing of recombinant HB " has been reported [Lind et al. (1995) Eur. J. Biochem. 232: 19-271. Characterization of Porcine factor VIII We have determined the cDNA sequence of porcine fVIII corresponding to 137 bp of the 5' UTR, the signal peptide coding region (57 bp), and the Al (1119 bp), A3 (990 bp), CI (456 bp), and C2 (483 bp) domains. Along with previously published sequence of the B domain and light chain activation peptide regions [Toole et al. (1986) supra] and the A2 domain [Lubin et al. (1994) supra], the sequence reported here completes the determination of the porcine fVIII cDNA corresponding to the translated product. A fragment that included the 5' UTR region, signal peptide, and Al domain cDNA was cloned using a 5'-RACE RT-PCR protocol. A primer based on human C2 sequence was successful in producing an RT-PCR product that led to cloning of the A3 , C 1 , and 5 ' half of the C2 domain. The cDN A corresponding to the 3' half of the C2 domain and 3' UTR cDNA proved difficult to clone. The remainder of the C2 domain ultimately was cloned by a targeted gene walking PCR procedure [Parker et al. (1991) supra] . The sequence reported herein SEQ ID NO: 36 was unambiguous except at nt 7045 near the 3' end of the C2 domain, which is either A or G as described hereinabove. The corresponding codon is GAC (Asp) or AAC (Asn). The human and mouse codons are GAC and CAG (Gin), respectively. Whether this represents a polymorphism or a reproducible PCR artifact is unknown. Recombinant hybrid human/porcine B-domainless fVIII cDNAs containing porcine C2 domain substimtions corresponding to both the GAC and AAC codons have been stably expressed with no detectable difference in procoagulant activity. This indicates that there is not a functional difference between these two C2 domain variants. The alignment of the predicted amino acid sequence of full-length porcine fVIII SEQ ID NO:37 with the published human [Wood et al. (1984) supra] and murine [Elder et al. (1993) supra] sequences is shown in Fig. 1A-1H along with sites for post-translational modification, proteolytic cleavage, and recognition by other macromolecules. The degree of identity of the aligned sequences is shown in Table VII. As noted previously, the B domains of these species are more divergent than the A or C domains. This is consistent with the observation that the B domain has no known function, despite its large size [Elder et al. (1993) supra; Toole et al. (1986) supra]. The results of the present invention confirm that the B domain or porcine fVTII is not necessary for activity. Based on the sequence data presented herein, porcine fVIII having all or part of the B-domain deleted can be synthesized by expressing the porcine fVIII coding DNA having deleted therefrom all or part of codons of the porcine B domain. There is also more divergence of sequences corresponding to the Al domain APC/factor IXa cleavage peptide (residues 337-372) and the light chain activation peptide (Table VII). The thrombin cleavage site at position 336 to generate the 337-372 peptide is apparently lost in the mouse since this residue is glutamine instead of arginine [Elder et al. (1993) supra]. The relatively rapid divergence of thrombin cleavage peptides (or in mouse fVHJ a possibly vestigial 337-372 activation peptide) has been previously noted for the fibrinopeptides [Creighton, T. E. (1993) In Proteins: Strucmres and Molecular Properties, W.H. Freeman, New York, pp. 105-138]. Lack of biological function of these peptides once cleaved has been cited as a possible reason for the rapid divergence. Arg562 in human fVTII has been proposed to be the more important cleavage site for activated protein C during the inactivation of fVIII and fVIIIa [Fay, P. J. et al. (1991) 7. Biol Chem. 266:20139-20145] . This site is conserved in human, porcine and mouse fVIII. Potential N-linked glycosylation sites (NXS/T where X is not proline) can be seen in Fig. 1A-1H. There are eight conserved N-linked glycosylation sites: one in the Al domain, one in the A2 domain, four in the B domain, one in the A3 domain, and one in the CI domain. The 19 A and C domain cysteines are conserved, whereas there is divergence of B domain cysteines. Six of the seven disulfide linkages in fVTII are found at homologous sites in factor V and ceruloplasmin, and both C domain disulfide linkages are found in factor V [McMullen, B. A. et al. (1995) Protein Sci. 4:740-746] . Human fVIII contains sulfated tyrosines at positions 346, 718, 719, 723, 1664, and 1680 [Pittman, D.D. et al. (1992) Biochemistry 31 :3315-3325; Michnick, D.A. et al. (1994) 7. Biol. Chem. 269:20095-201021. These residues are conserved in mouse JVffl and porcine fVIII (Fig. 1), although the CLUSTNLW program failed to align the mouse tyrosine corresponding to Tyr346 in human fVIII. Mouse and pig plasma can correct the clotting defect in human hemophilia A plasma, which is consistent with the level of conservation of residues in the A and C domains of these species. The procoagulant activity of porcine fVIII is superior to that of human fVIII [Lollar, P. et al. (1992) 7. Biol. Chem. 267:23652-23657]. The recombinant porcine factor VIII (B domain-deleted) expressed and purified as herein described also displays greater specific coagulant activity than human fVIII, being comparable to plasma-derived porcine fVIII. This may be due to a decreased spontaneous dissociation rate of the A2 subunit from the active A1/A2/A3-C1-C2 fVIIIa heterotrimer. Whether this difference in procoagulant activity reflects an evolutionary change in function as an example of species adaptation [Perutz, M.F. (1996) Adv. Protein Chem. 36:213-244] is unknown. Now that the porcine fVTII cDNA sequence corresponding to the translated product is complete, homolog scanning mutagenesis [Cunningham, B.C. , et al. (1989) Science 243: 1330-13361 may provide a way to identify strucmral differences between human and porcine fVIII that are responsible for the superior activity of the latter. Porcine fVIII is typically less reactive with inhibitory antibodies that arise in hemophiliacs who have been transfused with fVIII or which arise as autoantibodies in the general population. This is the basis for using porcine fVIII concentrate in the management of patients with inhibitory antibodies [Hay and Lozier (1995) supra]. Most inhibitors are directed against epitopes located in the A2 domain or C2 domain [Fulcher, CA. et al. (1985) Proc. Natl. Acad. Sci. USA 82:7728-7732; Scandella, D. et al. (1988) Proc. Natl. Acad. Sci. USA 85:6152-6156; Scandella, D. et al. (1989) Blood 14:1618-1626]. Additionally, an epitope of unknown significance has been identified that is in either the A3 or CI domain [Scandella et al. (1989) supra; Scandella, D. et al. (1993) Blood 82: 1767-1775; Nakai, H. et al. (1994) Blood 84:224a] . The A2 epitope has been mapped to residues 484-508 by homolog scanning mutagenesis [Healey et al. (1995) supra]. In this 25 residue segment, there is relatively low proportion of identical sequence (16/25 or 64%). It is interesting that this region, which appears to be functionally important based on the fact that antibodies to it are inhibitory, apparently has been subjected to relatively more rapid genetic drift. Alignment of the porcine A2 domain and A3 domains indicate that the A2 epitope shares no detectable homology with the corresponding region in the A3 domain. The C2 inhibitor epitope of human fVIII has been proposed to be located to within residues 2248-2312 by deletion mapping [Scandella, D. et al. (1995) Blood 86: 1811-1819]. Human and porcine fVIII are 83% identical in this 65 residue segment. However, homolog scanning mutagenesis of this region to characterize the C2 epitope has revealed that a major determinant of the C2 epitope was unexpectedly located in the region corresponding to human amino acids 2181-2243 (SEQ ID NO:2) and Fig. 1H. Human-porcine hybrid factor VIII proteins were made in which various portions of the C2 domain of human factor VIII were replaced by the corresponding portions of porcine factor VIII, using the strategy herein described. (Example 8) The synthesis of the various C2-hybrid factor VIIIs was accomplished by constructing hybrid coding DNA, using the nucleotide sequence encoding the porcine C2 region given in SEQ ID NO.37. Each hybrid DNA was expressed in transfected cells, such that the hybrid factor VIIIs could be partially purified from the growth medium. Activity, in the absence of any inhibitor, was measured by the one-stage clotting assay. A battery of five human inhibitors was used to test each hybrid factor VIII. The inhibitor plasmas containing anti factor VIII antibody had been previously shown to be directed against human C2 domain, based on the ability of recombinant human C2 domain to neutralize the inhibition. In all the test plasmas, the inhibitor titer was neutralized greater than 79% by C2 domain or light chain but less than 10% by recombinant human A2 domain. In addition the C2-hybrid factor VIIIs were tested against a murine monoclonal antibody, which binds the C2 domain, and like human C2 inhibitor antibodies, it inhibited the binding of factor VIII to phospholipid and to von Willebrand factor. By comparing the antibody inhibitor titers against the C2-hybrid factor VIIIs, the major determinant of the human C2 inhibitor epitope was shown to be the region of residues 2181- 2243 (SEQ ID NO:2, see also Fig. 1H). Anti-C2 antibodies directed to a region COOH- terminal to residue 2253 were not identified in four of the five patient sera. In comparing hybrids having porcine sequence corresponding to human amino acid residues numbers 2181- 2199 and 2207-2243, it was apparent that both regions contribute to antibody binding. The porcine amino acid sequence corresponding to human residues 2181-2243 is numbered 1982- 2044 in SEQ ID NO: 37. The sequence of porcine DNA encoding porcine amino acids numbered 1982-2044 is nucleotides numbered 5944-6132 in SEQ ID NO:35. Referring to Fig. 1H, it can be seen that in the region 2181-2243, there are 16 amino acid differences between the human and porcine sequences. The differences are found at residues 2181, 2182, 2188, 2195-2197, 2199, 2207, 2216, 2222, 2224-2227, 2234, 2238 and 2243. Amino acid replacement at one or more of these numbered residues can be carried out to make a modified human factor VIII non-reactive to human anti-C2 inhibitor antibodies. Alanine scanning mutagenesis provides a convenient method for generating alanine substimtions for naturally-occurring residues, as previously described. Amino acids other than alanine can be substimted as well, as described herein. Alanine substimtions for individual amino acids, especially those which are non- identical between human/porcine or human/mouse or which are most likely to contribute to antibody binding, can yield a modified factor VIII with reduced reactivity to inhibitory antibodies. In addition, the strategy of inserting amino acids with lower potential to be immunogenic in the defined region of residues 2181-2243 yields modified factor VIIIs having reduced immunogenicity. Reduced immunogenicity factor VIII is useful as a factor VIII supplement for treatment of hemophilia A patients in preference to natural-sequence factor VIII. Patients treated with reduced immunogenicity factor VIII are less likely to develop inhibitory antibodies, and are therefore less likely to suffer from reduced effectiveness of treatment over their lifetimes. Figs.1A-1H taken together provide an aligned sequence comparison of the human, pig and mouse factor VIII amino acid sequences. Fig.1 A compares signal peptide regions (human. SEQ ID NO:40; porcine, SEQ ID NO:37, amino acids 1-19; murine, SEQ ID NO:6. amino acids 1-19). Note that the amino acids in Fig. 1A-1H are numbered at the first Alanine of the mature protein as number 1 , with amino acids of the signal peptide assigned negative numbers. The Human fVIII sequence in SEQ ID NO:2 also begins with the first Alanine of the mature protein as amino acid number 1. In the amino acid sequences of mouse fVTII (SEQ ID NO: 6) and porcine fVIII (SEQ ID No:37), the first amino acid (alanine) of the mature sequence is amino acid number 20. Fig. 1A-1H shows an alignment of the corresponding sequences of human, mouse and pig fVIII, such that the regions of greatest amino acid identity are juxtaposed. The amino acid numbers in Fig. 1 A- IH apply to human fVIII only. Fig. IB gives the amino acid sequences for the Al domain of human (SEQ ID NO:2, amino acids 1-372), porcine (SEQ ID NO: 37, amino acids 20-391), and murine (SEQ ID NO:6, amino acids 20- 391). Fig. IC provides amino acid sequences for the Factor VIII A2 domains from human (SEQ ID NO:2, amino acids 373-740), pig (SEQ ID NO:37, amino acids 392-759) and mouse (SEQ ID NO:6, amino acids 392-759). Fig. ID provides the amino acid sequences of B domains of human factor VIII (SEQ ID NO:2, amino acids 741-1648), pig (SEQ ID NO:37, amino acids 760-1449) and mouse (SEQ ID NO:6, amino acids 760-1640). Fig. IE compares the amino acid sequences of Factor VIII light chain activation peptides of human, pig and mouse (SEQ ID NO:2, amino acids 1649-1689; SEQ ID NO:37, amino acids 1450-1490; and SEQ ID NO:6, amino acids 1641-1678, respectively). Fig. IF provides the sequence comparison for human, pig and mouse Factor VIII A3 domains (SEQ ID NO:2. amino acids 1690-2019; SEQ ID NO:37, amino acids 1491-1820; and SEQ ID NO:6. amino acids 1679- 2006, respectively. Fig. IG provides the amino acid sequences of the Factor VIII CI domains of human, pig and mouse (SEQ ID NO:2, amino acids 2020-2172; SEQ ID NO:37, amino acids 1821-1973; and SEQ ID NO:6, amino acids 2007-2159, respectively). Fig. IH provides sequence data for the C2 domains of the Factor VIII C2 domains of human, pig and mouse (SEQ ID NO:2, amino acids 2173-2332; SEQ ID NO:37, amino acids 1974-2133; and SEQ ID NO:6, amino acids 2160-2319, respectively). The diamonds represent tyrosine sulfation sites, proposed binding sites for Factor IXa, phospholipid and Protein C are double-underlined, and regions involved in binding anti-A2 and anti-C2 inhibitory antibodies are italicized. Asterisks highlight amino acid sequences which are conserved. See also SEQ ID NO:36 (porcine factor VIII cDNA) and SEQ ID NO:37 (deduced amino acid sequence of porcine factor VIII). The human numbering system is used as the reference [Wood et al. (1984) supra] . The Al, A2, and B domains are defined by thrombin cleavage sites at positions 372 and 740 and an unknown protease cleavage site at 1648 as residues 1-372, 373-740, and 741-1648, respectively [Eaton, D.L. et al. (1986) Biochemistry 25:8343-8347]. The A3, CI , and C2 domains are defined as residues 1690-2019, 2020-2172, and 2173-2332, respectively [Vehar et al. (1984) supra] . Cleavage sites for thrombin (factor Ila), factor IXa, factor Xa and APC [Fay et al. (1991) supra; Eaton, D. et al. (1986) Biochemistry 25:505-512; Lamphear, BJ. et al. (1992)Rtooa"80:3120-3128] are shown by placing the enzyme name over the reactive arginine. An acidic peptide is cleaved from the fVIII light chain by thrombin or factor Xa at position 1689. Proposed binding sites for factor IXa [Fay, P.J. et al. (1994) 7. Biol. Chem. 269:20522-20527; Lenting, P.J. et al. (1994) 7. Biol. Chem. 269:7150-7155), phospholipid (Foster, P.A. et al. (1990) Blood 75: 1999-2004) and protein C (Walker, F.J. et al. (1990) J. Biol. Chem. 265: 1484-1489] are doubly underlined. Regions involved in binding anti-A2 [Lubin et al. (1994) supra; Healey et al. (1995) supra]; and previously proposed for anti-C2 inhibitory antibodies are italicized. The C2 inhibitor epitope identified as herein described (human amino acids 2181-2243) is shown by a single underline in Fig. IH. Tyrosine sulfation sites [Pittman et al. (1992) supra; Michnick et al. (1994) supra] are shown by ♦ . The nucleotide sequence encoding the factor VIII protein lacking the B domain is given in SEQ ID NO: 38, and the corresponding deduced amino acid sequence is provided in SEQ ID NO:39.

Claims: CLAIMS 1. A modified human factor VIII comprising an amino acid substimtion at one or more of positions 484-508 according to SEQ ID NO:2, said substimtion being an insertion of an immunoreactivity-reducing amino acid for the naturally-occurring amino acid, said modified factor VIII having procoagulant activity. 2. A modified factor VIII according to claim 1 wherein the modified factor VIII has reduced reactivity to an inhibitory antibody as compared to unmodified factor VIII. 3. The modified factor VIII of claim 1 wherein the amino acid substimtion is made at one or more of positions selected from the group consisting of 490, 493, 496. 499, 500, 502, 503, 505 and 507. 4. The modified factor VIII of claim 1 wherein the amino acid substimtions is made at one or more of positions selected from the group consisting of 486, 491 , 494, 498, 504 and 506. 5. The modified factor VIII of claim 1 wherein an amino acid substimtion is made at position 497. 6. The modified factor VIII of claim 1 wherein an amino acid substimtion is made at position 490. 7. DNA encoding modified human factor VIII A2 domain, said DNA having one or more nucleotide substimtions resulting in a coding change at one or more amino acid positions chosen from positions 484-508 according to SEQ ID NO:2, said change encoding an immunoreactivity-reducing amino acid at the chosen position. 8. DNA according to claim 7 wherein the amino acid position chosen is selected from the group consisting of 490, 493, 496, 499, 500, 502, 503, 505 and 507. 9. DNA according to claim 7 wherein the amino acid position chosen is selected from the group consisting of 486, 491, 494, 498, 504 and 506. 10. DNA according to claim 7 wherein the amino acid position chosen is amino acid position 497. 11. DNA according to claim 7 wherein the amino acid position chosen is amino acid position 490. 12. Expression product of DNA encoding human or hybrid human/mammalian factor VIII, said DNA comprising DNA encoding a modified A2 domain, the DNA having one or more nucleotide substimtions resulting in a coding change at one or more of amino acid positions chosen from positions 484-508 according to SEQ ID NO:2, said change encoding an immunoreactivity-reducing amino acid at the chosen position, occurring amino acid. 13. The expression product of claim 12 wherein the DNA encoding the modified A2 domain encodes an amino acid substimtion at one or more of amino acid positions selected from the group consisting of 490, 493, 496, 499, 500, 502, 503, 505 and 507. 14. The expression product of claim 12 wherein the DNA encoding the modified A2 domain encodes an amino acid substimtion at one or more of amino acid positions selected from the group consisting of 486, 491 , 494, 498, 504 and 506. 15. The expression product of claim 12 wherein the DNA encoding the modified A2 domain encodes an amino acid substimtion at position 497. 16. The expression product of claim 12 wherein the DNA encoding the modified A2 domain encodes an amino acid substimtion at position 490. 17. The method of making a modified mammalian factor VIII A2 domain comprising the steps of mutating DNA encoding the domain at one or more codons encoding amino acids at positions corresponding to 484-508 of human factor VIII whereby one or more mutated codons encoding an amino acid is substimted for a corresponding naturally- occurring codon, and expressing in a host cell DNA comprising the mutated codons, either independently or in contiguous translatable sequence with DNA encoding another domain of factor VIII, whereby a modified factor VIII A2 domain is made. 18. The method of claim 17 wherein the DNA to be mutated is human DNA. 19. The method of claim 17 wherein the DNA to be mutated is porcine DNA. 20. The method of claim 17 wherein the DNA to be mutated is murine DNA. 21. The method of claim 17 wherein a mutated codon encodes an immunoreactivity- reducing amino acid. 22. The method of claim 21 wherein a mutated codon encodes an amino acid at a position selected from the group consisting of 490, 493, 496, 499, 500, 502, 503, 505 and 507. 23. The method of claim 21 wherein a mutated codon encodes an amino acid at a position selected from the group consisting of 486, 491 , 494, 498, 504 and 506. 24. The method of claim 21 wherein a mutated codon encodes an amino acid at position 497. 25. The method of claim 21 wherein a mutated codon encodes an amino acid at position 490.

Inventors: LOLLAR JOHN S, LOLLAR, JOHN, S.
Applicants: UNIV EMORY, EMORY UNIVERSITY
Application year: 2000
Main classification: A61K35
IPC codes: A61K38, A61K48, A61P7, C07K14, C07K19, C12N15, C12P21