METHODS AND COMPOSITIONS FOR IMMUNOMODULATION

Abstract: Methods and compositions for inducing immune suppression are disclosed. The methods involve administering an effective amount of an OX-2 protein or a nucleic acid encoding an OX-2 protein. The methods are useful in preventing graft rejection, fetal loss, autoimmune disease, and allergies. Methods and compositions for preventing immune suppression are also disclosed. The methods involve administering an effective amount of an agent that inhibits OX-2.

Description: Title: Methods and Compositions for Immunomodulation FIELD OF THE INVENTION The present invention relates to methods and compositions for modulating an immune response. The invention includes the use of the protein OX-2 to suppress an immune response. BACKGROUND OF THE INVENTION The immune system protects the body from infectious agents and disease and is critical to our survival. However, in certain instances, the immune system can be the cause of illness. One example is in autoimmune disease wherein the immune system attacks its own host, tissues, in many instances causing debilitating illness and sometimes resulting in deafϊ . Examples of autoimmune diseases include multiple sclerosis, type 1 insulin- dependent diabetes mellitus, lupus erythematosus and arthritis. A second example where the immune system can cause illness is during tissue or organ transplantation. Except in the cases of genetically identical animals, such as monozygotic twins, tissue and organ transplants are rejected by the recipient's immune system as foreign. The immune reaction against transplants is even more pronounced in transplantation across species or xenotransplantation. A third example where the immune system harms the host is during an allergic reaction where the immune system is activated by a generally innocuous antigen causing inflammation and in some cases tissue damage. In order to inhibit the detrimental immune reactions during transplantation, autoimmune disease and allergic reactions, immunosuppressive drugs (such as cyclosporin A, tacrolimas, and corticosteroids) or antibody therapies (such as anti-T cell antibodies) are generally administered. Unfortunately, these non-specific modes of immunosuppression generally have undesirable side effects. For example, cyclosporin may cause decreased renal function, hypertension, toxicity and it must be administered for the life of the patient. Corticosteroids may cause decreased resistance to infection, painful arthritis, osteoporosis and cataracts. The anti-T cell antibodies may cause fever, hypertension, diarrhea or sterile meningitis and are quite expensive. In view of the problems associated with immunosuppression, there has been an interest in developing methods or therapies that induce unresponsiveness or tolerance in the host to a transplant, to "self" tissues in autoimmune disease and to harmless antigens associated with allergies. The inventor has been studying the mechanisms involved in transplant rejection and has developed methods for inducing a state of antigen-specific immunological tolerance in transplantation. In particular, in animal allograft models, the inventor has demonstrated that graft survival can been increased if the recipient animal is given a pre-transplant infusion via the portal vein of irradiated spleen cells from the donor animal. In contrast, a pre-transplant infusion via the tail vein does not prolong graft survival. Understanding the molecular mechanisms involved in the induction of tolerance following portal-venous (pv) immunization may lead to the development of methods of inducing immune tolerance that may be useful in transplant, autoimmune disease and allergies. SUMMARY OF THE INVENTION The present inventor has identified genes that show an increase in expression following portal venous immunization. One of the genes isolated encodes OX-2, a molecule with previously unknown function belonging to the Ig superfamily. The inventor has shown that administering antibodies to OX-2 inhibited the graft survival generally seen following pre-transplant pv immunization. The inventor has also shown that there is a negative association between levels of OX-2 and risk of fetal loss. The inventor has further shown that OX-2 inhibits cytotoxic cells and IL-2 production and induces IL-4 production. All of these results demonstrate that OX-2 is involved in immune suppression. Consequently, broadly stated, the present invention provides a method of suppressing an immune system comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to an animal in need of such treatment. In one embodiment, the present invention provides a method of inducing immune tolerance to a transplanted organ or tissue in a recipient animal comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to the recipient animal prior to the transplantation of the organ or tissue. In another embodiment, the present invention provides a method of preventing or inhibiting graft versus host disease in a recipient animal receiving an organ or tissue transplant comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to the organ or tissue prior to the transplantation in the recipient animal. In yet another embodiment, the present invention provides a method of preventing or inhibiting fetal loss comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to an animal in need thereof. In a further embodiment, the present invention provides a method of preventing or treating an autoimmune disease comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to an animal having, suspected of having, or susceptible to having an autoimmune disease. In yet a further embodiment, the present invention provides a method of preventing or treating an allergy comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to an animal having or suspected of having an allergy. The invention also includes pharmaceutical compositions containing OX-2 proteins or nucleic acids encoding OX-2 proteins for use in inducing tolerance in transplantation or autoimmune disease. The inventor has cloned and sequenced the murine OX-2 gene. Accordingly, the invention also includes an isolated nucleic acid sequence encoding a murine OX-2 gene and having the sequence shown in Figure 7 and SEQ.ID.NO.:l and an isolated murine OX-2 protein having the amino acid sequence shown in Figure 8 and SEQ.ID.NO.:2. As stated above, OX-2 can be used to induce immune suppression. Consequently, inhibiting OX-2 may also be useful in preventing immune suppression. Therefore, in another aspect, the present invention provides a method of preventing immune suppression comprising administering an effective amount of an agent that inhibits OX-2 to an animal in need thereof. In a preferred embodiment the OX-2 inhibitor is an antibody that binds OX-2 or an antisense oligonucleotide that inhibits the expression of OX-2. In one embodiment, the present invention provides a method of inducing fetal loss comprising administering an effective amount of an agent that inhibits OX-2 to an animal in need thereof. The invention also includes pharmaceutical compositions containing an OX- 2 inhibitor for use in inducing or augmenting an immune response. Other features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples while indicating preferred embodiments of the invention are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in relation to the drawings in which: Figure 1 illustrates PCR validation of suppressive subtractive hybridization using β-actin primers. Figure 2 illustrates PCR validation of suppressive subtractive hybridization using IL-10 primers. Figure 3 is an autoradiograph using 32 P-labeled probes from 4 clones obtained from the subtractive hybridization process. Figure 4 is flow cytometry profile of spleen adherent cells. Figure 5A and B are Western Blots illustrating the increased expression of OX-2 antigen after pv immunization. Figure 5A shows staining with a control mouse antibody, anti-mouse CD8a. Figure 5B shows staining with anti-rat MRC OX-2. Figure 6 is a graft showing percent survival versus days post renal transplantation. Figure 7 shows the cDNA sequence of rat, mouse and human MRC OX-2. Figure 8 shows the deduced protein sequence of rat, mouse and human MRC OX-2 protein. Figures 9A and 9B are bar graphs showing cytokine production and cell proliferation following stimulation by allogeneic DC using hepatic NPMC. Figures 10A, 10B and IOC are bar graphs showing inhibition of cell proliferation and cytokine production by hepatic NPMC. Figure 11 is a bar graph analysis of FACS data showing OX-2 expression in a subpopulation of NPC. Figure 12 shows PCR analysis mRNA expression of B7-1, B7-2 and OX-2 in various hepatic NPMC cell fractions. Figures 13A and 13B are bar graphs showing proliferation and cytokine production by NPMC from FH3L treated mice. Figure 14 is a bar graph showing cytokines produced from C3H mice with C57BL16 renal allografts and NPC from Flt3 treated C57BL16 donors. Figure 15 is a graph showing inhibition of graft rejection with NPC from FU3 treated mice. Figure 16 is a graph showing that anti-OX-2 reverses inhibition by NPC. The effect of anti-B7-l, anti-B7-2 and anti-OX-2 on primary allostimulation is shown. Figure 17 is a graph showing that anti-OX-2 mAb reverses inhibition by NPC and inhibits the development of immunoregulatory cells. Figure 18A is a photograph showing in situ hybridization with antisense OX-2 in a 8-11 day placenta from a mouse that has undergone fetal loss. Figure 18B is a photograph showing in situ hybridization with antisense OX-2 in a 8-11 day placenta from a mouse that is not susceptible to spontaneous fetal loss. DETAILED DESCRIPTION OF THE INVENTION The present inventor has identified genes that show an increase in expression following portal venous immunization. These genes play a role in the development of immune suppression or tolerance and may be useful in developing therapies for the prevention and treatment of transplant rejection, fetal loss, autoimmune disease or allergies. Using suppression subtractive hybridization (SSH), the inventor has isolated a clone that is preferentially expressed in mice receiving allogenic renal grafts along with pre-transplant donor-specific immunization and that encodes the protein OX-2. The OX-2 protein (also known as MRC OX-2) in rat was described as a 41Kd-47Kd glycoprotein which is expressed on the cell surface of thymocytes, follicular dendritic cells and endothelium, B cells and neuronal cells. Differences in apparent size of the molecule in different tissues is probably a function of differential glycosylation. The function of the molecule was previously unknown, but DNA and amino acid sequence analysis shows it has a high degree of homology to molecules of the immunoglobulin gene family, which includes molecules important in lymphocyte antigen recognition and cell-cell interaction (e.g. CD4, CD8, ICAMs, VCAMs), as well as adhesion receptor molecules (NCAMs) in the nervous system. Members of the immunoglobulin superfamily are distinct from other molecules of the integrin and selectin families, which, at least within the immune system, also seem to play critical role in cell recognition, migration and even development of the lymphocyte recognition repertoire (by regulating intra-thymic selection events). It has become increasingly evident that molecules of these different families play an important role in human disease. The inventor has shown that administering antibodies to OX-2 inhibited the graft survival generally seen following pre-transplant pv immunization. The inventor has also shown that there is negative association between levels of OX-2 and risk of fetal loss. The inventor has further shown that OX-2 inhibits cytotoxic cells and IL-2 production and induces IL-4 production. The data supports the role of OX-2 in immune suppression. THERAPEUTIC METHODS Inducing Immune Suppression In one aspect, the present invention provides a method of suppressing an immune response comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to an animal in need of such treatment. The invention includes a use of an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to suppress an immune response. The term "OX-2 protein" includes the full length OX-2 protein as well as fragments or portions of the protein. Preferred fragments or portions of the protein are those that are sufficient to suppress an immune response. The OX-2 protein also includes fragments that can be used to prepare antibodies. In a preferred embodiment, the OX-2 protein is prepared as a soluble fusion protein. The fusion protein may contain the extracellular domain of OX-2 linked to an irnmunoglobulin (Ig) Fc Region. The OX-2 fusion may be prepared using techniques known in the art. Generally, a DNA sequence encoding the extracellular domain of OX-2 is linked to a DNA sequence encoding the Fc of the Ig and expressed in an appropriate expression system where the OX-2 - Fclg fusion protein is produced. The OX-2 protein may be obtained from known sources or prepared using recombinant DNA techniques. The protein may have any of the known published sequences for OX-2. (The sequences can be obtained from GenBank. The human sequence accession no. M17226 X0523; the rat sequence accession no. X01785; and the mouse sequence accession no. AF029214.) The protein may also be modified to contain amino acid substitutions, insertions and/ or deletions that do not alter the immunosuppressive properties of the protein. Conserved amino acid substitutions involve replacing one or more amino acids of the OX-2 amino acid sequence with amino acids of similar charge, size, and/or hydrophobicity characteristics. When only conserved substitutions are made the resulting analog should be functionally equivalent to the OX-2 protein. Non-conserved substitutions involve replacing one or more amino acids of the OX-2 amino acid sequence with one or more amino acids which possess dissimilar charge, size, and /or hydrophobicity characteristics. The OX-2 protein may be modified to make it more therapeutically effective or suitable. For example, the OX-2 protein may be cyclized as cyclization allows a peptide to assume a more favourable conformation. Cyclization of the OX-2 peptides may be achieved using techniques known in the art. In particular, disulphide bonds may be formed between two appropriately spaced components having free sulfhydryl groups. The bonds may be formed between side chains of amino acids, non-amino acid components or a combination of the two. In addition, the OX-2 protein or peptides of the present invention may be converted into pharmaceutical salts by reacting with inorganic acids including hydrochloric acid, sulphuric acid, hydrobromic acid, phosphoric acid, etc., or organic acids including formic acid, acetic acid, propionic acid, glycolic acid, lactic acid, pyruvic acid, oxalic acid, succinic acid, malic acid, tartaric acid, citric acid, benzoic acid, salicylic acid, benzenesulphonic acid, and tolunesulphonic acids. Administration of an "effective amount" of the OX-2 protein and nucleic acid of the present invention is defined as an amount effective, at dosages and for periods of time necessary to achieve the desired result. The effective amount of the OX-2 protein or nucleic acid of the invention may vary according to factors such as the disease state, age, sex, and weight of the animal. Dosage regima may be adjusted to provide the optimum therapeutic response. For example, several divided doses may be administered daily or the dose may be proportionally reduced as indicated by the exigencies of the therapeutic situation. The term "animal" as used herein includes all members of the animal kingdom including humans. In one embodiment, the present invention provides a method of inducing immune tolerance to a transplanted organ or tissue in a recipient animal comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to the recipient animal prior to the transplantation of the organ or tissue. The invention includes a use of an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to induce immune tolerance to a transplanted organ or tissue. The term "inducing immune tolerance" means rendering the immune system unresponsive to a particular antigen without inducing a prolonged generalized immune deficiency. The term "antigen" means a substance that is capable of inducing an immune response. In the case of autoimmune disease, immune tolerance means rendering the immune system unresponsive to an auto-antigen that the host is recognizing as foreign, thus causing an autoimmune response. In the case of allergy, immune tolerance means rendering the immune system unresponsive to an allergen that generally causes an immune response in the host. In the case of transplantation, immune tolerance means rendering the immune system unresponsive to the antigens on the transplant. An alloantigen refers to an antigen found only in some members of a species, such as blood group antigens. A xenoantigen refers to an antigen that is present in members of one species but not members of another. Correspondingly, an allograft is a graft between members of the same species and a xenograft is a graft between members of a different species. The recipient can be any member of the animal kingdom including rodents, pigs, cats, dogs, ruminants, non-human primates and preferably humans. The organ or tissue to be transplanted can be from the same species as the recipient (allograft) or can be from another species (xenograft). The tissues or organs can be any tissue or organ including heart, liver, kidney, lung, pancreas, pancreatic islets, brain tissue, cornea, bone, intestine, skin and heamatopoietic cells. The method of the invention may be used to prevent graft versus host disease wherein the immune cells in the transplant mount an immune attack on the recipient's immune system. This can occur when the tissue to be transplanted contains immune cells such as when bone marrow or lymphoid tissue is transplanted when treating leukemias, aplastic anemias and enzyme or immune deficiencies, for example. Accordingly, in another embodiment, the present invention provides a method of preventing or inhibiting graft versus host disease in a recipient animal receiving an organ or tissue transplant comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to the organ or tissue prior to the transplantation in the recipient animal. The invention includes a use of an effective amount of an OX-2 protein or a nucleic acid molecule encoding an OX-2 protein to prevent or inhibit graft versus host disease. The present inventor has shown that there is an association between levels of OX-2 expression and fertiUty. In particular the inventor has shown that low levels (or no levels) of OX-2 is related to fetal loss. Accordingly, the present invention provides a method of preventing or inhibiting fetal loss comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to an animal in need thereof. The invention includes a use of an effective amount of an OX-2 protein on a nucleic acid molecules encoding an OX-2 protein to prevent or inhibit fetal loss. As stated previously, the method of the present invention may also be used to treat or prevent autoimmune disease. In an autoimmune disease, the immune system of the host fails to recognize a particular antigen as "self" and an immune reaction is mounted against the host's tissues expressing the antigen. Normally, the immune system is tolerant to its own host's tissues and autoimmunity can be thought of as a breakdown in the immune tolerance system. Accordingly, in a further embodiment, the present invention provides a method of preventing or treating an autoimmune disease comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to an animal having, suspected of having, or susceptible to having an autoimmune disease. The invention includes a use of an effective amount of an OX-2 protein on a nucleic acid molecule encoding an OX-2 protein to prevent or inhibit an autoimmune disease. Autoimmune diseases that may be treated or prevented according to the present invention include, but are not limited to, type 1 insulin-dependent diabetes mellitus, adult respiratory distress syndrome, inflammatory bowel disease, dermatitis, meningitis, thrombotic thrombocytopenic purpura, Sjόgren's syndrome, encephalitis, uveitic, leukocyte adhesion deficiency, rheumatoid arthritis, rheumatic fever, Reiter's syndrome, psoriatic arthritis, progressive systemic sclerosis, primary biniary cirrhosis, pemphigus, pemphigoid, necrotizing vasculitis, myasthenia gravis, multiple sclerosis, lupus erythematosus, polymyositis, sarcoidosis, granulomatosis, vasculitis, pernicious anemia, CNS inflammatory disorder, antigen-antibody complex mediated diseases, autoimmune haemolytic anemia, Hashimoto's thyroiditis, Graves disease, habitual spontaneous abortions, Reynard's syndrome, glomerulonephritis, dermatomyositis, chronic active hepatitis, celiac disease, autoimmune complications of AIDS, atrophic gastritis, ankylosing spondylitis and Addison's disease. As stated previously, the method of the present invention may also be used to treat or prevent an allergic reaction. In an allergic reaction, the immune system mounts an attack against a generally harmless, innocuous antigen or allergen. Allergies that may be prevented or treated using the methods of the invention include, but are not limited to, hay fever, asthma, atopic eczema as well as allergies to poison oak and ivy, house dust mites, bee pollen, nuts, shellfish, penicillin and numerous others. Accordingly, in a further embodiment, the present invention provides a method of preventing or treating an allergy comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to an animal having or suspected of having an allergy. The invention includes a use of an effective amount of an OX-2 protein or a nucleic acid molecule encoding an OX-2 protein to prevent or treat an allergy. Preventing Immune Suppression In another aspect, the present invention provides a method of preventing immune suppression comprising administering an effective amount of an agent that inhibits OX-2 to an animal in need thereof. There are a large number of situations whereby it is desirable to prevent immune suppression including, but not limited to, the treatment of infections, cancer and Acquired Immune Deficiency Syndrome. In one embodiment, the present invention provides a method of preventing immune suppression comprising administering an effective amount of an agent that binds OX-2 to an animal in need thereof. In a preferred embodiment, the agent that binds OX-2 is an OX-2 specific antibody. The present inventor has prepared antibodies to OX-2 which are described in Examples 4 and 5. Antibodies to OX-2 may also be prepared using techniques known in the art such as those described by Kohler and Milstein, Nature 256, 495 (1975) and in U.S. Patent Nos. RE 32,011; 4,902,614; 4,543,439; and 4,411,993, which are incorporated herein by reference. (See also Monoclonal Antibodies, Hybridomas: A New Dimension in Biological Analyses, Plenum Press, Kennett, McKearn, and Bechtol (eds.), 1980, and Antibodies: A Laboratory Manual, Harlow and Lane (eds.), Cold Spring Harbor Laboratory Press, 1988, which are also incorporated herein by reference). Within the context of the present invention, antibodies are understood to include monoclonal antibodies, polyclonal antibodies, antibody fragments (e.g., Fab, and F(ab') 2 ) and recombinantly produced binding partners. In another embodiment, the OX-2 inhibitor is an antisense oligonucleotide that inhibits the expression of OX-2. Antisense oligonucleotides that are complimentary to a nucleic acid sequence from an OX-2 gene can be used in the methods of the present invention to inhibit OX-2. The present inventor has prepared antisense oligonucleotides to OX-2 which are described in Example 3. Consequently, the present invention provides a method of preventing immune suppression comprising administering an effective amount of an antisense oligonucleotide that is complimentary to a nucleic acid sequence from an OX-2 gene to an animal in need thereof. The term antisense oligonucleotide as used herein means a nucleotide sequence that is complimentary to its target. In one embodiment of the invention, the present invention provides an antisense oligonucleotide that is complimentary to a nucleic acid molecule having a sequence as shown in Figure 7, wherein T can also be U, or a fragment thereof. The term "oligonucleotide" refers to an oligomer or polymer of nucleotide or nucleoside monomers consisting of naturally occurring bases, sugars, and intersugar (backbone) linkages. The term also includes modified or substituted oligomers comprising non-naturally occurring monomers or portions thereof, which function similarly. Such modified or substituted oligonucleotides may be preferred over naturally occurring forms because of properties such as enhanced cellular uptake, or increased stability in the presence of nucleases. The term also includes chimeric oligonucleotides which contain two or more chemically distinct regions. For example, chimeric oligonucleotides may contain at least one region of modified nucleotides that confer beneficial properties (e.g. increased nuclease resistance, increased uptake into cells), or two or more oligonucleotides of the invention may be joined to form a chimeric oligonucleotide. The antisense oligonucleotides of the present invention may be ribonucleic or deoxyribonucleic acids and may contain naturally occurring bases including adenine, guanine, cytosine, thymidine and uracil. The oligonucleotides may also contain modified bases such as xanthine, hypoxanthine, 2-aminoadenine, 6-methyl, 2-propyl and other alkyl adenines, 5-halo uracil, 5-halo cytosine, 6-aza uracil, 6-aza cytosine and 6-aza thymine, pseudo uracil, 4-thiouracil, 8-halo adenine, 8-aminoadenine, 8-thiol adenine, 8-thiolalkyl adenines, 8-hydroxyl adenine and other 8-substituted adenines, 8-halo guanines, 8-amino guanine, 8-thiol guanine, 8-thiolalkyl guanines, 8-hydroxyl guanine and other 8-substituted guanines, other aza and deaza uracils, thymidines, cytosines, adenines, or guanines, 5-trifluoromethyl uracil and 5-trifluoro cytosine. Other antisense oligonucleotides of the invention may contain modified phosphorous, oxygen heteroatoms in the phosphate backbone, short chain alkyl or cycloalkyl intersugar linkages or short chain heteroatomic or heterocyclic intersugar linkages. For example, the antisense oligonucleotides may contain phosphorothioates, phosphotriesters, methyl phosphonates, and phosphor odithioates. In an embodiment of the invention there are phosphorothioate bonds links between the four to six 3'-terminus bases. In another embodiment phosphorothioate bonds link all the nucleotides. The antisense oligonucleotides of the invention may also comprise nucleotide analogs that may be better suited as therapeutic or experimental reagents. An example of an oligonucleotide analogue is a peptide nucleic acid (PNA) wherein the deoxyribose (or ribose) phosphate backbone in the DNA (or RNA), is replaced with a polyamide backbone which is similar to that found in peptides (P.E. Nielsen, et al Science 1991, 254, 1497). PNA analogues have been shown to be resistant to degradation by enzymes and to have extended lives in vivo and in vitro. PNAs also bind stronger to a complimentary DNA sequence due to the lack of charge repulsion between the PNA strand and the DNA strand. Other oligonucleotides may contain nucleotides containing polymer backbones, cyclic backbones, or acyclic backbones. For example, the nucleotides may have morpholino backbone structures (U.S. Pat. No. 5,034,506). Oligonucleotides may also contain groups such as reporter groups, a group for improving the pharmacokinetic properties of an oligonucleotide, or a group for improving the pharmacodynamic properties of an antisense oligonucleotide. Antisense oligonucleotides may also have sugar mimetics. The antisense nucleic acid molecules may be constructed using chemical synthesis and enzymatic ligation reactions using procedures known in the art. The antisense nucleic acid molecules of the invention or a fragment thereof, may be chemically synthesized using naturally occurring nucleotides or variously modified nucleotides designed to increase the biological stability of the molecules or to increase the physical stability of the duplex formed with mRNA or the native gene e.g. phosphorothioate derivatives and acridine substituted nucleotides. The antisense sequences may be produced biologically using an expression vector introduced into cells in the form of a recombinant plasmid, phagemid or attenuated virus in which antisense sequences are produced under the control of a high efficiency regulatory region, the activity of which may be determined by the cell type into which the vector is introduced. COMPOSITIONS The invention also includes pharmaceutical compositions containing OX-2 proteins or nucleic acids for use in immune suppression as well as pharmaceutical compositions containing an OX-2 inhibitor for use in preventing immune suppression. Such pharmaceutical compositions can be for intralesional, intravenous, topical, rectal, parenteral, local, inhalant or subcutaneous, intradermal, intramuscular, intrathecal, transperitoneal, oral, and intracerebral use. The composition can be in liquid, solid or semisolid form, for example pills, tablets, creams, gelatin capsules, capsules, suppositories, soft gelatin capsules, gels, membranes, tubelets, solutions or suspensions. The pharmaceutical compositions of the invention can be intended for administration to humans or animals. Dosages to be administered depend on individual needs, on the desired effect and on the chosen route of administration. The pharmaceutical compositions can be prepared by per se known methods for the preparation of pharmaceutically acceptable compositions which can be administered to patients, and such that an effective quantity of the active substance is combined in a mixture with a pharmaceutically acceptable vehicle. Suitable vehicles are described, for example, in Remington's Pharmaceutical Sciences (Remington's Pharmaceutical Sciences, Mack Publishing Company, Easton, Pa., USA 1985). On this basis, the pharmaceutical compositions include, albeit not exclusively, the active compound or substance in association with one or more pharmaceutically acceptable vehicles or diluents, and contained in buffered solutions with a suitable pH and iso-osmotic with the physiological fluids. The pharmaceutical compositions may additionally contain other agents such as irnmunosuppressive drugs or antibodies to enhance immune tolerance or immunostimulatory agents to enhance the immune response. In one embodiment, the pharmaceutical composition for use in inducing immune tolerance comprises an effective amount of an OX-2 protein in admixture with a pharmaceutically acceptable diluent or carrier. The OX-2 protein is preferably prepared as an irnrnunoadhesion molecule in soluble form which can be administered to the patient. In the case of tissue or organ transplantation, the composition preferably contains OX-2 proteins in soluble form which may be injected intravenously or perfused directly at the site of the transplantation. In another embodiment, the pharmaceutical composition for use in inducing immune tolerance comprises an effective amount of a nucleic acid molecule encoding an OX-2 protein in admixture with a pharmaceutically acceptable diluent or carrier. The nucleic acid molecules of the invention encoding an OX-2 protein may be used in gene therapy to induce immune tolerance. Recombinant molecules comprising a nucleic acid sequence encoding an OX-2 protein, or fragment thereof, may be directly introduced into cells or tissues in vivo using delivery vehicles such as retroviral vectors, adenoviral vectors and DNA virus vectors. They may also be introduced into cells in vivo using physical techniques such as microinjection and electroporation or chemical methods such as coprecipitation and incorporation of DNA into liposomes. Recombinant molecules may also be delivered in the form of an aerosol or by lavage. The nucleic acid molecules of the invention may also be applied extracellularly such as by direct injection into cells. The nucleic acid molecules encoding OX-2 are preferably prepared as a fusion with a nucleic acid molecule encoding an immunoglobulin (Ig) Fc region. As such, the OX-2 protein will be expressed in vivo as a soluble fusion protein. In another aspect, the pharmaceutical composition for use in preventing immune suppression comprises an effective amount of an OX-2 inhibitor in admixture with a pharmaceutically acceptable diluent or carrier. Such compositions may be administered as a vaccine either alone or in combination with other active agents or antigens. When used in combination, the OX-2 inhibitors may act like an adjuvant by potentiating the immune response to the antigen in the vaccine. In one embodiment, the pharmaceutical composition for use in preventing immune suppression comprises an effective amount of an antibody to OX-2 in admixture with a pharmaceutically acceptable diluent or carrier. The antibodies may be delivered intravenously. In another embodiment, the pharmaceutical composition for use in preventing immune suppression comprises an effective amount of an antisense oligonucleotide nucleic acid complimentary to a nucleic acid sequence from an OX-2 gene in admixture with a pharmaceutically acceptable diluent or carrier. The oligonucleotide molecules may be administered as described above for the compositions containing OX-2 nucleic acid sequences. MURINE OX-2 The inventor has cloned and sequenced the murine OX-2 gene. Accordingly, the invention also includes an isolated nucleic acid sequence encoding a murine OX-2 gene and having the sequence shown in Figure 7 and SEQ.ID.NO.:l. The term "isolated" refers to a nucleic acid substantially free of cellular material or culture medium when produced by recombinant DNA techniques, or chemical precursors, or other chemicals when chemically synthesized. The term "nucleic acid" is intended to include DNA and RNA and can be either double stranded or single stranded. Preferably, the purified and isolated nucleic acid molecule of the invention comprises (a) a nucleic acid sequence as shown in SEQ.ID.NO.:l, wherein T can also be U; (b) nucleic acid sequences complementary to (a); (c) a fragment of (a) or (b) that is at least 15 bases, preferably 20 to 30 bases, and which will hybridize to (a) or (b) under stringent hybridization conditions; or (a) a nucleic acid molecule differing from any of the nucleic acids of (a) or (b) in codon sequences due to the degeneracy of the genetic code. It will be appreciated that the invention includes nucleic acid molecules encoding truncations of the murine OX-2 proteins of the invention, and analogs and homologs of the proteins of the invention and truncations thereof, as described below. It will further be appreciated that variant forms of the nucleic acid molecules of the invention which arise by alternative splicing of an mRNA corresponding to a cDNA of the invention are encompassed by the invention. An isolated nucleic acid molecule of the invention which is DNA can also be isolated by selectively amplifying a nucleic acid encoding a novel protein of the invention using the polymerase chain reaction (PCR) methods and cDNA or genomic DNA. It is possible to design synthetic oligonucleotide primers from the nucleic acid molecules as shown in Figure 7 for use in PCR. A nucleic acid can be amplified from cDNA or genomic DNA using these oligonucleotide primers and standard PCR amplification techniques. The nucleic acid so amplified can be cloned into an appropriate vector and characterized by DNA sequence analysis. It will be appreciated that cDNA may be prepared from mRNA, by isolating total cellular mRNA by a variety of techniques, for example, by using the guanidinium-thiocyanate extraction procedure of Chirgwin et al., Biochemistry, 18, 5294- 5299 (1979). cDNA is then synthesized from the mRNA using reverse transcriptase (for example, Moloney MLV reverse transcriptase available from Gibco/BRL, Bethesda, MD, or AMV reverse transcriptase available from Seikagaku America, Inc., St. Petersburg, FL). An isolated nucleic acid molecule of the invention which is RNA can be isolated by cloning a cDNA encoding a novel protein of the invention into an appropriate vector which allows for transcription of the cDNA to produce an RNA molecule which encodes a OX-2 protein of the invention. For example, a cDNA can be cloned downstream of a bacteriophage promoter, (e.g. a T7 promoter) in a vector, cDNA can be transcribed in vitro with T7 polymerase, and the resultant RNA can be isolated by standard techniques. A nucleic acid molecule of the invention may also be chemically synthesized using standard techniques. Various methods of chemically synthesizing polydeoxynucleotides are known, including solid-phase synthesis which, like peptide synthesis, has been fully automated in commercially available DNA synthesizers (See e.g., Itakura et al. U.S. Patent No. 4,598,049; Caruthers et al. U.S. Patent No. 4,458,066; and Itakura U.S. Patent Nos. 4,401,796 and 4,373,071). The sequence of a nucleic acid molecule of the invention may be inverted relative to its normal presentation for transcription to produce an antisense nucleic acid molecule. Preferably, an antisense sequence is constructed by inverting a region preceding the initiation codon or an unconserved region. In particular, the nucleic acid sequences contained in the nucleic acid molecules of the invention or a fragment thereof, preferably a nucleic acid sequence shown in Figure 7 may be inverted relative to its normal presentation for transcription to produce antisense nucleic acid molecules. The antisense nucleic acid molecules of the invention or a fragment thereof, may be chemically synthesized using naturally occurring nucleotides or variously modified nucleotides designed to increase the biological stability of the molecules or to increase the physical stability of the duplex formed with mRNA or the native gene e.g. phosphorothioate derivatives and acridine substituted nucleotides. The antisense sequences may be produced biologically using an expression vector introduced into cells in the form of a recombinant plasmid, phagemid or attenuated virus in which antisense sequences are produced under the control of a high efficiency regulatory region, the activity of which may be determined by the cell type into which the vector is introduced. The invention also provides nucleic acids encoding fusion proteins comprising an OX-2 protein of the invention and a selected protein, or a selectable marker protein. The invention further includes an isolated protein which has the amino acid sequence as shown in Figure 8 and SEQ.ID.NO.:2. Within the context of the present invention, a protein of the invention may include various structural forms of the primary protein which retain biological activity. For example, a protein of the invention may be in the form of acidic or basic salts or in neutral form. In addition, individual amino acid residues may be modified by oxidation or reduction. In addition to the full length amino acid sequence (Figure 8), the protein of the present invention may also include truncations of the protein, and analogs, and homologs of the protein and truncations thereof as described herein. Truncated proteins may comprise peptides of at least fifteen amino acid residues. Analogs of the protein having the amino acid sequence shown in Figure 8, and /or truncations thereof as described herein, may include, but are not limited to an amino acid sequence containing one or more amino acid substitutions, insertions, and/or deletions. Amino acid substitutions may be of a conserved or non-conserved nature. Conserved amino acid substitutions involve replacing one or more amino acids of the proteins of the invention with amino acids of similar charge, size, and/or hydrophobicity characteristics. When only conserved substitutions are made the resulting analog should be functionally equivalent. Non-conserved substitutions involve replacing one or more amino acids of the amino acid sequence with one or more amino acids which possess dissimilar charge, size, and/or hydrophobicity characteristics. One or more amino acid insertions may be introduced into the amino acid sequences shown in Figure 8. Amino acid insertions may consist of single amino acid residues or sequential amino acids ranging from 2 to 15 amino acids in length. For example, amino acid insertions may be used to render the protein is no longer active. This procedure may be used in vivo to inhibit the activity of a protein of the invention. Deletions may consist of the removal of one or more amino acids, or discrete portions from the amino acid sequence shown in Figures 8. The deleted amino acids may or may not be contiguous. The lower limit length of the resulting analog with a deletion mutation is about 10 amino acids, preferably 100 amino acids. Analogs of a protein of the invention may be prepared by introducing mutations in the nucleotide sequence encoding the protein. Mutations in nucleotide sequences constructed for expression of analogs of a protein of the invention must preserve the reading frame of the coding sequences. Furthermore, the mutations will preferably not create complementary regions that could hybridize to produce secondary mRNA structures, such as loops or hairpins, which could adversely affect translation of the receptor mRNA. Mutations may be introduced at particular loci by synthesizing oligonucleotides containing a mutant sequence, flanked by restriction sites enabling ligation to fragments of the native sequence. Following ligation, the resulting reconstructed sequence encodes an analog having the desired amino acid insertion, substitution, or deletion. Alternatively, oligonucleotide-directed site specific mutagenesis procedures may be employed to provide an altered gene having particular codons altered according to the substitution, deletion, or insertion required. Deletion or truncation of a protein of the invention may also be constructed by utilizing convenient restriction endonuclease sites adjacent to the desired deletion. Subsequent to restriction, overhangs may be filled in, and the DNA religated. Exemplary methods of making the alterations set forth above are disclosed by Sambrook et al (Molecular Cloning: A Laboratory Manual, 2nd Ed., Cold Spring Harbor Laboratory Press, 1989). The invention also contemplates isoforms of the proteins of the invention. An isoform contains the same number and kinds of amino acids as a protein of the invention, but the isoform has a different molecular structure. The isoforms contemplated by the present invention are those having the same properties as a protein of the invention as described herein. The present invention also includes a protein of the invention conjugated with a selected protein, or a selectable marker protein to produce fusion proteins. Additionally, immunogenic portions of a protein of the invention are within the scope of the invention. The proteins of the invention (including truncations, analogs, etc.) may be prepared using recombinant DNA methods. Accordingly, the nucleic acid molecules of the present invention having a sequence which encodes a protein of the invention may be incorporated in a known manner into an appropriate expression vector which ensures good expression of the protein. Possible expression vectors include but are not limited to cosmids, plasmids, or modified viruses (e.g. replication defective retroviruses, adenoviruses and adeno-associated viruses), so long as the vector is compatible with the host cell used. The expression vectors are "suitable for transformation of a host cell", means that the expression vectors contain a nucleic acid molecule of the invention and regulatory sequences selected on the basis of the host cells to be used for expression, which is operatively linked to the nucleic acid molecule. Operatively linked is intended to mean that the nucleic acid is linked to regulatory sequences in a manner which allows expression of the nucleic acid. The invention therefore contemplates a recombinant expression vector of the invention containing a nucleic acid molecule of the invention, or a fragment thereof, and the necessary regulatory sequences for the transcription and translation of the inserted protein-sequence. Such expression vectors may be useful in the above-described therapies using a nucleic acid sequence encoding an OX-2 protein. Suitable regulatory sequences may be derived from a variety of sources, including bacterial, fungal, or viral genes (For example, see the regulatory sequences described in Goeddel, Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, CA (1990). Selection of appropriate regulatory sequences is dependent on the host cell chosen, and may be readily accomplished by one of ordinary skill in the art. Examples of such regulatory sequences include: a transcriptional promoter and enhancer or RNA polymerase binding sequence, a ribosomal binding sequence, including a translation initiation signal. Additionally, depending on the host cell chosen and the vector employed, other sequences, such as an origin of replication, additional DNA restriction sites, enhancers, and sequences conferring inducibility of transcription may be incorporated into the expression vector. It will also be appreciated that the necessary regulatory sequences may be supplied by the native protein and /or its flanking regions. The invention further provides a recombinant expression vector comprising a DNA nucleic acid molecule of the invention cloned into the expression vector in an antisense orientation. That is, the DNA molecule is operatively linked to a regulatory sequence in a manner which allows for expression, by transcription of the DNA molecule, of an RNA molecule which is antisense to a nucleotide sequence comprising the nucleotides as shown in Figure 7. Regulatory sequences operatively linked to the antisense nucleic acid can be chosen which direct the continuous expression of the antisense RNA molecule. The recombinant expression vectors of the invention may also contain a selectable marker gene which facilitates the selection of host cells transformed or transfected with a recombinant molecule of the invention. Examples of selectable marker genes are genes encoding a protein such as G418 and hygromycin which confer resistance to certain drugs, β-galactosidase, chloramphenicol acetyltransf erase, or firefly lucif erase. Transcription of the selectable marker gene is monitored by changes in the concentration of the selectable marker protein such as β-galactosidase, chloramphenicol acetyltransferase, or firefly luciferase. If the selectable marker gene encodes a protein conferring antibiotic resistance such as neomycin resistance transformant cells can be selected with G418. Cells that have incorporated the selectable marker gene will survive, while the other cells die. This makes it possible to visualize and assay for expression of recombinant expression vectors of the invention and in particular to determine the effect of a mutation on expression and phenotype. It will be appreciated that selectable markers can be introduced on a separate vector from the nucleic acid of interest. The recombinant expression vectors may also contain genes which encode a fusion moiety which provides increased expression of the recombinant protein; increased solubility of the recombinant protein; and aid in the purification of a target recombinant protein by acting as a ligand in affinity purification. For example, a proteolytic cleavage site may be added to the target recombinant protein to allow separation of the recombinant protein from the fusion moiety subsequent to purification of the fusion protein. Recombinant expression vectors can be introduced into host cells to produce a transformant host cell. The term "transformant host cell" is intended to include prokaryotic and eukaryotic cells which have been transformed or transfected with a recombinant expression vector of the invention. The terms "transformed with", "transfected with", "transformation" and "transfection" are intended to encompass introduction of nucleic acid (e.g. a vector) into a cell by one of many possible techniques known in the art. Prokaryotic cells can be transformed with nucleic acid by, for example, electroporation or calcium-chloride mediated transformation. Nucleic acid can be introduced into mammalian cells via conventional techniques such as calcium phosphate or calcium chloride co- precipitation, DEAE-dextran-mediated transfection, lipofectin, electroporation or microinjection. Suitable methods for transforming and transfecting host cells can be found in Sambrook et al. (Molecular Cloning: A Laboratory Manual, 2nd Edition, Cold Spring Harbor Laboratory press (1989)), and other laboratory textbooks. Suitable host cells include a wide variety of prokaryotic and eukaryotic host cells. For example, the proteins of the invention may be expressed in bacterial cells such as E. coli, insect cells (using baculovirus), yeast cells or mammalian cells. Other suitable host cells can be found in Goeddel, Gene Expression Technology: Methods in Enzymology 185, Academic Press, San Diego, CA (199 1). The proteins of the invention may also be prepared by chemical synthesis using techniques well known in the chemistry of proteins such as solid phase synthesis (Merrifield, 1964, J. Am. Chem. Assoc. 85:2149-2154) or synthesis in homogenous solution (Houbenweyl, 1987, Methods of Organic Chemistry, ed. E. Wansch, Vol. 15 I and II, Thieme, Stuttgart). The following non-limiting examples are illustrative of the present invention: EXAMPLES Example 1 This example demonstrates the increased expression of certain genes following pv immunization. Mice: C3H/HEJ and C57BL/6 mice were purchased from The Jackson Laboratory, Bar Harbor, ME. Mice were housed five /cage and allowed food and water ad libitum . All mice were used at 8-12 weeks of age. Monoclonal antibodies: The following monoclonal antibodies (Mabs) from Pharmingen (San Diego, CA) were used: anti-IL-2 (JES6-1A12; biotinylated JES6-5H4); anti-IL-4 (11B11; biotinylated BVD6-24G2); anti-IFNγ (R4-6A2; biotinylated XMG1.2); anti-IL-10 (JES5-2A5; biotinylated SXC-1, Pharmingen, San Diego, CA); mouse IgGl isotype control (clone 107.3, BALB/c anti-TNP). Strepavidin horse radish peroxidase and recombinant mouse GM-CSF was also purchased from Pharmingen (San Diego, CA). NLDC-145 (anti-mouse dendritic cells), and F(ab') 2 rabbit anti-rat IgG FITC conjugate (non-cross reactive with mouse IgG), or F(ab') 2 rabbit anti-mouse IgG PE was obtained from Serotec, Canada. Rabbit complement, L3T4, anti-thyl.2, anti-Ly2.2, anti-Ly2.1 (mouse IgG3), FITC-MAC-1 and mouse IgGl anti-rat OX-2 were obtained from Cedarlane Labs, Hornby, Ontario. Anti-CD28 (PV-1) and anti-CTLA (UC10-4F10-11) were obtained from Drs. C. June and J. Bluestone respectively, while anti-B7-l, anti-B7-2 were obtained from Dr. G. Powers. High titres of all 4 of the latter antibodies were produced by in vitro culture in a CELLMAX system (CELLCO Inc., Germantown, MD, USA). Preparation of cells: Spleen, Peyer's Patch (PP) and mesenteric lymph node (MLN) cell suspensions were prepared aseptically from individual mice of the different treated groups in each experiment. Where dendritic cells were obtained by culture of bone marrow cells in vitro the following technique was used (Gorczynski et al., 1996a). Bone marrow plugs were aspirated from the femurs of donor male C57BL/6 (or BALB/c) mice, washed and resuspended in αFlO. Cells were treated sequentially with a mixture of antibodies (L3T4, anti-thyl.2, anti-Ly2.2) and rabbit complement and dead cells removed by centrifugation over mouse lymphopaque (Cedarlane Labs, Ontario). Cells were washed x3 in αFlO, and cultured in 10ml αFlO in tissue culture flasks, at a concentration of 2xl0 6 /ml with 500U/ml recombinant murine GM-CSF (Pharmingen, USA). Fresh GM-CSF was added at 36 hr intervals. Cells were separated over lymphopaque on days 3.5 and 7 of culture, again reculturing in αFlO with recombinant GM-CSF. At 10 days an aliquot of the sample was stained with NLDC-145 and FITC anti-rat IgG, anti-OX-2 and PE anti-mouse IgG, FITC-anti-B7-l or FITC anti-B7-2. Mean staining with these antibodies using cells harvested from such cultures has been 93%±7%, 14%+5%, 78%±9% and 27%±6 % respectively. Remaining cells were washed, and injected into the portal vein as described. Portal vein immunizations and renal transplantation: The pv immunizations and renal transplantation were performed as described earlier (Gorczynski et al., 1994). All C3H mice received pv/iv immunization with 15xl0 6 C57BL/6 10-day cultured, bone marrow derived, dendritic cells, followed by C57BL/6 kidney transplantation. Animals received 1 intramoscular (im) injection with lOmg/Kg cyclosporin A on the day of transplantation. Mice were sacrificed for tissue harvest and RNA preparation 5 days after transplantation. In other studies animals were sacrificed as described in the text. Where monoclonal antibodies were injected into transplanted mice, animals received lOOmg intravenous (iv) at 2 day intervals (x4 injections) beginning within 2 hours of transplantation. Cytokine production from spleen cells of transplanted mice: In cultures used to assess induction of cytokine production spleen responder cells stimulated with irradiated (2000R) C57BL/6 spleen stimulator cells in triplicate in αFlO have been used. In multiple studies significant quantitative or qualitative differences in cytokine production from spleen, lymph node or Peyer's Patch of transplanted mice have not been seen. (Gorczynski et al., 1994b). Supernatants were pooled at 40hr from replicate wells and assayed in triplicate in ELISA assays for lymphokine production. All capture antibodies, biotinylated detection antibodies, and recombinant cytokines were obtained from Pharmingen (San Diego, CA-see above). For IFNγ the assay used flat-bottomed 96-well Nunc plates (Gibco, BRL) coated with lOOng/ml R4-6A2. Varying volumes of supernatant were bound in triplicate at 4°C, washed x3, and biotinylated anti-IFNγ (XMG1.2) added. After washing, plates were incubated with strepavidin-horse radish peroxidase (Cedarlane Labs), developed with appropriate substrate and OD 0 5 determined using an ELISA plate reader. IL-10 was assayed using a similar ELISA system with JES5-2A5 as the capture antibody, and biotinylated SXC-1 as developing antibody. Each assay reliably detected cytokine levels in the range 0.01 to 0.1 ng/ml. ELISA assays for IL-2 and IL-4 used JES6-1A12 and 11B11 as capture antibodies, with biotinylated JES6-5H4 or BVD6-24G2 as developing antibodies. Sensitivity of detection was lOpg/ml for each cytokine. Oligonucleotide primers: The primers used for PCR amplification for β-actin, and different cytokines, are described in previous publications (Gorczynski, R.M., 1995a; Gorczynski, R.M., 1995b; Gorczynski, R.M., 1996a). In addition, the following oligonucleotides were synthesized. cDNA synthesis primer for driver ds cDNA (DP) : 5'-TTTTGTACAAGCTT 30 -3' Adapter 1 (Adl): 5'-CTAATACGACTCACTATAGGGCTCGAGCGGCCGCCCGGGCAGGT-3' Adapter 2 (Ad2): 5'-TGTAGCGTGAAGACGACAGAAAGGGCGTGGTGCGGAGGGCGGT-3' PCR Primerl (PI): 5'-CTAATACGACTCACTATAGGGC-3' Nested Primer 1 (NP1): 5'-TCGAGCGGCCGCCCGGGCAGGT-3' PCR Primer2 (P2): 5'-TGTAGCGTGAAGACGACAGAA-3' Nested Primer 2 (NP2): 5'-AGGGCGTGGTGCGGAGGGCGGT-3' Driver and Tester Preparation: RNA was extracted from pooled mesenteric lymph node (MLN) and Peyer's Patches (PP) of 5 /group renal transplant mice with iv or pv immunization. Poly(A) + mRNA was prepared from the driver (iv) group, and 2mg material used for ds cDNA synthesis with lng DP primer and a cDNA Synthesis Kit (Clontech) with T4 DNA polymerase. The final cDNA preparation was digested with Rsal in a 50ml reaction mixture with 15 units enzyme (GIBCO) for 3 hrs, and the cDNA phenol-extracted, ethanol precipitated, and resuspended in 7ml of deionized water (concentration approximately 300ng/ml). Rsal digested ds tester cDNA (pv group) was prepared in a similar fashion. 50ng of tester cDNA diluted in TE buffer was ligated with 2ml of Adl and Ad2 (each at lOmM) in separate ligation reactions at 16°C for 18hrs with 50 Units/ml T4 ligase. Thereafter 1ml of 0.2M EDTA was added, the mixture heated at 70°C for 5 min to inactivate the ligase, and the product stored at -70°C. Subtractive hybridization and PCR amplification: 600ng driver (iv) ds cDNA was added to each of two tubes containing 20ng Adl- and Ad2- ligated pv cDNA. The samples were mixed, precipitated with ethanol, resuspended in hybridization buffer, overlaid with mineral oil and denatured /annealed in standard fashion. The two independent samples were then combined, 200ng fresh driver cDNA added to allow for further enrichment of differentially expressed mRNAs, and the mixture again denatured and annealed for lOhrs at 68°C. The final sample was diluted in Hepes buffer with EDTA and stored at -20°C. After subtraction two PCR amplifications were performed on the subtracted cDNA. In the first 1ml of subtracted cDNA was amplified using 1ml each of PI and P2. The conditions for amplification were as described by Diatchenko. The amplified products were diluted 10-fold in deionized water and 1ml of product used for further amplification using the nested primers (NP1 and NP2) and a 10-cycle amplification reaction. Aliquots of the original driver /tester and subtracted cDNAs were used for PCR reactions with control oligonucleotide primers (β-actin) for known "housekeeping genes", and with primers for genes whose expression has been previously documented to be different in iv/pv immunized mice. These data are shown in Figures 1 and 2. Figure 1 shows PCR validation of suppressive subtractive hybridization. Samples from unsubtracted (lanes 1, 3, 5 and 7) or subtracted (lanes 2, 4, 6 and 8) mRNA were reverse transcribed and tested in PCR with b-actin primers for different PCR cycle times. Lanes 1 and 2: 15 cycles; lanes 3 and 4: 20 cycles; lanes 5 and 6: 25 cycles; lanes 7 and 8: 30 cycles. Figure 2 shows PCR validation of suppressive subtractive hybridization. Samples from unsubtracted (lanes 2 and 4) or subtracted (lanes 3 and 5) mRNA were tested as in Figure 1, except primers used were for IL-10, and different cycle times are shown. Lanes 2 and 3: 20 cycles; lanes 4 and 5: 30 cycles, lane 1: mol. wt. standard. In addition, cloning of the subtracted cDNA was performed as follows. Cloning and further analysis of subtracted cDNA: The PCR amplified cDNA was cloned with a TA cloning kit (Invitrogen, California) by directly ligating into the PCR II vector. Ligation was performed at an insert:vector ratio of 3:1 in lx ligation buffer with T4 ligase (3U/μl) overnight at 14°C. Ligation products were then inserted into INFocF' competent Escherichia Coli using a standard transformation protocol, and selected with ampicillin on plates containing X-gal (5-bromo-4-chloro-3-indolyl~D-galactoside). Miniprep plasmid DNA was purified with a Plasmid extraction Spin kit (Qiagen, Germany) and cut with EcoR I restriction enzyme to determine whether the plasmids contained the expected insert. Plasmids with inserts were sequenced by the dideoxy sequencing method using a T7 sequencing kit (Pharmacia Biotech, Canada). Nucleic acid homology searches were performed using the BLAST program at the National Center for Biotechnology Information (NIH, Bethesda, USA). Further analyses of cloned material, using Northern hybridization, was as follows. Inserts in pCRII were amplified for 12 cycles using nested PCR primers. The amplified material was purified using Qiaquick Spin PCR Purification Kits (Qiagen), 32 P-labeled by random priming, and used as a probe for Northern hybridization with 20mg samples of the original (and fresh) iv or pv total RNA. Hybridization was performed in 5ml of ExpressHyb solution (Clontech) with a minimum of 5xl0 6 cpm per lOOng cDNA probe and O.lmg/ml sonicated heat-denatured salmon sperm DNA. Filters were washed 4 times, each at 15 min at 27°C with lxSSC and 0.1% SDS, followed by a high stringency wash at 42°C for 30 min with 0.2xSSC and 0.1% SDS. Exposure times varied from 18hrs to 6 days. Figure 3 shows an autoradiograph using 32 8P-labeled probes prepared from 4 clones obtained using the subtraction hybridization approach described above (with pv cDNA as tester material and iv cDNA as driver). A labeled control probe was prepared with a PCR amplicon for mouse b-actin. Total RNA was prepared from mice receiving iv or pv immunization and equivalent amounts loaded in replicate lanes as shown, with gels developed from 18 hours (clone #28) to 6 days (#71). Clone 8 is most homologous with mouse poly (A) binding protein. Clone 16 is most homologous with rat MRC OX-2. Clone 28 is most homologous with human zinc-finger protein. Clone 71 has no homologous sequence. Western blotting protocol: The technique used was essentially that described by Sandhu et al. (1991) as modified by Bronstein et al. (1992). Samples were obtained 14 days post renal transplantation, using groups described in Figure 5. Fresh rat thymus cells were used as control. Samples were electrophoresed in 12%SDS-PAGE and transferred to PVDF membranes (Novex Co., San Diego, CA) prior to addition of primary antibody. A commercial anti-rat OX-2 was used as test reagent; control antibody was an antibody to mouse CD8a. The developing antibody used was a commercial horse-radish peroxidase labeled anti-mouse IgG. All reagents were obtained from Cedarlane Labs (Hornby, Ontario, Canada). DNA sequence homology comparison: Comparison of mouse OX-2 with known cDNA sequences for B7-1, B7-2, CD28 and CTLA-4 was performed using a DNASIS program (version 2.0). RESULTS Evaluation of suppression subtraction hybridization(SSH) technique In order to evaluate the efficacy of the SSH technique used, the inventor used his previous evidence that, by PCR analysis, increased expression of mRNA for IL-10 genes was evident in lymphoid tissue from pv immunized mice. Accordingly, a dilution analysis of cDNA from the tester, driver and subtracted material, using PCR primers for β-actin and IL-10 was performed. As shown in Figure 1, after SSH there was a detectable signal for β-actin in subtracted material only after 35 cycles of amplification. By contrast, a signal was present in the unsubtracted material after only 15 cycles. Using additional quantitative measures of template, it was found to correspond to some 1000-10,000 depletion of β-actin mRNA. In a separate study, analyzing IL-10 mRNA (Figure 2), significant enrichment of IL-10 mRNA was found as determined by comparison of the amplification detected at 30 cycles in subtracted /unsubtracted material (see lanes 4 and 5, Figure 2). In a further test of the efficiency of subtraction the mixture of unsubtracted and subtracted tester (pv) cDNA was labeled and hybridized to Northern blots of iv (tester) and pv (driver) total RNA. The results (data not shown) indicated that the subtracted tester cDNA probe did indeed produce a significantly stronger signal with the tester RNA. Given the evidence that for any cDNA species to produce a signal in a Norther blot it must represent a concentration greater than 0.1-0.3% of the cDNA mixture, these data are again consistent with our having produced a high level of enrichment of pv-specific cDNA, with a concomitant reduction in abundant cDNAs common between tester (pv) and driver (iv) material. Detection of unique cDNA fragments in tissue from py immunized mice The efficiency and validity of SSH for detection of cDNAs unique to the tissue sample from the pv immunized mice was further confirmed after cloning and sequence analysis of selected tester-specific cDNAs. 10 randomly selected cDNA clones (of 66 sequenced) were used to probe multiple preparations of pv or iv whole RNA. All revealed unique mRNAs expressed preferentially in the pv samples. Autoradiograms from 4 of these Northern blots, along with a β-actin probe as control, are shown in Figure 3. Exposure times from 18 hrs to 6 days were used which were interpreted as indicative of pv specific cDNAs of different abundance in the samples of interest. The cDNA inserts of the 4 clones shown, along with the other 62 clones, were partially sequenced and analyzed for homology in the GenBank and EMBL data bases. A summary of these data are shown in Table 1. Note that some 30 cDNA fragments had at least 50% homology (BLAST score >250 over at least 50 nt) with other described sequences. A further 14 clones showed similar homology with known rat/human genes. Both sets may represent members of different gene families. An additional 22 clones demonstrated no significant matches with entries in the database, and thus may represent novel genes up-regulated after pv immunization. That the data shown are a minimal estimate of such differentially expressed genes is evident from the fact that homology with IL-4 or IL-10 gene sequences (mRNAs known to be over-expressed following pv immunization-see also Figure 2) were NOT detected in any of the 66 clones analyzed. The sequence homology for the clones shown in Figure 3 (>80% homology over the compared sequence) led to the further characterization of these clones. Clone 8 was shown to be most homologous with mouse poly (A) binding protein; clone 16 was shown to be most homologous with rat MRC OX-2; and clone 28 was shown to be most homolgous with human zinc-finger protein. No homologous sequence was found for clone 71. In the data that follows, the analysis of one of these clones which showed homology to a rat cDNA (for OX-2, a molecule previously characterized as being preferentially expressed on rat thymocytes and dendritic cells) is described. The rationale for further investigation of this clone lies in data showing that infusion of dendritic cells via the portal vein is a potent method for prolonging allograft survival in our model systems. Note, however, that while the bone marrow derived dendritic cells that were infused via the portal vein themselves express OX-2 (see above), identical data has been obtained in Northern gels to those shown in Figure 3 using tissue harvested from mice receiving, as the earliest studies(l-5) irradiated spleen cells (OX-2" by FACS analysis) via the portal vein. In addition, in both situations, OX-2 mRNA was not detected by this suppression subtraction hybridization approach when we used tissue harvested at 0.5-2.5 days post transplantation. These results are consistent with the idea that the OX-2 signal detected is a result of novel increased expression in cells following pv immunization. Probing a cDNA library from tissue from py immunized mice for expression of the murine equivalent of rat OXA A cDNA library was constructed from mRNA prepared from a pool of 5 C3H mice receiving pv immunization with 25xl0 6 irradiated (2000Rads) C57BL/6 bone marrow cells followed by renal transplantation as described in the Materials and Methods, using a kit purchased from ClonTech. Clones were plated in LB medium and probed with the 32 SP-labeled amplicon described in Figure 3 as showing homology with rat OX-2. A 1.3Kb clone was detected, amplified, and shown after 2 P labeling to detect a differentially expressed product by Northern gel analysis. After sequencing using an automated DNA sequencer and fluorescent-labeled deoxynucleotides, this 1.3Kb fragment was found to share >95% homology with the region encoding the 3'untranslated region of the rat OX-2 mRNA as determined from the GeneBank sequence for rat OX-2. Using a primer construct program, a 5 'PCR primer representing positions 1-19 of the rat GeneBank sequence (corresponding to a portion of the 5'untranslated region, and the leader sequence) and 3' primers from our characterized mouse sequence were synthesized, and long-distance amplification performed to produce an amplicon predicted to encode the open-reading-frame (ORF) of the murine equivalent of the rat OX-2 gene. This amplicon was determined (as expected) to be of some 1.4Kb length. Automated sequencing produced a full-length sequence for the mouse homologue of the rat MRC OX-2 gene, including an ORF with >90% homology (predicted amino acid sequence) with the corresponding rat product, along with the 3'untranslated region. This sequence has been submitted to the Genebank (accession number AF004023). Using a DNASIS program the predicted mouse protein sequence has some 51% homology with B7-1 and B7-2, 48% with CD28 and 54% with CTLA4 (unpublished). Evidence for an important role for the expressed OXA homologue in prolonged graft survival following py immunization In an attempt to define the potential importance of the product encoded by the OX-2 gene we used a commercial antibody to rat OX-2 in a transplant model in mice receiving pv immunization and renal transplantation. In the first such study, it was asked whether there was evidence for specifically increased expression of the OX-2 molecule following pv immunization. By FACS analysis, using dual staining of hepatic mononuclear cells and spleen cells with OX-2 and NLDC145, similar numbers of NLDC145 + cells in liver or spleen samples from iv and pv immunized mice were found, (5xl0 5 and 6.5xl0 6 respectively), but a 4-fold increase in the numbers of OX-2 + NLDC145 + following pv immunization. Figure 4 shows a flow cytometry profile of spleen adherent cells from iv immunized/ grafted mice (panels A and B) or pv immunized /grafted mice (panels C and D). Cells were harvested 7 days after transplantation and stained with NLDC145 and F(ab') 2 FITC-anti-rat IgG, as well as with control (clone 107.3) mouse IgGl serum (left hand panels) or anti-OX-2 (right hand panels) and F(ab') 2 PE-anti-mouse IgG. Data are representative of one of three different studies. Values shown represent the total cell population in each quadrant. The absolute numbers (xlO 5 ) of double positive cells in the liver or spleen of pv immunized mice were 3.2+0.5 and 39±8 respectively (see Figure 4 for FACS profiles of spleen adherent cells). This 4-fold increase was seen regardless of the cells used for pv immunization, either bone marrow derived dendritic cells (some 20% OX-2 + -see above) or irradiated whole spleen lymphoid cells (OX-2"), suggesting that they were not merely detecting surviving OX-2 + (donor) cells, but novel expression of OX-2 in vivo. Western blot, Figure 5, shows increased expression of OX-2 antigen after pv imrnunization. The technique used for Western blotting is previously described. Samples were obtained 14 days post renal transplantation, using the groups described in Figure 6. Fresh rat thymus cells (lane 5) were used as control. Lanes 1 and 2 represent samples pooled from 3 donors /group (iv immunized; pv immunized +infusion of anti-OX-2 respectively). Samples in lanes 3 and 4 are from individual mice receiving pv immunization and renal transplantation only (no antibody treatment). Staining with anti-rat MRC OX-2 is shown in Figure 5B; with a control antibody (to mouse Ly2.1), anti-mouse cD8a, shown in Figure 5A. The developing antibody used was a commercial horse-radish peroxidase labeled anti-mouse IgG. No signal was seen using the mouse IgGl isotype control clone 107.3 (BALB/c anti-TNP)-data not shown. Data are representative of 1 of 3 equivalent studies. Western blotting (see Figures 5A and 5B) of samples prepared from the spleen of iv vs pv immunized and grafted mice 14 days following renal transplantation revealed staining of a band migrating with estimated molecular weight 43Kd, in agreement with data elsewhere reporting extensive glycosylation of this molecule in isolates from rat thymus. In mice receiving pv immunization along with in vivo treatment with anti-OX-2, no detectable signal was seen in Western blots (see lane 2, Figure 5). No staining was seen with a murine IgGl isotype control (BALB/c anti-TNP, clone 107.3: unpublished), making it unlikely that the band observed was Fc receptor. Figure 6 is a graft showing percent survival versus days post renal transplantation.Commercial anti-OX-2 monoclonal antibody, but not anti-mouse CD28 or anti-mouse CTLA4, reverses the graft prolongation following donor-specific pv immunization. Groups of 6 C3H mice received C57BL/6 renal allografts with no other treatment (cyclosporin A only, -0-), or additional pv immunization with 15xl0 6 C57BL/6 bone marrow derived dendritic cells (-D-)as described previously. Subsets of these latter mice received iv injection (every second day x4 injections) with lOOmg/mouse of a commercial anti-rat OX-2 monoclonal antibody (-A-) or the isotype control (clone 107.3, -V- ), or of antibodies to mouse CD28 (-•-) or CTLA4 (-*-)• The animal survival for the different groups shown are pooled from 2 studies. Note that the mouse isotype control itself produced no modification of the increased renal graft survival following pv immunization. * p<0.02, Mann-Whitney U-test). In two final studies mice received pv immunization and transplantation as before, but now also received iv injection with commercial anti-rat OX-2 (x4 injections; lOOmg/mouse at 2 day intervals). As shown in Figures 5A and B and 6 these infusions of anti-OX-2 significantly decreased the prolonged graft survival (Figure 6) and increased expression of OX-2 antigen (Western blotting-Figures 5A and 5B) seen following pv immunization. No perturbation of graft survival following pv immunization was seen using additional treatments with anti-CD28/anti-CTLA4 (see Figure 6), or, in studies not shown, using anti-B7-l or anti-B7-2. Again infusion of the IgGl isotype control Mab (clone 107.3) did not alter the increased graft survival seen following pv immunization (see Figure 6). In separate experiments cells were harvested from mice receiving pv immunization along with additional treatment with monoclonal antibodies as show (see Table 2). Following treatment with anti-OX-2 there was no longer the altered cytokine production (with polarization to production of IL-4 and IL-10) which the inventor has described in multiple model systems in which animals received pv donor-specific pre-transplant immunization. Treatment with any of the other 4 monoclonal antibodies tested did not produce this reversal in polarization of cytokine production seen following pv immunization-indeed, using these Mabs alone in the absence of pv immunization produced a trend to increased graft survival (not shown) and significant polarization in cytokine production to increased IL-4 and IL-10 production, akin to that produced by pv immunization itself (upper half of Table 2). OX-2 is a molecule previously characterized by Barclay et al. (1981, 1982) as being preferentially expressed on rat thymocytes and dendritic cells. Dendritic cells are known to be important signalling cells for lymphocytes, which also potentially regulate cytokine production and graft rejection, and infusion of dendritic cells is a potent means of inducing pv tolerance. The inventor has determined that OX-2 expression increased following pv immunization, and further studied whether this had any functional consequences. As shown in Figures 4 and 5, there is indeed significantly increased expression of OX-2 in spleen cells isolated from pv immunized mice, along with the increased graft survival and polarization in cytokine production (Figure 6 and Table 2). In contrast, in vivo infusion of anti-OX-2 abolishes increased expression of this molecule, simultaneously reversing the increased graft survival and altered cytokine profile seen. This data is consistent with the possible function of OX-2 + cells in promoting allograft survival. In the studies described the donor dendritic cells infused via the portal vein were themselves OX-2 + (see description of materials and methods above). However, identical data in FACS analysis (Figure 4) and Western Blots (Figure 5), and from suppression subtraction hybridization (Figure 3), have been obtained in studies in which we used irradiated whole spleen cells (OX-2 " by FACS) for pv infusion. This is consistent with the lack of evidence for increased mRNA expression of OX-2 early (1-2 days) post transplant, as noted above. Thus it seems most likely that an operationally important "OX-2 signal" detected in the spleen of the pv immunized mice can derive from new expression, rather than necessarily from infused OX-2 + cells. In the absence of a polymorphic marker for OX-2, however, it cannot be determined whether increased expression is from donor or host cells (or both). Indeed, it is perhaps somewhat surprising that the murine antibody to rat OX-2 cross-reacts in the fashion shown with murine OX-2. Definitive analysis of the in vivo role of OX-2 awaits similar studies to those above, using antibodies developed against the murine OX-2 homologue-these experiments are currently in progress. It is also important to point out that while pv immunization led to only a 4-fold alteration in the absolute number of detectable OX-2 + NLDC + cells in the spleen/liver (see text and Figure 4), nevertheless in the face of this 4-fold difference a clear difference in OX-2 signals in Northern gels using RNA from pv vs iv immunized mice (Figure 3), along with evidence for a role for this quantitative difference in the outcome of graft survival (Figure 6) were detected. Presumably these results reflect respectively the limitation to the sensitivity of the Northern assay used, and some function of the quantitation of "co-stimulation" occurring after OX-2:OX-2 ligand interaction. While there was some 50% homology of the predicted protein sequence of murine OX-2 with murine B7-1, B7-2, CD28 and CTLA4 (Borriello et al., 1997), antibodies to the latter molecules did not reverse the prolonged graft survival and altered cytokine production following pv immunization (Figure 6, Table 2- see also (Castle et al., 1993)). In fact these latter antibodies themselves, infused in the absence of pv immunization, produced some of the same changes in cytokine production induced by pv immunization (Table 2). EXAMPLE 2 Murine OX-2 This example describes the cloning and sequencing of murine MRC OX-2. A cDNA library was constructed from MLN cells derived from adult C3H mice, preimmunized 5 days earlier with 10x106 allogeneic B10.BR bone marrow-derived dendritic cells allogeneic cells by the portal venous (pv) route, using a Cap Finder PCR cDNA library construction kit (Clontech). The inventor had previously isolated, using a PCR-Select cDNA subtraction hybridization kit (Clontech) and RNAs obtained from pooled MLN of mice immunized by the pv route or via the lateral tail vein (iv), a 350bp amplicon which showed over 98% homology with the 3' untranslated region of rat MRC OX-2 cDNA. Northern blot analysis confirmed that this amplicon detected a differentially expressed product in RNAs prepared from iv vs pv immunized mice. This amplicon was used to screen 5x105 clones of the amplified library. The sequences of cDNA clones were established with an Applied Biosystems 377 Automated Sequencer, utilizing the Dye Terminator Cycle Sequencing method (Applied Biosystems, Foster City, CA). The nucleotide sequence reported in this paper has been submitted to the GenBank/EMBL Data Bank with accession number AF004023. The cDNA shown in Figure 7 has an open reading frame of 837 base pairs, and a deduced amino acid sequence (Figure 8) of 248 amino acids, of which 30 represent a cleaved leader sequence. The predicted molecular weight of this, and the equivalent molecules in rat and human, is approximately 25kDa. The measured molecular weight in rat thymocytes, where the molecule is highly gylcosylated, is 47kDa. The murine MRC OX-2 shows some 92% and 77% homology overall at the amino acid level with equivalent molecules in rat or human respectively. As noted for the rat molecule, the sequence from a 203-229 seems likely to represent a membrane spanning domain (highly hydrophobic region), while the region from 229-248 is likely the intracytoplasmic region, with a stretch of highly basic residues immediately C-terminal to position 229. Homology in the combined transmembrane and C-terminal regions with rat and human shows some 98% and 85% similarity respectively. As predicted from membership in the Ig supergene family, there are a number of conserved Cys residues forming the disulphide bonds between β-strands of Ig-like domains, (21 and 91; 130 and 184 respectively); residue 91 was previously found to be the most highly conserved among members of the irnrnunoglobulin superfamily. Homology between the N-teiminal Ig-domain with rat and human, versus the next Ig-domain, is 88% and 82%, or 97% and 73% respectively. This relative concentration in variability between rat and mouse in the V-terminal Ig-domain may be more understandable when the ligand specificity for the molecules in these species is clarified. Note that the presumed extracellular portion of the molecule (1-202) contains a number of sites for N-glycosylation which are preserved across species (44, 65, 73, 80, 94, 127, 130 and 151). This was previously reported for the rat cDNA sequence, and inferred from the measured size of the expressed material in rat thymocytes. The intracytoplasmic region of the molecules has no sequence identity with known signaling kinases, nor does it have the well-described consensus sequence for the immunoreceptor tyrosine activation motif (ITAM: DXXYXXLXXXXXXXYDXL). In addition, it lacks typical SH2 or SH3 domains to serve as "docking sites" for adapter molecules which might in turn co-opt other protein kinases in an activation cascade. Accordingly the ligand-binding activity of the extracellular domains presumably represent the biologically important region of the molecule. Some possible functions attributable to ligand interaction with OX-2 can be inferred from other data in the literature. A homologous molecule, Ng-CAM, has been reported to bind a protein-tyrosine phosphatase via N-linked oligosaccharide residues, and protein tyrosine phosphatases are known to play a key regulatory role in immune responses. More recently ALCAM, another adhesion molecule member of the Ig superfamily, the gene for which is located close to that for OX-2 on chromosome 3 in humans, has been shown to bind CD6 (a member of the scavenger receptor cystein rich family, SRCR), and antibodies to CD6 may themselves play a role in regulating immune function. EXAMPLE 3 OX-2 Positive Cells Inhibit Type-1 Cytokine Production The inventor has shown that hepatic mononuclear, non-parenchymal, cells (NPC) can inhibit the immune response seen when allogeneic C57BL/6 dendritic cells (DC) are incubated with C3H spleen responder cells. Cells derived from these cultures transfer increased survival of C57BL/6 renal allografts in C3H mice. The inventor also found that increased expression of OX-2 on dendritic cells was associated with inhibition of cytokine production and renal allograft rejection. The inventor further explored whether inhibition by hepatic NPC was a function of OX-2 expression by these cells. Fresh C57BL/6 spleen derived DC were cultured with C3H spleen responder cells and other putative co-regulatory cells. The latter were derived from fresh C3H or C57BL/6 liver NPC, or from C3H or C57BL/6 mice treated for 10 days by intravenous infusion of human FZf3 ligand (FZf3L). Different populations of murine bone-marrow derived dendritic cells from cultures of bone marrow with (IL-4+GM-CSF) were also used as a source of putative regulator cells. Supernatants of all stimulated cultures were examined for functional expression of different cytokines (IL-2, IL-4, IFNγ, TGFβ). It was found that fresh C57BL/6 splenic DC induced IL-2 not IL-4 production. Cells from the sources indicated inhibited IL-2 and IFNγ production, and promoted IL-4 and TGFβ production. Inhibition was associated with increased expression of OX-2 on these cells, as defined by semi-quantitative PCR and FACS analysis. By size fractionation, cells expressing OX-2 were a subpopulation of NLDC145+ cells. This data implies a role for cells expressing OX-2 in the regulation of induction of cytokine production by conventional allostimulatory DC. MATERIALS AND METHODS Mice: Male and female C3H/HEJ and B10.BR (H-2 k / k ), B10.D2 (H-2 d / d ) and C57BL/6 (H-2 b * /b ) mice were purchased from the Jackson laboratories, Bar Harbour, Maine. Mice were housed 5/cage and allowed food and water ad libitum. All mice were used at 8-12 weeks of age. Monoclonal antibodies: The following monoclonal antibodies (Mabs), all obtained from Pharmingen (San Diego, CA, USA) unless stated otherwise, were used: anti-IL-2 (JES6-1A12; biotinylated, JES6-5H4 ); anti-IL-4 (11B11, ATCC; biotinylated, BVD6-24G2 ); anti-IFNγ (R4-6A2, ATCC; biotinylated XMG1.2); anti-IL-10 (JES5-2A5; biotinylated SXC-1); PE anti-B7-l/B7-2 (Cedarlane Labs, Hornby, Ontario, Canada). Rat anti-mouse OX-2 monoclonal antibodies were prepared by Immuno-Precise Antibodies Ltd. (Victoria, BC, Canada) following immunization of rats with a crude membrane extract of LPS stimulated murine DC, followed by fusion with a non-secreting rat myeloma parent cell line (YB2/3Hl.P2.G11.16Ag.20). Hybridoma supernatants were screened in ELISA using plates pre-coated with a 40-45Kd preparation of DC extracts run on Western gels (Barclay, A.N. 1981. Immunology 44:727; Barclay, A.N., and H.A. Ward. 1982. Eur. J. Biochem. 129:447). Positive clones were re-screened using FACS analysis of CHO cells transduced with a cDNA clone encoding full-length murine OX-2 (Chen, Z., H. Zeng, and R. M. Gorczynski. 1997. BBA. Mol. Basis Dis. 1362:6-10). FITC-conjugated F(ab')2 rabbit anti-rat IgG (non cross-reactive with mouse IgG) from Serotec, Canada was used as second antibody. The Mab selected for further analysis (M3B5) was grown in bulk in a CELLMAX system (Cellco Inc., Germantown, MD). A crude preparation of rat immunoglobulin (30% saturated ammonium sulphate preparation) was used as a control Ig. In tissue culture assays where anti-cytokine Mabs were used to confirm the specificity of the assay used 10 μg/ml of the relevant Mabs was found to neutralize up to 5.0 ng/ml of the cytokine tested. NLDC145 (anti-mouse DC) was also obtained from Serotec. Recombinant mouse IL-4 was a kind gift from Dr. L. Yang (The Toronto Hospital); mouse rGM-CSF was purchased from Pharmingen. Recombinant human F/f3L (derived from CHO cells) was a kind gift from Dr. A.B. Troutt, Immunex Corp., Seattle, Washington, USA. Renal Transplantation Renal transplantation was performed essentially as described elsewhere (Gorczynski, R.M. et al. 1994a. Transplantation 58:816-820). Animals were anesthetized with a combination of halothane and nitrous oxide inhalation, using novogesic for post-op analgesia. Orthotopic renal transplantation was performed using routine procedures. In brief, Donor animals received 200 Units of heparin, and kidneys were flushed with 2ml of ice cold heparinized physiological saline solution, prior to removal and transplantation into recipient animals with left nephrectomy. The graft renal artery was anastomosed to the recipient's abdominal aorta, and the renal artery was anastomosed to the recipient's inferior vena cava. The ureter was sewn into the recipient bladder using a small donor bladder patch. All recipients received im injection with cefotetan (30mg/Kg) on the day of transplantation and for 2 succeeding days. The remaining host kidney was removed 2 days after transplantation, unless otherwise indicated. Treatment of recipients with pv immunization, by monoclonal antibodies, or by oral immunization was as described in individual studies. Portal Vein and Oral Immunization Portal vein and oral immunization was performed as described earlier (Gorczynski, R. M. 1995a. Cell. Immunol. 160:224-231; Gorczynski, R.M. et al. Transplantation 62:1592-1600). All animals were anaesthetized with nembutal. A midline abdominal incision was made and the viscera exposed. Cells were injected in 0.1ml through a superior mesenteric vein using a 30 gauge needle. After injection the needle was rapidly withdrawn and hemostasis secured without hematoma formation by gentle pressure using a 2mm 3 gel-foam. Bone-marrow derived dendritic cells (DC) for pv immunization were obtained by culture of T depleted bone marrow cells in vitro with rIL-4 and rGM-CSF (Gorczynski, R.M. et al. Transplantation 62:1592-1600). Staining with NLDC145 and FITC anti-rat IgG, or with FITC anti-CD3 confirmed >95% NLDC145+ and <5% CD3+ cells at day 10 of culture (Gorczynski, R.M. et al. Transplantation 62:1592-1600). These cells were washed and injected into mice or used for mixed leucocyte cultures. Preparation of cells: Spleen and bone marrow (Gorczynski, R.M. et al. Transplantation 62:1592- 1600) cell suspensions were prepared aseptically from individual mice in each experiment. Hepatic mononuclear nonparenchymal cells (NPC) were isolated essentially as described elsewhere (Gorczynski, R.M. 1994b. Immunology 81:27-35). Tissue was first digested at 37°C for 45 min with a mixture of collagenase/dispase, prior to separation (15 min at 17,000rpm at room temperature) over mouse lymphopaque (Cedarlane Labs). Mononuclear cells were resuspended in a-Minimal Essential Medium supplemented with 2-mercaptoethanol and 10% fetal calf serum (aF10). Where cells were obtained from Flt3L injected mice, animals were treated by iv injection of lOmg/mouse Flt3L daily for 10 days. After enzyme digestion recovery of liver/spleen cells from these mice was markedly increased compared with saline-injected controls (120x10 6 , 390x10 6 vs 7xl0 6 and 120xl0 6 respectively). Cytotoxicity and cytokine assays: In cultures used to assess induction of cytotoxicity or cytokine production responder cells were stimulated with irradiated (2000R) stimulator cells in triplicate in αFlO. Supernatants were pooled from replicate wells at 40 hrs for cytokine assays (below). No reproducible differences in cytokine levels have been detected from cultures assayed between 36 and 54 hrs of stimulation. In some experiments the cultures received lμCi/well (at 72 hrs) of 3 HTdR and proliferation was assessed by harvesting cells 14 hrs later and counting in a well-type β-counter. Where cytoxicity was measured cells were harvested and pooled from equivalent cultures at 5 days, counted, and recultured at different effector: target with 51 Cr EL4 (H2 b b ) or P815 (H2 d / d ) tumor target cells. Supernatants were sampled at 4 hrs for assessment of specific cytotoxicity. IL-2 and IL-4 activity were assayed by bioassay using the IL-2/IL-4 dependent cell lines, CTLL-2 and CT4.S respectively. Recombinant cytokines for standardization of assays was purchased from Genzyme (Cambridge, MA). IL-2 assays were set up in the presence of 11B11 to block potential stimulation of CTLL-2 with IL-4; IL-4 assays were set up in the presence of S4B6 to block IL-2 mediated stimulation. Both the IL-2 and IL-4 assays reproducibly detected 50 pg of recombinant lymphokine added to cultures. In addition, IL-2, IL-4, IFNγ and IL-10 were assayed using ELISA assays. For IFNγ the assay used flat-bottomed Nunc plates (Gibco, BRL) coated with lOOng/ml R4-6A2. Varying dilutions of supernatant were bound in triplicate at 4 C C, washed x3, and biotinylated anti-IFNγ (XMG1.2) added. After washing, plates were incubated with streptavidin-horse radish peroxidase (Cedarlane Labs, Hornby, Ontario), developed with appropriate substrate, and OD 405 determined using an ELISA plate reader. Recombinant IFNγ for standardization was from Pharmingen. IL-10 was similarly assayed by ELISA, using JES5-2A5 as a capture antibody and biotinylated SXC-1 as developing antibody. rIL-10 for standardization was from Pepro Tech Inc. (Rocky Hill, NJ). Each assay detected O.lng/ml cytokine. ELISA assays for IL-2 and IL-4 used JES6-1A12 and 11B11 as capture antibodies, with JAS6-5H4 or BVD6-24G2 as developing antibodies. Sensitivity of detection was 20pg/ml for each cytokine. Where checked the correlation between bioassay and ELISA for IL-2 or IL-4 was excellent (r>0.90). In all studies reported below, data are shown from ELISA assays only. Where cytokine data are pooled from several studies (e.g. Figures 14, 16, 17), absolute values of cytokine production were obtained as above using commercial recombinant cytokines to standardize the assays. In our hands, supernatants from C3Hanti-C57BL/6 cultures, under the conditions described, reproducibly contain 950+200 and 80+25 pg/ml IL-2 and IL-4 respectively. Preparation of RNA: Different sources of tissue from renal-grafted female mice receiving DC and kidney allografts from male mice were harvested for RNA extraction as described elsewhere (Gorczynski, R. M. 1995a. Cell. Immunol. 160:224-231). The OD280/260 of each sample was measured and reverse transcription performed using oligo (dT) primers (27-7858: Pharmacia, USA). The cDNA was diluted to a total volume of 100ml with water and frozen at -70°C until use in PCR reactions with primers for murine GAPDH, B7-1, B7-2 or OX-2. The sense (S) and antisense (AS) primers were synthesized by the Biotechnology Service Centre, Hospital for Sick Children, Toronto, using published sequences. 5' primers were 32 P end-labeled for PCR and had comparable levels of specific activity after purification by ethanol precipitation. 5ml cDNA was amplified for 35 cycles by PCR, and samples were analyzed in 12.5% polyacrylamide gels followed by overnight (18 hrs) exposure for autoradiography. In control studies, using H-Y primer sets, this technique reliably detects H-Y mRNA from extracts of female spleen cells to which male cells are added at a concentration of 1:105 (Gorczynski, R. M. 1995a. Cell. Immunol. 160:224-231; Gorczynski, R.M. et al. Transplantation 62:1592-1600). Quantitative comparison of expression of different PCR products used densitometric scanning of the autoradiograms. GAPDH Sense: 5TGATGACATCAAGAAGGTGGTGAAG3' GAPDH Antisense: 5TCCTTGGAGGCCATGTAGGCCAT3' B7-1 Sense: 5'CCTTGCCGTTACAACTCTCC3' B7-lAntisense: 5'CGGAAGCAAAGCAGGTAATC3' B7-2 Sense: 5'TCTCAGATGCTGTTTCCGTG3' B7-2 Antisense: 5'GGTTCACTGAAGTTGGCGAT3' OX-2 Sense: 5'GTGGAAGTGGTGACCCAGGA3' OX-2 Antisense: 5ΑTAGAGAGTAAGGCAAGCTG3' Statistical analysis: In studies with multiple groups, ANOVA was performed to compare significance. In some cases (as defined in individual circumstances) pairwise comparison between groups was also subsequently performed. RESULTS Antigen stimulation, in the presence of hepatic NPC, induces development of a cell population capable of inhibiting proliferation and IL-2 production on adoptive transfer: In a previous manuscript (Gorczynski, R. M. et al., Transplantation. 66: 000-008) it was reported that C3H spleen cells stimulated in the presence of syngeneic NPC and allogeneic (C57BL/6) DC produced a cell population able to inhibit generation of IL-2 from fresh spleen cells stimulated with C57BL/6 DC, and capable of inhibiting C57BL/6 renal allograft rejection in vivo. In order to ask whether this function of NPC was MHC restricted or not, the following study was performed. C57BL/6 (H2 b b ) spleen cells were stimulated in vitro with B10.BR (H2 k/k ) bone-marrow derived DC, in the presence /absence of the following NPC: C57BL/6; B10.BR; B10.D2 (H2 d / d ). In addition, control cultures were incubated with the NPC only. Proliferation and IL-2/IL-4 production was measured in one aliquot of these primary cultures. In addition, at 5 days, cells were harvested from another set of the primary cultures, washed, and 2xl0 5 cells added to cultures containing 5xl0 6 fresh C57BL/6 spleen cells and B10.BR DC. Proliferation and cytokine production was measured in these latter cultures in standard fashion. Data pooled from three equivalent studies are shown in panels A) and B) of Figure 9. Figure 9 is a bar graph showing regulation of proliferation and cytokine production following stimulation by allogeneic DC using hepatic NPC in accordance with the methods described herein. In panel A) cultures were initiated with 5xl0 6 C57BL/6 responder spleen cells alone (group 1), or with 2xl0 5 B10.BR DC (group 2). Further groups (3-5, and 6-8 respectively) contained C57BL/6 responder cells and 2x105 NPC from either C57BL/6, B10.D2 or B10.BR respectively (3-5) or these same NPC and B10.BR DC (6-8). Data show mean proliferation and cytokine production from triplicate cultures in three separate studies. In panel B) data show proliferation and cytokine production from cultures of 5xl0 6 C57BL/6 responder spleen cells stimulated in triplicate with 2x105 B10.BR DC alone, or with the addition also of 2x10 5 cells harvested from the cultures shown in the upper panel. Again data represent arithmetic means of 3 separate experiments. * p<0.05 compared with control cultures (far left in each panel). There are a number of points of interest. As previously documented, addition of NPC syngeneic with spleen responder cells (C57BL/6 in this case) to cells stimulated with allogeneic (BIO.BR) DC led to decreased proliferation and IL-2 production from those responder cells compared with cells stimulated by DC alone (compare groups 6 and 2 of upper panel of Figure 9, panel A). IL-4 production in contrast was enhanced. NPC alone, whether syngeneic or allogeneic to the responder cells, produced no obvious effect (groups 3-5, panel A) of Figure 9). Furthermore, cells from primary cultures receiving the DC+NPC mixture were able to inhibit proliferation and IL-2 production (while promoting IL-4 production) from fresh spleen cells stimulated in secondary cultures with the same (BIO.BR) DC (see panel B) of Figure 9). However, data in this Figure make another important point. The same inhibition of proliferation/IL-2 production in primary cultures was seen using either BIO.BR NPC (MHC matched with the DC stimulus-group 8, panel A) of Figure 9) or with third-party B10.D2 NPC (MHC-mismatched with both spleen responder cells and allogeneic stimulator DC-group 7, panel A) of Figure 9). Again no obvious effect was seen in cultures stimulated with BIO.BR or B10.D2 NPC alone (groups 4 and 5). Finally, cells taken from primary cultures stimulated with DC and NPC from either BIO.BR or B10.D2 could also inhibit proliferation/IL-2 production from secondary C57BL/6 spleen cell cultures stimulated with BIO.BR DC-again cells taken from primary cultures with NPC alone produced no such inhibition (see panel B) of Figure 9). Thus the inhibition of proliferation/IL-2 production and enhancement of IL-4 production seen in primary cultures, as well as the induction of suppression measured in secondary cultures, all induced by NPC, are not MHC-restricted. Specificity of inhibition/suppression induced by hepatic NPC: One interpretation of the data shown in Figure 9 and elsewhere is that NPC deliver a signal to DC-stimulated cells which is distinct from the antigen-signal provided by the DC themselves (and is MHC non-restricted). This signal modulates the antigen-specific signal provided by the DC. In order to assess the antigen-specificity of the immunoregulation described in Figure 9, the following experiment was performed. C57BL/6 spleen responder cells were stimulated with B10.D2 or BIO.BR bone marrow-derived DC, in the presence /absence of NPC from BIO.BR or B10.D2 mice. Proliferation and cytokine production was measured in aliquots of these cultures as before. In addition, further aliquots of cells harvested from these primary cultures were added to cultures of fresh C57BL/6 spleen cells stimulated with BIO.BR (panel B)-Figure 10) or B10.D2 (panel C)-Figure 10) DC. Again proliferation and cytokine production was measured. Data pooled from three such studies are shown in Figure 10. Figure 10 shows specificity of inhibition of proliferation of cytokine production by hepatic NPC (see Figure 9 and description of Figure 9 for more details). In panel A), 5x106 C57BL/6 spleen cells were stimulated in triplicate for 3 days with 2xl0 5 BIO.BR or B10.D2 DC, with/without 2xl0 5 NPC derived from B10.D2 or BIO.BR mice. Data shown are arithmetic means of 3 repeat studies. In panels B) and C), fresh C57BL/6 responder spleen cells were cultured in triplicate with either BIO.BR DC (panel B), or B10.D2 DC (Panel C), with/without 2xl0 5 additional cells from the primary cultures (groups 1-6 in panel A). Again data represent arithmetic means of proliferation/ cytokine production from 3 studies. * p<0.05 compared with control cultures (far left in each panel). Data from the primary cultures (panel A)) recapitulates the observations made in Figure 9, and show that NPC inhibit proliferation and IL-2 production from DC-stimulated responder cells in an antigen and MHC-unrestricted fashion. However, the data in panels B) and C) of this figure show clearly that adoptive transfer of inhibition using cells from these primary cultures occurs in an antigen-restricted fashion, dictated by the antigen-specificity of the DC used in the primary cultures, not of the NPC used for induction of suppression. These auxiliary cells in the NPC population thus have a functional property of being "facilitator cells for induction of suppression". Note that in other studies (data not shown) where the final assay system involved measuring cytotoxicity to allogeneic target cells, a similar inhibition of lysis (rather than cytokine production) was seen using cells harvested from primary cultures stimulated with DC and hepatic NPC (see Gorczynski, R. M., et al. 1998a. Transplantation. 66: 000-008). Hepatic cell preparations from Flt3L treated mice are a potent source of DC and "facilitator" cells: It has been reported at length that pv infusion of alloantigen, or iv infusion of liver-derived allogeneic mononuclear cells induces operational unresponsiveness in recipient animals (Gorczynski, R. M. 1995a. Cell. Immunol. 160:224-231; Gorczynski, R.M. et al. Transplantation 62:1592-1600; Gorczynski, R.M. et al. 1994a. Transplantation 58:816- 820.; Gorczynski, R. M., and D. Wojcik. 1992. Immunol. Lett. 34:177-182; Gorczynski, R. M. et al. 1995b. Transplantation. 60:1337-1341). The total hepatic mononuclear cell yield from normal mice is of the order of 5xl0 6 cells/mouse. In order to increase the yield, and explore the possibility that the liver itself might be a source both of allostimulatory DC and "facilitator" cells 2 C57BL/6 mice were exposed for 10 days to daily iv infusions of lOmg/mouse human CHO-derived FH3L, a known growth factor for DC (Steptoe, R. J. et al. 1997. / Immunol. 159:5483-5491). Liver tissue was harvested and pooled from these donors and mononuclear cells prepared as described in the Materials and Methods section above (mean 130xl0 6 cells/donor). These cells were further subjected to sub-fractionation by size using unit gravity sedimentation techniques (Miller, R. G., and R. A. Phillips. 1969. /. Cell. Comp. Physiol. 73:191-198). A typical size profile for recovered cells is shown in Figure 11 (one of 3 studies). Figure 11 shows OX-2 expression in a subpopulation of NPC. It is a sedimentation analysis (cell profile) and FACS analysis of cells isolated at 10 days from F/i3L-treated C57BL/6 mice. Two C57BL/67 mice received lOμg/mouse FU3 iv daily for 10 days. Hepatic NPC were sedimented for 3 hrs at 4°C, and the fractions shown collected (Fxs 1-4 with sedimentation velocities 2.5-3.8, 3.8-5.1, 5.1-6.4 and 6.4-8.0 mm/hr respectively). Aliquots of the cells were stained in triplicate with the Mabs shown. The remainder of the cells were used as in Figures 12-14. Data are pooled from 3 studies. In these same studies cells isolated from the various fractions shown in Figure 11 were tested as follows. Firstly, cells were stained with FITC-labeled Mabs to B7-1, B7-2, NLDC145 and rat anti-mouse OX-2 (M3B5) with FITC anti-rat IgG as second antibody. In addition, mRNA extracted from the different cell samples were assayed by PCR for expression of GAPDH, B7-1, B7-2 and OX-2. Data are shown in Figures 11 (pooled from 3 separate studies) and Figure 12 (representative PCR data from one experiment). Figure 12 shows PCR detection of B7-1, B7-2 and OX-2 in hepatic NPMC. It is a PCR analysis for mRNA expression of OX-2, B7-1 and B7-2 in various hepatic NPC cell fractions isolated from FU3L treated mice (see Figure 11). Data are representative from 1 of 3 studies. Further aliquots of the cells were used to stimulate fresh C3H spleen responder cells in culture. Proliferation and cytokine assays were performed as before (see Figure 9), and in addition cells were taken from these primary cultures and added to fresh secondary cultures of C3H spleen responder cells and C57BL/6 bone marrow-derived DC. Again proliferation and cytokine production was assayed from these secondary cultures. Data pooled from 3 studies of this type are shown in Figure 13 (panels A) and B). Figure 13 shows that hepatic NPMC from FU3L treated mice results IL-2 and IL-4 production. Stimulation of proliferation/cytokine production by NPC from Flt3L treated mice, and inhibition of the same (where stimulation is induced by a separate population of DC) is a function of different cell populations. (See text and Figures 11-12 for more details.) Hepatic NPC fractions were derived from FU3L treated C57BL/6 mice and were used to stimulate C3H spleen cells in triplicate cultures, alone or in the presence of bone-marrow derived C57BL/6 DC (see panel A). Data show arithmetic means for proliferation/cytokine production from 3 experiments. In addition, cells harvested from these primary cultures were added to fresh C3H spleen cells stimulated with C57BL/6 DC (panel B), and again proliferation/cytokine production assayed. * p<0.05 compared with control groups (far left of panel). Finally, cells from the various fractions were infused iv into 2 /group C3H mice which also received C57BL/6 renal allografts as antigen challenge. Spleen cells were harvested from these individual mice 10 days after transplantation and restimulated in culture with C57BL/6 or B10.D2 DC, again with cytokines measured at 40 hrs (see Figure 14). Figure 14 is a bar graph of cytokines produced from cells from C3H mice with C57BL/b renal allografts and NPC from FU3 treated C57BL/6 donors. OX-2 + NPC infused iv into renal transplant allograft recipients leads to polarization of cytokine production (to IL-4, IL-10 and TGFβ) in spleen cells harvested from those mice and restimulated in vitro. Fractions of NPC from FU3L treated C57BL/6 mice (from Figure 11) were infused iv into 2/group C3H recipients, receiving C57BL/6 renal allografts (along with CsA) in standard fashion (see Materials and Methods). Mice were sacrificed 14 days after transplantation and spleen cells stimulated in vitro in triplicate with C57BL/6 DC stimulator cells. Cytokines were assayed in the supernatants of these cultures at 60hrs. Data show arithmetic means pooled from cultures in 3 studies of this type. * p<0.05 compared with control groups (far left-no NPC infused). Data in Figure 11 show that distinct subpopulations of slow-sedimenting cells express OX-2 in the cells harvested from Flt3L treated mice, when compared with cells expressing B7-1 and /or B7-2. In general expression of OX-2 and B7-2 occured in equivalent subpopulations. Faster-sedimenting cells (Fx 3 and 4 in Figure 11), while staining for NLDC145, were positive by fluorescence mainly for B7-1, not B7-2 or OX-2. Similar conclusions were reached both by FACS analysis of cell populations (Figure 11) , and by PCR analysis of mRNA (Figure 12). When the functional capacity of these different cell populations was investigated (Figures 13 and 14) it was found that optimal direct stimulation (or proliferation and IL-2 production) was seen from B7-1 expressing cells (Fxs 3 and 4 in panel A) of Figure 13), while only OX-2 expressing cells (Fxs 1 and 2 in Figures 11 and 12) were capable of producing the inhibitory effects defined earlier (Figures 9 & 10) in the two-stage culture system (panel B) in Figure 13). These same cells (Fxs 1 and 2) were in turn able, after iv infusion, to polarize cells from mice given renal allografts to produce predominantly IL-4, IL-10 and TGFβ production on restimulation in vitro (Figure 14). These data are consistent with the notion that after FltL treatment of mice expansion of a population of immunostimulatory DC occurs within the liver, which also contains another distinct population of (facilitator) cells which promote immunoregulation. Evidence that cell populations with "facilitator" activity from the liver of Flt3L treated mice prolong graft survival in vivo: Since it has been reported elsewhere that there is a good correlation between treatments (such as pv immunization) which decrease IL-2 production and increase IL-4 production from restimulated cells and prolongation of graft survival (Gorczynski, R. M., and D. Wojcik. 1994. /. Immunol. 152:2011-2019; Gorczynski, R. M. 1995a. Cell. Immunol. 160:224-231; Gorczynski, R.M. et al. Transplantation 62:1592-1600), and that increased expression of OX-2 is also independently associated with increased graft survival after pv immunization (Gorczynski, R. M. et al. 1998b. Transplantation. 65:1106-1114), the next question was whether cells isolated from Flt3L treated mice which induced inhibitory function in vitro (see Figures 9, 10 and 13), and expressed increased amounts of OX-2 (Figures 11, 12) were themselves capable of promoting increased graft survival in vivo. Groups of 2 C57BL/6 mice received iv infusions of lOmg/mouse Flt3L for 10 days as before. Cells were isolated from the liver by enzyme digestion, and fractionated by unit gravity sedimentation. 4 pools of cells were recovered, and an aliquot stained as before in FACS with anti-OX-2. Groups of 2 C3H mice received 10x10 6 cells iv from the 4 separate pools. A control group received saline injections only. Over the next 48 hrs all mice received C57BL/6 renal transplants. All mice received CsA (lOmg/Kg) on the day of renal transplantation. Data in Figure 15 are pooled from 3 studies of this type (representing 6 mice/group), and show the animal survival in these 5 different groups. Figure 15 shows NPC from Flt3L treated C57BL/6 mice, infused iv into recipient C3H mice, inhibit C57BL/6 renal allograft rejection. Two mice groups received the different subpopulations of NPC derived from Flt3L treated mice shown in Figures 11 and 12. Fxs 1 and 2 were OX-2+. Mice received C57BL/6 renal allografts within 48 hrs along with CsA (see Materials and Methods). Animal survival was followed as an end point. Data shown are pooled from 3 studies (6 mice/group). *p<0.05 compared with mice receiving CsA only (■). It is quite clear from this Figure that only hepatic cells expressing OX-2 (Fxs 1 and 2-see Figures 11 and 12) were capable of promoting increased graft survival after iv infusion. Comparison of these data with those in Figure 13 confirm that these cell populations were also those identified, using a 2-stage culture assay system, as cells with functional "facilitator" activity (see also Figures 9 and 10). There was no significant difference in survival between groups receiving NPC-Fxl or NPC-Fx2 in this experiment, in keeping with relatively equivalent levels of OX-2 expression in these fractions (Figure 11). Anti-OX-2 monoclonal antibody in vitro reverses regulation induced by hepatic NPC: A final study was directed to whether anti-OX-2 monoclonal antibody M3B5, added to cultures of C3H spleen responder cells, allogeneic (C57BL/6) DC and NPC from C57BL/6 mice, could prevent the inhibition of IL-2 production in primary cultures, and the development of cells able to inhibit such cytokine responses from freshly stimulated responder cells in secondary cultures (see Figures 9, 10 and 13). Data in Figures 16 and 17 are pooled from 3 studies of this type. Figure 16 is a bar graph showing the effect of anti B7-1; B7-2; or OX-2 on primary allostimulation. It shows that anti-OX-2 Mab increases IL-2 cytokine production in vitro after stimulation of C3H responder spleen cells with C57BL/6 DC. Subgroups of cultures contained the Mabs shown. Cytokines were assayed at 60 hrs. AU data represent arithmetic means pooled from 3 repeat studies. *p<0.05 compared with control group (far left). Figure 17 is a bar graph showing that anti-OX-2 reverses inhibition by NPC. It shows that anti-OX-2 Mab inhibits development of immunoregulatory cells in vitro following incubation with hepatic NPC. C3H responder spleen ceUs were incubated in triplicate with C57BL/6 DC along with NPC (see Figures 9 and 10). Subgoups of these cultures contained the Mabs shown. Cytokines were assayed in cultures at 60 hrs (panel A). In addition, cells were harvested from all groups, washed and added to fresh C3H responder spleen cells and C57BL/6 DC (panel B). Cytokines in these groups were assayed 60 hrs later. All data represent arithmetic means pooled from 3 repeat studies. *p<0.05 compared with control group from cultures of NPC with no monoclonal antibodies (far left in Figure)-see also Figure 16. Primary cultures were of two types, containing C3H responder spleen cells and C57BL/6 DC alone (Figure 16), or the same mixture with added C57BL/6 NPC (Figure 17). Subsets of these cultures contained in addition either 5μg/ml of anti-B7-l, anti-B7-2 or anti-OX-2. Supernatants from responder cells stimulated in the presence of DC only were assayed after 60hrs for cytokine production (Figure 16). For the primary cultures incubated with both DC and NPC, supernatants were harvested at 60hrs and tested for cytokine production (Figure 17A). In addition, cells were harvested after 5 days, washed, and added to secondary cultures of fresh C3H responder ceUs with fresh C57BL/6 DC. No monoclonal antibodies were added at this second culture stage. Data for cytokine production these secondary cultures are shown in Figure 17B. Addition of anti-B7-l or anti-B7-2 to DC stimulated spleen cultures led to inhibition of cytokine production (Figure 16), while in contrast anti-OX-2 monoclonal antibody led an increase in IL-2 production in these primary cultures (Figure 16). We have reported similar findings elsewhere (Ragheb et al-submitted for publication). Interestingly, anti-OX-2 aboUshed the inhibition of cytokine production caused by NPC in these primary cultures (Figure 17A-see also Figures 9, 10 and 13). In addition, anti-OX-2 prevented the functional development of a cell population capable of transferring inhibition of cytokine production to freshly stimulated spleen cells (Figure 17B). DISCUSSION There is considerable theoretical as well as practical interest in understanding the mechanism(s) by which a state of antigen specific tolerance can be induced in lymphoid populations. Limits to the effective induction of tolerance represent a major challenge to more successful allo (and xeno) transplantation, to name but one example (Akatsuka, Y., C. Cerveny, and J. A. Hansen. 1996. Hum. Immunol. 48:125-134). Significant efforts have been invested into exploring how pre- (or peri-) transplant donor-specific immunization might produce such a state (Qian, J. H. et al. 1985. /. Immunol. 134:3656-3663; Kenick, S., et al. 1987. Transpl. Proc. 19:478-480; Gorczynski, R. M. 1992. Immunol. Lett. 33:67-77; Thelen, M., and U. Wirthmueller. 1994. Curr. Opin. Immunol. 6:106-112; Akolkar, P. N. et al. 1993. /. Immunol. 150 (April l):2761-2773; Ahvazi, B. C. et al. /. Leu. Biol. 58 (1):23-31; Albina, J. E. et al. 1991. /. Immunol. 147:144-152). There is good evidence that portal venous (pv) immunization somehow leads to tolerance induction, and this immunoregulation can apparently be monitored by foUowing changes in cytokine production from host cells, with decreased production of IL-2, IL-12 and IFNγ, and increased IL-4, IL-10, IL-13 and TGFβ ( Thelen, M., and U. Wirthmueller. 1994. Curr. Opin. Immunol. 6:106-112; Gorczynski, R. M. et al. 1998a. Transplantation. 66: 000-008). Which, if any, of these cytokine changes is directly and causally implicated nevertheless remains obscure. Further analysis of the cell population able to induce tolerance after pv immunization led to the somewhat paradoxical observation that donor dendritic (DC) ceUs represented an excellent tolerizing population (Gorczynski, R. M. 1995a. Cell. Immunol. 160:224-231; Gorczynski, R.M. et al. Transplantation 62:1592-1600). Since antigen-pulsed DC are conventionally thought of as representing an optimal immunizing regime, the mechanism(s) activated following DC pv immmunization which led to tolerance (Banchereau, J., and R. M. Steinman. 1998. Nature. 392:245-252) was of interest. It is already clear that DC themselves represent an extremely heterogeneous population, in terms of origin, ceU surface phenotype, turnover in vivo and possibly function (Salomon, B. et al. 1998. /. Immunol. 160:708-717; Leenen, P. J. M. et al. 1998. /. Immunol. 160:2166-2173). In the mouse lymph node at least 3 discrete populations were identified, one of which comprised small CD8α + NLDC145 + cells, likely of lymphoid origin, with an immature phenotype, and whose numbers were profoundly increased (lOOx) foUowing FH3L treatment in vivo (Salomon, B. et al. 1998. /. Immunol. 160:708-717) (administration of the latter has been reported to lead to proliferation of dendritic cells and other cells of hematopoietic origin (Maraskovsky, E. et al. 1996. /. Exptl. Med. 184:1953-1962)). These ceUs resembled the interdigitating DC found in the T cell areas of the splenic white pulp, and have been impUcated in regulation of immunity induced by other (myeloid derived) DC (Salomon, B. et al. 1998. /. Immunol. 160:708-717; Kronin, V. et al. 1996. /. Immunol. 157:3819-3827; Suss, G., and K. Shortman. 1996. /. Exptl. Med. 183:1789-1796). A variety of other studies have indicated that the induction of immunity (vs tolerance) following antigen presentation was intrinsically dependent upon the co-existence of other signaling ligands at the surface of DC (interacting with appropriate counter-ligands on the surface of other cells (e.g. stimulated T cells)) (Larsen, C. P. et al. 1994. /. Immunol. 152:5208-5219; Lenschow, D. J. et al. 1996. Annu. Rev. Immunol. 14:233-258; Larsen, C. P., and T. C. Pearson. 1997. Curr. Opin. Immunol. 9:641-647). It was speculated that infusion of DC via the portal vein induced tolerance by co-opting another cell population, distinguishable by expression of unique cell surface ligands, whose biological function was to facUitate induction of tolerance, not immunity, when antigen was presented in association with otherwise immunogenic DC. Some preliminary evidence supporting this hypothesis was recently reported (Gorczynski, R. M. et al. 1998a. Transplantation. 66: 000-008). Herein, this is referred to as a facilitator cell. Moreover, because pv immunization has been shown to be associated with increased expression of a novel molecule, OX-2, previously reported to be expressed on DC (Barclay, A.N. 1981. Immunology 44:727; Barclay, A.N., and H.A. Ward. 1982. Eur. }. Biochem. 129:447; Chen, Z. et al. 1997. BBA. Mol. Basis Dis. 1362:6-10; Gorczynski, R. M. et al. 1998b. Transplantation. 65:1106-1114), it was predicted that this molecule would in fact serve as a "marker" for the hypothetical facilitator cell described. Experiments reported herein are consistent with such a hypothesis. It is here shown that within the hepatic NPC population there is a subset of cells able to inhibit stimulation by allogeneic DC in a non-MHC restricted fashion (see Figures 9 and 10), and able to induce the development of an antigen-specific immunoregulatory cell population in vitro (see Figures 9 and 10). The non-MHC-restricted nature of this "facilitator" cell interaction indicates that it functions by providing an accessory signal (a regulatory not a co-stimulatory signal) to the DC which stimulate T cells in the allogeneic mixed leukocyte reaction described, in a fashion analogous to the original description of costimulatory interactions (Jenkins, M. K. et al. 1988. /. Immunol. 140:3324-3329). As a result the stimulated lymphocytes alter their cytokine production profile (with decreased IL-2 production and proliferation), and become able to regulate the immune response seen from freshly stimulated lymphocytes (see panel B in Figures 9 and 10). Most interestingly, following expansion of DC in vivo by F/i3L treatment, it is shown that in fact the liver itself contains both an immunostimulating population (large cells by velocity sedimentation analysis), and this putative "facilitator" cell population (see Figures 11-15). Furthermore, the latter biological activity resides within a slow-sedimenting (small size) NLDC145 + cell population expressing preferentially both cell surface B7-2 and OX-2 (see Figures 11 and 12). When it was investigated whether this same population of cells was active in vivo in regulating graft tolerance, it was found again that after prior FZt3L treatment, the liver contained a population of cells which transferred increased renal graft acceptance (Figure 15) and in parallel altered the cytokine production profile of immunized mice towards increased IL-4 and TGFβ, and decreased IL-2 and IFNγ production (Figure 14). In a final attempt to explore the role for OX-2 expression itself in this regulatory function, fresh spleen cells were stimulated with DC alone or in the presence of anti-B7-l, anti-B7-2 or anti-OX-2. Note that other studies (data not shown) have confirmed that even the bone-marrow derived DC used contains small numbers of OX-2 + cells (RMG-unpublished). Unlike anti-B7-l and anti-B7-2 which decreased cytokine production, a result in keeping with the hypothesized role for these as costimulator molecules (Hancock, W. W. et al. 1996. Proc. Natl. Acad. Sci. USA. 93:13967-13972; Freeman, G. J. et al. 1995. Immunity. 2:523-532; Kuchroo, V. K. et al. 1995. Cell. 80:707-718), anti-OX-2 produced a small but significant (1.7-2.5 fold in three studies) increase in IL-2 production in this system (Figure 16). Most important, however, inclusion of anti-OX-2 Mab in a system where exogenous "facilitator" cells were added (from NPC), blocked completely the induction of inhibition normally seen in such cultures (Figures 9 and 10; compare with lower panel of Figure 17). These data are consistent with the concept that OX-2 deUvers a regulatory, not a costimulatory, signal in this situation. How does the present data fit within the evolving framework of understanding in the heterogeneity of DC? As noted above, there has been speculation that a separate population of CD8α + NLDC145 + DC of lymphoid origin which proliferates in response to FH3L, might be responsible for immunoregulation. Other data have implicated IL-10 as a cytokine which might modify development/maturation of DC into a population expressing increased amounts of B7-2 and capable of inducing tolerance (Steinbrink, K. et al. 1997. J Immunol. 159:4772-4780). The role of regulation of expression of Fas as a controlling feature in this regard is unexplored (Suss, G., and K. Shortman. 1996. /. Exptl. Med. 183:1789-1796). The data disclosed herein is the first to implicate another molecule, OX-2, in the delivery of a tolerizing signal, perhaps in association with alterations in expression of B7-2, Fas etc. It is intriguing that while there is clearly a key role for intra-thymic DC in the regulation of self-tolerance (Banchereau, J., and R. M. Steinman. 1998. Nature. 392:245-252), natural expression of OX-2 was initially first described on thymic DC (as well as within the brain) (Barclay, A.N. 1981. Immunology 44:727)-there is as yet no evidence to suggest that this represents a functionally relevant expression for OX-2 in this location. However, other independent data have also implied an immunoregulatory role for OX-2 expression, again as assayed by altered cytokine production in vitro from cells stimulated in the presence /absence of expressed OX-2 (Borriello, F. et al. 1997. /. Immuno.. 158:4548). It has been reported that following pv immunization there is a measureable expansion in numbers of populations of γδTCR + cells capable of adoptive transfer of increased graft survival to naive recipients (Gorczynski, R. M. et al. 1996c. Immunology. 87 (3):381-389). Little is known concerning the nature of the antigen recognized by these cells, and why, as a population, their numbers are preferentially increased following pv immunization. It is speculated that this may be explainable ultimately in terms of a differential susceptibility of γδTCR + vs ocβTCR + cells to immunoregulatory signals delivered following OX-2 expression. In conclusion, the inventor has reported for the first time that functional heterogeneity in the DC pool may be understandable in terms of differential expression of OX-2 on the cell surface. Expression of this molecule seems to give cells the capabUity to induce immunoregulation, increased renal graft survival (and altered cytokine production both in vivo and in vitro). The present invention suggests that such OX-2 expressing cells are referred to as "facilitator" cells (for tolerance induction). EXAMPLE 4 Preparation of Murine Antibodies Mouse and rat hybridomas to a 43Kd molecule expressed in the thymus, on a subpopulation of dendritic ceUs, and in the brain, in mammalian tissue derived from mouse, rat and human were prepared. Using CHO cells transiently transfected with adenovirus vector(s) expressing a cDNA construct for the relevant OX-2 gene, the monoclonal antibodies (Mabs) detect a molecule encoded by this construct (rat OX-2 (rOX-2), mouse OX-2 (mOX-2) and human OX-2 (huOX-2) respectively). Furthermore, at least some of the anti-rat Mabs detect determinants expressed on the murine OX-2 molecule. MATERIALS AND METHODS Antigen preparation from tissues and Western blotting were performed as described in Gorczynski et al., Transplantation, 1998, 65:1106-1114: Spleen cells (human samples were obtained from cadavers at the time of organ retrieval for transplantation) were used for preparation of dendritic cells /macrophages. Tissue was digested with a mixture of collagenase and dispase and centrifuged over lymphopaque. Cells were adhered for 2 hr at 37°C, washed vigorously, and incubated for 14 hr at 37°C. Dendritic cells were isolated as non-adherent cells (Gorczynski et al., Transplantation, 1996. 62:1592-1600). Routine staining of mouse splenocytes with NLDC-145 and FITC anti-rat IgG, or FITC-MAC-1 before and after overnight incubation produced the following staining pattern in these adherent cells: 8%±2%, 90%±11% and 92%+9%, 9%+3% respectively. The crude (non-adherent) dendritic cell preparation was extracted with lysis buffer, titred to a protein concentration of lOmg/ml, and used for immunization. Some of the same material was used subsequently in screening ELISAs (below). When brain tissue was used in Western gel analysis, whole tissue extract was electrophoresed in 12%SDS-PAGE and transferred to PVDF membranes (Novex Co., San Diego, CA). Putative anti-OX-2 Mabs were used as test reagent, with isotypic antibodies (negative in ELISA tests) used as controls. Membranes were developed using either anti-rat or anti-mouse horse radish peroxidase and appropriate substrate. Immunization and production of Mabs: Four female BALB/c mice were initially immunized by intraperitoneal injections with lmg of human or rat dendritic antigen in Complete Freundis Adjuvant. Three subsequent boosts were administered as above, spaced at 3 week intervals, with Incomplete Freundis Adjuvant. When the serum titre had risen more than 10-fold from a pre-immune serum sample, as determined by ELISA, the 2 highest responders were boosted intravenously. Three days later the donor mice were sacrificed and the spleen ceUs were harvested and pooled. Fusion of the splenocytes with X63-Ag8.6.5.3 BALB/c parental myeloma cells was performed as previously described (Kohler, G. and C. Milstein. 1975. Nature. 25: p. 256-259), except that one-step selection and cloning of the hybridomas was performed in 0.8% methylcellulose medium (Immuno-Precise Antibodies Ltd., Victoria, BC). This proprietary semi-solid medium allows HAT selection and cloning in a single step and eliminates the overgrowth of slower growing desirable clones by faster growing, perhaps undesirable, hybridomas. Clones were picked and resuspended in wells of 96-well tissue culture plates in 200 μl of D-MEM medium containing 1% hypoxanthine/thymidine, 20% Fetal Bovine serum, 1% OPI, and 1 x 106/ml BALB/c thymocytes. After 4 days, the supernatants were screened by ELISA for antibody activity on plates coated with the immunizing antigen. Putative positive hybridomas were re-cloned by limited dilution cloning to ensure monoclonality and screened in FACS on extracts prepared from brain tissue (below). For the production of rat mAbs, 2 Fisher rats were immunized as above with mouse antigen. EssentiaUy the same procedure was followed, except the parental cell line used for the fusion was YB2/0. ELISA and FACS analysis of putative Mabs: ELISA assays used polystyrene plates pre-coated with lOOng/ml poly-L-lysine, followed by overnight incubation with the crude dendritic cell antigen (used for immunization) at lOmg/ml. Wells were developed after binding of hybridoma superntatants using the anti-rat/anti-mouse horse radish peroxidase antibodies above and plates were analysed in an automatic ELISA plate reader (TiterTek Multiskan, MCC/340, FlowLabs, Mississauga, Ontario, Canada). FACS analysis was performed using putative anti-OX-2 Mabs and the following cells. Fresh peripheral blood leucocytes (PBL), isolated over rat/mouse lymphopaque (Cedarlane laboratories) or Ficoll-Hypaque (human); fresh spleen dendritic cells (isolated after adherence and overnight incubation, as above); and CHO cells transduced with viral vectors engineered to contain a single copy of a cDNA inserted into the notl/bamHl sites, encoding the relevant species-specific OX-2, as per published sequences (Chen, Z. et al. 1997. BBA. Mol. Basis Dis. 1362:6-10; McCaughan, G.W., et al. 1987. Immunogenetics. 25: p. 133-135), or with control vector alone. FITC anti-mouse (or anti-rat) IgG was used as secondary antibody. Mixed leucocyte reactivity (MLR) and cytokine production: Allogeneic MLR cultures, using 1:1 mixtures of 2.5xl0 6 responder PBL and mitomycin C treated stimulator PBL, were set up in 24-well culture plates in 1ml of aMEM medium supplemented with 10% FCS. Cells were obtained from C3H responder mice (with stimulator C57BL/6), Lewis (LEW) rats (with Brown Norway, BN, as stimulator), and individual human donors. Culture supernatants were harvested at 60 hrs and tested for different cytokines using previously described ELISA assays (mouse), or using CTLL-2 as bioassay for IL-2 production from all responder cell sources (Gorczynski, R.M., et al. 1998c. Immunology. 93: p. 221-229). RESULTS Evaluation of a number of Mabs for staining of cell populations in fresh PBL or spleen: All Mabs tested in the experiments herein described were previously screened as described in the Materials and Methods above, and detected a molecule in Western gel of brain extracts with Molecular Weight 42-45 Kd, and also stained CHO transduced by OX-2 encoding viral vectors. Data in Table 3 show FACS analysis for these Mabs using fresh cells. The data are summed over several independent analyses, using a number of Mabs directed to rat, mouse or human OX-2, for staining of cells harvested from fresh PBL or spleen (adherent cells only were tested for the latter: these represented some 5%-8% of the total cell population in all cases). It is clear from Table 3 that PBL in all species tested contained some 1.3%-2.5% OX-2 + cells by FACS analysis, and that spleen adherent cells similarly contained 4%-8% OX-2 + cells. As confirmation of the inventor's previous work, spleen adherent cells taken from C3H mice or LEW rats treated 4 days earlier by portal venous immunization with 20x106 (or 50xl0 6 respectively) of C57BL/6 (or BN) bone marrow cells showed some 3.5-5 fold elevation in OX-2 + cells (see Table 3). Under these conditions specific increases in survival of subsequent allo- transplanted cells /tissue have been reported (Gorczynski, R.M. et al. 1996a. Transplantation 62:1592-1600). Ability of anti-OXA Mabs to modulate cytokine production in MLR in vitro: In a final study the issue of whether these Mabs can modify the immune response (as assayed by cytokine production) of cells stimulated in an allogeneic mixed leucocyte reaction (MLR) in vitro was addressed. The inventor has previously shown that ceUs taken from mice pretreated by portal allogeneic immunization produce predominantly type-2 cytokines, and that an anti-OX-2 Mab could apparently reverse this polarization in cytokine production (and indeed abolish the increased graft survival seen in such mice). Data in Table 4 confirm these results using 3 independent Mabs to mouse OX-2. Further, rat or human cells stimulated in the presence of anti-rat (or human) OX-2, sirmlarly show more pronounced IL-2 production than cells stimulated in the presence of isotypic control Ig (or no Ig), without a generalized increase in cytokine production (as analysed here by no change in IL-6 production in any group). DISCUSSION In the data in this example it is confirmed that using species specific Mabs, to human, rat or mouse OX-2, that Mabs to the molecule detected on the surface of host dendritic cells may play a role in regulating cytokine production after allostimulation in vitro, and more particularly that functionally blocking OX-2 expression leads to enhanced IL-2 production (a type-1 cytokine) after allostimulation (Table 4). Borriello et al also recently reported that OX-2 expression was not a costimulator for induction of IL-2 and IFNγ synthesis (Borriello, F. et al. 1997. /. Immuno.. 158:4548)-our data imply it is in fact a negative signal for type-1 cytokine production. In mice preimmunized by the portal vein, as reported earlier, there is a 4-fold increase in OX-2 expressing cells in PBL and spleen, and a reversal of polarization in cytokine production (from type-2 cytokines to type-1 cytokines) after stimulation of cells in the presence of OX-2 (see Tables 3 and 4) (Gorczynski, R. M. et al. 1998b. Transplantation. 65:1106-1114). EXAMPLE 5 Preparation of Rat Antibodies Five rats were immunized using GERBU adjuvant (GERBU Biotechnik, Gaiberg, Germany) with 500μg of membrane protein purified from the mouse dendritic cell (DC) line DC2.4 (a gift from K. Rock, Harvard). Serum from these rats was tested 7 days after the third immunization, and compared with a pre-immunization sample in an ELISA using plate-bound material of Mol. Wt. 40Kd-45Kd eluted from Western blots, and Alk Pase anti-rat Ig. Two rats with high titre antibody were re-immunized and sacrificed 4 days later for fusion of spleen cells with HAT-sensitive Sp2/0 parent cells for preparation of hybridomas. Hybridomas were screened by ELISA (56/960+ve), subcloned, and frozen (- 70°C). For further specificity testing of the anti-OX-2 Mabs will use CHO cells can be transfected with a pBK eukaryotic expression vector (Stratagene,CA) expressing OX-2. Full length OX-2 cDNA, including the leader sequence, was amplified from DC2.4 cells using sense and antisense primers constructed with Spel or Xbal sites respectively at their 5' ends for directional cloning into the vector. A band of the expected size (849bp) was obtained on agarose gel electrophoresis. The sequence of the cloned cDNA was confirmed by sequencing using an automated DNA sequencer (Chen, Z. and Gorczynski, R.M. 1997. Biochem. Biophys. Ada. 100, in press). CHO cells were transfected by electroporation (5x106 cells in 0.5ml were pulsed at 960MH 2 and 120V using a Bio-Rad Gene Pulser (BioRad, Hercules, CA), using the full length OX-2 expression plasmid along with a plasmid encoding puromycin resistance (100:1 ratio), foUowed by selection in puromycin (12μg/ml for 4 days). Puromycin resistant cells were cloned by limiting dilution. 5 CHO transfectant clones have been obtained expressing mRNA for OX-2 as confirmed by PCR. These clones can be used to screen the putative rat anti-mouse OX-2 Mabs. (a) FACS staining of cells from py immunized mice with anti-mouse OX-2 A 4-fold increase in staining of spleen and hepatic NLDC145+ (dendritic ceU marker) cells from pv immunized mice with anti-rat OX-290 was observed. Spleen and liver tissue of mice at various times (12 hours; 2, 7 and 14 days) following pv immunization can be sectioned and stained by immunohistochemistry, using anti-NLDC145, anti-OX-2 Mabs. Single cell suspensions from the same tissues can be stained, using 3-colour FACS, with FITC-anti-mouse OX-2, rhodamine-anti-NLDC145, and phycoerythrin-anti-T200 (mouse lymphocyte marker). In all cases (both FACS and immunohistochemistry) the appropriate irrelevant isotype control antibodies are included. Tissue from control mice receiving renal grafts alone, or following additional iv immunization, can also be examined. Detection of NLDC145+ (and/or MAC-1+ ) cells showing increased expression of OX-2 is predicted in pv immunized mice only (see Gorczynski, R.M. et al. 1998. /. Immunol. 160, in press). The inventor has shown DC-associated antigen persists only in animals with surviving grafts (Gorczynski, R.M., Chen, Z., Zeng, H. and Fu, X.M. 1998. Transplantation submitted). It was also assessed whether anti-OX-2, infused at different times post transplantation, causes rejection (b). (b) Modulation of graft rejection and cytokine production by anti-mouse OX-2 C3H mice receive pv immunization with cultured C57BL/6 bone-marrow derived dendritic cells (DC), CsA and renal aUografts. Groups of mice receive intravenous infusion of various rat anti-mouse OX-2 Mabs (100-500μg/mouse, x5, at 2 day intervals), beginning at different times post transplantation (this will be guided by data from (a)). Serum creatinine and animal survival are followed. Serum from Mab-treated mice are tested in ELISA and by FACS with OX-2 expressing CHO transfectants (above) to ensure antibody excess. If OX-2 expression is important for pv induced increased graft survival, the anti-OX-2 treated pv immunized mice will reject grafts like untreated controls, with similar polarization of cytokine production to type-1 cytokines (assayed by PCR; ELISA with cultured, restimulated cells). As controls pv immunized, grafted mice receive anti- CD28 and anti-CTLA4 these Mabs do not modify the effects of pv immunization as assayed by graft survival or polarization in cytokine production. It is expected that OX-2 treatment but not other Mabs, will simultaneously abolish expansion of γδTCR÷ cells after pv immunization. EXAMPLE 6 Preparation of a fusion protein linking the extracellular domain of OX-2 to mouse Fc Immunoadhesins, in which a hybrid molecule is created at the cDNA level by fusing the extracellular domain (ED) of an adhesion molecule with the carboxyl terminus of IgG heavy chain, the whole being expressed in mammalian cells or in a baculovirus system, have been powerful tools in the identification and isolation of the counter ligands for the adhesion molecule of interest. Ligands for a number of members of the TNFR family, were identified in this fashion (Goodwin, R.G. et al. 1993. Eur. J. Immunol. 23, 2631-2641; Gruss, H. and Dower, S. 1995. Blood 85, 3378-3404). Interest has developed in the potential application of immunoadhesins as therapeutic agents. A CTLA4 immunoadhesion, with the capacity to bind both B7-1 and B7-2, has been used to inhibit T cell costimulation and decrease rejection (Larsen, C.P. et al. 1996. Nature 381, 434-438). Note that CD28/CTLA4 are not counter ligands for OX-289. The fusion protein, is predicted to alter cytokine production (increased IL-4, IL-10; decreased IL-2, IFNγ) and increase renal graft survival like pv immunization. We expect that synergistic blockade of costimulation (e.g. by CTLA4-Fc) and triggering of a coregulatory pathway (by OX-2ED-Fc) will induce tolerance and produce indefinite graft survival. a) Construction of an OX-2 fusion protein with murine IgGFcla A cDNA encoding the extracellular region of OX-2 (OX-2ED) was amplified by PCR, using a 5' oligonucleotide primer which inserts a Sail site 5' immediately at the start of the V-region sequence and a 3' primer which creates a BamHl site at the 3' end (the site of junction with Fc). Using cDNA prepared from mouse ConA activated spleen cells, with a 5'primer containing an Spel site, and a 3' primer containing a Sail site, the signal peptide for IL-6 (SP-IL-6) was amplified by PCR and ligated to the OX-2ED amplicon. In frame ligation across the junction of SP-IL-6 and OX-2ED was checked by manual sequencing-the final cDNA amplified by the 5'SP-IL-6 primer and the 3OX-2ED primer was, as expected, 695bp. A plasmid expressing murine IgGFc2a (Fcγ2a), modified to create a unique BamHl site spanning the first codon of the hinge region, and with a unique Xbal site 3' to the termination codon, has been obtained from Dr. Terry Strom (Zheng, X.X. et al. 1995. journal of Immunology. 154, 5590-5600). The IgGFc2a in this insert has been further modified to replace the Clq binding motif (rendering it non-lytic) and inactivate the FcγRl binding site (see Zheng, X.X. et al. 1995. journal of Immunology . 154, 5590-5600). Ligation of OX-2ED and IgGFc2a in the correct reading frame at the BamHl site yields a 1446bp long open reading frame encoding a single 478-amino acid polypeptide (including the 24-amino acid IL-6 signal peptide). The homodimer has a predicted 105kDa Mol Wt, exclusive of glycosylation. The fusion gene is then cloned as an Spel-Xbal cassette into the eukaryotic expression plasmid pBK/CMV (Stratagene, CA). This plasmid has a CMV promoter/ enhancer and a neomycin-resistance gene for selection using G418. The appropriate genetic construction of the OX-2ED-Fc can be confirmed by direct sequencing after cloning into the plasmid vector (Chen, Z. and Gorczynski, R.M. 1997. Biochem. Biophys. Ada. 100, in press)-see also above. The plasmid is transfected into CHO cells by electroporation (see above), and selected in medium with 1.5mg/ml G418 (Geneticin:Life Technologies, Inc.). After subcloning, high producing clones are selected by screening culture supernatants in ELISA using anti-OX-2 Mabs as capture antibody, and Alk Pase coupled anti-IgGFc2a as detection antibody. OX-2ED-Fc fusion protein is purified from culture supernatants using protein A-Sepharose affinity chromatography, dialysed against PBS, filter-sterilized and stored in aliquots at -20°C. The size, and OX-2 (+IgGFc2a) specificity of the secreted product can be confirmed using Western blot analysis under reducing (+DTT) and non-reducing (-DTT) conditions, with Mabs to OX-2 and rat monoclonal anti-mouse IgGFc2a (Pharmingen). The product can be titrated as an inhibitor for FACS staining of OX-2 expressing CHO cells (see above) using rat Mabs to OX-2 as probe. As a prelude to studies (below) using OX-2ED-Fc in vivo, the half -life (tl/2) in mouse serum following injection of groups of 6 8-week C3H mice wiU be studied. This is carried out by subjecting mice to iv injections of 50 μg or 10 μg of OX-2ED-Fc, and obtains serial 50 μl blood samples at 0.3, 1, 6, 24, 48, 72 and 96 hours. The serum is analyzed in ELISA using plates coated with anti-OX-2 as capture antibody, and Alk Pase coupled monoclonal anti-IgGFc2a for detection (thus ensuring the assay detects only OX-2ED-Fc, not OX-2 or IgGFc2a alone). Based on earUer data in which Fc fusion proteins were used to extend the in vivo half-life, a tl /2 in the range of 30-40 hrs (Zheng, X.X. et al. 1995. Journal of Immunology. 154, 5590-5600) is predicted. b) OX-2: IgGFc Immunoadhesion Inhibits MLR CHO cells were transduced with a vector carry the OX-2:Fc cDNA insert. Supernatant was harvested from the CHO cells at 7 days and was cultured with 5x10 6 LEW spleen and 2.5xl0 6 irradiated LBNFI spleen cells. The supernatant contained 50ng/ml OX- 2:Fc. The results, shown in Table 5, demonstrate that the soluble OX-2:Fc immunoadhesion inhibits IL-2 production and generation of cytotoxic T cells and induces IL- 4 production. These results support the use of OX-2 as an immunosuppressant. c) Use of OX-2:Fc in vivo for Prevention of Graft Rejection It was shown in (b) that incubation in the presence of 50ng/ml OX-2:Fc can inhibit an in vitro MLR reaction. To detect inhibition of in vivo graft rejection, C3H mice received C57BL/6 skin grafts along with iv injection of OX-2:Fc (50μg/mouse) every 2 days x4 injections. Grafts were inspected daily after 10 days for rejection. In a separate study 3 mice/group (receiving saline or OX-2:Fc) were sacrificed at 10 days and spleen cells restimulated in vitro (x48 hrs) for analysis of cytokine production. Data for these studies is shown in Tables 6 and 7. It is clear from these data that OX-2:Fc has the potential for use as an immunosuppressant to prolong graft acceptance. Furthermore, in association with increased graft survival in this model, OX-2:Fc alters polarization in cytokine production, as already described for portal vein donor-specific immunization. EXAMPLE 7 OX-2 Prevents Fetal Loss Using in situ hybridization, the inventor has shown that OX-2 is not expressed in the placenta of mice with increased potential for fetal loss. In contrast, OX-2 is expressed in the placenta of normal, non-aborting mice. CBA/J and DBA/2J mice were used. Matings of CBA/J(females) with DBA/2J males show a high incidence of fetal loss (>80%), unlike the reverse scenario. Placental tissue was obtained from matings at 8-11 days of gestation. Uteri were snap frozen, 5 μm sections cut, and stained with a biotinylated anti-sense probe for murine OX-2. Data shown in Figures 18A and 18B indicate increased expression of OX-2 mRNA (in situ labeling) in the non-aborting strain combination, with essentially absent expression in the aborting combination. These data are consistent with the notion that OX-2 expression prevents spontaneous fetal loss syndrome. While the present invention has been described with reference to what are presently considered to be the preferred examples, it is to be understood that the invention is not limited to the disclosed examples. To the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. All publications, patents and patent applications are herein incorporated by reference in their entirety to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety. TABLE 1 Summary of sequences and clones detected in cDNA Ubrary from pv immunized mice Match category Number of clones represented (%) Known mouse genes 30 (45) Non-mouse genes (rat /human) 14 (21) No data base match 22 (34) Footnotes: Genes were considered a "match" with a BLAST score >250 with a minimum of 50bp alignment. TABLE 2 Cytokine production from ceUs of mice receiving pv immunization and anti-rat OX-2 Mabs given to recipients 3 Cytokine levels in culture supernatants IL-2 IFNγ IL-4 ΓL-10 No pv immunization (CsA only) None 750+125 85±18 29+8 130±40 +anti-rat OX-2 890±160 93±19 30±10 120±35 +anti-mouse CD28 415±88* 57±9* 105±22* 275±55* +anti-mouseCTLA4 505±125* 65±8 95±20* 190±45 +anti-B7-l 340±65* 35±7* 120±21* 285±60* +anti-B7-2 495+90* 64±7 90±20* 185±45 PV immunization + CsA None 190+55 25±8 107±21 780±150 +anti-rat OX-2 730±140* 60±16* 33±10* 220±40* +anti-mouse CD28 145±38 20±9 145±34 1140±245 + anti-mouseCTL A4 85±25 15±6 125±31 960±220 +anti-B7-l 110±30 20±6 144±28 885±180 +anti-B7-2 75+20 14+5 150+30 1230±245 Footnotes: a. 3 C3H mice /group were used in each experiment. All animals received CsA and C57BL/6 renal transplants as described in the Materials and Methods. Mice in the lower half of the Table also received pv infusions of 15xl0 6 C57BL/6 bone marrow derived dendritic cells on the day of transplantation. Where monoclonal antibodies were given the dose used was lOOmg/mouse, x4 doses at 2 day intervals. AU mice were sacrificed 14 days post transplantation. Spleen cells were cultured in triplicate from individual animals for 40hrs in a 1:1 mixture with irradiated C57BL/6 spleen stimulator cells. b. Arithmetic mean (±SD) for triplicate determinations from individual samples of the animals described in the first column. AU cytokines were assayed by ELISA. IL-2, IL-4 and IL-10 are shown as pg/ml, IFNg as ng/ml. Data are pooled from 2 such studies (total of 6 individual mice tested/group). * represents significantly different from control group with no Mab (p<0.02) TABLE 3 FACS staining of PBL and spleen adherent cells in different species, using anti-OX-2 Mabs SPECIES 3 Donor b Mab Percent stained cells c Treatment PBL Spleen H4B4 1.5±0.3 4.8±1.7 Human NONE H4A9A2 1.5±0.4 6.1±2.0 H4A9C7 1.3±0.4 4.3±1.7 M3B5 1.9+0.4 6.7+2.1 Mouse NONE M3B6 1.7+0.4 5.2±1.6 M2C8 1.4±0.4 4.2±1.4 M3B5 5.9±1.5 20±4.1 Mouse PV immune M3B6 5.2+1.4 17+3.6 M2C8 4.7 1.4 15+3.3 RC6A3 1.3±0.3 5.3±1.6 Rat NONE RC6C2 1.5±0.4 6.5±1.7 RC6D1 1.9±0.6 6.8±1.5 RC6A3 4.8+1.3 16±4.2 Rat PV immune RC6C2 4.9±1.6 18±3.9 RC6D1 5.3+1.7 20+4.5 Footnotes: a* Fresh cells were obtained from normal human donors (PBL), cadaveric transplant donors (human spleen), or from adult (8-10 week) mouse or rat donors. The same 3 separate tissue donors were used for each Mab tested. b * Donor pretreatment refers to infusion of allogeneic bone marrow cells into the portal vein (C57BL/6 for C3H mouse donors; BN for LEW rat donors) 4 days before harvest of PBL or spleen (see text and (6)). c* Arithmetic mean (+SD) for percent cells stained in 3 independent assays. Control antibodies (FITC anti-mouse IgG (for anti-human or anti-rat Mabs, or FITC anti-rat IgG for anti-mouse Mabs) gave no significant staining above background (<0.2%). TABLE 4 Type-1 cytokine production in MLR cultures is increased by anti-OX-2 Mabs Mabs in culture 3 Cytokine levels in culture supernatants b ELISA assays (murine only) Bioassay (CTTL-2) IL-2 IFNγ IL-4 IL-10 IL-2 IL-6 MOUSE MLR None 350±55 35±18 345±63 340+50 480+160 365±74 M3B5 890+160* 115±29* 130+10* 168±42* 820±200* 265+46 M3B6 915±155* 117±25* 135+32* 135±38* 850±175* 303+55 M2C8 855±155* 105±28* 120±32* 140±37* 830±165* 279161 control Ig 370±75 36+11 330±55 310+45 335±60 349159 None** 710±145 108±23 110±21 105±23 690±155 285154 RAT MLR None 490+145 360157 RC6A3 690±155* 295155 RC6C2 845 180* 345168 RC6D1 830±160* 370157 Control Ig 475±160 356158 HUMAN MLR None 395±85 295145 H4B4 570±125* 315150 H4A9A2 6301145* 320148 H4A9C7 625±140* 345156 Control Ig 360+120 320150 Footnotes: a* MLR cultures were set up as described in the Materials and Methods. For human MLR cultures the same 3 different responder preparations were used for each Mab, and stimulated with a pool of mitomycin C treated spleen stimulator cells (from a random mixture of 6 spleen donors). For mouse (C3H anti-C57BL/6) and rat (LEW anti-BN) MLR cultures aU assays were set up in triplicate for each Mab. Mouse responder spleen cells were from mice treated 4 days earlier by portal vein infusion of C57BL/6 bone marrow cells, except for data shown as (None**) where responder ceUs were from non-injected C3H mice. Mab was added as a 30% superntatant concentration. Supernatants were harvested for cytokine assays at 60 hrs. b* Data show arithmetic means (+SD) for each Mab. For mouse assays all supernatants were assayed for a number of cytokines (ELISA), and for IL-2/IL-6 using bioassays (proUferation of CTLL-2, B9 respectively). Supernatants from rat/human cultures were assayed in bioassays only. Note that cells incubated with isotype control Igs (non-reactive by ELISA or FACS) gave cytokine data indistinguishable from cultures incubated in the absence of Mab. p<0.05, compared with cultures without Mabs. TABLE 5 OX-2:FC Immunoadhesin Inhibits Mixed Leukocyte Reaction in vitro Added supernatant 3 Percent lysis 51 Cr targets* 3 Cytokines in culture (pg/ml) c (50:1, effectoπtarget) IL-2 IL-4 NONE (control) 3114.0 1005+185 60+20 Control CHO 3314.3 8101190 45120 (vector transduced) CHO transduced with 4.212.1 175145 245155 OX-2:Fc Footnotes: 3 Supernatant was harvested at 7 days from CHO cells transduced with control pbK vector, or vector carrying a cDNA insert encoding OX-2 linked to murine Fc. A 1:1 mixture of supernatant was used in cultures containing 5xl0 6 LEW spleen and 2.5xl0 6 irradiated LBNF1 spleen cells; this corresponded to 50ng/ml OX-2:Fc and Percent lysis with cells at 5 days, using lxlO 4 5iCr BN spleen ConA targets; cytokines in culture supernatants at 60hrs. 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Claims: I igjpt: 1. A use of an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to suppress an immune response. 2. A use according to claim 1 to induce immune tolerance to a transplanted organ or tissue. 3. A use according to claim 1 to prevent or inhibit fetal loss. 4. A use according to claim 1 to prevent or inhibit graft versus host disease. 5. A use according to claim 1 to prevent or treat an autoimmune disease. 6. A use according to claim 1 to prevent or treat an allergy. 7. A use according to claims 1-6 wherein the OX-2 protein has the amino acid sequence as shown in Figure 8 or a fragment thereof. 8. A use according to any one of claims 1 to 7 wherein the OX-2 protein is a soluble fusion protein. 9. A use according to claims 1-6 wherein the nucleic acid sequence encoding the OX-2 protein has the sequence shown in Figure 7 or a fragment thereof. 10. A use of an effective amount of an agent that inhibits OX-2 to prevent immune suppression. 11. A use according to claim 10 wherein the agent is a molecule that binds the OX-2 protein. 12. A use according to claim 11 wherein the molecule is an antibody. 13. A use according to claim 10 wherein the agent is an antisense oligonucleotide that is complimentary to a nucleic acid sequence from an OX-2 gene. 14. A pharmaceutical composition for use in suppressing an immune response comprising an OX-2 protein in admixture suitable dUuent or carrier. 15. A pharmaceutical composition according to claim 14 wherein the OX-2 protein is a soluble fusion protein. 16. A pharmaceutical composition for use in suppressing an immune response comprising an effective amount of a nucleic acid molecule encoding an OX-2 protein in admixture with a pharmaceutically acceptable diluent or carrier. 17. A pharmaceutical composition for use in preventing immune suppression comprising administering an effective amount of an agent that inhibits OX-2 to an animal in need thereof. 18. A composition according to claim 17 wherein the agent is a molecule that binds the OX-2 protein. 19. A composition according to claim 18 wherein the molecule is an antibody. 20. A composition according to claim 18 wherein the agent is an antisense oUgonucleotide that is complimentary to a nucleic acid sequence from an OX-2 gene. 21. An isolated nucleic acid sequence encoding a murine OX-2 gene having the sequence shown in SEQ.ID.NO.:l. 22. An isolated nucleic acid sequence encoding an OX-2 protein and comprising (a) a nucleic acid sequence as shown in Figure 7, wherein T can also be U; (b) nucleic acid sequences complementary to (a); (c) a fragment of (a) or (b) that is at least 15 bases, preferably 20 to 30 bases, and which will hybridize to (a) or (b) under stringent hybridization conditions; or (a) a nucleic acid molecule differing from any of the nucleic acids of (a) or (b) in codon sequences due to the degeneracy of the genetic code. 23. An isolated murine OX-2 protein having amino acid sequence shown in SEQ.ID.NO.:2 or a fragment thereof. 24. A method of inducing immune tolerance comprising adrninistering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to an animal in need thereof. 25. A method of inducing immune tolerance to a transplanted organ or tissue in a recipient animal comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to the recipient animal prior to the transplantation of the organ or tissue. 26. A method of preventing or inhibiting graft versus host disease in a recipient animal receiving an organ or tissue transplant comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to the organ or tissue prior to the transplantation in the recipient animal. 27. A method of preventing or treating an autoimmune disease comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to an animal having, suspected of having, or susceptible to having an autoimmune disease. 28. A method of preventing or treating an allergy comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to an animal having or suspected of having an allergy. 29. A method of preventing or inhibiting fetal loss comprising administering an effective amount of an OX-2 protein or a nucleic acid sequence encoding an OX-2 protein to an animal in need thereof. 30. A method according to any one of claims 22-29 wherein the OX-2 protein has the amino acid sequence as shown in Figure 8 or a fragment thereof. 31. A method according to any one of claims 22-29 wherein the OX-2 protein is a soluble fusion protein. 32. A method according to any one of claims 22-29 wherein the nucleic acid sequence encoding the OX-2 protein has the sequence shown in Figure 7 or a fragment thereof. 33. A method of preventing immune suppression comprising administering an effective amount of an agent that inhibits OX-2 to an animal in need thereof. 34. A method according to claim 33 wherein the agent is a molecule that binds the OX-2 protein. 35. A method according to claim 34 wherein the molecule is an antibody. 36. A method according to claim 33 wherein the agent is an antisense oUgonucleotide that is complimentary to a nucleic acid sequence from an OX-2 gene.

Inventors: GORCZYNSKI REGINALD M, GORCZYNSKI, REGINALD, M.
Applicants: GORCZYNSKI REGINALD M, GORCZYNSKI, REGINALD, M.
Application year: 1998
Main classification: C12N15
IPC codes: A61K31, A61K38, A61K39, A61K48, A61P15, A61P37, C07K14, C07K16, C12N15