GENETICALLY MODIFIED CELLS FOR USE IN TRANSPLANTATION

Abstract: Cells suitable for transplantation which can be used to deliver a gene product to an allogeneic or xenogeneic subject are disclosed. Such cells are modified to express a gene product and have at least one antigen on the cell surface which stimulates an immune response against the cell in an allogeneic or xenogeneic subject. Prior to administration of the modified cells, the antigen on the cell surface, such as an MHC class I antigen, is altered to inhibit rejection of the cell by an allogeneic or xenogeneic subject. Preferably, the antigen is altered by contact with an antibody, or fragment or derivative thereof, such as an F(ab)'2 fragment. Alteration of the antigen on the surface of the cells prior to administration inhibits immunological rejection of the cells and avoids the need for systemic immunosuppression of a subject.

Description: GENETICALLY MODIFIED CELLS FOR USE IN TRANSPLANTATION Background of the Invention The elucidation of the molecular basis for many inherited disorders together with the molecular isolation of the genes involved in the : =c. disorders now offer the potential for therapeutic treatments based upon providing a functional gene product to a patient having a defect in that gene product. Gene therapy, in which a gene encoding a functional gene product is introduced into cells of a patient to restore the activity of that gene product in the patient, is now a realistic option for many congenital diseases. Two patients with adenosine deaminase deficiency have already been treated for their disease by gene therapy, with encouraging results, and a number of other human gene therapy protocols have received approval for limited clinical use. Cystic fibrosis, Duchenne muscular dystrophy and hemophilia are just a few of the inherited diseases which are potentially treatable by gene therapy. Furthermore, gene therapy approaches are being applied to acquired disorders as well, for example by introducing into cells of a patient genes encoding gene products which enhance the responsiveness of the patient's immune system. Novel approaches to treating diseases such as cancer and AIDS are thus also possible by applying the principles of gene therapy. For reviews on gene therapy approaches see Anderson, W. F. (1992) Science 256: 808-813; Miller, A. D. (1992) Nature 357: 455-460; Friedmann, T. (1989) Science 244: 1275-1281; and Cournoyer, D., et al. (1990) Curr. Opin. Biotech. 1: 196-208. The general approach of gene therapy involves the introduction of exogenous genetic material (e. g., DNA or RNA) into a cell such that one or more gene products encoded by the introduced genetic material are produced in the cell, for example to restore or enhance a functional activity. Exogenous DNA has been successfully introduced into cells both ex vivo (i. e., in vitro) and in vivo. In recent years, many advances in gene therapy have been reported that address problems relating to the types of gene delivery systems that can be used, the different types of genes which can be introduced into cells and the kinds of cells which can be modified. However, gene therapy is still limited by the need to modify autologous cells. A patient's own cells must be modified because foreign cells, whether they are from the same species (allogeneic) or another species (xenogeneic), are recognized as foreign by the patient's immune system when introduced into the patient and are subsequently rejected. Thus, in the case of ex vivo gene therapy, cells must first be harvested from the patient, modified in culture and then reintroduced into the patient. This procedure is both time consuming to complete and invasive for the patient. While the ability to modify some cells in vivo may overcome some of these problems, certain cell types may not be accessible for modification in vivo, or may not be targeted specifically or efficiently modified in vivo. Furthermore, the cell modification procedure, whether performed ex vivo or in vivo, must be repeated for each individual patient. As a means of gene therapy, it would be beneficial to provide a patient with heterologous donor cells that have been modified to express a gene product. This would obviate the need to harvest cells from the patient for genetic modification, thereby reducing both the time and invasiveness of the procedure. In addition, a ready supply of modified donor cells that cc-l1d be cryopreserved and available for introduction into one, or multiple, patients could be prepared for use as needed. The ability to use modified heterologous donor cells from either the same species as the patient or from a different species would also expand the source of cells which could be used for gene therapy. However, transplantation of heterologous cells (i. e., allogeneic or xenogeneic cells) into a host elicits an immune response against the cells. Thus, use of modified heterologous cells for therapeutic purposes requires a means by which to avoid immunological rejection of the modified cells by the patient. Current approaches toward inhibiting immune responses against transplanted cells typically involve systemic treatment of the patient, for example with immunosuppressive agents. This has the disadvantage that the patient exhibits nonspecific immunosuppression. Additionally, immunosuppressive drugs are known to have side effects that include an increased susceptibility to infections, renal failure, hypertension and tumor growth. Thus, there is a need for an improved method which allows the use of heterologous donor cells for gene therapy purposes which avoids the detrimental effects of systemic immunosuppressants. Summarv of the Invention This invention provides a means by which allogeneic cells or xenogeneic cells are used to deliver a gene product to an individual without the need for systemic immunosuppression of the individual. The invention features cells which are modified to express a gene product and which have an antigen on the surface of the cell altered to inhibit rejection of the cell when the cell is transplanted into a subject. Prior to alteration, the antigen on the cell surface stimulates an immune response against the cell in the subject. However, the antigen on the cell surface is altered to inhibit immunological rejection of the cells by the subject. Specifically, the antigen is altered to modify an interaction between the antigen and a hematopoietic cell, preferably a T lymphocyte, in the subject. Since the antigen on the cell surface is altered prior to transplantation, the recipient subject does not require systemic treatment with an immunosuppressive agent to prevent rejection of the cell. Thus, this invention permits the use of allogeneic or xenogeneic cells as vehicles for delivery of a gene product to a subject. Moreover, this invention allows for the preparation of genetically modified cells which can be cryopreserved and administered to a subject when necessary, thereby circumventing the need to isolate and modify autologous cells from the subject. According to the invention, a cell is modified to express a gene product by, for example, introducing into the cell a nucleic acid having a nucleotide sequence which encodes the gene product in a form suitable for expression of the gene product in the cell. In a preferred embodiment, the nucleic acid encoding the gene product is introduced into the cell using a recombinant viral vector, such as an adenoviral vector, an adeno-associated viral vector or a retroviral vector. The gene product can be, for example, a secreted protein, a membrane-bound protein or an intracellular protein. Other gene products include active RNA molecules. In one embodiment of the invention, an antigen on the surface of the cell is altered by contacting the cell prior to transplantation (i. e., in vitro) with a molecule which binds to the antigen. A preferred molecule for altering an antigen on the cell is an antibody, or fragment or derivative thereof, such as an F (ab') 2 fragment. Alternatively, the molecule is a peptide or derivative thereof (e. g., a peptide mimetic) which binds the antigen and interferes with an interaction with a hematopoietic cell. In a preferred embodiment, the antigen on the cell surface which is altered is an MHC class I antigen. Other cell surface antigens which can be altered include adhesion molecules such as LFA-1, ICAM-1 and ICAM-2. In one embodiment of the invention, a non-human cell is modified to express a human gene product. A preferred non-human cell for use in providing a human gene product to a subject is a porcine cell. Cell types which are modified according to the invention include muscle cells, liver cells, neural cells, pancreatic islet cells and hematopoietic cells. Furthermore, the cell which is modified can be within a tissue or organ. The modified cells of the invention are administered to subjects to deliver a gene product expressed by the cell to the subject. Prior to administering the cell to the subject, one or more antigens on the cell surface are altered, e. g. by contacting the cell in vitro with a molecule which binds to the antigen. Although a cell can be modified to express a gene product in vivo, it is preferred that the cell is modified ex vivo, prior to administering the cell to the subject. The invention further provides kits for use in delivering a gene product to a subject which include a cell modified to express the gene product and a molecule (e. g., an antibody, or fragment or derivative thereof) which binds to an antigen on the cell. Alternatively, the kit includes a vector encoding a gene product with which to modify a cell and a molecule (e. g., an antibody, or fragment or derivative thereof) which binds to an antigen on the cell surface. Brief Description of the Drawings Figure 1 depicts the plasmid map of pCMVGH, which contains the gene encoding human growth hormone. Figure 2 depicts a p-galactosidase stain of cultured human myotubes transfected with plasmids pCMVss, which contains a gene encoding p-galactosidase, and pJK2Neo, which displays neomycin resistance. Figure 3 depicts a p-galactosidase stain of human myoblasts transfected with plasmids pCMVss, which contains a gene encoding p-galactosidase, and pJK2Neo, which displays neomycin resistance, which were transplanted into rat muscle. Figure 4 depicts a rabbit anti-desmin stain of human myoblasts modified with F (ab') 2 fragments of the monoclonal antibody W6/32 and transplanted into cyclosporin-treated mice. Detailed Description of the Invention This invention provides modified heterologous cells and methods for delivering a gene product to an allogeneic or xenogeneic subject without eliciting an immune response against the cell in the subject. The invention features a heterologous cell which is modified to express a gene product and which is treated such that an antigen on the cell surface which stimulates an immune response against the cell in an allogeneic or xenogeneic subject is altered to inhibit rejection of the cell when transplanted into the subject. Preferably, the antigen which is altered is an antigen which interacts with a T lymphocyte in an allogeneic or xenogeneic subject. Typically, the heterologous cell is treated to alter the antigen on its surface prior to administering the cell to the subject. Thus, it is not necessary to treat the subject systemically with an immunosuppressive agent to prevent rejection of the heterologous cell. Rather, following administration of the heterologous cell (which has been altered as described herein), the subject exhibits immunological non-responsiveness specific for the cell. Preferably, the heterologous cell is also modified to express a gene product prior to administering the cell to a subject (i. e., ex vivo). However, the cell can be modified to express a gene product in vivo, following administration of the cell to the subject. This invention enables gene therapy approaches to be extended to the use allogeneic and xenogeneic cells as donor cells to deliver a gene product to a subject. Since both the procedure to modify a heterologous cell to express a gene product and the procedure to alter an antigen on the surface of the cell can be perfomed ex vivo, prior to administering the cell to a subject, the invention allows for the preparation of genetically modified allogeneic or xenogeneic cells which can be cryopreserved and stored until use. When the cells are needed by a subject, the already modified cells can be thawed, treated to alter an antigen on their surface and immediately administered to the subject. Thus, a subject in need of gene therapy can receive immediate treatment, rather than having to wait until his or her own cells can be isolated, successfully modified and reintroduced. Additionally, if retreatment is necessary, a second aliquot of already modified cells can easily be thawed, altered and readministered. Moreover, the ability to use heterologous cells for gene therapy purposes greatly extends the supply of donor cells which can be used in this type of treatment. Accordingly, this invention provides a heterologous cell which is modified to express a gene product and which has an antigen on the cell surface altered to modify an interaction between the antigen and a hematopoietic cell (e. g., a T lymphocyte). The invention further provides methods for delivering a gene product to a subject by administering cells of the invention. The following subsections describe in detail: 1) the modification of a heterologous cell to express a gene product ; and 2) the alteration of an antigen on the cell to modify an interaction between the antigen and a hematopoietic cell. I. Modification of a Cell to Express a Gene Product A cell of the invention is"modified to express a gene product". As used herein, the term"modified to express a gene product"is intended to mean that the cell is treated in a manner that results ir the production of a gene product by the cell. Preferably, the cell does not express the gene product prior to modification. Alternatively, modification of the cell may result in an increased production of a gene product already expressed by the cell or result in production of a gene product (e. g., an antisense RNA molecule) which decreases production of another, undesirable gene product normally expressed by the cell. In a preferred embodiment, a cell is modified to express a gene product by introducing genetic material, such as a nucleic acid molecule (e. g., RNA or, more preferably, DNA) into the cell. The nucleic acid molecule introduced into the cell encodes a gene product to be expressed by the cell. The term"gene product"as used herein is intended to include proteins, peptides and functional RNA molecules. Generally, the gene product encoded by the nucleic acid molecule is the desired gene product to be supplied to a subject. Alternatively, the encoded gene product is one which induces the expression of the desired gene product by the cell (e. g., the introduced genetic material encodes a transcription factor which induces the transcription of the gene product to be supplied to the subject). A nucleic acid molecule introduced into a cell is in a form suitable for expression in the cell of the gene product encoded by the nucleic acid. Accordingly, the nucleic acid molecule includes coding and regulatory sequences required for transcription of a gene (or portion thereof) and, when the gene product is a protein or peptide, translation of the gene product encoded by the gene. Regulatory sequences which can be included in the nucleic acid molecule include promoters, enhancers and polyadenylation signals, as well as sequences necessary for transport of an encoded protein or peptide, for example N-terminal signal sequences for transport of proteins or peptides to the surface of the cell or for secretion. Nucleotide sequences which regulate expression of a gene product (e. g., promoter and enhancer sequences) are selected based upon the type of cell in which the gene product is to be expressed and the desired level of expression of the gene product. For example, a promoter known to confer cell-type specific expression of a gene linked to the promoter can be used. A promoter specific for myoblast gene expression can be linked to a gene of interest to confer muscle-specific expression of that gene product. Muscle-specific regulatory elements which are known in the art include upstream regions from the dystrophin gene (Klamut et al., (1989) Mol. Cell. Biol. 9: 2396), the creatine kinase gene (Buskin and Hauschka, (1989) Mol. Cell Biol. 9: 2627) and the troponin gene (Mar and Ordahl, (1988) Proc. Natl. Acad. Sci. USA. 85: 6404). Regulatory elements specific for other cell types are known in the art (e. g., the albumin enhancer for liver-specific expression; insulin regulatory elements for pancreatic islet cell-specific expression; various neural cell-specific regulatory elements, including neural dystrophin, neural enolase and A4 amyloid promoters). Alternatively, a regulatory element which can direct constitutive expression of a gene in a variety of different cell types, such as a viral regulatory element, can be used. Examples of viral promoters commonly used to drive gene expression include those derived from polyoma virus, Adenovirus 2, cytomegalovirus and Simian Virus 40, and retroviral LTRs. Alternatively, a regulatory element which provides inducible expression of a gene Hjked thereto can be used. The use of an inducible regulatory element (e. g., an inducible promoter) allows for modulation of the production of the gene product in the cell. Examples of potentially useful inducible regulatory systems for use in eukaryotic cells include hormoneregulated elements (e. g., see Mader, S. and White, J. H. (1993) Proc. Natl. Acad. Sci. USA 90: 5603-5607), synthetic ligand-regulated elements (see, e. g. Spencer, D. M. et al. (1993) Science 262: 1019-1024) and ionizing radiation-regulated elements (e. g., see Manome, Y. et al. (1993) Biochemistry 32: 10607-10613; Datta, R. et al. (1992) Proc. Natl. Acad. Sci. USA 89: 10149-10153). Additional tissue-specific or inducible regulatory systems which may be developed can also be used in accordance with the invention. There are a number of techniques known in the art for introducing genetic material into a cell that can be applied to modify a cell of the invention. In one embodiment, the nucleic acid is in the form of a naked nucleic acid molecule. In this situation, the nucleic acid molecule introduced into a cell to be modified consists only of the nucleic acid encoding the gene product and the necessary regulatory elements. Alternatively, the nucleic acid encoding the gene product (including the necessary regulatory elements) is contained within a plasmid vector. Examples of plasmid expression vectors include CDM8 (Seed, B., Nature 329: 840 (1987)) and pMT2PC (Kaufman, et al., EMBO J. 6: 187-195 (1987)). In another embodiment, the nucleic acid molecule to be introduced into a cell is contained within a viral vector. In this situation, the nucleic acid encoding the gene product is inserted into the viral genome (or a partial viral genome). The regulatory elements directing the expression of the gene product can be included with the nucleic acid inserted into the viral genome (i. e, linked to the gene inserted into the viral genome) or can be provided by the viral genome itself. Examples of methods which can be used to introduce naked nucleic acid into cells and viralmediated transfer of nucleic acid into cells are described separately in the subsections below. A. Introduction of Naked Nucleic Acid into Cells 1. Transfection mediated by CaP04 : Naked DNA can be introduced into cells by forming a precipitate containing the DNA and calcium phosphate. For example, a HEPES-buffered saline solution can be mixed with a solution containing calcium chloride and DNA to form a precipitate and the precipitate is then incubated with cells. A glycerol or dimethyl sulfoxide shock step can be added to increase the amount of DNA taken up by certain cells. CaP04mediated transfection can be used to stably (or transiently) transfect cells and is only applicable to in vitro modification of cells. Protocols for CaPO4-mediated transfection can be found in Current Protocols in Molecular Biology, Ausubel, F. M. et al. (eds.) Greene Publishing Associates, (1989), Section 9.1 and in Molecular Cloning: A Laboratory Manual. 2nd Edition. Sambrook et al. Cold Spring Harbor Laboratory Press, (1989), Sections 16.3216.40 or other standard laboratory manuals. 2. Transfection mediated b DEAE-dextran: Naked DNA can be introduced into cells by forming a mixture of the DNA and DEAE-dextran and incubating the mixture with the cells. A dimethylsulfoxide or chloroquine shock step can be added to increase the amount of DNA uptake. DEAE-dextran transfection is only applicable to in vitro modification of cells and can be used to introduce DNA transiently into cells but is not preferred for creating stably transfected cells. Thus, this method can be used for short term production of a gene product but is not a method of choice for long-term production of a gene product. Protocols for DEAE-dextran-mediated transfection can be found in Current Protocols in Molecular Biology, Ausubel, F. M. et al. (eds.) Greene Publishing Associates, (1989), Section 9.2 and in Molecular Cloning: A Laboratory Manual. 2nd Edition, Sambrook et al. Cold Spring Harbor Laboratory Press, (1989), Sections 16.41-16.46 or other standard laboratory manuals. 3. Electroporation: Naked DNA can also be introduced into cells by incubating the cells and the DNA together in an appropriate buffer and subjecting the cells to a high-voltage electric pulse. The efficiency with which DNA is introduced into cells by electroporation is influenced by the strength of the applied field, the length of the electric pulse, the temperature, the conformation and concentration of the DNA and the ionic composition of the media. Electroporation can be used to stably (or transiently) transfect a wide variety of cell types and is only applicable to in vitro modification of cells. Protocols for electroporating cells can be found in Current Protocols in Molecular Biology. Ausubel, F. M. et al. (eds.) Greene Publishing Associates, (1989), Section 9.3 and in Molecular Cloning: A Laboratory Manual, 2nd Edition. Sambrook et al. Cold Spring Harbor Laboratory Press, (1989), Sections 16.54-16.55 or other standard laboratory manuals. 4. Liposome-mediated transfection ("lipofection'): Naked DNA can be introduced into cells by mixing the DNA with a liposome suspension containing cationic lipids. The DNA/liposome complex is then incubated with cells. Liposome mediated transfection can be used to stably (or transiently) transfect cells in culture in vitro. Protocols can be found in Current Protocols in Molecular Biology, Ausubel, F. M. et al. (eds.) Greene Publishing Associates, (1989), Section 9.4 and other standard laboratory manuals. Additionally, gene delivery in vivo has been accomplished using liposomes. See for example Nicolau et al. (1987) Meth. Enz. 149: 157-176 ; Wang and Huang (1987) Proc. Natl. Acad. Sci. USA 84: 7851-7855; Brigham et al. (1989) Am. J. Med. Sci. 298: 278; and Gould-Fogerite et al. (1989) Gene 84: 429-438. 5. Direct Injection: Naked DNA can be introduced into cells by directly injecting the DNA into the cells. For an in v itro culture of cells, DNA can be introduced by microinjection. Since each cell is microinjected individually, this approach is very labor intensive when modifying large numbers of cells. However, a situation wherein microinjection is a method of choice is in the production of transgenic animals (discussed in greater detail below). In this situation, the DNA is stably introduced into a fertilized oocyte which is then allowed to develop into an animal. The resultant animal contains cells carrying the DNA introduced into the oocyte. Direct injection has also been used to introduce naked DNA into cells in vivo (see e. g., Acsadi et al. (1991) Nature 332: 815-818; Wolff et al. (1990) Science 247: 1465-1468). A delivery apparatus (e. g., a"gene gun") for injecting DNA into cells in vivo can be used. Such an apparatus is commercially available (e. g., from BioRad). 6. Receptor-Mediated DNA Uptake: Naked DNA can also be introduced into cells by complexing the DNA to a cation, such as polylysine, which is coupled to a ligand for a cellsurface receptor (see for example Wu, G. and Wu, C. H. (1988) J. Biol. Chem. 263: 14621; Wilson et al. (1992) J. Biol. Chem. 267: 963-967; and U. S. Patent No. 5,166,320). Binding of the DNA-ligand complex to the receptor facilitates uptake of the DNA by receptor-mediated endocytosis. Receptors to which a DNA-ligand complex have targeted include the transferrin receptor and the asialoglycoprotein receptor. A DNA-ligand complex linked to adenovirus capsids which naturally disrupt endosomes, thereby releasing material into the cytoplasm can be used to avoid degradation of the complex by intracellular lysosomes (see for example Curiel et al. (1991) Proc. Natl. Acad. Sci. USA 88: 8850; Cristiano et al. (1993) Proc. Natl. Acad. Sci. USA 90: 2122-2126). Receptor-mediated DNA uptake can be used to introduce DNA into cells either in vitro or in vivo and, additionally, has the added feature that DNA can be selectively targeted to a particular cell type by use of a ligand which binds to a receptor selectively expressed on a target cell of interest. Generally, when naked DNA is introduced into cells in culture (e. g., by one of the transfection techniques described above) only a small fraction of cells (about 1 out of 105) typically integrate the transfected DNA into their genomes (i. e., the DNA is maintained in the cell episomally). Thus, in order to identify cells which have taken up exogenous DNA, it is advantageous to transfect nucleic acid encoding a selectable marker into the cell along with the nucleic acid (s) of interest. Preferred selectable markers include those which confer resistance to drugs such as G418, hygromycin and methotrexate. Selectable markers may be introduced on the same plasmid as the gene (s) of interest or may be introduced on a separate plasmid. An alternative method for generating a cell that is modified to express a gene product involving introducing naked DNA into cells is to create a transgenic animal which contains cells modified to express the gene product of interest. A transgenic animal is an animal having cells that contain a transgene, wherein the transgene was introduced into the animal or an ancestor of the animal at a prenatal, e. g., an embryonic stage. A transgene is a DNA k molecule which is integrated into the genome of a cell from which a transgenic animal develops and which remains in the genome of the mature animal, thereby directing the expression of an encoded gene product in one or more cell types or tissues of the transgenic animal. Thus, a transgenic animal expressing a gene product of interest in one or more cell types within the animal can be created, for example, by introducing a nucleic acid encoding the gene product (typically linked to appropriate regulatory elements, such as a tissue-specific enhancer) into the male pronuclei of a fertilized oocyte, e. g., by microinjection, and allowing the oocyte to develop in a pseudopregnant female foster animal. Methods for generating transgenic animals, particularly animals such as mice, have become conventional in the art and are described, for example, in U. S. Patent Nos. 4,736,866 and 4,870,009 and Hogan, B. et al., (1986) A Laboratory Manual, Cold Spring Harbor, New York, Cold Spring Harbor Laboratory. A transgenic founder animal can be used to breed more animals carrying the transgene. Cells of the transgenic animal which express a gene product of interest can then be used to deliver the gene product to a subject in accordance with the invention. Alternatively, an animal containing a gene which has been modified by homologous recombination can be constructed to express a gene product of interest. For example, an endogenous gene carried in the genome of the animal can be altered by homologous recombination (for instance, all or a portion of a gene could be replaced by the human homologue of the gene to"humanize"the gene product encoded by the gene) or an endogenous gene can be"knocked out" (i. e., inactivated by mutation). For example, an endogenous gene in a cell can be knocked out to prevent production of that gene product and then nucleic acid encoding a different (preferred) gene product is introduced into the cell. To create an animal with homologously recombined nucleic acid, a vector is prepared which contains the DNA which is to replace or interrupt the endogenous DNA flanked by DNA homologous to the endogenous DNA (see for example Thomas, K. R. and Capecchi, M. R. (1987) Cell 51 : 503). The vector is introduced into an embryonal stem cell line (e. g., by electroporation) and cells which have homologously recombined the DNA are selected (see for example Li, E. et al. (1992) Cell 69: 915). The selected cells are then injected into a blastocyst of an animal (e. g., a mouse) to form aggregation chieras (see for example Bradley, A. in Teratocarcinomas and Embryonic Stem Cells : A Practical Approach, E. J. Robertson, ed. (IRL, Oxford, 1987) pp. 113-152). A chimeric embryo can then be implanted into a suitable pseudopregnant female foster animal and the embryo brought to term. Progeny harbouring the homologously recombined DNA in their germ cells can be used to breed animals in which all cells of the animal contain the homologously recombined DNA. Cells of the animal containing the homologously recombined DNA which express a gene product of interest can then be used to deliver the gene product to a subject in accordance with the invention. B. Viral-Mediated Gene Transfer A preferred approach for introducing nucleic acid encoding a gene product into a cell is by use of a viral vector containing nucleic acid, e. g. a cDNA, encoding the gene product. Infection of cells with a viral vector has the advantage that a large proportion of cells receive the nucleic acid, which can obviate the need for selection of cells which have received the nucleic acid. Additionally, molecules encoded within the viral vector, e. g., by a cDNA contained in the viral vector, are expressed ef it) to be integrated into the host genome to stably introduce nucleic acid into the cell. Thus, it may be necessary to stimulate replication of the target cell. 2. Adenoviruses: The genome of an adenovirus can be manipulated such that it encodes and expresses a gene product cr ; nterest but is inactivated in terms of its ability to replicate in a normal lytic viral life cycle. See for example Berner et al. (1988) BioTechniques 6: 616; Rosenfeld et al. (1991) Science 252: 431-434; and Rosenfeld et al. (1992) Cell 68: 143-155. Suitable adenoviral vectors derived from the adenovirus strain Ad type 5 dl324 or other strains of adenovirus (e. g., Ad2, Ad3, Ad7 etc.) are well known to those skilled in the art. Recombinant adenoviruses are advantageous in that they do not require dividing cells to be effective gene delivery vehicles and can be used to infect a wide variety of cell types, including airway epithelium (Rosenfeld et al. (1992) cited supra), endothelial cells (Lemarchand et al. (1992) Proc. Natl. Acad. Sci. USA 89: 6482-6486), hepatocytes (Herz and Gerard (1993) Proc. Natl. Acad. Sci. USA 90: 2812-2816) and muscle cells (Quantin et al. (1992) Proc. Natl. Acad. Sci. USA 89: 2581-2584). Additionally, introduced adenoviral DNA (and foreign DNA contained therein) is not integrated into the genome of a host cell but remains episomal, thereby avoiding potential problems that can occur as a result of insertional mutagenesis in situations where introduced DNA becomes integrated into the host genome (e. g., retroviral DNA). Moreover, the carrying capacity of the adenoviral genome for foreign DNA is large (up to 8 kilobases) relative to other gene delivery vectors (Berkner et al. cited supra; Haj-Ahmand and Graham (1986) J. Virol. 57: 267). Most replication-defective adenoviral vectors currently in use are deleted for all or parts of the viral E1 and E3 genes but retain as much as 80 % of the adenoviral genetic material. 3. Adeno-Associated Viruses: Adeno-associated virus (AAV) is a naturally occurring defective virus that requires another virus, such as an adenovirus or a herpes virus, as a helper virus for efficient replication and a productive life cycle. (For a review see Muzyczka et al. Curr. Topics in Micro. and Immunol. (1992) 158: 97-129). It is also one of the few viruses that may integrate its DNA into non-dividing cells, and exhibits a high frequency of stable integration (see for example Flotte et al. (1992) Am. J. Respir. Cell. Mol. Biol. 7: 349-356; Samulski et al. (1989) J Virol. 63: 3822-3828; and McLaughlin et al. (1989) J. Virol. 62: 1963-1973). Vectors containing as little as 300 base pairs of AAV can be packaged and can integrate. Space for exogenous DNA is limited to about 4.5 kb. An AAV vector such as that described in Tratschin et al. (1985) Mol. Cell. Biol. 5: 3251-3260 can be used to introduce DNA into cells. A variety of nucleic acids have been introduced into different cell types using AAV vectors (see for example Hermonat et al. (1984) Proc. Natl. Acad. Sci. USA 81: 6466-6470; Tratschin et al. (1985) Mol. Cell. Biol. 4: 2072-2081; Wondisford et al. (1988) Mol. Endocrinol. 2: 32-39; Tratschin et al. (1984) J. Virol. 51: 611-619; and Flotte et al. (1993) J. Biol. Chem. 268: 3781-3790). The efficacy of a particular expression vector system and method of introducing nucleic acid into a cell can be assessed by standard approaches routinely used in the art. For example, DNA introduced into a cell can be detected by a filter hybridization technique (e. g., Southern blotting) and RNA produced by transcription of introduced DNA can be detected, for example, by Northern blotting, RNase protection or reverse transcriptase-polymerase chain reaction (RT-PCR). The gene product can be detected by an appropriate assay, for example by immunological detection of a produced protein, such as with a specific antibody, or by a functional assay to detect a functional activity of the gene product, such as an enzymatic assay. If the gene product of interest to be expressed by a cell is not readily assayable, an expression system can first be optimized using a reporter gene linked to the regulatory elements and vector to be used. The reporter gene encodes a gene product which is easily detectable and, thus, can be used to evaluate the efficacy of the system. Standard reporter genes used in the art include genes encoding p-glactosidase, chloramphenicol acetyl transferase, luciferase and human growth hormone. When the method used to introduce nucleic acid into a population of cells results in modification of a large proportion of the cells and efficient expression of the gene product by the cells (e. g., as is often the case when using a viral expression vector), the modified population of cells may be used without further isolation or subcloning of individual cells within the population. That is, there may be sufficient production of the gene product by the population of cells such that no further cell isolation is needed. Alternatively, it may be desirable to grow a homogenous population of identically modified cells from a single modified cell to isolate cells which efficiently express the gene product. Such a population of uniform cells can be prepared by isolating a single modified cell by limiting dilution cloning followed by expanding the single cell in culture into a clonal population of cells by standard techniques. C. Other Methods for Modifying a Cell to Express a Gene Product Alternative to introducing a nucleic acid molecule into a cell to modify the cell to express a gene product, a cell can be modified by inducing or increasing the level of expression of the gene product by a cell. For example, a cell may be capable of expressing a particular gene product but fails to do so without additional treatment of the cell. Similarly, the cell may express insufficient amounts of the gene product for the desired purpose. Thus, an agent which stimulates expression of a gene product can be used to induce or increase expression of a gene product by the cell. For example, cells can be contacted with an agent in vitro in a culture medium. The agent which stimulates expression of a gene product may function, for instance, by increasing transcription of the gene encoding the product, by increasing the rate of translation or stability (e. g., a post transcriptional modification such as a poly A tail) of an mRNA encoding the product or by increasing stability, transport or localization of the gene product. Examples of agents which can be used to induce expression of a gene product include cytokines and growth factors. Another type of agent which can be used to induce or increase expression of a gene product by a cell is a transcription factor which upregulates transcription of the gene encoding the product. A transcription factor which upregulatef the expression of a gene encoding a gene product of interest can be provided to a cell, for example, by introducing into the cell a nucleic acid molecule encoding the transcription factor. Thus, this approach represents an alternative type of nucleic acid molecule which can be introduced into the cell (for example by one of the previously discussed methods). In this case, the introduced nucleic acid does not directly encode the gene product of interest but rather causes production of the gene product by the cell indirectly by inducing expression of the gene product. In yet another method, a cell is modified to express a gene product by coupling the gene product to the cell, preferably to the surface of the cell. For example, a protein can be obtained by purifying the cell from a biological source or expressing the protein recombinantly using standard recombinant DNA technology. The isolated protein can then be coupled to the cell. The terms"coupled"or"coupling"refer to a chemical, enzymatic or other means (e. g., by binding to an antibody on the surface of the cell or genetic engineering of linkages) by which a gene product can be linked to a cell such that the gene product is in a form suitable for delivering the gene product to a subject. For example, a protein can be chemically crosslinked to a cell surface using commercially available crosslinking reagents (Pierce, Rockford IL). Other approaches to coupling a gene product to a cell include the use of a bispecific antibody which binds both the gene product and a cell-surface molecule on the cell or modification of the gene product to include a lipophilic tail (e. g., by inositol phosphate linkage) which can insert into a cell membrane. II. Alteration of an Antigen on the Cell In addition to modification of a cell to express a gene product, this invention involves altering an antigen on the cell surface to reduce the immunogenicity of the cell and thereby inhibit rejection of the cell when transplanted into an allogeneic or xenogeneic subject. In an unaltered state, the antigen on the cell surface stimulates an immune response against the cell (also referred to herein as the donor cell) when the cell is administered to a subject (also referred to herein as the recipient or host). By altering the antigen, the normal immunological recognition of the donor cell by the immune system cells of the recipient is disrupted and additionally,"abnormal"immunological recognition of this altered form of the antigen can lead to donor cell-specific long term unresponsiveness in the recipient. Thus, alteration of an antigen on the donor cell prior to administering the cell to a recipient interferes with the initial phase of recognition of the donor cell by the cells of the host's immune system subsequent to administration of the cell. Furthermore, alteration of the antigen may induce immunological nonresponsiveness or tolerance, thereby preventing the induction of the effector phases of an immune response (e. g., cytotoxic T cell generation, antibody production etc.) which are ultimately responsible for rejection of foreign cells in a normal immune response. As used herein, the term"altered"encompasses changes that are made to a donor cell antigen which reduce the immunogenicity of the antigen to thereby interfere with immunological recognition of the antigen by the recipient's immune system. Preferably, immunological nonresponsiveness to the donor cells in the recipient subject is generated as a result of alteration of the antigen. The term altered is not intended to include complete elimination of the antigen on the donor cell since delivery of an inappropriate or insufficient signal to the host's immune cells (e. g., T lymphocytes) may be necessary to achieve immunological nonresponsiveness. Antigens to be altered according to this invention include antigens on a donor cell which can interact with a hematopoietic cell in an allogeneic or xenogeneic recipient and thereby stimulate a specific immune response against the donor cell in the recipient The interaction between the antigen and the hematopoietic cell may be an indirect interaction (e. g., mediated by soluble factors which induce a response in the hematopoietic cell) or, more preferably, is a direct interaction between the antigen and a molecule present on the surface of the hematopoietic cell. As used herein, the term hematopoietic cell is intended to include T lymphocytes, B lymphocytes, monocytes and other antigen presenting cells. Preferably, the antigen to be altered is one which interacts with a T lymphocyte in the recipient (e. g., the antigen normally binds to a receptor on the surface of a T lymphocyte). In a preferred embodiment, the antigen on the donor cell to be altered is an MHC class I antigen. MHC class I antigens are present on almost all cell types. In a normal immune response, self MHC molecules function to present antigenic peptides to a T cell receptor (TCR) on the surface of self T lymphocytes. In immune recognition of allogeneic or xenogeneic cells, foreign MHC antigens (most likely together with a peptide bound thereto) on donor cells are recognized by the T cell receptor on host T cells to thereby elicit an immune response. MHC class I antigens on a donor cell are altered to interfere with their recognition by T cells in an allogeneic or xenogeneic host (e. g., a portion of the MHC class I antigen which is normally recognized by the T cell receptor is blocked or"masked"such that normal recognition of the MHC class I antigen fails to occur). Additionally, an altered form of an MHC class I antigen which is exposed to host T cells (i. e., available for presentation to the host T cell receptor) may deliver an inappropriate or insufficient signal to the host T cell such that, rather than stimulating an immune response against the allogeneic or xenogeneic cell, donor cell-specific T cell non-responsiveness is induced. For example, it is known that T cells which receive an inappropriate or insufficient signal through their T cell receptor (e. g., by binding to an MHC antigen in the absence of a costimulatory signal, such as that provided by B7) become anergic rather than activated and remain refractory to restimulation for long periods of time (see for example Damle et al. (1981) Proc. Natl. Acad. Sci. USA 78: 5096 5100; Lesslauer et al. (1986) Eur. J. Immunol. 16: 1289-1295; Gimmi, et al. (1991) Proc. Natl. Acad. Sci. USA 88: 6575-6579; Linsley et al. (1991) J Exp. Med. 173: 721-730; Koulova et al. (1991) J. Exp. Med. 173: 759-762; Razi-Wolf, et al. (1992) Proc. Natl. ilcad. Sci. USA 89: 4210-4214). Alternative to MHC class I antigens, the antigen to be altered on a donor cell can be z-v.. MHC class II antigen. Similar to MHC class I antigens, MHC class II antigens function o present antigenic peptides to a T cell receptor on T lymphocytes. However, MHC class II antigens are present on a limited number of cell types (primarily B cells, macrophages, dendritic cells, Langerhans cells and thymic epithelial cells). In addition to or alternative to MHC antigens, other antigens on a donor cell which interact with molecules on host T cells and which are known to be involved in immunological rejection of allogeneic or xenogeneic cells can be altered. Other donor cell antigens known to interact with host T cells and contribute to rejection of a donor cell include molecules which function to increase the avidity of an interaction between a donor cell and a host T cell. Due to this property, these molecules are typically referred to as adhesion molecules (although they may serve other functions in addition to increasing the adhesion between a donor cell and a host T cell). Examples of preferred adhesion molecules which can be altered according to the invention include LFA-3 and ICAM-1. These molecules are ligands for the CD2 and LFA-1 receptors, respectively, on T cells. By altering an adhesion molecule on the donor cell, (such as LFA-3, ICAM-1 or a similarly functioning molecule), the ability of the host's T cells to bind to and interact with the donor cell is reduced. Both LFA-3 and ICAM-1 are found on endothelial cells within blood vessels in transplanted organs such as kidney and heart. Altering these antigens may facilitate transplantation of any vascularized implant, by altering recognition of those antigens by CD2+ and LFA-1+ host T-lymphocytes. The presence of MHC molecules or adhesion molecules such as LFA-3, ICAM-1 etc. on a particular donor cell can be assessed by standard procedures known in the art. For example, the donor cell can be reacted with a labeled antibody directed against the molecule to be detected (e. g., MHC molecule, ICAM-1, LFA-1 etc.) and the association of the labeled antibody with the cell can be measured by a suitable technique (e. g., immunohistochemistry, flow cytometry etc.). A preferred method for altering an antigen on a donor cell to inhibit an immune response against the cell is to contact the cell with a molecule which binds to the antigen on the cell surface. It is preferred that the cell be contacted with the molecule which binds to the antigen to be altered prior to administering the cell to a recipient (i. e., the cell is contacted with the molecule in vitro). For example, the cell can be incubated with the molecule which binds the antigen under conditions which allow binding of the molecule to the antigen and then any unbound molecule can be removed (such as described in the Examples below). Following administration of the modified cell to a recipient, the molecule remains bound to the antigen on the cell for a sufficient time to interfere with immunological recognition by host cells and induce non-responsiveness in the recipient. Preferably, the molecule for binding to an antigen on a donor cell is an antibody, or fragment or derivative thereof which retains the ability to bind to the antigen. For use in therapeutic applications, it is necessary that the antibody which binds the antigen to be altered be unable to fix complement, thus preventing donor cell lysis. Antibody complement fixation can be prevented by deletion of an Fc portion of an antibody, by using an antibody isotype which is not capable of fixing complement, or, less preferably, by using a complement fixing antibody in conjunction with a drug which inhibits complement fixation. Alternatively, amino acid residues within the Fc region of an antibody which are important for activating complement (see e. g., Tan et al. (1990) Proc. Natl. Acad. Sci. USA 87: 162-166 ; Duncan and Winter (1988) Nature 332: 738-740) can be mutated to reduce or eliminate the complementactivating ability of an intact antibody. Likewise, amino acids residues within the Fc region of an antibody which are necessary for binding of the Fc region to Fc receptors (see e. g. Canfield, S. M. and S. L. Morrison (1991) J Exp. Med. 173: 1483-1491 ; andLund, J. etal. (1991) J : Immunol. 147: 2657-2662) can also be mutated to reduce or eliminate Fc receptor binding if an intact antibody is to be used. A preferred antibody fragment for altering an antigen is an F (ab') 2 fragment. Antibodies can be fragmented using conventional techniques. For example, the Fc portion of an antibody can be removed by treating an intact antibody with pepsin, thereby generating an F (ab') 2 fragment. In a standard procedure for generating F (ab') 2 fragments, intact antibodies are incubated with immobilized pepsin and the digested antibody mixture is applied to an immobilized protein A column. The free Fc portion binds to the column while the F (ab') 2 fragments passes through the column. The F (ab') 2 fragments can be further purified by HPLC or FPLC. F (ab') 2 fragments can be treated to reduce disulfide bridges to produce Fab' fragments. An antibody, or fragment or derivative thereof, to be used to alter an antigen can be derived from polyclonal antisera containing antibodies reactive with a number of epitopes on an antigen. Preferably, the antibody is a monoclonal antibody directed against the antigen. Polyclonal and monoclonal antibodies can be prepared by standard techniques known in the art. For example, a mammal, (e. g., a mouse, hamster, or rabbit) can be immunized with the antigen or with a cell which expresses the antigen (e. g., on the cell surface) to elicit an antibody response against the antigen in the mammal. Alternatively, tissue or a whole organ which expresses the antigen can be used to elicit antibodies. The progress of immunization can be monitored by detection of antibody titers in plasma or serum. Standard ELISA or other immunoassay can be used with the antigen to assess the levels of antibodies. Following immunization, antisera can be obtained and, if desired, polyclonal antibodies isolated from the sera. To produce monoclonal antibodies, antibody producing cells (lymphocytes) can be harvested from an immunized animal and fused with myeloma cells by standard somatic cell fusion procedures thus immortalizing these cells and yielding hybridoma cells. Such techniques are well known in the art. For example, the hybridoma technique originally developed by Kohler and Milstein ( (1975) Nature 256: 495-497) as well as other techniques such as the human B-cell hybridoma technique (Kozbar et al., (1983) Immunol. Today 4: 72), and the EBV-hybridoma technique to produce human monoclonal antibodies (Cole et al. (1985) Monoclonal Antibodies in Cancer Therapy, Allen R. Bliss, Inc., pages 77-96) can be used. Hybridoma cells can be scr., --,. immunochemically for production of antibodies specifically reactive with the antigen and monoclonal antibodies isolated. Another method of generating specific antibodies, or antibody fragments, reactive with an antigen is by use of expression libraries encoding immunoglobulin genes, or portions thereof, expressed in bacteria which can be screened with the antigen (or a portion thereof). For example, complete Fab fragments, VH regions, FV regions and single chain antibodies can be expressed in bacteria using phage expression libraries. See for example Ward et al., (1989) Nature 341: 544-546; Huse et al., (1989) Science 246: 1275-1281 ; and McCafferty et al. (1990) Nature 348: 552-554. Alternatively, a SCID-hu mouse can be used to produce antibodies, or fragments thereof (available from Genpharm). Antibodies of the appropriate binding specificity which are made by these techniques can be used to alter an antigen on a donor cell. An antibody, or fragment thereof, produced in a non-human subject can be recognized to varying degrees as foreign when the antibody is administered to a human subject (e. g., when a donor cell with an antibody bound thereto is administered to a human subject), resulting in an immune response against the antibody in the subject. One approach for minimizing or eliminating this problem is to produce chimeric or humanized antibody derivatives, i. e., antibody molecules comprising portions which are derived from non-human antibodies and portions which are derived from human antibodies. Chimeric antibody molecules can include, for example, an antigen binding domain from an antibody of a mouse, rat, or other species, with human constant regions. A variety of approaches for making chimeric antibodies have been described. See, for example, Morrison et al., Proc. Natl. Acad. Sci. U. S. A. 81,6851 (1985); Takeda et al., Nature 314,452 (1985), Cabilly et al., U. S. Patent No. 4,816,567 ; Boss et al., U. S. Patent No. 4,816,397; Tanaguchi et al., European Patent Publication EP171496; European Patent Publication 0173494, United Kingdom Patent GB 2177096B. For use in therapeutic applications, it is preferred that an antibody used to alter a donor cell antigen not contain an Fc portion. Thus, a humanized F (ab') 2 fragment in which parts of the variable region of the antibody, especially the conserved framework regions of the antigen-binding domain, are of human origin and only the hypervariable regions are of non-human origin is a preferred antibody derivative. Such altered immunoglobulin molecules can be produced by any of several techniques known in the art, (e. g., Teng et al., Proc. Natl. Acad. Sci. U. S. A., 80,7308-7312 (1983); Kozbor et al., Immunology Today, 4, 7279 (1983); Olsson et al., Meth. Enzymol., 92,3-16 (1982)), and are preferably produced according to the teachings of PCT Publication W092/06193 or EP 0239400. Humanized antibodies can be commercially produced by, for example, Scotgen Limited, 2 Holly Road, Twickenham, Middlesex, Great Britain. Each of the cell surface antigens to be altered, e. g., the MHC class I antigens, MHC class II antigens, LFA-3 and ICAM-1 is well-characterized and antibodies reactive with these antigens are commercially available. For example, an antibody reactive with human MHC class I antigens (i. e., an anti-HLA class I antibody), W6/32, is available from the American Type Culture Collection (ATCC HB 95). This antibody was raised against human tonsillar lymphocyte membranes and binds to HLA-A, HLA-B and HLA-C (Barnstable, C. J. et al. (1978) Cell 14: 9-20). Another anti-MHC class I antibody which can be used is PT85 (see Davis, W. C. et al. (1984) Hybridoma Technology in Agricultural and Vetrinary Research. N. J. Stem and H. R. Gamble, eds., Rownman and Allenheld Publishers, Totowa, NJ, pl21; commercially available from Veterinary Medicine Research Development, Pullman WA). This antibody was raised against swine leukocyte antigens (SLA) and binds to class I antigens from several different species (e. g., pig, human, mouse, goat). An anti-ICAM-1 antibody can be obtained from AMAC, Inc., Maine. Hybridoma cells producing anti-LFA-3 antibodies can be obtained from the American Type Culture Collection, Rockville, Maryland. A suitable antibody, or fragment or derivative thereof, for use in the invention can be identified based upon its ability to inhibit the immunological rejection of allogeneic or xenogeneic cells using a protocol such as that described in the Examples. Briefly, an antibody (or antibody fragment) to be tested is incubated for a short period of time (e. g., 30 minutes at room temperature) with cells or tissue to be transplanted and any unbound antibody is washed away. The cells or tissue are then transplanted into a recipient animal. The ability of the antibody pretreament to inhibit or prevent rejection of the transplanted cells or tissue is then determined by monitoring for rejection of the cells or tissue compared to untreated controls. It is preferred that an antibody, or fragment or derivative thereof, which is used to alter an antigen have an affinity for binding to the antigen of at least 10¯7 M. The affinity of an antibody or other molecule for binding to an an antigen can be determined by conventional techniques (see Masan, D. W. and Williams, A. F. (1980) Biochem. J. 187: 1-10). Briefly, the antibody to be tested is labeled with I125 and incubated with cells expressing the antigen at increasing concentrations until equilibrium is reached. Data are plotted graphically as [bound antibody/ free antibody] versus [bound antibody] and the slope of the line is equal to the kD (Scatchard analysis). Other molecules which bind to an antigen on a donor cell and produce a functionally similar result as antibodies, or fragments or derivatives thereof, (e. g., other molecules which interfere with the interaction of the antigen with a hematopoietic cell and induce immunological nonresponsiveness) can be used to alter the antigen on the donor cell. One such molecule is a soluble form of a ligand for an antigen (e. g., a receptor) on the donor cell which can be used to alter the antigen on the donor cell. For example, a soluble form of CD2 (i. e., comprising the extracellular domain of CD2 without the transmembrane or cytoplasmic domain) can be used to alter LFA-3 on the donor cell by binding to LFA-3 on donor cells in a manner analogous to an antibody. Alternatively, a soluble form of LFA-1 can be used to alter ICAM-1 on the donor cell. A soluble form of a ligand can be made by standard recombinant DNA procedures, using a recombinant expression vector containing DNA encoding the ligand encompassing an extracellular domain (i. e., lacking DNA encoding the transmembrane and cytoplasmic domains). The recombinant expression vector encoding the extracellular domain of the ligand can be introduced into host cells to produce a soluble ligand, which can then be isolated. Soluble ligands of use have a binding affinity for the receptor on the donor cell sufficient to remain bound to the receptor to interfere with immunological recognition and induce non-responsiveness when the cell is administered to a recipient (e. g., preferably, the affinity for binding of the soluble ligand to the receptor is at least about 10¯7 M). Additionally, the soluble ligand can be in the form of a fusion protein comprising the receptor binding portion of the ligand fused to another protein or portion of a protein. For example, an immunoglobulin fusion protein which includes an extracellular domain, or functional portion of CD2 or LFA-1 linked to an immunoglobulin heavy chain constant region (e. g., the hinge, CH2 and CH3 regions of a human immunoglobulin such as IgGl) can be used. Immunoglobulin fusion proteins can be prepared, for example, according to the teachings of Capon, D. J. et al. (1989) Nature 337 : 525-531 and U. S. Patent No. 5,116,964 to Capon and Lasky. Another type of molecule which can be used to alter an MHC antigen (e. g., and MHC class I antigen) is a peptide which binds to the MHC antigen and interferes with the interaction of the MHC antigen with a T lymphocyte. In one embodiment, the soluble peptide mimics a region of the T cell receptor which contacts the MHC antigen. This peptide can be used to interfere with the interaction of the intact T cell receptor (on a T lymphocyte) with the MHC antigen. Such a peptide binds to a region of the MHC molecule which is specifically recognized by a portion specificity for two different epitopes on the same antigen can be used (e. g., two different anti MHC class I antibodies can be used in combination). Alternatively, two different types of molecules which bind to the same antigen can be used (e. g., an anti-MHC class I antibody and an MHC class 1-binding peptide). A preferred combination of anti-MHC class I antibodies which can be used with human donor cells is the W6/32 antibody and the PT85 antibody or F (ab') 2 fragments thereof. Another anti-MHC class I antibody which can be used is the monoclonal antibody 9-3 generated at Diacrin, Inc. 9-3 reacts with porcine MHC class I. The epitope for the monoclonal antibody 9-3 has been shown to be on the alpha-3 domain of MHC class I (the alpha-3 domain of porcine MHC class I is known-see, e. g., Satz, M. L. et al. (1985) J. Immunol. 135: 2167-2175) and is separate from the epitope for the monoclonal antibody PT85. When the donor cell to be administered to a subject bears more than one hematopoietic cell-interactive antigen, two or more treatments can be used together. For example, two antibodies, each directed against a different antigen (e. g., an anti-MHC class I antibody and an anti-ICAM-1 antibody) can be used in combination or two different types of molecules, each binding to a different antigen, can be used (e. g., an anti-ICAM-1 antibody and an MHC class I-binding peptide). Alternatively, a polyclonal antisera generated against the entire donor cell or tissue containing donor cells can be used, following removal of the Fc region, to alter multiple cell surface antigens of the donor cells. Alternative to binding a molecule (e. g., an antibody) to an antigen on a donor cell to inhibit immunological rejection of the cell, the antigen on the donor cell can be altered by other means. For example, the antigen can be directly altered (e. g., mutated) such that it can no longer interact normally with a hematopoietic cell (e. g., a T lymphocyte) in an allogeneic or xenogeneic recipient and induces immunological non-responsiveness to the donor cell in the recipient. For example, a mutated form of a class I MHC antigen or adhesion molecule (e. g., LFA-3 or ICAM-1) which does not contribute to T cell activation but rather delivers an inappropriate or insufficient signal to a T cell upon binding to a receptor on the T cell can be created by mutagenesis and selection. A nucleic acid encoding the mutated form of the antigen can then be inserted into the genome of a non-human animal, either as a transgene or by homologous recombination (to replace the endogenous gene encoding the wild-type antigen). Cells from the non-human animal which express the mutated form of the antigen can then be modified to express a gene product of interest according to one of the procedures described earlier. The modified cell expressing the gene product of interest and the mutated (i. e., altered) form of the antigen can then be used as a donor cell to deliver a gene product to an allogeneic or xenogeneic recipient. Alternatively, an antigen on the donor cell can be altered by downmodulating or altering its level of expression on the surface of the donor cell such that the interaction between the antigen and a recipient hematopoietic cell is modified. By decreasing the level of surface expression of one or more antigens on the donor cell, the avidity of the interaction between the donor cell and the hematopoietic cell (e. g., T lymphocyte) can be reduced. The level of surface expression of an antigen on the donor cell can be downmodulated by inhibiting the transcription, translation or transport of the antigen to the cell surface. Agents which decrease surface expression of the antigen can be contacted with the donor cell. For examp't, a number of oncogenic viruses have been demonstrated to decrease MHC class I expression in infected cells (see e. g., Travers et al. (1980) Int'l. Symp. on Aging in Cancer, 175180; Rees et al. (1988) Br. J. Cancer, 57: 374-377). In addition, it has been found that this effect on MHC class I expression can be achieved using fragments of viral genomes, in addition to intact virus. For example, transfection of cultured kidney cells with fragments of adenovirus causes elimination of surface MHC class I antigenic expression (Whoshi et al. (1988) J. Exp. Med. 168: 2153-2164). For purposes of decreasing MHC class I expression on the surfaces of donor cells, viral fragments which are non-infectious are preferable to whole viruses. Alternatively, the level of an antigen on the donor cell surface can be altered by capping the antigen. Capping is a term referring to the use of antibodies to cause aggregation and inactivation of surface antigens. To induce capping, a tissue is contacted with a first antibody specific for an antigen to be altered, to allow formation of antigen-antibody immune complexes. Subsequently, the tissue is contacted with a second antibody which forms immune complexes with the first antibody. As a result of treatment with the second antibody, the first antibody is aggregated to form a cap at a single location on the cell surface. The technique of capping is well known and has been described, e. g., in Taylor et al. (1971), Nat. New Biol. 233: 225-227; and Santiso et al. (1986), Blood, 67: 343-349. To alter MHC class I antigens, donor cells are incubated with a first antibody (e. g., W6/32 antibody, PT85 antibody) reactive with MHC class I molecules, followed by incubation with a second antibody reactive with the donor species, e. g., goat anti-mouse antibody, to result in aggregation. III. Genetically Modified Cells with Altered Surface Antigens as Donor Cells for Delivery of Gene Products to Allogeneic or Xenogeneic Subjects This invention provides a means for modifying a variety of cell types to express a gene product and for reducing the immunogenicity of such cells in an allogeneic or xenogeneic host. Depending on the type of cell to be modified, a gene expression system (e. g., vector with appropriate regulatory elements) is selected to allow expression of a gene product in that particular cell type. One or more appropriate molecule (s) is also selected to bind to antigen (s) on the cell surface to alter the antigen (e. g., one or more anti-class I antibodies which bind the MHC class I antigens on that particular cell type). Cells which can be modified and altered according to the invention include liver cells (e. g., hepatocytes), muscle cells (e. g., myoblasts, myocytes, myotubes), neural cell, pancreatic islet cells and hematopoietic cells. The use of hepatocytes for the expression of a particular gene product allows for the production of proteins that require a specific co-or post-translational modification, such as vitamin K-dependent y-carboxylation (e. g., many of the blood clotting factors require this modification for biological activity). Myoblasts have the advantage that injected myoblasts fuse with existing muscle fibers in a recipient (see e. g., Partridge, et al. (1989) Nature 337: 176; and. Karpati et al. (1989) Am. J. Pathology 1'5y27). Hematopoietic stem cells are advantageous in that they continue to divide and repopulate a number of cell types (see e. g., Chang and Johnson (1989) Int. J. Cell Cloning 7: 264; Williams (1990) Hum. Gene Ther. 1: 229; Karlsson et al. (1985) Proc. Natl. Acad. Sci. USA 82: 158; and Bodine et al. (1989) Proc. Natl. Acad. Sci. USA 86: 8897). Alternatively, mature hematopoietic cells, such as lymphocytes or monocytes, can be used. A gene product can be continuously produced by modifying a cell which continues to divide (e. g., a stem cell, such as a hematopoietic stem cell) or a gene product can be produced in a limited amounts by modifying a differentiated cell which does not divide (e. g., a myotubes or a neural cell). Recombinant retroviral vectors are suitable for modifying dividing cells but are not suitable for modifying non-dividing cells. The modified cells can be contained within a tissue or whole organ. For example, a tissue or organ can be modified to express a gene product by infecting the tissue or organ with a recombinant virus (e. g., retrovirus, adenovirus, adeno-associated virus etc.). One or more antigens on the tissue or organ can be altered by contacting the tissue or organ with a molecule which binds the antigen. For example, an organ can be perfused with a solution containing the molecule (e. g., an antibody) using conventional organ perfusion methods. This invention further allows cells to be modified to express a variety of gene products and thus allows many different types of gene products to be delivered to a subject. For example, the gene product can be a secreted protein. In this situation, the modified donor cell secretes the gene product in the subject, either locally or systemically. Non-limiting examples of secreted gene products of therapeutic interest which a cell can be modified to express include glucocerebrosidase, p-glucouronidase, al-antitrypsin, phenylalanine hydroxylase, tyrosine hydroxylase, ornithine transcarbamylase, arginosuccinate synthetase, UDP-glucuronysyl transferase, apoAl, TNF, soluble TNF receptor, human growth hormone, insulin, erythropoietin, anti-angiogenesis factors and interleukins. For example, the secreted protein can replace a missing function in a subject (e. g., insulin in a diabetic subject) or can stimulate a response in a subject (e. g., TNF or IL-2 can be produced in a tumor-bearing subject to stimulate an immune response against the tumor in the subject). Alternatively, the gene product can be a membrane-bound protein. In this case, the gene product remains associated with the membrane of the modified donor cell and functions, for example, by binding a soluble substance in a host (e. g., binding of LDL cholesterol by an LDL receptor) or by binding to another membrane-bound protein (e. g., a receptor) on cells of the host to trigger a signal within the recipient cells. Non-limiting examples of membrane-bound gene products which a cell can be modified to express include the LDL receptor, CFTR and CD 18. Alternatively, the gene product can be an intracellular protein. The intracellular protein within modified donor cells can be introduced into cells of a recipient by fusion of the donor cells to recipient cells (e. g., fusion of modified myoblasts or myocytes with muscle cells within the recipient, e. g., to deliver dystrophin). An intracellular protein can also function by acting upon substances within a recipient that are taken up by the modified cell (e. g., to detoxify substances within the recipier Non-limiting examples of intracellular proteins which a cell can be modified to express include dystrophin, p-globin and adenosine deaminase. In one embodiment, the cell to be modified is a non-human cell and the gene product is a human gene product. Human gene products have been expressed in non-human cells (see e. g., Dai, Y. et al. (1992) Proc. Natl. Acad. Sci. USA 89: 10892-10895; Armentano, et al. (1990) Proc. Natl. Acad. Sci. USA 87: 6141-6145; van Beusechem, V. W. et al. (1992) Proc. Natl. Acad. Sci. USA 89: 7640-7644). The human gene product expressed in a non-human cell can be the human version of a gene product typically expressed by that cell type, e. g., human insulin can be expressed in non-human islet cells or human Factor IX can be expressed in non-human hepatocytes (see, e. g. Armentano, et al. (1990) Proc. Natl. Acad. Sci. USA 87: 6141-6145). Alternatively, the human gene product expressed in the non-human cell can be a gene product which is not normally expressed by that cell type. For example, human growth hormone can be expressed in non-human myoblasts or tyrosine hydroxylase can be expressed in non-human myoblasts (see e. g. Jiao, S. et al. (1993) Nature 362: 450-453 and Dai, Y. et al. (1992) Proc. Natl. Acad. Sci. USA 89: 10892-10895 for examples of the expression in a cell of a gene product not normally expressed by that cell type). A preferred non-human cell for use in the methods of this invention is a porcine cell. Genetically inbred strains of pigs (e. g., miniature pigs), which have organs of approximately equivalent size to humans and have well characterized MHC antigens, are available in the art. Alteration of one or more antigens on the surface of a cell prior to transplantation reduces the immunogenicity of the cell such that rejection of the cell by an allogeneic or xenogeneic recipient is inhibited following transplantation. Accordingly, the invention provides a method for reducing the immunogenicity of a cell which is modified to express a gene product in which the cell is contacted, prior to transplantation (i. e., in vitro), with at least one molecule which binds to at least one antigen on the cell surface. The antigen to be altered stimulates an immune response against the cell in an allogeneic or xenogeneic subject. Thus, alteration of the antigen on the cell surface inhibits rejection of the cell when transplanted into a subject. It is preferred that the cell is contacted in vitro with a molecule, (e. g., antibody, or fragment or derivative thereof, such as an F (ab') 2 fragment) which binds to the antigen on the cell surface but does not activate complement or induce lysis of the cell. Preferably, the antigen on the cell surface which is altered is an MHC class I antigen. A modified cell of the invention is used to deliver a gene product to a subject by administering the cell to subject. The term"subject"is intended to include mammals, preferably humans, in which an immune response is elicited against allogeneic or xenogeneic cells. A cell can be administered to a subject by any appropriate route which results in delivery of the gene product to a desired location in the subject. For example, cells can be administered intravenously, subcutaneously, intramuscularly, intracerebrally, subcapsular (e. g., under the kidney capsule) or intraperitoneally. The cells can be administered in a physiologically compatible carrier, such as a buffered saline solution. When cells are within a tissue or organ, the tissue or organ can be transplanted into a suitable location in the subject by conventional techniques. It is preferable that a cell is modified to express a gene product prior to administering the cell to a subject (i. e., modified ex vivo). It is also preferable that the cell is modified to express the gene product prior to altering an antigen on the cell surface. However, a cell can be modified to express a gene product ex vivo after the antigen has been altered, so long as the modification method does not disrupt an association between the antigen on the cell surface and the molecule (e. g., an antibody) which is bound to the antigen. Furthermore, a cell can be modified to express a gene product in vivo following alteration of a surface antigen (s) ex vivo and administration to a subject. In vivo methods for genetically modifying a cell (e. g., using retroviral or adenoviral vectors) are known in the art. These embodiments are encompassed by the invention. The methods of the invention for delivering a gene product to a subject can further comprise additional treatments which inhibit rejection of the transplanted cells by the subject. For example, an immunosuppressive agent (e. g., a drug) can be administered to the subject at a dose and for a period of time sufficient to induce tolerance to the transplanted cells in the subject. A preferred immunsuppressive agent for administration to a subject is cyclosporin A. Other immunsuppressive agents which can be used include FK506 and RS-61443. Such immunosuppressive agents can be used in conjunction with a steroid (e. g., glucocorticoids such as prednisone, methylprednisolone and dexamethasone) or chemotherapeutic agent (e. g., azathioprine and cyclosphosphamide), or both. Alternatively, an agent which depletes or inhibits T cell activity in the subject can be administered to the subject. For example, an antibody which binds to a surface antigen on T cell in the subject can be used to deplete T cells within the subject. Preferred surface antigens to which a T cell-depleting antibody can bind include CD3, CD2, CD4 and CD8. Other antibodies which can be used to inhibit T cell activity in a subject include antibodies against IL-2 or other T cell growth factors and antibodies against the IL-2 receptor or other T cell growth factor receptors. Another aspect of the invention pertains to a kit for use in delivering a gene product to a subject. In one embodiment, the kit includes a cell which is modified to express a gene product; the cell having an antigen on the surface which stimulates an immune response against the cell in an allogeneic or xenogeneic subject. The kit further includes a molecule (e. g., an antibody, or fragment or derivative thereof) which binds to the antigen on the cell surface. In another embodiment, the kit includes a vector encoding a gene product in a form suitable for expression of the gene product in a cell and a molecule (e. g., an antibody, or fragment or derivative thereof) which binds to the antigen on the cell surface. In this embodiment, the kit can optionally include a cell which has the antigen to be altered on the cell surface. When cells are included in the kit (e. g., genetically modified cells or cells to be genetically modified), the cells can be cryopreserved (e. g., in liquid nitrogen or on dry ice) and thawed before use. The molecule (e. g., an antibody, or fragment or derivative thereof) which binds to an antigen on a cell can be provided in the kit in a physiologically acceptable carrier, such as a buffered saline solution. A preferred molecule for binding to an antigen, such as an MHC class I antigen on a cell is a F (ab') 2 fragment. The components of the kit can be supplied in appropriate containers (e. g., tubes, vials) for each component (e. g., cells, antibodies, vectors) and each component supplied within an appropriate holder (e. g., container). A kit of the invention can also include instructions for use of the kit to deliver a gene product to a subject. This invention is further illustrated by the following Examples which should not be construed as limiting. The contents of all references and published patents and patent applications cited throughout the application are hereby incorporated by reference. EXAMPLES EXAMPLE I: PRODUCTION OF GENETICALLY MODIFIED HUMAN MYOBLASTS AND TRANSPLANTATION OF THE MODIFIED MYOBLASTS INTO MICE Satellite myoblasts were isolated from a frozen biopsy of human muscle following 10 minute digestions in trypsin/collagenase/bovine serum albumin mix at 37 C. Released cells from each digestion were seeded in 100 mm plates in the following growth medium (GM): MCDB 120 (JRH Biosciences, Lenexa, KS) + epidermal growth factor + dexamethasone + 20% fetal bovine serum. Primary cultures were re-fed once with GM before being trypsinized for electroporation ten days after digestion. Cell harvests from digestions 3 through 10 were combined and washed twice with ice cold Hepes buffered saline pH 7.0, counted and resuspended at a final concentration of 2.6 x 105 cells/0.8 ml of Hepes buffered saline for placement in an 0.4 cm gap width electroporation cuvette. Cells were incubated on ice for 10 minutes with 20 pLg of ScaI digested pCMV (3 plasmid (Clontech, Palo Alto, CA), a plasmid which contains the gene encoding p-galactosidase, and 2 u. g of NsiI digested pkJ2Neo (Dinsmore, J. H. and Solomon, F. (1993) Neuroprotocols 2: 19-23), which displays neomycin resistance, or pCMVGH, a plasmid containing a gene encoding human growth hormone and which also displays neomycin resistance. pCMVGH was constructed as follows: plasmid pGH (Selden, R. F. et al. (1986) Mol. Cell. Biol. 6 (9): 3173-3179) was cut with BamHI and EcoRI and cloned into plasmid pcDNA3 (Clontech, Palo Alto, CA) previously cut with the same enzymes. The resultant plasmid, pCMVGH, which is shown in Figure 1, contains the gene encoding human growth hormone. The pCMVGH plasmid was then linearized with ScaI in preparation for introduction into human myoblasts via electroporation. Electroporation was performed with a BioRad electroporation device with capacitance extender at 240V, 50011F. After electroporation, cells were left to recover for 10 minutes at room temperature and seeded into four 6-well plates at 6.5 x 104 cells per weU (assuming no cell death during electroporation), and cultured in GM + 8001lgG418/ml for 11 days to select for stable transfectants. Cloning efficiency was 0.01-0.02%. Electroporation was determined to be more efficient for transfection than either lipofection (BRL) or calcium phosphate-mediated transfection (Rosenthal, N. (1987) Meth. Enzymol. 152: 704-720). For determing expression in vitro, cells transfected with pCMV and pJK2Neo were fixed in 0.05% glutaraldehyde, rinsed three times (5 minutes each rinse) in PBS, stained with X-GAL for 3-24 hours (Na2HP04-8OmM, NaH2PO4-20mM, MgCi-1.3mM, X-GALlmg/ml, K3Fe (CN) 6-3mM, K4Fe (CN) 6-3mM in dH20) according to standard protocols. Clones after G418 selection often showed sporadic expression of (3-gal when stained with X GAL substrate mix. However, expression increased with consecutive passages and was at 100% upon fusion of cells (Figure 2). For determining expression of human growth hormone in vitro, medium from cells transfected with pCMVGH was sampled and measured for human growth hormone content using a growth hormone radioimmunoassay from Nichols Labs, San Jan Capistrano, CA. Human growth hormone was produced at 800-1800 ng/106 cells/hour. For transplantation experiments, nude mice were anesthetized by intraperitoneal administration of Avertin (250 mg/kg body weight) and a flank incision was made to expose the kidney. A small incision was made on the kidney and a small fire polished glass rod was inserted between the kidney epithelium and the kidney tissue to create a space for cells to be transplanted. Prior to transplantation, 106 cells were spun down in an Eppendorf pipette tip, then were placed under the kidney capsule together with a piece of sponge blocking the tip. This type of transplant can be easily localized even with a bare eye. The skin incision was then closed with a wound clip and the animal transferred to a cage for recovery. Mice transplanted with myoblasts were sacrificed thirty days after transplantation and then the region of muscle which received the transplant was dissected and placed in 0.5% glutaraldehyde. After fixation, tissue was rinsed three times (five minutes each rinse) in PBS and then frozen. The frozen tissue was later thawed, sectioned and stained with X-GAL for 24-48h (X-GAL solution was the same as above except for increased K3Fe (CN) 6 and K4Fe (CN) 6 concentration to 50mM). (3-gal expression was detected 30 days after transplantation. EXAMPLE II: TRANSPLANTATION OF GENETICALLY MODIFIED HUMAN MYOBLASTS/MYOTUBES INTO CYCLOSPORIN TREATED RATS Satellite myoblasts were isolated from human muscle, cultured and transfected as described b Example I. (3-gal and human growth hormone expression in these cells were also measured as described in Example I. (3-gal expression was measured after 34 days in vitro (Figure 2). This was the latest time point examined and does not reflect a limit on the length of time in which expression is expected. Expression of hGH in stable transfectants in vitro was maintained over 5 passages up to 14 days in culture. Clones were never cultured till senescence to assay for GH but expression is stable in cultures from different frozen passages. In myoblasts, hGH was expressed at 200-500 ng/106/hour; in myotubes, hGH was expressed at 8001500ng/106/hour. The Lewis rats used for the transplantation experiment were obtained from Charles River, Wilmington, MA. Rat tibialis anterior muscle was damaged by injection of pivacaine and hyaluronidase three days prior to transplantation. One day prior to transplantation and daily thereafter, the rats were treated with cyclosporin (10-15mg/kg). Human myoblasts stably transfected with p-gal or hGH expression vectors as described above were transplanted as myoblasts or induced to form myotubes then transplanted to the damaged site in recipient muscle. Two weeks after transplantation, the muscle was removed for histological analysis to detect (3-gal expression. Rat serum was also sampled to measure circulating hGH levels at days 3 and 7 post transplantation. Production of human growth hormone in the rats, as a result of expression of human growth hormone by the introduced modified cells, is monitored by detecting the presence of human growth hormone in the circulation of the rats. Aliquots of blood from the rats were collected periodically, from the tail vein, and hGH present within the blood sample was detected using a growth hormone radioimmunassay from Nichols Labs, San Juan Capistrano, CA. (3-gal expression was measured 14 days after transplantation in vivo (Figure 3). This was the latest time point examined and does not reflect a limit on the length of time in which expression is expected. In vivo detection of human growth hormone production was observed to last for three days. After myoblast and myotube injection into normal Lewis rat TA muscle, a much reduced number of cells were detected with a human specific probe 14 days post implantation. Thus, the inability to detect hGH expression past three days post transplantation would appear to be due to poor cell survival rather than a loss of GH expression from the transgene. EXAMPLE III: TRANSPLANTATION OF MODIFIED HUMAN MYOTUBES INTO CYCLOSPORIN TREATED MICE Human myotubes were modified by incubation with purified F (ab') 2 fragments of the monoclonal antibody W6/32 (F (ab') 2 fragments were were generated using; the Immunopure F (ab') 2 preparation kit sold by Pierce Chemical Company, Rockford, Illinois) at a concentration of 10 u. g of antibody fragment for approximately 5 x 106 cells in 500 ul of PBS for one hour on ice with intermittent shaking. After the incubation, the treated myotubes were washed, spun down once and resuspended at transplant concentration in PBS and then immediately transplanted (5 x 106 nuclei per mouse) into the kidney capsules, as described in Example I, of four experimental Balb/c mice. Two of these mice were treated with cyclosporin (20 mg/kg) beginning one day prior to transplantation and continued daily thereafter. The other two mice were masked with F (ab') 2 fragments of the monoclonal antibody W6/32 and not subject to cyclosporin treatment. As a positive control, untreated myotubes (5 x 106 nuclei per mouse) were transplanted under the kidney capsules of two nude mice. As a negative control, untreated myotubes (5 x 106 nuclei per mouse) were transplanted under the kidney capsule of two normal Balb/c mice. The mice kidneys were removed 30 days after transplantation, fixed in 4% paraformaldehyde for 24 hours, and stained with rabbit anti-desmin from BioGenex Labs, San Ramon, CA (Figure 4). Human myotubes were detected in all experimental kidneys 30 days after transplantation including the kidneys from the negative control. The negative control, however, showed evidence of ongoing rejection of the myotubes, e. g., vacuolated areas with obvious cell necrosis. Evidence of ongoing rejection was not observed in the test mice (mice transplanted with myotubes and subject to cyclosporin treatment, mice transplanted with F (ab') 2 modified myotubes, and nude mice transplanted with myotubes). EXAMPLE IV: PRODUCTION AND TRANSPLANTATION OF GENETICALLY MODIFIED PORCINE MYOBLASTS SUITABLE FOR TRANSPLANTATION An expression vector containing a gene encoding human growth hormone is introduced into porcine myoblasts to create a modified cell which expresses human growth hormone. The modified porcine cell is then treated with an anti-MHC class I antibody F (ab') 2 fragment, thereby altering porcine MHC class I antigens (i. e., SLA antigens) on the cell. The genetically modified cells with altered MHC class I antigens are then administered to a mouse to demonstrate the delivery of a human gene product to a subject using a cell which is xenogeneic to the subject. Nucleic acid encoding the gene for human growth hormone (hGH) is cloned into the commercially available plasmid expression vector pCDNAIII. Nucleic acid encoding the human growth hormone gene can be obtained by a standard procedure based upon the reported nucleotide sequence of the gene (Goeddel et al. (1979) Nature 281: 544), for example either by designing PCR primers based upon the gene sequence and amplifying a fragment of DNA encompassing the coding region of the gene by PCR or by screening a cDNA or genomic DNA library with primers based upon the gene sequence. Nucleic acid encoding human growth hormone is cloned into the polyHnker ofpCDNAIII to create a vector (phGH) containing the human growth hormone gene under the transcriptional control of the cytomegalovirus promoter. The vector also contains a bacterial selectable marker (ampicillin resistance) and a mammalian selectable marker (neomycin resistance). Appropriately prepared phGH plasmid (e. g., cesium chloride purified and linearized) is introduced into porcine myoblasts by electroporation. In a general procedure, approximately 1 x 107 cells are suspended in 0.5 ml of ice-cold electroporation buffer (possible electroporation buffers include: PBS without Ca2+ or Mg2+ ; HEPES-buffered saline and tissue culture medium without fetal calf serum. e. g., RPMI) and phGH DNA (approximately 1-10 ug) is added. The DNA/cell suspension is placed in an electroporation cuvutte and incubated on ice for 5-10 minutes. The cuvette is then placed in an electroporation apparatus (e. g., commercially available from BioRad) and pulsed at a desired voltage and capacitance setting. The voltage and capacitance conditions for most efficient electroporation are determined empirically (for example by testing a variety of conditions, determining the percentage of cell death with each condition and selecting a condition that achieves approximately 50 % cell death) but typical conditions are between 200 to 300 V with 500 to 1000 uF capacitance. After the cells are shocked, the cells are again incubated on ice for 10 minutes and then washed and plated in an appropriate culture media. To select cells which have incorporated the introduced phGH plasmid, the antibiotic G418 is added to the culture media three days after electroporation and the cells are refed on alternate days with G418-containing media. The concentration of G418 used for efficient selection of cells is determined empirically (for example, by determining the dosage of G418 which is needed to kill mock transfected cells) but typically is in the range of 400 llg/ml to 800 pg/ml. Selective cell death of cells which have not taken up the phGH plasmid occurs beginning about on day 5 after the start of drug selection and by day 30 surviving cells are considered to be selected. Individual clones of cells are then isolated and expanded into specific cell lines and analyzed for expression of the gene product. Expression of human growth hormone by the transfected cells is detected using a commercially available two-site radioimmunoassay (RIA ; from Nichols Institute Diagnostics) with pure hGH standards for comparison to determine the level of hGH expression by the cells. Human growth hormone produced by the cells is secreted into the culture medium, thereby allowing assessment of hGH expression by sampling an aliquot of the culture media for the presence of hGH using the RIA. Thus, the production of hGH by the cells can be continuously monitored by repeatedly sampling the culture media over time. A stably tranfected cell line which produces maximal expression of the introduced hGH DNA is chosen by screening different clones that have been drug selected for hGH production. Such a cell line can then be prepared for transplantation into a xenogeneic recipient animal by treating the cells with an F (ab') 2 fragment of the PT85 anti-MHC class I antibody (commercially available from Veterinary Medicine Research Development, Pullman WA) which binds to SLA class I antigens. An F (ab') 2 fragment is prepared from intact antibodies as follows: Purified PT85 at 20 mg/ml is incubated with immobilized pepsin for 4 hours at 37 C in a pH 4.7 digestion buffer in a shaking water bath. The crude digest is removed from the pepsin and immediately neutralized with a pH 7.0 binding buffer. The crude digest is applied to an immobilized protein A column and the eluate containing the F (ab') 2 fragments is collected. The F (ab') 2 fragments are dialyzed against PBS for 24 hours using 50,000 MW cutoff dialysis tubing to remove any contaminating Fc fragments. CHAPS is added to the dialysis bag at a concentration of 10 mM. The completeness of the F (ab') 2 digest is monitored by silver staining of 15 % SDS gels. Final purification of the fragments is achieved by FPLC using a Superose 12 column (Pharmacia, Upsala, Sweden). Cells modified to express hGH are prepared for transplantation by incubating the cells with the purified F (ab') 2 fragments at a concentration of 1 mg of antibody for approximately 1 x 106 cells for 30 minutes at room temperature. After the incubation, the treated cells are washed once in Hank's buffer containing 2 % fetal calf serum and then immediately transplanted into Balb/c mice by syringe injection at an appropriate site (e. g., into the muscle of the hindleg). Production of human growth hormone in the mice, as a result of expression of human growth hormone by the introduced modified cells, is monitored by detecting the presence of human growth hormone in the circulation of the mice. An aliquot of blood from the mouse is collected periodically, for example from the tail vein, and hGH present within the blood sample is detected using the radioimmunassay described previously. EQUIVALENTS Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.

Claims: CLAIMS 1. A non-human cell suitable for transplantation which is modified to express a human gene product and which has at least one antigen on the cell surface which is capable of stimulating an immune response against the cell in an allogeneic or xenogeneic subject, wherein the antigen on the cell surface is altered to inhibit rejection of the cell when transplanted into a subject. 2. The non-human cell of claim 1, wherein the antigen is altered to modify an interaction between the antigen and a T lymphocyte in an allogeneic or xenogeneic subject. 3. The non-human cell of claim 2, wherein the cell is modified to express a human gene product by introducing into the cell a nucleic acid encoding the gene product in a form suitable for expression of the gene product in the cell. 4. The non-human cell of claim 3, wherein the gene product is a secreted protein. 5. The non-human cell of claim 3, wherein the gene product is a membranebound protein. 6. The non-human cell of claim 5, wherein the membrane-bound protein is a cell surface receptor. 7. The non-human cell of claim 3, wherein the gene product is an intracellular protein. 8. The non-human cell of claim 2, wherein the antigen on the cell surface which is altered is an MHC class I antigen. 9. The non-human cell of claim 8 which is contacted prior to transplantation with at least one anti-MHC class I antibody, or fragment or derivative thereof, which binds to the MHC class I antigen on the cell surface but does not activate complement or induce lysis of the cell. 10. The non-human cell of claim 9, wherein the anti-MHC class I antibody is an anti-MHC class IF (ab') 2 fragment. 11. The non-human cell of claim 8 which is contacted prior to transplantation with at least one peptide which binds an MHC class I antigen. 12. The non-human cell of claim 2 which is a muscle cell. 13. The non-human cell of claim 2 which is a liver cell. 14. The non-human cell of claim 2 which is a neural cell. 15. The non-human cell of claim 2 which is a pancreatic islet cell. 16. The non-human cell of claim 2 which is a hematopoietic cell. 17. A porcine cell suitable for transplantation which is modified to express a human gene product and which has at least one MHC class I antigen on the cell surface which is capable of stimulating an immune response against the cell in an allogeneic or xenogeneic subject, wherein the MHC class I antigen on the cell surface is altered to inhibit rejection of the cell when transplanted into a subject. 18. The porcine cell of claim 17 which is contacted prior to transplantation with at least one anti-MHC class I antibody, or fragment or derivative thereof, which binds to the MHC class I antigen on the cell surface but does not activate complement or induce lysis of the cell. 19. The porcine cell of claim 18, wherein the anti-MHC class I antibody is an anti MHC class I F (ab') 2 fragment. 20. The porcine cell of claim 19, wherein the anti-MHC class I F (ab') 2 fragment is a F (ab') 2 fragment of a monoclonal antibody PT85. 21. A cell suitable for transplantation which is infected with a recombinant virus comprising a nucleic acid encoding a gene product in a form suitable for expression of the gene product in the cell, the cell having at least one antigen on the cell surface which is capable of stimulating an immune response against the cell in an allogeneic or xenogeneic subject and wherein the antigen on the cell surface is altered to inhibit rejection of the cell when transplanted into a subject. 22. The cell of claim 21, wherein the antigen is altered to modify an interaction between the antigen and a T lymphocyte in an allogeneic or xenogeneic subject. 23. The cell of claim 22, wherein the recombinant virus is an adenovirus or an adeno-associated virus. 24. The cell of claim 22, wherein the recombinant virus is a retrovirus. 25. The cell of claim 22, wherein the antigen on the cell surface which is altered is an MHC class I antigen. 26. The cell of claim 25 which is contacted prior to transplantation with at least one anti-MHC class I antibody, or fragment or derivative thereof, which binds to the MHC class I antigen but does not activate complement or induce lysis of the cell. 27. The cell of claim 26, wherein the anti-MHC class I antibody is an anti-MHC class I F (ab') 2 fragment. 28. The cell of claim 27 which is contacted with a F (ab') 2 fragment of a monoclonal antibody W6/32 or a F (ab') 2 fragment of a monclonal antibody PT85 or F (ab') 2 fragments of both W6/32 and PT85. 29. The cell of claim 25 which is contacted prior to transplantation with at least one peptide which binds to an MHC class I antigen. 30. A kit for delivering a human gene product to a subject comprising: (a) a non-human cell which is modified to express the human gene product and which has an antigen on the cell surface which is capable of stimulating an immune response against the cell in an allogeneic or xenogeneic subject; and (b) an antibody, or fragment or derivative thereof, which binds to the antigen on the cell surface. 31. The kit of claim 30, wherein the antibody, or fragment or derivative thereof, is a F (ab') 2 fragment of the antibody. 32. The kit of claim 29, wherein the antigen is an MHC class I antigen. 33. A kit for delivering a human gene product to a subject comprising: (a) a vector encoding the human gene product in a form suitable for expression of the human gene product in a cell; and (b) an antibody, or fragment or derivative thereof, which binds to an antigen on a cell surface which is capable of stimulating an immune response against the cell in an allogeneic or xenogeneic subject. 34. The kit of claim 33 further comprising a non-human cell which hat an antigen on the cell surface which is capable of stimulating an immune response against the cell in an allogeneic or xenogeneic subject, wherein the antibody, or fragment or derivative thereof binds to the antigen. 35. A method for reducing the immunogenicity of a non-human cell for transplantation which is modified to express a human gene product comprising contacting the cell prior to transplantation with at least one molecule which binds to at least one antigen on the cell surface which is capable of stimulating an immune response against the cell in an allogeneic or xenogeneic subject to alter the antigen on the cell surface to inhibit rejection of the cell when transplanted into a subject. 36. The method of claim 35, wherein the antigen on the cell surface which is altered is an MHC class I antigen. 37. The method of claim 36, wherein the cell is contacted prior to transplantation with at least one anti-MHC class I antibody, or fragment or derivative thereof, which binds to the MHC class I antigen but does not activate complement or induce lysis of the cell. 38. The method of claim 37, wherein the anti-MHC class I antibody is an anti MHC class I F (ab') 2 fragment. 39. A method for delivering a human gene product to a subject comprising: (a) contacting a non-human cell which has been modified to express the human gene product with at least one molecule which binds to at least one antigen on the cell surface which is capable of stimulating an immune response against the cell in an allogeneic or xenogeneic subject to alter the antigen on the cell surface to inhibit rejection of the cell when transplanted into a subject; and (b) administering the cell to the subject. 40. The method of claim 39, wherein the antigen on the cell surface which is altered is an MHC class I antigen. 41. The method of claim 40, wherein the cell is contacted prior to transplantation with at least one anti-MHC class I antibody, or fragment or derivative thereof, which binds to the MHC class I antigen but does not activate complement or induce lysis of the cell. 42. The method of claim 41, wherein the awnti-MHC class I antibody is an anti MHC class I F (ab') 2 fragment.

Inventors: CHAPPEL SCOTT C, EGAN E MICHAEL, CHAPPEL, SCOTT, C., EGAN, E., MICHAEL
Applicants: DIACRIN INC, DIACRIN, INC.
Application year: 1995
Main classification: C12N5
IPC codes: A61K35, A61K39, A61K48, C07K16