A primary barrier to xenotransplantation has been the essentially immediate recognition of carbohydrate epitopes present in the foreign tissue causing hyperacute xenograft rejection (HAR). The reaction begins immediately upon reperfusion, and once initiated destroys the foreign tissue within minutes to a few hours. The presence of HAR in some donor/recipient combinations while not others is postulated to be related to two primary factors, a) the binding of xenoreactive natural antibodies of the recipient to antigens or endothelial cells in the graft and b) the incompatibility of complement regulatory proteins in the transplant with the complement system of the recipient, allowing uncontrolled activation of complement. Greater than 1% of the complement-fixing natural antibodies in human serum recognize a single structure-Galα(1-3)Galβ(1,4)GlcNAc-R. The synthesis of Galα(1-3)Galβ(1,4)GlcNAc-R is catalyzed by the enzyme α(1,3) galactosyltransferase (αGT).
This enzyme catalyzes the synthesis of α-galactosyl (αGal) epitopes in the Golgi apparatus of cells from various non-primate mammals by the following reaction:Galβ(1,4)GlcNAc-R+UDP-Gal→Galα(1-3)Galβ(1,4)GlcNAc-R
This enzyme was found to be active in new world monkeys but not in old world monkeys and humans. The αGT cDNA has been cloned from bovine and murine cDNA libraries. Larson, R. D. et al. (1989) “Isolation of a cDNA Encoding Murine UDP galactose; β-D-galactosyl-(1,4) N Acetyl-D-Glucosamine α-(1,3) Galactosyl Transferase Expression Cloning by Gene Transfer”, PNAS, USA 86:8227; and Joziasse, D. H. et al., (1989) “Bovine α-(1,3) Galactosyl Transferase: Isolation and Characterization of a cDNA Clone, Identification of Homologous Sequences in Human Genomic DNA”, J. Biol Chem 264:14290.
The gene is present in the human genome, although no transcription has been detected. Instead, two frame shift mutations were found (deletions generating premature stop codons) in the human exons encoding the enzyme. See generally, Galili, Uri “Evolution in Pathophysiology of the Human Natural anti-α-Galactosyl IgG (anti-αGal) Antibody”, Springer Semin. Immunopathol. (1993) 15:155-171.
anti-αGal, a naturally occurring antibody present in all humans, specifically interacts with the carbohydrate epitope Galα(1-3)Galβ(1,4)GlcNAc-R (αGal epitope). This antibody does not interact with any other known carbohydrate epitope produced by mammalian cells (Galili, 1993, Springer Seminar Immunopathology 15:153). anti-αGal constitutes approximately 1% of circulating IgG (Galili et al., 1984, J. Exp. Med. 160:1519) and is also found in the form of IgA and IgM (Davine et al., 1987, Kidney Int. 31:1132; Sandrin et al., 1993, Proc. Natl. Acad. Sci. USA 90:11391). It is produced by 1% of circulating B lymphocytes (Galili et al., 1993, Blood 82:2485). Production of this natural anti-αGal Ab in humans is constantly stimulated by the presence of αGal carbohydrate residues present in intestinal and pulmonary bacterial flora. In humans anti-αGal reacts to the presence of this epitope in hyperacute xenograft rejection and complement is swift and certain resulting in destruction of foreign tissues in minutes to hours.
It is an object of this invention to develop a therapeutic cancer vaccine by introducing the gene encoding for αGT into tumor cells, to drive the addition of αGal epitopes to such vaccine tumor cells in order to allow for enhanced opsonization of the vaccine cells by natural anti-αGal antibodies and stimulate tumor antigen presentation to induce a humoral and cellular immune response to the tumor specific antigens.
It is a further object of this invention to provide a therapeutic pharmaceutical composition comprising recombinant tumor cells which express and process αGT to engineer αGal epitopes on cells.
It is a further object of the invention to provide compositions and methods for treatment of tumors, viruses, neoplastic cells or other cells, which grow and evade the cellular and humoral immune response.
Other objects of the invention will become apparent from the description of the invention which follows.