Induction of molecular chimerism by gene therapy prevents antibody mediated heart transplant rejection
Introduction In primates, preformed natural antibodies that bind Galα1-3Galβ1-4GlcNAc-R (αGal), synthesized by UDP galactose:β-D-galactosyl-1,4-N-acetyl-D-glucosaminide α(1-3)galactosyltransferase (E.C. 220.127.116.11) or simply αGT, mediate vigorous rejection of pig organs. Using αGT knockout mice (GT0 mice), which like humans contain in their serum antibodies that bind αGal, we have shown using a gene therapy approach, that expression of a retrovirally transduced αGT gene in transplanted bone marrow hematopoietic progenitors resulting in the induction of molecular chimerism prevents production of αGal reactive antibodies. Here, we examine whether induction of molecular chimerism by gene therapy can prevent rejection of cardiac transplants that express αGal epitopes.
Materials and methods Lethally irradiated GT0 mice were reconstituted with syngeneic GT0 bone marrow cells transduced with either retroviruses carrying the gene encoding porcine αGT, control viruses carrying the neomycin resistance gene (Neor) or mock transduced bone marrow. Mice in each group were then sensitized to αGal by repeated immunizations with irradiated porcine peripheral blood mononuclear cells. Minor antigen mismatched hearts from C57BL/6 mice were then heterotopically transplanted into the abdomen of sensitized mice.
Results Following immunization, the average αGal antibody titer was 1/202 473 ± 103 171 in control mice, while αGal reactive antibodies were undetectable in the serum of mice reconstituted with αGT transduced bone marrow. Transplantation of wild type hearts into control mice resulted in rapid rejection (median survival time (MST): 6 h). In contrast, survival of wild-type hearts in mice reconstituted with αGT transduced bone marrow was markedly prolonged (MST: 29 days, P < 0.0005). Histological examination of rejected transplants suggested that antibody mediated hyperacute rejection led to rejection in control animals. In contrast, hearts transplanted into mice receiving αGT transduced bone marrow underwent T cell mediated rejection due to minor antigen disparities, rather than antibody mediated rejection.
Conclusions Reconstitution of lethally irradiated GT0 mice with αGT transduced bone marrow cells prevents antibody mediated rejection of cardiac transplants from wild type mice. Thus, gene therapy can be used to induce immunological tolerance to defined antigens and thereby overcome transplant rejection.