Achieving Xenograft Tolerance through Mixed Chimerism

Project 2 Summary Project 2 is directed toward the induction of xenotransplantation tolerance through mixed hematopoietic chimerism, which is the only treatment regimen to date that has been successful in translating allogeneic tolerance induction from animal models to the clinic. In previous funding periods, we have overcome two of the major barriers to establishment of pig-to-primate mixed chimerism through: 1) development of α1,3-galactosyltransferase null (GalT- KO) miniature swine, which avoid the problem of hyperacute rejection due to natural anti-Gal antibodies; 2) addition of the hCD47 transgene to these GalT-KO swine, thereby avoiding removal of bone marrow stem cells due to phagocytosis by macrophages bearing the species-specific SIRP-alpha receptor; and 3) development of a methodology for intra-bone administration of porcine HSC (IBBM), which increased the ability of GalT-KO/CD47 inocula to engraft. These efforts have led to markedly increased levels and duration of porcine chimerism in baboons, as well as to prolonged survival of donor swine skin xenografts, in the absence of immunosuppression. In addition, we have demonstrated that mixed chimerism can lower the levels of natural anti-pig non-Gal antibodies and that this effect is proportional to the duration of chimerism. Since renal xenograft survival remains markedly affected by the levels of recipient anti-pig nAbs and since humans, like baboons, show marked variability in levels of such nAbs, ways of improving the levels and prolonging the duration of mixed chimerism will be required in order to make xenografts available to all potential human recipients. Our Specific Aims are now directed toward optimizing the effect of mixed chimerism on elimination of nAbs and inducing tolerance by: 1) Testing our new human cytokine receptor transgenic GalT-KO miniature swine as HSC donors; 2) Utilizing recipient Tregs to enhance xenogeneic chimerism; and 3) Applying our optimized mixed chimerism approach to induction of renal xenograft tolerance.