Initial Evidence for Functional Immune Modulation in Primate Recipients of Porcine Skin Grafts Following Conditioning With Human CD47 Transgenic Pig Hematopoietic Stem Cells.

Abstract

Background: To inhibit phagocytosis of porcine cells by primate macrophages in tolerization protocols including mixed chimerism, we generated human CD47 transgenic swine on an α-1,3-galactosyltransferase null background (hCD47/GalT-KO). We report the initial results of hematopoietic stem (HSC) transplantation from this swine into baboons with respect to cell survival and immune response. Methods: Two baboons underwent a conditioning regimen consisting of non-myeloablative total body and thymic irradiation, T and B cell depletion, and infusion of HSC obtained by cytokine mobilization and leukapheresis of a CD47 transgenic GalT-KO swine. Immunosuppression consisted of clinical level administration of FK506 for the first 30 days and anti-CD154 antibody for the first 7 weeks following cell transplant. Recipients received challenge infusions of HSPC from the same donor swine 7 and 10 weeks following the initial infusion. Both baboons received split thickness skin grafts from the porcine donor on full thickness wound beds at 14 weeks without any additional immunosuppression. Results: Peripheral macrochimerism was detected following three cell transplants, with >10% levels observed at 24 hours declining over the next 3-7 days. Despite detectable macrochimerism no increase in anti-pig antibody levels or T-cell sensitization was observed prior to skin grafting. Both xenogeneic skin grafts developed extensive areas of bruising during the first week, while maintaining islands of apparently healthy tissue. These islands underwent expansion, reaching confluence over the entire wound bed in one recipient 6 weeks after graft placement and undergoing slow rejection over the following 5 weeks. In vitro measures of T cell responsiveness showed no evidence of sensitization to the xenografts prior to rejection. Conclusions: Assessment of the definitive role of hCD47 transgenic HSC transplantation in these results must await further ongoing studies of non-transgenic cell transplantation in similarly conditioned recipients. Nevertheless, the level and duration of chimerism using hCD47/GalT-KO mobilized HSC is very encouraging and may have been responsible for the greatly attenuated response against nonvascular components of xenogeneic skin grafts that we have observed in the absence of additional immunosuppression.