Co-transplantation of Vascularized Thymic Graft with Kidney in Pig-to-Nonhuman Primates for the Induction of Tolerance Across Xenogeneic Barriers

Using advanced gene editing technologies, xenotransplantation from multi-transgenic alpha-1,3-galactosyltransferase knockout pigs has demonstrated marked prolongation of renal xenograft survival, ranging from days to greater than several months for life-supporting kidneys and >2 years in a heterotopic non-life-supporting cardiac xenograft model. However, continuous administration of multiple immunosuppressive drugs continues to be required, and attempts to taper immunosuppression have been unsuccessful. These data are consistent with previous reports indicating that the human-anti-porcine T cell response is similar or stronger than that across allogeneic barriers. Due to the strength of both the innate and adaptive immune responses in xenotransplantation, the level of continuous immunosuppression needed to control these responses and prolong xenograft survival has been associated with prohibitive morbidity and mortality. These facts provide compelling rationale to pursue a clinically applicable strategy for the induction of tolerance. Mixed chimerism and thymic tissue transplantation have both achieved xenogeneic tolerance in pig-to-mouse models, and both have recently been extended to pig-to-baboon models. Although these strategies are promising in small animal models, neither direct intravenous injection of porcine bone marrow cells nor direct fetal thymic tissue transplantation into recipients was able to achieve >2 days chimerism following BM Tx or the engraftment of thymic tissues across xenogeneic barriers in pig-to-nonhuman primate models. Several innovative procedures have been largely developed by Kazuhiko Yamada to overcome these failures. These include vascularized thymic transplantation, combined with either thymokidney (TK) or vascularized thymic lobe (VTL) transplantation. Utilizing the strategy of transplanting vascularized thymic grafts with kidney from the same GalT-KO donor without further gene modification, we have achieved longer than 6 months survival of life-supporting kidneys in a baboon. Notably, the recipient became donor specific unresponsive and developed new thymic emigrants. In this chapter, we introduce a brief summary of our achievements to date toward the successful induction of tolerance by utilizing our novel strategy of vascularized thymic transplantation (including thymokidney transplantation), as well as describe the step-by-step methodology of surgical and in vitro procedures which are required for this experiment.