We have previously shown that peri-transplant infusion of donor splenocytes treated with 1-ethyl-3-(3’-dimethylaminopropyl)-carbodiimide (ECDI-SP) induces robust donor-specific tolerance and permanent allograft protection in a murine full MHC-mismatched allogeneic islet and heart transplant models. Tolerance mechanisms involve targeting recipient antigen-presenting cells and elaboration of PD-L1/PD-L2 negative costimulatory molecules, deletion of T cells with indirect donor-specificity, anergy of T cells with direct donor-specificity, as well as expansion of regulatory cell populations including CD4+CD25+Foxp3+ regulatory T cells as well as CD11b+Gr1+ myeloid derived suppressor cells. We have further shown that donor ECDI-SP in combination with peri-transplant anti-CD20 and a short course of rapamycin induce permanent survival of porcine xenogeneic islet grafts in diabetic murine recipients in rat-to-mouse and pig-to-mouse islet transplant models. However, recipient responses to donor ECDI-SP in xenogeneic combinations are unique compared to those in allogeneic combinations, consequently, mechanisms of tolerance are also different. Currently, we are using pig-to-mouse xenogeneic islet transplant/immunization models to delineate tolerance requirements and tolerance mechanisms in this stringent combination. In addition, we are working closely with our collaborators to apply our tolerance strategy in non-human primates to determine its efficacy in monkey-to-monkey allogeneic transplantation and in pig-to-monkey xenogeneic transplantation. The ultimate goal is to safely and efficiently translate this approach for human transplantation.