The application of Tregs in the context of organ
transplantation is supported further by the seminal work by signaling pathway Sakaguchi et al. , who showed that Tregs from naive mice prevented rejection of allogeneic skin grafts in T cell-deficient nude mice given CD25– T cells. Subsequently, a series of preclinical rodent models of skin and cardiac transplantation demonstrated that Tregs present in the recipient at the time of transplantation are critical in the induction and maintenance of tolerance (reviewed in ). In support of such studies we have also generated Treg lines in vitro, and shown that these Tregs are very effective at inducing survival of MHC-mismatched heart allografts . Furthermore, in a murine skin transplant model following thymectomy and partial T cell depletion, we have demonstrated previously the ability of in-vitro-expanded Tregs in inducing donor-specific transplantation tolerance in this system . JNK high throughput screening The importance of adoptive Treg therapy in transplantation is supported further in mouse models of bone marrow transplantation, where the transfer of freshly isolated Tregs together with the bone-marrow allograft has been shown to ameliorate GVHD and facilitate engraftment . GVHD was also the first model in which it was shown that the adoptive transfer of ex-vivo-expanded donor Tregs was highly
effective in preventing acute or chronic GVHD . Moreover, the adoptive transfer of Tregs has been shown to prevent rejection of pancreatic islet  and other organ allografts [46, 47]. The use of currently available humanized mouse models of GVHD and allotransplantation [48, 49] has reinforced further the importance of Tregs in these settings. These models are based on the reconstitution of immunodeficient mice with human immune
cells. More recently we have also shown the efficacy of human Tregs in preventing alloimmune dermal tissue injury in a humanized mouse model of skin transplantation . Furthermore, Nadig et al.  of developed a human vessel graft model to study the in-vivo function of Tregs. Their results showed convincingly that grafts from mice reconstituted with peripheral mononuclear cells (PBMCs) alone exhibited extensive vasculopathy, whereas the co-transfer of Tregs prevented this process. Such adoptive transfer experiments in rodents, therefore, support the notion that tolerance requires ‘tipping the balance’ between reactivity and regulation. Despite such data generated in preclinical animal models, showing successfully that Tregs can induce and maintain transplantation tolerance, we currently face many challenges in the laboratory that have hindered the widespread application of Treg cell therapy in the transplant setting. In addition, a number of different strategies have been proposed for the isolation and expansion of Tregs for cellular therapy.