PROJECT SUMMARY / ABSTRACT Achieving tolerance, defined as long-term allograft survival in the absence of ongoing immunosuppression, is the ultimate goal in transplantation. Tolerance of kidney allografts has been achieved in non-human primates (NHPs) and in humans using a combination of non-myeloablative conditioning and donor bone marrow transplantation (DMBT) that results in transient donor chimerism. However, until now, transient mixed-chimerism protocols that achieve long-term tolerance of kidney allografts in NHPs have consistently failed to induce tolerance in recipients of heart allografts. It is well known that some organs, such as kidney and liver, are “tolerance-prone” while others, such as heart and lung, are “tolerance-resistant.” Despite the immune barriers posed by the heart, our laboratory has recently developed novel protocols that have, for the first time, achieved long-term, stable tolerance of heart transplants in cynomolgus monkey recipients. This remarkable result was attained in heart recipients exhibiting only transient mixed chimerism by including donor kidney cotransplantation, which enhanced the contributions of host regulatory T cells following mixed chimerism conditioning. While the combination of kidney and heart transplantation produced remarkable results, this strategy does not have wide clinical feasibility due to the use of experimental agents and the ethical barrier of sacrificing a kidney simply to induce heart tolerance. Although lung allografts will not be studied in this Program, our recent findings in lung recipients provide proof of principle that durable mixed chimerism can be achieved in NHPs and that this state results in long term tolerance of resistant thoracic organs; these were the first NHPs to become tolerant of lung allografts. Thus, the unifying goal of this overall Program is to design innovative mixed chimerism strategies for heart alone recipients that either amplify the contributions of regulatory T cells and macrophages in transient mixed chimerism protocols (Project 1) or achieve durable mixed chimerism (Project 2) while using clinically-relevant agents for host conditioning (Project 3). Project 1 will focus on enhancing the contribution of regulatory T cells and macrophages in protocols which induce transient mixed chimerism. Project 2 will focus on achieving durable mixed chimerism using next-generation conditioning regimens, innovative immunosuppression platforms. Project 3 will investigate novel, clinically-applicable, agents which enhance costimulation blockade and diminish toxicity. Core A will investigate the mechanisms by which treatments described in Projects 1-3 influence the host immune response. The Program will be organized in such a way that early mechanistic discoveries/advances in Core A will inform and refine the aims of Projects 1-3. The complementary models and approaches described in this application are unique strengths of this program project. We anticipate that toget...