The objective of this Program Project is to improve outcomes following heart or lung transplantation by clarifying the mechanisms involved in donor-specific tolerance induction. Previous studies funded by this Program Project have provided the basis for successfully extending a tolerance induction approach using mixed chimerism to patients who received simultaneous kidney and bone marrow transplants from living donors. Our hypothesis is that with further mechanistic study, tolerance induced through mixed chimerism will prove applicable to recipients of deceased donor organs making tolerance accessible to heart and lung allograft recipients. The integrating themes in this Program are that 1) targeting newly recognized barriers to tolerance will allow us to alter the balance of an alloresponse away from alloaggression and towards deletion/regulation leading to long-term tolerance, 2) humoral responses and proinflammatory states are particularly detrimental to tolerance induction and may be important confounding factors limiting the duration of the state of mixed chimerism and 3) durable mixed chimerism will likely be required for tolerance in heart and lung recipients because, unlike kidneys, thoracic organs do not appear capable of maintaining tolerance once chimerism disappears. In the specific aims of each Project, the contributions of allo-, auto- and natural antibodies to graft loss after mixed chimerism induction are recognized and addressed in an innovative and complementary manner. Likewise, the deleterious effects of proinflammatory molecules are considered throughout the program and addressed with innovative and complementary approaches. We anticipate ongoing progress will continue to contribute to a reduction in the morbidity and mortality associated with solid organ transplantation.