With the continued introduction of new agents capable of intervening at different stages in the generation of an immune response, significant progress has been made in the field of transplantation over the past decade. One of the major challenges facing the field today concerns definition of protocols that result in the establishment of a state of donor specific tolerance, thus obviating the need for life-long immunosuppression of the recipient. Several strategies proven successful for tolerance induction in rodent models have not translated-well to pre-clinical models or man. Bone marrow transplantation and the establishment of mixed hematopoietic chimerism can lead to tolerance to allogeneic organs and ceils, as well as prevention or amelioration of autoimmune disease in rodents. Clinical application of this strategy has been hampered by the requirement for intensive conditioning of the recipient in order to attain donor HSC engraftment across MHC barriers. Variables associated with the ability to overcome this challenge are being defined in a rapid manner in basic research models. In this program project, we aim to bring together and test, in a rigorous pre-clinical model, several approaches that have been demonstrated to facilitate donor HSC engraftment in rodents. In Project I, the following concepts will be tested: I) can intraportal infusion of donor hematopoietic cells and/or MSCs lead to recruitment of peripheral regulatory mechanisms that predispose the host to become tolerant to islet allografts, 2) does intraportal transplantation of stromal cells result in the establishment of a microenvironment within the liver that facilitates donor HSC engraftment (augmented by i.v. HSC infusion), and 3) do HSC repopulate host tissues, thus leading to recruitment of central deletional mechanisms that, combined with peripheral regulatory processes, will lead to a state of robust tolerance. In Project 2, we propose to: I) manipulate donor stem cells to engineer a graft that exerts immunoregulatory effects on the host, 2) determine whether stem cells of the bone marrow microenvironment can be manipulated to further enhance graft survival, and 3) address issues related to residual host T and B cells in relation to late graft loss. Overlapping both projects 1 and 2 are newly emerging concepts regarding the role of regulatory cells in the induction and maintenance of tolerance (project 3). Animals transplanted in Projects 1 (islet) and 2 (kidney) will be studied in Project 3 to identify and analyze putative regulatory cells in monkeys with stable allografts, and we will then utilize this information to refine our transplant strategies. Core A will provide MSCs for transplantation in Projects 1 and 2.