Attempts to specifically induce in vivo immunological unresponsiveness in humans have been uniformly unsuccessful. Many treatment regimens which have resulted in tolerance in small laboratory animals have, at best, produced marginal immunosuppression in costly primate models and in human clinical trials. The focus of this project has been to develop an in vivo model of the human allograft response. In such a model, both human allograft immunity and its manipulation to produce immunological unresponsiveness in vivo can be studied in a non-clinical setting in a cost effective fashion. Mice with severe combined immunodeficiency (SCID) have been lightly irradiated and engrafted with human spleen cells (hu.Spl.SCID). Distinct from many other models of human lymphocyte engraftment in SCID mice, macrochimerism is established. Human T cells traffic to murine lymphoid organs and human "allografts" such as skin or pancreatic islets. T cell alloresponsiveness is maintained despite an intercurrent graft vs host response. Actual human "allograft" rejection is reproducably achieved. The focus of the proposed research will be to dissect and understand the components of the human T cell response to alloantigen in the in vivo xenogeneic environment. The human leukocyte populations necessary for engraftment and graft rejection will be identified. The question of why human spleen cells engraft better than PBL will be explored. Factors which regulate human T cell migration in the xenogeneic environment will be enumerated. The role of noncognate recognition in in vivo human transplant rejection will be examined. From an understanding of in vivo cellular interactions in human allograft rejection, strategies to elicit immunological unresponsiveness will be devised and tested, specifically focusing on agents affecting homing, adhesion, T cell receptor engagement and costimulation. The resulting in vivo intragraft events, with specific attention to T cell subpopulations, e.g., Th1 vs Th2, will be studied in unmodified rejection and in manipulated animals. These studies should provide a reliable, cost-effective approach to examine the effect of immunosuppressive regimens in vivo on human immunity. They allow extension of in vitro studies of human T cell interactions such as adhesion and signalling. More fundamentally, they provide a unique opportunity to study the manipulation of human allograft rejection in vivo in a controlled laboratory model in a way that allows investigation of the cellular events which lead to graft survival prolongation.