The applicant is a physician-scientist dedicated to investigating clinically-relevant questions with the goal of developing a career as an independent investigator. Through training in immunology and clinical pathology focusing on transplantation and clinical histocompatibility the candidate has developed clinical expertise necessary for discerning clinically-relevant and scientifically important questions and translating basic science findings into clinical medicine. With these skills, the candidate, with help from mentor Paul M. Allen at Washington University in St. Louis, has developed a research project to address the fundamental barrier to allogeneic transplantation, the strong primary alloreactive response of T cells to allogeneic major histocompatibility complex (MHC) molecules. While it has long been recognized that the wide array of polymorphisms in MHC in combination with the direct role of MHC in activation of T cells through the T cell receptor (TCR) makes MHC the primary antigenic barrier to transplantation, the molecular interactions between TCR and allogeneic MHC defining alloreactivity remain enigmatic. Alloreactivity is important both in fundamental understanding of T cell recognition of antigen, as well as in clinical allogeneic transplantation. The applicant and mentor hypothesize that the repertoire of alloreactive T cells mediating graft vs. host disease (GvHD), a highly clinically-relevant manifestation of alloreactivity, is expanded by T cells naturally expressing dual TCRs and that the TCR repertoire involved in GvHD is greatly influenced by CDR3 composition which is critical for specific or polyspecific recognition of presented peptide antigens. To address these questions, the applicant proposes research that continues investigation of the influence of the TCR repertoire in GvHD; the applicant proposes to translate preliminary results demonstrating the participation of highly alloreactive dual TCR T cells in GvHD to human models, including clinical feasibility studies for diagnostic test development. Additionally, the applicant proposes examining the breadth of the repertoire of T cells mediating GvHD, to identify molecular determinants of the TCR repertoire predisposing alloreactivity, and subsequent functional studies to determine critical molecular interactions of the TCR with allogeneic MHC and presented peptides. Together, these studies will improve understanding of molecular interactions defining the alloreactive T cell response responsible for GvHD, enabling further investigation and rational design of improved diagnostics, immunosuppressive agents, and donor matching strategies. The work proposed in this application, in combination with the mentorship of Dr. Allen and the support of the academic community at the Department of Pathology and Immunology at Washington University will enable the applicant to develop into an independent investigator physician scientist. Molecular Definition of the Alloreactive T Cell Receptor Repertoire