PROJECT SUMMARY/ABSTRACT Selection of the T cell receptor (TCR) repertoire is essential for producing T cells that are tolerant to self- antigens, but poised to defend the host against pathogens. Although this selection process is considered effective at removing self-reactive T cells from the repertoire, the high incidence of autoimmune diseases indicates that this process is not perfect. Thus, greater understanding of this process is needed to identify the mechanistic failures that predispose to autoimmunity. Heterogeneous populations of antigen presenting cells (APCs) in the thymus drive selection of the TCR repertoire. Strong TCR interactions with self-peptide–MHC complexes presented by these APCs result in clonal deletion or differentiation into regulatory T cells (Treg), whereas weak interactions produce the naïve TCR repertoire via positive selection. Despite the diversity in APCs that facilitate thymic selection, little is known about how they contribute independently to this process. Therefore, the aims proposed in this project are designed to clarify the respective contributions of distinct APC subsets to clonal deletion and Treg induction. Specifically, the results from these studies will further the understanding of central tolerance based on cell-specific differences in driving selection of the T cell repertoire. Aim 1 will use cell type specific MHC ablation to determine how distinct APC subsets impact clonal deletion and Treg differentiation at early and late stages in the thymus. Furthermore, the proposed experiments will distinguish if specific APC subsets are better equipped to contribute to negative selection of autoreactive thymocytes. Aim 2 will evaluate the spatial distribution of heterogeneous dendritic cell populations in the thymus. This aim will provide important insight into how subset-specific localization may dictate tolerance to non-overlapping antigens presented in the thymus. Collectively, these results will substantially contribute to the understanding of how specific APC populations drive both clonal deletion and regulatory T cell development in a non-redundant manner. Moreover, this application provides a rigorous, defined scientific framework to foster the career goals outlined in this application for becoming a successful physician scientist.