PROJECT SUMMARY/ ABSTRACT Autoimmunity is defined as the loss of tolerance to self-antigen resulting in the targeted destruction of tissue by immune cells. Autoimmune conditions are on the rise globally while our knowledge on how autoimmunity emerges is still limited. The thymus is a highly structured organ that is essential for T cell development and education. The nature of the gene rearrangements on the T cell receptor (TCR) allows for recognition of self and non-self-antigens. Positive selection occurs in the thymus cortex where thymocytes receive survival signals when selected against self-antigen by cortical thymic epithelial cells (cTECs). Negative selection occurs in the thymus medulla driven by medullary thymic epithelial cells (mTECs), and a diverse repertoire of antigen presenting cells (APCs) including different subsets of dendritic cells (DCs) and B cells. The signaling strength of the tri-molecular complex (TCR-peptide-MHC) between APCs and thymocytes dictates cell fate. Thymocytes who react too strongly to self-antigens are deleted, become anergic or divert to regulatory T cell (Treg) lineage. In optimal conditions T cell selection prevents the release of autoreactive T cell clones to the periphery a critical process to prevent the onset of autoimmunity. However, the mechanisms behind the escape of autoreactive T cells to the periphery in autoimmune conditions are poorly understood, and the study of T cell education in a primary lymphoid organ is not feasible in humans. Mouse models have demonstrated to be powerful tools in thymus studies, as several developmental and functional mechanisms are evolutionary conserved. Migratory and resident APCs are known to drive clonal deletion and Treg lineage in both the humans and mice. CX3CR1 is expressed in a subset of tolerogenic B cells and Mononuclear phagocytes (MNPs): DCs, macrophages and monocytes. CX3CR1+ APCs present antigen to CD4+ T cells in peripheral organs. CX3CR1+ APCs in the gut induce differentiation of peripheral Tregs via transcription factor Foxp3. The close interaction of CX3CR1+ APCs with CD4+ T cells in the periphery lead us to hypothesize whether a similar population of CX3CR1+ APCs resides in the thymus to drive selection of T cells and may play a role in thymic Treg development. Our preliminary data shows a population of CX3CR1+ APCs in the thymus capable of antigen specific stimulation of CD4+ T cells in vitro. The long-term goal of the proposed work is to understand how autoreactive T cells develop and escape selection mechanisms in the thymus driven by CX3CR1+ APCs and ultimately drive the emergence of autoimmune disease. Our studies aim to investigate the contribution of APCs to central T cell tolerance with the goal of finding potential new targets for treatment of autoimmune disease.