Molecular signature of recent thymic selection events on effector and regulatory CD4 T lymphocytes Running title: Thymic selection of effector and regulatory CD4 T lymphocytes

Natural CD4CD25 regulatory T lymphocytes are key protagonists in induction and maintenance of peripheral T cell tolerance. Their thymic origin and biased repertoire continue to raise important questions on the signals that mediate their development. We validated analysis of MHC class II-capture by developing thymocytes from thymic stroma as a tool to study quantitative and qualitative aspects of the cellular interactions involved in thymic T cell development, and used it to analyze regulatory T cell differentiation in wild type mice. Our data indicate that antigen-presenting cells of bone-marrow origin, but surprisingly and importantly not thymic epithelial cells, induce significant negative selection among the very autoreactive regulatory T cell precursors. This fundamental difference between thymic development of regulatory and effector T lymphocytes leads to development of a regulatory T cell repertoire enriched in cells specific for a selected subpopulation of self-antigens, i.e. those specifically expressed by thymic epithelial cells. H A L athor m anscript inerm -000915, version 1