The goal of this proposal is to define the key events ongoing in the thymus that shape the repertoire of autoreactive T cells. For this purpose, we will exploit a novel model system in which thymi from NOD female mice of varying ages are transplanted under the kidney capsule of NOD.scid recipients resulting in T cell reconstitution of the periphery. Thymi prepared from fetal and newborn NOD donors produce CD4+ and CD8+ T cells characterized by a type 1 phenotype. Importantly, the NOD.scid recipients develop overt diabetes, in addition to thyroiditis. In marked contrast, NOD.scid mice transplanted with thymi from 4 week-old or older NOD donors develop severe colitis but remain free of diabetes and thyroiditis. We hypothesize that age- dependent events regulate the efficacy of thymic positive and negative selection that in turn promote the temporal development of pathogenic T precursors. Three Specific Aims have been established to determine the mechanisms regulating the temporal development of T cell tissue-specificity. The first will determine how age-dependent thymic events shape the repertoire of autoreactive T precursors. The second Specific Aim will determine the cellular events ongoing in the thymus that mediate the temporal development of T precursors mediating autoimmunity and colitis. The final Specific Aim will ascertain the contribution of genetic background and thymic expressed tissue antigens in the temporal development of tissue-specific T cells. In this way, novel insight will be gained into the critical events that shape the repertoire of autoreactive thymocytes.