CD4 and CD8 T cells express αβ T cell receptors (TCR) and respectively function as "helpers" and "cytotoxic" effector cells during an immune response. Importantly, CD4 T cells recognize peptides presented by major histocompatibility complex class II (MHCII) molecules and CD8 T cells recognize peptides presented by MHCI molecules. Intriguingly, both CD4 and CD8 T cells arise from immature CD4 + CD8 + (double positive, DP) thymocytes in the thymus. However, how DP thymocytes successfully differentiate into CD4 lineage T cells expressing MHCII-specific TCR and CD8 lineage T cells expressing MHCI-specific TCR remains an unresolved and intensely debated aspect of T cell biology. Previous investigations on the mechanism of the CD4/CD8 lineage decision have either focused on signals contributed by individual coreceptors or by the TCR itself. Given their distinct regulatory elements, I hypothesized that the kinetics of Cd4 and Cd8 gene transcription during T cell differentiation in the thymus determines specification of the CD4 and CD8 T cell fates. In this dissertation, I found that the process of CD4/CD8 lineage choice in the thymus is not stochastic. Significantly, I revealed a novel integration of TCR and cytokine signals by cis -regulatory Cd8 gene enhancers during positive selection and CD8 T cell lineage commitment. Specifically, during positive selection, TCR signals terminated the activity of cis enhancers that promote CD8 expression in pre-selection thymocytes while Stat5-dependent cytokines (notably IL-7) promote the activity of cis -regulatory Cd8 enhancers in post-selection T cells. Indeed, cis -regulatory enhancers that promote CD8 expression in post-selection thymocytes are critical in reactivating Cd8 expression in CD4 + CD8 - intermediate thymocytes that give rise to mature T cells. Consistent with the hypothesis that CD4/CD8 lineage choice is determined by the Cd4 and Cd8 transcriptional machinery, MHCII-selected T cells in mice in which the Cd8a gene was engineered to encode CD4 protein differentiated into cytotoxic CD8 lineage T cells. Based on the data presented in this dissertation, I conclude that CD4/CD8 lineage specification in the thymus is dictated by the kinetics of coreceptor gene transcription and not by the identity of the coreceptor.
