Regulation of apoptosis-induction in thymocytes

Most T cells are produced in the thymus, where they proceed through several differentiation steps to shape the TCR repertoire. The first thymocyte subset to express a TCR complex is characterized by the expression of CD4 and CD8 co-receptors, CD4+CD8+ thymocytes. At this stage, the CD4+CD8+ thymocytes' fate relies on the affinity/avidity of their TCR towards self-peptide/MHC complexes. Indeed, elimination of thymocytes by apoptosis occurs in death by neglect, due to the absence of TCR-peptide/MHC interactions of sufficient affinity/avidity, and during negative selection, upon interactions of strong affinity/avidity. In contrast, weak affinity/avidity binding of TCR with its ligand would lead to positive selection followed by thymocyte maturation. This dissertation centers on understanding the factors involved in the elimination of thymocytes at the CD4+CD8+ stage of thymic differentiation, namely death by neglect and negative selection.Particularly, we address the role of CD45 in thymocyte deletion. Ligation of CD45 in the absence of TCR signaling has been shown to induce mature T cell death. Thus, we have evaluated the consequences of CD45 ligation on thymocytes in the absence of TCR triggering, and shown that it induces the death of thymocytes with rapid kinetics. Moreover, this non-necrotic form of cell death is independent of caspases and does not lead to DNA fragmentation. Based on these results, we propose a role for CD45 in the induction of death by neglect. Next, we have evaluated the contribution of factors involved in negative selection. In particular, the individual impact of TCR-ligand affinity, TCR-ligand density, and costimulation in the process of negative selection was studied, We found that, with ligands of physiological affinity, costimulation is required for the induction of negative selection. In addition, our results suggest that increasing TCR ligand density or costimulation does not always promote thymocyte deletion. Finally, we studied some aspects of the intracellular activation signaling pathways leading to thymocyte apoptosis. Specifically, caspase-3 was shown to participate in negative selection, and regulation of Nur77 activity on a physiological mammalian DNA response element was identified, In summary, this work contributes to understanding the regulation of the apoptosis-induction in developing thymocytes