Functions of Notch1, Notch3, and Fringe Glycosyltransferases during Thymocyte β-Selection and T-cell Leukemogenesis

T-cells play critical roles in the immune system, from fighting infection and tissue remodeling to anti-tumor immunity. Notch1 (N1) signaling is a conserved signaling pathway that is required for T-cell specification and commitment of multi-potent progenitors. Subsequently, N1 co-operates with pre-T-cell receptor (pre-TCR) signaling to induce self-renewal of TCRβ+ CD4/CD8 double negative (DN) progenitors. This “β-selection” process produces large numbers of CD4/CD8 double positive (DP) progenitors whose survival is N1-independent. Thus, N1 signaling plays key roles at several stages of intrathymic T-cell development. However, excessive N1 activation causes T-cell lymphoblastic leukemia/lymphoma (T-LL). Hence, the goal of my thesis was to elucidate how N1 signaling is regulated during thymocyte β-selection. There are four mammalian N receptors (N1-4), activated by ligands of the related Delta-like (DL1 and DL4), and Serrate/Jagged (Jag1 and Jag2) families. Fringe glycosyltransferases modify N receptors, enhancing activation by DL ligands and inhibiting activation by Jag1. My thesis goals were to elucidate: 1) the role of N3 downstream on N1 in thymocyte β-selection and T-cell leukemogenesis; 2) how Fringe glycosyltransferases modulate N1 sensitivity to DL vs Jag family ligands during β-selection; and 3) the global impact of ectopic N1 activation on T-cell development. In Chapter 3, I examined the role of N3 in thymocyte β-selection and N1 induced T-LL using N3–deficient mice, and showed that N3 is dispensable for both processes. In Chapter 4, I demonstrated that Lfng and Mfng have distinct functions in modulating N activation by DL1 and DL4 versus Jag1 during β-selection. Unexpectedly, Jag2-induced β-selection was uniquely insensitive to both Lfng and Mfng. Finally, in Chapter 5, I generated a new mouse model to ectopically activate N1 signaling in TCRβ+ DN progenitors, and used it to demonstrate that N1 signaling impacts thymocyte self-renewal during β-selection in vivo. As expected, ectopic N1 signaling in DP thymocytes altered CD4/CD8 lineage commitment and caused T-LL. However, ectopic N1 activation also prevented development of certain innate-like T-cells and caused T-cell lymphopenia, and eventually lead to auto-immune-like pathologies. Collectively, my findings expanded our understanding of N regulation in β-selection, and revealed unexpected effects of aberrant N1 activation in these stages.Ph