Role of Notch Signaling and GATA-3 in Establishing T-lineage Commitment and Survival in Developing Thymocytes

The adaptive immune response, including antigen-specific T cells, is crucial in resolving infections and protecting against tumourigenesis. However, the precise molecular signals needed for the development of these cells are still not clearly defined. The Notch signaling pathway is required to commit progenitors to the T-lineage and acts in concert with many transcription factors such as GATA-3 to fully induce this fate choice while inhibiting the B-cell lineage. Although both Notch and GATA-3 are indispensible for thymocyte development, their precise roles in directing these events remain to be elucidated. Examining such phenomena during ontogeny has been prohibited by genetic lethality caused by deletion of these genes early in development. Work presented in this Thesis provides means to circumvent this obstacle by establishing genetic systems, which allow to examine precise roles of Notch and GATA-3 in adult mice. Using RBPjind mice in which Notch responsiveness in all cells of hematopoietic lineage can be controlled with a Teton-inducible system, we demonstrate that Notch is absolutely required for generation of αβ and γδ thymocytes as well as marginal zone B-cells, while inhibiting B-lineage fate from bone marrow-derived progenitors. Similarly, conditional deletions of GATA-3 establish a profound need for this transcription factor in T-lineage specification for inducing expression of Bcl11b and directing early thymocyte proliferation and survival through repression of cell cycle-dependent kinase inhibitor 2b. Unlike Notch, GATA-3 was not required to inhibit B-lineage development but was crucial in preventing myeloid lineage outcome. These findings characterize a novel role for GATA-3 during T-lymphopoiesis and cement Notch signaling as a key lineage-commitment factor.Ph.D