Notch Signaling in Pre-thymic T-cell Differentiation and Intrathymic γδ T-cell Development

T-cell development in the postnatal thymus is contingent on the successful recruitment of bone marrow (BM)-derived thymus-seeding progenitors (TSPs). Upon entry into the thymus, TSPs are termed early T-cell progenitors (ETPs) and subsequently engage Notch ligands on thymic epithelial cells (TECs) to specify and commit to the T-cell lineage. Specification occurs at the CD4-CD8- double negative (DN)2 stage of T-cell development while commitment occurs at the DN3 stage. At the DN3 stage, T-cell progenitors also bifurcate into either the - or the -lineage. -lineage committed cells develop into CD4+CD8+ double positives (DPs) before maturing into CD4+ or CD8+ single positive (SP) T-cells. -lineage committed cells remain DN and mature through strong TCR signaling. In contrast to T-cells, T-cells are able to be programmed to specific effector fates during their thymic development. The role of Notch signaling throughout intrathymic T-cell development from T-lineage specification/commitment, to /-lineage bifurcation, and CD8+ SP differentiation is well understood. However, whether Notch plays a role in a pre-thymic setting to generate T-lineage competent TSPs remain unclear. In addition, the role of Notch signaling, along with other factors such as TCR signaling and temporal regulation, in intrathymic effector programming of T-cells remain to be further elucidated. Using an RBPJ-inducible (RBPJind) mouse model in which Notch responsiveness can be regulated in hematopoietic cells, I revealed that Notch signaling plays a critical pre-thymic role to generate T-cell competent TSPs and thus thymic appearance of ETPs. With the RBPJind system, I also revealed the influence of different temporal periods in the generation of specifc T-cell V subsets and effector subsets. Using a TCR transgenic mouse model (termed KN6tg), I revealed that strong TCR signaling promotes the generation of IL-4 producers, while weak TCR signaling promotes the generation of IL-17 producers. The combination of KN6tg and RBPJind approaches revealed that Notch signaling plays an inhibitory role in -lineage commitment and thus optimal IFN production in the presence of weak TCR signals. However, Notch signaling plays a positive role in promoting optimal generation of IL-4 producers in the presence of strong TCR signals. Taken together, this work highlights an important role for Notch in T-cell differentiation in a pre-thymic setting, and establishes the precise requirements for temporal timing, TCR, and Notch in the thymic programming of specific T-cell effector fates.Ph.D.2020-11-11 00:00:0