Investigating the role of TCR signalling during T cell development

Abstract

The thymus generates multiple T cell populations from an early common CD4(-) CD8(-) double negative (DN) progenitor. Conventional αβ T cells, such as CD4(+) and CD8(+) T cells undergo thymic positive selection on partial agonist interactions with self-peptide/MHC, whereas unconventional TCRαβ(+)CD8αα(+) intraepithelial lymphocytes (IELs) and regulatory T cells (Tregs), are thought to require full agonist interactions. By contrast, very little is known about how γδ T cells undergo selection. While it has been suggested that IL-17A-secreting γδ T cells rely on strong TCR signals resulting from potential interactions with a ligand, several studies have led to opposite conclusions. This thesis investigates the role of TCR signal strength during the early stages of thymocyte development, and how this dictates subsequent T cell fate. We show that FVB/n mice have relatively few unconventional TCRαβ(+)CD8αα(+) IELs in comparison to C57Bl/6 animals. This depleted IEL compartment correlated with reduced thymic IEL progenitors, an inefficient preTCR-driven DN-to-DP transition, and a relatively increased expression of full length pTαa transcripts in CD44(-)CD25(+) DN3 cells. Transgenic over-expression of pTαa in the absence of pTαb transcripts mimicked the phenotype of FVB/n animals. CD5 levels on DN3 cells from pTαa transgenic mice indicate that preTCR signalling was weaker than in C57Bl/6 mice, suggesting that signal strength at the β-selection checkpoint may influence subsequent selection of mature T cell populations. Consistent with this, we show that OT-II mice, which express a transgenic TCRαβ from an early DN stage of thymocyte 6 development, and which have high levels of CD5 on DN3 cells, have a relatively increased unconventional TCRαβ(+)CD8αα(+)IEL compartment. For γδ T cell development, we also show that weakening signal strength impairs the development of IL-17A-secreting γδ T cells. However, increasing signal strength, through the use of a cross-linking antibody, does not favour their development as predicted. These seemingly paradoxical observations are explored further. Taken together, these data suggest that the quality or quantity of TCR-mediated signals early in T cell development have fundamental consequences for subsequent T cell selection events that differentially generate distinct T cell subsets.