The role of TCR signals and downstream events for NKT and regulatory T cell identity and function in the mouse

T cells develop in the thymus from hematopoietic precursor cells. During maturation some T cells receive strong T cell receptor (TCR) stimulation required for the initiation of lineage defining developmental programs, a selection process referred to as agonist selection. This thesis explores the biology of two agonist selected T cell populations namely Natural Killer T (NKT) cells and regulatory T (Treg) cells by means of genetically modified mouse models. NKT cells differentiate in the thymus into NKT1, NKT2 and NKT17 cells. The expression of signature transcription factors orchestrates their cell fates and production of subset specific cytokines. The key molecules however, which regulate the differentiation into these NKT cell lineages, are only incompletely understood. Treg cells are characterized by their ability to suppress overshooting immune reactions. Their dependence on strong TCR stimulation during thymic development is well established, the role of continuous TCR signals for mature peripheral Treg cells however still needed to be thoroughly investigated. In Paper 1 we describe Roquin proteins as central suppressors of NKT17 cell polarization in the thymus. TCR signals induce the degradation of Roquin proteins via the paracaspase MALT1. Therefore, we aimed to elucidate the consequences of isolated ablation of Roquin proteins for the NKT cell lineage. Conditional knockout of Roquin 1 and Roquin 2 in T cells leads to a decrease of total NKT cells in the thymus with a selective expansion of NKT17 cells. In addition, the peripheral NKT cell pool is essentially abolished. These changes are mediated through cell intrinsic and extrinsic mechanisms. Interestingly, transgenic expression of the Vα14i TCR at the CD4 CD8 double positive (DP) stage in the thymus restores the development of NKT cells and mitigates the developmental bias towards NKT17 differentiation. Roquin deficient NKT17 cells are characterized by an increased TCR expression and are uniformely positive for all NKT17 specific markers tested. TCR signal transduction and subset specific cytokine production however depend on the expression of Roquin paralogs. In Paper 2 we analyze the distribution of functional NKT cell subsets in the thymus and lamina propria of the gut. It is known that RORγt expressing NKT17 cells constitute a main portion of NKT cells in peripheral lymph nodes. Here we show that NKT17 cells also represent the predominant NKT cell population in the lamina propria (LP) of the large intestine. The distribution of NKT cell subsets in the colon LP and thymus is also age dependent with a preponderance of early developmental stages and NKT2 cells soon after birth. By analyzing NKT cells in the gut and thymus of germ free (GF) mice we could show that the distribution of NKT cells in both organs is likewise affected by the microbiota. In Paper 3 we study the effects of induced TCR ablation on mature Treg cells. We show that once Treg cell identity, namely expression of the lineage defining transcription factor forkhead box P3 (Foxp3) and hypomethylation of Treg signature genes, has been established, it does not depend on continuous signal through the antigen receptor. However, Treg cells loose their activated phenotype after TCR ablation. In addition, the homeostasis, maintenance and suppressive functions of peripheral Treg cells depend on continuous TCR signals. Paper 4 is a review that gives a summary of the most recent data highlighting the importance of constant recognition of self for the biology and function of Treg cells.