Development Of Regulatory T Cells Capable Of Maintaining Immune Homeostasis

Abstract

The adaptive immune response, comprised of both T cells and B cells, is essential to control infections and eliminate transformed cancer cells. The success of the adaptive immune system relies on the ability to discriminate self from non-self-antigens. The thymus is the site of selection for T cells, where self-reactive T cells are eliminated, generating a non-self focused T cell compartment. However, this selection process is leaky and potentially pathogenic cells do escape thymic, or central, tolerance. Thus, a population of suppressor cells termed regulatory T cells (Treg cells) co-evolved in order to keep these self-reactive escapees in check. Treg cells that develop in the thymus as part of central tolerance induction are a critical population of T cells that are required to maintain immune homeostasis and prevent autoimmunity. Without intervention, mice or humans that lack the ability to generate Treg cells die shortly after birth from widespread autoimmune-mediated tissue destruction. Further, neonatal thymectomy in mice causes the development of an autoimmune wasting phenotype. These observations highlight the importance of thymic Treg cell selection in immune homeostasis. Thymic Treg cell development occurs via a two-step process. Step one involves developing CD4+ thymocytes receiving strong T cell receptor (TCR) stimulation via engagement of thymic self-antigens, leading to upregulation of CD25, the high affinity subunit of the IL-2 receptor, or FOXP3, the lineage defining transcription factor of Treg cells, generating either CD25+ or FOXP3lo Treg cell progenitors (TregP). Step two is driven by encounters between TregP cell and intrathymic STAT5 activating cytokines, predominantly IL-2, leading to co-expression of CD25 and FOXP3. These CD25+FOXP3+ cells represent fully mature Treg cells that disseminate from the thymus to mediate immune tolerance. While the framework of this two-step development process is understood, many details of each step remain incompletely understood. This thesis addresses several aspects of thymic Treg cell development. First, we identify that T cells are the critical source of IL-2 required to drive Treg differentiation. Second, we provide evidence that CD25+ and FOXP3lo TregP arise via distinct selection programs and contribute functionally distinct TCRs to the mature Treg compartment. Third, using single-cell RNA-sequencing analysis of conventional and Treg lineage thymocytes we provide a more detailed analysis of transcriptional signatures and intermediates of thymic Treg development. Finally, we gathered preliminary data to better understand the heterogeneity and function of recirculating or resident thymic Treg cells. Developing a holistic understanding of Treg development is essential to discern the etiology of immune disorders and properly modulate Treg cells to treat autoimmune disease, infections and cancer.