Thymic interferons and protein O-GlcNAcylation in regulatory T cells: two tales of T cell tolerance

Abstract

Immune tolerance mechanisms prevent the development of immune responses directed to the host. This is especially important for the adaptive immune system, whose potent and long-lasting responses would be extremely deleterious to the host if misguided. This work explores two aspects of immune tolerance: the role of protein O-GlcNAcylation in regulatory T (Treg) cells and the importance of interferons during T cell tolerance development in the thymus. In chapter 2 of this document, we show that the posttranslational modification by O- linked N-Acetylglucosamine (O-GlcNAc) stabilizes FOXP3 and activates STAT5, thus integrating these critical signaling pathways. O-GlcNAc-deficient Treg cells develop normally but display modestly reduced FOXP3 expression, strongly impaired lineage stability and effector function, and ultimately fatal autoimmunity in mice. Moreover, deficiency in protein O-GlcNAcylation attenuates IL-2/STAT5 signaling, while overexpression of a constitutively active form of STAT5 partially ameliorates Treg cell dysfunction and systemic inflammation in O-GlcNAc deficient mice. These data demonstrate that protein O-GlcNAcylation is essential for lineage stability and effector function in Treg cells. In chapter 3, we characterized the expression of interferons in the thymus. We found that developing thymocytes displayed a type I IFN signature that was mainly dependent on IFN-β. Using Ifnb tdtomato and luciferase reporter mouse strains, we found expression in a small population of medullary thymic epithelial cells (mTEC), which was AIRE dependent and peaked at 2-3 weeks of age. To study the cellular response to thymic interferon, we used an Mx1gfp reporter mouse strain and report that numerous thymic cell populations respond constitutively to IFN in vivo. The response in some cell populations was not abrogated unless both IFNAR and IFNLR, or STAT1 were deficient, suggesting that both type I and type III IFNs are at play. Indeed, single cell RNA sequencing analysis revealed dramatic transcriptional changes in all thymic APCs in IFNAR/ IFNLR deficient mice. These results show that steady state type I and type III IFN signaling drives a gene-expression program in thymic APCs that shapes the thymic microenvironment.