Regulatory T (Treg) cells express tumor necrosis factor receptor superfamily (TNFRSF) members, but their role in thymic Treg development is undefined. We demonstrate that Treg progenitors highly express the TNFRSF members GITR, OX40, and TNFR2. Expression of these receptors correlates directly with T cell receptor (TCR) signal strength, and requires CD28 and the kinase TAK1. Neutralizing TNFSF ligands markedly reduced Treg development. Conversely, TNFRSF agonists enhanced Treg differentiation by augmenting IL-2R/STAT5 responsiveness. GITR-ligand costimulation elicited a dose-dependent enrichment of lower-affinity cells within the Treg repertoire. In vivo, combined inhibition of GITR, OX40 and TNFR2 abrogated Treg development. Thus TNFRSF expression on Treg progenitors translates strong TCR signals into molecular parameters that specifically promote Treg differentiation and shape the Treg repertoire.

The developmental programs that generate a broad repertoire of regulatory T cells (T reg cells) able to respond to both self antigens and non–self antigens remain unclear. Here we found that mature T reg cells were generated through two distinct developmental programs involving CD25 + T reg cell progenitors (CD25 + T reg P) and Foxp3 lo T reg cell progenitors (Foxp3 lo T reg P). The CD25 + T reg P had higher rates of apoptosis and interacted with thymic self-antigens with higher affinity than Foxp3 lo T reg P, and had a T cell antigen receptor (TCR) repertoire and transcriptome distinct from that of Foxp3 lo T reg P. The development of CD25 + T reg P and Foxp3 lo T reg P was controlled by distinct signaling pathways and enhancers. Transcriptomic and histocytometric data suggested that CD25 + T reg P and Foxp3 lo T reg P arose by coopting negative and positive selection programs, respectively. T reg cells derived from CD25 + T reg P, but not Foxp3 lo T reg P, prevented experimental autoimmune encephalitis. Our findings indicate that T reg cells arise through two distinct developmental programs that are both required for a comprehensive T reg cell repertoire capable of establishing immune tolerance.

The cytokine IL-2 is critical for promoting the development, homeostasis, and function of regulatory T (Treg) cells. The cellular sources of IL-2 that promote these processes remain unclear. T cells, B cells, and dendritic cells (DCs) are known to make IL-2 in peripheral tissues. We found that T cells and DCs in the thymus also make IL-2. To identify cellular sources of IL-2 in Treg cell development and homeostasis, we used mice to selectively delete in T cells, B cells, and DCs. Because IL-15 can partially substitute for IL-2 in Treg cell development, we carried out the majority of these studies on an background. Deletion of in B cells, DCs, or both these subsets had no effect on Treg cell development, either in wild-type (WT) or mice. Deletion of in T cells had minimal effects in WT mice but virtually eliminated developing Treg cells in mice. In the spleen and most peripheral lymphoid organs, deletion of in B cells, DCs, or both subsets had no effect on Treg cell homeostasis. In contrast, deletion of in T cells led to a significant decrease in Treg cells in either WT or mice. The one exception was the mesenteric lymph nodes where significantly fewer Treg cells were observed when was deleted in both T cells and DCs. Thus, T cells are the sole source of IL-2 needed for Treg cell development, but DCs can contribute to Treg cell homeostasis in select organs.

Development of a comprehensive regulatory T (Treg) cell compartment in the thymus is required to maintain immune homeostasis and prevent autoimmunity. In this study, we review cellular and molecular determinants of Treg cell development in the thymus. We focus on the evidence for a self-antigen–focused Treg cell repertoire as well as the APCs responsible for presenting self-antigens to developing thymocytes. We also cover the contribution of different cytokines to thymic Treg development and the cellular populations that produce these cytokines. Finally, we update the originally proposed “two-step” model of thymic Treg differentiation by incorporating new evidence demonstrating that Treg cells develop from two Treg progenitor populations and discuss the functional importance of Treg cells generated via either progenitor pathway.

Cells arrested in mitosis by inactivation of the APC/C complex sometimes manage to exit mitosis in a process called mitotic slippage, which helps cancer cells circumvent chemotherapy drugs. Balachandran et al. show that mitotic slippage occurs as a result of targeting of cyclin B1 for degradation by the ligase CRL2ZYG11.