Tumor-Necrosis Factor Receptor Superfamily Costimulation Drives Thymic Regulatory T Cell Development

Abstract

Thymic-derived CD4+Foxp3+ regulatory T cells (Tregs) are critical for preventing autoimmune disease. Previous work has demonstrated that Tregs develop by an IL-2R/STAT5-dependent mechanism, and that the T cell antigen receptors (TCRs) collectively expressed by thymic-derived Tregs react with self-antigen with high-affinity. In this regard, Tregs are similar to autoreactive T cells undergoing negative selection. Relatively little is known about how the selection bias in the Treg TCR repertoire is imposed and how the decision to become a Treg as opposed to undergoing apoptosis is made. This thesis demonstrates that progenitors of thymic Tregs, which are CD25+ and Foxp3-, highly upregulate three family members of the TNF receptor superfamily (TNFRSF): GITR, OX40, and TNFR2. The expression level of these receptors directly mirrors perceived TCR signal strength by developing Tregs. Stimulating Treg progenitors with the ligands for these receptors enhances the expression of the IL-2 receptor alpha chain, CD25, resulting in increased sensitivity to IL-2, and ultimately increasing the likelihood of maturation into the Foxp3+ thymic Treg lineage. Loss of TNFRSF expression in Treg progenitors blocks Treg development in vivo. Finally, we demonstrate that TNFRSF costimulation directly shapes the Treg TCR repertoire. A second line of investigation aims to uncover the developmental and functional potential of a second, more recently described population of Treg progenitors which are CD25- and Foxp3lo. We find that despite lacking CD25, these cells do efficiently respond to IL-2, and that one potential explanation for this finding is that CD25-Foxp3lo Treg progenitors express higher levels of CD122 (the IL-2 receptor beta chain that mediates signaling upon ligand binding). CD25-Foxp3lo Treg progenitors are phenotypically `older' and have resided in the thymus longer than CD25+Foxp3- Treg progenitors, and this may also explain their ability to enter the Treg lineage given their elevated levels of CD122. Finally, we demonstrate that both populations of Treg progenitors are responsive to TNFRSF costimulation and that ligation of GITR in these cells shapes the resultant Treg TCR repertoire. Further pending studies involving TCR sequencing and in vivo competition experiments will fully elucidate the significance of this alternative population of Treg progenitors.