Conditional deletion of CD25 in Tregs abrogates their suppressive function independently of Foxp3

Abstract

Abstract IL-2R signaling is essential for thymic development and homeostasis of CD4+Foxp3+ Tregs, which are required to maintain self-tolerance. Although deletion of IL-2/IL-2R in mice leads to lethal autoimmunity, disease is delayed in comparison to mice lacking Foxp3. It is unclear whether this delay is due to redundant signaling in Tregs by other γc cytokines, or to loss of IL-2 activation of autoreactive T cells. To explore IL-2R function in Tregs, we evaluated the consequence of germline knockout (gKO) versus Treg-conditional knockout (cKO) of CD25. Both types of KO mice developed lethal systemic autoimmunity, but CD25 cKO mice exhibited a scurfy-like phenotype with a life span of 3–4 weeks, while death was delayed until 8–12 weeks in gKO mice. CD4+ and CD8+ T cells were more activated and produced higher levels of TNFα, IL-6, and IFNγ in CD25 cKO mice early in life when compared to CD25 gKO mice. This result in conjunction with normal thymic selection in both models indicates that the slower tempo of autoimmunity in CD25 gKO mice is due to impaired function of their autoreactive T cells, which does not occur in CD25 cKO mice. In addition, Foxp3 levels were only slightly reduced in thymocytes from both models, but they each showed diminished expression of Treg suppressive markers. This finding is consistent with the idea that IL-2R signaling is essential for functional programming of Tregs independent of Foxp3 expression. The effects noted above for nascent thymic Tregs in gKO mice were somewhat more striking than in cKO mice. This observation suggests another IL-2-dependent step in Treg development that is likely redundant. Overall, our data indicate that Tregs are completely non-functional without IL-2R signaling.