The Role of IL-2Rα-Dependent Signaling in Treg Development, Homeostasis, and Suppressive Function

Deletion of interleukin-2 (IL-2) or IL-2 receptor (IL-2R) subunits in mice causes lethal autoimmunity, establishing the vital contribution of IL-2R signaling to thymic development of Foxp3+ Tregs. However, precise mechanisms of IL-2-driven functional programming in Tregs and pathogenic autoreactive T cells remain undefined. To investigate these issues, we characterized a germline knockout (gKO) versus Treg-targeted conditional knockout (cKO) of the IL-2Rα chain (CD25) in mice. Both KO models developed lethal autoimmunity, but disease onset was delayed in CD25 gKO mice due to IL-2 deprivation among CD4+ and CD8+ T cells. Foxp3+ thymocytes from gKO mice showed greater reduction in Treg suppressive markers than cKO mice, along with unique defects in Ki67 and Bcl-2 expression, suggesting that an early IL-2-dependent programming step operates prior to Foxp3 induction. To evaluate IL-2R signaling in mature Tregs independently of thymic development, we bred mouse strains allowing tamoxifen-inducible genetic ablation of CD25 and fate mapping within the Treg pool. IL-2R signaling was universally required for persistence of mature Tregs in the periphery, as CD25 KO Tregs were undetectable in all tissues examined by ~15 weeks post-induction. CD25 KO Tregs showed increased apoptosis, oxidative stress, and signs of mitochondrial dysfunction, along with reduced transcription of key enzymes of lipid and cholesterol biosynthetic pathways, suggesting that IL-2 governs mature Treg survival through a non-conventional mechanism of metabolic regulation. Divergent transcriptional changes in CD25 KO Tregs versus CD25 cKO Foxp3+ thymocytes indicate that IL-2R signaling in the thymus and the periphery represent two fundamentally distinct categories of influence