Disruption of IL-33-mediated costimulation to CD4⁺ T cells leads to peripheral Treg generation and resolution of T cell-mediated pathology

Stromal signals are becoming appreciated as critical coordinators of immunity. We recently demonstrated that fibroblastic reticular cell (FRC)-derived IL-33 directly stimulates CD4 +T cells during priming to promote IL-12-independent Type 1 T helper cell (Th1) differentiation and expansion. Utilizing TCR transgenic (Tg) T cells lacking the IL-33 receptor, ST2, we established that IL-33 is a costimulatory molecule augmenting TCR signaling to low affinity or concentration antigens. Blockade of costimulatory molecules can lead to peripheral regulatory T cell (pTreg) induction and our RNAseq analysis suggested that the absence of early IL-33 stimulation increased CD4 +T cell expression of IL-10 and Foxp3. Thus, we hypothesized that disruption of the ST2/IL-33 pathway in CD4 +T cells would mediate pTreg generation and the resolution of T cell-mediated pathology. To test this hypothesis, we compared Tg B6 mice allowing inducible ST2 deletion (R26-CreERT 2xSt2 fl/fl(ST2 FL) to wildtype (WT) R26-CreERT 2(ST2 WT) as T cell donors in a CD4-driven, lethal Graft-vs-Host Disease (GVHD) model. Irradiated BALB/c recipients received WT CD45.1 +bone marrow and either ST2 WTor ST2 FL. Recipient mice received tamoxifen (TAM) on days 10–14 post-transplant. Assessment of CD45.1 −CD45.2 +K d−CD4 +T cells at day 21 revealed robust ST2 deletion associated with increased Foxp3 +expression in effector populations (CD4 +CD44 +CD69 +CD62L −CCR7 −and CD44 +CD69 −) in the lymphoid organs and tissues. TAM-treated recipients of ST2 FLT cells had improved GVHD clinical scores and weight gain compared to ST2 WT. Thus, IL-33 is a novel and potent stromal cell-derived costimulatory signal whose targeting may help resolve active CD4 +Th1 cell-mediated pathology. Supported by grants from NIH R01AR073527, R01HL22489, and R56AI139327 (to HRT); and T32CA082084 and F30AI147437 (to GKD).