IL-33 and TBI expands ST2⁺ Tregs that persist through day 4 post-TBI. B6 mice were administered IL-33 (1.0 μg per mouse per day) or PBS (control) for 10 days and then exposed to lethal TBI (1100 cGy). Mice were sacrificed on day 2 or day 4 post-TBI, and nonirradiated mice were sacrificed on day 0. (A) Representative contour plots of CD3⁺CD4⁺-gated splenocytes showing Foxp3 vs CD25 expression from control (PBS) and IL-33–treated mice before and after TBI. (B) CD4⁺CD25⁺-gated splenocytes showing Foxp3 vs ST2 expression. (C) Summary graphs for the frequency (%) and cell number with SD the mean for the indicated populations. Average from n = 3-4 mice per group, and statistical significance was calculated between each group using an unpaired Student t test (*P < .05, **P < .01, ***P < .001); *IL-33–treated vs control at each time point; #significance for day 2 or day 4 control vs control day 0. Data are representative of at least 4 independent experiments. (D) CD4⁺CD25⁺-gated BM cells showing Foxp3 vs ST2 expression. (E) Summary graphs for the frequency (%) and cell number of indicated BM populations. Statistical significances were calculated as in panel C. Three to 4 mice per group and data are representative of 2 independent experiments.