The IL-33–ST2 axis supports adoptively transferred Tregs and promotes their GVHD-protective capacity. (A) On day −1, WT B6 mice received lethal TBI (1100 cGy) followed by 1 × 10⁷ WT BALB/c TCD-BM alone (n = 9) or with 4 × 10⁶ BALB/c T cells (CD25-depleted) alone (BM+T; n = 13) or with 2 × 10⁶ CD4⁺CD25⁺ Tregs from WT (st2^+/+; n = 8) or st2^−/− (n = 12) BALB/c mice on day 0. Effective Treg-to-T-effector ratio was 1:2. (B) Survival is depicted with significant differences calculated using the log-rank (Mantel-Cox) test. (C) Clinical scores were also monitored and statistical differences determined between st2^−/− Tregs vs st2^+/+ groups as in Figures 1 and 3. (D-E) B6 recipients irradiated as above received CD4⁺CD25⁺ T cells from st2^+/+ or st2^−/− BALB/c mice, as well as Thy1.1⁺ BALB/c TCD-BM, and CD25-depleted CD45.1⁺CD3⁺ T cells. (D) Representative flow plots depict CD45.1 and CD4 expression on K^d+Thy1.1⁻CD3⁺-gated splenocytes (top panels) or SI LPLs (bottom panels) at day 5 post-alloHCT. Full gating strategy is shown in supplemental Figure 7B. (E) Averages and SD are shown for the frequency of the indicated T-cell population in the spleen (top graphs) or SI LPLs (bottom graphs). Four mice per group and these data are representative of 2 independent experimental repeats. Statistical differences between groups were calculated using an unpaired Student t test (*P < .05, **P < .01, ***P < .001, ****P < .0001).