Administration of IL-33 post–allo-HCT increases the severity of GVHD. (A) C57BL/6 recipient mice of either 5 × 10⁶ BALB/c (allo) or C57BL/6 (syngeneic) BM and 5 × 10⁶ T cells (CD90-sorted) were treated with recombinant IL-33 (from day 3 to day 7 after BMT; 0.4 μg/dose) or phosphate-buffered saline (PBS) as control and monitored for survival. Allo BM+T-IL-33 vs Allo BM+T-PBS. P = .017. (B) Clinical GVHD scores were monitored throughout the experiment (Bi) and statistical differences were assessed on day 8 after allo-HCT (Bii) for the experiment shown in (A). (C) Concentrations of TNF-α were measured on day 8 after allo-HCT in the serum of C57BL/6 allogeneic recipients (received BM or BM plus T cells from BALB/c mice) treated with IL-33 (5 × 0.2 μg) or PBS. (D-F) Number of BALB/c donor T cells (D), a₄β₇⁺ T cells (E), or IFN-γ⁺ T cells (F) isolated from lamina propria of the small intestine of C57BL/6 allogeneic recipients treated with IL-33 (5 × 0.2 μg) or PBS on day 8 after allo-HCT. NS, not significant.