Reduced GVHD histopathology in recipients of IL-21^−/− CD25⁻ T cells. (A) Lethally irradiated recipients were injected with BM only, BM + 2 × 10⁶ wt CD25⁻ T cells, or BM + IL-21^−/− CD25⁻ T cells, killed on day 14 after BMT, and hematoxylin and eosin–stained tissue sections were scored for GVHD: Colon (i,iii,v) and ileum (ii,iv,vi) from recipients of BM (i-ii), BM + wild-type T-effector cells (iii-iv) or BM + IL-21^−/− cells (v-vi). Mean ± SEM of histologic grade of graft-versus-host disease in target organs in mice injected with BM only (□), BM + 2 × 10⁶ wt CD25⁻ T cells (■), or BM + IL-21^−/− CD25⁻ T cells (; wt vs IL-21^−/−, P = .020 [colon] and P = .005 [small intestine], all others not significant [ns]; B).