AbstractInterleukin-21 (IL-21) enhances T helper 1 (Th1) and Th17 differentiation while inhibiting the conversion of inducible regulatory T cells (Tregs) from naive T cells. To determine the role of IL-21 in graft-versus-host disease (GVHD), anti–IL-21 antibody (Ab) was given to recipients of CD25−CD4+ or CD4+ and CD8+ T-effectors. IL-21 neutralization attenuated GVHD-related weight loss and prolonged survival. Likewise, a majority of mice receiving IL-21−/− CD25− T-effectors survived long term, whereas those receiving wild-type T cells died. The latter recipients had higher grades of GVHD in the ileum and colon. Surprisingly, disruption of IL-21 signaling did not affect IL-17 production, although colon-infiltrating T-effector cells had decreased interferon γ (IFNγ) and increased IL-4 production. FoxP3+ Tregs were increased in colons of anti–IL-21 Ab-treated recipients of FoxP3− IL-21−/− T cells, indicating Treg conversion. Recipients of FoxP3-deficient T-effectors isolated from chimeras were resistant to the GVHD protective effects of IL-21 blockade. Whereas graft-versus-leukemia (GVL) can occur in the absence of IL-21, loss of both IL-21 and perforin expression abrogated GVL. Together, these data indicate that IL-21 suppresses inducible Treg conversion and further suggest that IL-21 blockade is an attractive strategy to reduce GVHD-induced injury.