Heme oxygenase-1 (HO-1) has been viewed as a cytoprotective protein, ameliorating the effects of inflammatory cellular damage, and as beneficial in allograft protection from acute and chronic rejection, suggesting important functions in both innate and adaptive immune responses. Mice deficient in HO-1 exhibit defective immune regulation characterized by a proinflammatory phenotype. We examined if impaired regulatory T cell (Treg) function contributes to the immunoregulatory defects observed in HO-1-/- mice. HO-1 -/- mice exhibited a significantly higher proportion of Foxp3-expressing cells among total CD4+ and CD4+CD25 + cells in comparison to HO-1+/+ mice, and HO-1 -/- Treg cells were at least as effective as HO-1+/+ Treg cells in suppressing proliferation of effector T cells in vitro from either HO-1+/+ or HO-1-/- mice. However, the absence of HO-1 in antigen-presenting cells abolished the suppressive activity of Treg cells on effector T cells. These findings demonstrate that HO-1 activity in antigen-presenting cells is important for Treg-mediated suppression, providing an explanation for the apparent defect in immune regulation in HO-1 -/- mice. Copyright © American Society for Investigative Pathology.