Heme oxygenase-1 (HO-1) is a microsomal enzyme with antioxidant, antiapoptotic, and immunoregulatory functions. We studied the expression of HO-1 by bone marrow-derived dendritic cells (BMDCs) and splenic DC subpopulations under quiescent conditions or following lipopolysaccharide (LPS) stimulation. The kinetics of HO-1 expression by BMDCs depended on the conditions under which they were propagated. Expression of HO-1 in mouse BMDCs in 100 U/ml GM-CSF peaked at 16 hours after LPS treatment and maintained expression for at least 48 hours. But cultures in 800 U/ml granulocyte-macrophage colony-stimulating factor (GM-CSF) showed peak expression by 16 hours that disappeared by 48 hours after LPS stimulation, similar to BMDCs cultured in both 100 U/ml GM-CSF and IL-4 (10 ng/ml). By flow cytometry, a large proportion of CD8(+) splenic DCs strongly expressed HO-1, and this population significantly increased following LPS administration in vivo. In HO-1(-/-) mice, the proportion of splenic CD8(+) DCs was significantly decreased in comparison with HO-1(+/+) mice. In addition, a unique subpopulation of MHC II(-)CD11b(+)CD11c(+) cells was prominent in HO-1(-/-) spleens. Injection of GFP-labeled HO-1(+/+) splenic DC precursors into HO-1(+/+) mice resulted in the generation of GFP(+)CD8(+) DCs in the spleen after 5 days, but GFP(+) CD8(+) DCs failed to appear in HO-1(-/-) spleens. Conversely, GFP(+)HO-1(-/-) splenic cells also generated GFP(+)CD8(+) DCs in HO-1(+/+) mice. These results show that HO-1 is involved in splenic DC differentiation, and/or the homing of CD8(+) splenic DC precursors appears to be dependent on HO-1 expression by the host.