Direct UV irradiation of dendritic cells and Langerhans cells reduces their Ag presenting ability. However, the effects of UV on CD11c+ cells located distally to the point of irradiation are poorly understood. Three days after UV irradiation (8 kJ/m 2) of BALB/c mice, bone marrow cells were isolated and cultured for 7 d with IL-4 and GM-CSF for the propagation of CD11c+ cells. Bone marrow-derived CD11c+ cells from UV-irradiated or nonirradiated mice were loaded with dinitrobenzene sulfonic acid and injected into the ear pinnas of naive BALB/c mice. After 7 d, the ears were painted with 2, 4-dinitro-1-fluorobenzene and the ear swelling determined 24 h later. A reduced contact hypersensitivity response was found in mice injected with CD11c+ cells from the UV-irradiated animals compared with those injected with cells from the nonirradiated animals. Further, a long-lasting suppression of the memory response to 2, 4-dinitro-1-fluorobenzene was created. This suppressed response corresponded to increased IL-10 and PGE 2 secretion by freshly isolated bone marrow cells from UV-irradiated mice, and to increased myelopoiesis. The reduction in competence of bone marrow-derived CD11c+ cells from UV-irradiated mice was not due to delayed maturation, as it was maintained upon LPS exposure prior to CD11c+ cell purification. The UV-induced effect was reversed by the administration of indomethacin to mice prior to UV irradiation and could be reproduced by sc PGE 2. These results show that UV irradiation of mice can affect the function of bone marrow-derived CD11c+ cells via a mechanism inhibitable by indomethacin; this pathway is likely to contribute to systemic UV-induced immunosuppression.