We recently demonstrated that exposure of mice to a single high dose or multiple smaller doses of ultraviolet (UV) radiation decreased the induction of the delayed-type hypersensitivity (DTH) response to bacillus Calmette-Guerin (BCG) from Mycobacterium bovis injected into unexposed sites. In view of the limited ability of UV radiation to penetrate beyond the epidermis and upper layers of the dermis, it is not entirely clear how exposing the dorsal skin of mice to UV radiation causes systemic impairment of the immune response to BCG. In this study we report that mice injected with supernatants from keratinocyte cultures exposed to UV radiation in vitro impaired host resistance to BCG. Both induction and elicitation of the DTH reaction were suppressed after the intravenous injection of supernatants from UV-irradiated keratinocytes. Furthermore, these supernatants interfered with the elimination of viable bacteria from the lymphoid organs. To determine whether macrophages were the target of the UV-induced, keratinocyte-derived, suppressive cytokine, macrophages were isolated from mice injected with the suppressive cytokine or treated in vitro with the supernatants and tested for their ability to ingest and kill BCG in vitro. Injection of the suppressive factor significantly reduced the phagocytosis of BCG by the macrophages but did not alter the rate of intracellular killing. Similarly, phagocytosis was reduced when normal macrophages were treated in vitro with the suppressive factor. These findings suggest that the suppressive cytokine interferes with the elimination of bacteria in vivo by inhibiting the initial step in bacterial clearance, the uptake of the bacteria by host macrophages.(ABSTRACT TRUNCATED AT 250 WORDS)