Activated monocytes and lymphocytes secrete cytokines that act as autocrine and paracrine mediators to promote and regulate local immune processes. These cell types are abundant at the maternal-fetal interface, and cytokines may play a role in pregnancy maintenance or failure. The purpose of this study was to determine the effects of selected monocyte- and lymphocyte-derived cytokines on trophoblast progesterone and estradiol production. JEG-3 choriocarcinoma cells were cultured in supplemented medium alone or in various concentrations of selected recombinant monocyte or lymphocyte cytokines. The cytokines were evaluated both individually and in combination. After 48 h of incubation, the culture supernatant was aspirated and stored at -20 C. Samples were then analyzed for steroid concentration by specific RIAs. Specific interleukin-1 (IL-1)-and tumor necrosis factor (TNF)-neutralizing antibodies were evaluated for their ability to abrogate the cytokine's observed stimulatory effect. To evaluate the physiological relevance of the progesterone-stimulating effect observed with monocyte-derived cytokines, JEG-3 cells were incubated with activated monocyte supernatant or directly cocultured with activated monocytes, and supernatants from these cultures were analyzed for progesterone levels. The monocyte cytokines [IL-1 alpha (5 U/mL), IL-1 beta (5 U/mL), and TNF alpha (1000 U/mL) significantly stimulated trophoblast progesterone production (nanograms per mL): JEG-3 control, 4.1 +/- 0.5; IL-1 alpha, 7.8 +/- 0.9; IL-1 beta, 8.8 +/- 0.5; and TNF alpha 7.2 +/- 0.8 (P < 0.05). Neither the monocyte nor the lymphocyte cytokines altered trophoblast estradiol production. Activated monocyte supernatant and direct JEG-3-monocyte cocultures also significantly stimulated trophoblast progesterone production in vitro. The stimulatory effect of the monocyte-derived cytokines was specific, as demonstrated by neutralization assay. The increased trophoblast progesterone production was not due to enhanced cellular proliferation, but to enhance cellular steroidogenesis, as measured by quantitative DNA analysis. The lymphocyte cytokines (IL-2, interferon-gamma, and granulocyte-macrophage colony-stimulating factor had no effect on trophoblast progesterone production. We conclude that monocyte IL-1 alpha, IL-1 beta, and TNF alpha may regulate trophoblast progesterone production through paracrine effects. Monocyte-trophoblast interactions may be significant in normal pregnancy as well as pregnancy disorders.