OBJECTIVE Interferon gamma (IFN gamma) is found to be elevated in the synovial fluid of patients with rheumatoid arthritis and osteoarthritis, suggesting its implication in joint disease pathogenesis. In this study, we investigated the effects of IFN gamma on the production of cytokines (IL-6, IL-8, IL-10), prostaglandin E(2)(PGE(2)), proteoglycans (PG), nitric oxide (NO), interleukin-1 receptor antagonist (IL-1ra) and stromelysin by non-stimulated and IL-1 beta-treated human chondrocytes. The role played by NO in the responses of chondrocytes to IFN gamma was also examined by incubation of chondrocytes with N(G)-monomethyl-L-arginine (L-NMMA), a competitive inhibitor of NO synthase. METHODS Enzymatically isolated human chondrocytes were cultured for 48 h in the absence or presence of IL-1 beta, IFN gamma or N(G)-monomethyl-L-arginine (L-NMMA) added solely or in combination. The productions of IL-6, IL-8, IL-10, IL-1ra and stromelysin were measured by enzyme amplified sensitivity immunoassays (EASIA). PG and PGE(2)were quantified by specific radioimmunoassays (RIA). Nitrite concentrations in the culture supernatants were determined by a spectrophotometric method based upon the Griess reaction. RESULTS As expected, IL-1 beta highly stimulated NO, IL-6, IL-8, IL-10, IL-1ra, PGE(2)and stromelysin synthesis, but dramatically decreased PG production. NO, IL-6, IL-1ra and PGE(2)production by non-stimulated chondrocytes was dose-dependently increased by IFN gamma while PG production was inhibited. In the absence of IL-1 beta, IL-10 was undetectable in the culture supernatants. At the doses of 10 and 100 U/ml, IFN gamma markedly inhibited the constitutive and IL-1 beta-stimulated IL-8, IL-10 and stromelysin productions. Interestingly, IFN gamma synergized with IL-1 beta to increase NO, IL-6, IL-1ra and to depress PG production. As previously reported, the inhibition of NO synthesis by the competitive inhibitor L-NMMA led to enhancement of IL-6, IL-8 and PGE(2)production by IL-1 beta treated chondrocytes, but did not significantly modify IL-10, PG and MMP-3 productions. Inhibition of NO synthase significantly inhibited the stimulating effect of IFN gamma on IL-6 and IL-1ra but did not affect the inhibitory effect of IFN gamma on IL-8, PG or stromelysin production. CONCLUSIONS These findings suggest that IFN gamma and IL-1 synergistically stimulate the production of IL-6, IL-1ra, NO and PGE(2)and inhibit PG synthesis. By contrast, IL-1 beta and IFN gamma have opposite effects on IL-8, IL-10 and stromelysin productions. These effects are not reversed by L-NMMA, suggesting that NO is not the principal mediator involved in responses of chondrocytes to IFN gamma.