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Adenosine, acting at its receptors, particularly A2A receptors, is a potent endogenous anti-inflammatory agent that modulates the functions and differentiation of inflammatory and immune cells. Because the inflammatory milieu abounds in proinflammatory cytokines, we investigated the effects of Th1-inflammatory cytokines on function and expression of adenosine A2A receptors in the human monocytic cell line THP-1. We found that, consistent with previous reports, adenosine and 2-[p-(2-carnonylethyl)phenylethylamino]-5′-N-ethylcarboxamidoadenosine (CGS-21680), a selective A2A receptor agonist, suppress IL-12 production but increase IL-10 production in LPS-activated THP-1 cells. These effects were blocked by the A2A receptor antagonist 4-{2-[7-amino-2-(2-furyl)[1,2,4-triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl}phenol (ZM-241385). More importantly, the suppressive effect of adenosine and CGS-21680 on IL-12 production was significantly enhanced in cells pretreated with either IL-1 (10 U/ml) or TNF-α (100 U/ml) but markedly attenuated in cells pretreated with IFN-γ (100 U/ml). Similarly, IL-1 and TNF-α treatment potentiated the stimulatory effect of adenosine and CGS-21680 on IL-10 production, whereas IFN-γ treatment almost completely abolished this effect. CGS-21680 stimulated an increase in intracellular cAMP in a time- and dose-dependent manner in IL-1- and TNF-α-treated cells but not in control or IFN-γ-treated cells. Both IL-1 and TNF-α increased A2A receptor mRNA and protein. In parallel with its effect on A2A receptor function, IFN-γ down-regulated A2A receptor message and protein. Because adenosine mediates many of the antiinflammatory effects of drugs such as methotrexate, these observations suggest that local changes in the cytokine milieu may influence the therapeutic response to those drugs by altering the expression and function of adenosine receptors on inflammatory cells.