Synthesis of interferon (IFN)-gamma by natural killer (NK) cells is an important pro-inflammatory event with interleukin (IL)-12 and IL-18 playing major inductive roles. However, other temporal events are likely to regulate such processes and as prostaglandin E2 (PGE2) is ubiquitous during inflammation this study tested the hypothesis that PGE2 was capable of directly modulating cytokine-induced NK cell IFN-gamma synthesis in the absence of other immune cells. Using homogeneous NK cell lines to establish direct effects, PGE2 (0.1-1 micro m) was found to suppress NK cell IFN-gamma synthesis and antagonized the potent synergistic IFN-gamma-inducing effects of IL-12 and IL-18. The actions of PGE2 were mimicked by synthetic PGE2 analogues including misoprostol and butaprost. The selective EP2 receptor agonist butaprost, but not the EP1/EP3 agonist sulprostone, suppressed IFN-gamma synthesis and exclusively competed with PGE2 for receptor binding on NK cells. Further analysis showed that PGE2 did not modulate IL-12 receptor mRNA expression and the effects of PGE2 could be mimicked by the phosphodiesterase inhibitor 3-iosobutyl-1-methylxanthine. The absence of demonstrable receptor modulation coupled with the observed suppression of IFN-gamma synthesis by both EP2 receptor-selective agonists and IBMX suggest that PGE2 acts directly on NK cells via EP2 receptors with its downstream effects on cAMP metabolism. This conclusion is further supported by findings that PGE2 and its analogues consistently elevated levels of cAMP in NK cells. The ability of PGE2 to antagonize the potent inductive signal provided by the combination of IL-12 and IL-18 supports the concept that PGE2 may play an important role in limiting innate inflammatory processes in vivo through direct suppression of NK cell IFN-gamma synthesis.