Human neutrophils, plated on fibronectin-coated polystyrene wells, were found to exhibit a prolonged production of superoxide anion (O2-) in response to tumour necrosis factor-alpha (TNF). The TNF-triggered O2- production was significantly reduced by 10 microM prostaglandin E2 (PGE2), which was ineffective at lower doses. Moreover, the O2- production was slightly reduced by the phosphodiesterase type IV (PDE IV) inhibitor RO 20-1724. When PGE2 and RO 20-1724 were added together to TNF-triggered neutrophils they caused a marked synergistic inhibition of O2- production. The action of PGE2 could be mimicked by forskolin (FK), a well-known direct activator of adenylate cyclase. These results suggest that cyclic AMP (cAMP)-elevating agents (PGE2, FK, RO 20-1724) down-regulate the capacity of adherent neutrophils to mount the respiratory burst in response to TNF. Consistent with this interpretation, PGE2 and RO 20-1724 increased the intracellular levels of cAMP displaying synergistic activity. Moreover, the membrane-permeable analogue of cAMP, dibutyryl cAMP, was found to inhibit the TNF-induced O2- production in a dose-dependent manner. As all the aforementioned cAMP-elevating agents did not affect the O2- production in response to phorbol myristate acetate, they appear to act by interfering with the assembly of the O2(-)-generating NADPH oxidase complex rather than by directly inhibiting the activity of already working oxidase complex. In conclusion, taking into account the TNF capacity to promote PGE2 formation at sites of inflammation, our observations suggest the existence of a negative PGE2-dependent feed-back, potentially capable of controlling the neutrophil response to TNF and susceptible to amplification by PDE IV-inhibiting compounds.

SUMMARY Human neutrophils, plated on fibronectin‐precoated wells, were found to release large quantities of superoxide anion (O2−) in response to GM‐CSF. O2− production was reduced by prostaglandin E2 (PGE2) and the phosphodiesterase type IV (PDE IV) inhibitor RO 20–1724. Both agents are known to increase intracellular cyclic AMP (cAMP) levels by inducing its production (PGE.) or blocking its catabolism (RO 20–1724). When added in combination, PGE2 and RO 20–1724 had a marked synergistic inhibitory effect, which was reproduced by replacing PGE2 with a direct activator of adenylate cyclase, i.e. forskolin (FK). Moreover, the neutrophil response to GM‐CSF was inhibited by a membrane‐permeable analogue of cAMP in a dose‐dependent manner. As GM‐CSF and PGE2 are known to be generated at tissue sites of inflammation, the results suggest the existence of a PGE2‐dependent regulatory pathway potentially capable of controlling the neutrophil response to GM‐CSF, in turn limiting the risk of local oxidative tissue injury. Moreover, owing to its susceptibility to amplification by RO 20–1724, the PGE2‐dependent pathway and in particular PDE‐IV may represent a pharmacological target to reduce the generation of histotoxic oxidants by GM‐CSF‐responding neutrophils.