Nitric oxide regulates endotoxin-induced TNF-alpha production by human neutrophils

We studied the effect of nitric oxide on LPS-induced TNF-alpha production by human neutrophils. Human neutrophils exposed to LPS and IFN-gamma did not show measurable increases in intracellular cyclic GMP (cGMP). However, cGMP increased upto 30-fold (p < 0.01) in neutrophils incubated with both sodium nitroprusside (SNP), an exogenous source of nitric oxide, and N-acetylcysteine (NAC), which increases the bioavailability of nitric oxide; this increase indicates that neutrophils contain a nitric oxide-sensitive guanylate cyclase. SNP, with or without NAC, did not increase TNF-alpha production in human neutrophils cultured in medium alone. However, LPS-dependent TNF-alpha production was increased by exposure to SNP (p < 0.05); this effect was further increased by the addition of NAC (p < 0.02). IFN-gamma greatly increased LPS-mediated TNF-alpha production by human neutrophils (p < 0.01), and SNP plus NAC was found to further augment this production (p < 0.01). The up-regulation of TNF-alpha production by nitric oxide was not associated with increased amounts of LPS-induced TNF-alpha mRNA, and was not reproduced by exposing neutrophils to cGMP analogues. These data suggest that nitric oxide released by endothelial and vascular smooth muscle cells may exert a paracrine effect on human neutrophils and augment the inflammatory response in sepsis by increasing the production of cytokines. Although the mechanism of this effect remains unknown, it does not seem to be dependent on cGMP or increased levels of TNF-alpha mRNA.