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In highly purified human polymorphonuclear leukocyte (PMN) preparations containing less than 0.1% contaminating monocytes, significant amounts of interleukin-8 (IL-8) and small amounts of IL-1 alpha, IL-1 beta, and tumor necrosis factor-alpha (TNF-alpha) were produced by lipopolysaccharide (LPS) stimulation. Contrary to published reports, IL- 6 production could not be detected. IL-10 inhibited the production of IL-1 alpha, IL-1 beta, IL-8, and TNF-alpha in LPS-stimulated PMNs, as it did in human blood mononuclear cell (MNC) preparations enriched in monocytes. Subsequent investigation of cytokine synthesis inhibitory effect of IL-10 on PMNs was focused on IL-8. IL-10 inhibited IL-8 synthesis in a dose-dependent manner and, in this regard, it was more potent than IL-4 and transforming growth factor-beta 1 (TGF-B1). In both MNCs and PMNs, degradation of LPS-induced IL-8 mRNA was enhanced by IL-10. Furthermore, as determined by nuclear run-on assays, IL-10 inhibited LPS-induced transcription of IL-8 gene in MNCs. However, in PMNs, run-on assays could not reliably detect IL-8 gene transcription. These results provide the first evidence that the human peripheral neutrophil is a target for inhibition of cytokine synthesis by IL-10, and that IL-10 acts by affecting both gene transcription and mRNA stability.