Interleukin-10 Inhibits Interferon-γ–Induced Intercellular Adhesion Molecule-1 Gene Transcription in Human Monocytes

Interleukin-10 (IL-10) is a potent monocyte regulatory cytokine that inhibits gene expression of proinflammatory mediators. In this study, we investigated the mechanism by which IL-10 downregulates expression of intercellular adhesion molecule-1 (ICAM-1) on the cell surface of normal human monocytes activated with interferon-γ (IFN-γ). IL-10 inhibition of IFN-γ–induced ICAM-1 expression was apparent as early as 3 hours and was blocked by an anti–IL-10 antibody but not by an isotype-matched control antibody. Northern blot analysis showed that IL-10 reduced the accumulation of ICAM-1 mRNA in IFN-γ–stimulated monocytes. IL-10 inhibition of ICAM-1 steady-state mRNA was detected at 3 hours and remained at 24 hours. Nuclear run-on transcription assays showed that IL-10 inhibited the rate of IFN-γ–induced transcription of the ICAM-1 gene, and mRNA stability studies showed that IL-10 did not alter the half-life of IFN-γ–induced ICAM-1 message. Thus, IL-10 inhibits IFN-γ–induced ICAM-1 expression in monocytes primarily at the level of gene transcription. Activation of IFN-γ–responsive genes requires tyrosine phosphorylation of the transcriptional factor STAT-1α (signal transducer and activator of transcription-1α). However, IL-10 did not affect IFN-γ–induced tyrosine phosphorylation of STAT-1α or alter STAT-1α binding to the IFN-γ response element (IRE) in the ICAM-1 promoter. Instead, IL-10 prevented IFN-γ–induced binding activity at the NF-κB site of the tumor necrosis factor α (TNF-α)–responsive NF-κB/C-EBP composite element in the ICAM-1 promoter. These data indicate that IL-10 inhibits IFN-γ–induced transcription of the ICAM-1 gene by a regulatory mechanism that may involve NF-κB.