Interferon-γ (IFN-γ), a cytokine that is dysregulated in children with AIDS, has variable effects on HIV-1 infection in macrophages. The C-C(β) chemokine receptor CCR5 is a major fusion coreceptor for HIV-1 infection of macrophages. We hypothesized that IFN-γ regulates CCR5 expression in mononuclear phagocytes. Using flow cytometry and RT-PCR, we studied the effect of IFN-γ on CCR5 expression in mononuclear phagocytes isolated from cord (n=7) and adult blood (n=7) of healthy donors. Monocytes and monocyte-derived macrophages (MDM) were incubated for 24h under one of the following conditions: control, IFN-γ, IFN-γ plus anti-IFN-γ-antibody. Both cord and adult blood monocytes and MDM treated with IFN-γ showed increased CCR5 expression compared to control cells (% of CCR5 positive cells [range]:- IFN-γ: 50% to 94%, control: 1% to 30%, p<0.001; mean fluorescence intensity(MFI)[range]:- IFN-γ: 6 to 17.9, control: 2.6 to 10, p<0.05). This effect of IFN-γ was dose-dependent, with significant stimulation at 10u/ml (p<0.05) and maximum effect at 500u/ml (p<0.001). Polyclonal blocking anti-IFN-γ antibody inhibited this effect of IFN-γ. RT-PCR analysis at 3h and 12h after treatment confirmed increased CCR5 mRNA content in IFN-γ treated cells. Monocytes treated with LPS, IFN-α, IFN-β, TNF-α, TGF-β1, GMCSF, IL-2, IL-4 or IL-6 did not show changes in CCR5 expression. We conclude that IFN-γ causes specific, rapid and pronounced upregulation of CCR5 expression in cord and adult blood mononuclear phagocytes. We speculate that regulation of the C-C chemokine system and modulation of HIV-1 infection of macrophages by IFN-γ occur in part by regulation of CCR5 expression.