Induction of phosphorylated STAT1 by IFNα2a is inhibited in monocytes from HIV-1–infected persons. Monocytes were incubated for 15 minutes with IFNα2a at 0, 1000, 3000, or 10000 U/mL. Individual histograms are shown for uninfected (A) and HIV-1–infected (B) subjects. Dose response induction of phosphorylated STAT1 is evident in uninfected persons but is inhibited in HIV-1–infected persons (C). The difference in phosphorylated STAT1 response between uninfected and HIV-1–infected subjects is significant at 1000 U IFNα2a (P = .012) and 3000 U IFNα2a (P = .005) but not at 10000 U IFNα2a (P = .075). Results are expressed as Δ sMFI (sMFI of stimulated monocytes − sMFI of unstimulated monocytes). Horizontal bars in panel C represent median values.