(A) IL-10 inhibits the IFN-induced tyrosine phosphorylation of STAT1. Monocytes were treated with 10 ng/mL of IL-10 for 60 minutes, followed by treatment for 30 minutes with the indicated doses of IFN at 200 U/mL (lanes 3 and 4) or 1,000 U/mL (lanes 5 and 6). Extracts were prepared, and equal amounts of protein were applied to an SDS-PAGE gel. Separated proteins were transferred and a Western blot was performed with an antibody that recognizes tyrosine-phosphorylated STAT1. (B) The blot was stripped and reprobed for STAT1. (C) The inhibitory effects of IL-10 on IFN-stimulated phosphorylation of STAT1 are dose-dependent. Monocytes were prepared as in (A), and the resulting autoradiographs were scanned for phosphorylated STAT1 using a Laser densitometer (LKB-Brommo, Piscataway, NJ). The numbers in parentheses refer to the number of experiments performed with different donors. The data are presented as the mean ± standard deviation.