Type I IFN promotes NK cell activation and expression of effector molecules after MCMV infection. (A and B) WT:Ifnar^−/− chimeric mice were infected with MCMV. CD69, IFN-γ, and granzyme B are shown for splenic WT and Ifnar^−/− NK cells (compared with uninfected mice) at day 1.5 PI. (C) STAT1 phosphorylation of NK cells at day 1.5 PI is shown, and bar graph plots mean fluorescent intensity (MFI). (D and E) Ly49H⁺ NK cells from WT:Ifnar^−/− chimeric mice (D) or WT:Stat1^−/− chimeric mice (E) were transferred into Ly49H-deficient hosts and infected with MCMV. Percentages of Ly49H⁺ NK cells are shown. Data are mean ± SEM and representative of three independent experiments with at least n = 3 biological replicates per condition. *, P < 0.05 using paired Student’s t test.