Type I IFN is essential for a robust and protective antiviral NK cell response after MCMV infection. (A) WT and Ifnar^−/− NK cells were cotransferred into a Ly49H-deficient host and infected with MCMV. Percentages of Ly49H⁺ NK cells are shown. (B and C) CD27 versus CD11b and KLRG1 expression are shown for WT and Ifnar^−/− Ly49H⁺ NK cells at day 7 PI. (D) Neonatal mice received 10⁶ WT (n = 13) or Ifnar^−/− (n = 7) NK cells followed by MCMV infection. Control mice received PBS (n = 10). The percentage of surviving mice is shown for each group. Data were pooled from three experiments and represent mean ± SEM of at least three independent experiments with at least n = 3 biological replicates per condition.