IFNAR1 and influenza: Combining splenocytes from IFNAR+/− or IFNAR−/− (CD45.2+) mice with splenocytes from B6 WT (CD45.1+) mice in vitro, followed by infection with flu yielded similar results, i.e. STAT4 was activated to an intermediate level in IFNAR−/− NK cells (Figure 5B).