Macrophage (Mφ) activation, as measured by cell surface expression of MHC class II, was examined during infection of immunocompetent and immunocompromised mice with murine cytomegalovirus (MCMV). Intraperitoneal infection of CB17 SOlD mice with 106 PFU of MCMV elicited a large population of Mφ which expressed low levels of MHC class II. This was surprising since infection of SOID mice with lower doses (e.g., 104 PFU) of MCMV elicits Mφ expressing high levels of MHC class II (M. T. Heise and H. W. Virgin, J. Virol. (1995) 69, 904-909). In vivo administration of recombinant mouse IFNγ resulted in high levels of MHC class II expression on Mφ from control but not MCMV-infected SCID mice, suggesting that MCMV infection generates a state in which IFNγ is not effective at activating Mφ. The effect of MCMV infection was MHC Class II specific, since MHC Class I and ICAM-1 levels were increased on Mφ expressing low levels of MHC class II. Interference with IFNγ action was not due to productive or abortive infection of Mφ. This suggested that MCMV infection induces a soluble factor that alters Mφ responsiveness to IFNγ. Infection of SOlD mice with 106 PFU of MCMV induced higher levels of serum IFNαβ (one candidate for inhibition of IFNγ induction of MHC class II expression) than infection with 104 PFU. We therefore evaluated the role of MCMV-induced IFNαβ on IFNγ responses of bone marrow-derived (BMMφ) or thioglycollate-elicited Mφ in vitro. Infection of normal Mφ with MCMV at a low m.o.i. (0.1 to 0.2) impaired IFNγ-mediated induction of Mφ MHC class II expression, but not MHC class I expression. Inhibition of IFNγ responses was not observed in Mφ from mice with a null mutation in the IFNαβ receptor (IFNαβR-/-). To test the in vivo relevance of virus-induced IFNαβ to IFNγ-mediated responses, the kinetics of MHC class II induction during MCMV infection of IFNαβR-/- mice was evaluated. MCMV-infected IFNαβR-/- mice mounted an earlier Mφ MHC class II response than normal mice. We conclude that MCMV infection specifically impairs IFNγ-mediated MHC class II expression on Mφ and that induction of IFNαβ is one mechanism by which this inhibition occurs.