CD28/B7-mediated co-stimulation is critical for early control of murine cytomegalovirus infection.

Control of acute murine cytomegalovirus (MCMV) infection is dependent upon both innate and adaptive immune responses, relying primarily upon natural killer (NK) and T-cell responses for control. Although CD28/B7 plays a clear role in T-cell responses in many antigen systems including some viral infections, the importance of co-stimulation during MCMV infection is unconfirmed. In addition, recent data suggest that CD28/B7 co-stimulation might also be important to Ly49H+ NK-cell expansion. We therefore hypothesized that CD28/B7 co-stimulation is critical to viral control after MCMV infection, and further that CD28/B7 co-stimulation plays a role in MCMV-specific T- and NK-cell responses. To test these hypotheses, we utilized C57BL/6 mice lacking the co-stimulatory molecules B7-1 and B7-2 or CD28. After primary infection with MCMV, viral titers are significantly elevated in mice lacking CD28 or B7 compared with wild-type mice. Impaired viral control is associated with significant defects in peripheral T-cell responses to MCMV, which appear to be dependent upon CD28/B7 co-stimulation. Abnormal hepatic T-cell responses in CD28(-/-) mice are preceded by impaired MCMV-specific Ly49H+ NK-cell responses. Cytokine evaluations confirm that CD28/B7 co-stimulation is not required for non-specific antiviral responses. We conclude that CD28-mediated co-stimulation is critical for early viral control during acute MCMV infection.