Inhibition of pulmonary antibacterial defense by interferon-γ during recovery from influenza infection

Secondary bacterial infection often occurs after pulmonary virus infection and is a common cause of severe disease in humans, yet the mechanisms responsible for this viral-bacterial synergy in the lung are only poorly understood. We now report that pulmonary interferon-γ (IFN-γ) produced during T cell responses to influenza infection in mice inhibits initial bacterial clearance from the lung by alveolar macrophages. This suppression of phagocytosis correlates with lung IFN-γ abundance, but not viral burden, and leads to enhanced susceptibility to secondary pneumococcal infection, which can be prevented by IFN-γ neutralization after influenza infection. Direct inoculation of IFN-γ can mimic influenza infection and downregulate the expression of the class A scavenger receptor MARCO on alveolar macrophages. Thus, IFN-γ, although probably facilitating induction of specific anti-influenza adaptive immunity, suppresses innate protection against extracellular bacterial pathogens in the lung.