Influenza A virus exacerbates Staphylococcus aureus pneumonia in mice by attenuating antimicrobial peptide production.

Influenza A represents a significant cause of morbidity and mortality worldwide. Bacterial complications of influenza A confer the greatest risk to patients. T(H)17 pathway inhibition has been implicated as a mechanism by which influenza A alters bacterial host defense. Here we show that preceding influenza causes persistent Staphylococcus aureus infection and suppression of T(H)17 pathway activation in mice. Influenza does not inhibit S. aureus binding and uptake by phagocytic cells but instead attenuates S. aureus induced T(H)17 related antimicrobial peptides necessary for bacterial clearance in the lung. Importantly, exogenous lipocalin 2 rescued viral exacerbation of S. aureus infection and decreased free iron levels in the bronchoalveolar lavage from mice coinfected with S. aureus and influenza. These findings indicate a novel mechanism by which influenza A inhibits T(H)17 immunity and increases susceptibility to secondary bacterial pneumonia. Identification of new mechanisms in the pathogenesis of bacterial pneumonia could lead to future therapeutic targets.