RSV enhances <i>Staphylococcus aureus</i> bacterial growth in the lung

Patients coinfected with respiratory syncytial virus (RSV) and bacteria have longer hospital stays, higher risk of intensive care unit admission, and worse outcomes. We describe a model of RSV line 19F/methicillin-resistant <i>Staphylococcus aureus</i> (MRSA) USA300 coinfection that does not impair viral clearance, but prior RSV infection enhances USA300 MRSA bacterial growth in the lung. The increased bacterial burden post-RSV correlates with reduced accumulation of neutrophils and impaired bacterial killing by alveolar macrophages. Surprisingly, reduced neutrophil accumulation is likely not explained by reductions in phagocyte-recruiting chemokines or alterations in proinflammatory cytokine production compared with mice infected with <i>S. aureus</i> alone. Neutrophils from RSV-infected mice retain their ability to migrate toward chemokine signals, and neutrophils from the RSV-infected lung are better able to phagocytize and kill <i>S. aureus ex vivo</i> on a per cell basis. In contrast, while alveolar macrophages could ingest USA300 post-RSV, intracellular bacterial killing was impaired. The RSV/<i>S. aureus</i> coinfected lung promotes a state of overactivation in neutrophils, demonstrated by increased production of reactive oxygen species (ROS) that can drive formation of neutrophil extracellular traps (NETs), resulting in cell death. Mice with RSV/<i>S. aureus</i> coinfection had increased extracellular DNA and protein in bronchoalveolar lavage fluid and histological evidence confirmed NETosis <i>in vivo</i>. Taken together, these data highlight that prior RSV infection can prime the overactivation of neutrophils leading to cell death that impairs neutrophil accumulation in the lung. Additionally, alveolar macrophage killing of bacteria is impaired post-RSV. Together, these defects enhance USA300 MRSA bacterial growth in the lung post-RSV.