Novel Requirement for Staphylococcal Cell Wall-Anchored Protein SasD in Pulmonary Infection

Staphylococcus aureus can complicate preceding viral infections, including influenza virus. A bacterial infection combined with a preceeding viral infection, known as super-infection, leads to worse outcomes compared to single infection. Most of the super-infection literature focuses on the changes in immune responses to bacteria between homeostatic and virally infected lungs. However, it is unclear how much of an influence bacterial virulence factors have in super-infection. Staphylococcal species express a broad range of cell wall-anchored proteins (CWAs) that have roles in host adhesion, nutrient acquisition, and immune evasion. We screened the importance of these CWAs using mutants lacking individual CWAs in vivo in both bacterial pneumonia and influenza super-infection. In bacterial pneumonia, lacking individual CWAs led to varying decreases in bacterial burden, lung damage, and immune infiltration into the lung. However, the presence of a preceding influenza infection partially abrogated the requirement for CWAs. In the screen, we found that the uncharacterized CWA S. aureus surface protein D (SasD) induced changes in both inflammatory and homeostatic lung markers. We further characterized a SasD mutant (sasD A50.1) in the context of pneumonia. Mice infected with sasD A50.1 had decreased bacterial burden, inflammatory responses, and mortalty compared to wildtype S. aureus. Mice also had reduced levels of IL-1β compared with wildtype, likely derived from macrophages. Reductions in IL-1β transcript levels as well as increased macrophage viability implicate altered macrophage cell death pathways. These data identify a novel virulence factor for S. aureus that influences inflammatory signaling within the lung.