Infection of Primary Human Alveolar Macrophages Alters Staphylococcus aureus Toxin Production and Activity

Pulmonary pathogens encounter numerous insults, including phagocytic cells designed to degrade bacteria, while establishing infection in the human lung. Staphylococcus aureus is a versatile, opportunistic pathogen that can cause severe pneumonia, and methicillin-resistant isolates are of particular concern. Recent reports present conflicting data regarding the ability of S. aureus to survive and replicate within macrophages. However, due to use of multiple strains and macrophage sources, making comparisons between reports remains difficult. Here, we established a disease-relevant platform to study innate interactions between S. aureus and human lungs. Human precision-cut lung slices (hPCLS) were subjected to infection by S. aureus LAC (methicillin-resistant) or UAMS-1 (methicillin-sensitive) isolates. Additionally, primary human alveolar macrophages (hAMs) were infected with S. aureus, and antibacterial activity was assessed. Although both S. aureus isolates survived within hAM phagosomes, neither strain replicated efficiently in these cells. S. aureus was prevalent within the epithelial and interstitial regions of hPCLS, with limited numbers present in a subset of hAMs, suggesting that the pathogen may not target phagocytic cells for intracellular growth during natural pulmonary infection. S. aureus-infected hAMs mounted a robust inflammatory response that reflected natural human disease. S. aureus LAC was significantly more cytotoxic to hAMs than UAMS-1, potentially due to isolate-specific virulence factors. The bicomponent toxin Panton-Valentine leukocidin was not produced during intracellular infection, while alpha-hemolysin was produced but was not hemolytic, suggesting that hAMs alter toxin activity. Overall, this study defined a new disease-relevant infection platform to study S. aureus interaction with human lungs and to define virulence factors that incapacitate pulmonary cells.