Phagocytosis of Staphylococcus aureus by human neutrophils prevents macrophage efferocytosis and induces programmed necrosis1

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) pose a significant threat to human health. Neutrophils (PMN) are the first responders during staphylococcal infection, but 15-50% of the initial ingested inoculum survives within the PMN phagosome and likely contributes directly or indirectly to disease pathogenesis. We hypothesize that surviving intracellular CA-MRSA undermine effective phagocyte-mediated defense by causing a decrease in macrophage uptake of PMN containing viable S. aureus and by promoting PMN lysis. In support of this hypothesis, PMN harboring viable CA-MRSA strain USA300 (PMN-SA) upregulated the “don’t eat me” signal CD47, remained bound to the surface and were inefficiently ingested by macrophages. In addition, coculture with PMN-SA altered the macrophage phenotype. Compared to macrophages fed USA300 alone, macrophages challenged with PMN-SA produced more IL-8 and less IL1-RA, TNFα, activated caspase-1, and IL-1β. Although they exhibited some features of apoptosis within 3 hours following ingestion of S. aureus, including phosphatidylserine (PS)-exposure and mitochondrial membrane depolarization, PMN-SA had sustained levels of proliferating cell nuclear antigen (PCNA) expression, absence of caspase activation and underwent lysis within six hours following phagocytosis. PMN lysis was dependent on receptor-interacting protein 1 (RIP-1), suggesting that PMN-SA underwent programmed necrosis or necroptosis. These data are the first demonstration that bacteria can promote sustained expression of PCNA and that human PMN undergo necroptosis. Together, these findings demonstrate that S. aureus surviving within PMN undermine the innate immune response and may provide insight into the pathogenesis of S. aureus disease.