Staphylococcus aureus inhibits the NLRP3 inflammasome in macrophages to varying degrees during early and late stages of infection

Staphylococcus aureus ( S. aureus ) is a highly virulent pathogen responsible for chronic infections such as osteomyelitis. Although its interaction with the host immune system has been widely studied, the specific role of inflammasomes in regulating the infection within macrophages remains unclear. We investigated this question using bone marrow-derived macrophages (BMDMs) infected with S. aureus for up to 96 hours (h). We observed a significant reduction in intracellular bacterial load beginning at 18 h post-infection (hpi), which continued through 96 hpi; this response was partially attenuated in BMDMs lacking NLRP3. Mechanistically, activation of the NLRP3 inflammasome, including ASC speck formation, IL-1β secretion, and pyroptosis, was modest and detectable only after 18 hpi. Consistent with these findings, infected BMDMs failed to respond robustly to NLRP3 inflammasome activators, lipopolysaccharide (LPS) and nigericin, up to 18 hpi, with only partial restoration of responsiveness at later time points. We also found that NLRP3 deficiency did not significantly affect bacterial burden control or bone homeostasis in experimental S. aureus osteomyelitis. Thus, S. aureus is inadequately controlled by the NLRP3 inflammasome in macrophages and within the bone microenvironment. IMPORTANCE Staphylococcus aureus causes difficult-to-treat, long-lasting infections such as osteomyelitis. While many aspects of how the immune system responds to this pathogen are known, it is still unclear how the inflammasomes contribute to controlling the infection inside macrophages. In this study, we found that macrophages gradually reduced the number of bacteria they contained over time, but this ability was only partly dependent on one major inflammasome component, NLRP3. Importantly, the inflammasome response to S. aureus was weak and delayed, becoming noticeable only well after infection had begun. Even when stimulated with agents that normally trigger a strong inflammasome response, infected macrophages responded poorly. Consistent with these findings, the absence of NLRP3 did not significantly worsen bacterial control or bone damage in a mouse model of osteomyelitis. Together, these results suggest that S. aureus largely evades inflammasome-based defenses, limiting their effectiveness in controlling infection in immune cells and bone.