Staphylococcus aureus, a versatile Gram-positive bacterium, is the main cause of bone and joint infections (BJI), which are prone to recurrence. The inflammasome is an immune signaling platform that assembles after pathogen recognition. It activates proteases, most notably caspase-1 that proteolytically matures and promotes the secretion of mature IL-1β and IL-18. The role of inflammasomes and caspase-1 in the secretion of mature IL-1β and in the defence of S. aureus-infected osteoblasts has not yet been fully investigated. We show here that S. aureus-infected osteoblast-like MG-63 but not caspase-1 knockout CASP1-/-MG-63 cells, which were generated using CRISPR-Cas9 technology, activate the inflammasome as monitored by the release of mature IL-1β. The effect was strain-dependent. The use of S. aureus deletion and complemented phenole soluble modulins (PSMs) mutants demonstrated a key role of PSMs in inflammasomes-related IL-1β production. Furthermore, we found that the lack of caspase-1 in CASP1-/-MG-63 cells impairs their defense functions, as bacterial clearance was drastically decreased in CASP1-/-MG-63 compared to wild-type cells. Our results demonstrate that osteoblast-like MG-63 cells play an important role in the immune response against S. aureus infection through inflammasomes activation and establish a crucial role of caspase-1 in bacterial clearance.

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Methicillin-resistant Staphylococcus aureus (MRSA) is a significant public health issue causing both hospital and community-associated infections worldwide. Besides, some strains can also produce enterotoxins, which aggravate the patient clinical condition (Rabelo et al., 2014). MRSA harbours the gene mecA encoding PBP2a, a low-affinity penicillin-binding protein (PBP), that allows the bacteria to grow in the presence of drug concentrations that otherwise would be considered inhibitory (Paterson et al., 2014). This gene is