Inflammasome activation and an involvement of Caspase-1 in a bacterial clearance during S. aureus infection.

BactInflam IJL focuses on the bacterial components involved in some inflammatory diseases. In particular, in two of them: chronic inflammatory bowel disease (IBD) and mastitis, affecting human health and animal health, respectively.International audienceStaphylococcus aureus is a highly versatile Gram-positive bacterium that is carried asymptomatically by up to fifty percent of healthy people, while being a major cause of hospital-acquired infections including bone and joint infections, such as osteomyelitis, which are prone to recurrence. S. aureus-caused bovine mastitis is also significant problem in veterinary medicine. The innate immune response plays a pivotal role in the defense against S. aureus. This response is initiated through pattern recognition receptors, and involves an activation of multi-protein signaling complexes known as inflammasomes. It activates proteases, most notably caspase-1 that proteolytically matures and promotes the secretion of mature IL-1β and IL-18. We investigated the role of inflammasomes and caspase-1 in the secretion of mature IL-1β and in the defence of S. aureus-infected osteoblast-like MG-63 cells. Using CRISPR-Cas9 technology, we demonstrated that MG-63 but not caspase-1 knock-out CASP1−/−MG-63 cells, activate the inflammasome as monitored by the release of mature IL-1β. 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 investigated the role of caspase-1in S. aureus clearance. 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.Using a flow cytometric approach we isolated only S. aureus-bearing cells from mixed populations that allows to identify signals specific to intracellular infection. After RNA-seq and KEGG and Reactome pathway enrichment analysis, the remodeled transcriptomic profile of infected cells revealed exacerbated immune and inflammatory responses, as well as metabolic and epigenetic dysregulations that likely influence the intracellular life of bacteria. Collectively, our results demonstrate that human osteoblast-like cells induce an immune response against S. aureus through inflammasomes activation and processing of IL-1β. The outcome of the infection depends on the balance between the host immune response and the action of main S. aureus virulence factors, such as PSMs, whose production differ among the S. aureus strains. Our results showed that the active caspase-1 prevents exacerbated intracellular replication of S. aureus in non-professional phagocytes in addition to professional phagocytes, suggesting the crucial role of caspase-1 in S. aureus clearance independently from the type of cells. Furthermore, our results provide an atlas of deregulated host genes and biological pathways and identify novel markers and potential candidates for prophylactic and therapeutic approaches