Involvement of the inflammasome in the response of host cells during Staphylococcus aureus infection

Abstract

Staphylococcus aureus (S. aureus) is a highly adaptive and versatile Gram-positive bacterium that can cause a wide range of infectious diseases in humans or animals. In the early stages of infection, the interaction between S. aureus and the host cells causes inflammation, the process of which depends on several complex cellular mechanisms and a signaling cascade coordinated mainly by cytokines and their activation. The key factors determining the initiation of inflammation and its progression are pathogen/ microbe-associated molecular patterns (PAMPs/MAMPs), danger-associated molecular patterns (DAMPs), pattern recognition receptors (PRRs), innate and adaptive immunity. Inflammasomes, multi-protein signaling complexes that assemble after sensing PAMPs/ DAMPs, are key players of the innate immune response. Most inflammasomes are formed by an NLR receptor, an ASC adapter protein, and active caspase-1. Once assembled, inflammasomes act as activating platforms that promote the maturation of pro-inflammatory cytokines IL-1β and IL-18 in their active forms. Their role in different types of professional phagocytes during S. aureus infection has been extensively studied. Conversely, knowledge about the involvement of inflammasomes in non-professional phagocytes (e.g. epithelial cells, endothelial cells...) is very fragmented. This work aimed to study the role of inflammasome activation during S. aureus infection in non-occupational phagocytes. We adopted a model of S. aureus infection developed on osteoblastic cells (MG- 63 line). Western blot and ELISA analyses were used to control caspase-1 activation and IL- 1β maturation in the S. aureus MG-63 interaction. We generated MG-63 cells deleted from the caspase-1 gene (CASP1–/– MG-63 cells) using the CRISPR/Cas9 approach. This allowed us to demonstrate the involvement of inflammasomes in the innate immune response to S. aureus by non-professional phagocytes. Unexpectedly, we have shown that S. aureus proliferates in CASP1–/– MG-63 cells, suggesting that Casp1 (or Casp1-regulated phenomenon) is involved in the elimination of intracellular S. aureus. We found that the formation of the inflammasome in MG-63 cells is later than in professional phagocytes. Besides, the use of S. aureus strains (deletion mutants) expressing or not Phenol-Soluble Modulins (PSM) allowed us to determine the role of PSM in this trigger. Thus, we have demonstrated that S. aureus can activate inflammasome formation in non-professional phagocytes (MG-63) cells and that PSM is involved in this phenomenon.