SummaryStaphylococcus aureus is a major pathogen responsible for both nosocomial and communityacquired infections. Central to its virulence is its ability to secrete haemolysins, pore-forming toxins and cytolytic peptides. The large number of membrane-damaging toxins and peptides produced during S. aureus infections has hindered a precise understanding of their specific roles in diseases. Here, we used comprehensive libraries of recombinant toxins and synthetic cytolytic peptides, of S. aureus mutants and clinical strains to investigate the role of these virulence factors in targeting human macrophages and triggering IL-1b release. We found that the Panton Valentine leukocidin (PVL) is the major trigger of IL-1b release and inflammasome activation in primary human macrophages. The cytolytic peptides, d-haemolysin and PSMa3; the pore-forming toxins, g-haemolysin and LukDE; and b-haemolysin synergize with PVL to amplify IL-1b release, indicating that these factors cooperate with PVL to trigger inflammation. PVL + S. aureus causes necrotizing pneumonia in children and young adults. The severity of this disease is due to the massive recruitment of neutrophils that cause lung damage. Importantly, we demonstrate that PVL triggers IL-1b release in human alveolar macrophages. Furthermore, IL-1b released by PVL-intoxicated macrophages stimulates the secretion of the neutrophil attracting chemokines, IL-8 and monocyte chemotactic protein-1, by lung epithelial cells. Finally, we show that PVL-induced IL-8/monocyte chemotactic protein-1 release is abolished by the inclusion of IL-1 receptor antagonist (IL-1Ra) in a mixed culture of lung epithelial cells and macrophages. Together, our results identify PVL as the predominant S. aureus secreted factor for triggering inflammasome activation in human macrophages and demonstrate how PVL-intoxicated macrophages orchestrate inflammation in the lung. Finally, our work suggests that anakinra, a synthetic IL-1Ra, may be an effective therapeutic agent to reduce the massive neutrophils infiltration observed during necrotizing pneumonia and decrease the resulting host-mediated lung injury.

The molecular mechanism of Staphylococcus aureus phathogenicity is complex and involves several toxins, including the famous staphylococcal enterotoxin (SE) and toxic shock syndrome toxin-1 (TSST-1). Although these toxins were discovered in specific clinical contexts of food poisoning and menstrual toxic shock syndrome, they share common biochemical and biological properties. As superantigens they are able to massively activate mononuclear cells and T cells regardless of the antigenic specificity of the T cells. To date, 19 different enterotoxins and related toxins have been described in S. aureus with some differences in structure and biological activity. It has been clearly demonstrated that most human S. aureus isolates harbor at least one gene encoding for these toxins. It is suspected that S. aureus produces SEs and TSST-1 in humans from colonization to infection, whatever the clinical situation. It is proposed that the production of SEs plays a role not only in classical staphylococcal infections but also in noninfectious diseases. This review will focus on recent findings related to staphylococcal superantigens and their impact on human diseases.

Multiple Inflammatory Syndrome in Children (MIS-C) is a delayed and severe complication of SARS-CoV-2 infection that strikes previously healthy children. As MIS-C combines clinical features of Kawasaki disease and Toxic Shock Syndrome (TSS), we aimed to compare the immunological profile of pediatric patients with these different conditions. We analyzed blood cytokine expression, and the T cell repertoire and phenotype in 36 MIS-C cases, which were compared to 16 KD, 58 TSS, and 42 COVID-19 cases. We observed an increase of serum inflammatory cytokines (IL-6, IL-10, IL-18, TNF-α, IFNγ, CD25s, MCP1, IL-1RA) in MIS-C, TSS and KD, contrasting with low expression of HLA-DR in monocytes. We detected a specific expansion of activated T cells expressing the Vβ21.3 T cell receptor β chain variable region in both CD4 and CD8 subsets in 75% of MIS-C patients and not in any patient with TSS, KD, or acute COVID-19; this correlated with the cytokine storm detected. The T cell repertoire returned to baseline within weeks after MIS-C resolution. Vβ21.3+ T cells from MIS-C patients expressed high levels of HLA-DR, CD38 and CX3CR1 but had weak responses to SARS-CoV-2 peptides in vitro. Consistently, the T cell expansion was not associated with specific classical HLA alleles. Thus, our data suggested that MIS-C is characterized by a polyclonal Vβ21.3 T cell expansion not directed against SARS-CoV-2 antigenic peptides, which is not seen in KD, TSS and acute COVID-19.