STING Contributes to Host Defense Against Staphylococcus aureus Pneumonia Through Suppressing Necroptosis.

STING (Stimulator of interferon genes) is known as an important adaptor protein or direct sensor in the detection of nucleotide originating from pathogens or the host. The implication of STING during pulmonary microbial infection remains unknown to date. Herein, we showed that STING protected against pulmonary S.aureus infection by suppressing necroptosis. STING deficiency resulted in increased mortality, more bacteria burden in BALF and lungs, severe destruction of lung architecture, and elevated inflammatory cells infiltration and inflammatory cytokines secretion. STING deficiency also had a defect in bacterial clearance, but did not exacerbate pulmonary inflammation during the early stage of infection. Interestingly, TUNEL staining and LDH release assays showed that STING-/- mice had increased cell death than WT mice. We further demonstrated that STING-/- mice had decreased number of macrophages accompanied by increased dead macrophages. Our in vivo and in vitro findings further demonstrated this cell death as necroptosis. The critical role of necroptosis was detected by the fact that MLKL-/- mice exhibited decreased macrophage death and enhanced host defense to S.aureus infection. Importantly, blocking necroptosis activation rescued host defense defect against S.aureus pneumonia in STING-/- mice. Hence, these results reveal an important role of STING in suppressing necroptosis activation to facilitate early pathogen control during pulmonary S.aureus infection.

GSDMD contributes to host defence against Staphylococcus aureus skin infection by suppressing the Cxcl1-Cxcr2 axis.

Gasdermin D (GSDMD), a member of the gasdermin protein family, is a caspase substrate, and its cleavage is required for pyroptosis and IL-1ß secretion. To date, the role and regulatory mechanism of GSDMD during cutaneous microbial infection remain unclear. Here, we showed that GSDMD protected against Staphylococcus aureus skin infection by suppressing Cxcl1-Cxcr2 signalling. GSDMD deficiency resulted in larger abscesses, more bacterial colonization, exacerbated skin damage, and increased inflammatory cell infiltration. Although GSDMD deficiency resulted in defective IL-1ß production, the critical role of IL-1ß was counteracted by the fact that Caspase-1/11 deficiency also resulted in less IL-1ß production but did not aggravate disease severity during S. aureus skin infection. Interestingly, GSDMD-deficient mice had increased Cxcl1 secretion accompanied by increased recruitment of neutrophils, whereas Caspase-1/11-deficient mice presented similar levels of Cxcl1 and neutrophils as wild-type mice. Moreover, the absence of GSDMD promoted Cxcl1 secretion in bone marrow-derived macrophages induced by live, dead, or different strains of S. aureus. Corresponding to higher transcription and secretion of Cxcl1, enhanced NF-κB activation was shown in vitro and in vivo in the absence of GSDMD. Importantly, inhibiting the Cxcl1-Cxcr2 axis with a Cxcr2 inhibitor or anti-Cxcl1 blocking antibody rescued host defence defects in the GSDMD-deficient mice. Hence, these results revealed an important role of GSDMD in suppressing the Cxcl1-Cxcr2 axis to facilitate pathogen control and prevent tissue damage during cutaneous S. aureus infection.

Non-Hematopoietic MLKL Protects Against Salmonella Mucosal Infection by Enhancing Inflammasome Activation.

The intestinal mucosal barrier is critical for host defense against pathogens infection. Here, we demonstrate that the mixed lineage kinase-like protein (MLKL), a necroptosis effector, promotes intestinal epithelial barrier function by enhancing inflammasome activation. MLKL-/- mice were more susceptible to Salmonella infection compared with wild-type counterparts, with higher mortality rates, increased body weight loss, exacerbated intestinal inflammation, more bacterial colonization, and severe epithelial barrier disruption. MLKL deficiency promoted early epithelial colonization of Salmonella prior to developing apparent intestinal pathology. Active MLKL was predominantly expressed in crypt epithelial cells, and experiments using bone marrow chimeras found that the protective effects of MLKL were dependent on its expression in non-hematopoietic cells. Intestinal mucosa of MLKL-/- mice had impaired caspase-1 and gasdermin D cleavages and decreased interleukin (IL)-18 release. Moreover, administration of exogenous recombinant IL-18 rescued the phenotype of increased bacterial colonization in MLKL-/- mice. Thus, our results uncover the role of MLKL in enhancing inflammasome activation in intestinal epithelial cells to inhibit early bacterial colonization.