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ABSTRACT Streptococcus pneumoniae is a leading bacterial cause of a wide range of infections, and pneumococcal pneumosepsis causes high mortality in hosts infected with antibiotic-resistant strains and those who cannot resolve ongoing inflammation. The factors which influence the development and outcome of pneumosepsis are currently unclear. IL-6 is critical for maintaining immune homeostasis, and we determined that this cytokine is also essential for resisting pneumosepsis, as it inhibits macrophage pyroptosis and pyroptosis-related inflammation injury in the lung. IL-6 affected infection outcomes in mice and exerted a protective role, primarily via macrophages. We further found that IL-6 deficiency led to increased lung macrophage death and aggravated lung inflammation, and that exogenous administration of IL-6 protein could decrease macrophage death and alleviate lung tissue inflammation. IL-6 also protected Streptococcus pneumoniae-induced lung macrophage death and lung inflammation injury by inhibiting gasdermin E (GSDME)- and gasdermin D (GSDMD)-mediated pyroptosis. Together, these data reveal a novel mechanism for the development of pneumosepsis and the critical protective role of IL-6. These findings may assist in the early identification and treatment of pneumococcal pneumosepsis. IMPORTANCE Pneumococcal pneumonia has been a significant cause of morbidity and mortality throughout human history. Failing to control pneumococcal pneumonia and resolve ongoing inflammation in a host can cause sepsis, namely pneumococcal pneumosepsis, and death ensues. Few theories have suggested an optimally therapeutic option for this infectious disease. The interleukin-6 (IL-6, a cytokine featuring pleiotropic activity) theory, proposed here, implies that IL-6 acts as a protector against pneumococcal pneumosepsis. IL-6 prevents lung macrophage death and lung inflammation injury by inhibiting a caspase-3-GSDME-mediated switch from apoptosis to pyroptosis and inhibiting caspase-1-GSDMD-mediated classic pyroptosis during pneumococcal pneumosepsis. Thus, IL-6 is an important determinant for controlling bacterial invasion and a homeostatic coordinator of pneumococcal pneumosepsis. This study clarifies a novel mechanism of occurrence and development of pneumonia and secondary sepsis following a Streptococcus pneumoniae infection. It is important for the early identification and treatment of pneumococcal pneumosepsis.

Teichoic acid (TA), together with peptidoglycan (PG), represents a highly complex glycopolymer that ensures cell wall integrity and has several crucial physiological activities. Through an insertion-deletion mutation strategy, we show that ΔrafX mutants are impaired in cell wall covalently attached TA (WTA)-PG biosynthesis, as evidenced by their abnormal banding patterns and reduced amounts of WTA in comparison with wild-type strains. Site-directed mutagenesis revealed an essential role for external loop 4 and some highly conserved amino acid residues in the function of RafX protein. The rafX gene was highly conserved in closely related streptococcal species, suggesting an important physiological function in the lifestyle of streptococci. Moreover, a strain D39 ΔrafX mutant was impaired in bacterial growth, autolysis, bacterial division, and morphology. We observed that a strain R6 ΔrafX mutant was reduced in adhesion relative to the wild-type R6 strain, which was supported by an inhibition assay and a reduced amount of CbpA protein on the ΔrafX mutant bacterial cell surface, as shown by flow cytometric analysis. Finally, ΔrafX mutants were significantly attenuated in virulence in a murine sepsis model. Together, these findings suggest that RafX contributes to the biosynthesis of WTA, which is essential for full pneumococcal virulence. [ABSTRACT FROM AUTHOR]