Pneumococcal hydrogen peroxide production is associated with epithelial and pulmonary immune responses during pneumonia

Abstract Background Streptococcus pneumoniae (Spn) produces substantial amounts of hydrogen peroxide (H₂O₂), a metabolic byproduct with cytotoxic and immunomodulatory effects. Although pneumococcal H₂O₂ has been implicated in pathogenesis, its contribution to pulmonary immune responses and epithelial transcriptional responses during infection remains incompletely understood. Methods Host responses to TIGR4 H₂O₂-deficient mutant, and complemented strains were evaluated in a murine pneumonia model using bacterial burden and multiparameter flow cytometry. In parallel, transcriptional responses of human airway epithelial cells (Calu-3 and BEAS-2B) infected with wild-type or H₂O₂-deficient pneumococci were examined by RNA sequencing. Results In vivo infection with TIGR4 showed higher bacterial burdens and increased frequencies of neutrophils, macrophages, and dendritic cells, together with increased frequencies of effector-memory T cells and altered B-cell frequencies. In contrast, infection with the H₂O₂-deficient mutant was associated with more rapid bacterial clearance and a more limited pulmonary inflammatory response. In Calu-3 cells TIGR4 induced a broad inflammatory transcriptional response involving cytokine, chemokine, and innate immune pathways, whereas the H₂O₂-deficient mutant elicited a markedly attenuated response. A similar core inflammatory transcriptional pattern was observed in BEAS-2B cells. Conclusions Pneumococcal H₂O₂ production was associated with enhanced epithelial inflammatory transcription and altered pulmonary immune-cell composition during pneumonia. The H₂O₂-deficient mutant also exhibited reduced pulmonary and bloodstream bacterial burdens. These findings support a role for pneumococcal H₂O₂ in modulating host responses, although additional effects of the disruption of enzymes producing H₂O₂, SpxB/LctO, may contribute to the observed phenotypes.