Epithelial innate immune sensing of pneumococci is inherently restricted to a small cellular minority across species and infection niches

Abstract

Streptococcus pneumoniae colonises the nasopharynx asymptomatically yet causes life-threatening invasive disease. How it navigates early epithelial immune surveillance to cause disease remains unclear. Conventional innate immune models predict coordinated, population-wide epithelial responses to bacterial infection. Using single-cell RNA sequencing, RNA fluorescence in situ hybridization and in vivo mouse and zebrafish models, pneumococcal infection is instead shown to activate innate immune genes including chemokine, NF-{kappa}B regulatory, and prostaglandin pathway genes, in only 1-4% of lung epithelial cells. This restriction is seemingly pneumococcal-specific as Escherichia coli triggers responses in over 40% of the same cells. Strikingly, nasopharyngeal epithelial cells show complete immune silence to pneumococci while responding robustly to E. coli and Staphylococcus aureus, suggesting niche-specific immune evasion. Additionally, pharmacological inhibition of COX-2 significantly increased mortality in a zebrafish meningitis model, identifying prostaglandin signalling as a protective host response during invasive disease. Competence-associated surface remodelling contributes modestly and incrementally to immune restriction, while the predominant dampening is competence-independent. These findings challenge canonical epithelial immunity models against bacterial infection and provide a cellular framework for understanding pneumococcal commensalism and pathogenesis. Significance statementClassical innate immune models predict that bacterial infection triggers coordinated, population-wide transcriptional responses across the epithelium. Using single-cell RNA sequencing, RNA fluorescence in situ hybridization, and in vivo zebrafish and mouse models, we show that Streptococcus pneumoniae, responsible for over one million deaths annually, activates innate immune genes in only 1-4% of lung epithelial cells. Escherichia coli triggers responses in over 40% of the same cells, demonstrating this restriction is pneumococcal-specific. Nasopharyngeal epithelial cells, the bacteriums primary colonization niche, show complete immune silence to pneumococci, suggesting niche-specific evolutionary adaptation. The prostaglandin pathway is identified as a protective host response during invasive disease. These findings challenge canonical models of epithelial immunity and provide a cellular framework for pneumococcal commensalism and pathogenesis.