Epithelial micro-invasion by Streptococcus pneumoniae induces epithelial-derived innate immunity during colonisation at the human mucosal surface

Abstract

Control of Streptococcus pneumoniae colonisation at human mucosal surfaces is critical to reducing the burden of pneumonia and invasive disease, interrupting onward transmission, and in achieving herd protection. We hypothesised that the pattern of pneumococcal-epithelial engagement dictates the inflammatory response to colonisation, and that this epithelial sensing is linked to bacterial clearance. Here we have used nasal curette biopsies from a serotype 6B Experimental Human Pneumococcal Carriage Model (EHPC) to visualize S. pneumoniae colonisation and relate these interactions to epithelial surface marker expression and transcriptomic profile upregulation. We have used a Detroit 562 cell co-culture model to further understand these processes and develop an integrated epithelial transcriptomic module to interrogate gene expression in the EHPC model. We have shown for the first time that pneumococcal colonisation in humans is characterised by microcolony formation at the epithelial surface, microinvasion, cell junction protein association, epithelial sensing, and both epithelial endocytosis and paracellular transmigration. Comparisons with other clinical strains in vitro has revealed that the degree of pneumococcal epithelial surface adherence and microinvasion determines the host cell surface marker expression (ICAM-1 and CD107), cytokine production (IL-6, IL-8 and ICAM-1) and the transcriptomic response. In the context of retained barrier function, epithelial microinvasion is associated with the upregulation of a wide range of epithelial innate signalling and regulatory pathways, inflammatory mediators, adhesion molecules, cellular metabolism and stress response genes. The prominence of epithelial TLR4R signalling pathways implicates pneumolysin, a key virulence factor, but although pneumolysin gene deletion partially ameliorates the inflammatory transcriptional response in vitro, critical inflammatory pathways persist in association with enhanced epithelial adhesion and microinvasion. Importantly, the pattern of the host-bacterial interaction seen with the 6B strain in vitro is also reflected in the EHPC model, with evidence of microinvasion and a relatively silent epithelial transcriptomic profile that becomes most prominent around the time of bacterial clearance. Together these data suggest that epithelial sensing of the pneumococcus during colonisation in humans is enhanced by microinvasion, resulting in innate epithelial responses that are associated with bacterial clearance.\n\nHighlights\n\nO_LIColonisation of the human mucosa by Streptococcus pneumoniae is associated with microcolony formation, microinvasion, epithelial sensing and an epithelial innate response.\nC_LIO_LIFollowing adherence to the epithelial cell surface, microinvasion of the epithelium may occur by endocytosis and/or lateral migration between cells without necessarily compromising barrier integrity.\nC_LIO_LIThe pattern of pneumococcal epithelial surface adherence and microinvasion determines the host cell response through a range of innate signaling and regulatory pathways, inflammatory mediators, adhesion molecules, cellular metabolism and stress response genes.\nC_LIO_LIEpithelial sensing is triggered by, but not wholly dependent on pneumolysin, a key virulence factor of S. pneumoniae.\nC_LI