The epithelial cell in host-pathogen interactions in airways disease

Non-typeable Haemophilus influenzae (NTHi) is the most common lower respiratory tract bacterium, where it causes persistent infections and is associated with neutrophilic airway inflammation and increased risk of exacerbations in patients with asthma and chronic obstructive pulmonary disease (COPD). This work aimed to investigate the persistence of NTHi within the epithelium, the role of bacterial internalisation in the epithelial immune response, and evaluate the differential immune signalling in the epithelium of upper compared with lower airway epithelium. In an in vitro model of NTHi infection using cell lines and primary cells, differing internalisation rates between NTHi strains did not correlate with induction of proinflammatory cytokines. Treatment with cytochalasin D to block invasion diminished IL-1b production, but did not affect other cytokines. Imaging demonstrated that intracellular NTHi localised primarily within the late endosome of differentiated epithelial cells. Viable bacilli did not persist beyond 48 h in cell lines in submerged culture, or 24 h in primary cells grown at ALI, suggesting the NTHi does not persist in epithelial cells alone. Although NTHi causes inflammation in the lower airways, it is considered a commensal organism in the nasopharynx. Comparison of primary cells cultured at air- liquid interface inoculated with NTHi showed substantially more upregulation of TLR signalling, NOD signalling, and of ICAM1, Caspase1 and IL-8 in nasal epithelial cells than bronchial cells. This suggests that the increased pathogenicity of NTHi in the lower airways compared with the nasopharynx is not due to increased innate immune responsiveness of bronchial epithelial cells. These data demonstrate that NTHi potently induces an innate response in the bronchial epithelium independent of internalisation rate, with limited NLR inflammasome dependence, implicating surface immune sensing via TLR2 and 4. These data also demonstrate differential immune signalling in nasopharyngeal compared with lower airway epithelium. Overall, these data suggest that NTHi is internalised transiently but has the capacity to drive inflammation in airway epithelial cells.