Tolerance to Haemophilus influenzae infection in human epithelial cells: insights from a primary cell-based model

Haemophilus influenzae is a human respiratory pathogen and inhabits the human respiratory tract as its only niche. Despite this, the molecular mechanisms that allow H. influenzae to establish persistent infections of human epithelia are not well understood. Here, we have investigated how H. influenzae adapts to the host environment and triggers the host immune response using a human primary cell-based infection model that closely resembles human nasal epithelia (NHNE). Physiological assays combined with dualRNAseq revealed that NHNE from five healthy donors all responded to H. influenzae infection with an initial, unproductive inflammatory response that included a strong hypoxia signature but did not produce pro-inflammatory cytokines. Subsequently, an apparent tolerance to large extra- and intracellular burdens of H. influenzae developed, with NHNE transcriptional profiles resembling the pre-infection state. This occurred in parallel with the development of intracellular bacterial populations, and appears to involve interruption of NFkB signalling. This is the first time that large-scale, persistence-promoting immunomodulatory effects of H. influenzae during infection have been u. Interestingly, NHNE were able to re-activate pro-inflammatory responses towards the end of the 14-day infection resulting in release of pro-inflammatory cytokines (IL8, TNF). Our data further indicate the presence of infection stage-specific gene expression modules, highlighting fundamental similarities between immune responses in NHNE and canonical immune cells, which merit further investigation.