Idiopathic pulmonary fibrosis (IPF) is a progressive and fatal fibrotic lung disease of unknown aetiology. There is growing evidence that the lung microbiota may play a role in IPF. However, no study has investigated the functional impact of the short-chain fatty acids (SCFAs) on primary bronchial epithelial cells (PBECs) and disease pathogenesis. Therefore, we investigated the influence of acetate, propionate, and butyrate on PBECs from healthy controls and subjects with IPF.

Subjects diagnosed with IPF (n=201) and healthy controls (n=40) were prospectively recruited and underwent bronchoalveolar lavage. Bacterial DNA was isolated and 16S rRNA gene sequencing undertaken to characterise bacterial communities. Untargeted ¹H nuclear magnetic resonance spectroscopy-based metabolomics and targeted gas chromatography-mass spectrometry captured the metabolic profile of these samples. PBECs from healthy controls and subjects with IPF were differentiated at air-liquid interface (ALI) and either left untreated or exposed to the SCFAs.

The IPF microbiota was less diverse (P<0.01) and had increased proportions of Firmicutes (P<0.01) compared to healthy controls. Streptococcus and Staphylococcus were more abundant in IPF cases than controls (P<0.05). Metabolomics analysis revealed distinct differences between the cohorts. Relative concentrations of the SCFAs were increased in IPF compared to healthy controls, and in IPF, propionate positively correlated with bacterial burden (rho=0.47, P=8 × 10^-5). Exposure of healthy and IPF PBECs cultured at ALI to 1 mM of the SCFAs did not impact cell viability. Treatment of PBECs from IPF subjects but not healthy controls with the SCFAs led to morphological changes, a dose-dependent release of pro-inflammatory mediators in the cell supernatant, and a decrease in transepithelial electrical resistance (TEER) over time. Specifically, compared to baseline, exposure of IPF PBECs to 1 mM of propionate led to a 40% reduction in TEER and a 2-fold increase in the secretion of IL-6.

Subjects with IPF display an altered microbiome which is associated with a distinct metabolic signature in the lower airways. Differences in specific bacterial genera and an increased bacterial burden in IPF results in changes in the SCFAs in the airways. In vitro work demonstrates the potential of these SCFAs to shape immunological responses in the lung, mediating the pathogenesis of fibrosis.