Sputum Neutrophil Elastase associates with microbiota and P. aeruginosa in bronchiectasis

Introduction Neutrophilic inflammation is a major driver of bronchiectasis pathophysiology, and neutrophil elastase activity is the most promising biomarker evaluated in sputum to date. How active neutrophil elastase correlates with lung microbiome in bronchiectasis is still unexplored. We aimed at understanding if active neutrophil elastase is associated with low microbial diversity and distinct microbiome characteristics. Methods An observational, cross-sectional study was conducted at the Bronchiectasis Program of the Policlinico Hospital in Milan, Italy, where adults with bronchiectasis were enrolled between March 2017 and March 2019. Active neutrophil elastase was measured on sputum collected during stable state, microbiota analysed through 16S rRNA gene sequencing, molecular assessment of respiratory pathogens through real time PCR and clinical data collected. Measurements and Main Results Among 185 patients enrolled, decreasing alpha diversity, evaluated through the Shannon entropy (rho: −0.37; p-value <0.00001), Pielou’ evenness (rho: −0.36, p<0.00001) and richness (rho: −0.33; p<0.00001), was significantly correlated with increasing elastase. A significant difference in median levels of Shannon was detected between patients with neutrophil elastase ≥20 µg·mL−1 [3.82 (2.20–4.96)] versus neutrophil elastase <20 µg·mL−1 [4.88 (3.68–5.80)], p<0.0001. A distinct microbiome was found in these two groups, mainly characterised by enrichment with Pseudomonas in the high and with Streptococcus in the low elastase group. Further confirmation of the association of P. aeruginosa with elevated active neutrophil elastase was found based on standard culture and targeted real-time PCR. Conclusions High levels of active neutrophil elastase are associated to low microbiome diversity and specifically to P. aeruginosa infection.