Neutrophil extracellular traps and the lung microbiome in bronchiectasis

<b>Background:</b> Neutrophil Extracellular Traps (NETs) are composed of DNA, histones and over 30 granular proteins. NETs are ineffective at killing bronchiectasis pathogens such as <i>Pseudomonas aeruginosa</i>. We hypothesised that NETs would be present in the bronchiectasis lung and would be associated with an altered microbiome. <b>Methods:</b> 218 bronchiectasis patients were enrolled. We performed microbiome analysis on patient sputum using 16s rRNA PCR and sequencing. Lung inflammation was evaluated using LC/MS proteomics and NET quantification in sputum supernatant using immunoassays. <b>Results:</b> 218 patients were studied. Mean age was 68 years, 58% female. The bronchiectasis microbiome was heterogeneous but was dominated by proteobacteria dysbiosis, such as <i>Pseudomonas</i> and <i>Haemophilus</i> (n=94) and to a lesser extent firmicutes dysbiosis, such as <i>Streptococcus</i> (n=52). Proteobacteria dysbiosis was associated with moderate and severe bronchiectasis, and a higher frequency of exacerbations. Integration of inflammatory and microbiota data showed severe bronchiectasis associated with proteobacteria, MMP9, LL-37, Olfactomedin-4, neutrophil elastase, resistin and other NET components, while firmicutes were associated with eosinophilic inflammation, and high levels of epithelial markers such as cystatin and SLPI. NETs were abundant in the bronchiectasis sputum and were higher in patients with proteobacteria dysbiosis (mean histone-elastase complexes 35.8 sd 44.6 vs 11.6 sd 13.3, p=0.0002). Higher NETs were associated with lower diversity measured by the Shannon index (p=0.01), and Berger-Parker index (p=0.02). <b>Conclusion:</b> NETs are associated with proteobacteria dysbiosis and loss of microbiome diversity in bronchiectasis.