Endotypes of Exacerbation in Bronchiectasis: An Observational Cohort Study

Rationale: Bronchiectasis is characterized by acute exacerbations, but the biological mechanisms underlying these events are poorly characterized. Objectives: To investigate the inflammatory and microbial characteristics of exacerbations of bronchiectasis. Methods: A total of 120 patients with bronchiectasis were enrolled and presented with acute exacerbations within 12 months. Spontaneous sputum samples were obtained during a period of clinical stability and again at exacerbation before receipt of antibiotic treatment. A validated rapid PCR assay for bacteria and viruses was used to classify exacerbations as bacterial, viral, or both. Sputum inflammatory assessments included label-free liquid chromatography-tandem mass spectrometry and measurement of sputum cytokines and neutrophil elastase activity. 16 s rRNA sequencing was used to characterize the microbiome. Measurements and Main Results: Bronchiectasis exacerbations showed profound molecular heterogeneity. At least one bacterium was identified in 103 samples (86%), and a high bacterial load (total bacterial load > 10⁷ copies/g) was observed in 81 patients (68%). Respiratory viruses were identified in 55 (46%) patients, with rhinovirus being the most common virus (31%). PCR testing was more sensitive than culture. No consistent change in the microbiome was observed at exacerbation. Exacerbations were associated with increased neutrophil elastase, proteinase-3, IL-1β, and CXCL8. These markers were particularly associated with bacterial and bacterial plus viral exacerbations. Distinct inflammatory and microbiome profiles were seen between different exacerbation subtypes, including bacterial, viral, and eosinophilic events in both hypothesis-led and hypothesis-free analysis using integrated microbiome and proteomics, demonstrating four subtypes of exacerbation. Conclusions: Bronchiectasis exacerbations are heterogeneous events with contributions from bacteria, viruses, and inflammatory dysregulation.