Sputum proteomics identifies distinct endotypes of exacerbations in bronchiectasis

<b>Introduction:</b> Acute exacerbations are important events in the natural history of bronchiectasis (BE). Most are assumed to be infective but whether there are different subtypes of exacerbations is unknown. <b>Aim:</b> To investigate whether exacerbations are associated with distinct sputum protein profiles in BE. <b>Methods:</b> Patients with BE (n=52, age 67 (64-70) years, female 50%, idiopathic 54%, BE severity Index (BSI) score 8.5 (7.3-9.7)) were included. Sputum samples were collected in clinically stable condition and a second sample obtained at the onset of exacerbation. Sputum protein profiling was performed using a label-free proteomics workflow and validation was performed using immunoassay. <b>Results:</b> Sputum protein profiles from BE patients were highly variable. Principal component analysis showed that ~40% of samples expressed higher levels of proteins characteristic of neutrophilic inflammation (e.g. MPO, AZU1, ELANE; Cluster 1) and ~40% expressed higher levels of protective proteins (e.g. PIGR, IgHA1, CST3; Cluster 2). Surprisingly, the majority (79%) of patients did not change their sputum proteome cluster from a stable to an exacerbation state, suggesting that sputum proteome of each individual is relatively stable and that inflammatory state at baseline predicts endotype of exacerbation. Interestingly, a sub-group analysis of patients in Cluster 1 showed that neutrophilic inflammation was elevated during an exacerbation. In Cluster 2, no consistent changes between the two states was found. Patients in Cluster 1 had higher BSI whereas those in Cluster 2 had fewer symptoms. <b>Conclusion:</b> Acute exacerbation increases neutrophilic inflammation only in a subset of BE patients who already have a neutrophilic endotype.