Proteomic profiling of equine airway mucus reveals compositional changes in asthmatic phenotypes

Mucus hypersecretion and accumulation are hallmark features of equine asthma (EA), a meaningful respiratory disorder in horses occurring in mild to moderate (MEA) and severe (SEA) forms. Changes of the proteomic composition of airway mucus in EA are poorly understood. Using label-free quantitative liquid chromatography-mass spectrometry, we analyzed airway mucus from SEA (n = 10), MEA (n = 6), and healthy (n = 8) horses. We identified and quantified 2,275 proteins including gel-forming mucins MUC5AC and MUC5B and membrane-bound mucins MUC1 and MUC4. Compared with healthy controls, 130 proteins (SEA) and 103 (MEA) were significantly increased. 38 were elevated in SEA relative to MEA, 10 were higher in MEA. MUC4 was markedly increased in both, correlated with bronchoalveolar lavage neutrophils (ρ = 0.790, p = 4.9E-06), and distinguished excellently between healthy and asthmatics (AUC = 1.0, 95% CI: 1-1), similar to 23 other proteins. MUC5AC was elevated in both, whereas MUC5B only in SEA. MUC1 did not differ between groups. Changes in mucus-modifying proteins, including glycosyltransferases and aquaporins, suggest altered mucus properties in EA. Functional enrichment analyses revealed inflammation-, tissue remodeling- and coagulation-linked GO terms and pathways in EA. The distinct proteomic profiles add to the understanding of EA and may offer novel targets for phenotype-specific biomarkers and therapy.