Background: Muco-obstructive diseases lead to hypersecretion, changing the airway mucus properties, and impairing mucociliary transport, resulting in mucostasis, and increasing infection likelihood. Mucin structure may determine the water sorption properties of mucus and play a role in chronic obstructive pulmonary disease (COPD) and cystic fibrosis (CF).

Objectives: Investigate the sorption properties and mucin nanostructures in mucus collected from healthy, COPD and CF airways.

Methods: To investigate mucin nanostructures, small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM) were used. Sorption properties were determined by quartz crystal microbalance with dissipation (QCM-D). Cell-culture (cc) samples from healthy airway mucus (HAM), COPD and CF cultures, and patient HAM from endotracheal tubes and COPD from bronchoscopy were used. Patient mucus was oven-dried at 80°C for the solids content.

Results: SAXS and AFM revealed mucin monomers with typical dumbbell structures and varying chain lengths, CF mucins having the shortest chain lengths. Dry-weight solids reached 11wt% in COPD mucus. From QCM-D analysis, cc-COPD and COPD mucus had the highest water content (67 and 75%) during sorption at 99% humidity compared with cc-HAM and HAM (63 and 56%). The overall sorption isotherm for cc-CF mucus was notably lower than HAM and COPD. In low-humidity environments, no hydration-induced glass transition was observed, suggesting mucus remains in a rubbery state.

Conclusions: Mucin nanostructures observed in disease could explain the sorption properties where mucin chains affect water content in high humidity environments, which may play a role in protecting the epithelium.