Cigarette smoke impairs mucociliary transport via increased mucus viscosity and airway dehydration, according to research published in the European Respiratory Journal.
Using a model of ferrets with cigarette smoke-induced chronic obstructive pulmonary disease (COPD), researchers examined the effects of cigarette smoke on mucociliary transport, specifically mucus hydration and viscosity. Wild-type ferrets were age- and gender-matched and exposed to either room air or nose only smoke 5 h/d, 5 =d/wk, for 6 months.
Using Tc-DTPA retention, the investigators measured in vivo mucociliary clearance in the ferrets. Heat maps were then generated to represent clearance rates. After quantifying these images, the researchers identified reduced clearance in smoke-exposed ferrets in 60 minutes (2.8% vs 25.4% in air controls). In each group, the area under the curve demonstrated a significantly higher percent of retention after 60 minutes in smoke-exposed ferrets (2923±86 smoke vs 2543±75 air control; P <.01). A diminished rate of mucociliary clearance was also noted.
Further analysis found that in ferrets with COPD, there was a 29.4% reduction in mucociliary transport (6.8±0.8 vs 9.6±1.1 mm/min in air controls; P <.05); this paralleled the reduction noted in the in vivo models. A linear mixed model, adjusting for cohort and sex at baseline, “confirmed the deleterious effects of smoking” on mucociliary transport (32% reduction; P =.016; R²=0.368).
The researchers confirmed findings using primary human bronchial epithelial cells isolated from nonsmoking, healthy smoking, and COPD donors. Findings from both analyses confirmed that chronic cigarette smoke “diminishes mucociliary transport, consistent with prior observations.”
Carbachol stimulation of ferret trachea resulted in increased mucus fluid secretion, reflected in both airway surface liquid and periciliary layer depths (22.3% and 20.5%, respectively), as well as in stimulated ciliary beat frequency and accelerated mucociliary transport (25.6% and 43.0%, respectively). Following smoke-induced chronic bronchitis, cholinergic stimulation of ferret trachea demonstrated similar effects as in a normal trachea, with increased airway surface liquid and periciliary layer depth, stimulated ciliary beat frequency, and mucociliary transport (24.3%, 13.0%, 25.1%, and 91.4%).
Finally, histology was performed on ferret model airway tissue. The investigators found that ferrets exposed to cigarette smoke demonstrated evidence of goblet cell hyperplasia, submucosal gland hypertrophy, and “overall higher amounts” of mucus staining, indicative of a chronic bronchitis phenotype consistent with previous reporting.
To establish the role of viscosity in the human samples, histology was performed on the same human bronchial epithelial tissues. Both COPD and healthy smoker epithelia had signs of goblet cell hyperplasia and mucus impaction; conversely, these were minimal in nonsmoker tissues. Mucus viscosity was also analyzed and mean-squared displacement of 1-μm particles was “markedly reduced” in COPD mucus compared with both normal and healthy smoker donors.
“Given these findings, augmenting aspects of the airway functional microanatomy to facilitate mucus transport in chronic bronchitis may prove beneficial for improving overall mucus clearance,” the researchers concluded. “Due to the heterogeneity of COPD, it may be necessary to combine therapies targeting specific mechanisms underlying its pathogenesis on an individual basis to ultimately slow disease progression.”
Lin VY, Kaza N, Birket SE, et al. Excess mucus viscosity and airway dehydration impact COPD airway clearance [published online October 31, 2019]. Eur Respir J. doi:10.1183/13993003.00419-2019