TGFB1 and chronic obstructive pulmonary disease: Bronchial brushings from active smokers showed significantly increased mRNA expression of TGF-β and IL-8, and it was reduced after quercetin treatment.<h4>Conclusions</h4>These results indicate that quercetin may improve airway epithelial regeneration by increasing the expression of genes involved in epithelial development/differentiation in COPD.<h4>Trial registration</h4>This study was registered at ClinicalTrials.gov on 6-18-2019.