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Epithelial-mesenchymal transition (EMT) is the process by which differentiated epithelial cells undergo a phenotypic transition to mesenchymal cells. This process may occur in certain fibrotic diseases that involve airway remodelling. However, few studies have directly proved the occurrence of EMT in primary cultures of airway epithelial cells. The aim of this study was to clarify whether airway epithelial cells can differentiate into mesenchymal cells through EMT.Mouse tracheal epithelial cells (mTEC) were cultured in an air-liquid interface (ALI) culture system, in the presence or absence of transforming growth factor-β1 (TGF-β1). The expression of mesenchymal and epithelial cell markers was examined by immunofluorescence staining and western blotting. Secretion of matrix metalloproteinase (MMP)-9 into the culture medium was measured by ELISA. The phenotype of epithelial cells involved in EMT was also examined by immunofluorescence staining.Immunofluorescence staining and western blotting revealed that TGF-β1 treatment for 14 days induced the expression of the mesenchymal markers, α-smooth muscle actin (α-SMA) and vimentin, and reduced the expression of the epithelial markers, zonula occludens-1 (Zo-1) and occludin. In addition, α-SMA and Zo-1 were colocalized within individual cells treated with TGF-β1. Concentrations of MMP-9 in the culture medium were significantly higher in TGF-β1-treated mTEC than in untreated cells. The basal cell markers, cytokeratin-5 and cytokeratin-17, were colocalized within the cells expressing α-SMA.EMT was induced by TGF-β1 in primary cultures of mTEC, suggesting that airway epithelial cells, possibly the basal cells, may be involved in airway remodelling through EMT.