Tissue remodelling in chronic bronchial diseases: from the epithelial to mesenchymal phenotype

Airway remodelling is a critical feature of chronic bronchial diseases, characterised by aberrant repair of the epithelium and accumulation of fibroblasts, which contribute to extracellular matrix (ECM) deposition resulting in fixed bronchial obstruction. Recently, epithelial-mesenchymal transition (EMT) has been identified as a new source of fibroblasts that could contribute to the remodelling of the airways. This phenomenon consists of the loss of the epithelial phenotype by bronchial epithelial cells and the acquisition of a mesenchymal phenotype. These cells are then able to migrate and secrete ECM molecules. Herein, we review the different types of EMT. We will then focus on the signalling pathways that are involved, such as transforming growth factor-beta and Wnt, as well as the more recently described Sonic Hedgehog pathway. Finally, we will highlight the implication of EMT in airway remodelling in specific chronic bronchial pathologies, such as asthma, chronic obstructive pulmonary disease and bronchiolitis obliterans following lung transplantation. Despite the limitations of in vitro models, future studies of EMT in vivo are warranted to shed new light on the pathomechanisms of bronchial obstruction.