In addition, persistent active epithelial–mesenchymal transition (EMT) has been observed in the airway epithelial cells of patients with COPD; specifically, IL-17A markedly up-regulated the expression of GDF15 in cigarette smoke-treated HSAEpiC cells in a dose-and time-dependent manner, and IL-17A combined with GDF15 activated EMT in HSAEpiC cells after cigarette smoke exposure [112]. The gene discussed is GDF15; the disease is chronic obstructive pulmonary disease.