TGFB1 and chronic obstructive pulmonary disease: In vivo and in vitro, M2-directed polarization related to the TGF-β/Smad pathway in COPD was observed (29, 44), with the M2/M1 proportions generally negatively correlated with the lung function of mice (29), demonstrating the potential significance of the polarization state biased towards M2 macrophages in COPD.