MMP12 and chronic obstructive pulmonary disease: Various inflammatory mediators secreted by M2 in the lung tissues of COPD patients were reported, such as matrix metalloproteinase (MMP)-2, MMP-9, MMP-12, and cathepsin S, to cause lung parenchymal injury and eventually form emphysema, with the increase in M2 amount positively correlated with the emphysema severities (39–41).