As a pro-inflammatory mediator, aberrant activation of MMP-12 led to multiple diseases progression, including chronic obstructive pulmonary disease (COPD) (52), pneumonia (53), etc. Degradation of extracellular matrix components by MMP-12 is critical for the migration of macrophage, as Mmp12-/- macrophages were incapable of penetrating reconstituted basement membranes (54). Here, MMP12 is linked to pneumonia.