Human PCLS exposed to CSE was shown to increase proinflammatory cytokines MMP-9, IL-1α, and IL-1β and cause tissue damage which correlated with the previous mice data, where cigarette smoke treatment for 1 month increased the expression of MMP-9 and the specific inhibitor of MMP-9 ameliorated the small airway remodeling and emphysema-associated changes [167]. Here, MMP9 is linked to pulmonary emphysema.