Extracellular superoxide dismutase (ECSOD or SOD3) is highly expressed in lungs and functions as a scavenger of O 2 • ─ . ECM fragmentation, which can be triggered by oxidative stress, participates in the pathogenesis of chronic obstructive pulmonary disease (COPD) through attracting inflammatory cells into the lungs. The level of SOD3 is significantly decreased in lungs of patients with COPD. However, the role of endogenous SOD3 in the development/progression of emphysema is unknown. We hypothesized that SOD3 protects against emphysema by attenuating oxidative fragmentation of ECM in mice. To test this hypothesis, SOD3-deficient, SOD3-transgenic, and WT C57BL/6J mice were exposed to cigarette smoke (CS) for 3 d (300 mg total particulate matter/m 3 ) to 6 mo (100 mg/m 3 total particulate matter) or by intratracheal elastase injection. Airspace enlargement, lung inflammation, lung mechanical properties, and exercise tolerance were determined at different time points during CS exposure or after elastase administration. CS exposure and elastase administration caused airspace enlargement as well as impaired lung function and exercise capacity in SOD3-null mice, which were improved in mice overexpressing SOD3 and by pharmacological SOD mimetic. These phenomena were associated with SOD3-mediated protection against oxidative fragmentation of ECM, such as heparin sulfate and elastin, thereby attenuating lung inflammatory response. In conclusion, SOD3 attenuates emphysema and reduces oxidative fragmentation of ECM in mouse lung. Thus, pharmacological augmentation of SOD3 in the lung may have a therapeutic potential in the intervention of COPD/emphysema.antioxidants | cigarette smoke | chronic obstructive pulmonary disease | oxidants | glutathione E xtracellular superoxide dismutase (ECSOD or SOD3) is one of the three SOD antioxidant enzyme isoforms, and is highly expressed in lungs and vessels (1, 2). It is located in the ECM, the junctions of airway epithelial cells, the surface of airway smooth muscle, and the lining of vessels of the lung. SOD3 functions as a superoxide anion scavenger, thereby attenuating oxidative stress, which may play an important role in the pathogenesis of chronic obstructive pulmonary disease (COPD). Acute loss of SOD3 in adult mice causes death, whereas low mortality rates are seen in SOD3-overexpressing animals exposed to hyperoxia, suggesting an essential role of SOD3 for survival (3,4). Accumulating evidence shows that SOD3 polymorphism is associated with a decline in lung function in rodents and humans, and susceptibility to COPD, which may be a result of alteration of its encoding protein structure, function, and level (5-14). However, it is unclear whether endogenous SOD3 participates in the development/ progression of the inflammatory response, leading to COPD/ emphysema.Cigarette smoke (CS) is the major etiological factor in the pathogenesis of COPD, which is characterized by chronic lung inflammation, parenchymal destruction (i.e., emphysema), and accelerated decline in lung function....