LEUKOCYTE-DERIVED EXTRACELLULAR SUPEROXIDE DISMUTASE IN PULMONARY DISEASE

The antioxidant enzyme extracellular superoxide dismutase (EC-SOD) is abundant in the lung and limits inflammation and tissue injury in response to many pulmonary insults. Previous studies reported a loss of full-length EC-SOD from the lung parenchyma with accumulation of proteolyzed EC-SOD in the airspace after interstitial lung injury. However, following airspace only inflammation (pneumonia), EC-SOD accumulates in the airspace without a loss from the interstitium, suggesting this antioxidant may be released from an extrapulmonary source.Because leukocytes are known to express EC-SOD and are prevalent in the bronchoalveolarlavage fluid after injury, it was hypothesized that these cells may transport and release EC-SODinto airspaces. To investigate this, bone marrow chimeras were generated using wild-type andEC-SOD knockout (KO) mice. Following intratracheal treatment with asbestos, reconstituted mice without pulmonary EC-SOD expression, but with EC-SOD in infiltrating and resident leukocytes did not have detectable levels of EC-SOD in the airspaces. In addition, leukocytederived EC-SOD did not significantly lessen inflammation or early stage fibrosis.Although these results indicate that leukocyte-derived EC-SOD is not influential in asbestos-induced interstitial lung injury, EC-SOD in these cells may play a role in attenuating pneumonias and other inflammatory diseases. To test this hypothesis, wild-type and EC-SOD KO mice were given Escherichia coli pneumonia. Notably, even though EC-SOD KO mice had greater pulmonary inflammation than wild-type mice, there was less bacterial clearance from their lungs following infection. While EC-SOD expression has been previously reported in macrophages and neutrophils, its function and subcellular localization in these inflammatory cells is unclear. In this study, EC-SOD was found to be in membrane bound vesicles of phagocytes. This finding led to the hypothesis that inflammatory cell EC-SOD may play a role in antibacterial defense. To investigate this, phagocytes from wild-type and EC-SOD KO mice were evaluated. While macrophages lacking EC-SOD produced more oxidants than EC-SOD expressing cells after stimulation, they had significantly impaired phagocytosis and bacterial killing ability. Overall, these studies suggest that while EC-SOD inside leukocytes does not contribute to interstitial lung injuries, it plays a central role in mediating bacterial infections by facilitating bacterial clearance and limiting inflammation by promoting phagocytosis