Inflammatory cells as a source of airspace extracellular superoxide dismutase after pulmonary injury

Extracellular superoxide dismutase (EC-SOD) is an antioxidant abundant in the lung. Previous studies demonstrated depletion of lung parenchymal EC-SOD in mouse models of interstitial lung disease coinciding with an accumulation of EC-SOD in airspaces. EC-SOD sticks to the matrix by a proteolytically sensitive heparin-binding domain; therefore, we hypothesized that interstitial inflammation and matrix remodeling contribute to proteolytic redistribution of EC-SOD from lung parenchyma into the airspaces. To determine if inflammation limited to airspaces leads to EC-SOD redistribution, we examined a bacterial pneumonia model. This model led to increases in airspace polymorphonuclear leukocytes staining strongly for EC-SOD. EC-SOD accumulated in airspaces at 24 h without depletion of EC-SOD from lung parenchyma. This led us to hypothesize that airspace EC-SOD was released from inflammatory cells and was not a redistribution of matrix EC-SOD. To test this hypothesis, transgenic mice with lung-specific expression of human EC-SOD were treated with asbestos or bleomycin to initiate an interstitial lung injury. In these studies, EC-SOD accumulating in airspaces was entirely the mouse isoform, demonstrating an extrapulmonary source (inflammatory cells) for this EC-SOD. We also demonstrate that EC-SOD knockout mice possess greater lung inflammation in response to bleomycin and bacteria when compared with wild types. We conclude that the source of accumulating EC-SOD in airspaces in interstitial lung disease is inflammatory cells and not the lung and that interstitial processes such as those found in pulmonary fibrosis are required to remove EC-SOD from lung matrix.