Acute respiratory distress syndrome, a severe form of acute lung injury (ALI), is a major cause of death during influenza pneumonia. We have provided evidence for the involvement of recruited neutrophils, their toxic enzymes such as myeloperoxidase and matrix metalloproteinases (MMPs), and neutrophil extracellular traps in aggravating alveolar-capillary damage. In this study, we investigated the effects of doxycycline (DOX), an inhibitor of MMPs, on influenza-induced ALI. BALB/c mice were infected with a sublethal dose of mouse-adapted virulent influenza A/Aichi/2/68 (H3N2) virus, and administered daily with 20mg/kg or 60 mg/kg DOX orally. The effects of DOX on ALI were determined by measuring inflammation, capillary leakage, and MMP activities. Furthermore, levels of T1-α (a membrane protein of alveolar type I epithelium) and thrombomodulin (an endothelial protein) in the bronchoalveolar lavage fluid were evaluated by Western blot analysis. Our results demonstrate significantly decreased inflammation and protein leakage in the lungs after DOX treatment. Levels of MMP-2 and MMP-9 activity, T1-α and thrombomodulin were also diminished in the DOX-treated group. These findings were corroborated by histopathologic analyses, which demonstrated significant reduction in lung damage. Although DOX treatment reduced ALI, there were no effects on virus titers and body weights. Taken together, these results demonstrate that DOX may be useful in ameliorating ALI during influenza pneumonia. Further studies are warranted to determine whether DOX can be used in combination with anti-viral agents to alleviate severe influenza pneumonia.
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