Fas/Fas ligand pathway is involved in the resolution of type II pneumocyte hyperplasia after acute lung injury: evidence from a rat model.

Objective: We used a rat model of acute lung injury to evaluate the role of apoptosis of type II pneumocytes in alveolar remodeling during the resolution phase. Design: Controlled animal study. Setting: University research laboratory. Subjects: Sprague-Dawley rats. Interventions: Sprague-Dawley rats had Escherichia coli lipopolysaccharide instilled transtracheally to induce acute lung injury. Animals were killed on various days after lipopolysaccharide instillation. Lung specimens from all animals were examined for the presence of apoptosis in type II pneumocytes by an in situ apoptosis assay and for proliferative nuclear antigen, cytokeratin-18, Fas, and Fas ligand with an immunohistochemical stain. Fas and Fas ligand expression in both lung tissue and bronchoalveolar lavage fluid was examined by Western blot analysis. Measurements and Main Results: Histologic examination revealed that the lungs of rats with acute lung injury showed infiltration of numerous inflammatory cells in the intra-alveolar and/or interstitial space and hyperplasia of type II pneumocytes. Type II pneumocyte proliferation, detected by proliferative nuclear antigen staining, developed maximally around day 3 after acute lung injury. In the in situ apoptosis assay, positive signals in type II pneumocytes were obvious and were distributed diffusely in the lung parenchyma from day 1 after acute lung injury, became maximal around day 7, then declined until day 21. DNA fragmentation analysis revealed that a DNA ladder pattern was detectable from day 3, persisted until day 10, and disappeared after day 14. The major cell types expressing Fas ligand are macrophages and neutrophils. Western blot analysis showed that Fas ligand, both membrane-bound form and soluble form, was present from day 1 to day 21 after acute lung injury, with highest level occurring during the first week of acute lung injury. Fas expression in type II pneumocytes reached its maximum on days 3-5 and then gradually declined until day 21. Fas and Fas ligand expression appeared to proceed type II pneumocyte apoptosis. After the acute stage, Fas and Fas ligand expression declined, and type II pneumocyte apoptosis also decreased. These findings correlate with histologic resolution of type II pneumocyte hyperplasia. Conclusions: Our results confirm that type II pneumocyte proliferation in response to acute lung injury is mainly a reparative phenomenon. During the resolution phase of acute lung injury, extensive apoptosis of type II pneumocytes is the main cellular mechanism that accounts for the disappearance of these cells, and Fas/Fas ligand is involved in the resolution of type II pneumocytes. Our model may provide a useful tool to assess the mechanisms of tissue remodeling after acute lung injury.