Abnormal alveolar epithelial repair associated with failure of resolution in experimental streptococcal pneumonia.

We describe an experimental model in Wistar rats of non-resolving bronchopneumonia evoked by Streptococcus pneumoniae type 25. In contrast to a model of resolving streptococcal pneumonia that we have previously described, morphological studies reveal that in this model, there is significant early damage to type 1 pneumocytes which progresses to necrosis, leaving isolated areas of denuded alveolar basement membrane. Furthermore, there is accompanying degeneration and necrosis of a proportion of the type 2 pneumocytes, and alveolar epithelial repair by proliferation and differentiation of these cells appears to be retarded. Isolated, hypertrophic, and hyperplastic foci of type 2 pneumocytes persist as the acute inflammatory response subsides, and organization progresses with proliferation and emigration of fibroblasts into the lumina of alveoli and terminal bronchioles. The resultant lesion is morphologically indistinguishable from bronchiolitis obliterans organizing pneumonia. We hypothesize that the abnormal outcome in this model of pneumonia is a consequence of the failure of proliferating type 2 pneumocytes to transform into type 1 pneumocytes and thus maintain the integrity of the alveolar epithelial surface.