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To determine whether and by which pathway (via the death receptor or mitochondrial mediated pathway) lymphocyte apoptosis occurs in pneumonia and to determine if increased bronchial epithelial cell apoptosis occurs in pneumonia. Prospective randomized study in a university research laboratory. Male C57BL/6 mice (n = 30). Animals received an intratracheal injection of Streptococcus pneumoniae or Pseudomonas aeruginosa to induce gram-positive or gram-negative pneumonia, respectively and were killed 24, 30, or 48 h later. Presence of pneumonia was confirmed via gross visual examination of lungs and by histology. Lymphocyte apoptosis in spleen and thymus was analyzed by flow cytometry for active caspases 3, 8, and 9 and by immunohistochemical (IHC) staining for active caspase 3 and DNA strand breaks. Respiratory epithelial cell apoptosis was assessed by IHC. Histologically, pneumonia was present in all bacteria-treated animals but none in sham-treated mice. Extensive lymphocyte apoptosis in spleen and thymus was documented by characteristic morphological changes on hematoxylin and eosin staining and by IHC staining in both S. pneumonia and P. aeruginosa infection. Flow cytometry confirmed IHC and showed apoptotic lymphocytes positive for active caspases 3, 8, and 9 in both thymi and spleens in both infections. In contrast to the extensive lymphocyte apoptosis, only rare scattered apoptotic changes were seen in respiratory epithelial or endothelial cells in pneumonia due to either organism. Increased lymphocyte but not bronchial cell apoptosis occurs in both gram-positive and gram-neagtive pneumonia and probably involves both the extrinsic and intrinsic pathway.

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