Alveolar epithelial cell death adjacent to underlying myofibroblasts in advanced fibrotic human lung

Earlier work from this laboratory showed that abnormal fibroblast phenotype s isolated from fibrotic human lung produce factor(s) capable of inducing a poptosis and necrosis of alveolar epithelial cells in vitro [B. D. Uhal, I. Joshi, A. True, S. Mundle, A. Raza, A. Pardo, and M. Selman. Am. J. Physio l. 269 (Lung Cell. Mel. Physiol. 13): L819-L828, 1995]. To determine whethe r epithelial cell death is associated with proximity to abnormal fibroblast s in vivo, the spatial distribution of epithelial cell loss, DNA fragmentat ion, and myofibroblasts was examined in the same tissue specimens used prev iously for fibroblast isolation. Paraffin sections of normal and fibrotic h uman lung were subjected to in situ end labeling (ISEL) of fragmented DNA a nd simultaneous immunolabeling of alpha-smooth muscle actin (alpha-SMA); re plicate samples were subjected to electron microscopy and detection of coll agens by the picrosirius red technique. Normal human lung exhibited very li ttle labeling except for positive alpha-SMA immunoreactivity of smooth musc le surrounding bronchi and vessels. In contrast, fibrotic human lung exhibi ted moderate to heavy ISEL of interstitial, cuboidal epithelial, and free a lveolar cells. ISEL of the alveolar epithelium was not distributed uniforml y but was most intense immediately adjacent to underlying foci of alpha-SMA -positive fibroblast-like interstitial cells. Both electron microscopy and picrosirius red confirmed epithelial cell apoptosis, necrosis, and cell los s adjacent to foci of collagen accumulation surrounding fibroblast-like cel ls. These results demonstrate that the cuboidal epithelium of the fibrotic lung contains dying as well as proliferating cells and support the hypothes is that alveolar epithelial cell death is induced by abnormal lung fibrobla sts in vivo as it is in vitro.