Immunohistochemical identification of cell types in normal and in bleomycin-induced fibrotic rat lung. Cellular origins of interstitial cells.

There is substantial tissue reorganization in the interstitium in pulmonary fibrosis. In order to determine the origin of cells in the interstitium of normal or fibrotic rat lung, we stained lung tissue immunohistochemically with antiserums directed against intermediate filament proteins specific for cells of mesenchymal or muscle origin. Vimentin, specific for mesenchymal cells, was isolated from cultured 3T3 fibroblasts, and desmin, specific for muscle cells, was isolated from chicken gizzard and antiserums prepared in guinea pigs. Paraffin-embedded sections of normal or fibrotic rat lung from rats 28 days after treatment with bleomycin sulfate were reacted with each antiserum and stained by the peroxidase-antiperoxidase technique. Antivimentin antiserum stained discrete interstitial cells of both control and fibrotic lung. Smooth muscle surrounding large airways and vascular smooth muscle was not significantly stained with this antiserum but was heavily stained with antidesmin. However, antidesmin did not stain cells in the interstitium of normal or fibrotic rat lung. These results suggest that most cells in the normal as well as in the fibrotic interstitium, including contractile interstitial cells, are probably of mesenchymal (fibroblastic) rather than of smooth muscle origin.