Immunohistochemical Study of Fibrosis and S-100 Protein-positive Cells in Idiopathic Pulmonary Fibrosis and in Interstitial Pneumonia Associated with Rheumatoid Arthritis

: Immunohistochemical methods were used to distinguish idiopathic pulmonary fibrosis (IPF) from interstitial pneumonia associated with rheumatoid arthritis (RA lung). The subjects were six patients with IPF and seven with RA lung, in whom the pathological findings were consistent with usual interstitial pneumonia. Antibodies to vimentin (Vim), alpha-smooth muscle actin (alpha-SMA), and S-100 protein were used for immunohistochemical studies done by the streptavidin-biotin-peroxidase complex method. In fibrosis associated with RA lung, proliferation of both Vim and alpha-SMA-positive myofibroblasts was widely observed, despite pathological findings of honeycombing, usual interstitial pneumonia, and BOOP. Fibrosis in cases of IPF was found to be characterized mainly by Vim-positive fibroblasts, and on occasion was associated with hyperplasia of smooth muscle. Lung tissues from patients with acute exacerbations of RA lung, especially when associated with a BOOP pattern, had many cells positive for S-100 protein. However, such cells were generally hard to find in cases of IPE. Similar results were obtained with regard to the honeycomb pattern in both IPF and RA lung. These findings suggest that IPF and RA lung can be fairly clearly differentiated based on the proliferation of myofibroblasts and on the presence or absence of cells positive for S-100 protein.