Immunolocalization of SPARC, tenascin, and thrombospondin in pulmonary fibrosis

Abstract

Several biochemically unrelated multifunctional extracellular proteins, SPARC, thrombospondin 1, and tenascin-C (TN), have been grouped as antiadhesive glycoproteins because they inhibit the spreading of cells on extracellular matrix in vitro. Migration of fibroblasts and epithelial cells into the air spaces to organize inflammatory exudate is a feature common to several fibrosing lung diseases. We hypothesized that migration would be facilitated by loosening the adhesive interactions between cells and the pericellular matrix components of the alveolar wall and that one or more of the anti-adhesive glycoproteins could be involved. Immunohistochemistry was used to localize SPARC, TN, and thrombospondin 1 in biopsies of organizing pneumonia, idiopathic pulmonary fibrosis (nine cases of usual interstitial pneumonia, one of desquamative interstitial pneumonia), and control lungs. Each antigen had a distinctive distribution. Only TN was expressed in control lungs, where it strongly stained the basement membrane of large bronchi and weakly stained alveolar entrance rings and small veins. In organizing pneumonia, TN was heavily stained through the entire extracellular matrix of the Masson bodies. In idiopathic pulmonary fibrosis, TN was abundant in the fibroblast foci of active fibrosis but was also present in the basement membrane regions beneath the metaplastic epithelium lining honeycomb cysts. TN was abundant in the interstitium in desquamative interstitial pneumonia. SPARC was observed only intracellularly where it occurred in the fibroblasts of Masson bodies of organizing pneumonia and the fibroblast foci of usual interstitial pneumonia. In desquamative interstitial pneumonia, expression of SPARC was minimal, in rare interstitial fibroblasts. Thrombospondin 1 was found consistently in organizing pneumonia but only infrequently in idiopathic pulmonary fibrosis. In both, it was localized in the extracellular matrix immediately beneath reparative epithelium. These results are consistent with a role for SPARC in fibroblast migration. TN may function in both fibroblast migration and the adhesion of metaplastic bronchial-type epithelium. However, these proteins also have other activities that may be important in pulmonary fibrosis. The localization of thrombospondin 1 suggests that it may be synthesized by regenerating epithelium where it may aid in the adhesion or migration of the epithelial cells.