Recent evidence suggests that fibronectin, an adhesive major cell surface protein of cultured fibroblasts and a component of normal human connective tissue, is important in maintenance of normal cell to cell and cell to connective tissue relationships. The hypothesis that fibronectin is a sensitive target of products released from inflammatory cells was investigated. To evaluate this concept, fibronectin was purified from normal human fibroblasts (HFL-1 lung cells) and incubated with human polymorphonuclear leukocyte granule contents under physiologic conditions. Fibronectin exposed to neutral proteases present in leukocyte granules lost its characteristic biologic properties of promoting cell adhesion to a collagen substrate and agglutination of sheep red blood cells. Electrophoretic analysis revealed that extensive proteolysis accompanied this loss of biologic activity. When the total extracellular proteins secreted by fibroblasts were exposed to leukocyte granule contents, fibronectin was more sensitive to proteolysis than the majority of other extracellular proteins. Moreover, the fibrillar network of extracellular high MW cell surface fibronectin was as sensitive to proteolysis by leukocyte granule contents as was purified fibronectin. Fibronectin is apparently a sensitive target of leukocyte proteases under physiologic conditions. Inasmuch as proteases contained in leukocyte granules are released into the surrounding milieu following appropriate physiologic stimulation, some of the changes occurring in normal tissue architecture accompanying inflammation may reflect loss of normal cell to cell and cell to connective tissue matrix interactions mediated by this adhesive cell surface molecule.
