Immunoelectron microscopic demonstration of fibronectin in rheumatoid pannus and at the cartilage-pannus junction.

The presence of fibronectin (Fn) in rheumatoid pannus was investigated by an immunoelectron microscopic method using horseradish peroxidase-conjugated antibody to stain Fn. Large amounts of Fn were demonstrated in the interstitial collagenous tissue of invasive rheumatoid pannus. Fn was also observed on the surface of both fibroblast-like and macrophage-like pannus cells, suggesting that it was secreted by these cells and that the secreted Fn might facilitate the spreading of the pannus cells over the cartilage matrix. However, decreased amounts of Fn were observed in areas of contact between pannus cells and the cartilage matrix. The reduction in the amount of Fn demonstrable at the cartilage-pannus junction may be a consequence either of proteolysis of Fn by enzymes secreted at the junction, inhibition of Fn synthesis in cells in contact with cartilage matrix, or transfer of Fn from the pannus cell surface to collagen of the cartilage matrix.