Identification and characterization of opsonic fibronectin in synovial fluids of patients with active rheumatoid arthritis.

A cofactor that selectively opsonizes particulate activators of the human alternative complement pathway and enhances their phagocytosis by human monocytes was identified in synovial fluids of patients with rheumatoid arthritis. The active material was present in fluids treated with protease inhibitors, was heat stable, and was unaffected by incubation with hyaluronidase. Chromatographic isolation of synovial fluid fibronectin by gelatin affinity and by immunoaffinity on antifibronectin monoclonal antibody BD4 yielded similar quantities of protein for each of 3 fluids. Synovial fluid proteins with the BD4 fibronectin epitope accounted for essentially all of the phagocytosis-enhancing activity and expressed this activity by opsonizing target activators. Additional chromatographic analyses of synovial fluid fibronectin with the BD4 epitope were carried out using Sepharose-bearing gelatin and 4 additional antifibronectin monoclonal antibodies. The opsonic materials were characterized as having 2 distinct fibronectin epitopes, which always mapped from the cell adhesive domain to the carboxyl-terminus of plasma fibronectin, but only rarely contained the gelatin binding domain.