Properties of fibrinogen-antigenic material on the rheumatoid synovial membrane and in the rheumatoid synovial fluid.

Fibrin deposits on rheumatoid synovial membranes and fibrinogen-antigenic material in rheumatoid synovial fluid were found to be identical by crossed immunoelectrophoresis into immunoglobulin against fibrinogen, by SDS-polyacrylamide gel electrophoresis, and by gel filtration on Sepharose CL 6B. The material was found to be neither fibrinogen nor fibrin, but degradation products. One of the fragments was purified by preparative agarose electrophoresis, and the physicochemical properties of this fragment were found to be different from those obtained by plasmin digestion of fibrinogen or fibrin. This indicates that other proteases than plasmin are responsible for the degradation products. The material was easily degraded by plasmin to D- and E-antigenic end products, identical to those obtained by plasmin digest of fibrinogen. The solubility of the material was poor in synovial fluid compared to serum and buffer. On the basis of these results, it is suggested that the fibrinlike material on the synovial membrane represents fibrinogen degradation products from the inflamed tissue. These products are likely released into the synovial fluid, and when their concentration here exceeds their solubility, they precipitate on the synovial membrane.