Studies on the first component of complement (C1) and the inhibitor of C1 esterase in rheumatoid synovial fluids

The specific activity of C1 (haemolytic units per unit of C1q or C1s protein) was two to three times lower in synovial fluid of patients with sero-positive rheumatoid arthritis (RA) than in non RA or sero-negative RA fluids. These fluids contain a high ratio of C1s inhibitor to C1s. Experiments with radiolabelled C1s established that the C1s inhibitor in the synovial fluids of these patients is functional as it binds radiolabelled C1s. At the high ratios observed, it is possible that a complex of C1s and C1s inhibitor forms in these synovial fluids and alters the activation of the complement system. The potential for excess C1s inhibitor to slow or diminish complement utilization was shown by adding purified C1s inhibitor to serum in order to simulate the conditions in joint fluid (i.e. a high ratio of C1s inhibitor to C1s). Subsequent whole complement, C1 and C4 activites after incubation with selected immune complexes were higher as compared to controls without added inhibitor.