Inhibition of complement-mediated solubilization of antigen-antibody complexes by sera from patients with rheumatoid arthritis.

Sera and synovial fluids from patients with seropositive rheumatoid arthritis inhibit the ability of normal serum to prevent immune precipitation. Sera from patients with seronegative forms of arthritis contain little inhibitory activity. Studies of the mechanism of action of the inhibitor show that it reduces C4 consumption by antigen-antibody complexes. These findings suggest that the binding of C1 to complexes or activation of C1 is impaired. The possibility that the inhibitory activity may be mediated by rheumatoid factor is discussed. As inhibitory activity is more prevalent and of a higher level in patients with extra-articular features, it is possible that it may play a pathogenetic role.