Levels of SC5b--9 complement complex in plasma and synovial fluid of patients with rheumatic disease.

Activation of the terminal complement pathway leads to formation of the C5b--9 complex. The main effects of C5b--9 generation are tissue injury by cell lysis or by stimulation of proinflammatory mediators. In a study carried out in 42 patients, using polyclonal antibodies against C5b--9 neoantigens and C9 in an ELISA assay, we found significantly higher levels of SC5b--9 complex in plasma from the 18 patients with active systemic lupus erythematosus than those found in 10 healthy controls (p less than 0.005). In the 18 patients presenting rheumatoid arthritis and the 6 with progressive systemic sclerosis the plasma levels of SC5b--9 complex did not differ significantly from those in controls. The SC5b--9 levels found in the synovial fluid samples from the 16 rheumatoid arthritis patients were higher than the corresponding plasma ones. The ratio between synovial fluid and plasma levels was 1.2. Immunoperoxidase staining for C5b--9 was intense in three rheumatoid synovial membranes and absent in two normal synovial membranes obtained during meniscectomy. Increased levels of plasma and synovial fluid SC5b--9 reflect pathologic systemic or local activation of the complement carcase in systemic lupus erythematosus and respectively rheumatoid arthritis. Synovial membrane deposits of C5b--9 are indicative for the lytic and proinflammatory effects of complement activation.