Membrane attack complex of complement in rheumatoid synovial tissue demonstrated by immunofluorescent microscopy

Rheumatoid and osteoarthritic synovial membranes were examined for evidence of terminal complement pathway activation. Using antiserum highly specific for C9 neoantigen, immunofluorescent microscopy was employed to study synovial membrane specimens from 4 patients with rheumatoid arthritis, one patient with psoriatic arthritis, and 5 patients with osteoarthritis. C9 neoantigen was detected in 3 of the 4 rheumatoid membranes, the one psoriatic membrane, and one of the 5 osteoarthritic membranes. Activation of the terminal pathway of complement through C9 should be added to the spectrum of immunologic events known to occur in some rheumatoid synovial membranes.