Immune interferon in serum and synovial fluid in rheumatoid arthritis and related disorders.

Moderate titres of antiviral activity were demonstrated in 48-58% of sera obtained from patients suffering from seropositive and seronegative rheumatoid arthritis (RA), psoriatic arthritis, Reiter's syndrome, ankylosing spondylitis, and juvenile rheumatoid arthritis. Sera from blood donors and from patients with various noninflammatory diseases were positive in 16% of cases. The activity was species-specific, mediated by the homologous cells, and destroyed by treatment with trypsin and exposure to pH 2. Antibodies against human IFN-alpha did not neutralise the activity. These characteristics are compatible with those of IFN-gamma or immune interferon. Neither the presence nor the titre of IFN was correlated with disease activity defined by concentration of C-reactive protein, C3 concentration, and erythrocyte sedimentation rate. IFN-gamma was present in 4 of 10 synovial fluids from patients with RA. The titre in one of these was higher than in the corresponding serum, indicating local production in the rheumatoid joint.