Increased levels of soluble tumor necrosis factor receptors in the sera and synovial fluid of patients with rheumatic diseases

Objective. Recently, 2 classes of cytokine inhibitors have been defined at the molecular level. The largest group comprises the extracellular domains of cell surface cytokine receptors, and includes both tumor necrosis factor receptors (TNF‐R). The present study was conducted to investigate the role of TNF inhibitors in arthritis. Methods. We measured p55 and p75 soluble TNF‐R (sTNF‐R) in serum and synovial fluid (SF) samples from patients with rheumatic diseases and compared their levels with levels of soluble interleukin‐2 receptors (sIL‐2R). Sensitive enzyme‐linked immunosorbent assays (ELISA), specific for p55 and p75 sTNF‐R and for sIL‐2R, were used. Results. Serum levels of p75 sTNF‐R were 3–4‐fold higher than levels of p55 sTNF‐R, and both were significantly elevated in patients with osteoarthritis (OA) and rheumatoid arthritis (RA) compared with healthy controls. RA SF levels of sTNF‐R were 4–5‐fold higher than levels in serum, suggesting local production in the joint, and were significantly higher than levels in the SF of patients with seronegative arthropathy or OA. Furthermore, levels of p55 and p75 sTNF‐R, but not sIL‐2R or TNFα measured by ELISA, were increased in the SF of patients with clinically active RA. The soluble TNF‐R in RA and OA SF were functional since they inhibited TNF activity in a cytotoxicity assay in proportion to the levels of inhibitor present. Evaluation of serially obtained serum samples suggested that sTNF‐R may be a useful parameter for monitoring RA disease activity. Conclusion. Biologically active soluble TNF‐R are up‐regulated in patients with rheumatic disease and are produced locally in the joints. Measurement of serum levels of TNF‐R may be useful for monitoring of disease, and determination of SF levels could be of diagnostic value.