Synovial fluid and serum levels of soluble interleukin-6 receptor in patients with rheumatoid arthritis

Our objective was to measure both synovial fluid (SF) and serum levels of soluble interleukin-6 receptor (sIL-6R) in patients with rheumatoid arthritis (RA) and patients with osteoarthritis (OA), and to investigate the amounts of sIL-6R protein produced by cultured synovial cells, chondrocytes and mononuclear cells (MNCs). We measured levels of sIL-6R using a sensitive and specific enzyme-linked immunosorbent assay. Synovial cells, chondrocytes and MNCs were cultured, and the supernatants were also measured for sIL-6R. SF levels of sIL-6R in RA were significantly higher than those in OA. SF levels of sIL-6R significantly correlated with SF levels of IL-6 in RA. The serum level of sIL-6R was approximately 3-fold higher than the SF level of sIL-6R. sIL-6R protein was not detected in the supernatants of synovial cells and chondrocytes. As compared to the SF levels of sIL-6R, a small amount of sIL-6R protein was produced by SF MNCs. The above findings suggest that increased amounts of sIL-6R form IL-6-sIL-6R complexes which mediate IL-6 function in RA joints and that SF sIL-6R protein might be not only produced in affected joints, but also supplied from the serum.