IL-6 in synovial fluids, plasma and supernatants from cultured cells of patients with rheumatoid arthritis and other inflammatory arthritides

The purpose of this investigation was to measure interleukin 6 (IL-6) levels in synovial fluid (SF) and plasma from patients with rheumatoid arthritis (RA) and other inflammatory arthritides (non-RA) and to examine the in vitro production of IL-6 by T cells and monocytes from SF and peripheral blood (PB). All patients had high levels of IL-6 in SF. Although the median level was higher in the group of non-RA arthritides, the difference was not statistically significant. Supernatants from both unstimulated and stimulated highly purified CD4+ and CD8+ cells from SF and PB did not contain IL-6, while high levels of IL-6 were detectable in supernatants of mononuclear cells (MNC) and plastic adherent cells without any specific in vitro stimulant. Since IL-6 is so readily produced by normal mononuclear cells in vitro without any specific stimulant, spontaneous in vitro production by SF MNC cannot be considered as evidence for in vivo production by the same cells.