Relationship between IgG2b‐inducing activity in rheumatoid arthritis synovial fluid and other well‐known cytokines and inflammatory mediators

In the present study, we sought to identify the T cell-replacing factor which selectively induces IgG2b antibody formation in lipopolysaccharide-activated mouse spleen cells in vitro and in vivo, and which is present in the synovial fluid (SF) of rheumatoid arthritis (RA) patients. The protein A plaque assay was used to measure IgM, IgG1, IgG2b, and IgG3 plaque-forming cells. An enzyme-linked immunosorbent assay was used to measure interleukin-6 (IL-6) levels in RA SF. We found that IgG2b induction by RA SF is not caused by IL-6, IL-1, or any other inflammatory cytokines or mediators, such as transforming growth factor β, platelet-derived growth factor, nerve growth factor, fibroblast growth factor, epidermal growth factor, elastase, colla-genase, and phospholipase A2. IgG2b-inducing factor in RA SF has unique biological properties compared with those of the interleukins and inflammatory mediators known to be present in RA SF.