Synovial fluids from patients with rheumatoid arthritis possessed B cell differentiation activity contributing to synthesis of rheumatoid factor in vitro.

B cell differentiation activity assayed on B cells preactivated by Staphylococcus aureus Cowan I was found in synovial fluids (SF) of patients with rheumatoid arthritis (RA) (n = 14), while little, if any, activity was found in SF of patients without RA (n = 13) including patients with osteoarthritis and traumatic arthritis. There was a positive correlation between the RA hemagglutination assay titer and B cell differentiation activity (r = 0.7438). This molecule with molecular weight ranges 15-20 kDa as well as supernatant from phytohemaglutinin stimulated normal T cells could promote not only polyvalent immunoglobulin synthesis but also rheumatoid factor synthesis by lymphocytes from patients with RA in vitro, and was considered to be distinct from interleukin-1, interleukin-2, interferon r, or proteolytic lysosomal enzymes which have been shown to modulate B cell activation.