Objective: To understand the regulation of rheumatoid factor (RF) production in rheumatoid arthritis (RA), we studied IgM-RF production by B cells isolated from the synovial fluid (SF).

Methods: Highly purified SF and peripheral blood (PB) B cells were isolated by negative selection in a fluorescence-activated cell sorter (FACS) and then cultured with either L cells, CD40 ligand (CD40L)-transfected L cells, or type B synoviocytes in the presence or absence of interleukin-2 (IL-2), IL-4, or IL-10. Total IgM and IgM-RF were detected after 14 days by enzyme-linked immunosorbent assay. Enzyme-linked immunospot assays were performed to detect cells that spontaneously produced immunoglobulin. SF B cells were also phenotypically characterized by FACS analysis.

Results: Terminally differentiated CD20-,CD38+ synovial plasma cells (PC) present in the SF of RA patients secreted IgM-RF in the absence of a stimulus. IgM-RF production markedly increased when SF B cells were cultured in the presence of type B RA synoviocytes together with IL-10, but independently of CD40-CD40L interaction. Although CD20-,CD38+ PC could also be demonstrated in SF B cells from patients with other forms of arthritis, IgM-RF production was restricted to the SF B cell cultures of patients with seropositive RA. The frequency of IgM-RF-producing cells among IgM-producing PC in patients with seropositive RA was estimated to be as much as 50%.

Conclusion: These data demonstrate that terminally differentiated CD20-,CD38+ IgM-RF-producing B cells are specifically present in the inflamed joints of patients with seropositive RA. There is evidence that the local environment in the rheumatoid joint favors RF production. The relatively high frequency of IgM-RF PC in the SF B cell population provides evidence of a dominant RA-specific antigen-driven response in the development of the synovial PC repertoire.