LOW-MOLECULAR-WEIGHT IGM AND CD5 B-LYMPHOCYTES IN RHEUMATOID-ARTHRITIS

Objectives-To evaluate the role of low molecular weight (LMW) IgM and CD5 B cells in rheumatoid arthritis (RA) and to explore the possibilit y that LMW IgM I is derived selectively from this subset of B cells. M ethods-LMW IgM in sera and culture supernatants was detected by a sens itive immunoblot technique with an enhanced chemiluminescence detectio n system. CD5 B cells were determined by FACScan cytometry. In vitro s tudies were established in culture plates containing pokeweed mitogen with or without 2-mercapatoethanol (2-ME). Supernatants were obtained from CD5 positive hybridomas and CDS negative hybridomas. Other immuno logical indices were measured by laser nephelometry. Results-Circulati ng LMW IgM was detected in all rheumatoid patients with significantly higher levels being observed in sere-positive patients. LMW IgM correl ated significantly with total IgM and RF. Peripheral blood mononuclear cells (PBMC) from the majority of the patients with RA secreted LMW I gM in vitro as did mononuclear cells from a synovial fluid sample. The addition of low concentrations of 2-ME to the culture medium enhanced the proportions of secreted monomeric IgM. In contrast, PBMC from hea lthy subjects secreted only trace quantities of LMW IgM. In RA no sign ificant correlations were observed between CD5 B cells and LMW IgM and RF. LMW IgM could be detected in the supernatants from both CD5+ and CD5-B cell lines. Finally, CDS B cells were not significantly elevated in RA and levels remained constant over time. Conclusion-LMW IgM exis ts in high concentrations in RA sera and synovial fluid. Serum level c orrelates with RF and IgM. In vitro studies have suggested that the oc currence of LMW IgM may be due to an intrinsic defect(s) in the assemb ly of the IgM pentameric molecule. LMW IgM is unlikely to be derived s olely from CD5 B cells.