The objective was to study immune regulation in a mouse model of rheumatoid arthritis that exhibits considerable heterogeneity of disease activity.

T-cell receptor transgenic mice, in which nearly all CD4⁺ T cells recognize a single peptide of type II collagen, were immunized with collagen and observed for development of arthritis for 4 weeks. At 28 days post-immunization, splenocytes were analyzed by flow cytometry and in vitro assays for markers of immune activation and regulation.

Disease severities ranging from 0 to 12 (on a 12-point scale) were observed. Among splenic lymphocyte populations, only the CD5⁺ B-cell subset displayed a decrease in relative numbers as arthritis severity increased. Splenic CD5⁺ B cells expressed higher levels of Fas ligand (FasL) than did CD4⁺ T cells or CD5^- B cells in all mice, and antigen-dependent T-cell death correlated with higher levels of CD5⁺ B cells in cocultures. Ratios of interleukin (IL)-17 to interferon-gamma production were higher in antigen-driven cultures of splenocytes from severely arthritic mice compared to mildly or nonarthritic mice. A correlation was established between the reduced production of IL-17 in antigen-driven T-cell/B-cell cocultures and FasL, but not IL-10. Confirmation of the direct killing effect of B cells on T cells was demonstrated using an antigen-specific T hybridoma cell line.

Reduced numbers of splenic FasL⁺ CD5⁺ B cells correlated with increasing arthritis severity and decreased T-cell death in a T-cell receptor transgenic mouse model of collagen-induced arthritis. These 'killer' B cells may provide a novel mechanism for inducing T-cell death as a treatment for arthritis.