Anti-BR3 mAb decreases pathogenic B-cell subsets in the NZB/W F1 SLE mouse model. Therapeutic treatment of proteinuric NZB/W F1 mice (15/group) for 6 weeks with anti-BR3 mAb (600 μg/week) or BR3-Fc (300 μg × 3/week) reduced splenic anti-DNA-producing cells, kidney-infiltrating B cells (B220⁺), and T cells (CD3⁺) but not naive spleen T cells (A). Splenic germinal center B cells (B220⁺CD38^lo) were decreased only by anti-BR3 mAb treatment, whereas plasma cells (CD138⁺B220^lo) were decreased by both treatments (B). After 1 week, anti-BR3 mAb and BR3-Fc treatment showed significant reduction in both cycling and noncycling splenic B cells (B220⁺) and IgG1 plasma cells (cytoplasmic IgG1⁺CD138⁺), but only anti-BR3 mAb treatment resulted in significant reduction in IgM plasma cells (cytoplasmic IgM⁺CD138⁺) (C). Statistical significance compared with the control treated group is indicated with an asterisk. When 2 treatment groups are significantly different from each other, the P value is displayed.