CD19⁺ and CD19⁻ BMPC subsets exhibit different ex vivo proliferation and in vivo persistence. (A) CD19⁺ and CD19⁻ BMPC and peripheral blood PB/PC were analyzed for the expression of Ki-67 after cell fixation with formaldehyde and permeabilization with saponin. Frequencies of Ki-67⁺ cells are shown and were compared between blood PB/PC (6 samples, steady state) and BMPC subsets (Mann-Whitney U test; ***P < .001) and among pairs of BM CD19⁺ and CD19⁻ PC (Wilcoxon test; *P < .05). Filled circles, Ki-67 staining (10 samples); open circles, control staining (available for 7 samples). Both BMPC subsets show frequencies of Ki-67⁺ cells significantly exceeding frequencies of isotype control stained cells (Mann-Whitney U test; both P < .01). Median values are indicated. (B) BM samples of 23 patients with rheumatoid arthritis treated with 2 × 1 g RTX (a chimeric anti-CD20 antibody) were analyzed for the presence of CD138⁺CD19⁺ and CD138⁺CD19⁻ PC twice, once before, and once again 1 to 3 months after treatment. PC were detected by flow cytometry, and their numbers were extrapolated according to Trepel²⁰  and were analyzed using the Wilcoxon test; *P < .05. Lines between data points indicate the trend shown by individual patients. Mean numbers of 3.10 × 10⁸ (standard deviation [SD], 1.76 × 10⁸) CD19⁺ and 1.36 × 10⁸ (SD, 0.85 × 10⁸) CD19⁻ BMPC compare with 2.24 × 10⁸ (SD, 1.69 × 10⁸) CD19⁺ and 1.06 × 10⁸ (SD, 0.93 × 10⁸) CD19⁻ BMPC after therapy, respectively.