Interaction between BMSCs and MCs results in an increased mature B-cell proliferation. (A-B) CD19⁺CD21⁺IgM⁺ circulating B cells obtained from healthy donors increase their proliferative rate when cocultured with BMSC-MC in the absence of additional stimuli. (C-D) The number of Ki-67⁺ cells in the CD40 low and CD40 high groups indicates a higher proliferation rate in cases with high CD40 expression in the BM stroma. Immunohistochemistry, STREPTavidin–biotin–peroxidase complex method, DAB chromogen; original magnification ×400. Microphotographs are relative to 2 representative cases with high (score 3) and low (score 1) CD40 stromal expression. **P < .01; ***P < .001.