Activated tonsillar B cells have negative regulatory effects on mDC functions. Monocytes were purified from the peripheral blood of healthy donors and stimulated with GM-CSF (1000 U/mL) and IL-4 (500 U/mL) for 6 days. The differentiated immature DCs were stimulated with LPS (100 ng/mL) and IFN-γ (1000 U/mL) for 24 hours to generate mDCs. Tonsillar B cells were purified and stimulated for 5 days on CD40L-transfected NIH3T3 murine fibroblasts in the presence of 0.25μM CpG-ODN 2006. (A) mDCs were cultured alone (1:0) or in the presence of activated B cells with their supernatant (ratio 1 DC/4 B cells) for a further 48 hours. Expression of HLA-DR, CD80, CD86, and IL-12 was evaluated by flow cytometry on CD19-negative cells. A representative experiment is shown with dotted lines corresponding to isotype controls, open histograms to mDCs cultured alone, and gray histograms to mDCs cocultured with B cells. (B) MFI of HLA-DR (9 samples), and percentages of CD80^low (13 samples), CD86^low (11 samples), and IL-12^low (11 samples) mDCs are shown. **P < .01; ***P < .001. (C) Representative example of the expression of CD40, CD45RO, CD54, CD58, CD11b, CD11c, CD25, and CD83 evaluated by flow cytometry. (D) CFSE-labeled T cells were cultured alone or in the presence of mDCs for 5 days. The T-cell proliferative response was evaluated by the dilution of CFSE expression by flow cytometry. Activated B cells were also added at the onset of the coculture. A representative experiment is shown.