Only CD14-derived DCs can induce CD40-activated naive B cells to differentiate into IgM secreting cells. The CD1a- and CD14-derived DC subsets were obtained as described in Materials and Methods and in the legend to Fig 1. A total of 10⁴ highly purified IgD⁺ B cells were cultured over 2,500 irradiated CD40-L–transfected L cells in the presence or absence of DC subsets. Proliferation was measured by ³H-TdR incorporation at day 6. Supernatants were harvested after 15 days of culture and assayed for the presence of IgM. (A) DC subsets were recovered at day 12 and used as stimulator cells (3,000 cells/well) for IgD⁺ B-cell proliferation in the absence or presence of 20 U/mL IL-2. (B) DC subsets were recovered at day 12 and used as stimulator cells (3,000 cells per well) for IgD⁺ B cell differentiation in the absence or presence of 20 U/mL IL-2. (C) DC subsets were recovered at day 12, and 300, 1,000, and 3,000 cells/well were used as stimulator cells for IgD⁺ B-cell differentiation in the presence of 20 U/mL IL-2. (D) DC subsets were recovered at the time point indicated and used as stimulator cells (3,000 cells/well) for IgD⁺ B-cell differentiation in the presence of 20 U/mL IL-2. IgM levels are expressed as mean ± SD of triplicate cultures. Results of each panel are representative of three experiments or more.