PDCs trigger plasma cell differentiation of naive and memory B cells. Sorted naive or memory B cells (1 × 10⁵ B cells/200 μL/well) were incubated alone or in the presence of PDCs (1 × 10⁴ PDCs/200 μL/well) and were stimulated with anti-Ig (10 μg/mL) with or without CpG-C (3 μg/mL). (A) After 8 days the frequency of plasma cells (CD20^-CD38+) and the MHC II expression was determined by flow cytometry. The frequency of plasma cells (gate R3) is indicated in the upper left quadrant (left panel, naive B cells; right panel, memory B cells; upper panel, B cells alone; lower panel, B cells plus PDCs). The expression of MHC II on plasma cells and nonplasma cells is shown by histograms. (B) The mean plasma cell frequency is depicted (naive B cells, mean ± SEM of 2 independent experiments; memory B cells, one experiment; □, B cells alone; ▪, B cells plus PDCs). (C) After 13 days IgM and IgG were measured in the supernatants by ELISA. Data are shown as means ± SEM of 3 independent experiments.