Expression of surface and cytoplasmic Ig heavy chain isotypes by B cells and PCs generated in the 3-step culture system. MBCs were cultured as described in Figure 1. Starting MBCs, D4 actBCs, D7 PBs, and D10 PCs were labeled with fluorochrome-conjugated anti-CD20, CD38, and CD138 mAbs and with fluorochrome-conjugated anti–human IgM, IgA, IgG mAbs, or isotype-controlled mAbs before or after cell permeabilization. The bold histograms represent labeling with anti-IgM, IgA, or IgG mAb and the light ones with the control mAb. Histograms are those of 1 experiment representative of 5. The numbers in the panels are the mean ± SD of the percentage of labeled cells (ie, ≥ MFI + SD of the control mAb). *The mean percentage of labeled cells is different from that in D0 MBCs. **The mean percentage of labeled cells is different from that in D4 actBCs. ***The mean percentage of labeled cells is different from that in D7 PBs.