The use of an excess of unlabeled CD33 mAb before labeling with PE-conjugated CD33 mAb confirms CD33 expression on purified leukemic pDCs from patient no. 2. (A) Purified leukemic cells from patient no. 2 were incubated with unlabeled CD33 mAb WM53 (α-CD33, 5 or 20 μg/mL) or irrelevant IgG1 (20 μg/mL) 30 minutes before labeling with either PE-conjugated CD33, PE-CD123 (gray curves), or PE-isotype control mAb (bold open curves) and flow cytometry analysis. On each histogram, rfi (corresponding to MFI R = MFI obtained with PE-CD33 or PE-CD123 mAb/MFI obtained with isotype control mAb) is indicated. CD33⁺ AML (AML 1 or AML 2) was used as positive control. Cells not incubated with unlabeled mAb but analyzed in the same experiments are also shown as controls (Ctrl). (B) Results with rfi obtained for each condition are summarized on bar graphs. Percent of signal inhibition, calculated as 100 - (α-CD33 rfi × 100/IgG1 rfi), is indicated on the top of bar. Similar signal inhibitions are obtained for CD33 fluorescence of leukemic pDCs (lower left panel) and AML cells, whereas no signal inhibition is observed for CD123 molecules on leukemic pDCs (lower right panel).