Phenotype of CD4⁺CD56⁺ leukemic cells from patient no. 1 at diagnosis and after purification. (A) Circulating leukemic cells from patient no. 1 were stained using specific mAbs and analyzed by flow cytometry. Leukemic cells were identified on their low expression of CD45 (gate R in upper left panel). Dot plots show the expression of the pDC markers BDCA-2, BDCA-4, and CD123 on these CD45^low leukemic cells. Dot plot corresponding to CD4 versus CD56 expression after gating on R1 is shown as positive control. Histograms show the expression of CD33, but not other myeloid markers CD117 and CD13 on CD45^low leukemic cells. Histogram corresponding to BDCA-4 expression was shown as positive control (row 5 in lower panel). (B) Peroxidase expression was determined using standard benzidine staining. No peroxidase staining was observed in leukemic cells, whereas positive staining was observed in a myeloid cell (arrow). Original magnification × 1000. (C) Bone marrow–infiltrating leukemic cells from patient no. 1 were purified as described in “Patients, materials, and methods.” After purification, cells were stained with specific mAb or isotype control and analyzed by flow cytometry. Purified cells expressed high levels of CD123, CD4 (dot plot), CD56, BDCA-2, and BDCA-4 (histograms).