MDSCs are expanded in patients with AML and can be cytogenetically related to the malignant clone. PBMCs were isolated by Ficoll density-gradient centrifugation and stained with antibodies for CD11b, HLA-DR, CD14, CD15, and CD33 expression. The cells were then analyzed with flow cytometry. (A) Representative example of patient 2 is shown. CD11b⁺ HLA-DR⁺ CD14⁻ blasts are shown in gate I (light blue). Monocytic MDSCs (CD11b⁺ HLA-DR⁻ CD14^−/+ CD33⁺ CD15⁻) are shown in gate E (purple), granulocytic MDSCs (CD11b⁺ HLA-DR⁻ CD14⁻CD33⁺CD15⁺) are shown in gate F (orange). PBMCs from AML patients and healthy controls were isolated by Ficoll density-gradient centrifugation and stained with antibodies for CD11b, HLA-DR, CD14, CD15, and CD33 expression. The cells were then analyzed with flow cytometry. If present, tumor cells were gated out on the basis of forward scatter/side scatter and known blast phenotype, and total MDSCs (CD33⁺CD15⁻ or CD33⁺CD15⁺) were quantified as a percentage of total cells (n = 8; P < .05) (B) and as a percentage of gated immature CD11b⁺/HLA-DR⁻ myeloid cells (n = 7; P < .05) (C). MDSCs were further characterized as granulocytic, by the presence of CD15⁺, or monocytic, by CD15⁻ and side scatter. (D) Granulocytic and monocytic MDSCs in AML (n = 7) versus healthy donors (n = 9) are shown. MDSCs are shown as a percentage of gated immature CD11b⁺/HLA-DR⁻ myeloid cells. *P < .05 for both monocytic and granulocytic MDSCs in AML versus healthy donors. SSC, side scatter.