Fig 1: Histamine dihydrochloride (HDC) facilitates the differentiation of monocytic primary leukemic cells and may be preferentially efficacious in monocytic forms of leukemia. FACS-plots showing live peripheral blood mononuclear cells from representative newly diagnosed patients with (A) FAB-M0 acute myeloid leukemia (AML) with a dominant immature leukemic population (CD34+CD33−CD14−) and (B) FAB-M4 AML with two distinct populations: an immature blast population (CD34+CD33−CD14−) and a mature monocytic population (CD34−CD33+CD14+). The expression of (C) H2R, (D) NOX2, (E) FPR1, and (F) FPR2 on primary AML cells [gated as indicated in (A,B)] and monocytes from healthy donors was determined by flow cytometry. The M5 leukemia is represented by an open circle. One-way ANOVA. (G–I) Median fluorescence intensity as determined by flow cytometry of (G) HLA-DR, (H) FPR1, and (I) FPR2 on live primary monocytic AML cells (FAB: M4/M5) or non-monocytic AML cells (FAB: M0–M2) cultured for 5 days with GM-CSF/IL-4 in the presence or absence of HDC. Wilcoxon matched pair’s test. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001.