IRF8 is downregulated by PML/RARA in preleukemic promyelocytes and acts as a tumor suppressor in PML/RARA-driven APL. (A) Details of the experimental strategy used to investigate IRF8 protein levels in highly purified populations of sorted promyelocytes, in the presence or absence of PML/RARA. (B) Western blot analysis of IRF8 in sorted early and late promyelocytes of PML/RARA mice. Total spleen from WT animals was loaded as a positive control. IRF8 is detected at a size band of 48 kDa. Actin loading control is detected at a size band of 42 kDa. Images were obtained on LI-COR scanner. (C) Enumeration of c-Kit⁺ progenitor populations in the BM of PML/RARA, Irf8^−/−, and PML/RARA Irf8^−/− mice (n = 3 for each group). Error bar represents mean ± standard error of the mean. (D) Mating strategy used to investigate kinetics of APL initiation in PML/RARA, Irf8^−/−, and PML/RARA Irf8^−/− backgrounds. (E-F) Overall (E) and acute (F) leukemia-free survival of lethally irradiated recipients of BM from young PML/RARA, Irf8^−/−, and PML/RARA Irf8^−/− donor mice. Leukemia-free survival in the PML/RARA Irf8^−/− cohort was shorter than that of the PML/RARA cohort (P = .03, Gehan-Breslow-Wilcoxon test). APC, allophycocyanin; CMP, common myeloid progenitor; GMP, granulocyte/macrophage progenitor; FITC, fluorescein isothiocyanate; MEP, megakaryocyte-erythrocyte progenitor; PE, phycoerythrin; PR, PML/RARA.