IRF8-deficient MPs accumulate due to decreased apoptosis, continued proliferation, and a block in monocyte differentiation. (A-C) One thousand MPs (GMP-Ly6C⁺CD115^hi) isolated from the bone marrow of wild-type or IRF8^−/− mice were cultured in liquid media containing 50 ng/mL M-CSF for the indicated time points. Monocyte and macrophage differentiation was monitored by flow cytometry (A; see supplemental Figure 2 for gating strategy), MP apoptosis was assessed by Annexin V staining (B), and incompletely differentiated monocyte (CD11b⁺CD115⁺Ly6C^int) production was assessed by flow cytometry (C; see supplemental Figure 11A for full gating strategy). Dotted line indicates starting MP number. All data are presented as mean plus standard deviation of independent cultures of MPs from at least 5 wild-type and 5 IRF8^−/− mice (A, C: 5 mice per group; B, 11 mice per group). Statistical significance was assessed by Student t test (*P < .05, **P < .01, ***P < .001).