Irf8^−/− mice lack Ly6C⁺ monocytes and have fewer Ly6C⁻ monocytes than WT mice. (A-C) Monocytes (Mo) in the peripheral blood (A), spleen (B), and bone marrow (C) from WT or Irf8^−/− mice. Monocytes were detected as CD11b⁺ CD115⁺ cells and then further analyzed for Ly6C expression. Splenic monocytes were first gated as side scatter (SSC)^low cells. Bone marrow CD117⁺ cells were gated out to exclude progenitor cells. Absolute numbers (per 1 mL blood, per spleen, or per tibias plus femurs from a mouse) of Ly6C⁺ and Ly6C⁻ monocytes are shown in the bar graphs. The ratios of Ly6C⁺ monocytes to Ly6C⁻ monocytes are also shown. (D) Peripheral blood monocytes in mice that received bone marrow transplants. Irradiated WT recipients reconstituted using WT (WT→WT) or Irf8^−/− (Irf8^−/−→WT) bone marrow cells (1 × 10⁷) or irradiated Irf8^−/− recipients reconstituted using WT (WT→Irf8^−/−) bone marrow cells (1 × 10⁷) for 4 weeks were analyzed as in (A-C). All values are mean ± standard deviation from four mice of each genotype. (*) P < .05, (**) P < .01, and (***) P < .001 (Student t test).