Decreased numbers of IL-7 responders in the BM of STAT3KO mice. (A) BM cells of control (left panels) or STAT3KO (right panels) mice were stained with anti-B220 and anti–IL-7Rα followed by FACS analysis. Light scatter patterns are shown in the top panels, and percentages of IL-7Rα⁺B220^intermediate and IL-7Rα^–B220^high are shown in the bottom panels. (B) Percentage (left panel) and cellularity (right panel) of B220⁺IL-7Rα⁺ cells in control (•) and STAT3KO (○) mice were measured. Cellularity was calculated by multiplying the percentage with the total number of BM cells. n = 3; *P < .05; Student t test. (C) Reduced CFU–pre-B progenitor cells in the BM of STAT3KO mice. Colony-formation assay was performed by seeding 2 × 10⁵ BM cells of control (▪) or STAT3KO (□) mice in the complete medium without or with 10 or 100 ng/mL IL-7 for 7 days, followed by counting of the number of the pre-B colonies under a light microscope. Colony-formation assay is presented as mean ± SE of replicate samples.