IL-9 promotes MK lineage cell expansion via activation of IL-9R/Stat3 signaling. (A) Quantification of FACS analysis of IL-9R expression in CD41⁺ MK, CD42d⁺ MK, CD11b⁺ granulocytic, and Ter119⁺ erythroid lineages in BM of 3-week-old mice (n = 6, Student unpaired t test). (B) Femur sections of OSX-Raptor, OSX-TSC1, or control mice at 3 weeks probed for expression of CD41 (red) and p-Stat3 (green). Blue, nuclear DAPI staining. Scale bar = 30 µm. (See supplemental Figure 10.) (C) Femur section of mice bilaterally injected IP with IL-9 (1 µg/kg per day) for 14 days and probed for expression of CD41 (red) and p-Stat3 (green). Blue, nuclear DAPI staining. Scale bar = 30 µm. (See supplemental Figure 11.) (D) OSX-Raptor and control mice at 3 weeks bilaterally intratibially injected into the marrow cavity with IL-9R or control siRNA for 10 days. The effect of specific siRNAs for IL-9R in BM cells was detected by western blot. (E) Femur section of treated mice immunolabeled for CD41 (red) and p-Stat3 (green). Scale bar = 20 µm. (See supplemental Figure 12.) (F) Platelet and MK counts (± standard error of the mean) of IL-9R siRNA-treat mice (n = 8, Student unpaired t test). (G) Quantitative analysis of proliferation rate in CD41⁺Sca-1⁺ cells (CD41⁺Sca-1⁺Edu⁺) in BM of IL-9 siRNA-treated mice (n = 6, Student unpaired t test). Data are representative of 3 independent experiments and are represented as mean ± SD. **P < .01; ***P < .001. Con, control; NC, no significance.