Blocking Axl activation leads to inhibition of cell growth in AML cells. (A) Primary blasts from 3 FLT3-ITD⁺ AML patients (top, P1 to P3) and those from 2 FLT3-WT AML patients (bottom, P8 and P9) were treated with Ctrl-Fc or Axl-Fc (triplicates for each patient) for the indicated times. Cell numbers were counted using trypan blue. *P < .05, **P < .01. (B) Primary FLT3-ITD⁺ AML patient blasts were treated with Ctrl-Fc or Axl-Fc for 2 days and the cell cycle was analyzed by flow cytometry after PI staining of DNA of the cells treated as above. Numbers indicate the percentage of each stage (G1, S, and G2/M) in the cell cycle. This is the representative of 3 separate experiments. (C) Blasts from a FLT3-ITD⁺ AML patient were treated with Ctrl-Fc or Axl-Fc for 3 days, stained with annexin V-FITC and PI, and analyzed by flow cytometry. This is the representative of 3 separate experiments. (D-E) Primary FLT3-ITD⁺ (panel D top) and FLT3-WT (panel E top) AML patient blasts were treated with vehicle (dimethylsulfoxide) or different concentrations of the pharmacologic Axl inhibitor XL-880 (0.1, 1, or 10 μM) for 3 days (triplicates) and the cell numbers were counted. The cell numbers at the beginning of treatment was set arbitrarily as 100%. Primary blasts from FLT3-ITD⁺ (panel D bottom) or FLT3-WT (panel E bottom) AML patients were treated with vehicle or XL-880 (10 μM) for 3 days, stained with annexin V-FITC and PI, and analyzed by flow cytometry.