Axl represents a prognostic factor in Cn AML patients and is upregulated by AML cells, whereas its ligand Gas6 is produced by BM stroma cells. (A) Cn AML patients expressing Axl mRNA above the median survived significantly shorter than patients expressing Axl below the median (Kaplan-Meier survival analysis; n = 64; P < .05). (B-C) Quantification of Axl protein expression by flow cytometry of AML and healthy BM MNCs (n = 19/6) and CD34⁺ CD38⁻ cells (n = 9/6) indicated a higher percentage of Axl⁺ cells in both populations in AML compared with healthy cells, respectively (*P < .05). (D-F) Representative immunohistochemical Gas6 stainings showing higher percentage of Gas6⁺ stromal cells in AML compared with control BM (n = 7/5). Bar represents 50 µm. (G) Gas6 ELISA indicating upregulation of mGas6 by OP9 stromal cells induced by direct (DIR) and indirect (IND) coculture with hGas6⁻ HL60 leukemia cells as well as by HL60-conditioned medium (COND) (n = 3; *P < .05; #P < .05). (H) Abrogation of HL60-induced Gas6 upregulation in S17 stroma cells by hIL-10 and hM-CSF neutralizing antibodies (n = 3; *P < .05; #P < .05). (I) Neutralization of Gas6 by sAxl decreased stroma cell–induced resistance of cocultured HL60 cells to cytarabine (AraC) (data normalized to HL60 single culture without treatment; n = 3; *P < .05).