Use of Axl, a therapeutic target in AML, to mediate stroma-induced chemoresistance.

7027 Background: Axl, the receptor for Growth Arrest-specific protein 6 (Gas6) plays a role in AML pathobiology (Blood Suppl. Nov 2011; 118: 940). Here, we investigated whether Axl represents a therapeutic target in AML. Methods: Gas6 levels were measured by ELISA and immunohistochemistry. Axl expression was detected by flow cytometry. Co-cultures of (murine) BM stroma cells (primary, OP9, S17) with Mv4-11 and OCI-AML5 cell lines were performed. Results: We found (i) higher expression of Axl in AML BM compared to healthy BM donors (66.20 ± 10.87 vs. 0.65 ± 0.10 %; n=8/6; p<0.05); (ii) Axl expression by 68 ± 31% of AML blasts and (iv) higher expression of Axl by CD34 + CD38 - AML stem cells compared to healthy CD34 + CD38 - BM stem cells (58.43 ± 4.63 % vs. 6.00 ± 2.01 %; n=7/6; p<0.05). The Axl inhibitor BGB324 dose-dependently inhibited proliferation of primary AML cells with a mean IC50 of 1.8 µM. Sensitivity to BGB324 (i.e. a lower IC50) correlated with Axl expression on leukemia cells (Pearson’s r = -0.9656, p<0.05). Combination therapy with BGB324 and cytarabine exerted an additive therapeutic effect and BGB324 could chemosensitize cytarabine-resistant AML cells. Analyses of BM sections revealed that Gas6 expression was low in AML cells, similar to healthy hematopoietic cells while it was abundantly expressed in AML BM stromal cells with fibroblastic/mesenchymal morphology (BMDSCs). Gas6 expression was considerably lower in control BMDSCs (86 ± 14 % vs. 20 ± 20 %; n=5/7; p<0.05) thus suggesting a possible paracrine interaction between AML cells and BMDSCs leading to Gas6 upregulation in the stroma compartment. Co-culture experiments indicated specific upregulation of murine (m)Gas6 in BMDSCs via leukemia-cell derived IL-10 and M-CSF. This stroma-derived Gas6 could mediate chemoresistance of AML cells in co-culture, which was abrogated by sAxl or by BGB324. Thus, interaction between stroma-derived Gas6 and Axl + leukemia cells forms a chemoprotective niche for leukemia cells. In line with these findings Axl blockade chemosensitizes Mv4-11 cells for treatment with doxorubicine in vivo. Conclusions: Axl represents a therapeutic target in AML and Axl inhibition by BGB324 holds potential to treat chemosensitive and chemoresistant AML.