CCL5 mediates target-kinase independent resistance to FLT3 inhibitors in FLT3-ITD-positive AML.

FLT3-ITD tyrosine kinase inhibitors (TKI) show limited clinical activity in acute myeloid leukemia (AML) due to emerging resistance. TKI resistance is mediated by secondary FLT3-ITD mutations only in a minority of cases. We hypothesize that the cytokine CCL5 protects AML cells from TKI-mediated cell death and contributes to treatment resistance. We generated PKC412- and sorafenib-resistant MOLM-13 cell lines as an in vitro model to study TKI resistance in AML. Increased CCL5 levels were detected in supernatants from PKC412-resistant cell lines compared to TKI-sensitive cells. Moreover, CCL5 treatment of TKI-sensitive cells induced resistance to PKC412. In resistant cell lines with high CCL5 release, we observed a significant downregulation of the CCL5-receptor CCR5 and CXCR4. In these cell lines, TKI resistance could be partly overcome by addition of the CXCR4-receptor antagonist plerixafor. Microarray and intracellular flow cytometry analyses revealed increased p-Akt or p-Stat5 levels in PKC412-resistant cell lines releasing high amounts of CCL5. Treatment with the CXCR4 antagonist plerixafor, αCCL5, or CCR5-targeting siRNA led to a decrease of p-Akt-positive cells. Transient transfection of sensitive MOLM-13 cells with a CCL5-encoding vector mediated resistance against PKC412 and led to an increase in p-Akt-positive and p-Stat5-positive cells. Isolated AML blasts from patients treated with PKC412 revealed that CCL5 transcript levels increase significantly at relapse. Taken together, our findings indicate that CCL5 mediates resistance to FLT3-TKIs in FLT3-ITD-mutated AML and could possibly serve as a biomarker to predict drug resistance.