Acute myeloid leukaemia niche regulates response to L‐asparaginase

Eradicating the malignant stem cell is the ultimate challenge in the treatment of leukaemia. 35 Leukaemic stem cells (LSC) hijack the normal haemopoietic niche in which they are largely 36 protected from cytotoxic drugs. The anti-leukaemic effect of L-asparaginase (ASNase) has been 37 extensively investigated in acute lymphoblastic leukaemia, but only partially in acute myeloid 38 leukaemia (AML). We explored the susceptibility of AML-LSC to ASNase as well as the role of 39 the two major cell types that constitute the bone marrow (BM) microenvironment, i.e., 40 mesenchymal stromal cells (MSC) and monocytes/macrophages. Whilst ASNase was effective on 41 both CD34 + CD38 + and CD34 + CD38 - LSC fractions, MSC and monocytes/macrophages partially 42 counteracted the effect of the drug. Indeed, the production of cathepsin B, a lysosomal cysteine 43 protease, by BM monocytic cells and by AML cells of the FAB M5 subtype is related to the 44 inactivation of ASNase. Our work demonstrates that, while MSC and monocytes/macrophages may 45 provide a protective niche for AML cells, ASNase has a cytotoxic effect on AML blasts and, 46 importantly, LSC subpopulations. Thus, these features should be considered in the design of future 47 clinical studies aimed at testing ASNase efficacy in AML patients. 48 49 50 and allophycocyanin-Alexa 750-anti to perform the analysis gating on the non-blast cells within the sample, the bulk blast population and the leukaemic CD34 + CD38 - and CD34 + CD38 + subpopulations. Experiments were performed on a FACSCanto TM II (BD Biosciences) and analysed with FACSDiva TM software v.6.1.3 (BD Biosciences).