Acute Myeloid Leukemia Driven IL-3 Dependent Upregulation of BCL-2 in Non-Malignant Hematopoietic Progenitor Cells Increases Venetoclax Induced Cytopenias

Abstract

The BH3 mimetic venetoclax, in combination with low dose cytarabine, decitabine or azacitidine has shown clinical efficacy in newly diagnosed acute myeloid leukemia (AML) (1, 2). This has been a significant advance, particularly in the treatment of older AML patients, and those who are ineligible for intensive chemotherapy, who historically have been difficult to treat. Venetoclax selectively inhibits the BCL-2 protein which is overexpressed in AML in order to activate intrinsic apoptosis. However, this treatment regime is associated with cytopenias including neutropenia, febrile neutropenia, and thrombocytopenia which leaves patients immunocompromised (3). The underlying cause of cytopenias in the context of venetoclax-treated AML remains unexplained. Here we describe a mechanism for venetoclax-induced cytopenias in AML. Two established AML models were used to assess BCL-2 expression profile in HPCs. Isolated LSK cells were co-cultured with mouse AML (HOXA9/Meis1 or MN1 overexpressing cells). RT-qPCR analysis identified BCL-2 upregulation in LSKs co-cultured with both AML cell types. To understand the significance of this in vivo, MN1 cells were engrafted into C57BL/6 mice and LSKs were FACS purified. Analysis confirmed increased BCL-2 gene and protein expression by RT-qPCR and flow cytometry when compared to control LSKs. Next, to recapitulate the clinical cytopenias, MN1 engrafted mice or control mice were treated with venetoclax for 7 days by oral gavage. Flow cytometry analysis of bloods and bone marrow in venetoclax-treated AML mice determined neutrophil depletion and reduced numbers of HPCs, namely LSK, MPP, HSC and GMP, compared to untreated AML mice . Todetermine the mechanism of BCL-2 upregulation in LSKs we explored various signalling pathways using targeted inhibitors on LSKs cultured with MN1 conditioned media. Using this method, we established that interleukin 3 (IL-3) activates BCL-2 transcription in LSKs leading to a dependency on pro-survival BCL-2 protein. Furthermore, treatment of LSKs, cultured in MN1 conditioned media with IL-3 neutralising antibody MP2-8F8 reduced BCL-2 gene and protein expression in LSKs. Finally, MN1 engrafted mice were treated with venetoclax and IL-3 neutralising antibody in combination which restored neutrophil levels to numbers observed in untreated AML mice. Here, we demonstrate that BCL-2 is overexpressed in non-malignant HPCs during AML progression using two AML models. In addition, using our syngeneic mouse model we show that venetoclax induces HPC depletion. In vitro studies confirm that IL-3 mediates BCL-2 upregulation in HPCs - this sensitises HPCs to venetoclax and causes cytopenias. We also demonstrate that neutrophils can be recovered using IL-3 neutralisation in combination with venetoclax in AML engrafted mice. Taken together, these findings provide biologic insight for IL-3 inhibition alongside venetoclax to reduce the incidence of cytopenias observed in elderly AML patients.