Bcr-Abl1 Kinase-Independent Upregulation of FcγRIIb Mediates Malignant Signaling That Is Critical for Leukemogenesis

Abstract Chronic myeloid leukemia (CML) is provoked by the chromosomal translocation t(9;22) within the hematopoietic stem cell (HSC) compartment, and this mutation gives rise to the oncogenic tyrosine kinase Bcr-Abl1. Although tyrosine kinase inhibitor (TKI) therapy results in enormous clinical success, TKIs fail to eradicate the disease initiating leukemic stem cell population (LSC) in the majority of CML patients. In a transgenic CML mice (SCLtTA/Bcr-Abl), which we have previously demonstrated to serve as an excellent model to study Bcr-Abl1 independent stem cell persistence, we identified the ITIM carrying Fc gamma receptor IIb (FcγRIIb; CD32) to be 2.8-fold upregulated in Bcr-Abl1+ versus control bone-marrow (BM) derived LSK (lin-;Sca-1+;c-kit+) cells, using microarray and qRT-PCR. Aiming to evaluate the role of FcγRIIb function in Bcr-Abl1 mediated leukemogenesis, we previously retrovirally infected 5-FU treated SCLtTA/Bcr-Abl BM cells (CD45.1+) using FcγRIIb shRNA or scrambled control and transplanted these cells into FVB/N wildtype (WT) CD45.2+ recipients. Spleen weight in recipients receiving shRNA transduced stem and progenitor cells was significantly reduced (352 ± 59.1 mg), as compared to scrambled controls (568.1 ± 101.7 mg). Stem cell compartment analysis revealed decreased leukemic cells of shRNA versus scrambled control transplanted mice in the BM LSK compartment (lin-, c-kit+, Sca-1+, CD45.1+, GFP+, 1.38-fold, p≤0.05). In addition, these effects were even more pronounced upon total depletion of FcγRIIb using the retroviral Bcr-Abl1 model. In these mice, complete loss of FcγRIIb decreased malignant BM LSK cells by 3.5-fold (p Ectopic Bcr-Abl1 expression in human TF-1 cells increased FcγRIIb levels by 4-fold (p Disclosures Bruemmendorf: Novartis: Research Funding. Koschmieder: Roche: Other: Clinical Trial participation; Incyte: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel support; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel support, Research Funding; BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel support, Research Funding.