Tumor Associated Macrophages Promoted Fatal Patient Derived Acute Promyelocytic Leukemia In Vivo

With immune therapies on the rise, an in-depth understanding of the immunological changes in leukemic bone marrow (BM) niches becomes indispensable. Being an crucial part of the tumor microenvironment (TME) in solid tumours, tumour-associated macrophages are often associated with poor prognosis (Bruni et al. 2020). Yet, in acute myeloid leukaemia (AML) the role of macrophages has not been thoroughly studied. The expression of the M2-markers CD163 and CD206 in the AML BM cell population predicted poor clinical outcome. We identified that this expression emerges from a more mature (CD45 ^midSSC ^highHLA-DR ⁺CD14 ⁺CD16 ^+/-) myeloid cell population (hereafter called AML-associated macrophages - AAM) and not from the leukemic blasts. By employing flow cytometry analysis (FACS) we noted a decrease in the expression of the M1-marker (CD80) and an increase of the M2-markers CD163/CD206on AAM (n=70) compared to healthy donors (HD, n=10). Unsupervised clustering based on the CD163/CD206 levels detected on AAM generated 4 distinct clusters, whereby patients within the CD163 ^low/CD206 ^low cluster displayed better overall survival than the other clusters.

In vitro, the co-culture of HD-derived M1 macrophages and AML primary/cell lines reduced AML growth via apoptosis induction and cell cycle arrest, while M2-macrophages promoted AML survival and phagocytosis/drug-resistance when treated with FLT3/BCL2 inhibitors. Primary AML cells were also able to repolarize M1- into M2-macrophages, suggesting that leukemic cells actively remodel their microenvironment. Next, we evaluated the impact of M2-macrophages on leukemogenesis in a patient derived xenograft (PDX) model, using the notoriously difficult to engraft primary Acute Promyelocytic Leukaemia (APL) cells (n=7 patient samples). Intra-BM injection of M2-macrophages and retro-orbital transplant of primary APL cells induced full-blown APL in NSGS mice. More strikingly, ex vivo culture of APL cells on M2-macrophages (48h) was sufficient to "train" these cells to engraft and induce fatal APL. Maintenance of self-renewal was shown in a secondary transplant and an enhanced frequency of leukemic stem cells was assessed by in vivo LTC-IC assays.

To identify the biological changes acquired by leukemic blasts, we performed RNA sequencing comparing AML/APL samples at diagnosis to cells that were "trained" (48 h) on M2-macrophages or on MS5 mesenchymal BM stromal cells. Gene ontology and gene set enrichment analysis on the genes up-regulated upon M2 co-culture were significantly enriched for cell migration, cell cycle progression and oxidative phosphorylation (OXPHOS) signatures. In line with our RNAseq data, we noted improved in vivo homing of primary APL cells to the BM within 18 h post-transplant upon ex vivo M2 co-culture compared to diagnosis (n=7 APL blasts). Concurrently, we detected increased levels of surface protein expression Integrin-α4 (CD49d) and -α5 (CD49e) on APL/AML blast cells after M2 exposure. The CD49d expression remained high in primary and secondary transplants. Using seahorse measurements, we confirmed the increased respiration capacity (basal and maximum) of primary AML/APL cells (n=7) after exposure to M2 macrophages compared to MS5.FACS analysis revealed that M2-macrophages were able to transfer more mitochondria than MS5 cells to primary AML cells, which could underlie the observed increase in OXPHOS mitochondrial metabolism. Treatment with Etomoxir (50 µM), prevented the gain in functional respiration when AML blast were co-cultured on M2-macrophages, while no changes were observed for MS5 co-cultures, suggesting increased fatty acid oxidation to drive the OXPHO-like state. Finally, we noted that training on M2 macrophages significantly increased colony formation and endowed the cells with long term proliferation in liquid cultures for over 30 days.

Overall, we reveal that the frequency of M2-macrophages is up-regulated in a subgroup of AML patients representing a group with poor prognosis. M2 macrophages can support leukemic growth and therapy-resistance, and support fatal APL in PDX models. Even an in vitro exposure to M2 macrophages suffices to alter adhesion, homing and metabolic characteristics of leukemic blasts to allow efficient engraftment and fatal leukemogenesis. Our study uncovers how the TME can contribute to leukemic transformation which provides alternative avenues for therapeutic interventions.