Tumor Necrosis Factor from Leukemic Microenvironment Provides Non-Leukemic Hematopoietic Stem Cells with Megakaryocyte Lineage Bias

Background: Acute myeloid leukemia (AML) is characterized by proliferation of malignant cells and impairment of normal hematopoiesis, leading to the severe bone marrow failure. Although genetic abnormalities and pathogenesis of AML are now investigated in detail, the precise mechanism impairing normal hematopoiesis in AML is not fully elucidated yet. Hematopoietic stem cells (HSC) retain the self-renewal capacity together with multipotent differentiation potential into all mature blood cells, playing a major role in maintaining the normal hematopoiesis. The ability of HSC is maintained by complex network of intrinsic factors and external stimulation from microenvironment called as HSC niche. According to these backgrounds, we hypothesized that the impairment of normal hematopoiesis in AML is caused by abnormal function of non-leukemic (nl) HSC. To verify this hypothesis, we isolated nl-HSC and HSC niche cells from MLL-AF9-induced AML mice and further analyzed their function and gene expression profiles.

Results: The frequencies of nl-HSC/hematopoietic progenitor fraction (Lin-cKit+Sca1+ cells, HSPC) were significantly increased at the initiation phase of leukemic proliferation in AML mice (leukemic cells : 10-30% in BM)（normal: 0.76+/-0.34%, AML: 2.07 +/-0.93 %, p<0.001）. Surface expression level of CD41 was also upregulated (normal: 15.1+/- 0.526%, AML: 25.4+/- 9.28%, p<0.05). Interestingly, the results of in vitro culture of nl-HSPC demonstrated that they proliferated more efficiently than HSPC from normal mouse BM, and that they tended to differentiate into megakaryocyte (MgK) lineage. Gene set enrichment analysis revealed that MgK-related gene sets and several inflammatory cytokines signaling pathways, including tumor necrosis factor (TNF) pathway, were statistically enriched in nl-HSC. These data suggested that nl-HSC/HSPC retain the MgK-biased character.

On the other hands, the gene expression profiles of perivascular stromal cells (ICAM1+CD31-CD45-TER119-cells) in AML mice demonstrated the significantly decreased expression of HSC-supporting factors (e.g., lpl, ptn, rspo2, angptl4, kitl, cxcl12), when compared with those in normal mice. These data suggested that the changes of multiple external humoral factors from leukemic environment induce functional alternation of nl-HSC.

To further investigate the role of inflammatory humoral factors in HSC self-renew and differentiation, we next utilized polyvinyl alcohol (PVA)-based ex vivo HSC expansion system and focused on the role of TNF signaling. In the analysis of highly purified normal HSC (CD150+CD48-CD34-cKit+Sca1+Lin-cells) with PVA, TNF alpha increased both proliferation and MgK differentiation. UMAP analysis of single cell RNA sequence data for ex vivo-expanded HSCs with TNF alpha showed the increased frequencies of the clusters expressing MgK-related genes (e.g., itga2b and gp1ba) and relatively decreased immature HSC related genes (e.g., hlf and procr). The results of competitive repopulation assay using ex-vivo expanded normal HSC with TNF alpha stimulation in HSC promoted myeloid differentiation of HSC, rather than self-renew and expansion of HSC.

Conclusion: Our data suggested that TNF alpha from leukemic microenvironment may play a key role for the driver signaling of HSCs to MgK, eventually resulting in the impairment of normal HSC maintenance.

Sakata-Yanagimoto:Otsuka Pharmaceutical Co., Ltd.: Research Funding; Eisai Co., Ltd.: Research Funding; Bristol-Myers Squibb Company: Research Funding. Chiba:Kyowa Kirin Co., Ltd.: Research Funding; Chugai Pharmaceutical Co., Ltd.: Research Funding; Thyas Co., Ltd.: Research Funding; Bayer Yakuhin Ltd.: Research Funding; Eisai Co., Ltd.: Research Funding; Astellas Pharma Inc.: Research Funding.