Introduction: Hematopoietic Stem and Progenitor Cells (HSPC) reside in the Bone Marrow (BM), composed of other cell types such as osteoblasts, adipocytes, as well as mesenchymal stromal cells (MSC). While HSPC are mostly quiescents in the BM, they might differentiate upon signals from their niche. In this context, MSC are involved in hematopoiesis regulation, through direct contact with HSPC and cytokines regulation, including Bone Morphogenetic Pathway (BMP) molecules. While the implication of MSC in adult Acute Myeloid Leukemia (AML) pathogenesis is well described, their implication in pediatric AML (P-AML) is poorly understood. P-AML cells exhibit fewer driver mutations, indicating a more significant role for non-mutational transforming events, such as microenvironment contribution. To identify how MSC could contribute to P-AML, we characterized MSC from 142 P-AML patients and 41 pediatric healthy donors. After phenotypical assessment, we identified a subpopulation of MSC expressing CD117 (also known as cKIT), present in more than 35% of P-AML patients and less than 5% of pediatric healthy donors. Here we propose to investigate the intrinsic and extrinsic functions of CD117+ MSC in the BM niche.

Material and Methods: After functional, transcriptomic and proteomic characterization of MSC, we used a standardized 3D BM model developed in our team to decipher the dialogue between P-AML cells and CD117+ or CD117- BM niches.

Results: In P-AML patients, presence of CD117+ MSC correlates with poor prognosis. On one hand, CD117+ MSC demonstrated altered intrinsic functions with an up-regulation of BMP pathway and altered adipocytic differentiation. On the other hand, while both CD117+ and CD117- MSC equally support HSPC in normal conditions, CD117+ MSC better protect leukemic stem cells from chemotherapy, by increasing direct contact. Finally, transcriptomic and proteomic profiles further demonstrated an up-regulation of genes and proteins implicated in fetal processes.

Conclusion: We identified a subpopulation of MSC that reshapes the bone marrow niche of more than 35% of P-AML patients, supporting leukemic cells maintenance upon chemotherapy. Interestingly, preliminary results on longitudinal samples from P-AML patients identified that CD117+ MSC subset could also be increased at relapse compared to diagnosis. Thus, our results suggest that in P-AML, MSC represent a heterogeneous population that can foster leukemogenesis from early stages to treatment resistance.