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Introduction Myelodysplastic syndromes (MDS) are clonal hematologic disorders characterized by ineffective hematopoiesis, dysplasia and increased risk of progression to acute myeloid leukemia. The development of targeted therapies for MDS has been lagging behind and remains a key clinical challenge that has been hampered, at least in part, by difficulties to establish in vivo model systems that recapitulate disease heterogeneity and complexity. Attempts to generate a xenograft model of lower risk MDS have only achieved low and often transient levels of engraftment. Recent evidence from mouse studies suggests that MDS is a disease in which both the hematopoietic system and the bone marrow microenvironment might be involved. Thus, we hypothesized that a specific MDS microenvironment might be required for the successful modeling of low risk MDS in mice, proposing a dependency of the “disease propagating cells“ on their corresponding niche cells in human MDS. Methods Our study is based on xenotransplantation of material from 19 MDS patients classified as follows: IPSS low risk (n=6), intermediate-1 risk (n=13), WHO 2008 classification: MDS 5q- (n=7), MDS RCMD (n=7), MDS RAEB I (n=3), MDS-U (n=1), MDS RARS (n=1). MDS CD34+ cells were co-injected with patient-derived mesenchymal stromal cells (MSCs) directly in the bone marrow cavity (i.f) of NOD.Cg-Prkdscid Il2rgtm1Wjl/Szj (NSG) or NSGS (NSG mice expressing human SCF, IL3 and GM-CSF) mice. Molecular tracking of MDS cells was carried out by copy number analysis (Affymetrix SNP 6.0 Arrays), metaphase cytogenetics, interphase FISH, Roche 454 deep sequencing and pyrosequencing of known mutations. Mice were analyzed after a minimum of 16 weeks post transplantation. Results We show that co-injection of MDS CD34+ cells with their corresponding MSCs leads to significant and long-term engraftment of over 77% of the MDS patients analyzed, both in NSG (10/13 patients, range hCD45+= 1-18%) and NSGS mice (7/8 patients, range hCD45+=2.2-74%). In contrast, absence of MSCs or co-injection of healthy age-matched MSCs only gave rise to limited engraftment in NSG mice (2/7 patients (hCD45+=1-3.8%) and 1/2 patients (hCD45+=2%), respectively). Transplanted samples exhibited a clear myeloid bias and significant engraftment of cells with progenitor (CD34+CD38+) and stem cell phenotype (CD34+CD38-) that could be serially transplanted. In addition, presence of morphologically dysplastic cells was readily detectable in NSGS mice. Importantly, molecular analysis of the engrafted cells confirmed their “diseased” origin as they carried identical lesions to the ones present in the original MDS patient. Furthermore, we could demonstrate that disease-propagating stem cells in lower risk MDS exclusively reside within the lin-CD34+CD38- stem cell fraction. Finally, RNA sequencing analysis comparing MDS and age-matched healthy control MSCs revealed altered expression of key genes involved in cellular adhesion, extra-cellular matrix (ECM) remodeling and cellular cross-talk in diseased MSCs, strongly supporting the notion of a complex interplay between MDS hematopoietic cells and their corresponding stroma. In addition, patient MSCs exhibited clear molecular features of fibrosis, a clinical feature often associated with MDS. Conclusion In this study we have identified patient-derived MSCs as a critical functional component of lower risk MDS. Together with MDS stem cells, these patient MSCs form a functional stem cell-niche unit, which allows the propagation of the disease in a xenograft recipient. The striking changed expression in diseased MSCs of genes involved in processes like cytokine-cytokine receptor interaction, cellular adhesion, ECM remodeling as well as hypoxia further suggests that diseased MDS cells might alter the function of the normal HSC niche into one that can support the requirement of MDS cells. Studying the interaction of MDS stem cells and MSCs at the cellular and molecular level will provide a platform for unraveling the molecular basis of clonal dominance in MDS as well as allow the design of targeted strategies aimed to disrupt the MDS stem cell-MSC niche interactions. Disclosures: No relevant conflicts of interest to declare.