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Chronic myelomonocytic leukemia (CMML) is a myeloid hematological malignancy with overlapping features of myelodysplastic syndromes (MDSs) and myeloproliferative neoplasms (MPNs). The knowledge of the role of the tumor microenvironment (TME), particularly mesenchymal stromal cells (MSCs), in MDS pathogenesis is increasing. Generally, cancer is associated with a procoagulant state participating in tumor development. Monocytes release procoagulant, tissue factor (TF)–bearing microparticles. We hypothesized that MSCs and clonal monocytes release procoagulant extracellular vesicles (EVs) within the CMML TME, inducing a procoagulant state that could modify hematopoietic stem cell (HSC) homeostasis. We isolated and cultured MSCs and monocytes from CMML patients and MSCs from healthy donors (HDs). Their medium EVs and small EVs (sEVs) were collected after iterative ultracentrifugations and characterized by nanoparticle tracking analysis. Their impact on hemostasis was studied with a thrombin generation assay and fibrinography. CMML or HD HSCs were exposed to sEVs from either CMML or HD MSCs. CMML MSC sEVs increased HD HSC procoagulant activity, suggesting a transfer of TF from the CMML TME to HD HSCs. The presence of TF on sEVs was shown by electron microscopy and western blot. Moreover, CMML monocyte EVs conferred a procoagulant activity to HD MSCs, which was reversed by an anti-TF antibody, suggesting the presence of TF on the EVs. Our findings revealed a procoagulant “climate” within the CMML environment related to TF-bearing sEVs secreted by CMML MSCs and monocytes.

Introduction: Anemia is the most frequent cytopenia in myelodysplastic syndromes (MDS) and patients will require red blood cell transfusion. Most low-risk MDS patients with anemia are currently treated with erythropoietin stimulating agents (ESA). This treatment gives 45-60% of erythroid response but responders will lose their erythroid response after a median time of 20-24 months (Park, Blood, 2008). Recently, luspatercept (RAP-536), a trap ligand of transforming growth factor-β superfamily, including activins and growth differentiation factors (GDFs), has shown positive effects on erythroid response in MDS patients especially in SF3B1 mutated patients (Platzbecker, Lancet Oncology, 2017). The objective of our work was to evaluate the effect of RAP-536 on human engraftment in PDX mice models of MDS. Methods: We performed patient derived xenograft (PDX) NOD SCID gamma (NSG) murine model by intrabone injections of mesenchymal stromal cells (MSC) and CD34+ hematopoietic stem progenitor cells (HSPC) from 3 MDS patients with SF3B1 mutations and 3 MDS patients without SF3B1 mutations but carrying other recurrent mutations (ASXL1, TET2, SRSF2, CBL, RUNX1, STAG2) seen in MDS (Table A). Three months after xenograft, we have realized a bone marrow aspiration for monitoring human engraftment using flow cytometry to check that there was a human engraftment. Then, the mice were treated for 2 months with 30mg/kg/week of RAP-536. At the end of the procedure, serum, blood and bone marrow were collected. Results: As GDF11 is a target of RAP-536, we have measured GDF11 level in serum in order to confirm the expected effects of RAP-536. We observed decreased levels of GDF11 in RAP-536 treated mice versus mock treated (p Conclusion: RAP-536 improves hemoglobin and WBC levels and seems to enhance the engraftment of human myeloid hematopoiesis in this PDX murine model of MDS. Molecular analyses are ongoing to clarify the relative proportions of normal and pathological human hematopoiesis before and after RAP-536. Figure Disclosures Park: Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding. OffLabel Disclosure: RAP-536