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Background: Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is the only curative treatment for myeloid malignancies such as some acute myeloid leukemias (AML) and high-risk myelodysplastic syndromes (MDS). It aims to eradicate the malignant clone using immunocompetent donor cells (graft-versus-leukemia effect, GVL). Unfortunately, relapse is the primary cause of transplant failure mainly related on HLA loss or downregulation and upregulation of inhibitory ligands on blasts which result in donor immune effector dysfunctions., Methods: Between 2018 and 2021, we conducted a monocentric prospective study including 61 consecutive patients transplanted for AML or high-risk MDS. We longitudinally investigated immune cells at days + 30, + 90 and + 180 post-transplant from bone marrow and peripheral blood. We assessed the dynamics between myeloid derived suppressor cells (MDSCs) and T-cells., Results: Among the 61 patients, 45 did not relapse over the first 12 months while 16 relapsed during the first year post-transplant. Through months 1 to 6, comparison with healthy donors revealed an heterogenous increase in MDSC frequency. In all recipients, the predominant MDSC subset was granulocytic with no specific phenotypic relapse signature. However, in relapsed patients, in vitro and in vivo functional analyses revealed that MDSCs from peripheral blood were highly immunosuppressive from day + 30 onwards, with an activated NLRP3 inflammasome signature. Only circulating immunosuppressive MDSCs were statistically correlated to circulating double-positive Tim3+LAG3+ exhausted T cells., Conclusion: Our simple in vitro functional assay defining MDSC immunosuppressive properties might serve as an early biomarker of relapse and raise the question of new preventive treatments targeting MDSCs in the future. Trial registration NCT03357172., (© 2024. The Author(s).)