Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of cells that regulate immune responses in cancer and various pathological conditions. However, the phenotypic and functional heterogeneity of human MDSCs represents a major hurdle for the development of therapeutic strategies targeting or regulating MDSCs in tumor progression, inflammation, and graft-versus-host disease (GVHD). We previously shown that circulating HLA-DR(-)CD14(+) monocytic MDSCs are a major contributor to clinical outcomes after allogeneic hematopoietic stem cell transplantation (allo-HSCT). In this study, we identified, using high-throughput screening, a set of surface markers that are strongly expressed in HLA-DR-CD14+ monocytic MDSCs isolated from the peripheral blood (PB) of patients receiving allo-HSCT. Subsequent experiments showed the consistent dominant expression of CD1d in monocytic MDSCs of allo-HSCT PB in comparison with granulocytic MDSCs. In addition, CD1d-expressing cells isolated from PB of allo-HSCT patients showed the suppressive activity of T cell proliferation and higher expression of MyD88 and IDO compared with CD1d(-) cells. Our results suggest that CD1d could be a valuable marker for further therapeutic evaluation of human monocytic MDSCs for immune-related diseases, including GVHD. (C) 2017 Elsevier Inc. All rights reserved.