An increase in the frequency of MDSC in the peripheral blood of patients with different types of cancers has been demonstrated.1,2 Murine MDSC are characterized by co-expression of Gr-1 and CD11b, and can be further subdivided into two major groups: CD11b+ Gr-1high granulocytic MDSC (which can also be identified as CD11b+ Ly-6G+ Ly6Clow MDSC) and CD11b+ Gr-1low monocytic MDSC (which can also be identified as CD11b+ Ly-6G− Ly6Chigh MDSC). We have previously identified CD49d as another marker to distinguish these two murine cell populations from each

other.3 We could demonstrate that CD11b+ CD49d+ monocytic MDSC mTOR inhibitor were more potent suppressors of antigen-specific T cells in vitro than CD11b+ CD49d− granulocytic MDSC. S100A9 has recently been reported to be essential for MDSC accumulation in tumour-bearing mice. It was also Palbociclib shown that S100A9 inhibits dendritic cell differentiation by up-regulation of reactive oxygen species. Finally, no increase in the frequency of MDSC was observed in S100A9 knockout mice, which also showed strong anti-tumour immune responses and rejection of implanted tumours,4 indicating the relevance of S100A9+ MDSC in tumour settings. In contrast to murine MDSC, human MDSC are not so clearly defined because of the lack of specific markers. Human MDSC have been shown to be CD11b+, CD33+ and HLA-DR−/low.

In addition, interleukin-4 receptor α, vascular endothelial growth factor receptor, CD15 and CD66b have been suggested as more specific markers for human MDSC. However, these markers can only be found on some MDSC subsets.5 It has been suggested that 4-Aminobutyrate aminotransferase monocytic MDSC are CD14+ 2,6 and granulocytic MDSC express CD15,7,8 whereas both groups of MDSC are HLA-DR−/low and CD33+. The heterogeneous expression of these markers suggests that multiple subsets of human MDSC can exist. We have previously shown direct ex vivo isolation of a new subset of MDSC that are significantly

increased in the peripheral blood and tumours of patients with hepatocellular carcinoma. These cells express CD14, have low or no expression of HLA-DR and have high arginase activity. CD14+ HLA-DR−/low cells not only suppress the proliferation of and interferon-γ secretion by autologous T cells, but also induce CD25+ Foxp3+ regulatory T cells that are suppressive in vitro.9 Others have been able to detect CD14+ cells with suppressor activity in the peripheral blood from patients with other malignancies such as melanoma, colon cancer and head and neck cancer.8,10 We have been able to demonstrate their suppressor activity in patients with colon cancer (data not shown). Although many studies have shown the presence of human MDSC in different pathological conditions, understanding their biology in human cancer requires further characterization of these cells.