CD38+ M-MDSC expansion characterizes a subset of advanced colorectal cancer patients

BACKGROUND. Myeloid-derived suppressor cells (MDSCs) are a population of immature immune cells with several protumorigenic functions. CD38 is a transmembrane receptor-ectoenzyme expressed by MDSCs in murine models of esophageal cancer. We hypothesized that CD38 could be expressed on MDSCs in human colorectal cancer (CRC), which might allow for a new perspective on therapeutic targeting of human MDSCs with anti-CD38 monoclonal antibodies in this cancer. METHODS. Blood samples were collected from 41 CRC patients and 8 healthy donors, followed by peripheral blood mononuclear cell (PBMC) separation. Polymorphonuclear (PMN-) and monocytic (M-) MDSCs and CD38 expression levels were quantified by flow cytometry. The immunosuppressive capacity of M-MDSCs from 10 CRC patients was validated in a mixed lymphocyte reaction (MLR) assay. RESULTS. A significant expansion of CD38(+) M-MDSCs and a trend of expansion of CD38(+) PMN-MDSCs (accompanied by a trend of increased CD38 expression on both M-and PMN-MDSCs) were observed in PBMCs of CRC patients when compared with healthy donors. The CD38(+) M-MDSCs from CRC patients were found to be immunosuppressive when compared with mature monocytes. CD38(+) M-and PMN-MDSC frequencies were significantly higher in CRC patients who previously received treatment when compared with treatment-naive patients. CONCLUSIONS. This study provides a rationale for an attempt to target M-MDSCs with an anti-CD38 monoclonal antibody in metastatic CRC patients.