Preferential enrichment of myeloid-derived suppressor cell subsets in patients with metastatic melanoma.

Abstract

e19050 Background: Myeloid derived suppressor cells (MDSC) are key immunosuppressive cells enriched in peripheral blood during chronic inflammation. While these cells have been extensively studied in mice, their human counterparts are less well characterized. Several MDSC populations have been identified in cancer patients, depending on tumor type and experimental settings. MDSC include immature macrophages, granulocytes and dendritic cells, at different degree of maturation, from immature CD33+, to mature CD14+ and CD15+HLA-DR- cells. The goal of this study was to characterize myeloid cells in melanoma patients compared with healthy donors (HD) to identify inflammation-mediated alterations involved in melanoma progression. Methods: PBMC were isolated by Lymphoprep gradient centrifugation from fresh blood samples of patients with malignant melanoma. We performed 9-color FACS staining of the freshly isolated cells. Results: CD15+ granulocytic MDSC are enriched in the mononuclear cell layer after gradient centrifugation, together with the CD14+ monocytic MDSCs. They display significantly different sedimentation abilities compared with neutrophils. Furthermore, CD14 and CD15 were partially co-expressed by the monocytic and granulocytic MDSC subsets, respectively. Our data show that CD14+HLA-DR- monocytic and CD15+HLA-DR- granulocytic MDSC were significantly enriched in peripheral blood of metastatic melanoma patients, regardless of whether or not they received any prior therapy for metastatic melanoma. The more immature CD33+CD11b- subset was found at significantly lower frequencies in melanoma patients compared with HD. We found a trend for accumulation of non-classical CD14+CD16+ monocytes. However, the increased percentages did not differ significantly from HD. The frequencies of classical CD14+CD16- monocytes were unchanged. Conclusions: Melanoma is associated with increased frequencies of CD14+ monocytic and CD15+ granulocytic MDSC, and decreased frequencies of the more immature CD33+CD11b- cells. This finding demonstrates tumor-mediated changes in the composition of myeloid cells, which could influence response to treatment and clinical behavior of the disease.