Characterization of circulating dendritic cells in melanoma: role of CCR6 in plasmacytoid dendritic cell recruitment to the tumor.

Dendritic cells (DCs) are central cells in the development of antitumor immune responses, but the number and function of these cells can be altered in various cancers. Whether these cells are affected during the development of melanoma is not known. We investigated the presence, phenotype, and functionality of circulating myeloid DCs (MDCs) and plasmacytoid DCs (PDCs) in newly diagnosed melanoma patients, compared to controls. The frequencies of PDCs and MDCs were equivalent in melanoma patients as compared with normal subjects. Both circulating DC subsets were immature, but on ex vivo stimulation with R848 they efficiently upregulated their expression of costimulatory molecules. We found that circulating DCs from melanoma patients and controls displayed similar pattern of expression of the chemokine receptors CXCR3, CXCR4, CCR7, and CCR10. Strikingly, PDCs from melanoma patients expressed higher levels of CCR6 than control PDCs, and were able to migrate toward CCL20. Further data showed that CCR6-expressing PDCs were present in melanoma primary lesions, and that CCL20 was produced in melanoma tumors. These results suggest that PDCs and MDCs are functional in melanoma patients at the time of diagnosis, and that CCL20 may participate to their recruitment from the blood to the tumor.