Accumulation of Circulating CCR7 Natural Killer Cells Marks Melanoma Evolution and Reveals a CCL19-Dependent Metastatic Pathway

Immune checkpoint blockade therapy has changed prognoses for many melanoma patients. However, immune responses that correlate with clinical progression of the disease are still poorly understood. To identify immune responses correlating with melanoma clinical evolution, we analyzed serum cytokines as well as circulating NK and T-cell subpopulations from melanoma patients. The patients' immune profiles suggested that melanoma progression leads to changes in peripheral blood NK and T-cell subsets. Stage IV melanoma was characterized by an increased frequency of CCR7CD56 NK cells as well as high serum concentrations of the CCR7 ligand CCL19. CCR7 expression and CCL19 secretion were also observed in melanoma cell lines. The CCR7 melanoma cell subpopulation coexpressed PD-L1 and Galectin-9 and had stemness properties. Analysis of melanoma-derived cancer stem cells (CSC) showed high CCR7 expression; these CSCs were efficiently recognized and killed by NK cells. An accumulation of CCR7, PD-L1, and Galectin-9 melanoma cells in melanoma metastases was demonstrated Altogether, our data identify biomarkers that may mark a CCR7-driven metastatic melanoma pathway.