We demonstrate that a higher abundance of circulating monocyte-derived macrophages (MDMs) and cytotoxic T-cell subsets in the tumor microenvironment (TME) at baseline distinguishes metastatic melanoma patients with a favorable response to anti-PD1 treatment from non-responders, who featured co-localization of suppressive macrophages (M2) and T-cells. This evidence concerns the gene PDCD1 and metastatic melanoma.