ARG1 and metastatic melanoma: T cells specific to important immunoregulatory molecules such as PD-L1, PD-L2, indoleamine 2,3-dioxygenase (IDO), and ARG-1/2 have been described (88–92), and vaccination with IDO- and PD-L1-derived epitopes have already shown clinical benefit to patients with stage IV non-small-cell lung cancer and metastatic melanoma (93, 94).