PDCD1 and melanoma: Increased expression of CTLA-4 molecules on T cells negatively stimulates T cells leading to their anergy [10]Elevated production of PD-L1 in melanoma cells combined with the PD-1 overexpression in T cells results in T cell exhaustion [20]Interaction between PD-1 and PD-L1 induces the differentiation of CD4+ lymphocytes into regulatory T cells (Tregs), which secrete cytokines and chemokines with immunosuppressive activity (TGF-β, IL-10, and IL-3) [20,21]