PDCD1 and neoplasm: Of highest clinical relevance is common resistance mechanisms to anti-PD-1/PD-L1 and anti-CTLA-4 ICB, which include downregulation of DC recruitment, insufficient activated or tumor-specific T cells, downregulation of antigen presentation, downregulation of IFN- γ, exhaustion of TILs, and increased immunosuppressive cells or cytokines.