PDCD1 and neoplasm: Importantly, tumor PD-L1 expression in EMT states is associated with poor responses to PD-1 blockade in a subset of patients [80,82,83] due to tumor expression of additional inhibitory ligands [84,85,86,87], loss of differentiation antigens by tumor cells [88] and activation of T-cell exclusion mechanisms [89,90,91], collectively contributing to reduced T-cell infiltration and tumor immune escape [86,92,93].