PDCD1 and neoplasm: Immune checkpoint blockade therapy functions to restore the T cell-mediated anti-tumor immune responses by blocking the checkpoint receptors, such as PD-1 and CTLA-4, which are expressed in T cells, or their ligands, such as PD-L1, which is upregulated in tumor and antigen presenting cells (APCs) upon chronic antigen stimulation within the TME (Chen and Mellman, 2017; Wei et al., 2018; Callahan and Wolchok, 2019).