PDCD1 and neoplasm: In addition to promoting tumor angiogenesis, VEFG signaling also affects immune cells by inhibiting DCs (57) through the induced production of the tolerance enzyme indoleamine 2,3-dioxygenase (IDO) in DCs, preventing T cell infiltration into tumors, up-regulating programmed cell death protein 1 (PD-1) expression in CD8+ T cells, impeding the differentiation of T cells, and interfering with the function of cytotoxic T cells (CTLs) (56, 57).