PDCD1 and neoplasm: Additionally, its administration in combination with carboxyamidotriazole (CAI) potently increases the activity of cytotoxic CD8+ T cells and their ability to kill tumor cells, probably due to the reduced expression of programmed cell death protein-1 (PD-1) and increased IFN-γ secretion in the tumor microenvironment associated with decreased KYN level [83].