PDCD1 and cancer: In cancer patients, several factors may affect T cell priming and activation, including defective expression of MHC molecules in tumor cells, over-expression of inhibitory signals (CTLA-4, PD-1/PD-L1, TIM-3/phospholipids, BTLA, LAG3, IDO, Arginase), and stimulation of suppressive cells, such as Tregs, myeloid-derived suppressor cells, and M2 macrophages.