PDCD1 and gastrointestinal stromal tumor: In KitV558Δ/+ GIST mice, IDO inhibitors may enhance the antitumor effect of imatinib [95] or anti-PD1 antibody [35] through activating CD8+ T cells and inducing apoptosis of Treg cells [95, 165], suggesting that the IDO-targeted immunotherapy would be of great value.