CD274 and cholangiocarcinoma: The production of prostaglandin E2 and adenosine by cholangiocarcinoma cells directly restrains the function and activity of CTLs, further inhibiting CTL-mediated anti-tumor immunity through the overexpression of immune checkpoint ligands such as PD-L1 and B7-H7, or through the downregulation of MHC-I expression on their surface (114).