CTLA4 and cholangiocarcinoma: By investigating the CCA tissues, reduced cytotoxic immune cells and increased Treg together with the activation of two co-inhibitory receptors programmed death-1 (PD-1) and cytotoxic T lymphocyte antigen-4 (CTLA-4) on tumor-infiltrating T cells and their ligands on cancer cells indicate that immunosuppression within the TME that facilitates tumor recurrence in CCA [143,144].