CTLA4 and cancer: However, cancer cells can inactivate TILs to evade immunosurveillance by ligating their checkpoint receptors [e.g., programmed death 1 (PD-1), cytotoxic T lymphocyte-associated protein 4 (CTLA-4)], which are normally bound by specific ligands (e.g., programmed death-ligand 1 (PD-L1), B7) to control immune activity (152).