CD274 and head and neck squamous cell carcinoma: When the protein binds its ligand, PD-L1, T cells are inactivated either through anergy or by undergoing apoptosis resulting in the PD-L1-bound cell being immunologically privileged.17 Abnormal levels of PD-L1 expression have been found in many cancers, including HNSCC, which may result in unhindered tumour growth.18–20 Recent studies suggest an alternative function of PD-L1.