CD274 and head and neck squamous cell carcinoma: A key emerging mechanism of tumor immune evasion involves T-cell exhaustion, whereby T-cell reactivity is impaired due to activation of T-cell checkpoints, including PD-1 by its ligand, programmed death ligand 1 (PD-L1) that is expressed by macrophages and some cancer cells, including HNSCC, restraining T-cell activation (reviewed in ref. 12).