CD274 and head and neck squamous cell carcinoma: Programmed death-ligand 1 (PD-L1), overexpressed in ~50% of HNSCC cases, mediates immune evasion through dual mechanisms: membrane-bound PD-L1 engages PD-1 on T cells to induce exhaustion, while extracellular vesicle (EV)-encapsulated PD-L1 systemically suppresses T cell activity (18).