CTLA4 and head and neck squamous cell carcinoma: The TME of HNSCC typically shows high expression of inhibitory molecules such as PD-L1 on both tumor cells and immune-infiltrating cells, along with the upregulation of co-inhibitory receptors including programmed death-1 (PD-1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) on T cells [35–38].