CTLA4 and head and neck squamous cell carcinoma: There are more attractive targets for immunotherapy in HNSCC, including the infiltration of immunosuppressive cells, such as regulatory T cells (Tregs), and the upregulation of co-inhibitory receptors on T-cells, such as PD-1 and cytotoxic T lymphocyte-associated protein 4 (CTLA-4) [23,24].