CTLA4 and head and neck squamous cell carcinoma: Furthermore, HNSCC tumors exploit various immune evasion mechanisms, such as upregulation of immune checkpoint molecules (e.g., PD-L1 and cytotoxic T-lymphocyte-associated protein 4, CTLA-4), recruitment of immunosuppressive cells (e.g., regulatory T cells and myeloid-derived suppressor cells), and downregulation of major histocompatibility complex (MHC) class I molecules to escape cytotoxic T cell-mediated recognition [4,20].