IDO1 and head and neck squamous cell carcinoma: The main immune checkpoint pathways in HNSCC include PD-1/PD-L1, cytotoxic T lymphocyte antigen 4 (CTLA-4), T-cell immunoglobulin mucin 3 (TIM-3), lymphocyte activation gene 3 (LAG-3), T-cell immunoglobulin and immunoreceptor tyrosine-based inhibitory motif (TIGIT), and indoleamine 2,3-dioxygenase 1 (IDO1) (9, 10).