CD274 and head and neck squamous cell carcinoma: Mechanisms of immune escape in HNSCC include the formation of novel mutated antigens, disruption of the antigen-presenting machinery, overexpression of PD-L1 inhibitory molecule in CTLs, secretion of inflammatory cytokines and immunosuppressive factors (transforming growth factor-beta (TGF-β), IL-10, indoleamine 2,3-dioxygenase (IDO)) and recruitment of key immunosuppressive cell types in the TME, such as T regulatory cells (Tregs) and myeloid derived suppressor cells (MDSCs) [6,22].