CD274 and head and neck squamous cell carcinoma: In HNSCC, these mechanisms of immune escape include the generation of new mutant antigens, disruption of antigen presentation pathways, overexpression of PD-L1 inhibitory molecules on CTLs, recruitment of inflammatory cytokines and immunosuppressive agents such as TGF-β, IL-10, IDO, and the presence of key immunosuppressive cells within the TME, including Treg cells and MDSCs [12, 13].