PDCD1 and head and neck squamous cell carcinoma: Moreover, head and neck tumors cells secrete exosomes containing COX2, TGF-β, PD-1 and CTLA-4 (21) and those inhibitory molecules cause CD8 T cell apoptosis, inhibit CD4 T cell proliferation and increase the frequency of regulatory T-cell (Tregs), compromising the antitumor responses of HNSCC, which accounts for functional defects or apoptosis of T-cells, both circulating and tumor-infiltrating lymphocytes (22, 23).