PDCD1 and oral cavity squamous cell carcinoma: Recent studies confirmed that hypoxia via HIF-1α might selectively up-regulate PD-L1 on immunosuppressive cells [26] and solid tumors such as pulmonary pleomorphic carcinoma [37], and oral squamous cell carcinoma [38], which contributed to the evidence for hypoxia induced immune deficiency by PD-1 / PD-L1 axis [39].