PDCD1 and head and neck squamous cell carcinoma: Compared with HPV-negative patients, HPV-positive oropharyngeal cancer has a relatively less immunosuppressive TME, as evidenced by a higher CD4+ cell count, higher CD8+ cell count, lower number of Tregs, higher PD-1 mRNA level, and lower CD4+/CD8+ ratio than the respective levels in HPV-negative HNSCC [67,69].