PDCD1 and head and neck squamous cell carcinoma: However, Poropatich et al. found that a higher proportion of PD-1+ CD8+ T cells and cytotoxic T lymphocyte antigen 4 (CTLA-4)+ CD8+ T cells were present in the tumor tissues and peripheral blood of HPV-negative HNSCC, and PD-1+ CD8+ T cells were related to primary tumor size (62).