PDCD1 and head and neck squamous cell carcinoma: Percentages of CD8+ cytotoxic T cells in the PD-1+ fraction were comparably low in PBMC HNSCC (23.9 ± 10.9%), tumor tissue (27.8 ± 17.3%), non-cancerous mucosa (22.0 ± 7.2%) and PBMC HC (18.7 ± 6.7%; Figure 3A).