PDCD1 and head and neck squamous cell carcinoma: In HNSCC patients, PD-1 expression was significantly increased compared to healthy donors on CD8+ T cells (mean value 9.5 ± 7.8% versus 4.5 ± 2.6%) and Treg (mean value 14.5 ± 4.4% versus 11.3 ± 4.2%) (Figure 2A).