PDCD1 and head and neck squamous cell carcinoma: HNSCC-programmed cell death 1 (PD-1) expression is elevated compared to healthy donors on CD8+ T-cells (mean value 9.5 ± 7.8% vs. 4.5 ± 2.6%) and Treg (mean value 14.5 ± 4.4% vs. 11.3 ± 4.2%), both of which reduced significantly with the addition of nivolumab [68].