CD8A and head and neck squamous cell carcinoma: However, a trend towards higher percentages of PD-1+ T cells in the microenvironment of HPV positive HNSCC was observed compared to HPV negative tumors (48.4 ± 18.1% vs. 61.2 ± 14.1%; p = 0.12), due to significantly higher percentages of PD-1 expressing cytotoxic CD8+ T cells (75.4 ± 9.4% vs. 51.2 ± 24.6%, p < 0.05; Figure 4E, middle plot).