CD4 and head and neck squamous cell carcinoma: A statistically not significant trend towards decreased proportions of CTLA-4 expressing T cells was observed in HPV positive HNSCC (6.6 ± 5.2% vs. 11.6 ± 8.5%; p = 0.17), which was similarly present in CD4+ and CD8+ T cells of HPV positive and HPV negative tumors (14.6 ± 11.6% vs. 9.9 ± 8.5%; p = 0.36; 8.3 ± 8.6% vs. 5.5 ± 4.2%; p = 0.44; Figure 4E, right plot).