CD4 and head and neck squamous cell carcinoma: However, Lyford-Pike et al. found that PD-1 expression on CD4+ T-cells and CD8+ T-cells was higher in tonsil tissue, compared to the peripheral blood, of patients with HNSCC [48], which indicates that PD-1 expression on peripheral blood lymphocytes may not be clinically significant, and our results support that possibility.