CD4 and head and neck squamous cell carcinoma: In sharp contrast with the role of PD-1 on T-cell exhaustion, Badoual et al. (93) suggested that an increased infiltration of PD-1 expressing T cells, including PD-1+CD4+ and PD-1+CD8+ T cells, significantly correlated with a favorable clinical outcome of patients with HPV-positive HNSCC.