CD4 and head and neck squamous cell carcinoma: First, a correlation was observed between infiltration of TILs and PD-L1 expression in the tumor: PD-L1-positive tumors showed higher CD3 + , CD4 + , CD8 + and PD1 + TIL counts in the tumor epithelium than PD-L1-negative tumors, a phenomenon that was observed before in HNSCC and in other types of cancer [36–38].