PDCD1 and head and neck squamous cell carcinoma: Furthermore, high PD-L2 expression in tumor tissue was independently associated with better clinical response to pembrolizumab in patients with head and neck squamous cell carcinoma, suggesting that PD-L2 expression in tumor tissue may exert a vital role in response to PD-1 blockade therapy, although this study was not included in our meta-analysis (10).