PDCD1 and head and neck squamous cell carcinoma: Overall, both the phenotypic analyses for IFN-γ-expressing cells and the analyses of IFN-γ secretion show responsiveness to challenges of premalignant lesion and HNSCC lysates by lymph node cells of mice treated for one week with PD-1 antibody, but a waning of this responsiveness with continued PD-1 antibody treatment.