PDCD1 and head and neck squamous cell carcinoma: Additionally, when two sorted CD8 + T-cell subsets isolated from human papilloma virus (HPV)-positive HNSCC patients were cultured with HPV peptide-loaded autologous peripheral blood mononuclear cells, only CD28+Tim-3− PD-1+ CD8 + T cells could proliferate and generate Tim-3+ PD-1+ progeny cells21.