PDCD1 and head and neck squamous cell carcinoma: Our previous study in metastatic melanoma and the present findings in HNSCC have showed that elevated levels of circulating PD-L1+ sEVs, the interaction protein for both PD-1 and CD80, are also associated with the anti-PD-1 response and survival rate (Fig. 1d and Supplementary Fig. 4a).