CD274 and head and neck squamous cell carcinoma: In this context, it has been shown that exosomes contribute to the HNSCC immunosuppressive microenvironment via elevated levels of checkpoint molecule PD-L1 [23], whereas further investigations revealed distinct cellular origins of PD-L1-positive extracellular vesicles in HNSCC patients, including tumor cells, T cells, and B cells, but also monocytes and macrophages [34].