CD44 and head and neck squamous cell carcinoma: CD44 can positively regulate the expression of PD-L1 by activating PD-L1 transcription partly via the association between its intracytoplasmic domain and a regulatory region in PD-L1 (62), and CD44+ tumor-initiating cells in head and neck squamous cell carcinoma suppress antitumor immunity through inducible expression of PD-L1 (63).