PDCD1 and melanoma: PD-L1 overexpression in tumour cells can promote tumour formation.18 In non-small-cell lung carcinoma (NSCLC) and melanoma, high expression of PD-L1 on tumour cells is strongly associated with high tumour grade and poor prognosis of patients.19,20 HNSCC tissues produce PD-L1 through an abnormal PD-1 signalling pathway, which leads to tumour immunosuppression.21 The PD-1/PD-L1 signalling pathway can be activated in a chronic inflammatory environment.