PDCD1 and nasopharyngeal carcinoma: To evaluate whether the observed increase in the killing of NPC cells by IFNβ-activated NK cells when combined with nivolumab was due to the blockade of the PD-L1/PD-1 interaction, PD-L1 expression in NPC cells was silenced by specific siRNA; the efficiency of siRNAs in silencing PD-L1 expression was monitored by flow cytometry (Suppl.