High-dose irradiation in combination with toll-like receptor 9 agonist CpG oligodeoxynucleotide 7909 downregulates PD-L1 expression via the NF-κB signaling pathway in non-small cell lung cancer cells

OBJECTIVES: Irradiation resistance appears as local recurrence and distant metastasis in advanced stages of non-small cell lung cancer (NSCLC). High-dose irradiation combined with immunotherapy improved overall survival and local control of NSCLC. This study explored the underlying molecular mechanism by which the effect of high-dose irradiation plus toll-like receptor 9 (TLR9) agonist CpG oligodeoxynucleotide (CpG ODN) 7909 on NSCLC. MATERIALS AND METHODS: NSCLC H460 cells were exposed to constant high-dose irradiation (6.37 Gy) in irradiation (IR) group and the irradiation plus CpG group. Gene expression was assessed using quantitative reverse transcriptase-polymerase chain reaction and Western blot. Knockdown of nuclear factor kappa B (NF-κB) p65 expression was conducted using p65 siRNA. RESULTS: Expression of programmed death-ligand 1 (PD-L1) mRNA was significantly decreased in IR combined with CpG ODN 7909 group compared with the control or IR-alone groups (P<0.05). TLR9 expression was also obviously increased in the combination group compared with the control (P<0.05). Moreover, expression of NF-κB p65 was apparently reduced in the combination group compared with the control (P<0.05). However, expression of PD-L1 was significantly decreased after knockdown of p65 in IR group (P<0.05), but increased in the combination group (P<0.05) and slightly increased in CpG ODN-alone group (P<0.05), which was opposite to that without p65 knockdown group. CONCLUSION: This study demonstrated that radiotherapy combined with CpG ODN 7909 was able to downregulate PD-L1 expression through inhibition via the NF-κB signaling pathway.