The IL-6/STAT3 Signaling Pathway Is Involved in Radiotherapy-Mediated Upregulation of PD-L1 in Esophageal Cancer.

Objective: This study aimed to investigate the mechanism whereby ionizing radiation improves anti-programmed cell death protein 1 (PD-1)/programmed cell death 1 ligand 1 (PD-L1) immunotherapy for esophageal squamous cell carcinoma (ESCC).

Methods: The protein levels of interleukin 6 (IL-6), signal transducer, and activator of transcription 3 (STAT3), and PD-L1 in human ESCC specimens and histologically normal tissues were analyzed by western blot. The effects of radiation exposure on PD-L1, STAT3, and IL-6 were determined in three esophageal cancer (EC) cell lines. Radiation therapy plays an important role in the treatment of esophageal cancer, so cell proliferation, invasion, colony formation, and cell cycle were measured before and after radiation exposure.

Results: The protein expression levels of PD-L1, IL-6, and STAT3 were significantly greater in ESCC specimens versus normal tissues. Notably, the protein levels of PD-L1 and STAT3 were positively correlated with the T stage, and IL-6 was positively related to the T stage and tumor site. Cox proportional hazard model multivariate analysis indicated that the expression levels of PD-L1, IL-6, and STAT3 were not independent prognostic factors for ESCC. In vitro studies showed that the PD-L1, STAT3, and IL-6 mRNA and protein expression levels in EC cells exposed to radiation were markedly increased. Further mechanistic investigations found that blocking the IL-6/STAT3 pathway significantly abrogated the radiotherapy-mediated increase in p-STAT3 and PD-L1 levels.

Conclusion: Radiotherapy significantly upregulates PD-L1 through the IL-6/STAT3 signaling pathway, suggesting that anti-PD-1/PD-L1 immunotherapy combined with radiotherapy may improve the treatment for ESCC.