The combination treatment of radiotherapy with anti-PD-1/PD-L1 antibody has been shown to significantly improve the clinical outcome of various cancers. Recent studies showthat radiotherapy has multiple functions in modifying the tumor microenvironment, by inducing both immunostimulation and immunosuppression. The upregulation of PD-L1 expression in cancer cells interferes with the effector functions of interacting T cells. Preclinical studies demonstrate that radiotherapy induces PD-L1 upregulation by 4 pathways; (1)IFN-γ signaling,(2)EGFR pathway,(3)DNA damage signaling pathway, and(4)cGAS-STING pathway. All of these mechanisms are involved in the upregulation of PD-L1 expression in cancer cells via JAK/STAT pathway. Because the PD-1/PD-L1 interaction has been shown to be one of the major mechanisms of cancer immune escape, which leads to treatment failure, anti-PD-1/PD-L1 antibody may improve the efficacy of radiotherapy by enhancement of anti-tumor activity. In addition, PD-L1 expression is one of the biomarkers of good response to anti-PD-1/ PD-L1 antibody. Therefore, the comprehensive understanding of the mechanism underlying PD-L1 expression in response to radiotherapy is important for the establishment of optimal combination strategy. This approach could help to provide the basis for the combined therapies and promote personalized immuno-radiotherapy, although the signaling of PD-L1 upregulation induced by radiotherapy in tumors could be intricately regulated. In this article, we review previous researches which revealed the mechanisms of PD-L1 upregulation induced by radiotherapy.
