Epigenetic modifications: Critical participants of the PD-L1 regulatory mechanism in solid tumors (Review)

Immune checkpoint inhibitors targeting the programmed cell death protein 1 (PD-1)/programmed death ligand 1 (PD-L1) axis have achieved marked and durable efficacy in patients with different solid tumors and have improved their survival. However, the presence of primary or acquired resistance to immune checkpoint blockades results in only a small fraction of patients benefiting from the treatment. An increasing number of preclinical studies have reported that PD-L1 expression in tumor cells is involved in a number of epigenetic changes, including histone modifications, non-coding RNA regulation and DNA methylation. In addition, multiple epigenetic targeting drugs have been demonstrated to directly or indirectly interfere with PD-L1 expression in various cancer models. This provides opportunities to better characterize the regulatory mechanisms of PD-L1 expression and explore novel therapeutic strategies to improve immunosuppressant response rates and overcome drug resistance. The present review focuses on the latest findings and evidence on the epigenetic mechanism regulating PD-L1 expression and discusses the biological and clinical implications of this regulatory mechanism in solid tumors. A rational combination of epigenetic regulation and PD-1/PD-L1 axis blockade may improve the prognosis of patients with solid tumors.