USP7 Enables Immune Escape of Glioma Cells by Regulating PD-L1 Expression

Background Glioma is the most common primary intracranial tumor. This study investigated the mechanism of ubiquitin-specific processing protease 7 (USP7) on the immune escape of glioma cells via the regulation of programmed cell death ligand-1 (PD-L1). Methods USP7 and PD-L1 expressions in glioma and normal brain tissues were detected using quantitative reverse transcriptase polymerase chain reaction and Western blot. The glioma cells U-251 MG were transfected with si-USP7 and pcDNA3.1-PD-L1, or treated with anti-PD-L1, after which the cell viability, colony-forming ability, and apoptosis rate were evaluated using cell counting kit-8, colony formation, and TdT-mediated dUTP nick-end labeling assays. Then, CD8+ T cells were purified, extracted, then co-cultured with U-251 MG cells. CD8+ T cell proliferation and the concentrations of interferon (IFN-gamma), tumor necrosis factor (TNF)-alpha, transforming growth factor (TGF)-beta, interleukin (IL)-1 beta, and IL-10 were verified using carboxyfluorescein succinimidyl amino ester proliferation assay and enzyme-linked immunosorbent assay. Afterwards, the interaction between USP7 and PD-L1 and the ubiquitination level of PD-L1 were also assessed. Results USP7 was highly-expressed and PD-L1 mRNA levels did not change, while PD-L1 protein levels were up-regulated in the glioma cells. Silencing USP7 in U-251 MG cells limited the growth of the glioma cells, promoted glioma cell apoptosis, and facilitated the proliferation of CD8+ T cells, thus inhibiting immune escape. USP7 stabilized PD-L1 expression through deubiquitination. PD-L1 overexpression reversed the inhibitory effect of silencing USP7 on the immune escape of glioma cells. Conclusion USP7 stabilized PD-L1 through deubiquitination and accelerated the immune escape of glioma cells.