Deubiquitination and Stabilization of PD-L1 by USP21

Recent studies have shown that the expression level of PD-L1 in tumor cells positively correlates with tumor metastasis and recurrence rate. The effects of post-translational modifications (PTMs) of PD-L1 are related to immunosuppression. However, the degradation of PD-L1 in cancers has not yet been sufficiently defined. Here, we identified USP21 as a novel deubiquitinase of PD-L1. Overexpression of USP21 significantly increased PD-L1 abundance while its knockdown induced PD-L1 degradation. In vitro deubiquitination assay revealed that USP21-WT, but not USP21-C221A, reduced polyubiquitin chains of PD-L1. These results highlight the role of USP21 in the deubiquitination and stabilization of PD-L1. Furthermore, we show that USP21 is the frequently amplified deubiquitinase in lung cancer, especially in lung squamous cell carcinoma, and its amplification is accompanied by upregulation of PD-L1. This study reveals the mechanism of USP21-mediated PD-L1 degradation, and suggests that USP21 might be a potential target for the treatment of lung cancer.