Influence of USP15 and its derived-peptide on non-small cell lung cancer immune evasion via regulating PD-L1 stability.

[BACKGROUND] Immune checkpoint inhibitors (ICIs) therapy targeting programmed cell death protein 1 (PD-1)/programmed cell death-ligand 1 (PD-L1) shows promising clinical benefits in non-small cell lung cancer (NSCLC). However, the relatively low response rate highlights the need to elucidate the regulatory mechanism of PD-L1 expression, and develop an alternative strategy to target PD-1/PD-L1 immune checkpoint pathway. Our study focuses on the role and mechanism of ubiquitin-specific protease 15 (USP15) and its derived peptide U10 on NSCLC immune evasion.

[METHODS] USP15 as PD-L1's deubiquitinase was identified by screening a human USP complementary DNA (cDNA) library. The role and mechanism of USP15 and its derived peptide U10 on PD-L1 stability in NSCLC cells were analyzed. T cell-mediated tumor cell killing activity and a syngeneic mouse NSCLC model were used to assess the influence of USP15 and U10 on NSCLC immune evasion. The antitumor effect of U10 in combination with PD-1 monoclonal antibody (mAb) via suppressing NSCLC immune evasion was also evaluated in mice. The expression and clinicopathological significance of USP15 and PD-L1 in cancer tissues were evaluated by immunohistochemistry.

[RESULTS] We identify USP15 as a novel deubiquitinase of PD-L1. Mechanistically, USP15 binds and stabilizes PD-L1 in NSCLC cells by inhibiting its ubiquitination and degradation. Functionally, USP15 inhibits T cell ability of killing NSCLC cells in vitro, and promotes NSCLC immune evasion in mice via decreasing the population and activation of CD8 T cells in the tumor microenvironment. Based on the interacting regions of USP15 and PD-L1, we develop a 10 amino acid-long USP15-derived peptide U10, which successfully degrades PD-L1 via disrupting USP15 and PD-L1 interaction, dramatically suppresses NSCLC immune escape in vitro and in mice, and enhances the anti-NSCLC effect of PD-1 mAb in mice. Moreover, the expression levels of USP15 and PD-L1 are significantly higher in NSCLC than those in normal lung tissues and are positively correlated. The combination of USP15 and PD-L1 proteins was superior to individual proteins for predicting the efficacy of PD-1 mAb immunotherapy and patient prognosis in NSCLC.

[CONCLUSION] Our findings reveal a critical role for USP15 in PD-L1 stability regulation and NSCLC immune escape and develop a novel peptide as an alternative strategy for ICIs therapy of NSCLC.