Downregulation of B7-H4 contributes to the synergistic effect of USP2a-targeted/anti-PD-1 combination therapy in EGFR mutant lung cancer.

Anti-PD-1 treatment has shown clinical benefit in malignant cancers. However, EGFR-mutant (EGFR-MT) lung adenocarcinoma, an immune-cold tumor, shows poor response to this immunotherapy. This scenario is associated with elevated levels of B7-H4, an immune checkpoint demonstrating CD8 T cell inhibition activity. Our previous study has revealed that deubiquitinase USP2a could stabilize B7-H4 protein. Therefore, we further explore whether USP2a inhibition could remodel immune-cold microenvironment in this study. First, we confirmed that USP2a inhibitors effectively promoted proteasomal degradation of B7-H4 through blocking its deubiquitination process. To rule out the possiblity that USP2a might directly inhibit EGFR MT endocytosis so as to inhibit B7-H4 expression,we investigated the effect of USP2a inhibitor ML364 on EGFR mutants protein levels. It showed that ML364 did not inhibit EGFR mutant protein levels. ML364 could directly inhibit tumor cell proliferation in vitro. In immune-deficient nude mice ML364 inhibited tumor growth through inhibiting USP2a substrates Cyclin D1 and MDM2. In immune-competent C57BL/6 mouse model, ML364 could downregulate B7-H4 level thereby repress tumor growth through remodeling immune-cold microenvironment. Finally, we proved that ML364 could sensitize tumor to anti-PD-1 therapy in immune-competent mice. Immunohistochemical staining analysis showed that ML364 could inhibit B7-H4 expression, thereby enhanced Qa-1b (homolog of HLA-E in mouse) expression and CD8 T cell infiltration. This study proved that inhibition USP2a could suppress tumor growth through two mechanisms: directly inhibiting tumor cell proliferation and remodel immune-cold microenvironment. Therefore, inhibiting USP2a could sensitize tumor to anti-PD-1 therapy.