Druggable genome CRISPR screening identifies the KEAP1/NRF2 axis as a mediator of PD-L1 expression.

Cancer cells rapidly induce PD-L1 expression in response to inflammatory cytokines such as IFNγ from cytotoxic T cells. Increased surface PD-L1 is a primary mechanism of cancer cells evading cytotoxic T-cell-mediated immune clearance. Identifying how cancer cells increase PD-L1 expression may yield clinically relevant immune checkpoint regulators. However, the key regulators and molecular mechanisms mediating rapid PD-L1 induction are yet to be understood entirely. To identify targetable mechanisms controlling cytokine-induced PD-L1 expression, we performed functional CRISPR gene KO screening with a custom-designed sgRNA library that targets "druggable" genes. We performed the screening in 6 different cancer lines: 3 ovarian (OVCAR4, CaOV3, and SKOV3) and three pancreatic cancer (MiaPaca2, ASPC1 and KP4) cell lines. The screening recovered the known regulators of PD-L1 expression and uncovered several novel regulators of PD-L1 that control its expression in all cell lines or in a cancer-type-specific fashion. For example, while genetic or pharmacological depletion of CSNK1A1 results in reduced PD-L1 expression in ovarian cancer cells, CDK1 depletion modulates PD-L1 in pancreatic cancer cell lines. Significantly, we discovered that KEAP1 depletion or pharmacological inhibition diminishes PD-L1 in all cell lines tested (n = 6). Mechanistically, KEAP1 depletion-mediated reduced PD-L1 is due to transcriptional repression of the PD-L1 gene by NRF2 activation. As such, depletion of NRF2 restores PD-L1 expression, while its overexpression leads to diminished PD-L1 expression. Supporting this, pharmacological NRF2 activation resulted in significant antitumor immunity with increased cytotoxic effector T cell infiltration and reduced exhausted T cells, resulting in smaller xenografted tumors. These findings establish the KEAP1/NRF2 axis as a novel and potentially druggable mechanism of IFNγ-meditated PD-L1 expression in cancer cells.