LAUSR

Significance The PD-1/PD-L1 immunoinhibitory axis plays a key role in immune evasion of cancer cells, and therapies targeting PD-1/PD-L1 show high efficacy in certain cancer types. Understanding how cancer drivers regulate immune surveillance will enable development of novel therapeutic strategies targeting cancer-mediated immune evasion, and identification of new biomarkers of response. Here, we utilized genetic screening with a curated library of 500 tumor suppressor genes to identify cohesin subunits and CTCF among the most significant suppressors of PD-L1. We report upregulation of additional key immune-regulatory molecules PD-L2 and MHC-I, and describe transcriptional consequences of loss of the STAG2 cohesin subunit, including an induction of IFN and NF-κB responses, which have implications for biology and treatment of cohesin-deficient tumors. Immune evasion is a significant contributor to tumor evolution, and the immunoinhibitory axis PD-1/PD-L1 is a frequent mechanism employed to escape tumor immune surveillance. To identify cancer drivers involved in immune evasion, we performed a CRISPR-Cas9 screen of tumor suppressor genes regulating the basal and interferon (IFN)-inducible cell surface levels of PD-L1. Multiple regulators of PD-L1 were identified, including IRF2, ARID2, KMT2D, and AAMP. We also identified CTCF and the cohesin complex proteins, known regulators of chromatin architecture and transcription, among the most potent negative regulators of PD-L1 cell surface expression. Additionally, loss of the cohesin subunit RAD21 was shown to up-regulate PD-L2 and MHC-I surface expression. PD-L1 and MHC-I suppression by cohesin were shown to be conserved in mammary epithelial and myeloid cells. Comprehensive examination of the transcriptional effect of STAG2 deficiency in epithelial and myeloid cells revealed an activation of strong IFN and NF-κB expression signatures. Inhibition of JAK-STAT or NF-κB pathways did not result in rescue of PD-L1 up-regulation in RAD21-deficient cells, suggesting more complex or combinatorial mechanisms at play. Discovery of the PD-L1 and IFN up-regulation in cohesin-mutant cells expands our understanding of the biology of cohesin-deficient cells as well as molecular regulation of the PD-L1 molecule.