EZH2 Inhibition Activates a dsRNA-STING-Interferon Stress Mechanism that Potentiates Response to PD-1 Check-Point Blockade in Prostate Cancer

Prostate cancers are considered immunologically 'cold' tumors given the very few patients who respond to checkpoint inhibitor therapy (CPI). Recently, enrichment of interferon stimulated genes (ISGs) predicts a favorable response to CPI across various disease sites. The enhancer of zeste homolog-2 (EZH2) is over-expressed in prostate cancer and is known to negatively regulate ISGs. Here, we demonstrate that a majority of ISGs are not directly silenced by EZH2 mediated H3K27me3 deposition, but instead in a poised chromatin state associated with accessible chromatin (ATAC+) and H3K27ac deposition. Inhibition of EZH2 catalytic activity in prostate cancer models activates a dsRNA-STING-ISG cellular stress response upregulating genes involved in antigen presentation, Th-1 chemokine signaling, and interferon (IFN) response, including PD-L1 that is dependent on STING activation. EZH2 inhibition combined with PD-1 CPI significantly enhances anti-tumor response dependent on up-regulation of tumor PD-L1 expression. Further, combination therapy significantly increases intratumoral trafficking of activated CD8+ T-cells and M1 tumor associated macrophages (TAMs) with concurrent loss of M2 TAMs. Our study identifies EZH2 as a potent inhibitor of antitumor immunity and responsiveness to CPI. This data suggests EZH2 inhibition is a novel therapeutic direction to enhance prostate cancer response to PD-1 CPI. Overall design: Mouse prostate organoids generated from genetically engineered mouse models housing overexpression of cMYC, EZH2 floxed alleles, and an inducible PSA-CreERT2 were generated. The purpose was to compare control verse EZH2 inhibition effects on gene expression changes. Organoid EZH2 activity was inhibited chemically by treatment with the EZH2 inhibitor DZNep (verse DMSO control) or genetically using Tamoxifen (verse EtOH control). 3 days post treatment, organoids were used to generate RNA and be analyzed by RNA-seq.