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It has previously been shown that immunogenic tumors spontaneously activate the innate immune system through the STING pathway. Based on this observation, STING agonists have been developed and are being tested as a pharmacologic approach to activate the pathway and promote anti-tumor immunity. As part of these studies, we observed that tumor cells themselves usually fail to produce IFN-β in response to STING agonists or cytoplasmic DNA, arguing that loss of activation of this pathway might occur regularly as a component of oncogenesis. Understanding failed activation of this pathway might lead to new therapeutic strategies to re-engage tumor cell-intrinsic IFN-β production, which would be predicted to cause immune-mediated tumor control. We found that B16 tumor cells retained expression of each gene in the STING pathway, and that STING signal transduction was intact up to and including nuclear translocation of IRF3. In tumor cells, ChIP assays revealed that IRF3 failed to bind the IFN-β promoter but still bound to other gene promoters and induced expression of those genes. This was likely due to an epigenetic block because Mϕ epigenetically opened the IFN-β promoter region after STING activation, but tumor cells lacked this opening event. Previous work had implicated the NF-κB pathway in the epigenetic opening of the IFN-β locus, and so we examined this pathway in B16 tumor cells. Indeed, we found failed nuclear translocation of the p65 component of NF-κB following STING agonist application. Heterokaryon fusion studies indicated that a dominant-negative mechanism was operational, suggesting that an inhibitory factor was present in tumor cells that blunted NF-κB activation. Treatment with cycloheximide to allow degradation of a putative inhibitory factor restored both NF-κB signaling and IFN-β expression by B16 tumor cells. A genome-wide CRISPR screen identified multiple candidates that, when knocked out, augmented tumor cell-intrinsic NF-κB signaling and IFN-β expression.



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