Project Summary Immunotherapy through the blockade of immune checkpoints can produce durable clinical response, but not all patients respond. It has been suggested that the up-regulation of immune checkpoint genes by IFN? contributes to the resistance of anti-PD1 therapy. The molecular mechanism involved in the regulation of immune checkpoints in tumor cells is poorly understood. Indoleamine 2.3-dioxigenase 1 (IDO1) is the ?rst and rate-limiting enzyme that catalyzes tryptophan degradation to kynurenine. IDO1 is strongly induced by IFN?. As an immune checkpoint gene, IDO1 is an attractive target for cancer treatment. ARID2 is a component of the SW/SNF chromatin remodeling family. ARID2 is suggested to be a tumor suppressor, but the molecular basis involved is poorly understood. We have recently made an unexpected discovery of a role of ARID2 in the regulation of IDO1. We found that ARID2 is required for the induction of IDO1 in response to IFN? stimulation. This application is to explore a previously unrecognized role of the tumor suppressor ARID2 in the regulation of the immune checkpoint gene IDO1.The proposal will test a novel hypothesis that the newly discovered ARID2- IDO1 link may have an important role in cancer immunotherapy. We propose to use a comprehensive approach involving biochemical, genetic, and animal models to study the role of ARID2 in the regulation of IDO1. Specifically, we will study the molecular mechanism of the ARID2-dependent IDO1 induction in response to IFN? stimulation. ChIP assays and gene knockout by the CRISPR/Cas9 strategy will be used to test the hypothesis that the ARID2 containing PBAF complex directly binds to the chromatin to regulate the accessibility of the IDO1 gene promoter toward STAT1 and IRF1 transcription factors. We will examine and dissect the effect of patient-derived ARID2 mutations on the induction of IDO1 in response to IFN? stimulation as compared to other functions of ARID2 such as in DNA repair and apoptosis. Finally, we will examine the biological significance of the ARID2-IDO1 link in the regulation of the intrinsic tumor growth as well as in tumor response to anti-PD1 immune therapy using xenograft as well as syngeneic mouse models. These timely studies, if successfully completed, will advance our ability to design rationale immunotherapeutic strategy.