A Short Hairpin RNA Screen of Interferon-Stimulated Genes Identifies a Novel Negative Regulator of the Cellular Antiviral Response

ABSTRACT The type I interferon (IFN) signaling pathway restricts infection of many divergent families of RNA and DNA viruses by inducing hundreds of IFN-stimulated genes (ISGs), some of which have direct antiviral activity. We screened 813 short hair-pin RNA (shRNA) constructs targeting 245 human ISGs using a flow cytometry approach to identify genes that modulated infec-tion ofWest Nile virus (WNV) in IFN--treated human cells. Thirty ISGs with inhibitory effects againstWNVwere identified, including several novel genes that had antiviral activity against related and unrelated positive-strand RNA viruses. We also de-fined one ISG, activating signal cointegrator complex 3 (ASCC3), which functioned as a negative regulator of the host defense response. Silencing of ASCC3 resulted in upregulation of multiple antiviral ISGs, which correlated with inhibition of infection of several positive-strand RNA viruses. Reciprocally, ectopic expression of human ASCC3 or mouse Ascc3 resulted in downregula-tion of ISGs and increased viral infection. Mechanism-of-action and RNA sequencing studies revealed that ASCC3 functions to modulate ISG expression in an IRF-3- and IRF-7-dependent manner. Compared to prior ectopic ISG expression studies, our shRNA screen identified novel ISGs that restrict infection ofWNV and other viruses and defined a new counterregulatory ISG, ASCC3, which tempers cell-intrinsic immunity. IMPORTANCE West Nile virus (WNV) is a mosquito-transmitted virus that continues to pose a threat to public health. Innate im-mune responses, especially those downstream of type I interferon (IFN) signaling, are critical for controlling virus infection and spread. We performed a genetic screen using a gene silencing approach and identified 30 interferon-stimulated genes (ISGs) tha