Cell-type-specific need of Ddx3 and PACT for interferon induction by RNA viruses

Virus infection of mammalian cells causes induction of interferon (IFN) synthesis, which, in turn, inhibits virus replication through the actions of the proteins encoded by the IFN-stimulated genes (ISG). Although, in general, ISGs inhibit virus replication in all cell types, there are reports that a few ISGs may function in a cell-specific manner. To address this issue systemically, we used an ISG shRNA library to screen two mouse cell lines, AML-12 and LA-4, for cell-specific actions of ISGs; Ddx3, an RNA helicase, was identified as such an ISG. It was required for both IFN action and IFN induction by the RIG-like receptor (RLR) pathway in AML-12 cells but not LA4 cells. In AML-12 cells, Ddx3 was required for the efficient translation of STAT1 and PACT mRNAs; ectopic expression of STAT1 and PACT in Ddx3-deficient cells restored IFN action and IFN induction, respectively. PACT was required for the activation of RLR signaling in AML-12 cells infected with vesicular stomatitis virus, Sendai virus (SeV), encephalomyocarditis virus (EMCV), or mouse hepatitis virus. Moreover, like PKR activation, RLR activation by PACT required S246 and S287 in its domain 3. This study demonstrated not only cell-specific action of an ISG, Ddx3, but also cell-specific need of PACT to trigger RLR signaling in RNA virus-infected cells.IMPORTANCEInterferon-stimulated genes (ISGs) are induced in response to interferon expression due to viral infections. Role of these ISGs can be variable in different cells or organs. Our study highlights such cell-specific role of an ISG, Ddx3, which regulates the translation of mRNAs essential for interferon induction (PACT) and interferon signaling (STAT1) in a cell-specific manner. Our study also highlights the role of PACT in RNA virus-induced RLR signaling. Our study depicts how Ddx3 regulates innate immune signaling pathways in an indirect manner. Such cell-specific behavior of ISGs helps us to better understand viral pathogenesis and highlights the complexities of viral tropism and innate immune responses. Interferon-stimulated genes (ISGs) are induced in response to interferon expression due to viral infections. Role of these ISGs can be variable in different cells or organs. Our study highlights such cell-specific role of an ISG, Ddx3, which regulates the translation of mRNAs essential for interferon induction (PACT) and interferon signaling (STAT1) in a cell-specific manner. Our study also highlights the role of PACT in RNA virus-induced RLR signaling. Our study depicts how Ddx3 regulates innate immune signaling pathways in an indirect manner. Such cell-specific behavior of ISGs helps us to better understand viral pathogenesis and highlights the complexities of viral tropism and innate immune responses.