Fish DDX3 enhances antiviral immunity by up-regulating antiviral interferon responses

DEAD-box RNA helicase enhances innate immunity against viruses. For instance, DEAD-box RNA helicase 3 (DDX3) can participate in the antiviral immune response in mammals mediated by type I IFN. However, only a few studies have reported the regulation of innate immunity by fish DEAD-box RNA helicase. In this study, the DDX3 gene was cloned from chinook salmon (Oncorhynchus tshawytscha) (OtDDX3) and characterized. The OtDDX3 encoded a 711-amino acid protein with a 99.5% and 79.8% homology with rainbow trout (Oncorhynchus mykiss) and humans (Homo sapiens), respectively. OtDDX3 is mainly distributed in the cytoplasm with a small amount in the nucleus. It is characterized by nucleocytoplasmic shuttling. OtDDX3 overexpression inhibited salmonid alphavirus (SAV) replication, while its knockdown promoted virus replication. SAV infection also significantly down-regulated endogenous OtDDX3 expression. Moreover, the endogenous OtDDX3 expression level gradually decreased with increasing infection duration. These results show that SAV infection negatively regulates the expression of host OtDDX3, thus promoting viral proliferation. OtDDX3 activated the IFN1 promoter. Overexpression of OtDDX3 significantly up-regulated the expression levels of IFN-I, IRF7, MAVS, Mx-1, IL-1β, and IL-6. In summary, OtDDX3 potentially induces an antiviral effect by activating the type I IFN signalling pathway and inducing the expression of type I IFN. This study reveals the biological role of fish DDX3 in regulating innate immunity and antiviral effect, thus providing a scientific basis for better understanding and interpretation of the antiviral regulation mechanism of innate immunity in fish. Our findings can guide the breeding of disease-resistant salmon and prevent viral diseases.