Black carp DDX3b suppresses antiviral immunity by inhibiting IRF7 modifications and promoting its lysosomal degradation.

Type I interferons (IFNs) serve as pivotal functional molecules in the antiviral immune response of fish, and their signaling pathways require precise negative regulation to avoid tissue damage caused by excessive immune activation. In the present study, DEAD-box RNA helicase 3b (DDX3b), an ATP-dependent RNA helicase involved in cellular RNA metabolism and host-pathogen interaction, was identified in black carp (Mylopharyngodon piceus). Sequence and structural analyses showed that bcDDX3b was evolutionarily conserved and contained the characteristic DEXDc and HELICc domains of DEAD-box RNA helicases. Subcellular localization analysis showed that bcDDX3b was predominantly distributed in the cytoplasm and formed distinct cytoplasmic aggregates. Functional assays verified that black carp DDX3b (bcDDX3b) acted as a negative regulator of type I IFN signaling. Overexpression of bcDDX3b markedly suppressed both basal and virus-induced IFN promoter activities and decreased the expression levels of interferon-stimulated genes (ISGs), thereby blocking the establishment of cellular antiviral status. Mechanistically, bcDDX3b directly interacted with bcIRF7, the core transcription factor of IFN signaling pathway, and the DNA-binding domain (DBD) of bcIRF7 was indispensable for the interaction. The binding of bcDDX3b to bcIRF7 significantly impaired bcIRF7-mediated antiviral defenses against spring viraemia of carp virus (SVCV) and grass carp reovirus (GCRV). Further mechanistic investigations revealed that bcDDX3b inhibited the phosphorylation and ubiquitination of bcIRF7 and facilitated bcIRF7 degradation via a lysosome-dependent pathway. Collectively, bcDDX3b functions as a critical negative modulator of RLR-triggered type I IFN signaling in black carp. This study provides novel insights into the regulatory mechanism of IRF7 and the immune homeostasis underlying antiviral responses in teleost fish.