Overexpression of <scp>miR</scp>‐155 Modulates Interferon Response and Inhibits Viral Replication of <scp>IHNV</scp> and <scp>IPNV</scp> in <scp>EPC</scp> Cells

Infections caused by RNA viruses, including infectious haematopoietic necrosis virus (IHNV) and infectious pancreatic necrosis virus (IPNV), result in substantial economic losses in the aquaculture industry. MicroRNAs (miRNAs), particularly miR-155, play crucial roles in regulating host immune responses and viral infections. In this study, we investigated the overexpression effect of miR-155 in Epithelioma papulosum cyprini (EPC) cells infected with IHNV and IPNV and analysed the mechanisms underlying these antiviral activities. EPC cells were transfected with miR-155 at 40 pmol and then infected with IHNV or IPNV at an MOI of 0.01. The cytopathogenic effect (CPE) was observed for 5 days post-infection. The cells transfected with miR-155 did not show any signs of CPE or exhibit any viral growth over time after infection with both viruses. Additionally, real-time qPCR of type I interferon-related immune genes (ISG15 and Mx1) showed upregulation at 0, 24, and 48 h.p.i in cells transfected with miR-155. At 48 h post-infection, the cells transfected with miR-155 did not show any bands of viral protein by western blot. Furthermore, the overexpression of miR-155 in EPC cells significantly enhanced the expression of interferon response genes by targeting BCL2 and CYLD and suppressed the viral replication through directly targeting viral genes, including the L gene of IHNV and the VP2 gene of IPNV. These findings elucidate the dual mechanism of miR-155's antiviral effect through immune modulation and direct viral gene targeting, offering insights for developing novel antiviral strategies in aquaculture.