Antiviral effect of miR-155 in Epithelioma papulosum cyprini (EPC) cells against viral hemorrhagic septicemia virus (VHSV) infection

MicroRNAs (miRNAs) are small non-coding RNAs known to play a significant role in the regulation of gene expression in various living organisms including fish. MiR-155 is known to enhance immunity in cells and several reports have demonstrated the antiviral properties of miR-155 in mammals. In this study, we investigated the antiviral role of miR-155 in Epithelioma papulosum cyprini (EPC) cells with viral hemorrhagic septicemia virus (VHSV) infection. EPC cells were transfected with miR-155 mimic and then infected with VHSV at different MOIs (0.01 and 0.001). The cytopathogenic effect (CPE) was observed at 0, 24, 48, and 72 h post infection (h.p.i). CPE progression appeared at 48 h.p.i in mock groups (VHSV only infected groups) and the VHSV infection group transfected with miR-155 inhibitors. On the other hand, the groups transfected with the miR-155 mimic did not show any CPE formation after infection with VHSV. The supernatant was collected at 24, 48 and 72 h.p.i., and the viral titers were measured by plaque assay. The viral titers increased at 48 and 72 h.p.i in groups infected only with VHSV. In contrast, the groups transfected with miR-155 did not show any increase in the virus titer and had a similar titer to 0 h.p.i. Furthermore, the real-time RT-PCR of immune gene expression showed upregulation of Mx1 and ISG15 at 0, 24, and 48 h.p.i in groups transfected with miR-155, while the genes were upregulated at 48 h.p.i in groups infected only with VHSV. Based on these results, miR-155 can induce the overexpression of type I interferon-related immune genes in EPCs and inhibit the viral replication of VHSV. Therefore, these results suggest that miR-155 could possess an antiviral effect against VHSV.