MicroRNAs (miRNAs) have been widely demonstrated to be involved in fish immune process as important post- transcriptional regulators. trim25, a key member of the RLR pathway, performs a critical function in antiviral immunity in fish. However, the antiviral immune mechanism of miRNAs and trim25 in rainbow trout (Onco- rhynchus mykiss) remains largely unknown. Herein, we detected expression patterns of miR-203 and trim25 after rainbow trout infected with IHNV, and overexpressed and repressed miR-203 and trim25 to investigate their functions in vitro and in vivo. Expression patterns suggested that miR-203 and trim25 are potential regulators in interferon-mediated innate immune responses. In vitro, trim25 was verified as a target of miR-203 by luciferase reporter assay, and overexpression of miR-203 significantly inhibited trim25 expression and resulted in down- regulation of downstream immune genes including rig-I, irf3, irf7, ifn, mx1, and nf-kappa b, and co-transfection of trim25 diminished this inhibitory effect. miR-203 overexpression significantly enhanced IHNV n gene expression. On the contrary, the expression level of trim25 and downstream genes were markedly increased after inhibition of miR-203. In vivo, agomiR-203 significantly depressed the expression of trim25, and then reduced the expression levels of rig-I, irf3, irf7, ifn, and mx1. Furthermore, overexpression of trim25 significantly reduced IHNV copies. Overexpression of miR-203 inhibited cell proliferation and promoted apoptosis, which was attenuated by the introduction of trim25, while the opposite results were obtained upon miR-203 suppression. This study contributes to the understanding of post-transcriptional regulatory mechanism governing antiviral immunity in rainbow trout and provides a theoretical basis for using miRNAs as target drugs to treat viral diseases.
