Interferon regulatory factor 6 gene from Oreochromis niloticus is involved in resistance against tilapia lake virus

Tilapia lake virus (TiLV) is an emerging virus that seriously threatens the tilapia industries worldwide. Interferon regulatory factors (IRFs) are the crucial mediators regulating the response of interferon (IFN) to combat invading viruses. Here, a IRF6 homologs from tilapia (Oreochromis niloticus) were characterized and analyzed. The full length of OnIRF6 mRNA contains a 249 bp 5'untranslated region (UTR), an open reading frame (ORF) of 1473 bp and a 3' UTR of 1209 bp. The OnIRF6 typically shared the conserved domains and phylogenetic relationship with IRF homologs of other species and showed 66 %-99 % identity and 75 %-99 % similarity to other IRF protein domains, respectively. The mRNA of OnIRF6 was high expressed in gill, spleen and Head kidney and low expressed in the dorsal fin, caudal fin and heart. OnIRF6 was dispersedly presented in both the cytoplasm and nucleus, while it is primarily localized in the cytoplasm under normal conditions in tilapia hemocytes, and could be activated by Poly(I:C) and TiLV to translocate into the nucleus of E11 and 293T cells. Moreover, OnIRF6 significantly enhanced the promoter activity of type I interferon-a3 (IFNa3) in 293T cells, suggesting that OnIRF6 serve as positive regulators in activating IFNa3. Overexpression of IRF6 in E11 cells significantly inhibited TiLV propagation. The expression of OnIRF6 was significantly responsive to challenges by TiLV, Poly(I:C) and Streptococcus agalactiae, and could be activated by TiLV to translocate to the nucleus of tilapia hemocytes. Furthermore, silencing OnIRF6 in vivo, whose expression in the spleen and tilapia hemocyte nuclei was significantly downregulated, increased the mortality of TiLV-infected tilapia and the viral load in tissues, and downregulated the expression of multiple components of the IFNs and JAK-STAT pathways and ISGs.​ These results suggested that OnIRF6 could play an important role in defense against TiLV via activating interferon in tilapia.