A specific CpG oligodeoxynucleotide induces protective antiviral responses against grass carp reovirus in grass carp Ctenopharyngodon idella

•Found a CpG ODN, 1670A, containing the optimal sequence with strong immune stimulatory and antiviral activity in grass carp.•CpG ODN induced immune response by TLR9 signaling pathway largely.•TLR21 did not play a role in the immune response to CpG ODN.•RLRs participated in the immune response caused by the efficient CpG ODN.•IFNγ2 played an important role in response to the efficient CpG ODN.

CpG oligodeoxynucleotides (ODNs) show strong immune stimulatory activity in vertebrate, however, they possess specific sequence feature among species. In this study, we screened out an optimal CpG ODN sequence for grass carp (Ctenopharyngodon idella), 1670A 5′-TCGAACGTTTTAACGTTTTAACGTT-3′, from six published sequences and three sequences designed by authors based on grass carp head kidney mononuclear cells and CIK (C. idella kidney) cells proliferation. VP4 mRNA expression was strongly inhibited by CpG ODN 1670A in CIK cells with GCRV infection, showing its strong antiviral activity. The mechanism via toll-like receptor 9 (TLR9)-mediated signaling pathway was measured by real-time quantitative RT-PCR, and TLR21 did not play a role in the immune response to CpG ODN. The late up-regulation of CiRIG-I mRNA expression indicated that RIG-I-like receptors (RLRs) signaling pathway participated in the immune response to CpG ODN which is the first report on the interaction between CpG and RLRs. We also found that the efficient CpG ODN can activates interferon system. Infected with GCRV, type I interferon expression was reduced and type II interferon was induced by the efficient CpG ODN in CIK cells, especially IFNγ2, suggesting that IFNγ2 played an important role in response to the efficient CpG ODN. These results provide a theoretical basis and new development trend for further research on CpG and the application of CpG vaccine adjuvant in grass carp disease control.