Double-stranded RNA- and CpG DNA-induced immune responses in Atlantic salmon: comparison and synergies.

Several TLR agonists are shown to activate piscine immunity and are interesting adjuvant candidates in vaccine development. To test the outcome of stimulating Atlantic salmon with CpG DNA and poly I:C, ligands for TLR9 and 3, respectively, we have measured the in vivo expression of different immune molecules in spleen and head kidney. The expression profiles for individual treatments with CpGs or poly I:C not only showed similarities but they also displayed unique profiles. When combining them a synergistic up-regulation of the genes interferon (IFN)-alpha1/alpha2, Mx, CXCL10, IL-1beta, IFN-gamma and CD83 was detected. Interestingly, synergies between two different CpG ODNs classes also resulted in pronounced IFN-alpha1/alpha2 and IFN-gamma transcripts levels. To our knowledge this is the first study showing synergy by combining two different TLR9 ligands. In conclusion, detection of dsRNA and CpG DNA in fish may mimic viral recognition, resulting in an enhanced innate immune response that could be used for vaccination.