CpG oligodeoxynucleotides and plasmid DNA stimulate Atlantic salmon (Salmo salar L.) leucocytes to produce supernatants with antiviral activity.

Unmethylated CpG dinucleotides are more frequent in the genomes of bacteria and viruses than of vertebrates. We report herein that plasmid DNA and synthetic oliogodeoxynucleotides (ODNs) containing unmethylated CpG induce production of antiviral cytokine activity in Atlantic salmon leucocytes, whereas ODNs with an inverted motif (GpC) or with methylated cytosines have nearly no stimulatory effect. The adherent cell population, representing mainly macrophages, is directly activated by CpG-ODN, while the effect on the non-adherent population is weak. Since the peak antiviral activity in ODN-stimulated leucocytes is seen after 48h, this might indicate that the unmethylated DNA stimulates the adherent cells to produce co-stimulatory molecules, which in turn stimulates production of antiviral cytokines in the non-adherent cell population. The potent immune activation by CpG ODNs points to possible new applications as adjuvant in fish vaccines.