Effect of CpG-ODNs belonging to different classes on resistance of olive flounder (Paralichthys olivaceus) against viral hemorrhagic septicemia virus (VHSV) and Miamiensis avidus (Ciliata; Scuticociliatia) infections

The objective of this study was to investigate the effects of different classes of CpG oligodeoxynucleotides (CpG-ODNs) on resistance of olive flounder (Paralichthys olivaceus) against viral hemorrhagic septicemia virus (VHSV) and Miamiensis avidus infections. Fish administered CpG-ODN 2216 (A-class ODN) showed the highest resistance against VHSV infection and the strongest expression of Mx and ISG15 genes. Fish administered CpG-ODN 2395 (C-class ODN) showed intermediate resistance against VHSV between ODN 1668 and ODN 2216. Whereas, olive flounder intraperitoneally (i.p.) injected with CpG-ODN 1668 (B-class ODN) showed no enhanced resistance against VHSV challenge and no significant increase in expression of Mx and ISG15 genes. In contrast, in the M. avidus challenge experiment, fish injected with CpG-ODN 1668 showed the highest survival rate and the strongest scuticocidal activity of serum. Similar to the results in the VHSV infection experiment, administration of CpG-ODN 2395 elicited secondly higher survival rate against M. avidus challenge. Although fish administered CpG-ODN 2216 eventually showed 100% mortality by M. avidus challenge, the cumulative mortality until 2 weeks post-challenge was similar to the group of fish injected with CpG-ODN 1668. The present results suggest that CpG-ODNs belonging to different classes have different abilities to increase resistance against different pathogens in olive flounder. Therefore, selection of a CpG-ODN appropriate to the characteristics of a certain pathogen is crucial to induce immune responses that are optimal for defense against the pathogen.

► Effects of different classes of CpG-ODNs on diseases resistance were investigated.
► CpG-ODN 2216 (A-class) led to the highest resistance against VHSV infection.
► CpG-ODN 1668 (B-class) led to the highest survival rate against M. avidus challenge.
► Increased resistance against different pathogens was related to CpG-ODN classes.