CpG ODN mimicking CpG rich region of myxosporean Myxobolus supamattayai stimulates innate immunity in Asian sea bass (Lates calcarifer) and defense against Streptococcus iniae.

Oligodeoxynucleotides (ODNs) containing unmethylated cytosine-phosphate-guanine CpG dinucleotides within specific sequence contexts (CpG motifs) have been reported as pathogen-associated molecular patterns (PAMPs). Its immunostimulatory effects have been demonstrated in diverse vertebrate models. CpG ODN is typically found in bacterial or viral genome and recognized by a non-self recognition receptor Toll-like receptor9 (TLR9). Here, a new CpG ODN 1013 which mimics sequence of SSU rDNA of early eukaryotic organism myxosporidia, Myxobolus supamattayai, was employed to stimulate the immune responses of Asian sea bass Lates calcarifer. Its immunostimulant potentiality was comparatively compared with that of CpG ODN 1668, a widely used as functional immunostimulant. Both unmethylated CpG ODNs with some modified phosphorothioated positions were intraperitoneally injection (5 μg/fish). Hematological examination, immunological assays and immune-related genes expression were evaluated 12 h, 1, 3 and 5 d after post CpG ODN challenge. The immunosimulatory effect of these CpG ODNs on fish immunity to protect the bacterial pathogen Streptococcus iniae was also determined. The results demonstrated that these two CpG ODNs could induce immune responses in Asian sea bass including the significant (P < 0.05) increase level of WBC, peroxidase activity and oxidative radicals in head kidney (HK) leukocyte, serum innate immune parameters and up-regulation of four immune responsive genes compared with the control group. Most of immune responses induced by ODN 1668 were strong within 1 d but lesser extended while ODN 1013 prolonged the stimulatory effects during the whole experimental period. After challenge with S. iniae, the survival proportion in ODN 1013-treated fish was apparently higher than that treated with ODN 1668 and PBS, respectively. The results together suggested that CpG ODN 1013 enhanced innate immune responses, including humoral and cellular responses, through TLR9 mediated signaling pathway which is mainly contribute to the protective immunity in Asian sea bass against S. iniae infection. These findings can lead to a new approach in immunostimulant development by using the novel CpG ODN originating from the parasite M. supamattayai, besides those from bacterial and viral genomes, for disease control in fish host.