Class B CpG-ODN2006 is highly associated with IgM and antimicrobial peptide gene expression through TLR9 pathway in yellowtail Seriola lalandi

The purpose of this study was to characterize the TLR9 gene from yellowtail (Seriola lalandi) and evaluate its functional activity using the class B Cytosine-phosphate-guanine-oligodeoxynucleotide2006 (CpG-ODN2006) in an in vivo experiment after one-week immunostimulation. The gene expressions of TLR9, Immunoglobulin M (IgM), antimicrobial peptides and cytokines were evaluated by real time PCR, and humoral immune parameters were analyzed in serum. The TLR9 nucleotide sequence from yellowtail was obtained using the whole-genome shotgun sequencing method and bioinformatics tools. The yellowtail full-length cDNA sequence of SlTLR9 was 3789 bp in length, including a 66-bp 5′-untranslated region (UTR), a 3′-UTR of 528 bp, and an open reading frame (ORF) of 3192 bp translatable to 1064 amino acid showing a high degree of similarity with the counterparts of other fish species and sharing common structural architecture of the TLR family, including LRR domains, one C-terminal LRR region, and a TIR domain. Gene expression studies revealed the constitutive expression of TLR9 mRNA in all analyzed tissues; the highest levels were observed in intestine, liver and spleen where they play an important role in the fish immune system. The expression levels of TLR9 after B class CpG-ODN2006 (the main TLR9-agonist) was significantly up-regulated in all analyzed tissues, with the high expression observed in spleen followed by intestine and skin. The CpG-B has been shown as a potent B cell mitogen, and interestingly, IgM mRNA transcript was up-regulated in spleen and intestine, which was highly correlated with TLR9 after CpG-ODN2006 stimulation. The antimicrobial peptides, piscidin and NK-lysine, were up-regulated in spleen and gill after CpG-ODN2006 injection with a high correlation (r ≥ 0.82) with TLR9 gene expression. Cytokine genes were up-regulated in spleen, intestine and skin after CpG-ODN was compared with the control group. No significant correlation was observed between TLR9 and IL-1β, TNF-α and Mx gene expressions. The results showed that CpG-ODN2006 intraperitoneal injection enhanced lysozyme, peroxidase and superoxide dismutase activities in serum and demonstrated that CpG-ODN2006 can induce a specific immune response via TLR9 in which IgM and antimicrobial peptides must have an important role in the defense mechanisms against infections in yellowtail.