Isolation and characterization of Toll-like receptor 21 and 22 genes from Nile tilapia, Oreochromis niloticus (Linnaeus)

In the present study, we obtained the full-length cDNA sequences of TLR21 and TLR22 from Nile tilapia (Oreochromis niloticus) (OnTLR21 and OnTLR22, respectively). The gene structure and expression patterns of OnTLR21 and OnTLR22 were also studied in order to understand the function of these genes in tilapia immunity. The OnTLR21 gene was 3196 bp in length, with an open reading frame (ORF) of 2940 bp encoding 979 amino acid residues and with no introns. The OnTLR22 gene contained three exons and two introns, with an ORF of 2886 bp, encoding 961 amino acid residues. The TLR family motifs, that is leucine-rich repeat (LRR) domains and Toll/interleukin (IL)-1 receptor (TIR) domains, are conserved in the putative OnTLR21 and OnTLR22 proteins. Quantitative real-time PCR (qRT-PCR) analysis revealed the presence of the OnTLR21 transcript in 10 tissues of healthy fish, with the highest expression level in the gill and the lowest expression level in the liver. In contrast, the highest expression level of OnTLR22 was detected in the spleen, while the lowest expression level was in the liver. The expression pattern of TLR during embryonic development showed that the expression levels of OnTLR21 significantly increased from 2.5 to 6.5 days post fertilization (dpf), decreased at 7.5 dpf and increased at 8.5 dpf. While the expression levels of OnTLR22 significantly decreased from 3.5 dpf, and there were no significant differences from 3.5 to 8.5 dpf. Upon stimulation with an intraperitoneal injection of Streptococcus agalactiae, the expression levels of OnTLR21 and OnTLR22 were downregulated in the spleen, kidney and gill. These results may indicate that OnTLR21 and OnTLR22 play important roles in mediating the response protection against S. agalactiae in Nile tilapia.