Molecular cloning of orange-spotted grouper (Epinephelus coioides) TLR21 and expression analysis post Cryptocaryon irritans infection

TLR21, a non-mammalian Toll like receptor, was recently identified in chicken as a pattern recognition receptor of unmethyl-CpG ODN, functionally similar to that of mammalian TLR9. Its role in fish immune defense and whether it is involved in anti-parasite immunity has not yet been proven. In this study, we identified a cDNA sequence encoding orange-spotted grouper Toll-like receptor 21 (EcTLR21), the open reading frame (ORF) was 2937 bp encoding a putative polypeptide of 979 amino acid residues. Some conserved motifs in mammalian TLR9 were also conserved in grouper and other fish species’ TLR9 and TLR21. Tissue distribution analysis indicated that EcTLR21 is broadly expressed in all the tissue we tested except muscle. High expression levels were found in the head kidney, trunk kidney, spleen and heart. Post Cryptocaryon irritans infection, TLR21 and TLR9 transcripts were induced at the local infection sites (skin and gill), while suppressed in systemic immune organs (spleen and head kidney), indicating that these two receptors may play a role in host anti-parasitic immune responses.

► TLR21, identified in chicken to recognize CpG-ODN, was firstly cloned in grouper. ► Sequence analysis of EcTLR21 and EcTLR9 shows their possibility to recognize CpG-ODN. ► The expression of both receptors was regulated similarly post Cryptocaryon irritans infection. ► TLR21 (in birds) and TLR9 (in mammals) coexist in fish and may function similarly.