Molecular cloning, characterization and immunological response analysis of Toll-like receptor 21 (TLR21) gene in grass carp, Ctenopharyngodon idella

• The putative amino acid sequence of grass carp TLR21 had 17 LRR domains.
• The structure of grass carp TLR21 is very similar to the TLR21s from other species.
• The grass carp TLR21 is predominantly expressed in skin, spleen and intestine.
• The grass carp TLR21 is significantly increased in liver after bacterial infection.
• The grass carp TLR21 is significantly down-regulated in liver after viral infection.

TLR21, a non-mammalian Toll-like receptor, has been recently identified in fishes, frogs and birds. In the present study, the full-length cDNA sequence of TLR21 (CiTLR21) from Ctenopharyngodon idella has been isolated and characterized. The CiTLR21 full-length cDNA sequence consists of 3578 bp, with an open reading frame (ORF) of 2958 bp encoding 985 amino acid residues. The putative CiTLR21 protein contains a signal peptide sequence, 17 leucine-rich (LRR) motifs, a transmembrane region and a Toll/interleukin-1 receptor (TIR) domain. The CiTLR21 gene is expressed in a wide range of tissues with the highest expression in skin. Upon induction by Aquareovirus, CiTLR21 expression is significantly down-regulated in liver and spleen, whereas is significantly up-regulated in liver and spleen after Aeromonas hydrophila infection. These results suggest that CiTLR21 plays an important role in Aquareovirus and A. hydrophila-related diseases. This work may provide the basis for further investigations into the immune system of grass carp and other teleost fishes.