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.