Molecular identification and expression analysis of Toll-like receptor 3 in common carp Cyprinus carpio

The Toll-like receptor (TLR) family is an evolutionarily conserved component of the innate immune system that responds to specific pathogen-associated molecular patterns (PAMP) during infections. TLR3 is a putative pattern recognition receptor (PRR) for dsRNA, a common by-product of viral replication. This study reports the identification and characterization of a full-length orthologue of TLR3 (CcTLR3) in the common carp Cyprinus carpio. It is 2899 base pairs (bp) encoding 904 amino acid residues with a calculated molecular mass of 102 744 Da and an estimated isoelectric point of 6.89. The deduced amino acid sequence is characterized by a signal peptide, 14 leucine-rich repeat (LRR) motifs, a transmembrane domain and a Toll-interleukin (IL)-1 receptor (TIR) domain. Real-time polymerase chain reaction (RT-PCR) analysis indicated that CcTLR3 mRNA transcripts could be detected in a wide range of tissues. Although C. carpio is resistant to grass carp Ctenopharyngodon idella reovirus (GCRV), after injection with GCRV, CcTLR3 expression was downregulated from 24 h post-injection (P < 0.05) and returned to control levels at 72 h post-injection (P > 0.05). Antiviral effector Mx mRNA expression, however, was upregulated from 24 h post-injection (P < 0.05) and recovered to control levels at 96 h post-injection (P > 0.05). The results provide a better understanding of the anti-GCRV mechanism and antiviral immune responses in fish, and help to develop suitable strategies to reduce viral diseases in fish.