Toll-like receptor 3 and TICAM genes in catfish: species-specific expression profiles following infection with Edwardsiella ictaluri

Toll-like receptors (TLRs) are a family of transmembrane proteins that recognize specific pathogen-associated molecular patterns and use conserved signaling pathways to activate proinflammatory cytokines and type-1 interferons to fight infection. TLR3 in mammals is best known for its recognition of dsRNA as ligand and its MyD88-independent signaling. TLR3, upon recognition of dsRNA, recruits and binds its adaptor protein TIR domain-containing adapter molecule (TICAM) 1. Here we report the genomic sequences and structures of TLR3 and a TICAM adaptor from channel catfish (Ictalurus punctatus). Whereas a partial TLR3 cDNA sequence has been reported from channel catfish, and complete TLR3 genes are known from other teleost fish species, a complete TICAM sequence has not been previously reported from a nonmammalian species. Analysis of catfish TLR3 and TICAM expression after infection with Edwardsiella ictaluri, the causative agent of enteric septicemia of catfish (ESC), suggested a conserved TLR3-TICAM receptor-adaptor relation in catfish. Comparison of TLR3 and TICAM expression profiles in channel catfish with those from the closely related blue catfish species (Ictalurus furcatus), which exhibits strong resistance to ESC, revealed a striking pattern of species-specific expression. A dramatic downregulation of TLR3 and TICAM gene expression was observed in blue catfish head kidney and spleen, which we speculate may be the result of maturation and migration of different cell types to and from the lymphoid tissues following infection.