Structural characterisation and expression analysis of Toll-like receptor 2 gene from catfish

Toll-like receptors (TLRs) are important components of innate immunity. They were found to recognise specific structures on pathogens termed pathogen-associated molecular patterns (PAMPs) and utilise conserved signaling pathways to activate pro-inflammatory cytokines and type-1 interferons. In spite of much understanding gained from the mammalian systems, many fish TLRs are unknown. Recent studies in Japanese flounder as well as in zebrafish suggested that the ligand binding and activation of inflammatory responses in fish may be different from and more complex than those found in mammals. In channel catfish, the major aquaculture species in the United States, only partial sequences of TLR3 and TLR5 were reported. As a part of efforts to characterise the innate immune components in channel catfish, here we cloned and sequenced both the cDNA and the gene for TLR2, a receptor believed mostly responsible for recognition of lipopeptides on the surface of most Gram-positive bacteria. However, expression analysis after infection with a Gram-negative bacterium, Edwardsiella ictaluri indicated that TLR2 was modestly down-regulated in the head kidney tissue of blue catfish, and with a similar pattern in the head kidney of channel catfish though the down-regulation in channel catfish was not statistically significant. In the spleen, an insignificant down-regulation was initially observed early after infection, with an increase of TLR expression later after infection. These results suggest the involvement of TLR2 in the responses after the bacterial infection. As LPS is believed to be the major PAMP for Gram-negative bacteria, additional research is warranted to determine the functions and mechanisms of TLR2 in infections of Gram-negative bacteria.