Non-mammalian Toll-like receptor 18 (Tlr18) recognizes bacterial pathogens in common carp (Cyprinus carpio L.): Indications for a role of participation in the NF-κB signaling pathway

Toll-like receptors are important pattern recognition receptors that can recognize pathogen-associated molecular patterns (PAMPs) and play a critical role in innate immunity. In the present study, tlr18 was identified from common carp (Cyprinus carpio L) (named Cctlrl8). The deduced amino acid sequence contained only a signal peptide, eight LRR (leucine-rich repeat) motifs, a transmembrane region and a TIR (Toll/IL-1 receptor) domain. Phylogenetic analysis showed that CcTlr18 was most closely related to Ctenopharyngodon idella TIr18. Quantitative real-time PCR analysis showed that Cctlrl8 was constitutively expressed in all investigated tissues with the highest expression level in the skin and lowest expression in the gonad. After injection with inactivated Aeromonas hydrophila, Cctlrl8 expression was significantly up-regulated in the head kidney, foregut, hindgut and skin. Moreover, significant up-regulation of Cctlr8 was observed in the spleen, head kidney, hindgut and skin after immersion with live A. hydrophila. In addition, the expression of Cctlrl8 was up-regulated in PGN or flagellin-stimulated HKLs. Luciferase reporter assays showed that Cctlrl8 activated NF-kappa B in 293 T cells and that NF-kappa B activity was enhanced in Cctlrl8 and Canyd88 co-transfected cells. Furthermore, Cctlrl8 could induce the expression of cytokines genes, including ifn, il-1 beta and il-10, in EPC cells. The results suggested that Cctlrl8 plays an important role in the immune response and provides basic information for investigating the mechanisms of fish tlr18. (C) 2017 Elsevier Ltd. All rights reserved.