Characterization of Toll-like receptor 18 (TLR18) gene in striped catfish (Pangasianodon hypophthalmus)

Abstract

In the fish innate immune system, toll-like receptors (TLRs) are the first germline-encoded pattern recognition receptors (PRRs) to be identified and characterized in fish and play a vital role in the recognition of conserved pathogen-associated molecular patterns (PAMPs) and as a first line of defense against the appearance of pathogens. TLR18 is well known as one of the fish- specific TLRs and belongs to the TLR1 family. In the present study, the TLR18 in striped catfish (Pangasianodon hypophthalmus) was analyzed by bioinformatics to characterize the TLR18 structure in striped catfish. It was 2564 bp long, encoding for 854 amino acids. Analysis of the deduced amino acid sequence showed that striped catfish TLR18 encodes structures typical of the TLR family, including a signal peptide, twenty-one leucine-rich repeats (LRRs), a transmembrane region, and a toll-interleukin-1 receptor (TIR) domain. Quantitative RT-PCR analysis was applied to clarify gene expression in various tissues in healthy fish. The result demonstrated that striped catfish TLR18 was constitutively expressed in all investigated tissues, with abundant expression in muscle, followed by the head kidney, liver, and the lowest expression in the posterior intestine. The expression of TLR18 in the examined organs showed that striped catfish TLR18 contributed a key role in the innate immune system in immunobiology muscle and it could support further study of immunostimulants in striped catfish.