Expression profile of toll-like receptor 2 mRNA in selected tissues of shark (Chiloscyllium sp.)

Abstract Sharks are a species of delight for immunologists from the evolutionary perspective since it is considered as the first species to have evolved the adaptive immune responses in addition to the innate immune system. One of the components of the highly conserved innate immune system is the toll-like receptors (TLR) which has a conserved overall protein structure throughout deuterostome evolution. There is no report that demonstrates the expression of these receptors in sharks. In this study we successfully amplified a 270 bp amplicon using a degenerate primer design strategy that corresponded to the Toll/IL-1 receptor (TIR) domain of TLR2 (GenBank ID: JF792813 ). BLAST analysis revealed a maximum nucleotide identity of 87% and 76% with the TLR2 of higher mammals and teleost fishes respectively. Domain prediction revealed a TIR structure between 1 and 87 amino acids that had a maximum identity of 58% and 76% with TLR2 – TIR protein of teleost fishes and higher mammals respectively. Phylogenetic analysis revealed a closer clustering of the shark TIR sequence with those from human, cattle, goat, sheep and chicken than with other fish species. Basal expression levels of the TLR2-TIR mRNA were found to be significantly higher in kidneys followed by fins, spleen and intestinal spiral valve (ISV). In tissues such as spleen and kidney the expression of the TLR2-TIR mRNA could be localized to lymphoid and macrophages like cells and tubular epithelial cells respectively. In-vivo exposure of sharks to peptidoglycan (TLR 2 ligand) resulted in 9 folds higher expression of TLR2-TIR mRNA in gills followed by 5 folds in the fins. However, when inoculated with a TLR ligand pool, the expression levels significantly increased to 12 fold in skin followed by epigonal, kidneys and ISV. These findings not only support the presence of the TLRs in sharks but also their induction upon exposure to specific ligands. Further studies are needed to identify their numbers, their ligand specificity and downstream cytokine responses.