Toll-like receptor (TLR) 22, a non-mammalian TLR in Asian seabass, Lates calcarifer: Characterisation, ontogeny and inductive expression upon exposure with bacteria and ligands

Toll-like receptor (TLR) 22 is a non-mammalian TLR found mostly in teleosts and characterized initially as a cell surface surveillance receptor for detecting extracellular long dsRNA. In the current study, the full-length cDNA sequence consisting of 3312 nucleotides encoding for 960 amino acids in Asian seabass (Lates calcarifer) TLR22 (AsTLR22) was identified. From the putative protein sequence, signature TLR domains such as 18 LRR domains, two transmembrane domains, a single LRR_CT domain and an intracellular TIR domain could be predicted. Phylogenetic analysis showed that AsTLR22 is clustered with other teleost TLR22 and is distinctly different from the other TLR groups. The transcript of AsTLR22 was ubiquitously expressed in all the tissues tested of healthy juveniles with the highest expression in gill followed by hindgut, spleen and skin. The AsTLR22 mRNA transcript was also detected in all the developmental stages as early as unfertilized eggs with higher expression in later stages such as neurula and early embryo. The dsRNA viral analogue, poly (I:C) and Gram-negative bacterium, Vibrio alginolyticus, were found to modulate the AsTLR22 expression in different tissues with the highest expression in kidney and liver. Gram-positive bacterium, Staphylococcus aureus, was also found to regulate the AsTLR22 expression at certain time-points with the highest expression in gill. Similarly, noticeable change in AsTLR22 expression was detected in SISK cell line induced with different ligands such as poly (I:C), LPS and PGN. The findings indicate that AsTLR22 responds in transcript level towards bacteria-borne PAMPs and extracellular dsRNA in the euryhaline teleost Asian seabass. Further, this might act as an important pathogen surveillance receptor during early developmental stages.