Identification and characterization of scavenger receptor class B member 2 in Nile tilapia (Oreochromis niloticus)

The class B scavenger receptors (scarb) are members of important pattern recognition receptors which play key roles for host immune response to pathogen infection. In the present study, two scarb member 2 paralogous (Onscarb2a and Onscarb2c) were identified and characterized in Nile tilapia. Onscarb2a was identified with an open reading frame (ORF) of 1593 bp which encoded a polypeptide of 530 amino acids, and Onscarb2c was consisting of an ORF of 1446 bp which encoded a polypeptide of 461 amino acids. Both Onscarb2a and Onscarb2c share high sequence identity/similarity with their counterparts from other teleost species. Multiple sequence alignment and phylogenetic tree analyses show that scarb2c from teleost fish is more closely related to other vertebrate scarb2s. Gene expression studies revealed that both scarb2a and scarb2c were highly expressed in liver, with the lowest expression in muscle in healthy tilapia. After challenges with Streptococcus agalactiae and Aeromonas hydrophila, both Onscarb2a and Onscarb2c were markedly up-regulated in liver and head kidney. In addition, the up-regulation of Onscarb2a and Onscarb2c were also detected in head kidney monocytes/macrophages and hepatocytes. In conclusion, this study revealed that Onscarb2a and Onscarb2c might play important roles in the innate immune responses to bacterial invasion in Nile tilapia.