Molecular characterization of a novel C-type lectin receptors (CD302) in Nile tilapia (Oreochromis niloticus) and its functional analysis in host defense against bacterial infection

C-type lectin receptors (CLRs) are a group of carbohydrate-binding proteins which function by mediating multiple biological events, including adhesion between cells, the turnover of serum glycoproteins, and the innate immune system's reaction to prospective invaders. However, how CLRs affects bony fish's innate immunity to bacterial infection is ill-defined. Therefore, CD302, a member of the CLRs family, was found in Nile tilapia (Oreochromis niloticus) (OnCD302) and the functions in the immune activities of tilapia against bacterial infection were examined. The OnCD302 consists of a C-type lectin domain, a transmembrane domain, and a deduced protein encoded by 242 amino acids. Besides, the OnCD302 protein has a series of highly conserved amino acid sites with other CD302 proteins. Subcellular localization showed that OnCD302 is located in the cell membrane. Transcriptional level investigation showed that OnCD302 was extensively expressed in all selected organs and had high expression in the liver. The transcriptional levels of OnCD302 are induced by Streptococcus agalactiae and Aeromonas hydrophila in the liver, spleen, head kidney, brain, and intestine. Afterwards, in vitro study revealed that several kinds of pathogens could be bound by the recombinant protein of OnCD302 (rOnCD302). Meanwhile, rOnCD302 could inhibit the growth of S. agalactiae, but had no inhibitory effect on A. hydrophila. However, the agglutinate ability of rOnCD302 to A. hydrophila and S. agalactiae was not observed in this study. Additionally, rOnCD302 reduced the ability of macrophages to phagocytose S. agalactiae and A. hydrophila. Summarily, our results indicated that CD302 get involved in fish immunization activities to bacterial infection