Identification and characterization of a B-type mannose-binding lectin from Nile tilapia (Oreochromis niloticus) in response to bacterial infection

Lectins are a group of carbohydrate-binding proteins, which play an important role in innate immune system against pathogen infection. In this study, a B-type mannose-binding lectin (OnBML) was identified from Nile tilapia (Oreochromis niloticus), and characterized at expression patterns against bacterial infection and capability to promote phagocytosis by macrophages. The open reading frame of OnBML is 354 bp of nucleotide sequence encoding polypeptides of 117 amino acids. The deduced protein is highly homologous to other teleost BMLs, containing two repeats of the conserved mannose-binding motif QXDXNXVXY. Expression of OnBML was widely exhibited in all examined tissues, with the most abundance in spleen and following gill, peripheral blood, and head kidney. The OnBML expressions were significantly up-regulated following two major bacterial infections including a Gram-positive bacterium (Streptococcus agalactiae) and a Gram-negative bacterium (Aeromonas hydrophila) in vivo and in vitro. Recombinant OnBML protein possessed capacities of mannose-binding and calcium-dependent agglutination to S. agalactiae and A. hydrophila, and promoted the phagocytosis by macrophages. Taken together, the present study indicated that OnBML is likely to get involved in host defense against bacterial infection in Nile tilapia.