Three novel B-type mannose-specific lectins of Cynoglossus semilaevis possess varied antibacterial activities against Gram-negative and Gram-positive bacteria

• CsBML1, CsBML2, and CsBML3 possess three conserved mannose binding sites.
• The expression of all three lectins was upregulated by bacterial infection.
• Recombinant lectins bound to a wide arrange of bacteria with different affinities.
• All three lectins had mannose-specific and Ca2+-dependent agglutinating capacities.
• rCsBML1 and rCsBML2 are bactericidal and inhibited bacterial infection in vivo.

Lectins are a group of sugar-binding proteins that are important factors of the innate immune system. In this study, we examined, in a comparative manner, the expression and function of three Bulb-type (B-type) mannose-specific lectins (named CsBML1, CsBML2, and CsBML3) from tongue sole. All three lectins possess three repeats of the conserved mannose binding motif QXDXNXVXY. Expression of CsBML1, CsBML2, and CsBML3 was most abundant in liver and upregulated by bacterial infection. Recombinant (r) CsBML1, CsBML2, and CsBML3 bound to a wide arrange of bacteria in a dose-dependent manner and with different affinities. All three lectins displayed mannose-specific and calcium-dependent agglutinating capacities but differed in agglutinating profiles. rCsBML1 and rCsBML2, but not rCsBML3, killed target bacteria in vitro and inhibited bacterial dissemination in fish tissues in vivo. These results indicate for the first time that in teleost, different members of B-type mannose-specific lectins likely play different roles in antibacterial immunity.