CsCTL1, a teleost C-type lectin that promotes antibacterial and antiviral immune defense in a manner that depends on the conserved EPN motif

• CsCTL1, a tongue sole C-type lectin, possesses the mannose-binding motif EPN.
• rCsCTL1 exhibited mannose-specific agglutination against a narrow range of bacteria.
• rCsCTL1 with mutation at the EPN motif failed to exhibit agglutinating activity.
• rCsCTL1–bacteria interaction stimulated the antimicrobial activity of phagocytes.
• rCsCTL1 promoted tongue sole resistance against bacterial and viral infections.

Many C-type lectins (CTLs) have been identified in teleost, however, the in vivo function of fish CTLs is essentially unknown. In this study, we examined the function of a CTL (CsCTL1) from tongue sole. CsCTL1 possesses the conserved EPN motif required for mannose binding in mammals but unknown in function in fish. Recombinant CsCTL1 (rCsCTL1), but not the mutant rCsCTL1M bearing substitutions at EPN, interacted with and agglutinated a limited range of bacteria. The agglutinating ability of rCsCTL1 was abolished in the absence of calcium or presence of mannose. Binding of rCsCTL1 to bacteria promoted phagocytosis and antimicrobial activity of head kidney monocytes. Fish administered with rCsCTL1 exhibited enhanced resistance against bacterial and viral infections. These results provide the first evidence that the EPN site is essential to a fish CTL and that, in addition to antibacterial properties, a fish CTL promotes the immune defense against viral infection as well.