Molecular and functional characterization of CD59 from Nile tilapia (Oreochromis niloticus) involved in the immune response to Streptococcus agalactiae

• A mammalian CD59 homolog was identified and characterized in tilapia.
• On-CD59 was significantly up-regulated in response to S. agalactiae stimulus.
• Recombinant On-CD59 had a species-selective inhibition of complement.
• Recombinant On-CD59 was able to bind to PGN and LTA.
• Recombinant On-CD59 had slightly antimicrobial activity against S. agalactiae.

CD59, the major inhibitor of membrane attack complex, plays a crucial role in regulation of complement activation. In this paper, a CD59 gene of Nile tilapia, Oreochromis niloticus (designated as On-CD59) was cloned and its expression pattern under the stimulation of Streptococcus agalactiae was investigated. Sequence analysis showed main structural features required for complement–inhibitory activity were detected in the deduced amino acid sequence of On-CD59. In healthy Nile tilapia, the On-CD59 transcripts could be detected in all the examined tissues, with the most abundant expression in the brain. When immunized with inactivated S. agalactiae, there was a clear time-dependent expression pattern of On-CD59 in the skin, brain, head kidney, thymus and spleen, with quite different kinetic expressions. The assays for the complement–inhibitory activity suggested that recombinant On-CD59 protein had a species-selective inhibition of complement. Moreover, our works showed that recombinant On-CD59 protein may possess both binding activities to PGN and LTA and inhibiting activity of S. agalactiae. These findings indicated that On-CD59 may play important roles in the immune response to S. agalactiae in Nile tilapia.