Immunization with recombinant actin, enolase and 26S proteasome protects Epinephelus coioides against Cryptocaryon irritans

Abstract

Cryptocaryon irritans (Brown, 1951) is a ciliated protozoan that infects various marine fish, causing severe economic losses in the aquaculture industry. We identified, expressed and purified three candidate antigens – actin, enolase and 26S proteasome subunit P45 protein family (26S) – from Cryptocaryon irritans and determined their roles in protecting grouper (Epinephelus coioides) against C. irritans infections as subunit vaccines. Serum immobilization titer was also carried out at 2 and 4 weeks post-immunization. Results indicated that all putative proteins highly corresponded with their orthologs from other ciliate species and contained multi-antigenic determinants (8–12). The CiActin, CiEnolase and Ci26S were optimized for successful expression in the Escherichia coli BL21 system. We found that all recombinant CiActin, CiEnolase, and Ci26S proteins conferred protection against C. irritans, with the relative percentage survival (RPS) of 36.7%, 23.3% and 46.7%, respectively. Additionally, the parasite abundance of infected groupers reduced by 70.1%, 15.4% and 67.5% in rCiActin, rCiEnolase and rCi26S groups compared to the PBS control. Conversely, the serum immobilization titers increased in all immunization groups, with the highest groups being rCiActin and rCi26S. These findings indicate that the rCiActin and rCi26S can provide a high level of immune protection against C. irritans infection in grouper and are therefore considered candidate C. irritans vaccines.