High-Level Expression of Recombinant VHSV Glycoprotein Using Transformed C. vulgaris and Verification of Vaccine Efficacy.

The demand for aquaculture is increasing, but production is declining due to high feed costs and disease outbreaks. Viral hemorrhagic septicemia (VHS) is a viral disease that seriously affects seawater and freshwater fish in aquaculture, including the olive flounder ( Paralichthys olivaceus ), a major aquaculture fish in Korea. However, very few vaccines are currently available for viral hemorrhagic septicemia virus (VHSV). The nutrient-rich microalga Chlorella vulgaris has been used as a feed additive in aquaculture and as a host for the industrial production of recombinant VHSV glycoprotein as an oral vaccine. In this study, VHSV glycoprotein was cloned with a salt-inducible promoter, and high levels of expression up to 41.1 mg/g wet C . vulgaris , representing 27.4% of total extracted soluble protein, were achieved by growing the transformed C . vulgaris for 5 days in the presence of 250 mM NaCl. The production of a neutralizing antibody was detected in the serum of fish given feed containing 9% VHSV glycoprotein-expressing C . vulgaris . Furthermore, relative survival rates of 100% and 81.9% were achieved following challenges of these fish with VHSV at 10 6 and 10 7 pfu/fish, respectively, indicating that C . vulgaris could be used as a platform for the production of recombinant proteins for use as oral vaccines in the control of viral diseases in aquaculture.