Development of DNA vaccines for fish: vector design, intramuscular injection and antigen expression using viral haemorrhagic septicaemia virus genes as model

Disease control is one of the major concerns in the aquaculture industry. However, there are no vaccines available for the prevention of many piscine infectious diseases, especially those of viral and parasitic origin. DNA-based vaccination could circumvent several problems associated with traditional methods of immunization, but little is known on its efficacy in fish. The luciferase and lacZ reporter genes were used to characterize expression of plasmid-encoded genes in rainbow trout and zebra fish injected intramuscularly. For a given dose of DNA, the luciferase activity was higher in fish than in mouse muscle. The enzyme activity in fish peaked with 1 mu g of DNA and remained constant for over 12 weeks, but it was not limited to the injected muscle since luciferase activity was also detected in the gills. Thin sections of rainbow trout muscle injected with the lacZ reporter gene showed no permanent tissue damage. To further investigate the ability of DNA-based vaccines to induce protective immunity in fish, viral haemorrhagic septicaemia virus G and N genes were cloned individually into an expression plasmid. Both G and N proteins produced in transfected fish cells appeared identical to native viral proteins, as they were recognized by specific monoclonal antibodies. Coinjection of the G and luciferase genes in fish muscle resulted in a rapid decrease of the luciferase activity over time, when compared to the control, suggesting that fish raised a cellular immune response to the G protein, killing the transfected host cells and ablating further expression of CT protein and luciferase. Finally, young rainbow trout injected with the G construct, alone or together with the N construct, were strongly protected against challenge with live virus. These results suggest that DNA vaccines should be as successful for fish as they are for other animals. (C) 1998 Academic Press Limited.