Combined DNA immunization with the glycoprotein gene of viral hemorrhagic septicemia virus and infectious hematopoietic necrosis virus induces double-specific protective immunity and nonspecific response in rainbow trout.

Glycoprotein (G) of viral hemorrhagic septicemia virus (VHSV) and infectious hematopoietic necrosis virus (IHNV) contains several neutralizing epitopes. However, recombinant G protein never matches intact viral particles for immunogenicity. DNA immunization offers the possibility to deliver the antigen through the cellular machinery, thus mimicking natural infection. We constructed pCDNA gVHS and pCDNA gIHN plasmids with the G gene of VHSV and IHNV under the control of the CMV promoter, and we tested the plasmids for the accurate G protein expression prior to their use in fish immunization. Following intramuscular injection to adult rainbow trout, plasmid DNA was found inside the muscle cells shortly after injection and was still present 45 days later. mRNA of the G protein was detected in muscle tissue extracts, and the G protein was found within muscle cells at the site of injection. This resulted in the synthesis of high levels of specific neutralizing and protective antibodies. Fish injected with pCDNA gVHS and pCDNA gIHN in combination responded similarly to fish receiving one recombinant plasmid. In addition to the elicitation of a strong humoral response, DNA immunization was able to activate specialized cells of the immune system as well as nonspecific defense mechanisms, since mRNAs of MHC class II and Mx were strongly activated at the site of injection.