Protective immunity induced by DNA vaccination of channel catfish with early and late transcripts of the channel catfish herpesvirus (IHV-1)

Seven full-length transcripts encoding four early and three late genes of the channel catfish virus (CCV), ictalurid herpesvirus I (IHV-1), have been cloned following rt-PCR amplification and DNA sequencing. Transcripts were selected based on their predicted association with membrane structures, identification as an envelope glycoprotein, or as a viral capsid protein. The transcripts derived from ORF 6, ORF 7, ORF 8a, ORF 10, ORF 51, ORF 53, and ORF 59 were all shown to be complete and unspliced. Each of the seven ORFs was cloned into a vaccine expression vector designed to support high levels of expression of the inserted sequence in catfish tissues. Solutions of DNA containing one each of the seven CCV ORFs, vector alone or PBS were injected intramuscularly into 4–8cm catfish. Four to 6 weeks after injection each experimental group was challenged with one LD50 of CCV. Single injections of DNA expression constructs containing ORF 59, encoding the envelope glycoprotein, or ORF 6, encoding a presumptive membrane protein, were found to elicit the strongest resistance to challenge compared to uninjected, PBS injected or vector injected groups. Even more effective was a combination vaccine pair in which both ORF 59 and ORF 6 expression constructs were injected. Other ORFs did not provide consistent protection to challenge above that observed in control fish. Both percent survival and kinetics of cumulative deaths were improved using the combination DNA vaccine encoding ORF 6 and ORF 59. Both ORF 6 and ORF 59 were able to elicit virus neutralizing antibodies capable of an anamnestic response on viral challenge. We believe this evidence provides adequate proof of principle for the use of DNA vaccines in channel catfish and the effectiveness of the resistance to viral infection they elicit.