Enhancement of piscine orthoreovirus-2 DNA vaccine potency by linkage of antigen gene to a trigger factor gene or signal peptide genes

Coho salmon (Oncorhynchus kisutch) is a Pacific salmon species that is commercially farmed in Japan, and is susceptible to erythrocyte inclusion body syndrome (EIBS), which is characterized by anemia, erythrocyte inclusion bodies, and cardiovascular muscle necrosis. At present, no vaccine for this disease is available. Piscine orthoreovirus-2 (PRV-2) is the causative agent of EIBS in Japan, and is one of three genotypes of PRV that induce the formation of erythrocyte inclusion bodies. However, due to a deficiency in cell lines able to culture PRV-2, the development of vaccines for diseases caused by PRV-2 has been difficult. A recent strategy involved the combination of a DNA vaccine, a σ1, and a non-structural protein gene for moderate PRV-1 protection. In the present study, analysis of antiserum obtained from PRV-2 infection survivors identified σ1 as a PRV-2 antigenic protein. Since a DNA vaccine incorporated with only σ1 did not prove efficacious against PRV-2 infection in a preliminary study, the present study tested the enhanced protective effect of the DNA vaccine with various sequences linked to the σ1 gene. Antigenic PRV-2 gene products were first screened using recombinant PRV-2 proteins and serum from coho salmon with a history of EIBS. Six vaccine groups of DNA vaccines were then created by the linkage of several sequences to the PRV-2 gene. Two trials were conducted to identify effective vaccines and a third vaccination trial confirmed the reproducibility of an effective vaccine. Trial I showed insignificant differences in the mean antibody level after 42 days. However, an outlier test indicated significantly higher antibody levels in coho salmon vaccinated in signal-linked σ1 gene DNA vaccine group and chaperone-linked σ1 gene DNA vaccine group. Trial II evaluated the effects of individual DNA vaccines and showed that individuals in the Sec-σ1, TF-σ1, and mixed vaccine group had significantly higher antibody levels and significantly lower virus loads. Trial III confirmed the suppression of viral replication and increase in antibody levels of the TF-σ1 DNA vaccine. However, this DNA vaccine was only effective for increasing antibody levels and inhibiting viral growth in some individuals, indicating the necessity for further experimentation involving analyses of the effect of individual differences on the efficacy of the vaccine.