A polyprotein-expressing salmonid alphavirus replicon induces modest protection in atlantic salmon (Salmo salar) against infectious pancreatic necrosis

First Experimental Application of DNA-Layered Salmonid Alphavirus-Based Replicon Vaccine in Non-Salmonid Fish: Induced Early Semi-Specific Protection against Spring Viraemia of Carp Virus (SVCV) in Common Carp (Cyprinus carpio)

Our study demonstrates the first application of the salmonid alphavirus-based replicon vector system (pSAV) as a DNA vaccine in a non-salmonid fish species, in common carp (Cyprinus carpio) against spring viraemia of carp virus (SVCV). SAV replicon encoding the glycoprotein of the SVCV was used as a DNA-layered plasmid, and its efficacy was compared with a previously described conventional DNA vaccine construct (pcDNA3.1 based vector) and with a control group (pcDNA3.1-empty-plasmid) in an SVCV challenge at a water temperature of 14 ± 1 °C. Vaccine prototypes were administered intramuscularly at a dose of 0.1 µg/g of fish (n = 25 per group). The DNA-layered SAV replicon resulted in 88% survival, compared to around 50% in all other groups. The DNA-layered pSAV vaccination induced the innate immune genes at the injection site, and increased IgM upregulation was also observed. Our preliminary results show that the SAV-based replicon construct may serve as a potential vaccine candidate for the protection of non-salmonid fish in the future provided that further clinical and field trials confirm its efficiency.

Comparison of Selected Immune Parameters in a Single Infection and Co-Infection with Infectious Pancreatic Necrosis Virus with Other Viruses in Rainbow Trout

Infectious pancreatic necrosis virus (IPNV) often occurs in an aquatic environment in co-infection with other viruses. In this study, we wanted to investigate the effect of this virus on the course of co-infection with other viruses in rainbow trout. For co-infection we used viral hemorrhagic septicemia virus (VHSV), infectious hematopoietic necrosis virus (IHNV) and salmonid alphavirus (SAV) field strains and infected rainbow trout divided into eight groups; I; IPNV, II; IHNV, III; VHSV, I; SAV, V; IPNV+IHNV, VI; IPNV+VHSV, VII; IPNV+SAV, and the control group. We assessed apoptosis in white blood cells and used a real time RT-PCR to analyze RNA obtained from the internal organs of the fish. During single infection and co-infection the level of expression of immune genes such as interferon and toll-like receptor 3 (TLR-3) was assessed. The highest mortality during the experiment was observed in group III infected by VHSV. The average percentage of apoptotic cells was higher in groups without co-infection, especially in groups II and III. Interferon expression was higher in singly infected groups, the highest being in the heart in group III, while expression of the TLR-3 gene was generally raised in all tested organs in all groups. We found that co-infection with IPNV had a positive impact on the course of infection with the viruses listed because it lowered mortality, reduced apoptosis in co-infected cells, and positively affected fish health.

Phylogenetic characterization of Polish isolates of infectious pancreatic necrosis virus in salmonid fish

INTRODUCTION

Infectious pancreatic necrosis virus belongs to the genus Aquabirnavirus and family Birnaviridae. By VP2 gene similarity, aquatic birnavirus is clustered into seven genogroups. The aim of this study was to genetically analyse IPN viruses occurring on Polish fish farms.

MATERIALS AND METHODS

Samples from freshwater fish mostly from 2012 to 2013 and from northern Poland were examined for the presence of IPN virus using isolation on cell cultures, real-time RT-PCR and RT-PCR. Fragments of 1,377 and 1,079 bp of the VP2 and VP5 genes, respectively, were sequenced, and the results were assembled into one consensus and analysed by Geneious software. The same VP2 gene region was compared and a phylogenetic tree generated by the neighbour-joining method and MEGA6 software.

RESULTS

All tested Polish isolates belonged to genogroup 5, like other European Spajurup isolates.

CONCLUSION

Our findings prove that there is only one IPN virus genogroup in Poland. Polish isolates show close relationships with each other. There is a close relationship between Polish isolates and isolates from Turkey, Spain and Iran. Isolate 57 is a separate branch related to isolates from the United States and Taiwan. This points to the likelihood of past virus introduction via import of stock from those countries.

DNA vaccine expressing the non-structural proteins of Piscine orthoreovirus delay the kinetics of PRV infection and induces moderate protection against heart -and skeletal muscle inflammation in Atlantic salmon (Salmo salar)

Piscine orthoreovirus (PRV) causes heart- and skeletal muscle inflammation (HSMI) in farmed Atlantic salmon (Salmo salar). Erythrocytes are the main target cells for PRV. HSMI causes significant economic losses to the salmon aquaculture industry, and there is currently no vaccine available. PRV replicates and assembles within cytoplasmic structures called viral factories, mainly organized by the non-structural viral protein µNS. In two experimental vaccination trials in Atlantic salmon, using DNA vaccines expressing different combinations of PRV proteins, we found that expression of the non-structural proteins µNS combined with the cell attachment protein σ1 was associated with an increasing trend in lymphocyte marker gene expression in spleen, and induced moderate protective effect against HSMI.

Veterinary Replicon Vaccines

Vaccination is essential in livestock farming and in companion animal ownership. Nucleic acid vaccines based on DNA or RNA provide an elegant alternative to those classical veterinary vaccines that have performed suboptimally. Recent advances in terms of rational design, safety, and efficacy have strengthened the position of nucleic acid vaccines in veterinary vaccinology. The present review focuses on replicon vaccines designed for veterinary use. Replicon vaccines are self-amplifying viral RNA sequences that, in addition to the sequence encoding the antigen of interest, contain all elements necessary for RNA replication. Vaccination results in high levels of in situ antigen expression and induction of potent immune responses. Both positive- and negative-stranded viruses have been used to construct replicons, and they can be delivered as RNA, DNA, or viral replicon particles. An introduction to the biology and the construction of different viral replicon vectors is given, and examples of veterinary replicon vaccine applications are discussed.