Rational vaccine design as an approach to protect against PMCV infection in salmon

Totivirus represent en group of viruses that typically infect protozoa, fungi, and some vertebrate species. Salmon are infected with a virus called piscine myocarditis virus (PMCV), which resembles totivirus and has acquired the ability to spread between hosts, a capability lacking in most totiviruses. PMCV infection is a chronic infection in salmon and causes a severe heart disease called cardiomyopathy syndrome (CMS), where the wall of the atrium becomes so fragile from the damage caused by the virus that it can rupture when the fish is exposed to physical stress/handling. The goal of the project is to test prototype vaccines against PMCV infection in Atlantic salmon, based on knowledge of the virus's surface structure. This could form the basis for developing vaccines that specifically target various structures on the surface that are important for successful infection and/or the initial stages of viral infection. So far, it has been challenging to obtain sufficient amounts of purified virus, and for this reason, we have used IPN virus as a model for structural studies of double-stranded RNA viruses. The studies have been successful, and detailed information has been obtained about surface structures in IPNV, which are of comparative interest to other dsRNA viruses. Another part of the project has focused on the development of a hybrid virus, where parts of PMCV are expressed during the replication of VHS virus. This can be used to study the effects of vaccination and subsequent infection, and the hybrid virus will be tested as a pseudovirus, with a focus on characterizing neutralizing antibodies after infection or vaccination. So far, three different variants of the hybrid virus have been developed and are undergoing testing.

Totiviridae viruses infect protozoa and fungi while a handful of metazoan totivirus-like viruses infect vertebrates and invertebrates. Salmon piscine myocarditis virus (PMCV) is a totivirus-like viruses, and like dsRNA viruses, metazoan totivirus-like viruses have acquired capsid structures and/or bystander proteins that contribute to extracellular transmission and intra-particle genome transcription. PMCV is the causative agent of cardiomyopathy syndrome (CMS) of Atlantic salmon. PMCV has acquired a unique strategy to adapt to the metazoan host, including two fundamental functions different from the protozoan totiviruses, i.e., extracellular transmission and intra-particle genome transcription. Currently no prophylactic or treatment options exist for PMCV infection in salmon, and our hypothesis is that a better understanding of the PMCV structure will facilitate vaccine design. Further, can protection against infection and/or disease come from antibodies alone or is cell mediated immunity also required. Topological complementarity of the surface contours of the virus particle and the antibody variable domains decides if the antigen is recognized (WP1). Structure-based design relies on which specific sites on the surface protein to target, and we begin with identifying pathogen-specific antibodies with neutralizing properties (WP2). Peptide scanning using sera from survivors of infection will pinpoint domains important for binding, and with structure studies of virus particle and ORF3 protein, this will guide us towards optimal vaccine design. This will be used for recombinant protein design of ORF1 and ORF3 (WP3) encoded viral proteins, and tests for ability to protect against infection/pathology. Next, to explore involvement of cell mediated immunity, a vector vaccine approach is used (WP4) based on attenuated strain of VHS virus expressing ORF1 or ORF3 subdomains. Effects are assessed using immunisation and challenge followed by assessment of pathogenic events.