Characterisation of porcine cytomegalovirus and development as a vaccine vector for classical swine fever virus

Classical swine fever (CSF) is a highly infectious viral disease found in domestic pigs and wild boar populations. Outbreaks have a large economic cost for the swine industry and cause significant morbidity and mortality in pigs. Only one compatible vaccine that allows the differentiation between infected and vaccinated animals (DIVA), Suvaxyn® CSF Marker vaccine, is commercially available. However, due to e.g. trading issues caused by vaccination of animals this vaccine is not used prophylactically and is only used in emergency situations to prevent spread of CSFV to wider areas. An urgent need exists for development of a safe, effective, DIVA compatible vaccine. This study set out to address this need by taking the initial steps towards developing porcine cytomegalovirus (PCMV), as a vaccine platform. CMV-based vaccines have been shown to induce strong, durable immune responses against heterologous antigens, and also have the potential to be self-disseminating, which would facilitate vaccination of inaccessible animal populations such as wild boar. The aim of the present study was to further characterise the in vitro growth characteristics of a recent PCMV isolate. Our results show PCMV to have a slow growth, and low titre phenotype. As a further step towards cloning of PCMV as an infectious BAC, recombinant BAC shuttle vectors were constructed and then characterised. Initial experiments using these shuttle vectors combined with homologous recombination and CRISPR/Cas9 technology to insert the BAC cassette within the PCMV genome are also described. In summary, the cloning of the shuttle vectors was successful but the results around PCMV replication kinetics provide preliminary evidence that there are difficulties to overcome before PCMV can be developed as a vaccine vector platform

An attenuated herpesvirus vectored vaccine candidate induces T-cell responses against highly conserved porcine reproductive and respiratory syndrome virus M and NSP5 proteins that are unable to control infection

Porcine reproductive and respiratory syndrome virus (PRRSV) remains a leading cause of economic loss in pig farming worldwide. Existing commercial vaccines, all based on modified live or inactivated PRRSV, fail to provide effective immunity against the highly diverse circulating strains of both PRRSV-1 and PRRSV-2. Therefore, there is an urgent need to develop more effective and broadly active PRRSV vaccines. In the absence of neutralizing antibodies, T cells are thought to play a central role in controlling PRRSV infection. Herpesvirus-based vectors are novel vaccine platforms capable of inducing high levels of T cells against encoded heterologous antigens. Therefore, the aim of this study was to assess the immunogenicity and efficacy of an attenuated herpesvirus-based vector (bovine herpesvirus-4; BoHV-4) expressing a fusion protein comprising two well-characterized PRRSV-1 T-cell antigens (M and NSP5). Prime-boost immunization of pigs with BoHV-4 expressing the M and NSP5 fusion protein (vector designated BoHV-4-M-NSP5) induced strong IFN-γ responses, as assessed by ELISpot assays of peripheral blood mononuclear cells (PBMC) stimulated with a pool of peptides representing PRRSV-1 M and NSP5. The responses were closely mirrored by spontaneous IFN-γ release from unstimulated cells, albeit at lower levels. A lower frequency of M and NSP5 specific IFN-γ responding cells was induced following a single dose of BoHV-4-M-NSP5 vector. Restimulation using M and NSP5 peptides from PRRSV-2 demonstrated a high level of cross-reactivity. Vaccination with BoHV-4-M-NSP5 did not affect viral loads in either the blood or lungs following challenge with the two heterologous PRRSV-1 strains. However, the BoHV-4-M-NSP5 prime-boost vaccination showed a marked trend toward reduced lung pathology following PRRSV-1 challenge. The limited effect of T cells on PRRSV-1 viral load was further examined by analyzing local and circulating T-cell responses using intracellular cytokine staining and proliferation assays. The results from this study suggest that vaccine-primed T-cell responses may have helped in the control of PRRSV-1 associated tissue damage, but had a minimal, if any, effect on controlling PRRSV-1 viral loads. Together, these results indicate that future efforts to develop effective PRRSV vaccines should focus on achieving a balanced T-cell and antibody response