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Identification of in vitro and in vivo correlates for protection of pigs by candidate ASFV vaccines - SE1512

Description
African swine fever (ASF) is a highly contagious fatal acute haemorrhagic viral disease of pigs which causes major economic losses. ASF virus (ASFV) is currently endemic in many sub-Saharan African countries and in Madagascar and Sardinia. The virus was introduced to the Iberian peninsula in the 1960’s, to South America the Caribbean and other European countries during the 1970's. Disease has been erradicted from all countries outside Africa apart from Sardinia, where in 2004 it caused nearly 250 outbreaks and the loss of 12000 pigs. There is no vaccine and disease control relies on implementing movement restrictions and slaughter policies which is difficult to achieve in countries lacking a good infrastructure. The virus persists for long periods in pigs which recover from infection, its natural hosts, warthogs and bushpigs and in species of soft ticks (Ornithodoros moubata or erraticus) which inhabit warthog burrows and domestic pig premises. These can act as reservoirs for infecting healthy pigs. The virus can also persist in pork products and pigs may become infected by eating contaminated pork. A vaccine is urgently needed to aid disease control strategies.

To date, the use of empirical methods have failed to produce an effective vaccine, partly due to the complexity of the virus, the acute fatal nature of the disease and a poor understanding of the what types of immune response protects against infection. However, as a result of previous studies, we are now in a position to develop a safe and effective vaccine against ASFV. In the past decade we have been characterising a particular ASFV isolate called OURT88/3. This virus is unique as it lacks a number of characteristics observed in most other ASFV isolates. For example, the OURT88/3 strain lacks a number of genes observed in pathogenic ASFVs, it does not cause severe disease in the majority of pigs and is able to induce protection against closely related pathogenic ASFV infection. This observation clearly indicates that an ASFV vaccine is possible. However, the OURT88/3 strain itself is not suitable as a vaccine as it still retains the ability to cause disease in some pigs. Nevertheless, the OURT88/3 strain can be used as a model for further ASFV vaccine development. In another project ASFV vaccine candidates are being generated by sequential deletion of virus genes involved in causing disease or in enabling the virus to escape the pig’s immune response. ASFV encodes many proteins which, although not essential for replication in cells, interfere with the host's immune response and help the virus to avoid being eliminated from infected animals. Our understanding of these evasion proteins and of proteins involved in virus virulence has increased dramatically in recent years and we are now in a strong position to rationally construct ASFV vaccines by the sequential deletion of genes encoding these proteins from the virus genome. In this way we can construct candidate attenuated vaccine strains which do not cause disease but induce a strong protective immune response.

One objective of this project is to use a series of cell culture tests in order to predict the virulence of ASFV candidate vaccines and their ability to induce protective immunity against homologous and heterologous virus isolates. These laboratory studies will help to replace and reduce the number of animals used in research and will be applicable to development of vaccines for other pig diseases. The tests we will use will include analysis of the patterns of gene expression and activation of key signalling pathways in ASFV infected macrophages. This will help us to predict the effectiveness of the host's reponse to candidate vaccine strains. In addition we will investigate the ability of different ASFVs to induce non-specific B-cell proliferation; to inhibit T cell prolieration; to effect the function of natural killer (NK) cells, which are an important first line of defence against virus infection; and for their effects on dendritic cells (DC), which are important in initiating immune responses so that lymphocytes can recognise and destroy infected cells. Our preliminary observations indicate that virulent ASFV infects DCs and inhibits their function. The results from these studies will aid the identification of in vitro correlates of virus pathogenicity and activation of host defences and will therefore assist in the selection ASFV vaccine candidates for testing by inoculation in pigs.

A second objective is to improve our understanding of the mechanisms of and to identify correlates of protective immunity. This would mean we could predict which vaccinated pigs would be protected. These assay would also be applicable to other pig diseases. Although antibodies are important in protection, they are unable to protect pigs alone and at least one type of white blood cell (lymphocyte) is also needed to give complete protection against ASFV. Pig lymphocytes are complex and differ from those of humans in some respects. We propose to continue to characterise the role of various lymphocyte subtypes in protection against ASFV. These lymphocytes are able to kill ASFV-infected cells and we intend to investigate, in the laboratory, the ability of lymphocytes obtained from pigs immune to one strain of ASFV to kill cells infected with other strains of ASFV.

A third objective will be to improve the methods for culture of ASFV in cells. At the moment most strains of the virus have to be grown in cells which are freshly prepared from pigs. Development of continuously cultured cell lines which can support high level replication of all isolates including the vaccine strains is extremely important for the eventual licensing of vaccines as well as to facilitate the production of candidiate vaccines. These cell lines would also be useful in diagnostic tests. We will attempt to make continuous cell lines which support high level replication of ASFV by inserting additional genes into an existing pig cell line. The genes we will insert include two which interfere with the host's antiviral response and one for a putative cell surface receptor for ASFV.
Objective
1) To develop in vitro assays that will aid the selection of attenuated ASF vaccine candidates.

2) To identify protective immune response(s) induced by non-pathogenic ASF viruses.

3) To construct porcine cell lines expressing inhibitors of interferon and putative receptors for ASFV and test these for their ability to support high level replication of candidate attenuated African swine fever virus vaccines
Project Documents
• EVID4 - Final project report : sid5 DEFRA SE1512   (614k)
Time-Scale and Cost
From: 2006

To: 2010

Cost: £973,346
Contractor / Funded Organisations
Institute for Animal Health (BBSRC)
Keywords
African Swine Fever              
Animal Diseases              
Animal Health              
Biotechnology              
GM Non-Food              
Immunity              
Plants and Animals              
Vaccines              
Fields of Study
Animal Health