Bovine herpesvirus 4 as a vaccine platform for African swine fever virus antigens in pigs

African swine fever is a viral haemorrhagic disease of domestic swine that causes mortality rates of up to 100% in affected herds. African swine fever is endemic in much of Africa south of the Sahara and there was an outbreak in the Georgian port of Poti in 2007. From here the disease spread west through eastern and central Europe and east into eastern and south-eastern Asia killing millions of animals and threatening global food security. The virus threatens endangered species babirusa and pygmy hogs, as well as species of pigs native to Asia and Oceania. The epidemiological situation in Europe is characterised by a substantial number of outbreaks in free-roaming wild boar and backyard farms with low biosecurity. The disease is continuing to spread despite the imposition of movement restrictions and African swine fever is unlikely to be eradicated from Europe without a vaccine. Classical swine fever, a similar disease caused by a different virus, was eradicated from European wild boar through a targeted vaccination campaign, however there is no licensed vaccine to African swine fever and control is entirely dependent on rapid diagnosis and slaughter. A positive case of African swine fever in the UK would lead to movement restrictions in a 10 km zone around the farm and the culling of infected herds. As well as the serious effects on animal welfare and distress caused to farmers, the value of UK pig and pork exports was £623,154,225 in 2022 which would likely be lost in the advent of an outbreak of African swine fever. The effect on domestic consumption of pork is difficult to assess, but a negative impact due to a drop in consumer confidence is possible.

Traditional methods of generating vaccines have not been successful at protecting pigs against African swine fever. Inactivated virus provides no protection whatsoever, even in combination with modern adjuvants. Attenuated strains of the virus (African swine fever virus), generated by repeated passage through tissue culture, were deployed in the Iberian Peninsula during the early 1960s but had a number of unacceptable side effects including a chronic form of the disease. More recently, targeted genetic modification of the virus to delete genes associated with immune evasion has led to the development of a second generation of live attenuated virus vaccines. These have a much more promising safety profile than the first generation vaccines and extensive field trials with such a vaccine are currently underway in Vietnam, however this vaccine transmits in the field and it is not possible to distinguish vaccinated animals from those that have been naturally infected. Furthermore, mechanisms behind reversion to virulence are not understood and therefore, alternative, safer vaccines are still desperately needed.

Progress has been made in the development of an African swine fever subunit vaccine which would eliminate the safety concerns around deploying an attenuated strain of a normally lethal virus in the field. We have shown that a combination of eight African swine fever virus antigens can prevent severe disease against historical isolates of African swine fever virus. However, further development of subunit vaccines is limited by our poor understanding of the protective immune response and in particular the role of local immunity. We propose to determine sites of early virus replication in domestic pigs infected with the current Eurasian strain of African swine fever virus as well as the effect of infection on local immune responses. Bovine herpesvirus 4 represents an attractive vaccine platform for the development of an African swine fever subunit vaccine due to the induction of a particular type of immune response by herpesviruses in other mammals. Therefore, we will evaluate bovine herpesvirus 4 expressing model African swine fever virus genes to test their ability to induce specific immune responses in pigs as well as their ability to protect animals from disease.

African swine fever virus (ASFV) causes a lethal disease in domestic pigs which is endemic in low and middle income countries in Africa south of the Sahara and has become established in wild boar in Eastern and Central Europe. The disease has also spread across eastern and south-eastern Asia since 2018. Control is limited by the lack of a safe vaccine which will be essential to limit the spread of the disease in wild populations of animals and hence prevent spill over into domestic pigs.

Historical data suggests that natural infection with ASFV is primarily initiated in lymph nodes that drain oronasal passages, however this has not been confirmed after infection with the current Eurasian strain of the virus and the effects of natural infection on local immune responses have not been defined. Herpesviruses induce effective mucosal immune responses after intramuscular inoculation and The Vaccine Group have developed bovine herpesvirus 4 (BoHV-4) as a vector for use in domestic pigs. As well as examining the local and systemic immune responses to infection with ASFV we will test cellular and humoral immune responses to three well characterised ASFV genes vectored using BoHV-4. Cellular immune responses will be measured using ELISpot and flow cytometry panels and antibody responses using ELISA, as well as LACA to study the responses to individual proteins.

We hypothesize that herpesvirus vectored antigens will induce immune responses that will be advantageous in protection against ASFV and therefore we will test this in an immunisation and challenge model that represents natural infection. We propose to use pigs inbred at the major histocompatibility complex which will enable a detailed comparative analysis of the immune responses to vaccination and infection.