Pedro J. Sánchez‐Cordón

African swine fever (ASF) recently has spread beyond sub-Saharan Africa to the Trans-Caucasus region, parts of the Russian Federation and Eastern Europe. In this new epidemiological scenario, the disease has similarities, but also important differences, compared to the situation in Africa, including the substantial involvement of wild boar. A better understanding of this new situation will enable better control and prevent further spread of disease. In this article, these different scenarios are compared, and recent information on the pathogenesis of ASF virus strains, the immune response to infection and prospects for developing vaccines is presented. Knowledge gaps and the prospects for future control are discussed.

African swine fever virus (ASFV) encodes multiple copies of MGF360 and MGF530/505 gene families. These genes have been implicated in the modulation of the type I interferon (IFN) response. We investigated the effect of modulating the IFN response on virus attenuation and induction of protective immunity by deleting genes MGF360 (MGF360-10L, 11L, 12L, 13L, 14L) and MGF530/505 (MGF530/505-1R, 2R and 3R) and interrupting genes (MGF360-9L and MGF530/505-4R) in the genome of the virulent ASFV isolate Benin 97/1. Replication of this deletion mutant, BeninΔMGF, in porcine macrophages in vitro was similar to that of the parental virulent virus Benin 97/1 and the natural attenuated isolate OURT88/3, which has a similar deletion of MGF360 and 530/505 genes. Levels of IFN-β mRNA in macrophages infected with virulent Benin 97/1 isolate were barely detectable but high levels were detected in macrophages infected with OURT88/3 and intermediate levels in macrophages infected with BeninΔMGF. The data confirms that these MGF360 and MGF530/505 genes have roles in suppressing induction of type I IFN. Immunisation and boost of pigs with BeninΔMGF showed that the virus was attenuated and all pigs (5/5) were protected against challenge with a lethal dose of virulent Benin 97/1. A short transient fever was observed at day 5 or 6 post-immunisation but no other clinical signs. Following immunisation and boost with the OURT88/3 isolate 3 of 4 pigs were protected against challenge. Differences were observed in the cellular and antibody responses in pigs immunised with BeninΔMGF compared to OURT88/3. Deletion of IFN modulators is a promising route for construction of rationally attenuated ASFV candidate vaccine strains.

ABSTRACT Many of the approximately 165 proteins encoded by the African swine fever virus (ASFV) genome do not have significant similarity to known proteins and have not been studied experimentally. One such protein is DP148R. We showed that the DP148R gene is transcribed at early times postinfection. Deletion of this gene did not reduce virus replication in macrophages, showing that it is not essential for replication in these cells. However, deletion of this gene from a virulent isolate, Benin 97/1, producing the BeninΔDP148R virus, dramatically reduced the virulence of the virus in vivo . All pigs infected with the BeninΔDP148R virus survived infection, showing only transient mild clinical signs soon after immunization. Following challenge with the parental virulent virus, all pigs immunized by the intramuscular route (11/11) and all except one immunized by the intranasal route (5/6) survived. Mild or no clinical signs were observed after challenge. As expected, control nonimmune pigs developed signs of acute African swine fever (ASF). The virus genome and infectious virus were observed soon after immunization, coincident with the onset of clinical signs (∼10 6 genome copies or 50% tissue culture infective doses/ml). The levels of the virus genome declined over an extended period up to 60 days postimmunization. In contrast, infectious virus was no longer detectable by days 30 to 35. Gamma interferon (IFN-γ) was detected in serum between days 4 and 7 postimmunization, and IFN-γ-producing cells were detected in all pigs analyzed following stimulation of immune lymphocytes with whole virus. ASFV-specific antibodies were first detected from day 10 postimmunization. IMPORTANCE African swine fever (ASF) is endemic in Africa, parts of the Trans Caucasus, the Russian Federation, and several European countries. The lack of a vaccine hinders control. Many of the ASF virus genes lack similarity to known genes and have not been characterized. We have shown that one of these, DP148R, is transcribed early during virus replication in cells and can be deleted from the virus genome without reducing virus replication. The virus with the gene deletion, BeninΔDP148R, caused mild clinical signs in pigs and induced high levels of protection against challenge with the parental virulent virus. Therefore, deletion of this gene can provide a target for the rational development of vaccines.

African swine fever (ASF) is a lethal haemorrhagic disease of domestic pigs for which there is no vaccine. Strains of the virus with reduced virulence can provide protection against related virulent strains of ASFV, but protection is not 100% and there are concerns about the safety profile of such viruses. However, they provide a useful tool for understanding the immune response to ASFV and previous studies using the low virulent isolate OUR T88/3 have shown that CD8+ cells are crucial for protection. In order to develop a vaccine that stimulates an effective anti-ASFV T-cell response we need to know which of the >150 viral proteins are recognized by the cellular immune response. Therefore, we used a gamma interferon ELIspot assay to screen for viral proteins recognized by lymphocytes from ASF-immune pigs using peptides corresponding to 133 proteins predicted to be encoded by OUR T88/3. Eighteen antigens that were recognized by ASFV-specific lymphocytes were then incorporated into adenovirus and MVA vectors, which were used in immunization and challenge experiments in pigs. We present a systematic characterization of the cellular immune response to this devastating disease and identify proteins capable of inducing ASFV-specific cellular and humoral immune responses in pigs. Pools of viral vectors expressing these genes did not protect animals from severe disease, but did reduce viremia in a proportion of pigs following ASFV challenge.

Classical approaches to African swine fever virus (ASFV) vaccine development have not been successful; inactivated virus does not provide protection and use of live attenuated viruses generated by passage in tissue culture had a poor safety profile. Current African swine fever (ASF) vaccine research focuses on the development of modified live viruses by targeted gene deletion or subunit vaccines. The latter approach would be differentiation of vaccinated from infected animals (DIVA)-compliant, but information on which viral proteins to include in a subunit vaccine is lacking. Our previous work used DNA-prime/vaccinia-virus boost to screen 40 ASFV genes for immunogenicity, however this immunization regime did not protect animals after challenge. Here we describe the induction of both antigen and ASFV-specific antibody and cellular immune responses by different viral-vectored pools of antigens selected based on their immunogenicity in pigs. Immunization with one of these pools, comprising eight viral-vectored ASFV genes, protected 100% of pigs from fatal disease after challenge with a normally lethal dose of virulent ASFV. This data provide the basis for the further development of a subunit vaccine against this devastating disease.