When to get ASF vaccine?

African Swine Fever virus (ASFV) is the causal agent of a serious disease which affects domestic pigs and causes important economic losses to the affected countries. ASFV is the only DNA arbovirus known, and its perpetuation and transmission in Africa, where the virus remains endemic, involves a sylvatic cycle (i.e. occurring in the wild) between ticks and wild pigs that are resistant to the disease, becoming a continuous source of virus, therefore complicating its eradication.

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Why are ASF vaccines not available? To our understanding, there are two main reasons explaining the lack of available vaccines against ASF. First of all, the high complexity of the virus has complicated this issue. ASFV is a large double stranded DNA virus that encodes more than 150 different proteins, with the ASFV particle containing at least 50 proteins arranged in several layers. In comparison, Porcine Circovirus type 2 particles are composed by one only polypeptide, the cap protein.

Additionally, ASFV encodes multiple virulence factors allowing its replication in porcine macrophages and the concomitant evasion from the host immune response, therefore complicating the development of efficient antiviral strategies.

Second, little effort has been made up to now to obtain a safe and efficient vaccine against ASF, leading to the probable false perception that this was an impossible task to pursue.

Thus, compared with the deep knowledge that we have today about different aspects of the complex biology of ASFV, efforts made to obtain an efficient vaccine have usually been scarce and most of them were performed more than ten years ago. Three different strategies have been followed in the past:

1. Inactivated vaccines

Inactivated vaccines of ASFV were capable of inducing antibody responses that, however, did not translate to efficient protection.

2. Attenuated strains

In clear contrast, immunization of pigs with classically attenuated strains of ASFV (natural isolates or tissue culture adapted viruses) induced very solid protection against the homologous viral challenge. Safety issues made the application of these live-attenuated viruses as vaccines impossible, they have however provided us with the most useful data existing today about immune parameters involved in protection. Thus, both antibodies and cytotoxic specific CD8+ T-cells were demonstrated to play important roles in the protection afforded by live-attenuated vaccines.

2a. Antibodies and T-cells

Neither antibodies nor T-cells seemed to be able, by themselves, to confer complete and sterilizing protection, indicating that an ideal vaccine against ASFV should be able to confer both kinds of immune responses. While specific antibodies could more efficiently neutralize and/ or inhibit the virus particles found in suspension (blood and other corporal fluids); CD8+ T-cells (cytotoxic T-cells) would be able to recognize and destroy ASFV infected cells.

2b. Deletion of genes

Deletion of specific virulence genes by homologous recombination allowed the construction of live attenuated ASFV viruses, albeit several genes should be simultaneously deleted in order to comply with the minimum security issues requested for any commercial vaccine. This alternative will require further research on ASFV pathogenesis, allowing a more rational selection of the virulence factors to be eliminated from the virus in order to obtain the safest and most efficient recombinant vaccine.

2c. Replication deficient strains

An attractive alternative to the classical deletion of genes is the use of inducible viruses as vaccines, a strategy that is currently being explored by the group of Dr Salas, at the CBMSO in Madrid, Spain. The idea behind this technology is to develop replication deficient ASFV vaccine strains. So far, in vitro experiments have demonstrated that these cells produce, under restrictive conditions, ‘empty’ viral particles that lack the inner contents.

Such virus-like particles possess all the external domains (inner envelope and capsid) and can exit efficiently from the infected cell, but they are not infectious.

This strategy should comply with all the requisites for an ideal vaccine against ASFV. Again, further research is needed.

3. Subunit vaccines

In terms of safety, subunit vaccines should be the preferred choice. However, the complexity of ASFV influences the task of selecting the optimal antigens to be included in a vaccine. Several reports exist describing antigenic viral proteins, but little has been reported about their protective efficacy. Immunization with peptide ‘cocktails’ showed a slight delay in the mortality found after experimental infection, while vaccination with entire viral proteins yielded contradictory results. Work done in the mid-1990’s described the protective potential of three ASFV structural proteins: p54, p30 and hemagglutinin (HA), when expressed in a baculovirus system and administered without further purification.

4. Partial protection

Partial protection against ASFV lethal challenge was demonstrated after DNA immunization of pigs with a plasmid encoding only three ASFV antigens: p54, p30 and the extracellular domain of the hemagglutinin (sHA), fused to ubiquitin. This plasmid was called pCMV-UbsHAPQ. As expected, the protection was afforded in the absence of antibodies, correlating with the expansion of CD8 T-cells that specifically recognized two peptides of nine aminoacids from the sHA, one of the three antigens encoded by the vaccine. Preliminary immunization experiments with these two synthetic peptides confirmed their protective capabilities. The identification for the first time of specific protective CD8 T-cell epitopes not only confirmed the relevance of this kind of T-cell response in protection against ASFV but also opened the possibility of generating peptide-based vaccines using more potent expression vectors (see below).