DNA vaccination partially protects against African swine fever virus lethal challenge in the absence of antibodies

Roles of African Swine Fever Virus Structural Proteins in Viral Infection

African swine fever virus (ASFV) is a large, double-stranded DNA virus and the sole member of the Asfarviridae family. ASFV infects domestic pigs, wild boars, warthogs, and bush pigs, as well as soft ticks (Ornithodoros erraticus), which likely act as a vector. The major target is swine monocyte-macrophage cells. The virus can cause high fever, haemorrhagic lesions, cyanosis, anorexia, and even fatalities in domestic pigs. Currently, there is no vaccine and effective disease control strategies against its spread are culling infected pigs and maintaining high biosecurity standards. African swine fever (ASF) spread to Europe from Africa in the middle of the 20^th century, and later also to South America and the Caribbean. Since then, ASF has spread more widely and thus is still a great challenge for swine breeding. The genome of ASFV ranges in length from about 170 to 193 kbp depending on the isolate and contains between 150 and 167 open reading frames (ORFs). The ASFV genome encodes 150 to 200 proteins, around 50 of them structural. The roles of virus structural proteins in viral infection have been described. These proteins, such as pp220, pp62, p72, p54, p30, and CD2v, serve as the major component of virus particles and have roles in attachment, entry, and replication. All studies on ASFV proteins lay a good foundation upon which to clarify the infection mechanism and develop vaccines and diagnosis methods. In this paper, the roles of ASFV structural proteins in viral infection are reviewed.

African Swine Fever Virus Biology and Vaccine Approaches

African swine fever (ASF) is an acute and often fatal disease affecting domestic pigs and wild boar, with severe economic consequences for affected countries. ASF is endemic in sub-Saharan Africa and the island of Sardinia, Italy. Since 2007, the virus emerged in the republic of Georgia, and since then spread throughout the Caucasus region and Russia. Outbreaks have also been reported in Belarus, Ukraine, Lithuania, Latvia, Estonia, Romania, Moldova, Czech Republic, and Poland, threatening neighboring West European countries. The causative agent, the African swine fever virus (ASFV), is a large, enveloped, double-stranded DNA virus that enters the cell by macropinocytosis and a clathrin-dependent mechanism. African Swine Fever Virus is able to interfere with various cellular signaling pathways resulting in immunomodulation, thus making the development of an efficacious vaccine very challenging. Inactivated preparations of African Swine Fever Virus do not confer protection, and the role of antibodies in protection remains unclear. The use of live-attenuated vaccines, although rendering suitable levels of protection, presents difficulties due to safety and side effects in the vaccinated animals. Several African Swine Fever Virus proteins have been reported to induce neutralizing antibodies in immunized pigs, and vaccination strategies based on DNA vaccines and recombinant proteins have also been explored, however, without being very successful. The complexity of the virus particle and the ability of the virus to modulate host immune responses are most likely the reason for this failure. Furthermore, no permanent cell lines able to sustain productive virus infection by both virulent and naturally attenuated African Swine Fever Virus strains exist so far, thus impairing basic research and the commercial production of attenuated vaccine candidates.

BA71ΔCD2: a New Recombinant Live Attenuated African Swine Fever Virus with Cross-Protective Capabilities

African swine fever is a highly contagious viral disease of mandatory declaration to the World Organization for Animal Health (OIE). The lack of available vaccines makes its control difficult; thus, African swine fever virus (ASFV) represents a major threat to the swine industry. Inactivated vaccines do not confer solid protection against ASFV. Conversely, live attenuated viruses (LAV), either naturally isolated or obtained by genetic manipulation, have demonstrated reliable protection against homologous ASFV strains, although little or no protection has been demonstrated against heterologous viruses. Safety concerns are a major issue for the use of ASFV attenuated vaccine candidates and have hampered their implementation in the field so far. While trying to develop safer and efficient ASFV vaccines, we found that the deletion of the viral CD2v (EP402R) gene highly attenuated the virulent BA71 strain in vivo. Inoculation of pigs with the deletion mutant virus BA71ΔCD2 conferred protection not only against lethal challenge with the parental BA71 but also against the heterologous E75 (both genotype I strains). The protection induced was dose dependent, and the cross-protection observed in vivo correlated with the ability of BA71ΔCD2 to induce specific CD8⁺ T cells capable of recognizing both BA71 and E75 viruses in vitro. Interestingly, 100% of the pigs immunized with BA71ΔCD2 also survived lethal challenge with Georgia 2007/1, the genotype II strain of ASFV currently circulating in continental Europe. These results open new avenues to design ASFV cross-protective vaccines, essential to fight ASFV in areas where the virus is endemic and where multiple viruses are circulating.

IMPORTANCE African swine fever virus (ASFV) remains enzootic in most countries of Sub-Saharan Africa, today representing a major threat for the development of their swine industry. The uncontrolled presence of ASFV has favored its periodic exportation to other countries, the last event being in Georgia in 2007. Since then, ASFV has spread toward neighboring countries, reaching the European Union's east border in 2014. The lack of available vaccines against ASFV makes its control difficult; so far, only live attenuated viruses have demonstrated solid protection against homologous experimental challenges, but they have failed at inducing solid cross-protective immunity against heterologous viruses. Here we describe a new LAV candidate with unique cross-protective abilities: BA71ΔCD2. Inoculation of BA71ΔCD2 protected pigs not only against experimental challenge with BA71, the virulent parental strain, but also against heterologous viruses, including Georgia 2007/1, the genotype II strain of ASFV currently circulating in Eastern Europe.

Approaches and Perspectives for Development of African Swine Fever Virus Vaccines

African swine fever (ASF) is a complex disease of swine, caused by a large DNA virus belonging to the family Asfarviridae. The disease shows variable clinical signs, with high case fatality rates, up to 100%, in the acute forms. ASF is currently present in Africa and Europe where it circulates in different scenarios causing a high socio-economic impact. In most affected regions, control has not been effective in part due to lack of a vaccine. The availability of an effective and safe ASFV vaccines would support and enforce control-eradication strategies. Therefore, work leading to the rational development of protective ASF vaccines is a high priority. Several factors have hindered vaccine development, including the complexity of the ASF virus particle and the large number of proteins encoded by its genome. Many of these virus proteins inhibit the host's immune system thus facilitating virus replication and persistence. We review previous work aimed at understanding ASFV-host interactions, including mechanisms of protective immunity, and approaches for vaccine development. These include live attenuated vaccines, and "subunit" vaccines, based on DNA, proteins, or virus vectors. In the shorter to medium term, live attenuated vaccines are the most promising and best positioned candidates. Gaps and future research directions are evaluated.

Gaps in African swine fever: Analysis and priorities

African swine fever (ASF) causes greater sanitary, social and economic impacts on swine herds than many other swine diseases. Although ASF was first described in 1921 and it has affected more than fifty countries in Africa, Europe and South America, several key issues about its pathogenesis, immune evasion and epidemiology remain uncertain. This article reviews the main characteristics of the causative virus, its molecular epidemiology, natural hosts, clinical features, epidemiology and control worldwide. It also identifies and prioritizes gaps in ASF from a horizontal point of view encompassing fields including molecular biology, epidemiology, prevention, diagnosis and vaccine development. The purpose of this review is to promote ASF research and enhance its control.

Adenovirus-vectored novel African Swine Fever Virus antigens elicit robust immune responses in swine

African Swine Fever Virus (ASFV) is a high-consequence transboundary animal pathogen that often causes hemorrhagic disease in swine with a case fatality rate close to 100%. Lack of treatment or vaccine for the disease makes it imperative that safe and efficacious vaccines are developed to safeguard the swine industry. In this study, we evaluated the immunogenicity of seven adenovirus-vectored novel ASFV antigens, namely A151R, B119L, B602L, EP402RΔPRR, B438L, K205R and A104R. Immunization of commercial swine with a cocktail of the recombinant adenoviruses formulated in adjuvant primed strong ASFV antigen-specific IgG responses that underwent rapid recall upon boost. Notably, most vaccinees mounted robust IgG responses against all the antigens in the cocktail. Most importantly and relevant to vaccine development, the induced antibodies recognized viral proteins from Georgia 2007/1 ASFV-infected cells by IFA and by western blot analysis. The recombinant adenovirus cocktail also induced ASFV-specific IFN-γ-secreting cells that were recalled upon boosting. Evaluation of local and systemic effects of the recombinant adenovirus cocktail post-priming and post-boosting in the immunized animals showed that the immunogen was well tolerated and no serious negative effects were observed. Taken together, these outcomes showed that the adenovirus-vectored novel ASFV antigen cocktail was capable of safely inducing strong antibody and IFN-γ⁺ cell responses in commercial swine. The data will be used for selection of antigens for inclusion in a multi-antigen prototype vaccine to be evaluated for protective efficacy.

Different routes and doses influence protection in pigs immunised with the naturally attenuated African swine fever virus isolate OURT88/3

This study compares different combinations of doses and routes of immunisation of pigs with low virulent African swine fever virus (ASFV) genotype I isolate OURT88/3, including the intramuscular and intranasal route, the latter not previously tested. Intranasal immunisations with low and moderate doses (10³ and 10⁴ TCID₅₀) of OURT88/3 provided complete protection (100%) against challenge with virulent genotype I OURT88/1 isolate. Only mild and transient clinical reactions were observed in protected pigs. Transient moderate virus genome levels were detected in blood samples after challenge that decreased, but persisted until the end of the experiment in some animals. In contrast, pigs immunised intramuscularly with low and moderate doses (10³ and 10⁴ TCID₅₀) displayed lower percentages of protection (50-66%), and low or undetectable levels of virus genome were detected in blood samples throughout the study. In addition, clinical courses observed in protected pigs were asymptomatic. In pigs that were not protected and developed acute ASF, an exacerbated increase of IL-10 sometimes accompanied by an increase of IFNγ was observed before euthanasia. These results showed that factors including delivery route and dose determine the outcome of immunisation with the naturally attenuated isolate OURT88/3.

Safety and immunogenicity of mammalian cell derived and Modified Vaccinia Ankara vectored African swine fever subunit antigens in swine

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Reverse vaccinology was applied to identify and rank ASFV immunogenic candidates .

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Selected ASFV immunogenic candidate proteins were expressed in HEK-293 mammalian cells and MVA constructs .

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Immunizations with antigens purified from HEK-293 cells and MVA constructs in swine were safe .

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Immunizations with selected antigens induced ASFV-specific antibodies and T-cell responses in swine.

A reverse vaccinology system, Vaxign, was used to identify and select a subset of five African Swine Fever (ASF) antigens that were successfully purified from human embryonic kidney 293 (HEK) cells and produced in Modified vaccinia virus Ankara (MVA) viral vectors. Three HEK-purified antigens [B646L (p72), E183L (p54), and O61R (p12)], and three MVA-vectored antigens [B646L, EP153R, and EP402R (CD2v)] were evaluated using a prime-boost immunization regimen swine safety and immunogenicity study. Antibody responses were detected in pigs following prime-boost immunization four weeks apart with the HEK-293-purified p72, p54, and p12 antigens. Notably, sera from the vaccinees were positive by immunofluorescence on ASFV (Georgia 2007/1)-infected primary macrophages. Although MVA-vectored p72, CD2v, and EP153R failed to induce antibody responses, interferon-gamma (IFN-γ⁺) spot forming cell responses against all three antigens were detected one week post-boost. The highest IFN-γ⁺ spot forming cell responses were detected against p72 in pigs primed with MVA-p72 and boosted with the recombinant p72. Antigen-specific (p12, p72, CD2v, and EP153R) T-cell proliferative responses were also detected post-boost. Collectively, these results are the first demonstration that ASFV subunit antigens purified from mammalian cells or expressed in MVA vectors are safe and can induce ASFV-specific antibody and T-cell responses following a prime-boost immunization regimen in swine.

Simultaneous Deletion of the 9GL and UK Genes from the African Swine Fever Virus Georgia 2007 Isolate Offers Increased Safety and Protection against Homologous Challenge

African swine fever virus (ASFV) is the etiological agent of a contagious and often lethal viral disease of domestic pigs that has significant economic consequences for the swine industry. The control of African swine fever (ASF) has been hampered by the unavailability of vaccines. Successful experimental vaccines have been derived from naturally occurring, cell culture-adapted, or genetically modified live attenuated ASFV. Recombinant viruses harboring engineered deletions of specific virulence-associated genes induce solid protection against challenge with parental viruses. Deletion of the 9GL (B119L) gene in the highly virulent ASFV isolates Malawi Lil-20/1 (Mal) and Pretoriuskop/96/4 (Δ9GL viruses) resulted in complete protection when challenged with parental isolates. When similar deletions were created within the ASFV Georgia 2007 (ASFV-G) genome, attenuation was achieved but the protective and lethal doses were too similar. To enhance attenuation of ASFV-G, we deleted another gene, UK (DP96R), which was previously shown to be involved in attenuation of the ASFV E70 isolate. Here, we report the construction of a double-gene-deletion recombinant virus, ASFV-G-Δ9GL/ΔUK. When administered intramuscularly (i.m.) to swine, there was no induction of disease, even at high doses (10⁶ HAD₅₀). Importantly, animals infected with 10⁴ 50% hemadsorbing doses (HAD₅₀) of ASFV-G-Δ9GL/ΔUK were protected as early as 14 days postinoculation when challenged with ASFV-G. The presence of protection correlates with the appearance of serum anti-ASFV antibodies, but not with virus-specific circulating ASFV-specific gamma interferon (IFN-γ)-producing cells. ASFV-G-Δ9GL/ΔUK is the first rationally designed experimental ASFV vaccine that protects against the highly virulent ASFV Georgia 2007 isolate as early as 2 weeks postvaccination.

IMPORTANCE

Currently, there is no commercially available vaccine against African swine fever. Outbreaks of the disease are devastating to the swine industry and are caused by circulating strains of African swine fever virus. Here, we report a putative vaccine derived from a currently circulating strain but containing two deletions in two separate areas of the virus, allowing increased safety. Using this genetically modified virus, we were able to vaccinate swine and protect them from developing ASF. We were able to achieve protection from disease as early as 2 weeks after vaccination, even when the pigs were exposed to a higher than normal concentration of ASFV.

Induction of Robust Immune Responses in Swine by Using a Cocktail of Adenovirus-Vectored African Swine Fever Virus Antigens

The African swine fever virus (ASFV) causes a fatal hemorrhagic disease in domestic swine, and at present no treatment or vaccine is available. Natural and gene-deleted, live attenuated strains protect against closely related virulent strains; however, they are yet to be deployed and evaluated in the field to rule out chronic persistence and a potential for reversion to virulence. Previous studies suggest that antibodies play a role in protection, but induction of cytotoxic T lymphocytes (CTLs) could be the key to complete protection. Hence, generation of an efficacious subunit vaccine depends on identification of CTL targets along with a suitable delivery method that will elicit effector CTLs capable of eliminating ASFV-infected host cells and confer long-term protection. To this end, we evaluated the safety and immunogenicity of an adenovirus-vectored ASFV (Ad-ASFV) multiantigen cocktail formulated in two different adjuvants and at two immunizing doses in swine. Immunization with the cocktail rapidly induced unprecedented ASFV antigen-specific antibody and cellular immune responses against all of the antigens. The robust antibody responses underwent rapid isotype switching within 1 week postpriming, steadily increased over a 2-month period, and underwent rapid recall upon boost. Importantly, the primed antibodies strongly recognized the parental ASFV (Georgia 2007/1) by indirect fluorescence antibody (IFA) assay and Western blotting. Significant antigen-specific gamma interferon-positive (IFN-γ⁺) responses were detected postpriming and postboosting. Furthermore, this study is the first to demonstrate induction of ASFV antigen-specific CTL responses in commercial swine using Ad-ASFV multiantigens. The relevance of the induced immune responses in regard to protection needs to be evaluated in a challenge study.

Association of the Host Immune Response with Protection Using a Live Attenuated African Swine Fever Virus Model

African swine fever (ASF) is a lethal hemorrhagic disease of swine caused by a double-stranded DNA virus, ASF virus (ASFV). There is no vaccine to prevent the disease and current control measures are limited to culling and restricting animal movement. Swine infected with attenuated strains are protected against challenge with a homologous virulent virus, but there is limited knowledge of the host immune mechanisms generating that protection. Swine infected with Pretoriuskop/96/4 (Pret4) virus develop a fatal severe disease, while a derivative strain lacking virulence-associated gene 9GL (Pret4Δ9GL virus) is completely attenuated. Swine infected with Pret4Δ9GL virus and challenged with the virulent parental virus at 7, 10, 14, 21, and 28 days post infection (dpi) showed a progressive acquisition of protection (from 40% at 7 dpi to 80% at 21 and 28 dpi). This animal model was used to associate the presence of host immune response (ASFV-specific antibody and interferon (IFN)-γ responses, or specific cytokine profiles) and protection against challenge. With the exception of ASFV-specific antibodies in survivors challenged at 21 and 28 dpi, no association between the parameters assessed and protection could be established. These results, encompassing data from 65 immunized swine, underscore the complexity of the system under study, suggesting that protection relies on the concurrence of different host immune mechanisms.

Challenges for African swine fever vaccine development-"… perhaps the end of the beginning."

African swine fever (ASF), an acute, viral hemorrhagic disease in domestic swine with mortality rates approaching 100%, is arguably the most significant emerging disease threat for the swine industry worldwide. Devastating ASF outbreaks and continuing epidemic in the Caucasus region and Russia (2007-to date) highlight significance of this disease threat. There is no vaccine for ASF, thus leaving animal slaughter the only effective disease control option. It is clear, however, that vaccination is possible since protection against reinfection with the homologous strain of African swine fever virus (ASFV) has been clearly demonstrated. Vaccine development has been hindered by large gaps in knowledge concerning ASFV infection and immunity, the extent of ASFV strain variation in nature and the identification of viral proteins (protective antigens) responsible for inducing protective immune responses in the pig. This review focuses on the challenges surrounding ASF vaccine design and development, with an emphasis on existing knowledge gaps.

A Mathematical Model that Simulates Control Options for African Swine Fever Virus (ASFV)

A stochastic model designed to simulate transmission dynamics of African swine fever virus (ASFV) in a free-ranging pig population under various intervention scenarios is presented. The model was used to assess the relative impact of the timing of the implementation of different control strategies on disease-related mortality. The implementation of biosecurity measures was simulated through incorporation of a decay function on the transmission rate. The model predicts that biosecurity measures implemented within 14 days of the onset of an epidemic can avert up to 74% of pig deaths due to ASF while hypothetical vaccines that confer 70% immunity when deployed prior to day 14 of the epidemic could avert 65% of pig deaths. When the two control measures are combined, the model predicts that 91% of the pigs that would have otherwise succumbed to the disease if no intervention was implemented would be saved. However, if the combined interventions are delayed (defined as implementation from > 60 days) only 30% of ASF-related deaths would be averted. In the absence of vaccines against ASF, we recommend early implementation of enhanced biosecurity measures. Active surveillance and use of pen-side diagnostic assays, preferably linked to rapid dissemination of this data to veterinary authorities through mobile phone technology platforms are essential for rapid detection and confirmation of ASF outbreaks. This prediction, although it may seem intuitive, rationally confirms the importance of early intervention in managing ASF epidemics. The modelling approach is particularly valuable in that it determines an optimal timing for implementation of interventions in controlling ASF outbreaks.

African swine fever virus serotype-specific proteins are significant protective antigens for African swine fever

African swine fever (ASF) is an emerging disease threat for the swine industry worldwide. No ASF vaccine is available and progress is hindered by lack of knowledge concerning the extent of ASFV strain diversity and the viral antigens conferring type-specific protective immunity in pigs. Available data from vaccination/challenge experiments in pigs indicate that ASF protective immunity may be haemadsorption inhibition (HAI) serotype-specific. Recently, we have shown that two ASFV proteins, CD2v (EP402R) and C-type lectin (EP153R), are necessary and sufficient for mediating HAI serological specificity (Malogolovkin et al., 2015).. Here, using ASFV inter-serotypic chimeric viruses and vaccination/challenge experiments in pigs, we demonstrate that serotype-specific CD2v and/or C-type lectin proteins are important for protection against homologous ASFV infection. Thus, these viral proteins represent significant protective antigens for ASFV that should be targeted in future vaccine design and development. Additionally, these data support the concept of HAI serotype-specific protective immunity.

Recombinant Newcastle disease virus expressing African swine fever virus protein 72 is safe and immunogenic in mice

African swine fever (ASF) is a lethal hemorrhagic disease that affects wild and domestic swine. The etiological agent of ASF is African swine fever virus (ASFV). Since the first case was described in Kenya in 1921, the disease has spread to many other countries. No commercial vaccines are available to prevent ASF. In this study, we generated a recombinant Newcastle disease virus (rNDV) expressing ASFV protein 72 (p72) by reverse genetics and evaluated its humoral and cellular immunogenicity in a mouse model. The recombinant virus, rNDV/p72, replicated well in embryonated chicken eggs and was safe to use in chicks and mice. The p72 gene in rNDV/p72 was stably maintained through ten passages. Mice immunized with rNDV/p72 developed high titers of ASFV p72 specific IgG antibody, and had higher levels of IgG1 than IgG2a. Immunization also elicited T-cell proliferation and secretion of IFN-γ and IL-4. Taken together, these results indicate that rNDV expressing ASFV p72 might be a potential vaccine candidate for preventing ASF.

DNA Vaccines: Experiences in the Swine Model

DNA vaccination is one of the most fascinating vaccine-strategies currently in development. Two of the main advantages of DNA immunization rely on its simplicity and flexibility, being ideal to dissect both the immune mechanisms and the antigens involved in protection against a given pathogen. Here, we describe several strategies used to enhance the immune responses induced and the protection afforded by experimental DNA vaccines tested in swine and provide with very basic protocol describing the generation and in vivo application of a prototypic DNA vaccine. Only time will tell the last word regarding the definitive implementation of DNA vaccination in the field.

African swine fever: a global view of the current challenge

African Swine Fever (ASF) is an important contagious haemorrhagic viral disease affecting swine whose notification is mandatory due to its high mortality rates and the great sanitary and socioeconomic impact it has on international trade in animal and swine products.

This disease only affects porcine species, both wild and domestic, and produces a variety of clinical signs such as fever and functional disorders of the digestive and respiratory systems. Lesions are mainly characterized by congestive-haemorrhagic alterations. ASF epidemiology varies significantly between countries, regions and continents, since it depends on the characteristics of the virus in circulation, the presence of wild hosts and reservoirs, environmental conditions and human social behaviour. Furthermore, a specific host will not necessarily always play the same active role in the spread and maintenance of ASF in a particular area.

Currently, ASF is endemic in most sub-Saharan African countries where wild hosts and tick vectors (Ornithodoros) play an important role acting as biological reservoirs for the virus. In Europe, the disease has been endemic since 1978 on the island of Sardinia (Italy) and since 2007, when it was first reported in Georgia, in a number of Eastern European countries. It is also endemic in certain regions of the Russia Federation, where domestic pig and wild boar populations are widely affected. By contrast, in the affected eastern European Union (EU) countries where ASF is currently as epidemic, the on-going spread of the disease affects mainly wild boar populations located in restricted areas and, to a much less extent, domestic pigs. Unlike most livestock diseases, no vaccine or specific treatment is currently available for ASF. Therefore, disease control is mainly based on early detection and the application of strict sanitary and biosecurity measures. Epidemiology of ASF is very complex by the existence of different virus circulating, reservoirs and a number of scenarios, and the on-going spread of the disease through Africa and Europe. Survivor pigs can remain persistently infected for months which may contribute to virus transmission and thus the spread and maintenance of the disease, thereby complicating attempts to control it.

Deletion of the thymidine kinase gene induces complete attenuation of the Georgia isolate of African swine fever virus

African swine fever virus (ASFV) is the etiological agent of a contagious and often lethal viral disease of domestic pigs. There are no vaccines to control Africa swine fever (ASF). Experimental vaccines have been developed using genetically modified live attenuated ASFVs obtained by specifically deleting virus genes involved in virulence, including the thymidine kinase (TK) gene. TK has been shown to be involved in the virulence of several viruses, including ASFV. Here we report the construction of a recombinant virus (ASFV-G/V-ΔTK) obtained by deleting the TK gene in a virulent strain of ASFV Georgia adapted to replicate in Vero cells (ASFV-G/VP30). ASFV-G/P-ΔTK demonstrated decreased replication both in primary swine macrophage cell cultures and in Vero cells compared with ASFV-G/VP30. In vivo, intramuscular administration of up to 10(6) TCID50 of ASFV-G/V-ΔTK does not result in ASF disease. However, these animals are not protected when challenged with the virulent parental Georgia strain.

Live attenuated African swine fever viruses as ideal tools to dissect the mechanisms involved in viral pathogenesis and immune protection

African swine fever virus (ASFV) is the causal agent of African swine fever, a hemorrhagic and often lethal porcine disease causing enormous economical losses in affected countries. Endemic for decades in most of the sub-Saharan countries and Sardinia, the risk of ASFV-endemicity in Europe has increased since its last introduction into Europe in 2007. Live attenuated viruses have been demonstrated to induce very efficient protective immune responses, albeit most of the time protection was circumscribed to homologous ASFV challenges. However, their use in the field is still far from a reality, mainly due to safety concerns. In this study we compared the course of the in vivo infection caused by two homologous ASFV strains: the virulent E75 and the cell cultured adapted strain E75CV1, obtained from adapting E75 to grow in the CV1 cell-line. Interestingly, the kinetics of both viruses not only differed on the clinical signs that they caused and in the virus loads found, but also in the immunological pathways activated throughout the infections. Furthermore, E75CV1 confirmed its protective potential against the homologous E75 virus challenge and allowed the demonstration of poor cross-protection against BA71, thus defining it as heterologous. The in vitro specificity of the CD8(+) T-cells present at the time of lethal challenge showed a clear activation against the homologous virus (E75) but not against BA71. These findings will be of utility for a better understanding of ASFV pathogenesis and for the rational designing of safe and efficient vaccines against this virus.

African Swine Fever Virus Georgia 2007 with a Deletion of Virulence-Associated Gene 9GL (B119L), when Administered at Low Doses, Leads to Virus Attenuation in Swine and Induces an Effective Protection against Homologous Challenge

African swine fever virus (ASFV) is the etiological agent of an often lethal disease of domestic pigs. Disease control strategies have been hampered by the unavailability of vaccines against ASFV. Since its introduction in the Republic of Georgia, a highly virulent virus, ASFV Georgia 2007 (ASFV-G), has caused an epizootic that spread rapidly into Eastern European countries. Currently no vaccines are available or under development to control ASFV-G. In the past, genetically modified ASFVs harboring deletions of virulence-associated genes have proven attenuated in swine, inducing protective immunity against challenge with homologous parental viruses. Deletion of the gene 9GL (open reading frame [ORF] B119L) in highly virulent ASFV Malawi-Lil-20/1 produced an attenuated phenotype even when administered to pigs at 10(6) 50% hemadsorption doses (HAD50). Here we report the construction of a genetically modified ASFV-G strain (ASFV-G-Δ9GLv) harboring a deletion of the 9GL (B119L) gene. Like Malawi-Lil-20/1-Δ9GL, ASFV-G-Δ9GL showed limited replication in primary swine macrophages. However, intramuscular inoculation of swine with 10(4) HAD50 of ASFV-G-Δ9GL produced a virulent phenotype that, unlike Malawi-Lil-20/1-Δ9GL, induced a lethal disease in swine like parental ASFV-G. Interestingly, lower doses (10(2) to 10(3) HAD50) of ASFV-G-Δ9GL did not induce a virulent phenotype in swine and when challenged protected pigs against disease. A dose of 10(2) HAD50 of ASFV-G-Δ9GLv conferred partial protection when pigs were challenged at either 21 or 28 days postinfection (dpi). An ASFV-G-Δ9GL HAD50 of 10(3) conferred partial and complete protection at 21 and 28 dpi, respectively. The information provided here adds to our recent report on the first attempts toward experimental vaccines against ASFV-G.

IMPORTANCE

The main problem for controlling ASF is the lack of vaccines. Studies on ASFV virulence lead to the production of genetically modified attenuated viruses that induce protection in pigs but only against homologous virus challenges. Here we produced a recombinant ASFV lacking virulence-associated gene 9GL in an attempt to produce a vaccine against virulent ASFV-G, a highly virulent virus isolate detected in the Caucasus region in 2007 and now spreading though the Caucasus region and Eastern Europe. Deletion of 9GL, unlike with other ASFV isolates, did not attenuate completely ASFV-G. However, when delivered once at low dosages, recombinant ASFV-G-Δ9GL induces protection in swine against parental ASFV-G. The protection against ASFV-G is highly effective after 28 days postvaccination, whereas at 21 days postvaccination, animals survived the lethal challenge but showed signs of ASF. Here we report the design and development of an experimental vaccine that induces protection against virulent ASFV-G.

Expression library immunization can confer protection against lethal challenge with African swine fever virus

African swine fever is one of the most devastating pig diseases, against which there is no vaccine available. Recent work from our laboratory has demonstrated the protective potential of DNA vaccines encoding three African swine fever viral antigens (p54, p30, and the hemagglutinin extracellular domain) fused to ubiquitin. Partial protection was afforded in the absence of detectable antibodies prior to virus challenge, and survival correlated with the presence of a large number of hemagglutinin-specific CD8(+) T cells in blood. Aiming to demonstrate the presence of additional CD8(+) T-cell determinants with protective potential, an expression library containing more than 4,000 individual plasmid clones was constructed, each one randomly containing a Sau3AI restriction fragment of the viral genome (p54, p30, and hemagglutinin open reading frames [ORFs] excluded) fused to ubiquitin. Immunization of farm pigs with the expression library yielded 60% protection against lethal challenge with the virulent E75 strain. These results were further confirmed by using specific-pathogen-free pigs after challenging them with 10(4) hemadsorbing units (HAU) of the cell culture-adapted strain E75CV1. On this occasion, 50% of the vaccinated pigs survived the lethal challenge, and 2 out of the 8 immunized pigs showed no viremia or viral excretion at any time postinfection. In all cases, protection was afforded in the absence of detectable specific antibodies prior to challenge and correlated with the detection of specific T-cell responses at the time of sacrifice. In summary, our results clearly demonstrate the presence of additional protective determinants within the African swine fever virus (ASFV) genome and open up the possibility for their future identification.

IMPORTANCE

African swine fever is a highly contagious disease of domestic and wild pigs that is endemic in many sub-Saharan countries, where it causes important economic losses and is currently in continuous expansion across Europe. Unfortunately, there is no treatment nor an available vaccine. Early attempts using attenuated vaccines demonstrated their potential to protect pigs against experimental infection. However, their use in the field remains controversial due to safety issues. Although inactive and subunit vaccines did not confer solid protection against experimental ASFV infection, our DNA vaccination results have generated new expectations, confirming the key role of T-cell responses in protection and the existence of multiple ASFV antigens with protective potential, more of which are currently being identified. Thus, the future might bring complex and safe formulations containing more than a single viral determinant to obtain broadly protective vaccines. We believe that obtaining the optimal vaccine formulation it is just a matter of time, investment, and willingness.

BacMam immunization partially protects pigs against sublethal challenge with African swine fever virus

Lack of vaccines and efficient control measures complicate the control and eradication of African swine fever (ASF). Limitations of conventional inactivated and attenuated virus-based vaccines against African swine fever virus (ASFV) highlight the need to use new technologies to develop efficient and safe vaccines against this virus. With this aim in mind, in this study we have constructed BacMam-sHAPQ, a baculovirus based vector for gene transfer into mammalian cells, expressing a fusion protein comprising three in tandem ASFV antigens: p54, p30 and the extracellular domain of the viral hemagglutinin (secretory hemagglutinin, sHA), under the control of the human cytomegalovirus immediate early promoter (CMVie). Confirming its correct in vitro expression, BacMam-sHAPQ induced specific T-cell responses directly after in vivo immunization. Conversely, no specific antibody responses were detectable prior to ASFV challenge. The protective potential of this recombinant vaccine candidate was tested by a homologous sublethal challenge with ASFV following immunization. Four out of six immunized pigs remained viremia-free after ASFV infection, while the other two pigs showed similar viremic titres to control animals. The protection afforded correlated with the presence of a large number of virus-specific IFNγ-secreting T-cells in blood at 17 days post-infection. In contrast, the specific antibody levels observed after ASFV challenge in sera from BacMam-sHAPQ immunized pigs were indistinguishable from those found in control pigs. These results highlight the importance of the cellular responses in protection against ASFV and point towards BacMam vectors as potential tools for future vaccine development.

Standardization of pathological investigations in the framework of experimental ASFV infections

African swine fever is still one of the major viral diseases of swine for which a commercial vaccine is lacking. For the design and development of such preventive products, researchers involved in African swine fever virus (ASFV) vaccinology need standardized challenge protocols and well characterized clinical, pathological and immunological responses of inbreed and outbreed pigs to different viral strains and vaccine-like products. The different approaches used should be assessed by immunologist, virologist and pathologist expertise. The main objectives of this guideline are to (1) briefly contextualize the clinical and pathological ASFV presentations focusing on points that are critical for pathogenesis, (2) provide recommendations concerning the analysis of clinical, gross and microscopic observations and (3) standardize the pathological report, the terminology employed and the evaluation of the severity of the lesions between the ASFV research groups for comparing inter-group data. The presented guidelines establish new approaches to integrate such relevant pathological data with virological and immunological testing, giving support to the global interpretation of the findings in the future experiments of ASFV-related vaccinology and immunology.

Cellular immunity in ASFV responses

African swine fever virus (ASFV) infection usually results in an acute haemorrhagic disease with a mortality rate approaching 100% in domestic pigs. However, pigs can survive infection with less-virulent isolates of ASFV and may become chronically infected. Surviving animals are resistant to challenge with homologous or, in some cases, closely related isolates of the virus indicating that pigs can develop protective immunity against ASFV. During asymptomatic, non-virulent ASFV infections natural killer cell activity increases in pigs, suggesting this cell type plays a role in ASFV immunity. Furthermore, depletion of CD8(+) lymphocytes from ASFV immune pigs demolishes protective immunity against related virulent viruses. This suggests that ASFV specific antibody alone is not sufficient for protection against ASFV infection and that there is an important role for the CD8(+) lymphocyte subset in ASFV protective immunity. These results were supported by DNA immunization studies, demonstrating a correlation between the protection afforded against lethal challenge and the detection of a large number of vaccine-induced antigen-specific CD8(+) T-cells. Peripheral blood mononuclear cells (PBMCs) from ASF immune pigs protected from clinical disease show higher proportions of ASFV specific CD4(+)CD8(high+) double positive cytotoxic T cells than PBMCs from ASF immune but clinically diseased pig. The frequency of ASFV specific IFNγ producing T cells induced by immunization correlates to the degree of protection from ASFV challenge, and this may prove to be a useful indicator of any potential cross-protection against heterologous ASFV isolates.