Novel virulence and host range genes of African swine fever virus

SY18ΔL60L: a new recombinant live attenuated African swine fever virus with protection against homologous challenge

Introduction

African swine fever (ASF) is an acute and highly contagious disease and its pathogen, the African swine fever virus (ASFV), threatens the global pig industry. At present, management of ASF epidemic mainly relies on biological prevention and control methods. Moreover, due to the large genome of ASFV, only half of its genes have been characterized in terms of function.

Methods

Here, we evaluated a previously uncharacterized viral gene, L60L. To assess the function of this gene, we constructed a deletion strain (SY18ΔL60L) by knocking out the L60L gene of the SY18 strain. To evaluate the growth characteristics and safety of the SY18ΔL60L, experiments were conducted on primary macrophages and pigs, respectively.

Results

The results revealed that the growth trend of the recombinant strain was slower than that of the parent strain in vitro. Additionally, 3/5 (60%) pigs intramuscularly immunized with a 105 50% tissue culture infectious dose (TCID50) of SY18ΔL60L survived the 21-day observation period. The surviving pigs were able to protect against the homologous lethal strain SY18 and survive. Importantly, there were no obvious clinical symptoms or viremia.

Discussion

These results suggest that L60L could serve as a virulence- and replication-related gene. Moreover, the SY18ΔL60L strain represents a new recombinant live-attenuated ASFV that can be employed in the development of additional candidate vaccine strains and in the elucidation of the mechanisms associated with ASF infection.

[Problems of specific prevention of African swine fever]

This review presents the current state of the problem of development and application of the specific prevention of African swine fever (ASF) with a brief description of its etiology and pathogenesis. The unique nature of the ASF virus (ASFV) determines some limitations and the complexity of solving the problem of vaccine development. Such situation stimulated the development of highly specific diagnostic methods for rapid and accurate detection of the ASFV. In this regard, results of studies, including our own, concerning the comparative analysis of the genome of vaccine and virulent strains of the ASFV, as well as immunodiagnostic approaches to determine causes of high virulence and low protective activity of the ASFV, are briefly presented. Special attention is given to the issue related to the development of safe and effective vaccines against ASF. In this context disadvantages and possible advantages of live attenuated (LAV) and recombinant (RV) vaccines are considered in details. Results of recent studies on the assessment of the immunogenicity of genetically modified vaccines (GMV) which developed in various laboratories around the world are presented. The obtained data indicate that ASF vaccination is currently the most promising measure to stop the spread of this disease in our country and in the world, however, previous experience with ASF vaccination has revealed some problems in its development and application. The significant contribution of foreign researchers to the study of the basics of virulence of this pathogen and the study of its genes functions are noted. The possible further expansion of ASF in Europe and Asia in bordering Russia territories, as well as the established fact of the persistence of ASFV in wild boar population indicate a constant threat of its re-introduction into our country. In conclusion, the importance of developing a safe effective vaccine against ASF and the assessing of the possible risks of creating the artificial sources of the infection in nature as a result of its use is emphasized.

Transcriptome profiling in swine macrophages infected with African swine fever virus at single-cell resolution

African swine fever virus (ASFV) causes a severe and highly contagious disease in pigs and wild boars, but no commercial vaccines or antivirals are available currently. Understanding the mutual antagonism between virus and host factors during ASFV infection may facilitate the development of new vaccines and antivirals. Our work profiled transcriptomes of swine macrophages infected with ASFV through single-cell RNA-sequencing technology. Identified dynamic transcriptome events of viral genes provide molecular characteristics of ASFV during infection. Moreover, virus–host interactions imply the regulation pathway of viral replication in host cells, which may guide research on antiviral strategies and dissection of ASFV pathogenesis.

African swine fever virus (ASFV) is the causative agent of African swine fever, a highly contagious and usually fatal disease in pigs. The pathogenesis of ASFV infection has not been clearly elucidated. Here, we used single-cell RNA-sequencing technology to survey the transcriptomic landscape of ASFV-infected primary porcine alveolar macrophages. The temporal dynamic analysis of viral genes revealed increased expression of viral transmembrane genes. Molecular characteristics in the ASFV-exposed cells exhibited the activation of antiviral signaling pathways with increased expression levels of interferon-stimulated genes and inflammatory- and cytokine-related genes. By comparing infected cells with unexposed cells, we showed that the unfolded protein response (UPR) pathway was activated in low viral load cells, while the expression level of UPR-related genes in high viral load cells was less than that in unexposed cells. Cells infected with various viral loads showed signature transcriptomic changes at the median progression of infection. Within the infected cells, differential expression analysis and coregulated virus–host analysis both demonstrated that ASFV promoted metabolic pathways but inhibited interferon and UPR signaling, implying the regulation pathway of viral replication in host cells. Furthermore, our results revealed that the cell apoptosis pathway was activated upon ASFV infection. Mechanistically, the production of tumor necrosis factor alpha (TNF-α) induced by ASFV infection is necessary for cell apoptosis, highlighting the importance of TNF-α in ASFV pathogenesis. Collectively, the data provide insights into the comprehensive host responses and complex virus–host interactions during ASFV infection, which may instruct future research on antiviral strategies.

Transcriptome Profiling Reveals Features of Immune Response and Metabolism of Acutely Infected, Dead and Asymptomatic Infection of African Swine Fever Virus in Pigs

African swine fever virus (ASFV) infection can result in lethal disease in pigs. ASFV encodes 150-167 proteins, of which only approximately 50 encoded viral structure proteins are functionally known. ASFV also encodes some nonstructural proteins that are involved in the regulation of viral transcription, viral replication and evasion from host defense. However, the understanding of the molecular correlates of the severity of these infections is still limited. The purpose of this study was to compare host and viral gene expression differences and perform functional analysis in acutely infected, dead and cohabiting asymptomatic pigs infected with ASFV by using RNA-Seq technique; healthy pigs were used as controls. A total of 3,760 and 2,874 upregulated genes and 4,176 and 2,899 downregulated genes were found in healthy pigs vs. acutely infected, dead pigs or asymptomatic pigs, respectively. Additionally, 941 upregulated genes and 956 downregulated genes were identified in asymptomatic vs. acutely infected, dead pigs. Different alternative splicing (AS) events were also analyzed, as were gene chromosome locations, and protein-protein interaction (PPI) network prediction analysis was performed for significantly differentially expressed genes (DEGs). In addition, 30 DEGs were validated by RT-qPCR, and the results were consistent with the RNA-Seq results. We further analyzed the interaction between ASFV and its host at the molecular level and predicted the mechanisms responsible for asymptomatic pigs based on the selected DEGs. Interestingly, we found that some viral genes in cohabiting asymptomatic pigs might integrate into host genes (DP96R, I73R and L83L) or remain in the tissues of cohabiting asymptomatic pigs. In conclusion, the data obtained in the present study provide new evidence for further elucidating ASFV-host interactions and the ASFV infection mechanism and will facilitate the implementation of integrated strategies for controlling ASF spread.

An extensive evaluation of codon usage pattern and bias of structural proteins p30, p54 and, p72 of the African swine fever virus (ASFV)

African swine fever virus (ASFV) belongs to the family of Asfarviridae to the genus Asfivirus. ASF virus causes hemorrhage illness with a high mortality rate and hence, commercial loss in the swine community. The ASFV has been categorized by variation in codon usage that is caused by high mutation rates and natural selection. The evolution is caused mainly due to the mutation pressure and regulating the protein gene expression. Based on publicly accessible nucleotide sequences of the ASFV and its host (pig & tick), codon usage bias analysis was performed since an approved effective vaccination is not available to date, it is very important to analyze the codon usage bias of the p30, p54, and p72 proteins of ASFV to produce an effective and efficient vaccine to control the disease. Even though the codon usage bias analyses have been evaluated earlier, the evaluation of the codon usage pattern specific to p30, p54, and p72 of ASFV is inadequate. In all the protein-coding sequences, nucleotide base and codons terminating with base T were most frequent and the mean effective number of codons (Nc) was high, indicating the presence of codon usage bias. The GC contents and dinucleotide frequencies also indicated the codon usage bias of the ASFV pig and tick. The Nc plot, parity plot, neutrality plot analysis, revealed natural selection, as well as mutation pressure, were the major constraints in altering the codon bias of ASF virus. codon usage bias analysis was performed with no substantial differences in codon usage of the ASFV in pig and tick.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13337-021-00719-x.

Adaptation of African swine fever virus to HEK293T cells

African swine fever (ASF), caused by African swine fever virus (ASFV), is a highly contagious disease with high morbidity and mortality in domestic pigs. Although adaptation of ASFV to Vero cells has been investigated, the phenotypic changes and the corresponding genomic variations during adaptation of ASFV to other cell lines remain unclear. To obtain a cell-adapted ASFV strain, different cell lines were tested to determine whether they support ASFV infection. Interestingly, the ASFV wild-type strain ASFV-HLJ/18 can infect HEK293T cells and replicate at a low level. After continuous passaging, the adapted ASFV strain can replicate efficiently in both HEK293T and Vero cells. However, the adapted ASFV strain displayed reduced infectivity in primary porcine alveolar macrophages compared to the corresponding wild-type strain. Furthermore, stepwise losses at the left variable end of the MGF genes and accumulative mutations were identified during passaging, indicating that the ASFV strain gradually adapted to HEK293T cells. Comparison of MGF deletions in other cell culture-adapted ASFV strains revealed that the deletions of MGF300 (1L, 2R and 4L) and MGF360 genes (8L, 9L, 10L and 11L) play an important role for the adaptation of ASFV to HEK293T cells at the early stage. The biological functions of the deletions and mutants associated with ASFV infection in HEK293T cells and pigs warrant further study. Overall, our findings provide new targets to elucidate the molecular mechanism of adaptation of ASFV to cell lines.

Thoughts on African Swine Fever Vaccines

African swine fever (ASF) is an acute viral hemorrhagic disease of domestic swine with mortality rates approaching 100%. Devastating ASF outbreaks and continuing epidemics starting in the Caucasus region and now in the Russian Federation, Europe, China, and other parts of Southeast Asia (2007 to date) highlight its significance. ASF strain Georgia-07 and its derivatives are now endemic in extensive regions of Europe and Asia and are "out of Africa" forever, a situation that poses a grave if not an existential threat to the swine industry worldwide. While our current concern is Georgia-07, other emerging ASFV strains will threaten for the indefinite future. Economic analysis indicates that an ASF outbreak in the U.S. would result in approximately $15 billion USD in losses, assuming the disease is rapidly controlled and the U.S. is able to reenter export markets within two years. ASF's potential to spread and become endemic in new regions, its rapid and efficient transmission among pigs, and the relative stability of the causative agent ASF virus (ASFV) in the environment all provide significant challenges for disease control. Effective and robust methods, including vaccines for ASF response and recovery, are needed immediately.

African Swine Fever Virus MGF360-12L Inhibits Type I Interferon Production by Blocking the Interaction of Importin α and NF-κB Signaling Pathway

African swine fever (ASF) is an infectious transboundary disease of domestic pigs and wild boar and spreading throughout Eurasia. There is no vaccine and treatment available. Complex immune escape strategies of African swine fever virus (ASFV) are crucial factors affecting immune prevention and vaccine development. MGF360 genes have been implicated in the modulation of the IFN-I response. The molecular mechanisms contributing to innate immunity are poorly understood. In this study, we demonstrated that ASFV MGF360-12L (MGF360 families 12L protein) significantly inhibited the mRNA transcription and promoter activity of IFN-β and NF-κB, accompanied by decreases of IRF3, STING, TBK1, ISG54, ISG56 and AP-1 mRNA transcription. Also, MGF360-12L might suppress the nuclear localization of p50 and p65 mediated by classical nuclear localization signal (NLS). Additionally, MGF360-12L could interact with KPNA2, KPNA3, and KPNA4, which interrupted the interaction between p65 and KPNA2, KPNA3, KPNA4. We further found that MGF360-12L could interfere with the NF-κB nuclear translocation by competitively inhibiting the interaction between NF-κB and nuclear transport proteins. These findings suggested that MGF360-12L could inhibit the IFN-I production by blocking the interaction of importin α and NF-κB signaling pathway, which might reveal a novel strategy for ASFV to escape the host innate immune response.

African Swine Fever: Lessons to Learn From Past Eradication Experiences. A Systematic Review

Prevention, early detection, prompt reaction, and communication play a crucial role in African swine fever (ASF) control. Appropriate surveillance capable of early detection of the disease in both domestic and wild animals, and the implementation of consolidated contingency plans, are currently considered the best means of controlling this disease. The purpose of this study was to understand the lessons to be learned through the global disease eradication history. To establish which strategies were successful for prevention, control, and eradication of ASF, and which errors should not be repeated, we conducted a systematic review. A query was defined to search for surveillance and control strategies applied by countries worldwide for ASF eradication in the past. Inclusion and exclusion criteria were defined. Decisions on study eligibility and data extraction were performed by two independent reviewers and the differences were resolved by consensus or by a third reviewer. From 1,980 papers, 23 were selected and included in the qualitative analysis. Reports from Belgium, Brazil, Cuba, the Dominican Republic and Haiti, France, mainland Italy, Malta, Portugal, and Spain were included. Despite the economic resources allocated and the efforts made, eradication was possible in only eight countries, between the 50s and 90s in the twentieth century, in different epidemiological and cultural contexts, in some instances within <1 year, and in others in about 40 years. Classical surveillance strategies, such as active and passive surveillance, both at farm and slaughterhouse levels, targeted surveillance, together with conventional biosafety and sanitary measures, led to eradication even in countries in which the tick's epidemiological role was demonstrated. Historical surveillance data analysis indicated that eradication was possible even when technological tools either were not available or were used less than they are currently. This emphasizes that data on surveillance and on animal population are crucial for planning effective surveillance, and targeting proper control and intervention strategies. This paper demonstrates that some strategies applied in the past were effective; these could be implemented and improved to confront the current epidemiological wave. This offers encouragement for the efforts made particularly in Europe during the recent epidemics.

African Swine Fever Virus Armenia/07 Virulent Strain Controls Interferon Beta Production through the cGAS-STING Pathway

African swine fever, a devastating disease for domestic pigs and wild boar, is currently spreading in Europe, Russia, and China, becoming a global threat with huge economic and ecological consequences. One interesting aspect of ASFV biology is the molecular mechanism leading to high virulence of some strains compared to more attenuated strains, which produce subclinical infections. In this work, we show that the presently circulating virulent Armenia/07 virus blocks the synthesis of IFN-β, a key mediator between the innate and adaptive immune response. Armenia/07 inhibits the cGAS-STING pathway by impairing STING activation during infection. In contrast, the cGAS-STING pathway is efficiently activated during NH/P68 attenuated strain infection, leading to the production of large amounts of IFN-β. Our results show for the first time the relationship between the cGAS-STING pathway and ASFV virulence, contributing to uncover the molecular mechanisms of ASFV virulence and to the rational development of ASFV vaccines.

African swine fever virus (ASFV) is a complex, cytoplasmic double-stranded DNA (dsDNA) virus that is currently expanding throughout the world. Currently, circulating virulent genotype II Armenia/07-like viruses cause fatal disease in pigs and wild boar, whereas attenuated strains induce infections with various levels of chronic illness. Sensing cytosolic dsDNA, mainly by the key DNA sensor cyclic GMP-AMP synthase (cGAS), leads to the synthesis of type I interferon and involves signaling through STING, TBK1, and IRF3. After phosphorylation, STING translocates from the endoplasmic reticulum to the Golgi compartment and to the perinuclear region, acting as an indispensable adaptor connecting the cytosolic detection of DNA to the TBK1-IRF3 signaling pathway. We demonstrate here that attenuated NH/P68, but not virulent Armenia/07, activates the cGAS-STING-IRF3 cascade very early during infection, inducing STING phosphorylation and trafficking through a mechanism involving cGAMP. Both TBK1 and IRF3 are subsequently activated and, in response to this, a high level of beta interferon (IFN-β) was produced during NH/P68 infection; in contrast, Armenia/07 infection generated IFN-β levels below those of uninfected cells. Our results show that virulent Armenia/07 ASFV controls the cGAS-STING pathway, but these mechanisms are not at play when porcine macrophages are infected with attenuated NH/P68 ASFV. These findings show for the first time the involvement of the cGAS-STING-IRF3 route in ASFV infection, where IFN-β production or inhibition was found after infection by attenuated or virulent ASFV strains, respectively, thus reinforcing the idea that ASFV virulence versus attenuation may be a phenomenon grounded in ASFV-mediated innate immune modulation where the cGAS-STING pathway might play an important role.

IMPORTANCE African swine fever, a devastating disease for domestic pigs and wild boar, is currently spreading in Europe, Russia, and China, becoming a global threat with huge economic and ecological consequences. One interesting aspect of ASFV biology is the molecular mechanism leading to high virulence of some strains compared to more attenuated strains, which produce subclinical infections. In this work, we show that the presently circulating virulent Armenia/07 virus blocks the synthesis of IFN-β, a key mediator between the innate and adaptive immune response. Armenia/07 inhibits the cGAS-STING pathway by impairing STING activation during infection. In contrast, the cGAS-STING pathway is efficiently activated during NH/P68 attenuated strain infection, leading to the production of large amounts of IFN-β. Our results show for the first time the relationship between the cGAS-STING pathway and ASFV virulence, contributing to uncover the molecular mechanisms of ASFV virulence and to the rational development of ASFV vaccines.

Molecular studies on African swine fever virus from Brazilian isolates

ABSTRACT: African swine fever (ASF) is a devastating viral infirmity that affects domestic and wild swine caused by the ASF virus (ASFV) that belongs to the family Asfaviridae in the Asfavirus genus. Studies for genotypic and antigenic determination of ASFV including samples from Brazilian outbreaks were carried out outside Brazil. Here, we have reviewed studies on the molecular aspects of Brazilian isolates from 1978 and 1979. Results obtained from restriction fragment analysis, cloning and gene sequencing display the genotypic variation of viral samples. Viral genotyping based on sequences of the 3’ region of the p72 gene included in genotype I Brazilian samples, reinforcing the suggestion of the European origin for the virus that infected Brazilian herds and having low virulence potential. Corroborating those findings, at the American station PIADC, the infection of healthy pigs with the Brazilian strain induced ASF sub acute disease with low mortality and a low-virulence. Those results were similar with epidemiological vigilance forms of Brazilian swineherd in good health conditions having at least one ASFV isolation, and the ASF pioneer’s studies on the low mortality in the Brazilian herds affected by ASF. The ASFV spreading in Eastern Europe and Russia triggered a greater concern with intensifying the risk of viral dissemination from country to country. The low virulence ASF strains can increase the problem because of hidden viral reservoirs - which further reinforces the need for safety and preventive measures in virus-free countries. Finally, the problem is further compounded by the lack of vaccines and other immunological resources.

A Review of African Swine Fever and the Potential for Introduction into the United States and the Possibility of Subsequent Establishment in Feral Swine and Native Ticks

African swine fever (ASF) is caused by African swine fever virus (ASFV), which can cause substantial morbidity and mortality events in swine. The virus can be transmitted via direct and indirect contacts with infected swine, their products, or competent vector species, especially Ornithodoros ticks. Africa and much of Eastern Europe are endemic for ASF; a viral introduction to countries that are currently ASF free could have severe economic consequences due to the loss of production from infected animals and the trade restrictions that would likely be imposed as a result of an outbreak. We identified vulnerabilities that could lead to ASFV introduction or persistence in the United States or other ASF-free regions. Both legal and illegal movements of live animals, as well as the importation of animal products, byproducts, and animal feed, pose a risk of virus introduction. Each route is described, and current regulations designed to prevent ASFV and other pathogens from entering the United States are outlined. Furthermore, existing ASFV research gaps are highlighted. Laboratory experiments to evaluate multiple species of Ornithodoros ticks that have yet to be characterized would be useful to understand vector competence, host preferences, and distribution of competent soft tick vectors in relation to high pig production areas as well as regions with high feral swine (wild boar or similar) densities. Knowledge relative to antigenic viral proteins that contribute to host response and determination of immune mechanisms that lead to protection are foundational in the quest for a vaccine. Finally, sampling of illegally imported and confiscated wild suid products for ASFV could shed light on the types of products being imported and provide a more informed perspective relative to the risk of ASFV importation.

Deletion of the African Swine Fever Virus Gene DP148R Does Not Reduce Virus Replication in Culture but Reduces Virus Virulence in Pigs and Induces High Levels of Protection against Challenge

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 (∼106 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.

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.

Evolution of African swine fever virus genes related to evasion of host immune response

African swine fever (ASF) is a notifiable and one of the most complex and devastating infectious disease of pigs, wild boars and other representatives of Suidae family. African swine fever virus (ASFV) developed various molecular mechanisms to evade host immune response including alteration of interferon production by multigene family protein (MGF505-2R), inhibition of NF-κB and nuclear activating factor in T-cells by the A238L protein, or modulation of host defense by CD2v lectin-like protein encoded by EP402R and EP153R genes. The current situation concerning ASF in Poland seems to be stable in comparison to other eastern European countries but up-to-date in total 106 ASF cases in wild boar and 5 outbreaks in pigs were identified. The presented study aimed to reveal and summarize the genetic variability of genes related to inhibition or modulation of infected host response among 67 field ASF isolates collected from wild boar and pigs. The nucleotide sequences derived from the analysed A238L and EP153R regions showed 100% identity. However, minor but remarkable genetic diversity was found within EP402R and MGF505-2R genes suggesting slow molecular evolution of circulating ASFV isolates and the important role of this gene in modulation of interferon I production and hemadsorption phenomenon. The obtained nucleotide sequences of Polish ASFV isolates were closely related to Georgia 2007/1 and Odintsovo 02/14 isolates suggesting their common Caucasian origin. In the case of EP402R and partially in MGF505-2R gene the identified genetic variability was related to spatio-temporal occurrence of particular cases and outbreaks what may facilitate evolution tracing of ASFV isolates. This is the first report indicating identification of genetic variability within the genes related to evasion of host immune system which may be used to trace the direction of ASFV isolates molecular evolution.

African swine fever virus infection in Ornithodoros ticks

African swine fever virus (ASFV) is an arbovirus which is vectored by soft ticks of the Ornithodoros spp. and in the sylvatic cycle infects wart hogs and bush pigs. ASFV infection of domestic swine causes a high mortality disease. On the other hand, ASFV infection of the tick can result in a high-titered and persistent infection depending upon the ASFV isolate and the tick combination. Recently, morphological, classical virology (titration) and recombinant ASFV have been used to study the cellular, molecular and genetic interactions that occur between ASFV and its host tick.

Interferon status and white blood cells during infection with African swine fever virus in vivo

African swine fever virus (ASFV) is the causative agent of African swine fever that is the significant disease of domestic pigs, with high rates of mortality. ASFV is double-stranded DNA virus whose genes encode some proteins that are implicated in the suppression of host immune response. In this study, we have modeled in vivo infection of ASFV for determination of interferon (IFN) status in infected pigs. We measured the level of IFN-α, -β and -γ by enzyme-linked immunosorbent assay and showed that the level of IFN-α sharply decreased during infection. Unlike IFN-α, the level of IFN-β and -γ increased from the 2nd and 4th days post-infection, respectively. Also, we analyzed the population dynamics of peripheral white blood cells of infected pigs due to their important role in host immune system. We showed that the atypical lymphocytes appeared after short time of infection and this result is in accordance with our previous study done in vitro. At the last day of infection about 50% of the total white blood cells were destroyed, and the remaining cells were represented mainly by small-sized lymphocytes, reactive lymphocytes and lymphoblasts.

Comparison of the genome sequences of non-pathogenic and pathogenic African swine fever virus isolates

The genomic coding sequences, apart from the inverted terminal repeats and cross-links, have been determined for two African swine fever virus (ASFV) isolates from the same virus genotype, a non-pathogenic isolate from Portugal, OURT88/3, and a highly pathogenic isolate from West Africa, Benin 97/1. These genome sequences were annotated and compared with that of a tissue culture-adapted isolate, BA71V. The genomes range in length between 170 and 182 kbp and encode between 151 and 157 open reading frames (ORFs). Compared to the Benin 97/1 isolate, the OURT88/3 and BA71V isolates have deletions of 8-10 kbp that encode six copies of the multigene family (MGF) 360 and either one MGF 505/530 copy in the BA71V or two copies in the OURT88/3 isolate. The BA71V isolate has a deletion, close to the right end of the genome, of 3 kbp compared with the other isolates. The five ORFs in this region include an additional copy of an ORF similar to that encoding the p22 virus structural protein. The OURT88/3 isolate has interruptions in ORFs that encode a CD2-like and a C-type lectin protein. Variation between the genomes is observed in the number of copies of five different MGFs. The 109 non-duplicated ORFs conserved in the three genomes encode proteins involved in virus replication, virus assembly and modulation of the host's defences. These results provide information concerning the genetic variability of African swine fever virus isolates that differ in pathogenicity.

The MyD116 African swine fever virus homologue interacts with the catalytic subunit of protein phosphatase 1 and activates its phosphatase activity

The DP71L protein of African swine fever virus (ASFV) shares sequence similarity with the herpes simplex virus ICP34.5 protein over a C-terminal domain. We showed that the catalytic subunit of protein phosphatase 1 (PP1) interacts specifically with the ASFV DP71L protein in a yeast two-hybrid screen. The chimeric full-length DP71L protein, from ASFV strain Badajoz 71 (BA71V), fused to glutathione S-transferase (DP71L-GST) was expressed in Escherichia coli and shown to bind specifically to the PP1-alpha catalytic subunit expressed as a histidine fusion protein (6xHis-PP1alpha) in E. coli. The functional effects of this interaction were investigated by measuring the levels of PP1 and PP2A in ASFV-infected Vero cells. This showed that infection with wild-type ASFV strain BA71V activated PP1 between two- and threefold over that of mock-infected cells. This activation did not occur in cells infected with the BA71V isolate in which the DP71L gene had been deleted, suggesting that expression of DP71L leads to PP1 activation. In contrast, no effect was observed on the activity of PP2A following ASFV infection. We showed that infection of cells with wild-type BA71V virus resulted in decreased phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2alpha). ICP34.5 recruits PP1 to dephosphorylate the alpha subunit of eukaryotic translational initiation factor 2 (also known as eIF-2alpha); possibly the ASFV DP71L protein has a similar function.

Protein pE120R of African swine fever virus is post-translationally acetylated as revealed by post-source decay MALDI mass spectrometry

Post-translational modification of proteins is a key regulatory event in many cellular processes. African swine fever virus (ASFV) is a large DNA virus that contains about 150 open reading frames (ORF) which encode for more than 150 polypeptides, most of them without assigned function. Two-dimensional gel electrophoresis (2DE) followed by Post-Source Decay Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry (PSD-MALDI-MS) revealed that ASFV protein pE120R, essential for virus transport from assembly sites to plasma membranes, is acetylated at the N-terminal Ala residue during infection. To our knowledge, this is the first acetylated ASFV protein described and this modification might be relevant to ASFV life cycle since many viruses use the acetylation signaling pathway as a primary target for viral proteins after infection.

Nuclear export of African swine fever virus p37 protein occurs through two distinct pathways and is mediated by three independent signals

Nucleocytoplasmic shuttling activity of the African swine fever virus p37 protein, a major structural protein of this highly complex virus, has been recently reported. The systematic characterization of the nuclear export ability of this protein constituted the major purpose of the present study. We report that both the N- and C-terminal regions of p37 protein are actively exported from the nucleus to the cytoplasm of yeast and mammalian cells. Moreover, experiments using leptomycin B and small interfering RNAs targeting the CRM1 receptor have demonstrated that the export of p37 protein is mediated by both the CRM1-dependent and CRM1-independent nuclear export pathways. Two signals responsible for the CRM1-mediated nuclear export of p37 protein were identified at the N terminus of the protein, and an additional signal was identified at the C-terminal region, which mediates the CRM1-independent nuclear export. Interestingly, site-directed mutagenesis revealed that hydrophobic amino acids are critical to the function of these three nuclear export signals. Overall, our results demonstrate that two distinct pathways contribute to the strong nuclear export of full-length p37 protein, which is mediated by three independent nuclear export signals. The existence of overlapping nuclear export mechanisms, together with our observation that p37 protein is localized in the nucleus at early stages of infection and exclusively in the cytoplasm at later stages, suggests that the nuclear transport ability of this protein may be critical to the African swine fever virus replication cycle.

Biological transmission of arboviruses: reexamination of and new insights into components, mechanisms, and unique traits as well as their evolutionary trends

Among animal viruses, arboviruses are unique in that they depend on arthropod vectors for transmission. Field research and laboratory investigations related to the three components of this unique mode of transmission, virus, vector, and vertebrate host, have produced an enormous amount of valuable information that may be found in numerous publications. However, despite many reviews on specific viruses, diseases, or interests, a systematic approach to organizing the available information on all facets of biological transmission and then to interpret it in the context of the evolutionary process has not been attempted before. Such an attempt in this review clearly demonstrates tremendous progress made worldwide to characterize the viruses, to comprehend disease transmission and pathogenesis, and to understand the biology of vectors and their role in transmission. The rapid progress in molecular biologic techniques also helped resolve many virologic puzzles and yielded highly valuable data hitherto unavailable, such as characterization of virus receptors, the genetic basis of vertebrate resistance to viral infection, and phylogenetic evidence of the history of host range shifts in arboviruses. However, glaring gaps in knowledge of many critical subjects, such as the mechanism of viral persistence and the existence of vertebrate reservoirs, are still evident. Furthermore, with the accumulated data, new questions were raised, such as evolutionary directions of virus virulence and of host range. Although many fundamental questions on the evolution of this unique mode of transmission remained unresolved in the absence of a fossil record, available observations for arboviruses and the information derived from studies in other fields of the biological sciences suggested convergent evolution as a plausible process. Overall, discussion of the diverse range of theories proposed and observations made by many investigators was found to be highly valuable for sorting out the possible mechanism(s) of the emergence of arboviral diseases.

Molecular epidemiology of African swine fever in East Africa

African swine fever (ASF) a lethal, viral hemorrhagic disease of domestic pigs, first reported from East Africa in 1921, is still widespread in this region. In order to assess field heterogeneity at the regional level, nucleotide sequences corresponding to the C-terminal end of the p72 gene were determined for 77 ASF viruses of diverse temporal and species origin occurring in eight East African countries. The number of sites completely conserved across all East African sequences characterized in this study was 84.2% and 86.8% on nucleotide and amino acid level, respectively. Phylogenetic analysis of a homologous 404 bp region revealed the presence of thirteen East African genotypes, of which eight appear to be country specific. An East African, pig-associated, homogeneous virus lineage linked to outbreaks in Mozambique, Zambia and Malawi over a 23 year period was demonstrated. In addition, genotype I (ESACWA) viruses were identified in East African sylvatic hosts for the first time which is significant as this genotype was previously thought to be restricted to the West African region where it occurs only in domestic pigs. The presence of discrete epidemiological cycles in East Africa and recovery of multiple genotypes affirms the epidemiological complexity of ASF in this region.

African swine fever virus proteins involved in evading host defence systems

African swine fever virus (ASFV) can cause an acutely fatal haemorrhagic fever in domestic pigs although in its natural hosts, warthogs, bushpigs and the soft tick vector, Ornithodoros moubata, ASFV causes inapparent persistent infections. The virus is a large, cytoplasmic, double-stranded DNA virus which has a tropism for macrophages. As it is the only member of the Asfarviridae family, ASFV encodes many novel genes not encoded by other virus families. The ability of the virus to persist in its natural hosts and in domestic pigs, which recover from infection with less virulent isolates, shows that the virus has effective mechanisms to evade host defence systems. This review focuses on recent progress made in understanding the function of ASFV-encoded proteins, which are involved in modulating the host response to infection. Growing evidence suggests that a major strategy used by the virus is to modulate signalling pathways in infected macrophages, thus interfering with the expression of a large number of immunomodulatory genes. One potent immunomodulatory protein, A238L, inhibits both activation of the host NFkappaB transcription factor and inhibits calcineurin phosphatase activity. Calcineurin-dependent pathways, including activation of the NFAT transcription factor, are therefore inhibited. Another ASFV-encoded protein, CD2v, resembles the host CD2 protein, which is expressed on T cells and NK cells. This virus protein causes the adsorption of red blood cells around virus-infected cells and extracellular virus particles. Expression of the CD2v protein aids virus dissemination in pigs and the protein also has a role in impairing bystander lymphocyte function. This may be mediated either by a direct interaction of CD2v extracellular domain with ligands on lymphocytes or by an indirect mechanism involving interaction of the CD2v cytoplasmic tail with host proteins involved in signalling or trafficking pathways. Two ASFV proteins, an IAP and a Bcl2 homologue, inhibit apoptosis in infected cells and thus facilitate production of progeny virions. The prediction is that half to two-thirds of the approximately 150 genes encoded by ASFV are not essential for replication in cells but have an important role for virus survival and transmission in its hosts. These genes provide an untapped repository, and will be valuable tools for deciphering not only how the virus manipulates the host response to infection to avoid elimination, but also useful for understanding important host anti-viral mechanisms. In addition, they may provide leads for discovery of novel immunomodulatory drugs.

Two African swine fever virus proteins derived from a common precursor exhibit different nucleocytoplasmic transport activities

African swine fever virus (ASFV), a large icosahedral deoxyvirus, is the causative agent of an economically relevant hemorrhagic disease that affects domestic pigs. The major purpose of the present study was to investigate the nuclear transport activities of the ASFV p37 and p14 proteins, which result from the proteolytic processing of a common precursor. Experiments were performed by using yeast-based nucleocytoplasmic transport assays and by analysis of the subcellular localization of different green fluorescent and Myc fusion proteins in mammalian cells. The results obtained both in yeast and mammalian cells clearly demonstrated that ASFV p14 protein is imported into the nucleus but not exported to the cytoplasm. The ability of p37 protein to be exported from the nucleus to the cytoplasm of both yeast and mammalian cells was also demonstrated, and the results clearly indicate that p37 nuclear export is dependent on the interaction of the protein with the CRM-1 receptor. In addition, p37 was shown to exhibit nuclear import activity in mammalian cells. The p37 protein nuclear import and export abilities described here constitute the first report of a nucleocytoplasmic shuttling protein encoded by the ASFV genome. Overall, the overlapping results obtained for green fluorescent protein fusions and Myc-tagged proteins undoubtedly demonstrate that ASFV p37 and p14 proteins exhibit nucleocytoplasmic transport activities. These findings are significant for understanding the role these proteins play in the replication cycle of ASFV.

Characterization of pathogenic and non-pathogenic African swine fever virus isolates from Ornithodoros erraticus inhabiting pig premises in Portugal

Ten African swine fever virus isolates from the soft tick Ornithodoros erraticus collected on three farms in the province of Alentejo in Portugal were characterized by their ability to cause haemadsorption (HAD) of red blood cells to infected pig macrophages, using restriction enzyme site mapping of the virus genomes and by experimental infection of pigs. Six virus isolates induced haemadsorption and four were non-haemadsorbing (non-HAD) in pig macrophage cell cultures. The restriction enzyme site maps of two non-HAD viruses, when compared with a virulent HAD isolate, showed a deletion of 9.6 kbp in the fragment adjacent to the left terminal fragment and of 1.6 kbp in the right terminal fragment and an insertion of 0.2 kbp in the central region. The six HAD viruses isolated were pathogenic and produced typical acute African swine fever in pigs and the four non-HAD isolates were non-pathogenic. Pigs that were infected with non-HAD viruses were fully resistant or had a delay of up to 14 days in the onset of disease, after challenge with pathogenic Portuguese viruses. Non-HAD viruses could be transmitted by contact but with a lower efficiency (42-50 %) compared with HAD viruses (100 %). The clinical differences found between the virus isolates from the ticks could have implications for the long-term persistence of virus in the field because of the cross-protection produced by the non-pathogenic isolates. This may also explain the presence of seropositive pigs in herds in Alentejo where no clinical disease had been reported.

African swine fever virus multigene family 360 and 530 genes affect host interferon response

African swine fever virus (ASFV) multigene family 360 and 530 (MGF360/530) genes affect viral growth in macrophage cell cultures and virulence in pigs (L. Zsak, Z. Lu, T. G. Burrage, J. G. Neilan, G. F. Kutish, D. M. Moore, and D. L. Rock, J. Virol. 75:3066-3076, 2001). The mechanism by which these novel genes affect virus-host interactions is unknown. To define MGF360/530 gene function, we compared macrophage transcriptional responses following infection with parental ASFV (Pr4) and an MGF360/530 deletion mutant (Pr4 Delta 35). A swine cDNA microarray containing 7,712 macrophage cDNA clones was used to compare the transcriptional profiles of swine macrophages infected with Pr4 and Pr4 Delta 35 at 3 and 6 h postinfection (hpi). While at 3 hpi most (7,564) of the genes had similar expression levels in cells infected with either virus, 38 genes had significantly increased (>2.0-fold, P < 0.05) mRNA levels in Pr4 Delta 35-infected macrophages. Similar up-regulation of these genes was observed at 6 hpi. Viral infection was required for this induced transcriptional response. Most Pr Delta 35 up-regulated genes were part of a type I interferon (IFN) response or were genes that are normally induced by double-stranded RNA and/or viral infection. These included monocyte chemoattractant protein, transmembrane protein 3, tetratricopeptide repeat protein 1, a ubiquitin-like 17-kDa protein, ubiquitin-specific protease ISG43, an RNA helicase DEAD box protein, GTP-binding MX protein, the cytokine IP-10, and the PKR activator PACT. Differential expression of IFN early-response genes in Pr4 Delta 35 relative to Pr4 was confirmed by Northern blot analysis and real-time PCR. Analysis of IFN-alpha mRNA and secreted IFN-alpha levels at 3, 8, and 24 hpi revealed undetectable IFN-alpha in mock- and Pr4-infected macrophages but significant IFN-alpha levels at 24 hpi in Pr4 Delta 35-infected macrophages. The absence of IFN-alpha in Pr4-infected macrophages suggests that MGF360/530 genes either directly or indirectly suppress a type I IFN response. An inability to suppress host type I IFN responses may account for the growth defect of Pr4 Delta 35 in macrophages and its attenuation in swine.

African swine fever virus polyproteins pp220 and pp62 assemble into the core shell

African swine fever virus (ASFV), a complex enveloped DNA virus, expresses two polyprotein precursors, pp220 and pp62, which after proteolytic processing give rise to several major components of the virus particle. We have analyzed the structural role of polyprotein pp62, the precursor form of mature products p35 and p15, in virus morphogenesis. Densitometric analysis of one- and two-dimensional gels of purified virions showed that proteins p35 and p15, as well as the pp220-derived products, are present in equimolecular amounts in the virus particle. Immunoelectron microscopy revealed that the pp62-derived products localize at the core shell, a matrix-like domain placed between the DNA-containing nucleoid and the inner envelope, where the pp220-derived products are also localized. Pulse-chase experiments indicated that the processing of both polyprotein precursors is concomitant with virus assembly. Furthermore, using inducible ASFV recombinants, we show that pp62 processing requires the expression of the pp220 core precursor, whereas the processing of both precursors pp220 and pp62 is dependent on expression of the major capsid protein p72. Interestingly, when p72 expression is blocked, unprocessed pp220 and pp62 polyproteins assemble into aberrant zipper-like elements consisting of an elongated membrane-bound protein structure reminiscent of the core shell. Moreover, the two polyproteins, when coexpressed in COS cells, interact with each other to form zipper-like structures. Together, these findings indicate that the mature products derived from both polyproteins, which collectively account for about 30% of the virion protein mass, are the basic components of the core shell and that polyprotein processing represents a maturational process related to ASFV morphogenesis.

Swine fever: classical swine fever and African swine fever

Because of the clinical and pathologic similarity to common endemic diseases, introduction of CSFV or ASFV strains of moderate to low virulence represents the greatest risk to North American swine herds. Producers, veterinarians, and diagnosticians should increase their awareness of these devastating diseases and request specific diagnostic testing whenever they are suspected. Production practices that improve biosecurity will reduce the risk of introduction of CSF and ASF and limit the spread if an incursion occurs. Additional resources. The following Web sites contain excellent color photographs that will assist producers and practitioners in identifying clinical signs and gross lesions associated with CSFV and ASFV: http://www.vet.uga.edu/vpp/gray_book/FAD and http://www.pighealth.com. The latter Web site and the OIE Web site (http://www.oie.int) offer updated information on current worldwide epizootics of ASF and CSF and other swine diseases. Details of biosecurity procedures can be found at http://www.agebb.missouri.edu; see publication G2340.

Repression of African swine fever virus polyprotein pp220-encoding gene leads to the assembly of icosahedral core-less particles

African swine fever virus (ASFV) polyprotein pp220, encoded by the CP2475L gene, is an N-myristoylated precursor polypeptide that, after proteolytic processing, gives rise to the major structural proteins p150, p37, p34, and p14. These proteins localize at the core shell, a matrix-like virus domain placed between the DNA-containing nucleoid and the inner envelope. In this study, we have examined the role of polyprotein pp220 in virus morphogenesis by means of an ASFV recombinant, v220i, containing an inducible copy of the CP2475L gene regulated by the Escherichia coli repressor-operator system. Under conditions that repress pp220 expression, the virus yield of v220i was about 2.6 log units lower than that of the parental virus or of the recombinant grown under permissive conditions. Electron microscopy revealed that pp220 repression leads to the assembly of icosahedral particles virtually devoid of the core structure. Analysis of recombinant v220i by immunoelectron microscopy, immunoblotting, and DNA hybridization showed that mutant particles essentially lack, besides the pp220-derived products, a number of major core proteins as well as the viral DNA. On the other hand, transient expression of the CP2475L gene in COS cells showed that polyprotein pp220 assembles into electron-dense membrane-bound coats, whereas a mutant nonmyristoylated version of pp220 does not associate with cellular membranes but forms large cytoplasmic aggregates. Together, these findings indicate that polyprotein pp220 is essential for the core assembly and suggest that its myristoyl moiety may function as a membrane-anchoring signal to bind the developing core shell to the inner viral envelope.