Cell culture propagation modifies the African swine fever virus replication phenotype in macrophages and generates viral subpopulations differing in protein p54

A blocking ELISA based on virus-like nanoparticles chimerized with an antigenic epitope of ASFV P54 for detecting ASFV antibodies

African swine fever virus (ASFV) is a highly lethal pathogen of domestic and wild pigs. Due to no vaccines or drugs available, early accurate diagnosis and eradication of infected animals are the most important measures for ASFV prevention and control. Bluetongue virus (BTV) core-like particles (CLPs) are non-infectious hollow nanoparticles assembled from the BTV VP3 and VP7 proteins, which could be used as a platform for presenting foreign epitopes. In this study, the secondary structure of BTV VP7 protein was analyzed and predicted using the IEDB Analysis resource. Based on the prediction results of the VP7 protein, the chimeric CLPs with an ASFV P54 epitope were successfully prepared through the BAC-to-BAC baculovirus expression system and sucrose gradient centrifugation. Based on the chimeric CLPs and mAb 2E4 against AFSV P54 epitope, a blocking ELISA for detecting AFSV antibodies was established, and its reaction conditions were optimized. Through comprehensive evaluation of the method, the results showed the chimeric CLPs-based blocking ELISA displayed the best detection performance, with an AUC of 0.9961, a sensitivity of 97.65%, and a specificity of 95.24% in ROC analysis. Compared with western blot and a commercial c-ELISA for detecting anti-ASFV antibodies, this method had an excellent agreement of 96.35% (kappa value = 0.911) and 97.76% (kappa value = 0.946) with the other tests, respectively. This ELISA also had high repeatability, with CV < 10%, and no cross-reaction with the serum antibodies against other swine viruses or Orbivirus. In brief, this was the first report on developing a blocking ELISA based on virus-like nanoparticles chimerized with an antigenic epitope of ASFV P54 for serological diagnosis of ASFV.

In silico and pepscan analysis of African swine fever virus p54 protein

Due to the great importance and economic impact of African swine fever, great emphasis is placed on the diagnosis of its causative agent and the development of new types of vaccines. However, a prerequisite for the shift in these areas is a more thorough understanding of the structure and characteristics of virus proteins. The aim of this work was to analyze the location of B-cell epitopes in the structure of the African swine fever virus p54 protein, combining the results of in silico and pepscan analysis. In silico analysis revealed the presence of potential B-cell epitopes primarily in the C-terminal part of the protein, pepscan technique confirmed these immunogenic regions between amino acids 24-61 and 151-182.

Cell Lines for the Development of African Swine Fever Virus Vaccine Candidates: An Update

African swine fever virus (ASFV) is the etiological agent of a highly lethal disease in both domestic and wild pigs. The virus has rapidly spread worldwide and has no available licensed vaccine. An obstacle to the construction of a safe and efficient vaccine is the lack of a suitable cell line for ASFV isolation and propagation. Macrophages are the main targets for ASFV, and they have been widely used to study virus–host interactions; nevertheless, obtaining these cells is time-consuming and expensive, and they are not ethically suitable for the production of large-scale vaccines. To overcome these issues, different virulent field isolates have been adapted on monkey or human continuous cells lines; however, several culture passages often lead to significant genetic modifications and the loss of immunogenicity of the adapted strain. Thus, several groups have attempted to establish a porcine cell line able to sustain ASFV growth. Preliminary data suggested that some porcine continuous cell lines might be an alternative to primary macrophages for ASFV research and for large-scale vaccine production, although further studies are still needed. In this review, we summarize the research to investigate the most suitable cell line for ASFV isolation and propagation.

Characterization of Anti-p54 Monoclonal Antibodies and Their Potential Use for African Swine Fever Virus Diagnosis

African swine fever (ASF) is a highly lethal hemorrhagic viral disease of domestic pigs caused by African swine fever virus (ASFV). Although a good advance has been made to understand the virus, a safe and effective vaccine against ASFV is still lacking and its eradication solely depends on its early and accurate diagnosis. Thus, improving the available diagnostic assays and adding some validated techniques are useful for a range of serological investigations. The aim of this study was to produce and characterize p54 monoclonal antibodies with an ultimate goal of developing a monoclonal antibody-based enzyme-linked immunosorbent assay (ELISA) for ASFV antibody detection. Five monoclonal antibodies against p54 protein expressed in Escherichia coli was generated and their characterizations were investigated. Furthermore, a competitive enzyme-linked immunosorbent assay (cELISA) based on a monoclonal antibody designated as 2A7 was developed. To evaluate the performance of the assay, a total of 365 pig serum samples (178 negative and 187 positive samples) were tested and a receiver-operating characteristic (ROC) analysis was applied to determine the cut-off value. Based on the ROC analysis, the area under the curve (AUC) was 0.982 (95% confidence interval: 96.9% to 99.4%), besides a sensitivity of 92.5% and a specificity of 98.9% was achieved when the percent inhibition of 20% was selected as a threshold. Moreover, the result showed an excellent agreement when compared to other commercially available blocking ELISA (kappa value = 0.912) and showed no reaction to other swine pathogens. Overall, the newly developed cELISA method offers a promising approach for a rapid and convenient ASFV serodiagnosis, which could be used as alternative to other serological assays for screening possible ASFV infection.

Epitope mapping of African swine fever virus (ASFV) structural protein, p54

In the absence of a vaccine for African swine fever virus (ASFV), diagnostic tools are critical for early detection and implementation of control measures. Along with other immunogenic proteins, p54 is a good serological target for conducting ASF detection and surveillance. In this study, a panel of 12 mouse monoclonal antibodies (mAbs) was prepared against a baculovirus-expressed p54(60-178) polypeptide. Further screening showed that five mAbs were positive for reactivity against ASFV-infected cells and recombinant p54 proteins. Mapping studies using five polypeptides and 12 oligopeptides, showed that mAb #154-1 recognized a conserved polypeptide sequence, p54(65-75), and was placed into Group 1. Mabs #143-1 and #7 recognized a region covered by p54(93-113) and were placed into Group 2. Group 3 consisted of mAbs #101 and #117, which recognized p54(118-127). Sera from pigs infected with the low virulent OURT 88/3 strain recognized the same p54 region covered by the Group 3 mAbs. When tested in a neutralization format, only mAb #143-1 showed neutralization activity above background. Together, the results identify important antigenic and immunogenic regions located on p54, which provide new tools for improving ASFV diagnostics.

An Update on African Swine Fever Virology

Animal diseases constitute a continuing threat to animal health, food safety, national economy, and the environment. Among those, African swine fever (ASF) is one of the most devastating viruses affecting pigs and wild suids due to the lack of vaccine or effective treatment. ASF is endemic in countries in sub-Saharan Africa, but since its introduction to the Caucasus region in 2007, a highly virulent strain of ASF virus (ASFV) has continued to circulate and spread into Eastern Europe and Russia, and most recently into Western Europe, China, and various countries of Southeast Asia. Given the importance of this disease, this review will highlight recent discoveries in basic virology with special focus on proteomic analysis, replication cycle, and some recent data on genes involved in cycle progression and viral–host interactions, such as I215L (E2 ubiquitin-conjugating enzyme), EP402R (CD2v), A104R (histone-like protein), QP509L, and Q706L (RNA helicases) or P1192R (Topoisomerase II). Taking into consideration the large DNA genome of ASFV and its complex interactions with the host, more studies and new approaches are to be taken to understand the basic virus–host interaction for ASFV. Proteomic studies are just paving the way for future research.

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.

Antibody-mediated neutralization of African swine fever virus: myths and facts

Almost all viruses can be neutralized by antibodies. However, there is some controversy about antibody-mediated neutralization of African swine fever virus (ASFV) with sera from convalescent pigs and about the protective relevance of antibodies in experimentally vaccinated pigs. At present, there is no vaccine available for this highly lethal and economically relevant virus and all classical attempts to generate a vaccine have been unsuccessful. This failure has been attributed, in part, to what many authors describe as the absence of neutralizing antibodies. The findings of some studies clearly contradict the paradigm of the impossibility to neutralize ASFV by means of monoclonal or polyclonal antibodies. This review discusses scientific evidence of these types of antibodies in convalescent and experimentally immunized animals, the nature of their specificity, the neutralization-mediated mechanisms demonstrated, and the potential relevance of antibodies in protection.

African swine fever virus serodiagnosis: a general review with a focus on the analyses of African serum samples

African swine fever (ASF) is an infectious disease that causes heavy mortality in domestic pigs. At present there is no vaccine against ASF, and eradication in countries where the disease is endemic is based only on competent diagnosis programs and the sacrifice of infected animals. Due to the presence of natural attenuated strains, certain infection conditions may result in reduced mortality. In these situations, the disease can be diagnosed by detection of specific antibodies. The use of classical and validated diagnosis assays, such as ELISA and Indirect Immunofluorescence or Immunoblotting, allowed the eradication of ASF in the Iberian Peninsula in the 1990s. However, given that conventional tests include the use of antigens obtained from ASF virus (ASFV)-infected cells, they have several disadvantages, such as difficulties to achieve standardization and also the risks associated with the manipulation of live virus. Such drawbacks have led to the development of alternative and more robust systems for the production of ASFV antigens for use in anti-ASFV antibody detection systems. In the present review, we provide an update on current knowledge about antigen targets for ASFV serodiagnosis, the significant progress made in recombinant antigen production, and the refinement of ASF serological diagnostic assays. Moreover, we describe the accuracy of an ELISA developed for the serodiagnosis of ASFV in Africa. This assay is based on a novel p30 recombinant protein (p30r) obtained from an Eastern African viral isolate (Morara strain), which shares 100% amino acid sequence identity with the Georgia virus isolate. That study included the analyses of 587 field sera collected from domestic pigs and warthogs in Senegal (West Africa), the Democratic Republic of Congo (Central Africa), Mozambique (South-East Africa), and South Africa. The results revealed that the novel p30r-based ELISA allows the accurate detection of antibodies against ASFV, independently of the geographical origin of the sera.

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

The lack of available vaccines against African swine fever virus (ASFV) means that the evaluation of new immunization strategies is required. Here we show that fusion of the extracellular domain of the ASFV Hemagglutinin (sHA) to p54 and p30, two immunodominant structural viral antigens, exponentially improved both the humoral and the cellular responses induced in pigs after DNA immunization. However, immunization with the resulting plasmid (pCMV-sHAPQ) did not confer protection against lethal challenge with the virulent E75 ASFV-strain. Due to the fact that CD8⁺ T-cell responses are emerging as key components for ASFV protection, we designed a new plasmid construct, pCMV-UbsHAPQ, encoding the three viral determinants above mentioned (sHA, p54 and p30) fused to ubiquitin, aiming to improve Class I antigen presentation and to enhance the CTL responses induced. As expected, immunization with pCMV-UbsHAPQ induced specific T-cell responses in the absence of antibodies and, more important, protected a proportion of immunized-pigs from lethal challenge with ASFV. In contrast with control pigs, survivor animals showed a peak of CD8⁺ T-cells at day 3 post-infection, coinciding with the absence of viremia at this time point. Finally, an in silico prediction of CTL peptides has allowed the identification of two SLA I-restricted 9-mer peptides within the hemagglutinin of the virus, capable of in vitro stimulating the specific secretion of IFNγ when using PBMCs from survivor pigs. Our results confirm the relevance of T-cell responses in protection against ASF and open new expectations for the future development of more efficient recombinant vaccines against this disease.

Enhancing DNA immunization by targeting ASFV antigens to SLA-II bearing cells

One of the main criticisms to DNA vaccines is the poor immunogenicity that they confer on occasions, at least in large animals. Confirming this theory, immunization with plasmid DNA encoding two African swine fever virus genes in frame (pCMV-PQ), failed in inducing detectable immune responses in pigs, while it was successful in mice. Aiming to improve the immune responses induced in swine, a new plasmid was constructed, encoding the viral genes fused in frame with a single chain variable fragment of an antibody specific for a swine leukocyte antigen II (pCMV-APCH1PQ). Our results clearly demonstrate that targeting antigens to antigen professional cells exponentially enhanced the immune response induced in pigs, albeit that the DNA vaccine was not able to confer protection against lethal viral challenge. Indeed, a viremia exacerbation was observed in each of the pigs that received the pCMV-APCH1PQ plasmid, this correlating with the presence of non-neutralizing antibodies and antigen-specific SLA II-restricted T-cells. The implications of our discoveries for the development of future vaccines against African swine fever virus and other swine pathogens are discussed.

Small peptide inhibitors disrupt a high-affinity interaction between cytoplasmic dynein and a viral cargo protein

Several viruses target the microtubular motor system in early stages of the viral life cycle. African swine fever virus (ASFV) protein p54 hijacks the microtubule-dependent transport by interaction with a dynein light chain (DYNLL1/DLC8). This was shown to be a high-affinity interaction, and the residues gradually disappearing were mapped on DLC8 to define a putative p54 binding surface by nuclear magnetic resonance (NMR) spectroscopy. The potential of short peptides targeting the binding domain to disrupt this high-affinity protein-protein interaction was assayed, and a short peptide sequence was shown to bind and compete with viral protein binding to dynein. Given the complexity and number of proteins involved in cellular transport, the prevention of this viral-DLC8 interaction might not be relevant for successful viral infection. Thus, we tested the capacity of these peptides to interfere with viral infection by disrupting dynein interaction with viral p54. Using this approach, we report on short peptides that inhibit viral growth.

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.

Analysis of porcine peripheral blood mononuclear cells proteome by 2-DE and MS: Analytical and biological variability in the protein expression level and protein identification

In this paper, we present the protein map corresponding to the porcine peripheral blood mononuclear cells (PBMC) to better understand the role of these cells in the pig immune system. To conform the map, the proteins were separated by 2-DE using a 5-8 range pH gradient in IEF and approximately 800 spots were detected. Due to the high level of indeterminate variability associates to the 2-DE, analytical and biological variances were analyzed. The analytical variance was calculated for 50 proteins in three replicate 2-DE gels from the same protein extract whereas the biological variance was determined by comparison of the patterns obtained for the same 50 proteins in different animals. Values of 15.13 and 33.70% were determined for analytical and biological variances, respectively. These average variances will provide a quantified and statistical basis for future proteomic studies directed to evaluate relevant quantitative changes in the biological response. A representative set of the major proteins was subjected to MALDI-TOF analysis and over 75% of the proteins were identified on the basis of their similarity with its human homologue proteins. A large number of cytoskeletal and metabolic proteins were found as well as some proteins related to cell mobility and immunological functions. Finally, other proteins implicated in the cell signaling process, transport or apoptosis were also identified giving a wide overview of the porcine PBMC protein map.

African swine fever virus structural protein p54 is essential for the recruitment of envelope precursors to assembly sites

The assembly of African swine fever virus (ASFV) at the cytoplasmic virus factories commences with the formation of precursor membranous structures, which are thought to be collapsed cisternal domains recruited from the surrounding endoplasmic reticulum (ER). This report analyzes the role in virus morphogenesis of the structural protein p54, a 25-kDa polypeptide encoded by the E183L gene that contains a putative transmembrane domain and localizes at the ER-derived envelope precursors. We show that protein p54 behaves in vitro and in infected cells as a type I membrane-anchored protein that forms disulfide-linked homodimers through its unique luminal cysteine. Moreover, p54 is targeted to the ER membranes when it is transiently expressed in transfected cells. Using a lethal conditional recombinant, vE183Li, we also demonstrate that the repression of p54 synthesis arrests virus morphogenesis at a very early stage, even prior to the formation of the precursor membranes. Under restrictive conditions, the virus factories appeared as discrete electron-lucent areas essentially free of viral structures. In contrast, outside the assembly sites, large amounts of aberrant zipper-like structures formed by the unprocessed core polyproteins pp220 and pp62 were produced in close association to ER cisternae. Altogether, these results indicate that the transmembrane structural protein p54 is critical for the recruitment and transformation of the ER membranes into the precursors of the viral envelope.

Identification of the principal serological immunodeterminants of African swine fever virus by screening a virus cDNA library with antibody

Protective immunity to African swine fever virus (ASFV) may involve a combination of both serological and cellular mechanisms. This work is focused on the identification of the possible relevant serological immunodeterminants of immunity. Thus, 14 serological immunodeterminants of ASFV have been characterized by exhaustive screening of a representative lambda phage cDNA expression library of the tissue culture-adapted Ba71V strain of ASFV. The library was constructed using RNA extracted from Vero cells infected for 3, 6, 9 and 12 h. A total of 150 clones was selected arbitrarily by antibody screening of the library with a polyclonal antiserum from a domestic pig surviving infection with the virulent Malta isolate of ASFV. Sequencing of these clones permitted identification of 14 independent viral proteins that stimulated an antibody response. These included six proteins encoded by previously unassigned open reading frames (ORFs) (B602L, C44L, CP312R, E184L, K145R and K205R) as well as some of the more well-studied structural (A104R, p10, p32, p54 and p73) and non-structural proteins (RNA reductase, DNA ligase and thymidine kinase). Immunogenicity of these proteins was confirmed by demonstrating the corresponding antibodies in sera from pigs infected either with the Malta isolate or with the OURT88/3-OURT88/1 isolate combination. Furthermore, the majority of these ORFs were also recognized by immune antiserum from the natural host, the bush pig, following secondary challenge with the virulent Malawi (SINT90/1) isolate of ASFV. Thus, it is possible that some of these determinants may be important in protection against virus infection.

Neutralization susceptibility of African swine fever virus is dependent on the phospholipid composition of viral particles

In this study we have investigated the generation of African swine fever (ASF) virus variants resistant to neutralizing antibodies after cell culture propagation. All highly passaged ASF viruses analyzed were resistant to neutralization by antisera from convalescent pigs or antibodies generated against individual viral proteins which neutralized low-passage viruses. A molecular analysis of neutralizable and nonneutralizable virus isolates by sequencing of the genes encoding for neutralizing proteins revealed that the absence of neutralization of high-passage viruses is not due to antigenic variability of critical epitopes. A comparative analysis of phospholipid composition of viral membranes between low- and high-passage viruses revealed differences in the relative amount of phosphatidylinositol in these two groups of viruses, independent of the cells in which the viruses were grown. Further purification of low- and high-passage viruses by Percoll sedimentation showed differences in the phospholipid composition identical to those found with the partially purified viruses and confirmed the susceptibility of these viruses to neutralization. The incorporation of phosphatidylinositol into membranes of high-passage viruses rendered a similar neutralization susceptibility to low-passage viruses, in which this is a major phospholipid. In contrast, other phospholipids did not interfere with high-passage virus neutralization, suggesting that phosphatidylinositol is essential for a correct epitope presentation to neutralizing antibodies. Additionally, the removal of phosphatidylinositol form a low-passage virus by a specific lipase transformed this virus from neutralizable to nonneutralizable. These data constitute clear evidence of the importance of the lipid composition of the viral membranes for the protein recognition by antibodies and may account in part for the past difficulties in reproducibly demonstrating ASF virus-neutralizing antibodies by using high-passage viruses.

Neutralizing antibodies to different proteins of African swine fever virus inhibit both virus attachment and internalization

African swine fever virus induces in convalescent pigs antibodies that neutralized the virus before and after binding to susceptible cells, inhibiting both virus attachment and internalization. A further analysis of the neutralization mechanisms mediated by the different viral proteins showed that antibodies to proteins p72 and p54 are involved in the inhibition of a first step of the replication cycle related to virus attachment, while antibodies to protein p30 are implicated in the inhibition of virus internalization.

The structural protein p54 is essential for African swine fever virus viability

Protein p54, one of the most antigenic structural African swine fever virus (ASFV) proteins, has been localized by immuno-electron microscopy in the replication factories of infected cells, mainly associated with membranes and immature virus particles. Attempts to inactivate the p54 gene from ASFV by targeted insertion of beta-galactosidase selection marker was uniformly unsuccessful, suggesting that this gene is essential for virus viability. To demonstrate that, we inserted in the TK (thymidine kinase) locus of the virus a construction containing a second copy of the p54 gene and beta-glucuronidase selection marker under the control of p54 and p73 promoters, respectively. Virus mutant clones expressing a second copy of p54 and beta-glucuronidase were used to achieve deletion mutants of the original copy of the gene. Virus mutants expressing only the second inserted copy of p54 and the two selection markers mentioned above were successfully obtained. Therefore, we have demonstrated that the p54 gene product plays an essential role in virus growth, characterizing for the first time in ASFV an essential virus gene.

Application of a monoclonal antibody recognizing protein p30 to detect African swine fever virus-infected cells in peripheral blood

Monoclonal antibody (MAb) 174F11.8 recognizes an epitope of the African swine fever (ASF) virus-induced protein, p30, a protein expressed on the plasma membrane of infected cells. This MAb has been used to analyze infected cell populations in peripheral blood of experimentally inoculated pigs with a virulent or attenuated ASF virus. Flow cytometric analysis of peripheral blood at different days postinfection using this MAb, showed expression of p30 mainly in the monocyte/macrophage cell lineage. Additionally, a small percentage of granulocytes also expressed p30 during infection. This methodology allowed the quantification of fluctuations in the pool of infected monocyte/macrophage cells in the inoculated pigs, maximum percentages ranging between 6 and 31%. Significant differences in the percentages of cell populations expressing p30 were not found between virulent or attenuated virus infection. However, a 2- to 4-day delay in the maximum percentage of cells expressing p30 was observed during infection with the attenuated virus when compared to virulent virus infection. Percentages of infected cells detected by the expression of p30 and viral titres obtained in peripheral blood showed positive correlation. Consequently, MAb 174F11.8 constitutes a marker to follow evolution of ASF virus infection, allowing quantification of percentages of infected cells in peripheral blood.

Protein mutations revealed by two-dimensional electrophoresis

High-resolution two-dimensional electrophoresis (2DE) can resolve many hundreds of proteins present in complex mixtures depending on the method of detection. These proteins can be characterised qualitatively, with respect to their electrophoretic mobilities (i.e. charge and apparent molecular mass) and quantitatively, using densitometry, to determine their amounts. There has been a widespread application of 2DE in the analysis and characterisation of protein mutations for a range of organisms. This review presents examples of the use of 2DE to study naturally occurring protein mutations and polymorphisms as well as the characterisation of induced protein mutations in prokaryotes and eukaryotes. Examples are presented to illustrate the use of 2DE to detect mutations affecting the electrophoretic mobility and biosynthesis of individual proteins as well as mutations leading to global alterations in cellular protein synthesis. The advantages and disadvantages of 2DE in the detection of protein mutations are discussed.

Characterization and molecular basis of heterogeneity of the African swine fever virus envelope protein p54

It has been reported that the propagation of African swine fever virus (ASFV) in cell culture generates viral subpopulations differing in protein p54 (C. Alcaraz, A. Brun, F. Ruiz-Gonzalvo, and J. M. Escribano, Virus Res. 23:173-182, 1992). A recombinant bacteriophage expressing a 328-bp fragment of the p54 gene was selected in a lambda phage expression library of ASFV genomic fragments by immunoscreening with antibodies against p54 protein. The sequence of this recombinant phage allowed the location of the p54 gene in the EcoRI E fragment of the ASFV genome. Nucleotide sequence obtained from this fragment revealed an open reading frame encoding a protein of 183 amino acids with a calculated molecular weight of 19,861. This protein contains a transmembrane domain and a Gly-Gly-X motif, a recognition sequence for protein processing of several ASFV structural proteins. In addition, two direct tandem repetitions were also found within this open reading frame. Further characterization of the transcription and gene product revealed that the p54 gene is translated from a late mRNA and the protein is incorporated to the external membrane of the virus particle. A comparison of the nucleotide sequence of the p54 gene carried by two virulent ASFV strains (E70 and E75) with that obtained from virus Ba71V showed 100% similarity. However, when p54 genes from viral clones generated by cell culture passage and coding for p54 proteins with different electrophoretic mobility were sequenced, they showed changes in the number of copies of a 12-nucleotide sequence repeat. These changes produce alterations in the number of copies of the amino acid sequence Pro-Ala-Ala-Ala present in p54, resulting in stepwise modifications in the molecular weight of the protein. These duplications and deletions of a tandem repeat sequence array within a protein coding region constitute a novel mechanism of genetic diversification in ASFV.

Flow cytometric analysis of African swine fever virus-induced plasma membrane proteins and their humoral immune response in infected pigs

African swine fever (ASF) virus-induced plasma membrane proteins may contribute to the protective immune response against the disease since they can be involved in the antibody-mediated lysis of infected cells. In this study we describe the regulation of ASF virus-induced plasma membrane protein expression and its antibody induction in pigs after viral infection by flow cytometric analysis. More than 80% of infected cells contained viral antigens on the surface membranes at 6 hr postinfection (hpi), and the relative amount of viral antigen expression was increased at 12 and 20 hpi. The kinetics of individual viral protein expression on cell surfaces was studied by a collection of monospecific antibodies directed against the six viral plasma membrane proteins p12, p15, p16, p23.5, p30, and p35. Most of these proteins were expressed at 6 hpi, with the exception of p35, which was first detected at 12 hpi. The percentage of cells expressing each antigen at different hpi was also determined. The immune response against virus-induced plasma membrane proteins in pigs infected with an attenuated ASF virus strain was studied. Antibodies against viral epitopes exposed on plasma membranes reached a plateau at 20 days postinfection (dpi). The relative amount of antibodies induced during infection with these specificities was not directly related to the antibody titer of the sera. Sera obtained at 20 and 40 dpi contained antibodies against most of the viral plasma membrane proteins and were most efficient in recognition of viral antigens exposed on the surface of infected cells at early times.