Enhancement of baculovirus plaque assay in insect cell monolayers by DEAE-dextran.

The presence of DEAE-dextran in the agarose overlay, when titrating wild-type or recombinant baculoviruses by plaque assay, resulted in a higher definition and contrast of viral plaques and increased the plaque number and mean diameter at least by a factor of 2x on day 5 post infection. This increase was related to neither a larger production of infectious virus nor of recombinant protein and did not occur when the polycation was only present in the virus inoculum or when insect cell monolayers were preincubated for 60 min with it. The extension of the observations to a number of different recombinant viruses from varied sources, including several baculoviruses that could not consistently produce plaques in the absence of the polycation, substantiates the use of DEAE-dextran to perform a more reliable, faster and reproducible plaque assay of recombinant baculoviruses.

Protein cell receptors mediate the saturable interaction of African swine fever virus attachment protein p12 with the surface of permissive cells.

Previous studies have demonstrated that the entry of African swine fever virus (ASFV) into Vero cells and swine macrophages is mediated by saturable binding sites located on the plasma membrane. The ASFV protein p12 has been implicated in virus attachment to the host cell, but the cellular component responsible for the interaction with the virus is largely unknown. We have studied the binding of recombinant p12 and ASFV to different cell lines. Permissive cells were able to bind p12 in saturable and nonsaturable interactions, as reported for ASFV. Experiments of binding recombinant p12 have been used for the initial characterization of the specific receptors on Vero cells. The treatment of cell surfaces with different enzymes and lectins resulted in the inhibition of the p12 binding activity by several proteases, but not by glycosidases or lipase, suggesting that the receptor is composed of protein, with no carbohydrates or lipids involved in the virus attachment to the cellular membrane. The recovery of receptor activity after pronase treatment was completed in 6 h in culture medium containing tunicamycin, and could not be restored in the presence of cycloheximide, confirming that synthesis of new proteins, but not glycosylation, was required for the recovery of the receptor activity. These data support the idea that membrane protein(s) on the surface of permissive cells act as receptors for ASFV and that this specific interaction is, at least, one necessary step in a productive virus infection.

Interaction of African swine fever virus with macrophages.

Morphological data obtained by electron microscopy have shown that African swine fever virus adapted to VERO cells enters swine macrophages, its natural host cell, by a mechanism of receptor-mediated endocytosis. Binding studies with 3H-labeled virus and competition experiments with UV-inactivated virus have shown that the virus entry that leads to a productive infection in swine macrophages is mediated by saturable binding sites on the plasma membrane. The virus also penetrated into rabbit macrophages that do not produce infectious virus and initiated the synthesis of some early viral proteins; however, the viral replication cycle was aborted since viral DNA synthesis did not occur. The interaction of ASF virus particles with rabbit macrophages was mediated by nonsaturable binding sites, suggesting that the lack of specific receptors in these cells may be related to the absence of a productive infection. A similar abortive infection was detected in macrophages from other virus-resistant animal species.

Production and titration of African swine fever virus in porcine alveolar macrophages.

The broncho-alveolar lavage of a pig (20-40 kg) contains about 1.6 x 10(9) alveolar cells, half of which were macrophages. The number of cells in the lavage of bacille Calmette Guerin (BCG)-treated pigs increased about 4-fold. Both African swine fever virus-infected porcine alveolar macrophages and blood monocytes produced about 1000 hemadsorption units/cell, a value 10-fold larger than that obtained in virus-infected Vero cells. Porcine alveolar cells could be stored frozen and, after thawing, they could be infected with African swine fever virus, producing the same amount of virus as the unfrozen cells. With the number of alveolar macrophages obtained from a single pit it is possible to titer about 3000 virus samples with the same stock of alveolar macrophages.

Production and purification of recombinant African swine fever virus attachment protein p12.

The conditions for cultivation of Spodoptera frugiperda (Sf9) insect cells for production of recombinant baculoviruses have been studied, to scale-up and improve the efficiency of the process for production of the African swine fever virus attachment protein p12 in the baculovirus expression system. It was shown that the total virus and recombinant protein production in insect cells infected with the Acp12 recombinant baculovirus were slightly dependent on cell density, but largely dependent on the serum concentration, in the case of suspended cells, but not in static monolayer cultures. The yield of recombinant protein p12 exceeded 50 mg per 1 of 2 x 10(9) cells, representing more than 10% of total cell proteins, a level > 20-fold higher than that observed with other eukaryotic expression systems. The presence of p12 in the cytoplasmic fraction of infected cells has allowed the purification of the protein by a simple two-step procedure of aqueous phase partition and octyl-glucoside solubilization. The recombinant protein p12 was able to inhibit, in a dose-dependent manner, the African swine fever virus production in swine macrophages infected with a number of different virus isolates, including attenuated, virulent, highly passaged on tissue culture, and non-haemadsorbing strains, indicating a fundamental role for p12 in the early interaction of the virus with the natural target cell receptors. However, pigs immunized with purified recombinant p12 did not develop protective immunity against African swine fever.

Comparative evaluation of novel African swine fever virus (ASF) antibody detection techniques derived from specific ASF viral genotypes with the OIE internationally prescribed serological tests.

The presence of antibodies against African swine fever (ASF), a complex fatal notifiable OIE disease of swine, is always indicative of previous infection, since there is no vaccine that is currently used in the field. The early appearance and subsequent long-term persistence of antibodies combined with cost-effectiveness make antibody detection techniques essential in control programmes. Recent reports appear to indicate that the serological tests recommended by the OIE for ASF monitoring are much less effective in East and Southern Africa where viral genetic and antigenic diversity is the greatest. We report herein an extensive analysis including more than 1000 field and experimental infection sera, in which the OIE recommended tests are compared with antigen-specific ELISAs and immuno-peroxidase staining of cells (IPT). The antibody detection results generated using new antigen-specific tests, developed in this study, which are based on production of antigen fractions generated by infection and virus purification from COS-1 cells, showed strong concordance with the OIE tests. We therefore conclude that the lack of success is not attributable to antigenic polymorphism and may be related to the specific characteristics of the local breeds African pigs.

African swine fever virus attachment protein.

Treatment of African swine fever virus particles with nonionic detergents released proteins p35, p17, p14, and p12 from the virion. Of these proteins, only p12 bound to virus-sensitive Vero cells but not to virus-resistant L or IBRS2 cells. The binding of p12 was abolished by whole African swine fever virus and not by similar concentrations of subviral particles that lacked the external proteins. A monoclonal antibody (24BB7) specific for p12 precipitated a protein that, when analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the absence of 2-mercaptoethanol, showed a molecular mass of 17 kDa (p17*) instead of 12 kDa as found in the presence of 2-mercaptoethanol. The relationship between these two proteins was confirmed by the conversion of p17* to p12 when the former was isolated from polyacrylamide gels in the absence of 2-mercaptoethanol and subsequently treated with the reducing agent. The supernatant obtained after immunoprecipitation with the p12-specific antibody lacked the virus-binding protein.

Localization of the African swine fever virus attachment protein P12 in the virus particle by immunoelectron microscopy.

The African swine fever virus attachment protein p12 was localized in the virion by immunoelectron microscopy. Purified virus particles were incubated, before or after different treatments, with p12-specific monoclonal antibody 24BB7 and labeled with protein A-colloidal gold. Untreated virus particles showed labeling only in lateral protrusions that followed the external virus envelope. Mild treatment of African swine fever virions with the nonionic detergent octyl-glucoside or with ethanol onto the electron microscope grid resulted in a heavier and more homogeneous labeling of the virus particles. In contrast, the release of the external virus proteins by either octyl-glucoside or Nonidet-P40 and beta-mercaptoethanol generated a subviral fraction that was not labeled by 24BB7. Preembedding, labeling, and thin-sectioning experiments confirmed that the antigenic determinant recognized by 24BB7 was localized into the external region of the virus particle but required some disruption to make it more accessible. From these results we conclude that protein p12 is situated in a layer above the virus capsid with, at least, one epitope predominantly not exposed in the virion surface; this epitope may not be related to the virus ligand-cell receptor interaction.

The entry of African swine fever virus into Vero cells.

The entry of African swine fever virus into Vero cells has been investigated by both biochemical and morphological techniques. A quantitative electron microscopy analysis of the early steps of the infection has shown that African swine fever virus enters Vero cells by a receptor-mediated endocytosis mechanism. The internalization of virus particles is a temperature- and energy-dependent process, since it did not take place at 4 degrees or in the presence of NaF and 2,4-dinitrophenol. To determine the involvement of acidic intracellular vacuoles in the virus entry pathway we have tested the effect of lysosomotropic agents in the infection. Chloroquine, dansylcadaverine, amantadine, methylamine, and ammonium chloride inhibited African swine fever virus production in Vero cells. Dansylcadaverine and chloroquine did not inhibit virus adsorption and internalization; however, in the presence of these drugs, virus particles were retained in cytoplasmic vacuoles and early viral RNA and protein synthesis were not detected, indicating that these compounds inhibit an early step in the infectious cycle, probably the uncoating of the virus particle.

Characterization of the african swine fever virus protein p49: a new late structural polypeptide.

The open reading frame B438L, located within the EcoRI B fragment of the African swine fever virus genome, is predicted to encode a protein of 438 amino acids with a molecular mass of 49.3 kDa. It presents a cell attachment RGD (Arg-Gly-Asp) motif but no other significant similarity to protein sequences in databases. Northern blot and primer extension analysis showed that B438L is transcribed only at late times during virus infection. The B438L gene product has been expressed in Escherichia coli, purified and used as an antigen for antibody production. The rabbit antiserum specific for pB438L recognized a protein of about 49 kDa in virus-infected cell extracts. This protein was synthesized late in infection by all the virus strains tested, was located in cytoplasmic virus factories and appeared as a structural component of purified virus particles.

Isolation and properties of the DNA of African swine fever (ASF) virus.

African swine fever (ASF) virus was grown either in swine macrophages or in VERO cells and purified free of cell DNA. Virus DNA was isolated from virions as a molecule with a sedimentation coefficient of 60S and a contour of 58 +/- 3 mum. .these two values give a mol. wt. of 102 +/- 5 X 10(6) and 107 +/- 5 X 10(6), respectively, for the genome of ASF virus. Denatured DNA fragments from ASF virus reassociate with a C0t1/2 value of 0-60 +/- 0-05 MS, which compared with the corresponding value for T4 DNA gives for the molecular mass of ASF virus DNA a value of 102 +/- 8 X 10(6) daltons. Only virus DNA is synthesized ASF virus-infected swine macrophages.

Saturable binding sites mediate the entry of African swine fever virus into Vero cells.

Binding experiments of 3H-labeled African swine fever virus to susceptible VERO cells have shown the presence of saturable binding sites for African swine fever virus on the plasma membrane. The Scatchard analysis of the binding data at equilibrium indicates the existence of about 10(4) cellular receptor sites per cell with a dissociation constant (Kd) of 70 pM. Virus entry into VERO cells is mediated by a saturable component, since tritiated African swine fever virus saturable binding and uptake were competed by the same amounts of unlabeled virus. Similarly, early viral protein synthesis and virus production were inhibited by concentrations of uv-inactivated virus that competed virus attachment to saturable binding sites, suggesting that specific receptors mediate the entry of African swine fever virus particles that initiate a productive infection in VERO cells. African swine fever virus binding to virus-resistant L cells was not mediated by saturable binding sites. As a result of the nonsaturable interaction the virus was not able to enter L cells and neither early viral protein synthesis nor viral DNA synthesis was detected, indicating that the absence of specific receptors for African swine fever virus is a factor that determines the resistance of L cells to the infection.

African swine fever virus EP153R open reading frame encodes a glycoprotein involved in the hemadsorption of infected cells.

The open reading frame EP153R, located within the EcoRI E' fragment of the African swine fever (ASF) virus genome, is predicted to encode a membrane protein of 153 amino acids that presents significant homology to the N-terminal region of several CD44 molecules. EP153R contains multiple putative sites for N-glycosylation, phosphorylation, and myristoylation, a central transmembrane region, a C-type animal lectin-like domain, and a cell attachment sequence. Transcription of EP153R takes place at both early and late times during the virus infection. The disruption of the gene, achieved by insertion of the marker gene LacZ within EP153R, did not change either the in vitro virus growth rate or the virus-sensitive/resistant condition of up to 17 established cell lines, but abrogated the hemadsorption phenomenon induced in ASF virus-infected cells. As the sequence and expression of the ASF virus protein pEP402R, a CD2 homolog responsible for the adhesion of erythrocytes to susceptible cells, was unaffected in cultures infected with the EP153R deletion mutant, we conclude that the gene EP153R is needed to induce and/or maintain the interaction between the viral CD2 homolog and its corresponding cell receptor.

Inhibition of African swine fever (ASF virus replication by phosphonoacetic acid.

Phosphonoacetic acid (PAA) inhibits the multiplication of African swine fever (ASF) virus in VERO cells. The observed inhibition of the in vivo DNA synthesis could be related to the in vitro inhibition of a virus-induced DNA polymerase activity present in cytoplasmic extracts from infected VERO cells.

Virus-specific cell receptors are necessary, but not sufficient, to confer cell susceptibility to African swine fever virus.

The entry of African swine fever (ASF) virus into Vero cells and swine macrophages is mediated by saturable binding sites located in the plasma membrane, which have been related, as in other virus-cell systems, to the sensitivity of the cell to the virus. In order to define this correlation, we have analyzed up to 16 cell lines derived from different species for their sensitivity to virus infection, to determine the step in the virus infective cycle that was blocked in each resistant cell, the presence of saturable cell receptors and the percentage of bound and internalized virus in these cells. Specific ASF virus receptors were found in different quantities in many sensitive and resistant cell lines. The most restricted cells showed a reduced efficiency of virus binding and virus internalization, as well as a lower amount of cell receptors for the virus attachment protein p12. Other resistant cells were restricted only after early virus translation or virus DNA replication, proving that the presence of virus-specific receptors may be necessary, but not sufficient, to guarantee the cell permissiveness to the virus, and that the ASF virus infection can be arrested at different steps on the infective cycle.

Titration of African swine fever (ASF) virus.

A haemadsorption microtest for African swine fever (ASF) virus is described. This assay is as sensitive and its response is faster than the conventional assay which uses buffy coat cultures in Leighton tubes. The method can also process a larger number of samples by using smaller amounts of swine blood and laboratory space. A plaque assay for ASF virus adapted to grow in VERO cells gives a titre similar to that obtained using the haemadsorption microtest. In both the micromethod and the plaque assay infection may be produced by a single infective particle.

Plaque assay for African swine fever virus on swine macrophages.

A plaque assay developed to detect the infection of African Swine Fever Virus on swine macrophages is described. Plaques were generated by all of the virus isolates tested. The method is suitable not only for virus titration but also for the selection of clones in protocols for isolation/purification of recombinant viruses.

Purification and properties of African swine fever virus.

We describe a method for African swine fever (ASF) virus purification based on equilibrium centrifugation in Percoll density gradients of extracellular virions produced in infected VERO cells that yielded about 15 +/- 9% recovery of the starting infectious virus particles. The purified virus preparations were essentially free of a host membrane fraction (vesicles) that could not be separated from the virus by previously described purification methods. The purified virus sedimented as a single component in sucrose velocity gradients with a sedimentation coefficient of 3,500 +/- 300S, showed a DNA-protein ratio of 0.18 +/- 0.02 and a specific infectivity of 2.7 X 10(7) PFU/micrograms of protein, and remained fully infectious after storage at -70 degrees C for at least 7 months. The relative molecular weights of the 34 polypeptides detected in purified virus particles ranged from 10,000 to 150,000. Some of these proteins were probably cellular components that might account for the reactivity of purified virus with antiserum against VERO cells.

Localization of structural proteins in African swine fever virus particles by immunoelectron microscopy.

Seven African swine fever virus structural proteins were localized in the virion by immunoelectron microscopy. African swine fever virus-infected cells were incubated, before or after embedding and thin sectioning, with monoclonal antibodies specific for different structural proteins, and after labeling with protein A-gold complexes, the samples were examined in the electron microscope. Proteins p14 and p24 were found in the external region of the virion, proteins p12, p72, p17, and p37 were found in the intermediate layers, and protein p150 was found in the nucleoid and in one vertex. A monoclonal antibody that recognized protein p150 as well as p220, a virus-induced, nonstructural protein, could also bind to a component present in the nucleus of both uninfected and virus-infected cells.

Amino acid sequence and structural properties of protein p12, an African swine fever virus attachment protein.

The gene encoding the African swine fever virus protein p12, which is involved in virus attachment to the host cell, has been mapped and sequenced in the genome of the Vero-adapted virus strain BA71V. The determination of the N-terminal amino acid sequence and the hybridization of oligonucleotide probes derived from this sequence to cloned restriction fragments allowed the mapping of the gene in fragment EcoRI-O, located in the central region of the viral genome. The DNA sequence of an EcoRI-XbaI fragment showed an open reading frame which is predicted to encode a polypeptide of 61 amino acids. The expression of this open reading frame in rabbit reticulocyte lysates and in Escherichia coli gave rise to a 12-kDa polypeptide that was immunoprecipitated with a monoclonal antibody specific for protein p12. The hydrophilicity profile indicated the existence of a stretch of 22 hydrophobic residues in the central part that may anchor the protein in the virus envelope. Three forms of the protein with apparent molecular masses of 17, 12, and 10 kDa in sodium dodecyl sulfate-polyacrylamide gel electrophoresis have been observed, depending on the presence of 2-mercaptoethanol and alkylation with 4-vinylpyridine, indicating that disulfide bonds are responsible for the multimerization of the protein. This result was in agreement with the existence of a cysteine-rich domain in the C-terminal region of the predicted amino acid sequence. The protein was synthesized at late times of infection, and no posttranslational modifications such as glycosylation, phosphorylation, or fatty acid acylation were detected.