Characterization of African swine fever virus antigenic proteins by immunoprecipitation

Expression in vivo and in vitro of the major structural protein (VP73) of African swine fever virus

The VP73 protein was produced by in vitro transcription and translation from the Xho I-Bam HI fragment located between the Cla I-N and Cla I-H fragments of the viral genome. This DNA fragment encodes a late mRNA of about 2.6 kb detected in infected MS monkey and BHK hamster cells. The transcript was initiated at a site within two bases upstream of the translation initiation codon. The in vitro synthesized polypeptide shows the same molecular weight as the in vivo synthesized polypeptide, suggesting that VP73 has no post-translational modification. There are two internal AUG initiation codons for in vitro translation, one of which is functional in vivo, as well as a possible GUG initiator codon detected by expression of the protein in E. coli cells.

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.

Inhibition of African swine fever virus in cultured swine monocytes by phosphonoacetic acid (PAA) and by phosphonoformic acid (PFA)

The use of phosphonoacetic (PAA) and phosphonoformic acid (PFA) as inhibitors of African swine fever virus (ASFV) replication in porcine monocytes/macrophages (MO) was investigated. At concentrations sufficient to inhibit replication, hemadsorption, and cytopathogenic damage by high inocula of ASFV, both antiviral agents were cytostatic and suppressed the DNA-synthetic growth response of porcine MO to the MO-specific colony-stimulating factor-1 (CSF-1). PAA and PFA inhibited ASFV-associated DNA-synthesis in the cytoplasm of infected swine MO. Using ASFV-specific monoclonal antibodies in immunebinding assays and in immunoprecipitation analysis of radiolabeled proteins of infected MO, PAA and PFA inhibited the synthesis of ASFV proteins of 13, 73, and 150/220 kDa, and caused a variable inhibition in the synthesis of a 12 kDa ASFV protein. These antiviral drugs, however, did not prevent the appearance of an early 32 kDa ASFV protein. The cytostatic and virus-suppressive effects of PAA and PFA could be reversed. ASFV resumed growth in infected MO cultures, if the cells maintained in medium with CSF-1 were removed from the antivirals before 1 week of drug exposure. With prolonged exposure to PAA or PFA (beyond 1 week), ASFV could not be recovered from infected MO cultures.

Inhibition of African swine fever virus DNA synthesis by (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)adenine

The acyclic nucleotide analogue (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)adenine [(S)-HPMPA] is a potent and selective inhibitor of African swine fever virus (ASFV) replication. Using the DNA-DNA hybridization technique with plasmid pRPEL-2 as probe, we have shown that (S)-HPMPA exerts a specific, dose-dependent, inhibitory effect on viral DNA synthesis. Also, (S)-HPMPA inhibits the production of late viral proteins, especially IP-73, in ASFV-infected MS and Vero cells. When evaluated under the same experimental conditions, phosphonoacetic acid (PAA) also caused an inhibition of viral DNA and late viral protein synthesis but only so at a concentration which was 10- to 20-fold higher than that required for (S)-HPMPA.

Epitopic diversity of African swine fever virus

African swine fever (ASF) is caused by an icosahedral cytoplasmic, double stranded DNA virus. In the acute form of the disease, pigs die from disseminated intravascular coagulation (DIC) with extensive damage of the free and fixed macrophage systems and the reticular epithelial cells of the thymus; mortality is virtually 100%. In recent years, subacute and chronic forms of ASF have become more prevalent in the field, especially in outbreaks occurring outside the continent of Africa, and virus isolated from these outbreaks have often been of lesser virulence. In pigs experimentally infected with such isolates, a number of immunopathological manifestations have been encountered, e.g. hypergammaglobulinemia associated with necrotizing pneumonia, persistent infection in the presence of ASF-specific antibodies, and lack of demonstrable virus neutralizing antibodies. Nevertheless, the immune systems of pigs that have clinically recovered have not been impaired by the infection. We suggest that the heterogeneous composition of the virus population in a given isolate may be one of the causes of the anomalous immune responses. When a number of biological markers, i.e., hemadsorption characteristics, plaque size, infectivity, virulence, antigenic determinants, and genomic structure, were used to characterize the virus clones derived from various ASF virus (ASFV) isolates, considerable heterogeneity was apparent. In the present investigation, 20 monoclonal antibodies (MAb), which specifically identified the 14 kDa viral protein within the cytoplasmic membrane of the infected cells, were used to determine epitopic differences among a number of virus clones derived from various isolates. All of the non-African isolates examined contained two epitopically different groups of virus clones, and the reaction profiles obtained were distinctly different from those obtained with the clones of an African isolate (Tengani). It was concluded that an ASFV isolate is composed of a biologically diverse virus population with distinctly different members which are only identified after cloning.

Proteins specified by African swine fever virus: V. Identification of immediate early, early and late proteins

Autoradiographic analysis of polypeptides separated by polyacrylamide gel electrophoresis revealed that, after infection with ASFV, MS cells synthesized at least 44 new polypeptides, that could be classified as immediate early, early or late proteins. A new viral polypeptide (IP 78) was detected by treatment of cells with cycloheximide for 14 hours.

Two-dimensional analysis of African swine fever virus proteins and proteins induced in infected cells

Two-dimensional (2D) analysis of African swine fever (ASF) virus purified by Percoll gradient centrifugation resolves 54 structural proteins, 30 in conventional IEF gels and 24 in NEPHGE gels, while only 26 structural proteins are separated by SDS-PAGE. The two main bands separated by SDS-PAGE, with mol wt 150K and 72K, correspond to single spots in 2D gels. Other bands, including major bands of 38K, 35K, 24K, 17K, and 15.5K mol wt, correspond to multiple proteins of the same molecular weight but different pI. One hundred six virus-specific proteins were resolved by 2D analysis, 59 in conventional IEF gels and 47 in NEPHGE gels. Thirty-five of the virus-specific proteins are early proteins, synthesized before DNA replication, and the remaining 71 proteins are late proteins. Early proteins belong to two groups: 11 transient early proteins are synthesized only early in infection and the other 24 are persistent early proteins, synthesized at both early and late phases. Treatment with cytosine arabinoside prevents the synthesis of late proteins and blocks the shut-off of the synthesis of transient early proteins. Eleven structural proteins are major early proteins and 28 are late proteins. The remaining 15 structural proteins migrate in 2D gels like cellular proteins. Three of these cellular proteins, with mol wt 58K, 56K, and 45K were identified by immunoblotting as alpha-tubulin, beta-tubulin, and actin, respectively.

Polypeptides and structure of African swine fever virus

Extracellular and intracellular African swine fever virus (ASFV) was purified using a two-phase aqueous polymer system. Both the structure of the virus and the polypeptides present during the purification procedure were studied. After PEG/dextran phase separation and centrifugation through 20% (w/v) Ficoll, 79% of input infectivity was recovered as semi-purified virus. The density of the virus after equilibrium centrifugation in sucrose was 1.19 g/ml. The envelope of the virion consisting of a unit membrane was removed from the virion after centrifugation in sucrose. Removal of envelope was associated with the loss of a 230 kilodalton (kd) glycoprotein from the virion. Disruption of the viral surface structure resulted in a loss of infectivity. Eighteen of the most prominent of the 33 polypeptides of extracellular or cell free (CF) virus were those with molecular weights of 230, 195, 165, 155, 150, 125, 116, 97, 92, 73, 62, 58, 50, 45, 35, 33, 25 and 11 kd, while the fourteen most prominent polypeptides in intracellular or cell associated (CA) virus were 103, 97, 92, 84, 73, 62, 58, 54, 47, 45, 35, 33, 25 and 17 kd. The 45 kd polypeptide may be actin which copurifies with the virus. No major differences were found in the number or size of proteins among three isolates of ASFV. Electron micrographs of thin sections of ASFV show the capsid to consist of a distinct double layer of closely packed capsomeres enclosed on both sides with a semi-transparent layer. Cell associated virus measured from side-to-side 188 nm and vertex-to-vertex 212 nm. The capsid encloses an inner core composed of a dense nucleoid surrounded by a 40-48 nm layer of core protein.

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.

Monoclonal antibodies against African swine fever viral antigens

Monoclonal antibodies specific for African swine fever (ASF) viral proteins of 14, 32, 73, 174, and 240 kDa were produced and characterized. Immunoelectron microscopy detected the 73 kDa but not the 14-, 32-, or 240-kDa proteins at the surface of the virion. The 32-kDa protein was detected by radioimmunoassay 2 hr after infection of porcine monocytes and Vero cells, was detected in the seven widely divergent ASFV isolates tested, and stained brilliantly virus-infected cells in indirect immunofluorescence suggesting that monoclonal antibodies directed against this protein may be useful in ASFV diagnosis. Two monoclonal antibodies detected heterogeneity between ASF viruses.