Structural Insights into the Assembly of the African Swine Fever Virus Inner Capsid

African swine fever virus (ASFV), the cause of a highly contagious hemorrhagic and fatal disease of domestic pigs, has a complex multilayer structure. The inner capsid of ASFV located underneath the inner membrane enwraps the genome-containing nucleoid and is likely the assembly of proteolytic products from the virally encoded polyproteins pp220 and pp62. Here, we report the crystal structure of ASFV p150△NC, a major middle fragment of the pp220 proteolytic product p150. The structure of ASFV p150△NC contains mainly helices and has a triangular plate-like shape. The triangular plate is approximately 38 Å in thickness, and the edge of the triangular plate is approximately 90 Å long. The structure of ASFV p150△NC is not homologous to any of the known viral capsid proteins. Further analysis of the cryo-electron microscopy maps of the ASFV and the homologous faustovirus inner capsids revealed that p150 or the p150-like protein of faustovirus assembles to form screwed propeller-shaped hexametric and pentametric capsomeres of the icosahedral inner capsids. Complexes of the C terminus of p150 and other proteolytic products of pp220 likely mediate interactions between the capsomeres. Together, these findings provide new insights into the assembling of ASFV inner capsid and provide a reference for understanding the assembly of the inner capsids of nucleocytoplasmic large DNA viruses (NCLDV). IMPORTANCE African swine fever virus has caused catastrophic destruction to the pork industry worldwide since it was first discovered in Kenya in 1921. The architecture of ASFV is complicated, with two protein shells and two membrane envelopes. Currently, mechanisms involved in the assembly of the ASFV inner core shell are less understood. The structural studies of the ASFV inner capsid protein p150 performed in this research enable the building of a partial model of the icosahedral ASFV inner capsid, which provides a structural basis for understanding the structure and assembly of this complex virion. Furthermore, the structure of ASFV p150△NC represents a new type of fold for viral capsid assembly, which could be a common fold for the inner capsid assembly of nucleocytoplasmic large DNA viruses (NCLDV) and would facilitate the development of vaccine and antivirus drugs against these complex viruses.

Unpicking the Secrets of African Swine Fever Viral Replication Sites

African swine fever virus (ASFV) is a highly contagious pathogen which causes a lethal haemorrhagic fever in domestic pigs and wild boar. The large, double-stranded DNA virus replicates in perinuclear cytoplasmic replication sites known as viral factories. These factories are complex, multi-dimensional structures. Here we investigated the protein and membrane compartments of the factory using super-resolution and electron tomography. Click IT chemistry in combination with stimulated emission depletion (STED) microscopy revealed a reticular network of newly synthesized viral proteins, including the structural proteins p54 and p34, previously seen as a pleomorphic ribbon by confocal microscopy. Electron microscopy and tomography confirmed that this network is an accumulation of membrane assembly intermediates which take several forms. At early time points in the factory formation, these intermediates present as small, individual membrane fragments which appear to grow and link together, in a continuous progression towards new, icosahedral virions. It remains unknown how these membranes form and how they traffic to the factory during virus morphogenesis.

The cryo-EM structure of African swine fever virus unravels a unique architecture comprising two icosahedral protein capsids and two lipoprotein membranes

African swine fever virus (ASFV) is a complex nucleocytoplasmic large DNA virus (NCLDV) that causes a devastating swine disease currently present in many countries of Africa, Europe, and Asia. Despite intense research efforts, relevant gaps in the architecture of the infectious virus particle remain. Here, we used single-particle cryo-EM to analyze the three-dimensional structure of the mature ASFV particle. Our results show that the ASFV virion, with a radial diameter of ∼2,080 Å, encloses a genome-containing nucleoid surrounded by two distinct icosahedral protein capsids and two lipoprotein membranes. The outer capsid forms a hexagonal lattice (triangulation number T = 277) composed of 8,280 copies of the double jelly-roll major capsid protein (MCP) p72, arranged in trimers displaying a pseudo-hexameric morphology, and of 60 copies of a penton protein at the vertices. The inner protein layer, organized as a T = 19 capsid, confines the core shell, and it is composed of the mature products derived from the ASFV polyproteins pp220 and pp62. Also, an icosahedral membrane lies between the two protein layers, whereas a pleomorphic envelope wraps the outer capsid. This high-level organization confers to ASFV a unique architecture among the NCLDVs that likely reflects the complexity of its infection process and may help explain current challenges in controlling it.

Cryo-EM Structure of the African Swine Fever Virus

African swine fever virus (ASFV) is a large double-stranded DNA virus with an icosahedral multilayered structure. ASFV causes a lethal swine hemorrhagic disease and is currently responsible for widespread damage to the pork industry in Asia. Neither vaccines nor antivirals are available and the molecular characterization of the ASFV particle is outstanding. Here, we describe the cryogenic electron microscopy (cryo-EM) structure of the icosahedral capsid of ASFV at 4.6-Å. The ASFV particle consists of 8,280 copies of the major capsid protein p72, 60 copies of the penton protein, and at least 8,340 minor capsid proteins, of which there might be 3 different types. Like other nucleocytoplasmic large DNA viruses, the minor capsid proteins form a hexagonal network below the outer capsid shell, functioning as stabilizers by "gluing" neighboring capsomers together. Our findings provide a comprehensive molecular model of the ASFV capsid architecture that will contribute to the future development of countermeasures, including vaccines.

African Swine Fever Virus: A Review

African swine fever (ASF) is a highly contagious viral disease of swine which causes high mortality, approaching 100%, in domestic pigs. ASF is caused by a large, double stranded DNA virus, ASF virus (ASFV), which replicates predominantly in the cytoplasm of macrophages and is the only member of the Asfarviridae family, genus Asfivirus. The natural hosts of this virus include wild suids and arthropod vectors of the Ornithodoros genus. The infection of ASFV in its reservoir hosts is usually asymptomatic and develops a persistent infection. In contrast, infection of domestic pigs leads to a lethal hemorrhagic fever for which there is no effective vaccine. Identification of ASFV genes involved in virulence and the characterization of mechanisms used by the virus to evade the immune response of the host are recognized as critical steps in the development of a vaccine. Moreover, the interplay of the viral products with host pathways, which are relevant for virus replication, provides the basic information needed for the identification of potential targets for the development of intervention strategies against this disease.

[Virosphere and giruses]

Novel findings and concepts in the field of virology particularly regarding virosphere and giruses--a group of large nuclear-cytoplasmic deoxyriboviruses are briefly summarized. In the context of novel understanding the major taxonomic features and virus pathogenicity including African swine plague are interpreted.

African swine fever virus induces filopodia-like projections at the plasma membrane

When exiting the cell vaccinia virus induces actin polymerization and formation of a characteristic actin tail on the cytosolic face of the plasma membrane, directly beneath the extracellular particle. The actin tail acts to propel the virus away from the cell surface to enhance its cell-to-cell spread. We now demonstrate that African swine fever virus (ASFV), a member of the Asfarviridae family, also stimulates the polymerization of actin at the cell surface. Intracellular ASFV particles project out at the tip of long filopodia-like protrusions, at an average rate of 1.8 microm min(-1). Actin was arranged in long unbranched parallel arrays inside these virus-tipped projections. In contrast to vaccinia, this outward movement did not involve recruitment of Grb2, Nck1 or N-WASP. Actin polymerization was not nucleated by virus particles in transit to the cell periphery, and projections were not produced when the secretory pathway was disrupted by brefeldin A treatment. Our results show that when ASFV particles reach the plasma membrane they induce a localized nucleation of actin, and that this process requires interaction with virus-encoded and/or host proteins at the plasma membrane. We suggest that ASFV represents a valuable new model for studying pathways that regulate the formation of filopodia.

The African swine fever virus nonstructural protein pB602L is required for formation of the icosahedral capsid of the virus particle

African swine fever virus (ASFV) protein pB602L has been described as a molecular chaperone for the correct folding of the major capsid protein p72. We have studied the function of protein pB602L during the viral assembly process by using a recombinant ASFV, vB602Li, which inducibly expresses the gene coding for this protein. We show that protein pB602L is a late nonstructural protein, which, in contrast with protein p72, is excluded from the viral factory. Repression of protein pB602L synthesis inhibits the proteolytic processing of the two viral polyproteins pp220 and pp62 and leads to a decrease in the levels of protein p72 and a delocalization of the capsid protein pE120R. As shown by electron microscopy analysis of cells infected with the recombinant virus vB602Li, the viral assembly process is severely altered in the absence of protein pB602L, with the generation of aberrant "zipper-like" structures instead of icosahedral virus particles. These "zipper-like" structures are similar to those found in cells infected under restrictive conditions with the recombinant virus vA72 inducibly expressing protein p72. Immunoelectron microscopy studies show that the abnormal forms generated in the absence of protein pB602L contain the inner envelope protein p17 and the two polyproteins but lack the capsid proteins p72 and pE120R. These findings indicate that protein pB602L is essential for the assembly of the icosahedral capsid of the virus particle.

Generation of filamentous instead of icosahedral particles by repression of African swine fever virus structural protein pB438L

The mechanisms involved in the construction of the icosahedral capsid of the African swine fever virus (ASFV) particle are not well understood at present. Capsid formation requires protein p72, the major capsid component, but other viral proteins are likely to play also a role in this process. We have examined the function of the ASFV structural protein pB438L, encoded by gene B438L, in virus morphogenesis. We show that protein pB438L associates with membranes during the infection, behaving as an integral membrane protein. Using a recombinant ASFV that inducibly expresses protein pB438L, we have determined that this structural protein is essential for the formation of infectious virus particles. In the absence of the protein, the virus assembly sites contain, instead of icosahedral particles, large aberrant tubular structures of viral origin as well as bilobulate forms that present morphological similarities with the tubules. The filamentous particles, which possess an aberrant core shell domain and an inner envelope, are covered by a capsid-like layer that, although containing the major capsid protein p72, does not acquire icosahedral morphology. This capsid, however, is to some extent functional, as the filamentous particles can move from the virus assembly sites to the plasma membrane and exit the cell by budding. The finding that, in the absence of protein pB438L, the viral particles formed have a tubular structure in which the icosahedral symmetry is lost supports a role for this protein in the construction or stabilization of the icosahedral vertices of the virus particle.

Apoptosis of thymocytes in experimental African Swine Fever virus infection

This paper report on the lesions occurred in the thymus in experimental acute African swine fever (ASF). Twenty-one pigs were inoculated with the highly virulent ASF virus (ASFV) isolate Spain-70. Animals were slaughtered from 1 to 7 days post infection (dpi). Three animals with similar features were used as controls. Thymus samples were fixed in 10% buffered formalin solution for histological and immunohistochemical study and in 2.5% glutaraldehyde for ultrastructural examination. For immunohistochemical study, the avidin-biotin-peroxidase complex (ABC) technique was used to demonstrate viral protein 73 and porcine myeloid-histiocyte antigen SWC3 using specific monoclonal antibodies. Cell apoptosis was evaluated by the TUNEL assay. Blood samples were taken daily from all pigs and were used for leukocyte counts. The results of this study show a severe thymocyte apoptosis not related to the direct action of ASFV on these cells, but probably to a quantitative increase in macrophages in the thymus and their activation. A decrease in the percentage of blood lymphocytes was observed at the same time No significant vascular changes were observed in the study. With these results we suggest that ASFV infection of the thymus does not seem to play a critical role in the acute disease. Although severe apoptosis was observed, animals died because of the severe lesions found in the other organs.

African swine fever virus multigene family 360 genes affect virus replication and generalization of infection in Ornithodoros porcinus ticks

Recently, we reported that African swine fever virus (ASFV) multigene family (MGF) 360 and 530 genes are significant swine macrophage host range determinants that function by promoting infected-cell survival. To examine the function of these genes in ASFV's arthropod host, Ornithodoros porcinus porcinus, an MGF360/530 gene deletion mutant (Pr4Delta35) was constructed from an ASFV isolate of tick origin, Pr4. Pr4Delta35 exhibited a significant growth defect in ticks. The deletion of six MGF360 and two MGF530 genes from Pr4 markedly reduced viral replication in infected ticks 100- to 1,000-fold. To define the minimal set of MGF360/530 genes required for tick host range, additional gene deletion mutants lacking individual or multiple MGF genes were constructed. The deletion mutant Pr4Delta3-C2, which lacked three MGF360 genes (3HL, 3Il, and 3LL), exhibited reduced viral growth in ticks. Pr4Delta3-C2 virus titers in ticks were significantly reduced 100- to 1,000-fold compared to control values at various times postinfection. In contrast to the parental virus, with which high levels of virus replication were observed in the tissues of infected adults, Pr4Delta3-C2 replication was not detected in the midgut, hemolymph, salivary gland, coxal gland, or reproductive organs at 15 weeks postinfection. These data indicate that ASFV MGF360 genes are significant tick host range determinants and that they are required for efficient virus replication and generalization of infection. The impaired virus replication of Pr4Delta3-C2 in the tick midgut likely accounts for the absence of the generalized infection that is necessary for the natural transmission of virus from ticks to pigs.

High-pressure freezing in the study of animal pathogens

High-pressure freezing is applicable to both morphological and immunocytochemical studies. We are investigating the morphogenesis of foot-and-mouth disease virus and African swine fever virus by the use of high-pressure freezing of infected cells. Foot-and-mouth disease virus particles are not detected in sections of conventionally immersion-fixed infected cells, but when the cells are prepared by high-pressure freezing, newly formed virions are readily seen throughout the cell. We report two methods for high-pressure freezing of virally infected cells: first, two sapphire discs frozen 'face to face' with a narrow spacer to prevent cell damage and, second, a fibrous filter substrate that can be easily cut into discs to fit into the freezing planchettes. Cells readily adhere to the fibres in vitro, and the complete disc can be rapidly transferred to the planchettes for freezing. Immunolabelling studies of the microneme proteins of the parasite Eimeria tenella indicate that high-pressure freezing followed by freeze-substitution in acetone with uranyl acetate allows high-sensitivity immunolabelling for these proteins.

African swine fever: Expression of interleukin-1 alpha and tumour necrosis factor-alpha by pulmonary intravascular macrophages

To determine, in the acute form of African swine fever (ASF), the relationship between the appearance of pulmonary oedema and viral replication and expression of cytokines by pulmonary intravascular macrophages (PIMs), 14 pigs were inoculated intramuscularly with ASF virus (strain España'70) and killed in pairs on days 1-7 post-inoculation. Samples of lung were examined immunohistochemically and ultrastructurally. The immunohistochemical study was carried out with antibodies against interleukin-1 alpha (IL-1alpha), tumour necrosis factor-alpha (TNF-alpha), viral antigen of ASF (Vp73) and a myeloid marker (SWC3). Viral replication was observed mainly in PIMs, which at the same time showed intense activation, accompanied by the expression of IL-1alpha and TNF-alpha. The occurrence of interstitial oedema, neutrophil sequestration and fibrin microthrombi in septal capillaries coincided with high degrees of cytokine expression by infected PIMs. Alveolar macrophages did not show a significant change in cytokine expression as a result of ASF infection, and viral replication was detected in only a low percentage of these cells.

African swine fever virus structural protein pE120R is essential for virus transport from assembly sites to plasma membrane but not for infectivity

This report examines the role of African swine fever virus (ASFV) structural protein pE120R in virus replication. Immunoelectron microscopy revealed that protein pE120R localizes at the surface of the intracellular virions. Consistent with this, coimmunoprecipitation assays showed that protein pE120R binds to the major capsid protein p72. Moreover, it was found that, in cells infected with an ASFV recombinant that inducibly expresses protein p72, the incorporation of pE120R into the virus particle is dependent on p72 expression. Protein pE120R was also studied using an ASFV recombinant in which E120R gene expression is regulated by the Escherichia coli lac repressor-operator system. In the absence of inducer, pE120R expression was reduced about 100-fold compared to that obtained with the parental virus or the recombinant virus grown under permissive conditions. One-step virus growth curves showed that, under conditions that repress pE120R expression, the titer of intracellular progeny was similar to the total virus yield obtained under permissive conditions, whereas the extracellular virus yield was about 100-fold lower than in control infections. Immunofluorescence and electron microscopy demonstrated that, under restrictive conditions, intracellular mature virions are properly assembled but remain confined to the replication areas. Altogether, these results indicate that pE120R is necessary for virus dissemination but not for virus infectivity. The data also suggest that protein pE120R might be involved in the microtubule-mediated transport of ASFV particles from the viral factories to the plasma membrane.

African swine fever virus is enveloped by a two-membraned collapsed cisterna derived from the endoplasmic reticulum

During the cytoplasmic maturation of African swine fever virus (ASFV) within the viral factories, the DNA-containing core becomes wrapped by two shells, an inner lipid envelope and an outer icosahedral capsid. We have previously shown that the inner envelope is derived from precursor membrane-like structures on which the capsid layer is progressively assembled. In the present work, we analyzed the origin of these viral membranes and the mechanism of envelopment of ASFV. Electron microscopy studies on permeabilized infected cells revealed the presence of two tightly apposed membranes within the precursor membranous structures as well as polyhedral assembling particles. Both membranes could be detached after digestion of intracellular virions with proteinase K. Importantly, membrane loop structures were observed at the ends of open intermediates, which suggests that the inner envelope is derived from a membrane cisterna. Ultraestructural and immunocytochemical analyses showed a close association and even direct continuities between the endoplasmic reticulum (ER) and assembling virus particles at the bordering areas of the viral factories. Such interactions become evident with an ASFV recombinant that inducibly expresses the major capsid protein p72. In the absence of the inducer, viral morphogenesis was arrested at a stage at which partially and fully collapsed ER cisternae enwrapped the core material. Together, these results indicate that ASFV, like the poxviruses, becomes engulfed by a two-membraned collapsed cisterna derived from the ER.

Intracellular virus DNA distribution and the acquisition of the nucleoprotein core during African swine fever virus particle assembly: ultrastructural in situ hybridisation and DNase-gold labelling

African swine fever virus (ASFV) is a large complex icosahedral double-stranded DNA virus that replicates in the cytoplasm of susceptible cells. Assembly of new virus particles occurs within the perinuclear viroplasm bodies known as virus factories. Two types of virus particle are routinely observed: "fulls," which are particles with an electron-dense DNA-containing nucleoid, and "empties," which consist of the virus protein and membrane icosahedral shell but are without the incorporation of the virus genome. The objective of this study was to understand ASFV morphogenesis by determining the distribution of intracellular viral DNA in the virus factory and during virus particle assembly. The ultrastructural localisation of DNA within ASFV-infected cells was achieved using two complementary methods: with an ASFV-specific DNA probe to the major capsid protein (p73) gene (B646L) hybridised in situ or through detection of all forms of DNA (viral and cellular) with gold-labelled DNase. Conditions for in situ hybridisation at the electron microscopic level were optimised for infected cells in two Lowicryl resins (K4M and HM20) and using two nonradioactive probe labels (digoxygenin and biotin). The morphological data indicate that the viral DNA, perhaps from specialised storage sites within the factory, begins to condense into a pronucleoid and is then inserted, at a single vertex, into an "empty" particle. Further maturation of the viral particle, including closure of the narrow opening in the icosahedron, gives rise to "intermediate" particles, where the nucleoprotein core undergoes additional consolidation to produce the characteristic mature or "full" virions. The site of particle closure may represent a "weak point" at one vertex, but the mechanisms and structures involved in the packaging and release of the virus genome via such a port are yet to be determined.

Inducible gene expression from African swine fever virus recombinants: analysis of the major capsid protein p72

A method to study the function of individual African swine fever virus (ASFV) gene products utilizing the Escherichia coli lac repressor-operator system has been developed. Recombinant viruses containing both the lacI gene encoding the lac repressor and a strong virus late promoter modified by the insertion of one or two copies of the lac operator sequence at various positions were constructed. The ability of each modified promoter to regulate expression of the firefly luciferase gene was assayed in the presence and in the absence of the inducer isopropyl beta-D-thiogalactoside (IPTG). Induction and repression of gene activity were dependent on the position(s) of the operator(s) with respect to the promoter and on the number of operators inserted. The ability of this system to regulate the expression of ASFV genes was analyzed by constructing a recombinant virus inducibly expressing the major capsid protein p72. Electron microscopy analysis revealed that under nonpermissive conditions, electron-dense membrane-like structures accumulated in the viral factories and capsid formation was inhibited. Induction of p72 expression allowed the progressive building of the capsid on these structures, leading to assembly of ASFV particles. The results of this report demonstrate that the transferred inducible expression system is a powerful tool for analyzing the function of ASFV genes.

African swine fever virus is wrapped by the endoplasmic reticulum

African swine fever (ASF) virus is a large DNA virus that shares the striking icosahedral symmetry of iridoviruses and the genomic organization of poxviruses. Both groups of viruses have a complex envelope structure. In this study, the mechanism of formation of the inner envelope of ASF virus was investigated. Examination of thin cryosections by electron microscopy showed two internal membranes in mature intracellular virions and all structural intermediates. These membranes were in continuity with intracellular membrane compartments, suggesting that the virus gained two membranes from intracellular membrane cisternae. Immunogold electron microscopy showed the viral structural protein p17 and resident membrane proteins of the endoplasmic reticulum (ER) within virus assembly sites, virus assembly intermediates, and mature virions. Resident ER proteins were also detected by Western blotting of isolated virions. The data suggested the ASF virus was wrapped by the ER. Analysis of the published sequence of ASF virus (R. J. Yanez et al., Virology 208:249-278, 1995) revealed a reading frame, XP124L, that encoded a protein predicted to translocate into the lumen of the ER. Pulse-chase immunoprecipitation and glycosylation analysis of pXP124L, the product of the XP124L gene, showed that pXP124L was retained in the ER lumen after synthesis. When analyzed by immunogold electron microscopy, pXP124L localized to virus assembly intermediates and fully assembled virions. Western blot analysis detected pXP124L in virions isolated from Percoll gradients. The packaging of pXP124L from the lumen of the ER into the virion is consistent with ASF virus being wrapped by ER cisternae: a mechanism which explains the presence of two membranes in the viral envelope.

Entry of African swine fever virus into Vero cells and uncoating

African swine fever virus (ASFV) enters Vero cells by adsorptive endocytosis [Valdeira, M.L., Geraldes, A., 1985. Morphological study on the entry of African swine fever virus into cells, Biol Cell. 55, 35-40]. Electron microscopy of a lysosomotropic drug-controlled penetration indicated that this step takes place in the endosomes, after fusion between the viral envelope and the limiting membrane of the endosome. Inhibition studies with colcemid, cytochalasin B, sodium azide, dinitrophenol, lysosomotropic weak bases, and the ionophore monensin, showed that the virus uptake is largely independent of cytoskeletal and lysosomal function, but dependent on oxidative phosphorylation. Some protease inhibitors inhibited viral replication at an early step, indicating that the initiation of infection depends on a viral proteolytic cleavage.

Assembly of African swine fever virus: role of polyprotein pp220

Polyprotein processing is a common strategy of gene expression in many positive-strand RNA viruses and retroviruses but not in DNA viruses. African swine fever virus (ASFV) is an exception because it encodes a polyprotein, named pp220, to produce several major components of the virus particle, proteins p150, p37, p34, and p14. In this study, we analyzed the assembly pathway of ASFV and the contribution of the polyprotein products to the virus structure. Electron microscopic studies revealed that virions assemble from membranous structures present in the viral factories. Viral membranes became polyhedral immature virions after capsid formation on their convex surface. Beneath the lipid envelope, two distinct domains appeared to assemble consecutively: first a thick protein layer that we refer to as core shell and then an electron-dense nucleoid, which was identified as the DNA-containing domain. Immunofluorescence studies showed that polyprotein pp220 is localized in the viral factories. At the electron microscopic level, antibodies to pp220 labeled all identifiable forms of the virus from the precursor viral membranes onward, thus indicating an early role of the polyprotein pp220 in ASFV assembly. The subviral localization of the polyprotein products, examined on purified virions, was found to be the core shell. In addition, quantitative studies showed that the polyprotein products are present in equimolar amounts in the virus particle and account for about one-fourth of its total protein content. Taken together, these results suggest that polyprotein pp220 may function as an internal protein scaffold which would mediate the interaction between the nucleoid and the outer layers similarly to the matrix proteins of other viruses.

African swine fever virus infection of bone marrow: lesions and pathogenesis

The effects of African swine fever (ASF) virus infection on bone marrow hematopoiesis and microenvironment were determined by studying the sequential development of ultrastructural lesions of bone marrow and blood cell changes. Eight pigs (two pigs/infected group) were inoculated by intramuscular route with 10(5) 50% hemadsorbing doses (HAD50) of the Malawi'83 ASF virus isolate. Two uninfected pigs were used as controls. Ultrastructural changes developed by day 3 postinoculation (PI), persisted through day 7 PI, and were characterized by activation of macrophages. From day 5 PI, viral replication was observed in monocytes/macrophages, reticular cells, immature neutrophils, and promonocytes. Also viral replication was detected in megakaryocytes, endothelial cells, and pericytes at day 7 PI. Vascular alterations consisted of activation of sinusoidal endothelial cells, intravascular coagulation, and fibrin strands interspersed among microenvironment and hematopoietic cells. No significant changes were observed in total white blood cells counts, percentage of monocytes, and platelet counts; however, severe lymphopenia and neutrophilia were detected from day 3 PI. Results of this experiment indicate that there is increased hematopoiesis in bone marrow during acute ASF, coinciding with macrophage activation. Neither vascular changes nor viral replication in different bone marrow cell populations gave rise to impaired bone marrow function. Increased hematopoiesis would exert a positive influence by preventing the early onset of thrombocytopenia and would exert a negative influence by stimulating the spread of the virus via neutrophils. Increased hematopoiesis would be unable to compensate for the lymphopenia.

Assembly of African Swine fever virus: quantitative ultrastructural analysis in vitro and in vivo

African swine fever virus is an icosahedral double-standed DNA virus which replicates in the cytoplasm of porcine monocytic cells. Progeny virus particles, like poxviruses and iridoviruses, are assembled in cytoplasmic inclusions, termed virus factories. The formation of these structures in susceptible cells infected in vitro with pathogenic and attenuated ASFV isolates has been studied by semiquantitative and qualitative electron microscopy. Recognizable virus factory elements were detected by B hr postinfection (hpi) and increased significantly in profile area between 12 and 18 hpi. The number of virus particles associated with the virus factories also increased significantly between 12 and 24 hpi. These included particles with ("full") and without ("empty") a nucleo-protein core. Similar results were obtained for both pathogenic (Malawi) and attenuated (Uganda) virus isolates, but the replication of pathogenic virus in the macrophage was more efficient than that of the attenuated virus in a porcine kidney cell line, where a relatively higher percentage of defective "empty" particles were observed. Analysis of virus factory formation was also done directly on lung and liver tissue from a pig infected with the highly pathogenic Malawi virus isolate. The in vivo data for virus factory area and particle number in both pulmonary intravascular macrophages and liver Kupffer cells at Day 4 postinfection were similar to those observed in vitro. As expected, using postembedding immunoelectron microscopy, DNA (both cellular and viral) was detected in the cell nucleus, cytoplasmic virus, and mature extracellular virus. More interestingly, DNA was absent in the cytoplasm, but readily observed in virus factories. This DNA, which we presume to be viral, was found in close association with membrane-like material and "empty" particles, suggesting that the virus DNA may enter a partially formed capsid which is then sealed in order to develope into a fully assembled particle. According to this hypothesis, ASFV morphogenesis involves the initial formation of "empty" particles within the virus factory. The adjacently formed nucleo-protein material is then inserted into the "empty" particles, as has been described for poxviruses. These particles then mature in to the "full" particles and leave the virus factory as a prelude to release from the cell by budding.

A structural DNA binding protein of African swine fever virus with similarity to bacterial histone-like proteins

Here we describe an African swine fever virus (ASFV) protein encoded by the open reading frame 5-AR that shares structural and functional similarities with the family of bacterial histone-like proteins which include histone-like DNA binding proteins, integration host factor, and Bacillus phage SPO1 transcription factor, TF1. The ASFV 5-AR gene was cloned by PCR and expressed in E. coli. Monospecific antiserum prepared to the 5-AR bacterial expression product specifically immunoprecipitated a protein of approximately 11.6 kDa from ASFV infected swine macrophages at late times post infection. Additionally, the 5-AR expression product was strongly recognized by ASFV convalescent pig serum, indicating its antigenicity during natural infection. Cloned p11.6 bound both double and single stranded DNA-cellulose columns. Consistent with a DNA binding function, immunoelectronmicroscopy localized p11.6 to the virion nucleoid, To our knowledge, p11.6 is the first bacterial histone-like DNA-binding protein found in an animal virus or eukaryotic cell system.

Characterization of a ubiquitinated protein which is externally located in African swine fever virions

An antiserum was raised against the African swine fever virus (ASFV)-encoded ubiquitin-conjugating enzyme (UBCv1) and used to demonstrate by Western blotting (immunoblotting) and immunofluorescence that the enzyme is present in purified extracellular virions, is expressed both early and late after infection of cells with ASFV, and is cytoplasmically located. Antiubiquitin serum was used to identify novel ubiquitin conjugates present during ASFV infections. This antiserum stained virus factories late after infection, suggesting that virion proteins may be ubiquitinated. This possibility was confirmed by Western blotting, which identified three major antiubiquitin-immunoreactive proteins with molecular masses of 5, 18, and 58 kDa in purified extracellular virions. The 18-kDa protein was solubilized from virions at relatively low concentrations of the detergent n-octyl-beta-D-glucopyranoside, indicating that it is externally located and is possibly in the virus capsid. The 18-kDa protein was purified, and N-terminal amino acid sequencing confirmed that the protein was ubiquitinated and was ASFV encoded. The ASFV gene encoding this protein (PIG1) was sequenced, and the encoded protein expressed in an Escherichia coli expression vector. Recombinant PIG1 was ubiquitinated in the presence of E. coli expressed UBCv1 in vitro. These results suggest that PIG1 may be a substrate for UBCv1. The predicted molecular masses of the PIG1 protein and recombinant ubiquitinated protein were larger than the 18-kDa molecular mass of the ubiquitinated protein present in virions. Therefore, during viral replication, a precursor protein may undergo limited proteolysis to generate the ubiquitinated 18-kDa protein.

African swine fever virus infection of skin-derived dendritic cells in vitro causes interference with subsequent foot-and-mouth disease virus infection

Highly purified skin-derived dendritic cells (SDDCs) isolated from swine skin by a simple novel method were cultured for 24 hours before independent or sequential inoculation with African swine fever virus (ASFV) and foot-and-mouth disease virus (FMDV). By avidin-biotin immunohistochemical staining, ASFV antigen was detected in 50% of SDDCs as early as 1.5 hours postinfection (HPI) and in 80% by 3 HPI when cytopathic effect was noted. Cell lysis was detected with FMDV infection as early as 8 HPI; immunostaining for FMDV antigen was found in 10% of the cells. African swine fever virus replication was detected by transmission electron microscopy in a high percentage of SDDCs by 11 HPI. Sequential infection with FMDV 3 hours after ASFV inoculation blocked FMDV infection. These findings demonstrate that both ASFV and FMDV infect dendritic cells of Langerhans cell type in vitro and ASFV interferes with FMDV infection.

African swine fever virus interaction with microtubules

The role of microtubules in intracellular transport of African swine fever virus (ASFV) and virus-induced inclusions was studied by immunofluorescence using anti-ASFV and anti-tubulin antibodies, by electron microscopy of infected Vero cells and by in vitro binding of virions to purified microtubules. MTC, a reversible colchicine analogue, was used to depolymerize microtubules. In cells treated with MTC multiple large inclusions containing ASFV antigens and particles were observed in the cytoplasm. Removal of the drug lead to migration and fusion of the inclusions at a perinuclear location. To study the effect of microtubule repolymerization on virus particle distribution, the particles were counted in thin sections of MTC treated cells and at different times after removal of the drug. In cells treated with MTC 6.8% and 3.6% of the virus particles were found respectively in the cytoplasm and at the cell membrane while 38% of the particles were located around the virosome. With reversal of the drug effect the number of virus particles around the virosomes progressively decreased to 10% at 2 h while the number of particles in the cytoplasm and at the cell membrane increased. At 2 h after removal of the drug 33.5% of the particles were found budding from the cell membrane. Virus particles were found closely associated with microtubules in cytoskeletons obtained by Triton X-100 extraction of taxol treated cells. The association of virus particles with microtubules was also observed in vitro using purified microtubules and virus particles.(ABSTRACT TRUNCATED AT 250 WORDS)

[The functional role of the glycosylation of viral components]

An inhibitory analysis of the role of glycosylation and glycoproteins in the manifestations of the most important properties of African swine virus was carried out using a set of strains and variants with contrasting characteristics. Glycoproteins were shown to realize some virus functions such as virulence and its variability, intracellular transport and exocytosis of virions, hemadsorption, but not immunological recognition of the infected cells by cytotoxic T-lymphocytes.

Morphogenesis of African swine fever virus in monkey kidney cells after reversible inhibition of replication by cycloheximide

The late cytoplasmic phases of African swine fever virus (ASFV) morphogenesis in monkey kidney cells have been studied by transmission electron microscopy, focusing attention on the synthesis of viral envelopes. Morphogenesis was studied after reversible cycloheximide blockage of monkey kidney cells infected with ASFV. ASFV appears to synthesize its external and internal envelopes within the cellular cytoplasm, at the same time as the capsid is formed, with intracellular and extracellular virions showing similar structure and polypeptide composition.

[Light and electron microscopic findings in the intestine of spontaneous and experimentally-produced African swine fever]

Light and electron microscopical studies were carried out after experimental induced and spontaneous infection with African swine fever virus. The experimental infection was performed in 18 pigs divided into two groups consisting of 9 animals. The pigs of group I were inoculated with virulent isolate E 70, those of group II with attenuated isolate E 75. Two infected pigs of group I and one control animal were killed on days 3, 5 or 7 p.i., two pigs of group II and one control animal were killed on days 9, 11 or 13 p.i. Additionally 19 spontaneous infected and seropositive animals and two seronegative pigs without clinical signs were examined. Infection with African swine fever virus resulted in slight morphological alterations of the intestine. The pathological findings showed a more intense involvement of the large intestine, especially the caecum and colon, than the small intestine, where the ileum was mostly affected. In all three groups edema and vacuolisation could be observed in endothelial cells. In spite of beginning degenerative signs, especially after spontaneous infection, the fenestrated structure of the endothelium was conserved in most of the cases. In animals infected with virulent isolate the vascular lumina contained aggregations of fibrin, which was severe pronounced in the pigs of the other groups. In the area of these alterations disturbance of permeability with extravasation could be found. In all groups single virions or virus aggregates could be identified in concave depressions of the erythrocyte membrane or free within the blood plasma, in some cases enveloped by a plasma-like material.(ABSTRACT TRUNCATED AT 250 WORDS)

African swine fever virus: purification of capsomeres released by lipase

The virus capsomeres of the outer and inner layers of capsids were effectively released simultaneously from purified virions by lipase digestion and were purified by a linear gradient ultracentrifugation. The capsid consisted of an array of double layers of uniformly arranged individual capsomeres where a lipid(s) served as a matrix in between the capsomeres.

Evidence for an acid phosphatase in African swine fever virus

An acid phosphatase activity has been detected in purified preparations of African swine fever virus. Purified viral cores obtained after Nonidet-P40 and 2-mercaptoethanol treatment of the virus retained the activity as assayed with nitrophenyl phosphate as substrate at pH 5. Enzyme cytochemistry by electron microscopy showed that the acid phosphatase activity is localized mainly inside the core of the virion. The molecular weight and the isoelectric point of the virus acid phosphatase activity confirmed that it was distinct from the host cellular enzyme.

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.

Association of African swine fever virus with the cytoskeleton

The association of African swine fever virus (ASFV) with the cytoskeleton was investigated. Immunofluorescent studies of ASFV infected cells with anti-ASFV serum showed a temporal and spatial development of viral inclusions which moved from a peripheral to a perinuclear location and fused to give a single large perinuclear factory. The migration and fusion of viral inclusions was inhibited by colchicine suggesting a function for microtubules in assembly site organization not previously described. Accumulation of virions outside the inclusions and inhibition of viral release was also observed in colchicine treated cells. Viral antigens and structural elements were retained on the cytoskeleton fraction of Triton X-100 extracted cells. Reorganization of cytoskeletal elements around the assembly sites was demonstrated by transmission electronmicroscopy and by immunofluorescent studies using monoclonal antibodies against actin, tubulin and vimentin. Intermediate filaments accumulated around the viral factories, microtubules were greatly decreased in number and microfilaments were reorganized in association with the plasma membrane. Bundles of 15 nm tubules of unknown origin were also observed around the assembly sites. The distribution of viral proteins in soluble, cytoskeleton and detergent insoluble nuclear fractions was studied by pulse-chase experiments with [35S]methionine. SDS-PAGE analysis showed the presence in the cytoskeletal and nuclear fractions of 150, 72, 38, 28, 19 and 15 kDa virus structural proteins which increased after a 5 h chase. Our results indicate a close association of ASFV replication with the cytoskeleton similar to events described during FV3 replication but which differ from those occurring in poxvirus-infected cells.

Effect of inhibitors of the host cell RNA polymerase II on African swine fever virus multiplication

The role of the host cell RNA polymerase II in African swine fever (ASF) virus growth has been examined using inhibitors of this enzyme. The adenosine analog 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), an inhibitor of mRNA precursor synthesis in mammalian cells, strongly inhibits the production of infectious progeny virus in Vero cells, but does not significantly affect the synthesis of virus-specific macromolecules. On the other hand, virion assembly seems to proceed normally in the presence of DRB, as virus particles can be seen in electron micrographs with a morphology indistinguishable from that observed in the absence of the inhibitor. However, taking into account the inhibition of the infectivity caused by the drug, most of these particles must be defective. In contrast with this effect of DRB on ASF virus replication, the toxin alpha-amanitin does not inhibit the production of infectious ASF virus in Vero cells or porcine alveolar macrophages. This result indicates that the host RNA polymerase II does not transcribe viral genes and that active transcription of the cell genome is not needed for ASF virus replication.

African swine fever. I. Morphological changes and virus replication in blood platelets of pigs infected with virulent haemadsorbing and non-haemadsorbing isolates

Replicating and mature viral particles were detected with the transmission electron microscope in blood platelets of pigs infected with virulent haemadsorbing and non-haemadsorbing African swine fever virus isolates. Although platelet numbers decreased terminally in infected pigs, the most noticeable morphological damage to these cells apparent in the last 2 days of the disease included cytoplasmic swelling, vacuolation, fragmentation and loss of dense granules.

Establishment of a Vero cell line persistently infected with African swine fever virus

A Vero cell line persistently infected with African swine fever virus was established by infecting the cells in the presence of 10 mM NH4Cl (Vero-P cell line). The virus derived from the Vero-P cultures infected Vero cells, and virus titers were comparable to those obtained in Vero cells acutely infected with African swine fever virus. The structural proteins of the virus from Vero-P cells were similar to those of the virus produced in lytic infections. Virus production was low when the Vero-P cells were growing logarithmically and increased considerably in confluent cultures when lysis appeared in a fraction of the cell population.

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.

In vitro and in vivo association of African swine fever virus with swine erythrocytes

The association of African swine fever virus (ASFV) with swine erythrocytes in vivo, in high titers, was verified by inoculating 30 pigs with 17 ASFV isolates and assaying their plasma and washed erythrocyte fractions for residual virus. Viral antigens were specifically localized on the surface of in vitro and in vivo swine erythrocytes, using the fluorescent antibody technique and 3 monoclonal antibodies specific for ASFV. The same monoclonal antibodies immunoprecipitated virus-specific polypeptides of molecular weights 13 kd and 73 kd from ASFV-infected Vero cells. Erythrocytes from viremic swine infected with Lisbon-60, Dominican Republic, Badajoz-M98, or Cameroon isolates of ASFV were studied by transmission electron microscopy. Virus was found in membrane depressions at the surface of erythrocytes. These surface depressions resembled stages of smooth surfaced pits. Erythrocytes from viremic pigs were fragile osmotically.

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.

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.

Porcine leukocyte cellular subsets sensitive to African swine fever virus in vitro

African swine fever virus infected most, if not all, of the macrophages (monocytes) and ca. 4% of the polymorphonuclear leukocytes from porcine peripheral blood. B and T lymphocytes, either resting or stimulated with phytohemagglutinin, lipopolysaccharide, or pokeweed mitogen, were not susceptible to the virus. All of the mitogens used inhibited African swine fever multiplication in susceptible cells. The number of virus passages in vitro and the virulence degree of the virus did not affect the susceptibility of porcine B or T lymphocytes to African swine fever virus.

General morphology and capsid fine structure of African swine fever virus particles

The structure of African swine fever virus particles has been examined by electron microscopy. The analysis of virions prepared by negative staining, thin sectioning, and freeze-drying and shadowing showed that the virus particle was composed of several concentric structures with an overall icosahedral shape. The inner region of the virus particles was a nucleoid that was surrounded by a membrane covered by the capsid. The capsid had side-to-side dimensions of 172 to 191 nm and was built up by capsomers arranged in an hexagonal lattice. Computer-filtered electron micrographs of either negatively stained or freeze-dried and shadowed capsids revealed capsomers with a hexagonal outline and a hole in the center. The intercapsomer distance ranged from 7.4 to 8.1 nm. The triangulation number of the capsid was estimated to be 189 to 217, indicative of 1892 to 2172 capsomers. Extracellular African swine fever virus particles had an external membrane that resembled the cytoplasmic unit membrane.

Photographic defocusing: a means to render recognizable line traced models of negatively stained biological specimens

A simple method of obtaining analogue images from traced models of biological specimens is presented. It consists of the photographic defocusing of traced models and it is illustrated with negatively stained cylindrical forms of the ASFV; the black lines of the trace in the model correspond to the negative stain surrounding the viral morphological subunits as seen in the electron micrograph. The photographic defocusing is the means by which the traced model is filtered and is used to introduce grey levels on an otherwise black and white image. The right amount of defocusing is attained when the width of the trace of the model equals the width of the rim of the negative stain appearing between the morphological subunits in the electron micrograph.

Sedimentation coefficient of African swine fever virus

The sedimentation coefficient of the infective unit of African swine fever in tissue culture harvest fluids was measured in a preparative ultracentrifuge. The boundary locator method used also permitted making an estimate of heterogeneity. The sedimentation coefficient ranged from 3,000 to 8,000 Svedberg units, representing many classes of infective particles. Electron microscopy on culture fluids from infected cells showed many kinds of virus-containing units. Sucrose-CsCl gradient centrifugation was used to concentrate and to purify (partly) African swine fever virus for analytical ultracentrifugation. The optical patterns of the physical particles revealed a range of coefficients from 1,800 to 3,200 Svedberg units in tris-buffered saline solution at 20 C and buoyant densities from 1.19 to 1.24 g/ml in CsCl. The disparity of these values from those obtained by preparative ultracentrifugation indicates a change in the virus structure or a selection of viral populations on purification (or both).

Electron microscopic observation of African swine fever virus development in Vero cells

African swine fever virus emerges from infected Vero cells either from areas of smooth cell surface or from microvilli. The two patterns may occur at different sites on the same cell and are unique for this virus. The scanning electron micrographs supplement regular thin section views.

African swine fever virus replication in porcine lymphocytes

Purified preparation of porcine lymphocytes were infected with three isolates of virulent African swine fever virus (ASFV). Electron microscopy showed the presence of small numbers of mature virus particles in degenerating cells. The titres of infective virus released were low and reached a maximum by 24 h after infection.

Negative staining of a non-haemadsorbing strain of African swine fever virus

Since the application of negative staining, preceded by fixation, prevents the disruption and distortion of the capsid of the African swine fever virus, improved contrast and evaluation of the appearance and size of virus particles in the electron microscope is possible and, in addition, the icosahedral shape of the virus is demonstrable. The mature virus particle contains at least 2 capsid layers and an outer envelope.