Mapping the genetic determinants of pathogenicity and plaque phenotype in swine vesicular disease virus

An immortalized porcine macrophage cell line competent for the isolation of African swine fever virus

African swine fever virus (ASFV) is the etiological agent of African swine fever (ASF), a fatal hemorrhagic disease of domestic pigs and wild boar. The virus primarily infects macrophage and monocyte host cells, these do not grow in vitro. Many attempts have been made to establish sustainable ASFV-sensitive cell lines, but which supported only low viral replication levels of limited, mostly artificially attenuated strains of ASFV. Here, we examined the competence of a novel cell line of immortalized porcine kidney macrophages (IPKM) for ASFV infection. We demonstrated that IPKM cells can facilitate high levels (> 10^7.0 TCID₅₀/mL) of viral replication of ASFV, and hemadsorption reactions and cytopathic effects were observed as with porcine alveolar macrophages when inoculated with virulent field isolates: Armenia07, Kenya05/Tk-1, and Espana75. These results suggested that IPKM may be a valuable tool for the isolation, replication, and genetic manipulation of ASFV in both basic and applied ASF research.

Characterization of Plaque Variants and the Involvement of Quasi-Species in a Population of EV-A71

Viral plaque morphologies in human cell lines are markers for growth capability and they have been used to assess the viral fitness and selection of attenuated mutants for live-attenuated vaccine development. In this study, we investigate whether the naturally occurring plaque size variation reflects the virulence of the variants of EV-A71. Variants of two different plaque sizes (big and small) from EV-A71 sub-genotype B4 strain 41 were characterized. The plaque variants displayed different in vitro growth kinetics compared to the parental wild type. The plaque variants showed specific mutations being present in each variant strain. The big plaque variants showed four mutations I97L, N104S, S246P and N282D in the VP1 while the small plaque variants showed I97T, N237T and T292A in the VP1. No other mutations were detected in the whole genome of the two variants. The variants showed stable homogenous small plaques and big plaques, respectively, when re-infected in rhabdomyosarcoma (RD) and Vero cells. The parental strain showed faster growth kinetics and had higher viral RNA copy number than both the big and small plaque variants. Homology modelling shows that both plaque variants have differences in the structure of the VP1 protein due to the presence of unique spontaneous mutations found in each plaque variant This study suggests that the EV-A71 sub-genotype B4 strain 41 has at least two variants with different plaque morphologies. These differences were likely due to the presence of spontaneous mutations that are unique to each of the plaque variants. The ability to maintain the respective plaque morphology upon passaging indicates the presence of quasi-species in the parental population.

Emergence of a Large-Plaque Variant in Mice Infected with Coxsackievirus B3

Coxsackieviruses are enteric viruses that frequently infect humans. To examine coxsackievirus pathogenesis, we orally inoculated mice with the coxsackievirus B3 (CVB3) Nancy strain. Using HeLa cell plaque assays with agar overlays, we noticed that some fecal viruses generated plaques >100 times as large as inoculum viruses. These large-plaque variants emerged following viral replication in several different tissues. We identified a single amino acid change, N63Y, in the VP3 capsid protein that was sufficient to confer the large-plaque phenotype. Wild-type CVB3 and N63Y mutant CVB3 had similar plaque sizes when agarose was used in the overlay instead of agar. We determined that sulfated glycans in agar inhibited plaque formation by wild-type CVB3 but not by N63Y mutant CVB3. Furthermore, N63Y mutant CVB3 bound heparin, a sulfated glycan, less efficiently than wild-type CVB3 did. While N63Y mutant CVB3 had a growth defect in cultured cells and reduced attachment, it had enhanced replication and pathogenesis in mice. Infection with N63Y mutant CVB3 induced more severe hepatic damage than infection with wild-type CVB3, likely because N63Y mutant CVB3 disseminates more efficiently to the liver. Our data reinforce the idea that culture-adapted laboratory virus strains can have reduced fitnessin vivo N63Y mutant CVB3 may be useful as a platform to understand viral adaptation and pathogenesis in animal studies.

IMPORTANCE

Coxsackieviruses frequently infect humans, and although many infections are mild or asymptomatic, there can be severe outcomes, including heart inflammation. Most studies with coxsackieviruses and other viruses use laboratory-adapted viral strains because of their efficient replication in cell culture. We used a cell culture-adapted strain of CVB3, Nancy, to examine viral replication and pathogenesis in orally inoculated mice. We found that mice shed viruses distinct from input viruses because they formed extremely large plaques in cell culture. We identified a single mutation, VP3 N63Y, that was sufficient for large-plaque formation. N63Y mutant viruses have reduced glycan binding and replication in cell culture; however, they have enhanced replication and virulence in mice. We are now using N63Y mutant CVB3 as an improved system for viral pathogenesis studies.

Genome sequence analysis of in vitro and in vivo phenotypes of Bunyamwera and Ngari virus isolates from northern Kenya

Biological phenotypes of tri-segmented arboviruses display characteristics that map to mutation/s in the S, M or L segments of the genome. Plaque variants have been characterized for other viruses displaying varied phenotypes including attenuation in growth and/or pathogenesis. In order to characterize variants of Bunyamwera and Ngari viruses, we isolated individual plaque size variants; small plaque (SP) and large plaque (LP) and determined in vitro growth properties and in vivo pathogenesis in suckling mice. We performed gene sequencing to identify mutations that may be responsible for the observed phenotype. The LP generally replicated faster than the SP and the difference in growth rate was more pronounced in Bunyamwera virus isolates. Ngari virus isolates were more conserved with few point mutations compared to Bunyamwera virus isolates which displayed mutations in all three genome segments but majority were silent mutations. Contrary to expectation, the SP of Bunyamwera virus killed suckling mice significantly earlier than the LP. The LP attenuation may probably be due to a non-synonymous substitution (T858I) that mapped within the active site of the L protein. In this study, we identify natural mutations whose exact role in growth and pathogenesis need to be determined through site directed mutagenesis studies.

Mapping genetic determinants of the cell-culture growth phenotype of enterovirus 71

Enterovirus 71 (EV71) is a member of the species Human enterovirus A within the family Picornaviridae and is a major causative agent of epidemics of hand, foot and mouth disease associated with severe neurological disease. Three EV71 genogroups, designated A, B and C, have been identified, with 75-84 % nucleotide sequence similarity between them. Two strains, EV71-26M (genogroup B) and EV71-6F (genogroup C), were found to have distinct cell-culture growth (26M, rapid; 6F, slow) and plaque-formation (26M, large; 6F, small) phenotypes. To identify the genome regions responsible for the growth phenotypes of the two strains, a series of chimeric viruses was constructed by exchanging the 5' untranslated region (UTR), P1 structural protein or P2/P3 non-structural protein gene regions plus the 3'UTR using infectious cDNA clones of both virus strains. Analysis of reciprocal virus chimeras revealed that the 5'UTRs of both strains were compatible, but not responsible for the observed phenotypes. Introduction of the EV71-6F P1 region into the EV71-26M clone resulted in a small-plaque and slow-growth phenotype similar to that of EV71-6F, whereas the reciprocal chimera displayed intermediate-growth and intermediate-sized plaque phenotypes. Introduction of the EV71-26M P2-P3-3'UTR regions into the EV71-6F clone resulted in a large-plaque and rapid-growth phenotype identical to that of strain EV71-26M, whereas the reciprocal chimera retained the background strain large-plaque phenotype. These results indicate that, although both the P1 and P2-P3-3'UTR genome regions influence the EV71 growth phenotype in cell culture, phenotype expression is dependent on specific genome-segment combinations and is not reciprocal.

Expression and immunological analysis of capsid protein precursor of swine vesicular disease virus HK/70

VP1, a capsid protein of swine vesicular disease virus, was cloned from the SVDV HK/70 strain and inserted into retroviral vector pBABE puro, and expressed in PK15 cells by an retroviral expression system. The ability of the VP1 protein to induce an immune response was then evaluated in guinea pigs. Western blot and ELISA results indicated that the VP1 protein can be recognized by SVDV positive serum, Furthermore, anti-SVDV specific antibodies and lymphocyte proliferation were elicited and increased by VP1 protein after vaccination. These results encourage further work towards the development of a vaccine against SVDV infection.

Identification of virulence determinants of porcine reproductive and respiratory syndrome virus through construction of chimeric clones

In order to determine virulence associated genes in porcine reproductive and respiratory syndrome virus (PRRSV), a series of chimeric viruses were generated where specific genomic regions of a highly virulent PRRSV infectious clone (FL12) were replaced with their counterparts of an attenuated vaccine strain (Prime Pac). Initial genome-wide scanning using a sow reproductive failure model indicated that non-structural (ORF 1a and 1b) and structural (ORF2-7) genomic regions appear to be sites where virulence determinants of PRRSV may reside. These results thus confirm the multigenic character of PRRSV virulence. Additional chimeras containing each individual structural ORFs (2 through 7) of Prime Pac and ORF5 of Neb-1 (parental strain of Prime Pac) within the FL12 backbone were generated and tested individually for further mapping of virulence determinants. Our results allow to conclude that NSP3-8 and ORF5 are the location of major virulence determinants, while other virulence determinants may also be contained in NSP1-3, NSP10-12 and ORF2.

Significance of arginine 20 in the 2A protease for swine vesicular disease virus pathogenicity

Pathogenic and attenuated strains of swine vesicular disease virus (SVDV), an enterovirus, have been characterized previously and, by using chimeric infectious cDNA clones, the key determinants of pathogenicity in pigs have been mapped to the coding region for 1D-2A. Within this region, residue 20 of the 2A protease is particularly significant. Inoculation of pigs with mutant viruses containing single amino acid substitutions at this residue leads to the appearance of revertants, often containing an arginine at this position encoded by an AGA codon, one of six codons for this residue. The properties in pigs of two chimeric viruses, each with an arginine residue at this position but encoded by different codons, have been investigated in parallel with the parental pathogenic and attenuated strains. Presence of the arginine residue, but not of the AGA codon, is essential for induction of high viraemia and appearance of significant disease.

Infective viruses produced from full-length complementary DNA of swine vesicular disease viruses HK/70 strain

The full-length cDNA clone of swine vesicular disease virus HK/70 strain named pSVOK₁₂ was constructed in order to study the antigenicity, replication, maturation and pathogenicity of swine vesicular disease virus. In vitro transcription RNA from pSVOK₁₂ transfected IBRS-2 cells and the recovered virus RNA were isolated and sequenced, then indirect hemagglutination test, indirect immunofluorescence assays, eleectron microscope test, 50% tissue culture infecting dose (TCID₅₀) assays and mouse virulence studies were performed to study the antigenicity and virulence of the recovered virus. The result showed that the infectious clones we obtained and the virus derived from pSVOK₁₂ had the same biological properties as the parental strain HK/70. The full-length infectious cDNA clone, pSVOK₁₂, will be very useful in studies of the antigenicity, virulence, pathogenesis, maturation and replication of SVDV.

Evolution of virulence in picornaviruses

The Picornaviridae encompass many positive-strand RNA viruses, all of which share a generally similar genome design and capsid structure, but which induce quite diverse diseases in humans and other animals. Picornavirus strains of the same serotype have been shown to express different virulence (or pathogenic) phenotypes when studied in animal models, demonstrating that key elements of pathogenesis reside in the viral genome. However, the genetics that determine the virulence phenotype of any picornavirus are poorly understood. Picornaviruses do not have virulence genes per se, but the design ofthe capsid andhow it interacts with the virus receptor expressed on the host cell surface, specific sequences within the nontranslated regions of the viral genome, as well as coding sequences that result in different protein sequences may all have a part in determining the virulence phenotype. Virulence may be better understood as a continuum from an apparent inability to induce disease to the ability to cause severe pathogenic changes. Ultimately, the ability of a picornavirus to induce disease depends upon viral genetics and how they are modulated by the host environment.

Sequence analysis of the 5′ untranslated region of swine vesicular disease virus reveals block deletions between the end of the internal ribosomal entry site and the initiation codon

Swine vesicular disease virus (SVDV) is a picornavirus closely related to the human pathogen coxsackievirus B5. In common with other picornaviruses, the 5′ untranslated region (5′ UTR) of SVDV contains an internal ribosomal entry site (IRES) that plays an important role in cap-independent translation. The aim of this study was to use RT-PCR and sequencing to characterize a fragment of the 5′ UTR encompassing the entire IRES. Sequence analysis demonstrated high nucleotide identities within the IRES between 33 representative SVDV isolates. These data support the choice of this region as a diagnostic target and provide information for the improvement of laboratory-based molecular assays to detect SVDV. In contrast to the relative conservation of the IRES element, there was considerable nucleotide variability in the spacer region located between the cryptic AUG at the 3′ end of the IRES and the initiation codon of the polyprotein. Interestingly, 11 SVDV isolates had block deletions of between 6 and 125 nt in this region. Nine of these isolates were of recent European origin and were phylogenetically closely related. In vitro growth studies showed that selected isolates with these deletions had a significantly reduced plaque diameter and grew to a significantly lower titre relative to an isolate with a full-length 5′ UTR. Further work is required to define the significance of these deletions and to assess whether they impact on the pathogenesis of SVD.

Importance of arginine 20 of the swine vesicular disease virus 2A protease for activity and virulence

A major virulence determinant of swine vesicular disease virus (SVDV), an Enterovirus that causes an acute vesicular disease, has been mapped to residue 20 of the 2A protease. The SVDV 2A protease cleaves the 1D-2A junction in the viral polyprotein, induces cleavage of translation initiation factor eIF4GI, and stimulates the activity of enterovirus internal ribosome entry sites (IRESs). The 2A protease from an attenuated strain of SVDV (Ile at residue 20) is significantly defective at inducing cleavage of eIF4GI and the activation of IRES-dependent translation compared to the 2A protease from a pathogenic strain (J1/73, Arg at residue 20), but the two proteases have similar 1D-2A cleavage activities (Y. Sakoda, N. Ross-Smith, T. Inoue, and G. J. Belsham, J. Virol. 75:10643-10650, 2001). Residue 20 has now been modified to every possible amino acid, and the activities of each mutant 2A protease has been analyzed. Selected mutants were reconstructed into full-length SVDV cDNA, and viruses were rescued. The rate of virus growth in cultured swine kidney cells reflected the efficiency of 2A protease activity. In experimentally infected pigs, all four of the mutant viruses tested displayed much-reduced virulence compared to the J1/73 virus but a significant, albeit reduced, level of viral replication and excretion was detected. Direct sequencing of cDNA derived from samples taken early and late in infection indicated that a gradual selection-reversion to a more efficient protease occurred. The data indicated that extensive sequence change and selection may introduce a severe bottleneck in virus replication, leading to a decreased viral load and reduced or no clinical disease.

Characterization of an infectious cDNA copy of the genome of a naturally occurring, avirulent coxsackievirus B3 clinical isolate

Group B coxsackieviruses (CVB) cause numerous diseases, including myocarditis, pancreatitis, aseptic meningitis and possibly type 1 diabetes. To date, infectious cDNA copies of CVB type 3 (CVB3) genomes have all been derived from pathogenic virus strains. An infectious cDNA copy of the well-characterized, non-pathogenic CVB3 strain GA genome was cloned in order to facilitate mapping of the CVB genes that influence expression of a virulence phenotype. Comparison of the sequence of the parental CVB3/GA population, derived by direct RT-PCR-mediated sequence analysis, to that of the infectious CVB3/GA progeny genome demonstrated that an authentic copy was cloned; numerous differences were observed in coding and non-coding sequences relative to other CVB3 strains. Progeny CVB3/GA replicated similarly to the parental strain in three different cell cultures and was avirulent when inoculated into mice, causing neither pancreatitis nor myocarditis. Inoculation of mice with CVB3/GA protected mice completely against myocarditis and pancreatitis induced by cardiovirulent CVB3 challenge. The secondary structure predicted for the CVB3/GA domain II, a region within the 5′ non-translated region that is implicated as a key site affecting the expression of a cardiovirulent phenotype, differs from those predicted for cardiovirulent and pancreovirulent CVB3 strains. This is the first report characterizing a cloned CVB3 genome from an avirulent strain.

Structure of swine vesicular disease virus: mapping of changes occurring during adaptation of human coxsackie B5 virus to infect swine

The structure of swine vesicular disease virus (SVDV) was solved and refined at a 3.0-A resolution by X-ray crystallography to gain information about the role of sequence changes that occurred as this virus evolved from the parental human pathogen coxsackievirus B5 (CVB5). These amino acid substitutions can be clustered in five distinct regions: (i) the antigenic sites, (ii) the hydrophobic pocket of the VP1 beta-sandwich, (iii) the putative CAR binding site, (iv) the putative heparan sulfate binding site, and (v) the fivefold axis. The VP1 pocket is occupied by a branched pocket factor, apparently different from that present in the closely related virus CVB3 and in other picornaviruses. This finding may be relevant for the design of new antiviral compounds against this site. Density consistent with the presence of ions was observed on the fivefold and threefold axes. The structure also provided an accurate description of the putative receptor binding sites.

Crystal structure of Swine vesicular disease virus and implications for host adaptation

Swine vesicular disease virus (SVDV) is an Enterovirus of the family Picornaviridae that causes symptoms indistinguishable from those of foot-and-mouth disease virus. Phylogenetic studies suggest that it is a recently evolved genetic sublineage of the important human pathogen coxsackievirus B5 (CBV5), and in agreement with this, it has been shown to utilize the coxsackie and adenovirus receptor (CAR) for cell entry. The 3.0-A crystal structure of strain UK/27/72 SVDV (highly virulent) reveals the expected similarity in core structure to those of other picornaviruses, showing most similarity to the closest available structure to CBV5, that of coxsackievirus B3 (CBV3). Features that help to cement together and rigidify the protein subunits are extended in this virus, perhaps explaining its extreme tolerance of environmental factors. Using the large number of capsid sequences available for both SVDV and CBV5, we have mapped the amino acid substitutions that may have occurred during the supposed adaptation of SVDV to a new host onto the structure of SVDV and a model of the SVDV/CAR complex generated by reference to the cryo-electron microscopy-visualized complex of CBV3 and CAR. The changes fall into three clusters as follows: one lines the fivefold pore, a second maps to the CAR-binding site and partially overlaps the site for decay accelerating factor (DAF) to bind to echovirus 7 (ECHO7), and the third lies close to the fivefold axis, where the low-density lipoprotein receptor binds to the minor group of rhinoviruses. Later changes in SVDV (post-1971) map to the first two clusters and may, by optimizing recognition of a pig CAR and/or DAF homologue, have improved the adaptation of the virus to pigs.

Molecular determinants of virulence, cell tropism, and pathogenic phenotype of infectious bursal disease virus

Infectious bursal disease viruses (IBDVs), belonging to the family Birnaviridae, exhibit a wide range of immunosuppressive potential, pathogenicity, and virulence for chickens. The genomic segment A encodes all the structural (VP2, VP4, and VP3) and nonstructural proteins, whereas segment B encodes the viral RNA-dependent RNA polymerase (VP1). To identify the molecular determinants for the virulence, pathogenic phenotype, and cell tropism of IBDV, we prepared full-length cDNA clones of a virulent strain, Irwin Moulthrop (IM), and constructed several chimeric cDNA clones of segments A and B between the attenuated vaccine strain (D78) and the virulent IM or GLS variant strain. Using the cRNA-based reverse-genetics system developed for IBDV, we generated five chimeric viruses after transfection by electroporation procedures in Vero or chicken embryo fibroblast (CEF) cells, one of which was recovered after propagation in embryonated eggs. To evaluate the characteristics of the recovered viruses in vivo, we inoculated 3-week-old chickens with D78, IM, GLS, or chimeric viruses and analyzed their bursae for pathological lesions 3 days postinfection. Viruses in which VP4, VP4-VP3, and VP1 coding sequences of the virulent strain IM were substituted for the corresponding region in the vaccine strain failed to induce hemorrhagic lesions in the bursa. In contrast, viruses in which the VP2 coding region of the vaccine strain was replaced with the variant GLS or virulent IM strain caused rapid bursal atrophy or hemorrhagic lesions in the bursa, as seen with the variant or classical virulent strain, respectively. These results show that the virulence and pathogenic-phenotype markers of IBDV reside in VP2. Moreover, one of the chimeric viruses containing VP2 sequences of the virulent strain could not be recovered in Vero or CEF cells but was recovered in embryonated eggs, suggesting that VP2 contains the determinants for cell tropism. Similarly, one of the chimeric viruses containing the VP1 segment of the virulent strain could not be recovered in Vero cells but was recovered in CEF cells, suggesting that VP1 contains the determinants for cell-specific replication in Vero cells. By comparing the deduced amino acid sequences of the D78 and IM strains and their reactivities with monoclonal antibody 21, which binds specifically to virulent IBDV, the putative amino acids involved in virulence and cell tropism were identified. Our results indicate that residues Gln at position 253 (Gln253), Asp279, and Ala284 of VP2 are involved in the virulence, cell tropism, and pathogenic phenotype of virulent IBDV.

Virulence of swine vesicular disease virus is determined at two amino acids in capsid protein VP1 and 2A protease

To identify the genetic determinants of virulence for swine vesicular disease virus, a panel of recombinant and site-directed mutant viruses were constructed from cDNA clones of a virulent J1'73 strain and an avirulent H/3'76 strain. Initial studies mapped the genetic determinants of virulence to either or both of the two sites at nucleotide (nt) 2842, encoding VP1-132, and nt 3355, encoding 2A-20. To determine their relative importance with regard to virulence, viruses mutated at either of these two sites from the avirulent to the virulent genotype and vice versa were tested in pigs. Viruses, mutated at nt 2842 to the virulent genotype (vSVLS104MJ1) or mutated at nt 3355 to the virulent genotype (vSVLS201MJ1), slightly recovered virulence but were very weak compared with viruses with site-directed mutations at both sites (vSVLS104/201MJ1). On the other hand, viruses, mutated at nt 2842 to the avirulent genotype (vSVLS104M00) or mutated at nt 3355 to the avirulent genotype (vSVLS201M00), did not have attenuated virulence. Sequence analysis of viruses recovered from inoculated pigs revealed that reversion at nt 3355 to the virulent genotype occurred in pigs which had been inoculated with vSVLS201M00. These results suggested that both amino acids determined the virulent phenotype, but that the 2A-20 site might be the major determinant for virulence.

An attenuating mutation in the 2A protease of swine vesicular disease virus, a picornavirus, regulates cap- and internal ribosome entry site-dependent protein synthesis

Virulent and avirulent strains of swine vesicular disease virus (SVDV), a picornavirus, have been characterized previously. The major determinants for attenuation have been mapped to specific residues in the 1D-2A-coding region. The properties of the 2A proteases from the virulent and avirulent strains of SVDV have now been examined. Both proteases efficiently cleaved the 1D/2A junction in vitro and in vivo. However, the 2A protease of the avirulent strain of SVDV was much less effective than the virulent-virus 2A protease at inducing cleavage of translation initiation factor eIF4GI within transfected cells. Hence the virulent-virus 2A protease is much more effective at inhibiting cap-dependent protein synthesis. Furthermore, the virulent-virus 2A protease strongly stimulated the internal ribosome entry sites (IRESs) from coxsackievirus B4 and from SVDV, while the avirulent-virus 2A protease was significantly less active in these assays. Thus, the different properties of the 2A proteases from the virulent and avirulent strains of SVDV in regulating protein synthesis initiation reflect the distinct pathogenic properties of the viruses from which they are derived. A single amino acid substitution, adjacent to His21 of the catalytic triad, is sufficient to confer the characteristics of the virulent-strain 2A protease on the avirulent-strain protease. It is concluded that the efficiency of picornavirus protein synthesis, controlled directly by the IRES or indirectly by the 2A protease, can determine virus virulence.

Swine vesicular disease: an overview

Swine vesicular disease (SVD) is a notifiable viral disease of pigs included on the Office International des Epizooties List A. The first outbreak of the disease was recognized in Italy in 1966. Subsequently, the disease has been reported in many European and Asian countries. The causative agent of the disease is SVD virus which is currently classified as a porcine variant of human coxsackievirus B5 and a member of the genus enterovirus in the family picornaviridae. From a clinical point of view, SVD is relatively unimportant, rarely causing deaths and usually only a minor setback to finishing schedules. However, the clinical signs which it produces are indistinguishable from those caused by foot-and-mouth disease, and its presence prevents international trade in pigs and pig products. This article reviews recent findings on all aspects of the virus and the disease which it causes.

Construction of a full-length infectious cDNA clone of swine vesicular disease virus strain NET/1/92 and analysis of new antigenic variants derived from it

The Dutch swine vesicular disease virus (SVDV) isolate NET/1/92 was one of the first isolates belonging to a new SVDV antigenic group. This strain was completely sequenced and was shown to have 93% similarity with the UKG/27/72 isolate. To enable antigenicity, replication, maturation and pathogenicity studies of NET/1/92, an infectious full-length cDNA clone, designated pSVD146, was prepared. The in vitro and in vivo biological properties of the virus derived from pSVD146 were studied by analysing antigenicity, plaque morphology, growth curves and virulence in pigs. The epitopes of newly prepared monoclonal antibodies were roughly mapped by fusion-PCR. Fine mapping of epitopes at the amino acid level was achieved by introducing single amino acid mutations in pSVD146. Two new amino acids important in epitope formation were located in VP1; one was mapped in the C-terminal end and the second is thought to be located in the H-I loop. Growth curve and plaque sizes in vitro were similar between virus derived from pSVD146 and the parent wild-type virus. In virulence studies in pigs, the lesions score, neutralization titres and the seroconversion rates were comparable between virus derived from pSVD146 and the parent strain. Since virus derived from pSVD146 had the same biological properties as the parent strain NET/1/92, the full-length infectious cDNA clone pSVD146 will be very useful in studies of the antigenicity, virulence, pathogenesis, maturation and replication of SVDV.

Swine vesicular disease, studies on pathogenesis, diagnosis, and epizootiology: a review

Swine vesicular disease (SVD) is a contagious viral disease of swine. It causes vesicular lesions indistinguishable from those observed of foot-and-mouth disease. Infection with SVD virus (SVDV) can lead to viraemia within 1 day and can produce clinical signs 2 days after a pig has come into contact with infected pigs or a virus-contaminated environment. Virus can be detected 3.5 hours after infection using immunohistochemistry. In these in vitro studies, this technique was superior to in-situ hybridization. In SVDV-infected tissues, however, more infected cells were positive using in-situ hybridization, and these were already seen 4.5 hours after infection. For serological diagnosis of SVD several new enzyme-linked immunosorbent assays (ELISA's) have been developed. The newest ELISAs, based on monoclonal antibodies, are superior to the previous tests. The new tests produce fewer less false-negative results and enable large-scale serological screening. In screening programmes a small percentage of false positive reactors have been detected. The cause of these false-positive reactions has not been identified, though infections with human Coxsackie B5 virus can be excluded.

Chimeric swine vesicular disease viruses produced by fusion PCR: a new method for epitope mapping

A new method of epitope mapping based on chimeric swine vesicular disease (SVD) viruses produced by fusion PCR (polymerase chain reaction). Seven out of 16 neutralising and non-neutralising newly produced monoclonal antibodies (MAbs) discriminated between SVD isolate ITL/1/66 and NET/1/92. Using fusion PCR eight chimeric viruses were produced containing different supplementary pieces of the P1 region of both parent strains. Using these chimeric viruses we were able to map the epitope regions recognised by these seven neutralising and non-neutralising Mabs. This new method, using chimeric viruses produced by fusion PCR, is particularly valuable for the epitope mapping of non-neutralising MAbs.