Antigenic comparison of the polypeptides of foot-and-mouth disease virus serotypes and other picornaviruses

Diagnostic potential of recombinant nonstructural protein 3B to detect antibodies induced by foot-and-mouth disease virus infection in bovines

Detection of antibodies to nonstructural proteins (NSP) of foot-and-mouth disease virus is the preferred diagnostic method to differentiate infected from vaccinated animals. In India, an endemic region practising preventive biannual vaccination, 3AB3 indirect ELISA (r3AB3 I-ELISA) has been employed as the primary screening test for serosurveillance. However, because of the variability observed in the immune response to the NSPs, the likelihood of detecting or confirming an infected animal is increased if an antibody profile against multiple NSPs is considered for diagnosis. In this study, all three copies of NSP 3B were expressed in a prokaryotic system to develop an indirect ELISA (r3B I-ELISA). At the decided cutoff of 40 percent positivity, the diagnostic sensitivity and specificity of the r3B I-ELISA were estimated to be 92.1% (95% CI: 89.0-94.5) and 98.1% (95% CI: 96.9-98.8), respectively, as compared to 97.04% and 95.04% for r3AB3 I-ELISA. Although r3B I-ELISA displayed lower sensitivity compared to the screening assay, which could possibly be attributed to additional relevant B-cell epitopes in the carboxy-terminal half of the 3A protein, the former achieved considerably higher specificity on repeatedly vaccinated animals. NSP antibodies could be detected from 10 to as late as 998 days postinfection in experimental calves. Substantial agreement in the test results (90.6%) was found between the two ELISAs. The r3B I-ELISA, when used in conjunction with the r3AB3 I-ELISA as an integrated system, can potentially augment the efficiency and confidence of detection of infected herds against the backdrop of intensive vaccination.

Granulocyte-macrophage colony-stimulating factor does not increase the potency or efficacy of a foot-and-mouth disease virus subunit vaccine

Foot-and-mouth disease (FMD) is one of the most feared diseases of livestock worldwide. Vaccination has been a very effective weapon in controlling the disease, however a number of concerns with the current vaccine including the inability of approved diagnostic tests to reliably distinguish vaccinated from infected animals and the need for high containment facilities for vaccine production, have limited its use during outbreaks in countries previously free of the disease. A number of FMD vaccine candidates have been tested and a replication-defective human adenovirus type 5 (Ad5) vector containing the FMDV capsid (P1-2A) and 3C protease coding regions has been shown to completely protect pigs against challenge with the homologous virus (FMDV A12 and A24). An Ad5-P1-2A+3C vaccine for FMDV O1 Campos (Ad5-O1C), however, only induced a low FMDV-specific neutralizing antibody response in swine potency tests. Granulocyte-macrophage colony-stimulating factor (GM-CSF) has been successfully used to stimulate the immune response in vaccine formulations against a number of diseases, including HIV, hepatitis C and B. To attempt to improve the FMDV-specific immune response induced by Ad5-O1C, we inoculated swine with Ad5-O1C and an Ad5 vector containing the gene for porcine GM-CSF (pGM-CSF). However, in the conditions used in this trial, pGM-CSF did not improve the immune response to Ad5-O1C and adversely affected the level of protection of swine challenged with homologous FMDV.

Molecular basis of pathogenesis of FMDV

Current understanding of the molecular basis of pathogenesis of foot-and-mouth disease (FMD) has been achieved through over 100 years of study into the biology of the etiologic agent, FMDV. Over the last 40 years, classical biochemical and physical analyses of FMDV grown in cell culture have helped to reveal the structure and function of the viral proteins, while knowledge gained by the study of the virus' genetic diversity has helped define structures that are essential for replication and production of disease. More recently, the availability of genetic engineering methodology has permitted the direct testing of hypotheses formulated concerning the role of individual RNA structures, coding regions and polypeptides in viral replication and disease. All of these approaches have been aided by the simultaneous study of other picornavirus pathogens of animals and man, most notably poliovirus. Although many questions of how FMDV causes its devastating disease remain, the following review provides a summary of the current state of knowledge into the molecular basis of the virus' interaction with its host that produces one of the most contagious and frightening diseases of animals or man.

Absence of protein 2C from clarified foot-and-mouth disease virus vaccines provides the basis for distinguishing convalescent from vaccinated animals

We have recently reported that cattle and pigs which have been vaccinated against foot-and-mouth disease can be distinguished from convalescent animals by the absence of antibodies to viral non-structural protein 2C (Lubroth and Brown, Res. Vet. Sci., 1995, 59, 70-78(1)). In this study, we show that the absence of 2C antibodies from the sera of vaccinated animals can be explained by the association of this viral protein with cellular debris which is separated from the virus harvest prior to inactivation of the supernatant for vaccine production. This serological marker can be of great value in countries where the disease occurs or in the veterinary regulatory arena when livestock are transported across borders, since it can be used to identify convalescent, persistently infected animals and vaccinates exposed to wild-type virus variants which have infected the vaccinated animals.

Identification of native foot-and-mouth disease virus non-structural protein 2C as a serological indicator to differentiate infected from vaccinated livestock

Cattle and pigs which have been vaccinated against foot-and-mouth disease can be distinguished from convalescent animals by radio-immunoprecipitation and sodium dodecyl sulphate polyacrylamide gel electrophoresis of the virus-induced proteins reacting with the respective sera. Baby hamster kidney cells infected with foot-and-mouth disease virus (FMDV) (serotype A24) were labelled with 35S-methionine and the virus-induced proteins were precipitated with sera from vaccinated and subsequently challenged animals, convalescent animals retained for over 300 days, animals vaccinated or infected with viruses belonging to all serotypes of FMDV, and animals infected with encephalomyocarditis (EMC) or porcine or bovine enteroviruses. In addition to the structural proteins of the virus, the non-structural proteins 2C, 3ABC, 3C, 3CD and 3D were precipitated by convalescent sera, but only 3D was precipitated by serum from vaccinated animals. Proteins L, 2C and 3C were precipitated only after challenge with a heterotypic virus (serotype O1 Tunisia), indicating that virus replication of the challenge virus had taken place. No precipitation was detected with sera from EMC or enterovirus-infected animals. The results indicate that protein 2C, and to a lesser extent the polypeptide 3ABC, could be used to differentiate potential carrier convalescent animals from vaccinated livestock.

Antiviral effects of a thiol protease inhibitor on foot-and-mouth disease virus

The thiol protease inhibitor E-64 specifically blocks autocatalytic activity of the leader protease of foot-and-mouth disease virus (FMDV) and interferes with cleavage of the structural protein precursor in an in vitro translation assay programmed with virion RNA. Experiments with FMDV-infected cells and E-64 or a membrane-permeable analog, E-64d, have confirmed these results and demonstrated interference in virus assembly, causing a reduction in virus yield. In addition, there is a lag in the appearance of virus-induced cellular morphologic alterations, a delay in cleavage of host cell protein p220 and in shutoff of host protein synthesis, and a decrease in viral protein and RNA synthesis. The implications of using E-64-based compounds as potential antiviral agents for FMDV are discussed.

Expression of the aphthovirus RNA polymerase gene in Escherichia coli and its use together with other bioengineered nonstructural antigens in detection of late persistent infections

A plasmid has been constructed containing the DNA sequences that direct the expression of the aphthovirus RNA-dependent RNA polymerase (virus infection-associated antigen, VIAA) in its native form. The aphthovirus polypeptide was designed to contain only a single additional amino acid, the N-terminal methionine. The recombinant protein has been purified and used in enzyme-linked immunoelectrotransfer blots to detect aphthovirus-specific antibodies in the sera of persistently infected animals. Furthermore, studies were carried out to test the hypothesis that antibodies against other nonstructural antigens appear in the sera of these animals. It was established that antibodies against polypeptides 3A and 3B can serve as complementary markers for late aphthovirus-carrier state detection. The considerable potential of this approach to detect aphthovirus-specific antibodies, when the isolation of infectious virus is not possible, was demonstrated. Negative results were obtained in animals from virus-free areas and in vaccinated cattle. This assay has the added advantage that no infectious or noninfectious virus is involved during antigen production.

Identification of foot-and-mouth disease virus replication in vaccinated cattle by antibodies to non-structural virus proteins

Antibodies raised in cattle against foot-and-mouth disease virus by vaccination or by experimental infection were distinguished. Vaccination elicited only antibodies to virus capsid proteins and the polymerase 3D. Virus replication in cattle elicited additional antibodies directed against the non-structural proteins 2B, 2C, 3AB1, and/or 3C irrespective of prior vaccination or whether the cattle exhibited symptoms of disease. Non-permissive mice inoculated with virus responded in the same way, indicating that antibodies raised due to the transient presence of antigen are safely recognized by the method applied which was radioimmunoprecipitation. All kinds of infections were thus detected and it was possible to differentiate between cattle exposed or not exposed to challenge in the field, and further between protected animals and possible virus carriers.

The detection and differentiation of foot-and-mouth disease viruses using solid-phase nucleic acid hybridization

Thirteen complementary DNA (cDNA) probes were used to detect the presence of foot-and-mouth disease virus (FMDV) RNA extracted from cell cultures. When labelled with 32P, these probes enabled the detection of 1 pg of FMDV-RNA, or 1 virus copy per cell. Two FMDV A12 probes that coded for the leader, structural protein VP1 region and part of the polymerase gene respectively, showed no hybridization with other closely related picornaviruses. Differentiation between FMDV serotypes A, O and C was possible, using cDNA probes from individual serotypes that corresponded to structural protein VP1.

Use of in situ hybridization for the detection of foot-and-mouth disease virus in cell culture

Biotinylated complementary DNA (cDNA) and RNA probes were prepared from a specific and highly conserved section of the foot-and-mouth disease virus (FMDV) genome coding for the RNA-dependent RNA polymerase. Hybridization was conducted on FMDV-infected, bovine enterovirus (BEV)-infected, and noninfected swine kidney cell cultures. The detection system utilized the enzyme system streptavidin-alkaline phosphatase, the substrate phosphate, and the chromogen nitroblue tetrazolium. Intense cytoplasmic granular staining was present at 2 and 4 hr postinfection (hpi), with less staining observed at 24 hpi. The staining was specific for FMDV, as indicated by a lack of staining of noninfected cells and BEV-infected cells. With the RNA probe, positive cells were detected up to the highest viral dilution assayed, which was approximately 96 TCID50. The cDNA probe was slightly less sensitive, detecting positive cells at 10-fold lower dilutions. This technique could prove useful in the diagnosis of foot-and-mouth disease in animals or in the detection of FMDV in biologics submitted for importation.

Serological probes for some foot-and-mouth disease virus nonstructural proteins

Foot-and-mouth disease virus (FMDV) O1 Kaufbeuren-specific cDNA fragments were subcloned into the E. coli expression vector pRIT.2T. Fusion proteins thus produced in bacteria were purified by affinity chromatography and inoculated into rabbits. Three sera thus obtained were found to be monospecific for FMDV proteins 3A, 3C, and 3D, respectively. Two others were prevalently directed against protein 2C, but in addition, either to protein 2B or to protein 3A. Five out of six mature nonstructural virus proteins can therefore be separately investigated in FMDV-infected cells, either by indirect immunofluorescence or by radioimmunoprecipitation. Immunofluorescence shows all investigated proteins to be located exclusively in the cytoplasm. One of them, protein 2C, transiently forms aggregates at the periphery of cells. Radioimmunoprecipitation confirmed current knowledge on maturation of FMDV proteins. It was further used to characterize postinfectional sera with regard to FMDV-specific antibodies. Cattle and guinea pig were found to have responded differently to FMDV nonstructural antigens. Furthermore, antigenicity of yet to be described FMDV polypeptides was observed in the guinea pig.

Relationship of p220 cleavage during picornavirus infection to 2A proteinase sequencing

Infection of HeLa cells by poliovirus results in an abrupt inhibition of host cell protein synthesis. It is thought that the mechanism of this inhibition involves proteolytic cleavage of the p220 component of the cap-binding protein complex, thereby causing functional inactivation of the cap-binding protein complex and preventing capped (cellular) mRNAs from binding ribosomes. Current data suggest that the viral proteinase 2A indirectly induces p220 cleavage via alteration or activation of a second proteinase of cellular origin. We present evidence that translation of poliovirus proteinase 2A sequences in vitro activates p220 cleavage. We have also aligned published picornavirus 2A amino acid sequences for maximum homology, and we show that the picornaviruses can be divided into two classes based on the presence or absence of a highly conserved 18-amino acid sequence in the carboxy-terminal portion of 2A. This conserved 2A sequence is homologous with the active site of the cysteine proteinase 3C common to all picornaviruses. We show that picornaviruses which contain the putative 2A active site sequence (e.g., enteroviruses and rhinoviruses) will induce cleavage of p220 in vivo. Conversely, we show that two cardioviruses (encephalomyocarditis virus and Theiler's encephalomyelitis virus) do not encode this putative proteinase sequence in the 2A region and do not induce cleavage of p220 in vivo. The foot-and-mouth disease virus (FMDV) 2A sequence represents an apparent deletion and consists of only 16 amino acids, most homologous with the carboxy terminus of the cardiovirus 2A sequence. It does not contain the putative cysteine proteinase active site. However, FMDV infection induces complete cleavage of BK cell p220, and translation of FMDV RNA in vitro induces an activity that cleaves HeLa cell p220. The data predict that an alternate FMDV viral protease is responsible for the induction of p220 cleavage.

Leader protein of foot-and-mouth disease virus is required for cleavage of the p220 component of the cap-binding protein complex

Suppression of host protein synthesis in cells infected by poliovirus and certain other picornaviruses involves inactivation of the cap-binding protein complex. Inactivation of this complex has been correlated with the proteolytic cleavage of p220, a component of the cap-binding protein complex. Since picornaviral RNA is not capped, it continues to be translated as the cap-binding protein complex is inactivated. The cleavage of p220 can be induced to occur in vitro, catalyzed by extracts from infected cells or by reticulocyte lysates translating viral RNA. Expression of polioviral protease 2A is sufficient to induce p220 cleavage, and the presence in 2A of an 18-amino-acid sequence representing a putative cysteine protease active site correlates with the ability of different picornaviruses to induce p220 cleavage. Foot-and-mouth disease virus (FMDV) infection induces complete cleavage of p220, yet the FMDV genome codes for a 2A protein of only 16 amino acids, which does not include the putative cysteine protease active site. Using cDNA plasmids encoding various regions of the FMDV genome, we have determined that the leader protein is required to initiate p220 cleavage. This is the first report of a function for the leader protein, other than that of autocatalytic cleavage from the FMDV polyprotein.

Prediction of three-dimensional models for foot-and-mouth disease virus and hepatitis A virus

Atomic models of foot-and-mouth disease virus and hepatitis A virus have been predicted using amino acid sequence alignments with the known structures of Mengo virus and human rhinovirus 14. The structural models are consistent with results of biochemical and immunological studies. The two viruses appear to have surface features exceedingly different than those of other picornaviruses. They also have large hydrophobic cavities within VP1 suggesting that it may be possible to inhibit their infectivity with suitably designed antiviral agents that block uncoating.