Virus-host interactions in persistently FMDV-infected cells derived from bovine pharynx

Foot-and-mouth disease virus (FMDV) produces a disease in cattle characterized by vesicular lesions and a persistent infection with asymptomatic low-level production of virus in pharyngeal tissues. Here we describe the establishment of a persistently infected primary cell culture derived from bovine pharynx tissue (PBPT) infected with FMDV serotype O1 Manisa, where surviving cells were serially passed until a persistently infected culture was generated. Characterization of the persistent virus demonstrated changes in its plaque size, ability to grow in different cell lines, and change in the use of integrins as receptors, when compared with the parental virus. These results demonstrate the establishment of persistently infected PBPT cell cultures where co-adaptation has taken place between the virus and host cells. This in vitro model for FMDV persistence may help further understanding of the molecular mechanisms of the cattle carrier state.

Thermal inactivation of foot-and-mouth disease virus in milk using high-temperature, short-time pasteurization

Previous studies of laboratory simulation of high temperature, short time pasteurization (HTST) to eliminate foot-and-mouth disease virus (FMDV) in milk have shown that the virus is not completely inactivated at the legal pasteurization minimum (71.7 degrees C/15 s) but is inactivated in a flow apparatus at 148 degrees C with holding times of 2 to 3 s. It was the intent of this study to determine whether HTST pasteurization conducted in a continuous-flow pasteurizer that simulates commercial operation would enhance FMDV inactivation in milk. Cows were inoculated in the mammary gland with the field strain of FMDV (01/UK). Infected raw whole milk and 2% milk were then pasteurized using an Arm-field pilot-scale, continuous-flow HTST pasteurizer equipped with a plate-and-frame heat exchanger and a holding tube. The milk samples, containing FMDV at levels of up to 10(4) plaque-forming units/mL, were pasteurized at temperatures ranging from 72 to 95 degrees C at holding times of either 18.6 or 36 s. Pasteurization decreased virus infectivity by 4 log10 to undetectable levels in tissue culture. However, residual infectivity was still detectable for selected pasteurized milk samples, as shown by intramuscular and intradermal inoculation of milk into naïve steers. Although HTST pasteurization did not completely inactivate viral infectivity in whole and 2% milk, possibly because a fraction of the virus was protected by the milk fat and the casein proteins, it greatly reduced the risk of natural transmission of FMDV by milk.

The ability of integrin alpha(v)beta(3) To function as a receptor for foot-and-mouth disease virus is not dependent on the presence of complete subunit cytoplasmic domains

The integrin alpha(v)beta(3) has been shown to function as one of the integrin receptors on cultured cells for foot-and-mouth disease virus (FMDV), and high-efficiency utilization of the bovine homolog of this integrin is dependent on the cysteine-rich repeat region of the bovine beta(3) subunit. In this study we have examined the role of the cytoplasmic domains of the alpha(v) and beta(3) subunits in FMDV infection. We have found that truncations or extensions of these domains of either subunit, including deletions removing almost all of the cytoplasmic domains, had little or no effect on the ability of the integrin to function as a receptor for FMDV. The lysosomotropic agent monensin inhibited viral replication in cells transfected with either intact or cytoplasmic domain-truncated alpha(v)beta(3). In addition, viral replication in transfected cells was inhibited by an alpha(v)beta(3) function-blocking antibody but not by function-blocking antibodies to three other RGD-directed integrins, suggesting that these integrins are not involved in the infectious process. These results indicate that alterations to the cytoplasmic domains of either subunit, which lead to the inability of the integrin receptor to function normally, do not abolish the ability of the integrin to bind and internalize this viral ligand.

High-efficiency utilization of the bovine integrin alpha(v)beta(3) as a receptor for foot-and-mouth disease virus is dependent on the bovine beta(3) subunit

We have previously reported that Foot-and-mouth disease virus (FMDV), which is virulent for cattle and swine, can utilize the integrin alpha(v)beta(3) as a receptor on cultured cells. Since those studies were performed with the human integrin, we have molecularly cloned the bovine homolog of the integrin alpha(v)beta(3) and have compared the two receptors for utilization by FMDV. Both the alpha(v) and beta(3) subunits of the bovine integrin have high degrees of amino acid sequence similarity to their corresponding human subunits in the ectodomains (96%) and essentially identical transmembrane and cytoplasmic domains. Within the putative ligand-binding domains, the bovine and human alpha(v) subunits have a 98.8% amino acid sequence similarity while there is only a 93% similarity between the beta(3) subunits of these two species. COS cell cultures, which are not susceptible to FMDV infection, become susceptible if cotransfected with alpha(v) and beta(3) subunit cDNAs from a bovine or human source. Cultures cotransfected with the bovine alpha(v)beta(3) subunit cDNAs and infected with FMDV synthesize greater amounts of viral proteins than do infected cultures cotransfected with the human integrin subunits. Cells cotransfected with a bovine alpha(v) subunit and a human beta(3) subunit synthesize viral proteins at levels equivalent to those in cells expressing both human subunits. However, cells cotransfected with the human alpha(v) and the bovine beta(3) subunits synthesize amounts of viral proteins equivalent to those in cells expressing both bovine subunits, indicating that the bovine beta(3) subunit is responsible for the increased effectiveness of this receptor. By engineering chimeric bovine-human beta(3) subunits, we have shown that this increase in receptor efficiency is due to sequences encoding the C-terminal one-third of the subunit ectodomain, which contains a highly structured cysteine-rich repeat region. We postulate that amino acid sequence differences within this region may be responsible for structural differences between the human and bovine beta(3) subunit, leading to more efficient utilization of the bovine receptor by this bovine pathogen.

Foot-and-mouth disease virus virulent for cattle utilizes the integrin alpha(v)beta3 as its receptor

Adsorption and plaque formation of foot-and-mouth disease virus (FMDV) serotype A12 are inhibited by antibodies to the integrin alpha(v)beta3 (A. Berinstein et al., J. Virol. 69:2664-2666, 1995). A human cell line, K562, which does not normally express alpha(v)beta3 cannot replicate this serotype unless cells are transfected with cDNAs encoding this integrin (K562-alpha(v)beta3 cells). In contrast, we found that a tissue culture-propagated FMDV, type O1BFS, was able to replicate in nontransfected K562 cells, and replication was not inhibited by antibodies to the endogenously expressed integrin alpha5beta1. A recent report indicating that cell surface heparan sulfate (HS) was required for efficient infection of type O1 (T. Jackson et al., J. Virol. 70:5282-5287, 1996) led us to examine the role of HS and alpha(v)beta3 in FMDV infection. We transfected normal CHO cells, which express HS but not alpha(v)beta3, and two HS-deficient CHO cell lines with cDNAs encoding human alpha(v)beta3, producing a panel of cells that expressed one or both receptors. In these cells, type A12 replication was dependent on expression of alpha(v)beta3, whereas type O1BFS replicated to high titer in normal CHO cells but could not replicate in HS-deficient cells even when they expressed alpha(v)beta3. We have also analyzed two genetically engineered variants of type O1Campos, vCRM4, which has greatly reduced virulence in cattle and can bind to heparin-Sepharose columns, and vCRM8, which is highly virulent in cattle and cannot bind to heparin-Sepharose. vCRM4 replicated in wild-type K562 cells and normal, nontransfected CHO (HS+ alpha(v)beta3-) cells, whereas vCRM8 replicated only in K562 and CHO cells transfected with alpha(v)beta3 cDNAs. A similar result was also obtained in assays using a vCRM4 virus with an engineered RGD-->KGE mutation. These results indicate that virulent FMDV utilizes the alpha(v)beta3 integrin as a primary receptor for infection and that adaptation of type O1 virus to cell culture results in the ability of the virus to utilize HS as a receptor and a concomitant loss of virulence.

Tissue culture adaptation of foot-and-mouth disease virus selects viruses that bind to heparin and are attenuated in cattle

Isolates of foot-and-mouth disease virus (FMDV) exist as complex mixtures of variants. Two different serotype O1 Campos preparations that we examined contained two variants with distinct plaque morphologies on BHK cells: a small, clear-plaque virus that replicates in BHK and CHO cells, and a large, turbid-plaque virus that only grows in BHK cells. cDNAs encoding the capsids of these two variants were inserted into a genome-length FMDV type A12 infectious cDNA and used to produce chimeric viruses that exhibited the phenotype of the original variants. Analyses of these viruses, and hybrids created by exchanging portions of the capsid gene, identified codon 56 in VP3 (3056) as the critical determinant of both cell tropism and plaque phenotype. Specifically, the CHO growth/clear-plaque phenotype is dependent on the presence of the highly charged Arg residue at 3056, and viruses with this phenotype and genotype were selected during propagation in tissue culture. The genetically engineered Arg 3056 virus was highly attenuated in bovines, but viruses recovered from animals inoculated with high doses of this virus had lost the ability to grow in CHO cells and contained either an uncharged residue at 3056 or a negatively charged Glu substituted for a Lys at a spatially and antigenically related position on VP2 (2134). Comparison of these animal-derived viruses to other natural and engineered viruses demonstrated that positively charged residues are required at both 2134 and 3056 for binding to heparin. Taken together, these results indicate that in vitro cultivation of FMDV type O selects viruses that bind to heparin and that viruses with the heparin-binding phenotype are attenuated in the natural host.

Characterization of synthetic foot-and-mouth disease virus provirions separates acid-mediated disassembly from infectivity

One of the final steps in the maturation of foot-and-mouth disease virus (FMDV) is cleavage of the VP0 protein to produce VP4 and VP2. The mechanism of this cleavage is unknown, but it is thought to function in stabilizing the virus particle and priming it for infecting cells. To investigate the cleavage process and to understand its role in virion maturation, we engineered synthetic FMDV RNAs with mutations at Ala-85 (A85) and Asp-86 (D86) of VP0, which border the cleavage site. BHK cells transfected with synthetic RNAs containing substitutions at position 85 (A85N or A85H) or at position 86 (D86N) yielded particles indistinguishable from wild-type (WT) virus in sedimentation and electrophoretic profiles. Viruses derived from these transfected cells were infectious and maintained their mutant sequences upon passage. However, BHK cells transfected with synthetic RNAs encoding Phe and Lys at these positions (A85F/D86K) or a Cys at position 86 (D86C) produced noninfectious provirions with uncleaved VP0 molecules. Despite their lack of infectivity, the A85F/D86K provirions displayed cell binding and acid sensitivity similar to those of WT virus. However, acid breakdown products of the A85F/D86K provirions differed in hydrophobicity from the comparable WT virion products, which lack VP4. Taken together, these studies are consistent with a role for soluble VP4 molecules in release of the viral genome from the endosomal compartment of susceptible cells.

Cloning and expression of a single-chain antibody fragment specific for foot-and-mouth disease virus

The gene for a single-chain antibody (VHK) to a conformational epitope on the type A12 foot-and-mouth disease virus (FMDV) particle was assembled and expressed in Escherichia coli. The VHK, purified from periplasmic extracts immunoprecipitated virus as efficiently as its parental monoclonal antibody (MAb) and exhibited the same binding specificity when tested against panel of natural and genetically engineered virus particles. The VHK neutralized type A12 virus in the presence of goat anti-mouse IgG; however, in the absence of the second antibody, only weak neutralizing activity was detected. Preliminary analysis of the mechanism of viral neutralization indicated that both the MAb and the VHK neutralize by the same mechanism. Small amounts of the VHK allowed infection of cells via Fc receptor-mediated adsorption in the presence of the second antibody. These data represent the first report of a single-chain neutralizing antibody for a picornavirus and provide insights into the mechanisms of viral neutralization and virus uptake.

Propagation of an attenuated virus by design: engineering a novel receptor for a noninfectious foot-and-mouth disease virus

To gain entry into cells, viruses utilize a variety of different cell-surface molecules. Foot-and-mouth disease virus (FMDV) binds to cell-surface integrin molecules via an arginine-glycine-aspartic acid (RGD) sequence in capsid protein VP1. Binding to this particular cell-surface molecule influences FMDV tropism, and virus/receptor interactions appear to be responsible, in part, for selection of antigenic variants. To study early events of virus-cell interaction, we engineered an alternative and novel receptor for FMDV. Specifically, we generated a new receptor by fusing a virus-binding, single-chain antibody (scAb) to intracellular adhesion molecule 1 (ICAM1). Cells that are normally not susceptible to FMDV infection became susceptible after being transfected with DNA encoding the scAb/ICAM1 protein. An escape mutant (B2PD.3), derived with the mAb used to generate the genetically engineered receptor, was restricted for growth on the scAb/ICAM1 cells, but a variant of B2PD.3 selected by propagation on scAb/ICAM1 cells grew well on these cells. This variant partially regained wild-type sequence in the epitope recognized by the mAb and also regained the ability to be neutralize by the mAb. Moreover, RGD-deleted virions that are noninfectious in animals and other cell types grew to high titers and were able to form plaques on scAb/ ICAM1 cells. These studies demonstrate the first production of a totally synthetic cell-surface receptor for a virus. This novel approach will be useful for studying virus reception and for the development of safer vaccines against viral pathogens of animals and humans.

Strategy for producing new foot-and-mouth disease vaccines that display complex epitopes

Widely used inactivated vaccines for foot-and-mouth disease (FMD) induce protective immunity, but vaccine production plants and residual virus in the vaccine itself have been implicated in disease outbreaks. The structure of the FMD virion has been determined, and although much of the surface of the viral particle is produced by complex folding of the three surface-exposed capsid proteins (VP1-3), some surface regions representing important linear epitopes can be mimicked by recombinant proteins or synthetic peptides. Vaccine candidates based on these products stimulate immune responses to foot-and-mouth virus (FMDV), but do not always protect livestock from disease. The basis of protective immunity to FMDV has been explored using genetic engineering to produce antigenic chimeras of the virus. Studies with these chimeras have shown that a strong and protective immune response can be generated in livestock to epitopes outside the sequential epitopes incorporated into previous subunit vaccine candidates. Genetic engineering of the virus has also been used to demonstrate that changes within the sequence encoding an arginine-glycine-aspartic acid (RGD) sequence in VP1 abrogate virus binding to cells in culture, confirming the role of RGD as the receptor binding site. Based on this information, genetically stable viruses which cannot bind to cells have been created by deleting the nucleotides coding the RGD sequence. The receptor binding site-deleted viruses have been shown to be non-infectious in tissue culture, mice, and swine. Cattle vaccinated with these viruses are protected from disease when challenged with virulent FMDV, demonstrating that they could serve as the basis for safer FMD vaccines.

Receptor binding site-deleted foot-and-mouth disease (FMD) virus protects cattle from FMD

Binding of foot-and-mouth disease virus (FMDV) to cells requires an arginine-glycine-aspartic acid (RGD) sequence in the capsid protein VP1. We have genetically engineered an FMDV in which these three amino acids have been deleted, producing a virus particle which is unable to bind to cells. Cattle vaccinated with these receptor binding site-deleted virions were protected from disease when challenged with a virulent virus, demonstrating that these RGD-deleted viruses could serve as the basis for foot-and-mouth disease vaccines safer than those currently in use. This strategy may prove useful in the development of vaccines for other viral diseases.

Antibodies to the vitronectin receptor (integrin alpha V beta 3) inhibit binding and infection of foot-and-mouth disease virus to cultured cells

The amino acid sequence Arg-Gly-Asp (RGD) is highly conserved on the VP1 proteins of different serotypes and subtypes of foot-and-mouth disease virus (FMDV) and is essential for cell attachment. This sequence is also found in certain extracellular matrix proteins that bind to a family of cell surface receptors called integrins. Within the Picornaviridae family, enterovirus coxsackievirus A9 also has an RGD motif on its VP1 capsid protein and has recently been shown to utilize the vitronectin receptor integrin alpha V beta 3 as a receptor on monkey kidney cells. Competition binding experiments between type A12 FMDV and coxsackievirus A9 using BHK-21 and LLC-MK2 cells revealed shared receptor specificity between these two viruses. Polyclonal anti-serum to the vitronectin receptor and a monoclonal antibody to the alpha V subunit inhibited both FMDV binding and plaque formation, while a monoclonal antibody to the beta 3 subunit inhibited virus binding. In contrast, antibodies to the fibronectin receptor (alpha 5 beta 1) or to the integrin (alpha V beta 5) had no effect on either binding or plaque formation. These data demonstrate that the alpha V beta 3 vitronectin receptor can function as a receptor for FMDV.

Foot-and-mouth disease virus undergoes restricted replication in macrophage cell cultures following Fc receptor-mediated adsorption

We have previously reported that foot-and-mouth disease virus (FMDV) can enter an Fc receptor (FcR)-expressing cell line by antibody-dependent enhancement. Since FMDV can establish a persistent infection in animals in the presence of high levels of neutralizing antibodies (carrier state), we examined macrophages for their ability to be infected by the virus in the presence of antibody. The murine macrophage cell line P388D1 or porcine macrophage-monocytes isolated from peripheral blood were incubated with antibody-complexed virus. Under these conditions, host protein synthesis was rapidly inhibited in both cell types, but not in cells incubated either with virus alone or with imine-inactivated antibody-complexed virus. Virus-specific structural and nonstructural proteins were synthesized in antibody-complexed virus-infected P388D1 cells, while only nonstructural proteins were detected in porcine macrophage cultures. Negative-strand RNAs were detected in both cell types, indicating that RNA replication had taken place. Cultures of P388D1 cells transfected with viral RNA produced very low levels of infectious virus, and infection with virus-antibody complexes, followed by a brief wash with pH 6.0 buffer to remove residual input virus, allowed the detection of low levels of productive replication. Thus, macrophages can be infected with FMDV via FcR-mediated adsorption, and infection of these cells could contribute to pathology or provide a reservoir of infectious virus in carrier animals.

Vaccines prepared from chimeras of foot-and-mouth disease virus (FMDV) induce neutralizing antibodies and protective immunity to multiple serotypes of FMDV

The G-H loop of VP1 (residues 132 to 159) of foot-and-mouth disease virus (FMDV) is a prominent feature on the virion surface and has an important role in vaccine efficacy, generation of antigenic variants, and cell binding. Using an infectious cDNA of FMDV, we have constructed serotype A viruses in which the G-H loop has been substituted with the homologous sequences from serotype O or C. These chimeric viruses replicated to high titer and displayed plaque morphologies similar to those of wild-type viruses, demonstrating that the functions provided by the loop can be readily exchanged between serotypes. Monoclonal antibody analyses showed that epitopes contained within the loop were transferred to the chimeras and that epitopes encoded by the type A backbone were maintained. Chemically inactivated vaccines prepared from chimeric viruses induced antibodies in guinea pigs that neutralized both type A and either type O or type C viruses. Swine inoculated with the A/C chimera vaccine also produced cross-reactive antibodies, were protected from challenge with the type A virus, and partially protected against challenge with type C. These studies emphasize the importance of epitopes outside of the G-H loop in protective immunity in swine, which is a natural host of FMDV.

Animal-derived antigenic variants of foot-and-mouth disease virus type A12 have low affinity for cells in culture

We recently have shown that binding of foot-and-mouth disease virus (FMDV) to cells in culture requires an arginine-glycine-aspartic acid (RGD) sequence in the G-H loop of the capsid protein VP1 (P. W. Mason, E. Rieder, and B. Baxt, Proc. Natl. Acad. Sci. USA 91:1932-1936, 1994). In this report, we show that FMDV type A12 viruses found in infected bovine tongue tissue (BTT) differ from their tissue culture-grown derivatives at amino acid residues near the RGD. Viruses genetically engineered to contain VP1 sequences found in animal tissue (BTT viruses) were antigenically different from their tissue culture derivatives and bound to BHK cells more poorly than did the tissue culture-adapted viruses. Passage of the genetically engineered BTT viruses in BHK cells resulted in the rapid selection of variants with cell-binding properties, antigenic characteristics, and sequences typical of tissue culture-adapted viruses. These data indicate that residues near the RGD are critical for cell binding and that interpretations of antigenic variation of FMDV can be affected by virus cultivation in vitro.

RGD sequence of foot-and-mouth disease virus is essential for infecting cells via the natural receptor but can be bypassed by an antibody-dependent enhancement pathway

Foot-and-mouth disease virus appears to initiate infection by binding to cells at an Arg-Gly-Asp (RGD) sequence found in the flexible beta G-beta H loop of the viral capsid protein VP1. The role of the RGD sequence in attachment of virus to cells was tested by using synthetic full-length viral RNAs mutated within or near the RGD sequence. Baby hamster kidney (BHK) cells transfected with three different RNAs carrying mutations bordering the RGD sequence produced infectious viruses with wild-type plaque morphology; however, one of these mutant viruses bound to cells less efficiently than wild type. BHK cells transfected with RNAs containing changes within the RGD sequence produced noninfectious particles indistinguishable from wild-type virus in terms of sedimentation coefficient, binding to monoclonal antibodies, and protein composition. These virus-like particles are defined as ads- viruses, since they were unable to adsorb to and infect BHK cells. These mutants were defective only in cell binding, since antibody-complexed ads- viruses were able to infect Chinese hamster ovary cells expressing an immunoglobulin Fc receptor. These results confirm the essential role of the RGD sequence in binding of foot-and-mouth disease virus to susceptible cells and demonstrate that the natural cellular receptor for the virus serves only to bind virus to the cell.

Cross-reactive idiotopes among anti-foot and mouth disease virus neutralizing antibodies

Foot and mouth disease virus (FMDV) viral protein 1 is the only one of the four viral proteins (VP) that induces neutralizing antibodies as an isolated protein. A 32 amino acid (AA) residue (32dimer) of FMDV subtype A12 Lp ab VP1 (AA 137-168) was immunogenic against the A12 subtype. Three antibody populations each recognizing different epitopes on 32dimer were isolated by affinity chromatography (AFC) from the serum of a steer which had been immunized with the 32dimer. The 32dimer contains an AA sequence that is recognized by a protective paratope carried on a murine monoclonal antibody (mAb) (7SF-3.H3.1). Polyclonal anti-7SF-3 idiotype antibodies specifically inhibited the binding activity of one of these anti-32dimer antibody populations suggesting the existence of cross-reactive paratopic-related idiotopes between mAb 7SF-3 and antibodies elicited by the 32dimer. These anti-idiotypic antibodies were used in AFC to purify antibodies from the anti-32dimer serum. The purified antibody population has characteristics that resemble those of the mAb 7SF-3, i.e. its reactivity with FMDV A subtypes in ELISA, radioimmunoassay (RIA), mouse neutralization and its lack of reactivity with a mAb 7SF-3 neutralizing escape virus variant. Furthermore, these antibodies were specifically inhibited by either anti-mAb 7SF-3 idiotypic antibodies or peptides containing the mAb 7SF-3 epitope. Using the same experimental approach, mAb 7SF-3 idiotope-bearing antibodies were shown to be present in serum from bovine and swine convalescent from FMDV A12 Lp ab infection. Thus, the highly immunogenic area between residues 137 and 168 of FMDV VP1 elicited a cross-reactive neutralizing idiotope response conserved amongst several animal species.

Antibody-complexed foot-and-mouth disease virus, but not poliovirus, can infect normally insusceptible cells via the Fc receptor

Poliovirus and foot-and-mouth disease virus (FMDV) initiate infection by binding to specific cell surface receptors, which is followed by a poorly understood disassembly process. To probe these early steps of infection, the ability of poliovirus and FMDV to infect cells following binding through an alternative receptor was examined. For these studies, a Chinese hamster ovary (CHO) cell line expressing the B2 isoform of the murine Fc receptor (FcR) was used. Both viruses were able to bind to this cell line in an antibody-dependent manner, but only FMDV was able to productively infect these cells following binding through the FcR. These results suggest that the natural poliovirus receptor has dual functions in binding and destabilizing the virus particle, whereas the putative FMDV receptor may only be necessary for virion binding. These findings are consistent with differences in virion architecture which predict a more intimate virion-receptor association for poliovirus than for FMDV.

The effect of peptides containing the arginine-glycine-aspartic acid sequence on the adsorption of foot-and-mouth disease virus to tissue culture cells

Sequencing of the VP1 of a large number of subtypes of foot-and-mouth disease virus (FMDV) has revealed the presence of a conserved arginine-glycine-aspartic acid (RGD) sequence located in a highly exposed region. This sequence has been shown to be essential for the interaction of certain extracellular matrix and adhesion proteins with a superfamily of cell-surface receptors called integrins. We have examined the effects of synthetic peptides containing the RGD sequence on the binding of eight different subtypes of FMDV to tissue culture cells. The results showed that such peptides inhibited viral adsorption by 50-80%. The inhibition was dose dependent but not as great as that achieved by using a saturating amount of virus as an inhibitor. Substitution of other amino acids for any of the three main residues lowered the inhibitory properties of the peptides. These results suggest that the RGD sequence in FMDV VP1 appears to be important for the interaction of virus with cellular receptor sites.

Analysis of foot-and-mouth disease virus-neutralizing idiotypes from immune bovine and swine with anti-murine idiotype antibody probes

Rabbit anti-idiotypic antibodies (a-IdAb) induced by foot-and-mouth disease virus (FMDV) neutralizing mAb were used as probes to identify anti-FMDV Id in immune serum from bovine and swine. In a competitive RIA, at least two of the a-IdAb exhibited a dose-dependent capacity to compete with labeled virus for anti-FMDV antibodies from a convalescent bovine serum. These a-IdAb were immobilized on activated Sepharose and used to isolate anti-viral Id from bovine, swine, and murine FMDV immune sera. Both the bovine and swine antibodies recovered from the a-IdAb/Sepharose columns reacted with virus, and to a lesser extent with corresponding mAb-resistant virus variants. The binding of affinity isolated bovine and swine antibodies to virus was specifically inhibited by the homologous a-IdAb, and in addition, both were capable of neutralizing FMDV in suckling mouse protection and plaque reduction neutralization assays. Therefore, by means of a-IdAb probes generated against FMDV murine Id, two neutralizing Id were identified in bovine and swine. These results suggest that FMDV-neutralizing epitopes recognized by murine systems play a role in the overall immunity of foot-and-mouth disease-susceptible animals.

Analysis of foot-and-mouth disease virus-neutralizing idiotypes from immune bovine and swine with anti-murine idiotype antibody probes

Rabbit anti-idiotypic antibodies (a-IdAb) induced by foot-and-mouth disease virus (FMDV) neutralizing mAb were used as probes to identify anti-FMDV Id in immune serum from bovine and swine. In a competitive RIA, at least two of the a-IdAb exhibited a dose-dependent capacity to compete with labeled virus for anti-FMDV antibodies from a convalescent bovine serum. These a-IdAb were immobilized on activated Sepharose and used to isolate anti-viral Id from bovine, swine, and murine FMDV immune sera. Both the bovine and swine antibodies recovered from the a-IdAb/Sepharose columns reacted with virus, and to a lesser extent with corresponding mAb-resistant virus variants. The binding of affinity isolated bovine and swine antibodies to virus was specifically inhibited by the homologous a-IdAb, and in addition, both were capable of neutralizing FMDV in suckling mouse protection and plaque reduction neutralization assays. Therefore, by means of a-IdAb probes generated against FMDV murine Id, two neutralizing Id were identified in bovine and swine. These results suggest that FMDV-neutralizing epitopes recognized by murine systems play a role in the overall immunity of foot-and-mouth disease-susceptible animals.

Foot-and-mouth disease virus-neutralizing antibodies induced in mice by anti-idiotypic antibodies

A neutralizing monoclonal antibody (nmAb) to foot-and-mouth disease virus (FMDV) was used as antibody-1 (AB1) to induce anti-idiotypic antibodies (a-IdAb) in rabbits. The rabbit a-IdAb (AB2) were isolated on protein A-Sepharose, followed by cycles of separation on idiotype and isotype affinity columns. The specificity of the AB2 for the paratope of AB1 was determined by direct binding to AB1 in solid-phase radioimmunoassay (SP-RIA), and by competition RIA (C-RIA) with virus for binding to the AB1. The AB2, termed a-2PD11, was utilized to immunize six groups of female Swiss mice at weekly intervals with either one of three formulations, in doses of 50 micrograms or 5 micrograms, given in single subcutaneous (s.c.) spots. Anti-viral antibody (AB3) was first detected by RIA at the fifth week in the 50 micrograms/dose groups, and maximum levels were reached at the sixth week in the 50 and 5 micrograms/dose groups. The AB3 levels were at least three times higher for mice given 50 micrograms doses. In addition, the AB3 were also shown to neutralize FMDV infectivity in tissue culture and in a suckling mouse protection assay. Overall, mice exhibited variable responses to immunization with AB2. In a subsequent trial, mice received multispot s.c. and footpad injections of 50 micrograms of a-2PD11 coupled to keyhole limpet haemocyanin (KLH) on a weekly basis. In these mice, AB3 was detected earlier than in mice immunized with single s.c. injections. These results support the use of a-IdAb as potential surrogates of critical determinants for FMD vaccines.

Analysis of neutralizing antigenic sites on the surface of type A12 foot-and-mouth disease virus

A series of seven neutralizing monoclonal antibodies (nMAbs) directed against type A12 foot-and-mouth disease virus was used to generate neutralization-resistant variants. Both plaque reduction neutralization and microneutralization assays showed that the variants were no longer neutralized by the nMAbs used to generate them, although some of the variants still reacted with the nMAbs at high antibody concentrations. Results of cross-neutralization studies by both plaque reduction neutralization and microneutralization assays suggested the presence of at least one immunodominant antigenic site on the surface of type A12 foot-and-mouth disease virus, along with evidence of a second antigenic site on the viral surface. Two of the variants had reduced virulence in tissue culture as evidenced by their inability to inhibit cellular protein synthesis and a marked reduction in virus-induced cellular morphological alterations. Nucleotide sequencing of the variant genomes placed three epitopes of the major antigenic site on VP1 and the fourth epitope on VP3 and VP1. The one epitope of the minor site appears to reside only on VP1.

Characterization of anti-idiotypic antibodies generated against foot-and-mouth disease virus neutralizing monoclonal antibodies

A series of seven neutralizing monoclonal antibodies (nMAbs) against type A12 foot-and-mouth disease virus (FMDV) was used to induce polyclonal anti-idiotypic antibodies (anti-ids) in rabbits. The anti-ids were semi-purified through isotype affinity columns and assayed by solid-phase radioimmunoassay for cross-reactivity. nMAbs which map to the same epitope on the virion appear to contain a common idiotype, and the corresponding anti-ids competitively inhibited the virus-nMAb reaction. Using a modified ELISA assay, it was possible to demonstrate binding of purified anti-ids to FMDV susceptible tissue culture cells. Such antibodies however, did not interfere with the binding of virus to cells, and the binding of anti-ids to FMDV receptor-negative cells could also be demonstrated. Mice were inoculated with purified anti-ids, and two elicited anti-viral antibodies, although these antibodies were non-neutralizing. Thus anti-ids to anti-FMDV nMAbs failed to react with cellular receptors for the virus, but were able to induce anti-viral antibody and thus should be examined as an alternative vaccine strategy for this virus.

Effect of lysosomotropic compounds on early events in foot-and-mouth disease virus replication

The effect of three lysosomotropic compounds, chloroquine, monensin and NH4Cl, on the replication of foot-and-mouth disease virus (FMDV) type A12 was studied. Viral replication was almost totally inhibited by 0.5 mM chloroquine, 50 microM monensin, or 25 mM NH4Cl. Monensin and NH4Cl affected replication when added either before or within the first hour of infection. Chloroquine, however, still inhibited viral replication when added up to 2.5 h after infection. Assays of binding of radiolabeled virus to cells showed that these compounds had no effect on viral adsorption. Neither monensin nor NH4Cl had any significant effect on cellular protein synthesis, but there was no evidence of viral protein synthesis in cells infected in the presence of these compounds. In contrast, chloroquine inhibited both cellular and viral protein synthesis. Eclipse assays, performed in the presence of the compounds, showed that while chloroquine and NH4Cl had little effect on cell-induced degradation of incoming virions to 12 S protein subunits, monensin inhibited this reaction. The replication of representative members of all seven serotypes of FMDV was inhibited by monensin although some types were less sensitive to the compound than others. These results are consistent with a model which postulates that viral eclipse is the result of acidification of endocytic vesicles which degrade entrapped virions to 12 S protein subunits resulting in the release of genome RNA.

Epitopes on foot-and-mouth disease virus outer capsid protein VP1 involved in neutralization and cell attachment

Foot-and-mouth disease virus structural protein VP1 elicits neutralizing and protective antibody and is probably the viral attachment protein which interacts with cellular receptor sites on cultured cells. To study the relationships between epitopes on the molecule related to neutralization and cell attachment, we tested monoclonal antibodies prepared against type A12 virus, isolated A12 VP1, and a CNBr-generated A12 VP1 fragment for neutralization and effect on viral absorption. The antibodies selected for analysis neutralized viral infectivity with varying efficiencies. One group of antibodies caused a high degree of viral aggregation and inhibited the adsorption of virus to cells by 50 to 70%. A second group of antibodies caused little or no viral aggregation but inhibited the adsorption of virus to cells by 80 to 90%. One antibody, which is specific for the intact virion, caused little viral aggregation and had no effect on the binding of virus to specific cellular receptor sites. Thus, at least three antigenic areas on the surface of foot-and-mouth disease virus which were involved in neutralization were demonstrated. One of the antigenic sites appears to have been responsible for interaction with the cellular receptor sites on the surface of susceptible cells.

Competition for cellular receptor sites among selected aphthoviruses

The competition between different types of aphthoviruses (foot-and-mouth disease virus [FMDV]) for receptor site utilization was determined. The Southern African Territories (SAT) types of FMDV absorbed poorly to BHK-21 cells as measured by a radioactivity binding assay but grew to relatively high titers on these cells. On BK cells, however, all three SAT types bound well and competed with each other for receptor sites. In addition, unlabeled FMDV types A12 and O1B were able to completely inhibit the binding of 3H-uridine labeled SAT types. Unlabeled SAT, however, was only slightly able to inhibit the adsorption of labeled A12 and moderately inhibit the binding of labeled O1B. Saturation binding studies with homologous virus showed that BK cells contain at least 100-fold more receptor sites for types A12 and O1B than for the SAT types. Competitive binding analysis between type A12 FMDV and poliovirus and encephalomyocarditis virus revealed that these three viruses all used different receptor sites. Thus, different FMDV serotypes appear to utilize both common and unique receptor sites which are different from those of at least two other picornaviruses.

Identification of a protein kinase activity in purified foot- and-mouth disease virus

Purified preparations of foot-and-mouth disease virus types A, O, and C contain a protein kinase activity which can transfer the gamma phosphate of [32P]ATP to virion structural proteins VP2 and VP3 and exogenous acceptor proteins. Utilizing protamine sulfate as an acceptor, the kinase activity can be demonstrated in disrupted virus but not in intact virus. The enzyme is heat labile with optimal activity at pH 7 or greater. Serine residues of protamine sulfate were identified as the amino acid phosphorylated by the protein kinase. Treatment of purified virus with trypsin, which cleaves VP3, did not affect the protein kinase activity. The results indicate that the protein kinase activity found in FMDV is present in an internally located protein of viral or host origin.

The structural polypeptides of aphthovirus are phosphoproteins

Analysis of aphthovirus A12, strain 119ab, grown in the presence of inorganic 32P revealed that two of the major viral polypeptides, VP4 and trypsin-sensitive protein VP3, were highly phosphorylated. The other major polypeptides, VP1 and VP2, were also phosphorylated but to a much lesser extent. Polypeptides VP0 and P56, of which there are approximately one of two copies per aphthovirion, were also labeled with 32P. Phosphoserine and phosphothreonine appeared to be the amino acids labeled with 32P.

Studies on the mechanisms of vaccina virus cytopathic effects. I. Inhibition of protein synthesis in infected cells is associated with virus-induced RNA synthesis

The mechanism of vaccinia virus-induced inhibition of protein synthesis was studied in LLC-MK2, HeLa and L cells. Removal of cycloheximide (300 microgram/ml) from cells infected at a multiplicity of infection (m.o.i.) of 300 particles/cell at 4 h after infection resulted in the resumption of both host and virus protein synthesis in LLC-MK2 cells, but not in HeLa and L cells. In order to determine whether virus-induced RNA synthesis, which occurs in infected cells in the presence of cycloheximide, is related to the inhibition of protein synthesis, (cut-off), the rate of virus-induced RNA synthesis in the presence of cycloheximide was measured in all three cell types. In L cells and HeLa cells the rate of virus-induced RNA synthesis increased with time, whereas in LLC-MK2 cells it remained constant for at least 4 h. However, when higher multiplicities (900 and 2700 particles/cell) were used to infect LLC-MK2 cells, the rate of RNA synthesis in the presence of cycloheximide did increase with time and was greater at the higher multiplicity. Under these conditions there was a direct relationship between the extent of virus RNA synthesis and the degree of cut-off after the removal of cycloheximide. In HeLa and L cells infected at 300 particles/cell, the longer the exposure to cycloheximide, the greater was the cut-off observed upon removal of the drug. As was the case the LLC-MK2 cells, there was a direct relationship between the rate of RNA synthesis and the degree of inhibition of protein synthesis. Since virus-induced RNA synthesis occurs in the presence of cycloheximide, the effects of actinomycin D and cordycepin on host polypeptide synthesis were tested. Inhibition of host cell protein synthesis was virtually abolished when HeLA cells were infected in the presence of cordycepin (50 and 25 microgram/ml) or actinomycin D (20 microgram/ml). These results indicate that, as the rate of virus-induced RNA synthesis increased, regardless of the type of cell used, protein synthesis was inhibited at earlier times and to a greater extent. These observations are consistent with the hypothesis that the cut-off phenomenon is related to the synthesis of an early virus-induced RNA(s).

Studies on the mechanisms of vaccinia virus cytopathic effects. II. Early cell rounding is associated with virus polypeptide synthesis

Vaccinia virus-induced morphological lesions were studied in LLC-MK2, HeLa and L cells. In LLC-MK2 cells, cell rounding occurs within 30 to 60 min after infection with 300, 900 or 2700 particles/cell and the time of appearance of these changes is dependent on the multiplicity of infection (m.o.i.). When cycloheximide (300 microgram/ml) is added to cultures at the time of infection, early cell rounding is prevented regardless of the m.o.i. However, cell rounding does occur when cycloheximide is removed, and its time of appearance and extent depends upon the time of removal of the compound and the m.o.i. Upon removal of cycloheximide at I or 2 h after infection early cell rounding occurs, and virus polypeptide synthesis is evident in cells infected at all three multiplicities. However, when the drug is removed at 4 hr after infection, cell rounding and virus polypeptide synthesis occur only in cultures infected at 300 particles/cell. Early morphological changes are also prevented in HeLa and L cells infected at 300 particles/cell in the presence of cycloheximide. These changes occur only if the compound is removed up to 2 h after infection in HeLa cells and up to 40 min after infection in L cells. Early morphological lesions are not seen if the compound is removed at later times. The occurrence of early morphological changes in HeLa and L cells is also correlated with the synthesis of virus polypeptides. All cell types, when infected at 2700 particles/cell in the presence of cycloheximide, or inhibitors of RNA synthesis, display cell fusion. Thus, whereas early morphological changes require virus protein synthesis to become manifest, cell fusion occurs in the absence of virus RNA or protein synthesis and may be mediated by a component of the virion.

Effects of interferon on vesicular stomatitis virus transcription and translation

The effect of interferon on the synthesis of the RNA species and proteins of vesicular stomatitis virus has been studied in two cell types. Virus protein synthesis is inhibited by interferon despite the apparent presence of near normal amounts of virus RNA with sedimentation values characteristic of virus messenger RNA. The synthesis of those virus RNA species which are completely dependent on virus protein synthesis is preferentially inhibited in interferon-treated cells. These results are most consistent with a model of interferon action postulating a primary effect on translation of virus messenger RNA.