Genetic and immunogenic variations among closely related isolates of foot-and-mouth disease virus

A simple and rapid colloidal gold-based immunochromatogarpic strip test for detection of FMDV serotype A

A sandwich format immunochromatographic assay for detecting foot-and-mouth disease virus (FMDV) serotypes was developed. In this rapid test, affinity purified polyclonal antibodies from Guinea pigs which were immunized with sucking-mouse adapted FMD virus (A/AV88(L) strain) were conjugated to colloidal gold beads and used as the capture antibody, and affinity purified polyclonal antibodies from rabbits which were immunized with cell-culture adapted FMD virus (A/CHA/09 strain) were used as detector antibody. On the nitrocellulose membrane of the immunochromatographic strip, the capture antibody was laid on a sample pad, the detector antibody was printed at the test line(T) and goat anti-guinea pigs IgG antibodies were immobilized to the control line(C). The lower detection limit of the test for a FMDV 146S antigen is 11.7 ng/ml as determined in serial tests after the strip device was assembled and the assay condition optimization. No cross reactions were found with FMDV serotype C, Swine vesicular disease (SVD), Vesicular stomatitis virus (VSV) and vesicular exanthema of swine virus (VES) viral antigens with this rapid test. Clinically, the diagnostic sensitivity of this test for FMDV serotypes A was 88.7% which is as same as an indirect-sandwich ELISA. The specificity of this strip test was 98.2% and is comparable to the 98.7% obtained with indirect-sandwich ELISA. This rapid strip test is simple, easy and fast for clinical testing on field sites; no special instruments and skills are required, and the result can be obtained within 15 min. To our knowledge, this is the first rapid immunochromatogarpic assay for serotype A of FMDV.

Location of the amino acid differences in the S1 spike glycoprotein subunit of closely related serotypes of infectious bronchitis virus

Four UK strains of three different serotypes were found to differ by only 2-3% of their S1 amino acids. The S1 sequences were also very similar to those of three Dutch isolates (D207, D274 and D3896), the greatest difference between two of the seven isolates being 4.4%. The few amino acid differences between the seven isolates were located largely between residues 19-122 and 251-347 of the mature S1 subunit. The seven isolates could be differentiated using 16 monoclonal antibodies in an enzyme-linked immunosorbent assay. Some virus neutralizing (VN) antibody-inducing epitopes were common to all seven isolates even though the strains had been differentiated into three serotypes by polyclonal sera. The results indicate that the most antigenic of the VN antibody-inducing epitopes are formed by very few amino acids and that these occur in the first and third quarters of the S1 subunit. We suggest that serology-based epizootiological studies of IBV should, therefore, be augmented by the inclusion of nucleic acid sequencing and/or monoclonal antibody analysis.

Emergence of a new strain of type O foot-and-mouth disease virus: its phylogenetic and evolutionary relationship with the PanAsia pandemic strain

In India, Foot-and-mouth disease virus (FMDV) serotype O has been associated with more than 75% of the outbreaks. Previous studies with this serotype have indicated that the viruses circulating in India belong to a single genotype. Recent (February 2001) FMD epidemics in Europe have focussed global attention on the source of the virus and have been traced to a strain, PanAsia (serotype O), which is present in India since 1990. In this study, to further characterize the isolates belonging to the PanAsian strain, we sequenced the complete VP1-encoding (1 D) gene for 71 FMDV serotype O isolates from India recovered from the field outbreaks during the last 4 decades (1962-2001). All the isolates in the tree were distributed in to three major branches (designated as A, B and C); the branch C is further divided into four groups (I-IV), of which the group IV belongs to the PanAsia strain. Furthermore, we show that the PanAsia strain has been circulating endemically since 1982 (not 1990 as reported earlier) and has been the most dominant outbreak strain in the recent years and distributed at least in 17 states of the country. During the year 2001, another new group (group III) of virus with genetic divergence of 5.4-11.1% at nucleotide level from the PanAsia strain is found to co-circulate endemically, and is slowly replacing it. At amino acid level this strain differed from PanAsia strain at five amino acid positions in the VP1. Although these strains are divergent at nucleotide level, they maintained a good antigenic relationship with one of the vaccine strains (IND R2/75) widely used in the country. Given the ability of the PanAsia virus to persist, spread and to outcompete other strains, the present trend could be of serious concern as the newly emerging virus is replacing it. If this is true, then there is another equally divergent strain as PanAsia that may pose a serious threat to the global dairy and meat industries.

Serotype C foot-and-mouth disease virus isolates from India belong to a separate so far not described lineage

Complete 1D gene sequences of 13 Indian foot-and-mouth disease virus (FMDV) type C field isolates and a vaccine strain (C-Bombay/64) were determined. All the field isolates showed a greater genetic homogeneity (95-100%) among themselves and were 19.7-21.2% divergent from the vaccine strain. In the phylogenetic analysis, the Indian field isolates formed a separate lineage (lineage VII) different from the previously identified six lineages (lineage I-VI) in type C FMDV [J. Virol. 66 (1992) 3557]. The vaccine strain was grouped with European lineage (lineage II). Comparison of the deduced amino acid sequences of antigenic sites A and C of field isolates showed no significant variation from the vaccine strain. One-way serological relationship determined in ELISA showed antigenic closeness of the field isolates with C-Bombay/64.

Stable expression of antisense RNAs targeted to the 5' non-coding region confers heterotypic inhibition to foot-and-mouth disease virus infection

The antiviral potential of transcripts targeted to the non-coding regions (NCRs) of foot-and-mouth disease virus (FMDV) RNA have been studied during transient and constitutive expression in susceptible BHK-21 cells. Transient expression of antisense transcripts corresponding to the 5' and 3'NCRs, alone or in combination, confers specific inhibition of homologous (serotype C) virus infection in BHK-21 cells. Constitutive expression of antisense 5'NCR transcripts (5'AS) exerted higher levels of inhibition to homologous and heterologous (serotypes O, A, Asia, SAT 1, SAT 2 and SAT 3) FMDV infection, as estimated by a 10-fold reduction in virus titre in the supernatants from infected clones and by a plaque reduction assay. These inhibitions were also observed, albeit to a lesser extent, in clones stably expressing antisense 3'NCR transcripts. The antiviral response was specific for FMDV, as the picornavirus encephalomyocarditis virus was not inhibited in any of the transformed cell lines. In all cases, a correlation was found between the level of transcript expression and the extent of virus inhibition. The potential to efficiently inhibit FMDV, including isolates representing the seven serotypes, by expressing interfering 5'AS transcripts opens the possibility of developing transgenic animals with a reduced susceptibility to FMDV.

Evolution of foot-and-mouth disease virus

Foot-and-mouth disease virus evolution is strongly influenced by high mutation rates and a quasispecies dynamics. Mutant swarms are subjected to positive selection, negative selection and random drift of genomes. Adaptation is the result of selective amplification of subpopulations of genomes. The extent of adaptation to a given environment is quantified by a relative fitness value. Fitness values depend on the virus and its physical and biological environment. Generally, infections involving large population passages result in fitness gain and population bottlenecks lead to fitness loss. Very different types of mutations tend to accumulate in the foot-and-mouth disease virus (FMDV) genome depending on the virus population size during replication. Quasispecies dynamics predict higher probability of success of antiviral strategies based on multivalent vaccines and combination therapy, and this has been supported by clinical and veterinary practice. Quasispecies suggest also new antiviral strategies based on virus entry into error catastrophe, and such procedures are under investigation. Studies with FMDV have contributed to the understanding of quasispecies dynamics and some of its biological implications.

Foot-and-mouth disease virus

Foot-and-mouth disease virus (FMDV) is an aphthovirus of the family Picornaviridae and the etiological agent of the economically most important animal disease. As a typical picornavirus, FMD virions are nonenveloped particles of icosahedral symmetry and its genome is a single stranded RNA of about 8500 nucleotides and of positive polarity. FMDV RNA is infectious and it replicates via a complementary, minus strand RNA. FMDV RNA replication is error-prone so that viral populations consist of mutant spectra (quasispecies) rather than a defined genomic sequence. Therefore FMDV in nature is genetically and antigenically diverse. This poses important challenges for the diagnosis, prevention and control of FMD. A deeper understanding of FMDV population complexity and evolution has suggested requirements for a new generation of anti-FMD vaccines. This is relevant to the current debate on the adequacy of non-vaccination versus vaccination policies for the control of FMD.

Phylogenetic analysis of foot-and-mouth disease viruses isolated in Argentina

We have analysed complete or partial VPI sequences of 31 foot-and-mouth disease (FMD) viruses belonging to serotypes A, O and C to determine the genetic relatedness of field strains of FMD virus (FMDV) that have circulated in Argentina between 1961 and 1994. Phylogenetic analysis, which also included 15 previously published Argentinean sequences and six reference strains, revealed that (i) FMD type A strains showed the highest genetic heterogeneity and could be divided into five lineages with a sequence divergence of 0.9-18.5% between strains (ii) most of the FMD type O viruses grouped in two clusters (within cluster sequence divergence ranging from 0.2% to 6.0%) circulating in Argentina since the early 1960s, and (iii) FMD type C viruses were grouped in two clusters with a 13.4% nucleotide sequence divergence between each cluster. The availability of sequence data for many more field isolates from the region will enable us to understand the genetic relationships between FMDV strains and to rapidly trace the source of an FMD outbreak for epidemiological surveillance.

Extensive antigenic and genetic variation among foot-and-mouth disease type A viruses isolated from the 1994 and 1995 foci in São Paulo, Brazil

Nine foot-and-mouth disease virus (FMDV) type A isolates recovered from the field FMD foci in São Paulo State, Brazil, during 1994 and 1995 (a period preceding the last reported focus of FMD in 1996 in this state) were compared among themselves and with the reference vaccine strain A(24)Cruzeiro. The techniques used were sandwich ELISA, virus neutralization (VN), polyacrylamide gel electrophoresis (PAGE) of the structural polypeptides and direct sequencing of the VP1-coding region (1D gene). Results of VN were recorded as serological relationships "R" and those from ELISA were expressed as percentage of the homologous reaction "r". ELISA and VN gave comparable results (correlation coefficient, 0.936) allowing assignment of these field viruses to four groups which were distinct from the A(24)Cruzeiro strain. PAGE and 1D nucleotide sequencing were also able to distinguish between these viruses. The high level of genetic and antigenic variation found when comparing the A(24)Cruzeiro vaccine strain and type A strains recovered from the last identified foci of FMD came from a formerly endemic area where vaccination with polyvalent vaccines (O(1)Campos, A(24)Cruzeiro and C(3)Indaial) had been extensively applied. The similarity between the results of the serological and genetic analyses suggest that the antigenic differences found are mainly located in the 1D protein.

Foot-and-mouth disease type O viruses exhibit genetically and geographically distinct evolutionary lineages (topotypes)

Serotype O is the most prevalent of the seven serotypes of foot-and-mouth disease (FMD) virus and occurs in many parts of the world. The UPGMA method was used to construct a phylogenetic tree based on nucleotide sequences at the 3' end of the VP1 gene from 105 FMD type O viruses obtained from samples submitted to the OIE/FAO World Reference Laboratory for FMD. This analysis identified eight major genotypes when a value of 15% nucleotide difference was used as a cut-off. The validity of these groupings was tested on the complete VP1 gene sequences of 23 of these viruses by bootstrap resampling and construction of a neighbour-joining tree. These eight genetic lineages fell within geographical boundaries and we have used the term topotype to describe them. Using a large sequence database, the distribution of viruses belonging to each of the eight topotypes has been determined. These phylogenetically based epidemiological studies have also been used to identify viruses that have transgressed their normal ecological niches. Despite the high rate of mutation during replication of the FMD virus genome, the topotypes appear to represent evolutionary cul-de-sacs.

Conformational flexibility in a highly mobile protein loop of foot-and-mouth disease virus: distinct structural requirements for integrin and antibody binding

The G-H loop of foot-and-mouth disease virus VP1 protein is a highly mobile peptide, that extends from the capsid surface and that in native virions is invisible by X-ray crystallography. In serotype C, this segment contains a hypervariable region with several continuous, overlapping, B-cell epitopes that embrace the conserved Arg-Gly-Asp (RGD) cell attachment motif. The solvent-exposed positioning of this peptide by selective insertion into different structural frameworks of E. coli beta-galactosidase, generates a spectrum of antigenic variants which react distinctively with a panel of anti-VP1 monoclonal antibodies and exhibit different efficiencies as cell ligands. The cell attachment efficiency is much less restricted by the different positioning of the viral segment at the insertion sites. A molecular model of an inserted stretch reveals a highest flexibility of the RGD tripeptide segment compared with the flanking sequences, that could allow a proper accommodation to integrin receptors even in poorly antigenic conformations. The non-converging structural requirements for RGD-mediated integrin binding and antibody recognition, explains the dynamism of the generation of neutralisation-resistant antigenic variants in the viral quasi-species, arising from a conformational space of integrin-binding competent peptides. This might be of special relevance for foot-and-moth disease virus evolution, since unlike in other picornaviruses, the cell binding motif and the major neutralising B-cell epitopes overlap in a solvent-exposed peptide accessible to the host immune system, in a virion lacking canyons and similar hiding structures.

The foot and mouth disease virus type O outbreak of 1992 is not related to vaccine strain (O/R2/75)

Vaccination is the only pragmatic approach to control foot and mouth disease in India. Strict quality control measures are essential to supply potent vaccine to the field application, in addition to monitoring the performance of the vaccine in the field. During the process of monitoring, an outbreak of FMD in vaccinated animals caused by type "O" virus in Tanjavur district of Tamil Nadu and a type "O" virus from unvaccinated herd of Karnataka were studied. Field isolates and vaccine virus were sequenced and analyzed. Data indicated that the virus from the outbreak in vaccinated cattle was a variant which could escape neutralization by antibodies against vaccine virus.

Genetic heterogeneity of Indian field isolates of foot-and-mouth disease virus serotype O as revealed by partial sequencing of 1D gene

The sequence of 165 nucleotides at the 3' end of the 1D gene, determined from RT PCR amplified cDNA fragments, of 25 type O strains isolated from different parts/regions of India during 1987 1995 and the vaccine strain (R2/75) currently in use in India were subjected to phylogenetic analysis. One isolate from the neighbouring country Nepal was also included in the study. The virus/ field strains showed high degree of genetic heterogeneity among themselves with % divergence in nucleotide sequence ranging from 1.2 to 19.4%. The Indian strains were much away (13.3 20.6%) from the exotic type O strains of O1BFS, O1K, and O1Campos. The type O strains analyzed were classified into three genotypes basing on level of divergence observed in nucleotide sequence. The type O vaccine virus (R2/75) was > 71% divergent (7.3-15.2%) from the field strains which revealed significant ( > 5%) genetic heterogeneity between the two. The phylogenetic analysis identified three distinct lineages, viz., (i) lineage 1 represented by the exotic strains, (ii) lineage 2 represented by 25 of the field strains which clustered into seven subgroups/sublines (2a-2g), and (iii) lineage 3 represented by a unique field isolate which shared the branching/origin with the vaccine strain. The lineage 2 which comprised of 25 of the 26 type O field strains analyzed, was placed almost at equidistance from the lineages 1 and 3 in the phylogenetic tree. The vaccine strain was closer to the viruses in lineage 2. Though there was no specific distribution pattern of sequences in different geographical regions of India, the viruses/ sequences in subgroup 2f appeared to be restricted to the southern states. Comparison of deduced amino acid sequence in the immunodominant regions 133-160 and 200-208 of the 1D gene product (VP1) showed that the two viruses in lineage 3 had unique amino acid residues at the positions 138 (D), 139 (G), 144 (I), and 158 (A) compared to rest of the strains including the exotic ones. Comparison of amino acid residues at critical positions 144, 148, 149, 151, 153, 154, and 208 revealed similarity between the type O strains analyzed. The virus strains showed variation (V/L/I) at position 144. One field strain showed replacement from Q149-->E and another from P208-->L. Thus, the study revealed that the type O FMD virus populations circulating in India and causing disease outbreaks are genetically much heterogeneous but related at the immunodominant region of VP1 polypeptide, and there are more than one genetically distinct virus populations in almost every region of the country which is possible due to unrestricted animal movement in the country. The involvement of vaccine virus in disease outbreaks was ruled out as the field strains (excluding the one in lineage 3) were phylogenetically distinct from it.

Antigenic variation in Foot and Mouth Disease Virus type Asia 1 isolates circulated during 1993-95 in India

The antigenic variation in Foot and Mouth Disease Virus (FMDV) is very high. The effective strategy to control the Foot and Mouth Disease (FMD) in India which is a habitat of four serotypes O, A, C and Asia 1, is by regular vaccination, using the vaccine strain most suitable for the local situation. India is an endemic country with the disease being widely distributed. Selection of vaccine strain should therefore need the information on the circulating viruses. Asia 1 causes the second largest number of disease outbreaks in India. As there is no information available with respect to the extent of antigenic variation in FMDV type Asia 1, we have studied FMDV isolates from vaccinated and unvaccinated animals from different parts of the country and compared their relationship with Asia 1 vaccine virus. The immunogenic, hypervariable region of viral protein 1 (VP1) gene was amplified by RT-PCR and sequenced. Analysis of sequence data showed that the viruses from two field outbreaks of Southern India were closely related to each other when compared to the isolate from the North and all the three isolates are away from the vaccine virus.

Neutralization antigenic sites on type Asia-1 foot-and-mouth disease virus defined by monoclonal antibody-resistant variants

Seven neutralizing monoclonal antibodies (nMAbs) produced against serotype Asia-1 foot-and-mouth disease virus (FMDV) were used to select neutralization-resistant variants. Seven single and six multiple antibody-resistant variants were selected to identify neutralization antigenic sites on FMDV Asia-1. The variants no longer reacted with nMAbs which were used to select them when tested by microneutralization test (MNT), radioimmunoassay (RIA) and agar gel immunodiffusion (AGID) assay. Based on the binding and neutralization patterns of the variants, the nMAbs could be divided into discrete groups indicating the presence of three independent antigenic sites with evidence for occurrence of possibly a fourth site on the virus surface. Site 1 was present on 140S, 12Sps and VP1 and thus was conformation-independent. Sites 2 and 3 were restricted to the intact virion (140S) and thus were more conformation-dependent. Site 4 present on 140S virions and 12S protein subunits was less conformation-dependent. The site 3 nMAbs neutralized the infectivity of all the ten different Asia-1 virus isolates tested indicating that this site is conserved in Asia-1 virus serotype. Both cross-neutralization of different Asia-1 viruses with the nMAbs and cross-inhibition assays between MAbs demonstrated that the nMAbs recognized at least six different epitopes on Asia-1 virus.

A recombinant, arginine-glycine-aspartic acid (RGD) motif from foot-and-mouth disease virus binds mammalian cells through vitronectin and, to a lower extent, fibronectin receptors

The cell-binding abilities of a recombinant, RGD-containing peptide from foot-and-mouth disease virus (FMDV) have been characterized in HeLa and BHK cells. This peptide represents the aa sequence of the solvent-exposed G-H loop of protein VP1 which is involved in cell recognition and infection. The efficiency of the viral motif in promoting cell attachment and spreading is comparable to that shown by fibronectin or vitronectin. Cell binding is inhibited by a monoclonal antibody directed against a viral, RGD-involving B-cell epitope and also by sera against vitronectin (alpha V beta 3/beta 5) and fibronectin (alpha 5 beta 1) receptors. In addition, a synthetic RGD peptide, which is a ligand for both integrins, prevents the cell binding mediated by the FMDV domain. These data demonstrate that the FMDV RGD motif is a potent ligand for cell-receptor integrins and sufficient to promote cell attachment to susceptible cells mainly through the vitronectin receptor.

Serological and molecular analysis of serotype O foot-and-mouth disease virus isolated from disease outbreaks in India during 1987-91

Foot-and-mouth disease virus (FMDV) type O outbreaks have been reported frequently in vaccinated cattle in India. Twenty-five field isolates, recovered from outbreaks in vaccinated and unvaccinated cattle between 1987 and 1991, were analyzed in relation to the vaccine strain (R2/75) by complement fixation, serum neutralization and partial nucleotide sequencing of the VP1 gene. These sequences were compared with the viral sequences in GenEMBL database. Although the Indian type O viruses were close to the European type O1 viruses, they constituted a separate group of type O FMDVs. One of the field viruses, isolated from an outbreak in vaccinated cattle and designated as BAK/90, showed significant serological and nucleotide sequence variations from the vaccine strain. Phylogenetic analysis showed that the BAK/90 and R2/75 viruses belong to separate subgroups. The other isolates were found to be serologically related to both the BAK/90 and the vaccine strain. The BAK/90 strain gave broader antigenic coverage, showed better immunogenicity, and yielded larger amounts of 146S particles in suspension cultures as compared with R2/75. Taken together, these results favour inclusion of the BAK/90 strain in the vaccine to provide adequate protection against the field variants of type O FMDV currently circulating in India.

Response of foot-and-mouth disease virus C3 Resende to immunological pressure exerted in vitro by antiviral polyclonal sera

The foot-and-mouth disease virus (FMDV) shows a remarkable antigenic variability. Like other RNA viruses, FMDV has a high mutation rate and it has been proposed that selection exerted by antibodies of the host could play a major role in its evolution. In this work, antiserum-resistant variants of FMDV (Nr variants) were selected upon 25 serial passages of a cloned C3 Resende strain on secondary monolayers of fetal bovine kidney (FBK-2) cells in the presence of subneutralizing levels of antiviral polyclonal sera (APS). After serial passage under immune selective pressure, the five Nr variant populations selected from five independent serial passages--their controls remaining unmodified--acquired the following characteristics: (i) increased resistance to neutralization by APS; (ii) five different antigenic specificities detected by enzyme-linked and neutralization assays using monoclonal antibodies; (iii) the same modification (residue 146, S to L) at the major antigenic site of VP1 (G-H loop, the 135-160 region); and (iv) specific changes for each Nr population outside the major antigenic site of VP1 at residues 46, 48 and 49 of the 40-60 region of VP1 (B-C loop). These results extend our previous work on selection of Nr variants using polyclonal sera, and add new information with regard to antigenic variation, mainly concerning the involvement of the 40-60 region of VP1 in the process of immune selection.

Large deletions in the 5'-untranslated region of foot-and-mouth disease virus of serotype C

Nucleotide sequences of the 5'-untranslated region (5'-UTR), at the 3'-side of the poly C tract, have been compared for 21 isolates of foot-and-mouth disease virus (FMDV) of serotype C from Europe, South America and The Philippines. A deletion of 43 nucleotides is present in the European isolates as compared with most American isolates. A larger deletion of 86 nucleotides is present in some viruses from South America and The Philippines. These deletions include the loss of one or two pseudoknot structures predicted in this region of the 5'-UTR. In addition, multiple point mutations have allowed the derivation of a phylogenetic tree which defines a grouping of isolates very similar to that derived from the capsid gene sequences of the same viruses. The study provides evidence that deletion (or addition) events must be very frequent during evolution of FMDV type C, since viruses which are phylogenetically very closely related (they belong to the same tree branch) may differ in the presence or absence of these deletions. Implications for FMDV evolution are discussed.

Specific inhibition of aphthovirus infection by RNAs transcribed from both the 5' and the 3' noncoding regions

RNA molecules containing the 3' terminal region of foot-and-mouth disease virus (FMDV) RNA in both antisense and sense orientations were able to inhibit viral FMDV translation and infective particle formation in BHK-21 cells following comicroinjection or cotransfection with infectious viral RNA. Antisense, but not sense, transcripts from the 5' noncoding region including the proximal element of the internal ribosome entry site and the two functional initiation AUGs were also inhibitory, both in in vitro translation and in vivo in comicroinjected or cotransfected BHK-21 cells. This effect was not observed with nonrelated RNA transcripts from lambda phage. The inhibitions found were permanent, sequence specific, and dose dependent; an inverse correlation between the length of the transcript and the extent of the antiviral effect was seen. In all cases, the extent of inhibition increased when viral RNAs and transcripts were allowed to reanneal before transfection, concomitant with a decrease in the doses required. The antiviral effect was specific for FMDV, since transcripts failed to inhibit infective particle formation by other picornavirus, such as encephalomyocarditis virus. These results indicate that the ability of RNA transcripts to inhibit viral multiplication depends on their efficient hybridization with target regions on the viral genome. Furthermore, cells transfected with the 5'1as transcript, which is complementary to the 5' noncoding region, showed a significant reduction of plaque-forming ability during the course of a natural infection. RNA 5'1as was able to inhibit FMDV RNA translation in vitro, suggesting that the inhibitions observed are mediated by a blockage of the viral translation initiation. Conversely, hybridization of short sequences of both sense and antisense transcripts from the 3' end induces distortion of predicted highly ordered structural motifs, which could be required for the synthesis of negative-stranded viral RNA, and correlates with inhibition of viral propagation.

Genetic variation of foot-and-mouth disease virus during persistent infection in cattle

Genetic variation of foot-and-mouth disease virus O1 Campos has been analyzed in consecutive isolates recovered over a one- or two-year period from four cattle with experimental persistent infection. Comparisons of RNase T1 two-dimensional maps and nucleotide sequences of the VP1-coding region revealed a continual, although irregular, increase in the fixation of mutations as the infection progressed. Most changes were not conserved in consecutive isolates. These results, together with the substantial rates of genomic variation observed between some pairs of strains recovered at close time periods, suggested the coexistence of heterogeneous populations in which variants evolve independently from each other, and predominate at irregular time intervals. Furthermore, non-related patterns of variation were observed in the four animals. Similarly, genetic diversity of representative strains from major serotype O outbreaks in endemic disease regions of southeastern Brazil and central eastern Argentina which occurred between 1958 and 1983, suggested that outbreak strains are also likely to represent fluctuations of heterogeneous populations which evolve independently from each other. The possible role of persistent infections in the introduction of variant populations in the field is discussed.

A comparative study of serological and biochemical methods for strain differentiation of foot-and-mouth disease type A viruses

Three serological and three biochemical methods were used to compare five field isolates of foot-and-mouth disease virus (FMDV) from Western India with nine reference vaccine strains and five field isolates from other countries. The serological tests (liquid-phase ELISA and virus neutralization) were able to distinguish between the three reference vaccine strains examined, but the five Indian field isolates reacted poorly with antisera produced against these vaccine strains. Analysis of monoclonal antibody (mAb) data was difficult to interpret although clearly the field isolate A/IND/5/87 reacted to a lesser extent with one of the mAb panels (A10/Holland/42) than the other four Indian isolates. The A22/Iraq/24/64 mAbs did not react with any of the Indian field isolates and only significantly with one of the reference vaccine strains, A/IND/57/79. Polyacrylamide gel electrophoresis distinguished the reference vaccine strains from each other and from the field isolates. Additionally, one of the Indian isolates (A/IND/5/87) could be differentiated from the other four. Electrofocusing showed similarities between the reference vaccine strain A22/Iraq/24/64 and three of the Indian field isolates (A/IND/1/87, A/IND/2/87 and A/IND/3/87), however, A/IND/4/87 and A/IND/5/87 were distinct. Nucleotide sequencing showed that the isolates A/IND/1/87, A/IND/2/87 and A/IND/3/87 were very closely related to each other and related to A/IND/4/87, however, A/IND/5/87 was different.

Antigenic heterogeneity of a foot-and-mouth disease virus serotype in the field is mediated by very limited sequence variation at several antigenic sites

Antigenic variation in a major discontinuous site (site D) of foot-and-mouth disease virus (FMDV) of serotype C has been evaluated with neutralizing monoclonal antibodies. Isolates representing the major evolutionary sublines previously defined for serotype C were compared. Extensive variation, comparable to that of continuous epitopes within the hypervariable immunodominant site A (the VP1 G-H loop), was found. The amino acid sequences of the complete capsids of three antigenically highly divergent FMDVs (C1 Haute Loire-Fr/69, C5 Argentina/69, and C3 Argentina/85) have been determined and compared with the corresponding sequences previously determined for seven additional type C viruses. Differences in antigenicity are due to a very limited number of substitutions of surface amino acids accessible to antibodies and located within antigenic sites previously identified on FMDV. A significant number of residues at these positions were also replaced in monoclonal antibody escape mutants. Depending on the variants compared, replacements within site A or at site D, or at both sites, contributed significantly to their antigenic differences. Examples of divergence mediated by a few amino acid replacements were found among FMDVs of Europe and South America. The results suggest that within a serotype of FMDV, antigenically highly divergent viruses can arise in the field by very limited sequence variation at exposed key residues of each of several antigenic sites.

Genetic relationships between foot-and-mouth disease type Asia 1 viruses

The sequence of 165 nucleotides at the 3' end of the 1D (VP1) gene of foot-and-mouth disease (FMD) virus was determined for 44 type Asia 1 strains isolated from throughout Asia between 1954-92. Analysis of the relationships between the virus genomes showed epidemiological links not previously evident. The possible origin of the only outbreak of FMD Asia 1 to have occurred in Europe, in Greece in 1984, was identified because the nucleotide sequence of this virus was closely-related to the sequences of those present in the Middle East between 1983-5. Variation in the region sequenced was not as great as that seen in the other FMDV serotypes and all viruses shared greater than 85% nucleotide identity. Thus all the virus isolates examined were considered to belong to a single genotype. A database of Asia 1 virus sequences has been established which will facilitate the rapid analysis of new outbreaks strains.

Genetic relationships between southern African SAT-2 isolates of foot-and-mouth-disease virus

Sequencing of part of the 1D gene of foot-and-mouth disease virus was used to determine the relationships between SAT-2 viruses isolated from outbreaks which occurred in cattle in Zimbabwe and Namibia and in impala in South Africa between 1979 and 1989. The results demonstrated that the outbreaks in different countries were unrelated. Surprisingly close relationships were shown between all SAT-2 viruses isolated from cattle in Zimbabwe since 1983 but the two major epizootics which occurred in 1989 were caused by viruses which were clearly different. Conversely, two apparently unrelated outbreaks in impala in South Africa were caused by viruses which could not be distinguished.

Genetic mechanisms of antigenic variation in infectious bursal disease virus: analysis of a naturally occurring variant virus

The major immunogenic protein VP2 from a pathogenic field isolate (variant A virus) of infectious bursal disease virus (IBDV) was cloned and sequenced to examine antigenic variations. The VP2 open reading frame consists of 1509 nucleotides and codes for a 503 amino acid protein. Overall, the VP2 amino acid sequence of the variant A virus shares 98.6% identity with VP2 genes from other published IBDV strains. However, within the central region of VP2 (amino acids 222-334) lies a highly divergent area that we have termed the variable domain. Relative to five other IBDV isolates, a total of six amino acid changes occur within the variable domain of the variant A virus. At positions 284-288, a substitution of isoleucine to threonine, a decrease in the number of Chou and Fasman beta turns, and a switch from a hydrophilic to a hydrophobic region are found only in the variant A virus. Together these changes predict a decrease in antigenicity as determined by calculation of potential antigenic sites. This suggests that only minor changes within VP2 contributed to the emergence of a variant virus that can cause disease in immunized birds.

Evolution of the capsid protein genes of foot-and-mouth disease virus: antigenic variation without accumulation of amino acid substitutions over six decades

The genetic diversification of foot-and-mouth disease virus (FMDV) of serotype C over a 6-decade period was studied by comparing nucleotide sequences of the capsid protein-coding regions of viruses isolated in Europe, South America, and The Philippines. Phylogenetic trees were derived for VP1 and P1 (VP1, VP2, VP3, and VP4) RNAs by using the least-squares method. Confidence intervals of the derived phylogeny (significance levels of nodes and standard deviations of branch lengths) were placed by application of the bootstrap resampling method. These procedures defined six highly significant major evolutionary lineages and a complex network of sublines for the isolates from South America. In contrast, European isolates are considerably more homogeneous, probably because of the vaccine origin of several of them. The phylogenetic analysis suggests that FMDV CGC Ger/26 (one of the earliest FMDV isolates available) belonged to an evolutionary line which is now apparently extinct. Attempts to date the origin (ancestor) of the FMDVs analyzed met with considerable uncertainty, mainly owing to the stasis noted in European viruses. Remarkably, the evolution of the capsid genes of FMDV was essentially associated with linear accumulation of silent mutations but continuous accumulation of amino acid substitutions was not observed. Thus, the antigenic variation attained by FMDV type C over 6 decades was due to fluctuations among limited combinations of amino acid residues without net accumulation of amino acid replacements over time.

Comparison of capsid protein VP1 of the viruses used for the production and challenge of foot-and-mouth disease vaccines in Spain

A significant frequency of amino acid substitutions, that affect important antigenic sites on capsid protein VP1, has been found among viral preparations used for the production and for challenge, in protection assays, of foot-and-mouth disease (FMD) vaccines. The amino acid substitutions present in one of the viruses studied abolished its reactivity with two neutralizing monoclonal antibodies that recognized different epitopes on VP1. Thus, the results obtained illustrate the high potential for antigenic variation introduced by the multiple cycles of growth usually undergone by the strains used for the production and challenge of FMD vaccines.

Two mechanisms of antigenic diversification of foot-and-mouth disease virus

The amino acid replacements that underlay the diversification of the main antigenic site A (VP1 residues 138 to 150) of foot-and-mouth disease virus (FMDV) of serotype C have been identified. Sixteen new VP1 sequences of isolates from 1926 until 1989 belonging to subtypes C1, C2, C3, C4, C5, and unclassified are reported. The reactivities in enzyme-linked immunoelectrotransfer blot assays of capsid protein VP1 with a panel of neutralizing monoclonal antibodies that recognize sites A or C (the VP1 carboxy-terminus) have been correlated with the amino acid sequence at the relevant epitopes. The analyses involving the immunodominant site A reveal two mechanisms of antigenic change. One is a gradual increase in antigenic distance brought about by accumulation of amino acid replacements at two hypervariable segments within site A. A second mechanism consists of an abrupt antigenic change manifested by loss of many epitopes, caused by one replacement at a critical position (particularly Ala (145)----Val or His (146)----Gln). The identification of the amino acid substitutions responsible for such large antigenic changes provides new information for the design of synthetic anti-FMD vaccines. However, the screening of isolates from six decades suggests that the virus, even within the confines of a single serotype, has exploited a minimum of its potential for antigenic variation.

Molecular cloning and expression of the VP1 gene of foot-and-mouth disease virus C1 in E. coli: effect on bacterial cell viability

The VP1 gene of foot-and-mouth disease virus (serotype C1) has been cloned in Escherichia coli Clts cells, under the control of the bacteriophage lambda pL promoter. The expressed VP1 protein was complete and non-fused, and its molecular weight was indistinguishable from that of the VP1 obtained from virions. Cells harbouring the recombinant vectors exhibited symptoms of plasmid instability and toxicity and died in a few weeks even when never exposed to inducing conditions. A new plasmid clone in which a segment of the VP1 gene was fused with contiguous genes of the viral genome was very stable. The expressed partial VP1 protein contains the two major immunogenic domains of the virion. This system can be used as a tool to design an immunogenic VP1, and to explore possible synthetic vaccines against foot-and-mouth disease.

Detection and localization of single-base sequence differences in foot-and-mouth disease virus genomes by the RNase mismatch cleavage method

The RNase mismatch cleavage method was examined for its efficiency of indicating single-base sequence differences in the capsid protein-coding regions of different foot-and-mouth disease virus subtype O1 strains. The method was found suitable for indicating such differences. RNase A as well as RNase T1 contributed to substrate conversion. Examples for the cleavage of eleven different single-base mismatches in RNA double-strands are now known. All virus genomes found to differ from each other exhibited three or more non-neighboured single-base sequence differences. Other genomes found to be indistinguishable by this method were those of a recent field isolate adapted to cell culture, and those of a vaccine production strain; its progeny was transmitted to pig and cow and then analyzed. The results suggest that host change does not necessarily select for antigenic variant virus, and that virus submitted to some kind of selection pressure is changed at more than one position.

Fixation of mutations at the VP1 gene of foot-and-mouth disease virus. Can quasispecies define a transient molecular clock?

The number of nucleotide (nt) substitutions found in the VP1 gene (encoding viral capsid protein) between any two of 16 closely related isolates of foot-and-mouth disease virus (FMDV) has been quantified as a function of the time interval between isolations [Villaverde et al., J. Mol. Biol. 204 (1988) 771-776]. One of them (isolate C-S12) includes some replacements found in isolates that preceded it and other replacements found in later isolates. The study has revealed alternating periods of rapid evolution and of relative genetic stability of VP1. During a defined period of acute disease, the rate of fixation of replacements at the VP1 coding segment was 6 x 10(-3) substitutions per nt per year. Only small differences in the rate of evolution were observed between subsegments within the VP1 gene. The observation of a relatively constant rate of evolution during a disease episode was unexpected. We propose that such constancy may be a consequence of random sampling of mutants from the FMDV quasispecies, followed by their amplification in susceptible hosts (to generate a new quasispecies). Successive sampling and amplification events may result in a steady accumulation of mutations.

Fitness alteration of foot-and-mouth disease virus mutants: measurement of adaptability of viral quasispecies

We document the rapid alteration of fitness of two foot-and-mouth disease virus (FMDV) mutants resistant to a neutralizing monoclonal antibody. Both mutants showed a selective disadvantage in BHK-21 cells when passaged in competition with their parental FMDV. Upon repeated replication of the mutants alone, they acquired a selective advantage over the parental FMDV and fixed additional genomic substitutions without reversion of the monoclonal antibody-resistant phenotype. Thus, variants that were previously kept at low frequency in the mutant spectrum of a viral quasispecies rapidly became the master sequence of a new genomic distribution and dominated the viral population.

Antigenic stability of foot-and-mouth disease virus variants on serial passage in cell culture

Two neutralizing monoclonal antibody (MAb)-resistant variants selected from an isolate of foot-and-mouth disease virus (FMDV) type A5 were repeatedly passaged in cell culture and monitored for susceptibility to neutralization by the selecting MAb. A variant isolated with a MAb to a conformational epitope (1-OG2) lost resistance in 20 passages, while a variant isolated with a MAb to a linear epitope (1-HA6) persisted for 30 passages. In both cases, the virus population emerging after passage was antigenically and genetically indistinguishable from the original wild-type parental virus (FMDV A5 Spain-86). Coinfection assays with the wild type and each variant, and between the variants, showed rapid conversion to a homogeneous population. Wild-type virus prevailed over the variants and for coinfection between the variants, the linear epitope variant 1-HA6. While both variants arose from a single nucleotide substitution and reversion to wild type occurred for each, it appears that the variant based on the continuous epitope (1-HA6) was more stable. We discuss the implications of these results for the antigenic diversity of FMDV and its relationship to virus evolution.

Identification of neutralizing antigenic sites on VP1 and VP2 of type A5 foot-and-mouth disease virus, defined by neutralization-resistant variants

Five neutralizing monoclonal antibodies (nMAbs) obtained against type A5 Spain-86 foot-and-mouth disease virus were used to generate a series of neutralization-resistant variants. In vitro and in vivo assays showed that the variants were fully refractory to neutralization by the selecting nMAb. On the basis of cross-neutralization and binding assays, two neutralizing antigenic sites have been located on the virus surface; one, located near the C-terminus of VP1, displayed a linear epitope, and the second, located on VP2, displayed two conformational epitopes. Nucleotide sequencing of RNA of the parental and variant capsid protein-coding region P1 has placed the amino acid changes at position 198 of VP1 for the first site and at positions 72 and 79 of VP2 for the related epitopes in the second site. The relative importance of these two sites in the biological properties of foot-and-mouth disease virus is discussed.

The capsid protein-encoding sequence of foot-and-mouth disease virus O2Brescia

Nucleotide sequences encoding the four capsid proteins of foot-and-mouth disease virus subtype O2Brescia/1947 have been determined. These and the deduced amino acid sequences were compared with those of a subtype O1 virus strain. The nucleotide sequences differed at 259 positions, causing only 35 amino acid changes. VP4 and VP2 differed by 2.4 and 1.8%, whereas VP1, known as major viral antigen, and VP3 differed by 8% and 5.5%, respectively. The differences occur mainly in protein domains not involved in the formation of alpha-helices and beta-sheets, suggesting that the surfaces of both viruses are more variable than their scaffolds. The O2Brescia sequence has been submitted to the GenBank data base and has the accession number M 55287.

Genetic and phenotypic variability during replication of foot-and-mouth disease virus in swine

A plaque-purified preparation of foot-and-mouth disease virus (FMDV) of serotype C1 (C-S8c1-1), grown in cell culture, was used to infect nonimmunized pigs. No variant genomes were detected in the average populations of 50 viruses isolated from infected animals by direct RNA sequencing of the carboxy-terminal half of the VP1 gene. However, a mutant with altered phenotypic properties was present in low proportion in an infected animal. The frequency of mutants resistant to neutralization by SD6 monoclonal antibody (MAb) [SD6 epitope MAb-resistant mutants (MARMs)], directly estimated in virus from lesions of infected animals (without passage in cell culture), depended on the procedure used for its determination and ranged from 2.9 x 10(-6) (when the virus was incubated with the MAb prior to plating) to 2.6 x 10(-5) (when incubation with MAb was avoided and the MAb was maintained in the agar overlay of the titration assay). Such a difference was not found for C-S8c1-1, which consistently showed frequencies of about 4 x 10(-5). In addition, the repertoire of amino acid substitutions was similar among SD6 epitope MARMs isolated directly both from vesicles of infected animals and from C-S8c1-1. Thus, in spite of the conservation of the average sequence in the region of VP1 RNA analyzed, antigenic heterogeneity has been found in viral populations of FMDV upon replication in nonimmunized swine.

Unique amino acid substitutions in the capsid proteins of foot-and-mouth disease virus from a persistent infection in cell culture

Maintenance of a persistent foot-and-mouth disease virus (FMDV) infection in BHK-21 cells involves a coevolution of cells and virus (J. C. de la Torre, E. Martínez-Salas, J. Díez, A. Villaverde, F. Gebauer, E. Rocha, M. Dávila, and E. Domingo, J. Virol. 62:2050-2058, 1988). The resident FMDV undergoes a number of phenotypic changes, including a gradual decrease in virion stability. Here we report the nucleotide sequence of the P1 genomic segment of the virus rescued after 100 passages of the carrier cells (R100). Only 5 of 15 mutations in P1 of R100 were silent. Nine amino acid substitutions were fixed on the viral capsid during persistence, and three of the variant amino acids are not represented in the corresponding position of any picornavirus sequenced to date. Cysteine at position 7 of VP3, that provides disulfide bridges at the FMDV fivefold axis, was substituted by valine, as determined by RNA, cDNA, and protein sequencing. The modified virus shows high buoyant density in cesium chloride and depicts the same sensitivity to photoinactivation by intercalating dyes as the parental FMDV C-S8c1. Amino acid substitutions fixed in VP1 resulted in altered antigenicity, as revealed by reactivity with monoclonal antibodies. In addition to defining at the molecular level the alterations the FMDV capsid underwent during persistence, the results show that positions which are highly invariant in an RNA genome may change when viral replication occurs in a modified environment.

Comparison of vaccine strains and the virus causing the 1986 foot-and-mouth disease outbreak in Spain: epizootiological analysis

RNAs of the most recent foot-and-mouth disease virus isolated in Spain (A5Sp86) during the 1986 outbreak, and of the three vaccine strains in use at that time in that country, have been compared. Although these viruses are serologically indistinguishable, differences have been found among them by T1 fingerprinting. This genetic heterogeneity affects the immunogenic VP1 gene, with amino acid changes located at the carboxyterminal end of the molecule. VP1-coding sequences obtained have been compared with those previously reported for European A5 FMDVs and it has been possible to trace their phylogenetic origin. The most parsimonious evolutionary tree obtained shows that the viruses analyzed are closely related to those previously isolated in 1983 in Spain, Portugal and Morocco. In spite of the VP1 sequence homology shown by this group of viruses, the genetic distances among field isolates and vaccine strains are significantly shorter than the distances found among field isolates. Thus, a significant relationship among virus recovered from recent outbreaks and the vaccine strains in use at that time in Spain, has been obtained.

Implications of a quasispecies genome structure: effect of frequent, naturally occurring amino acid substitutions on the antigenicity of foot-and-mouth disease virus

We provide evidence that the quasispecies nature (extreme genetic heterogeneity) of foot-and-mouth disease virus is relevant to the virus evading an immune response. A monoclonal antibody neutralizing the viral infectivity (clone SD6) recognizes an epitope located around a highly conserved sequence (amino acid sequence Arg-Gly-Asp-Leu-Ala at positions 141-145) in the capsid protein VP1 of foot-and-mouth disease virus of serotype C1. The amino acid substitutions Ala-138----Thr and Leu-147----Ile (or ----Val) reduced 100-fold the binding titer of monoclonal antibody SD6 to virions or to VP1. The effect of those substitutions was quantitatively reproduced with synthetic peptides representing the relevant sequences. This provides evidence that the two chemically conservative amino acids replacements--and not other substitutions present in the virus quasispecies--are responsible for the modified interaction with neutralizing monoclonal antibody SD6. The three substitutions were fixed in the viral capsid during one occurrence of foot-and-mouth disease and, furthermore, they are of a type found frequently among independent foot-and-mouth disease virus isolates. The results implicate the extreme heterogeneity of foot-and-mouth disease virus as an important element of viral pathogenesis.

Antigenic comparison of different foot-and-mouth disease virus types using monoclonal antibodies defining multiple neutralizing epitopes on FMDV A5 subtypes

Thirteen monoclonal antibodies (MAbs) were elicited with A5 Spain-86 virus, the cause of the most recent foot-and-mouth disease virus (FMDV) outbreak in Spain. The MAbs were tested for ability to bind 140S virions and 12S protein subunits by liquid-phase radioimmunoassay (RIA), and to bind VP1 capsid protein by Western immunoblot assay. One of the thirteen MAb was virion (140S) specific, seven recognized 140S and 12S subunits, one bound to 140S, 12S and VP1 and four were 12S specific. These MAbs presented varying binding patterns when tested against different FMDV subtypes and serotypes, indicating the presence of conserved and non-conserved epitopes among FMDV serotypes and subtypes. Neutralization assays, in vivo and in vitro, showed that none of the 140S specific MAbs or 12S specific MAbs were neutralizing, but notably several of the 12S specific MAbs bound to all the different FMDV serotypes and can be useful diagnostic reagent for the detection of FMDV. The remaining MAbs showed varying behavior with the different types tested: not all types to which the MAbs bound were neutralized, demonstrating that the presence of an epitope and subsequent neutralization of infectivity are not necessarily correlated. Five type A12 neutralizing MAbs, previously characterized, have been used in this work. Four bound to A5 Spain-86 virus, but only one neutralized viral infectivity. On the basis of differential reactivity and neutralization among various FMDV subtypes and serotypes, and cross-inhibition binding assays between these MAbs, seven neutralization related epitopes have been defined on A5 Spain-86 virus.

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.

RNA virus evolution and the control of viral disease

RNA viruses and other RNA genetic elements must be viewed as organized distributions of sequences termed quasi-species. This means that the viral genome is statistically defined but individually indeterminate. Stable distributions may be maintained for extremely long time periods under conditions of population equilibrium. Perturbation of equilibrium results in rapid distribution shifts. This genomic organization has many implications for viral pathogenesis and disease control. This review has emphasized the problem of selection of viral mutants resistant to antiviral drugs and the current difficulties encountered in the design of novel synthetic vaccines. Possible strategies for antiviral therapy and vaccine development have been discussed.

3D gene of foot-and-mouth disease virus. Conservation by convergence of average sequences

The nucleotide sequence of the 3D (polymerase) gene of eight epidemiologically related isolates of foot-and-mouth disease virus of serotype C1 is reported. The genetic heterogeneity of 3D RNA is compared with that of the VP1-coding RNA of the same viruses. Regression lines of substitutions per nucleotide that distinguish any pair of viruses as a function of the time interval between the corresponding isolations show: (1) the slope (substitutions/nucleotide per month) is 2.1 times larger for the VP1 RNA than for the 3D RNA region; (2) the intercept with the ordinate (substitutions/nucleotide) for VP1 RNA is indistinguishable from that for 3D RNA. Thus, the average heterogeneity of the VP1-coding region is very similar to that of the 3D-coding region only among co-circulating viruses. Nine mutations and points of heterogeneity occurred within nucleotide residues 883 to 1026, which encode an amino acid segment, extremely conserved among many different RNA viruses. The results suggest that, rather than due to inherently lower mutability, the conservation of 3D genes is caused by a limitation in the fixation of substitutions in viable genomes.

Extensive antigenic heterogeneity of foot-and-mouth disease virus of serotype C

The antigenic behavior of 46 field isolates of foot-and-mouth disease virus (FMDV) of serotype C has been studied with a panel of 24 monoclonal antibodies (MAbs) prepared against FMDV C1 or FMDV C3 Indaial. Reactivities were assayed by immunodot, immunoelectrotransfer blot, and neutralization of infectivity. The epitopes recognized by the 10 nonneutralizing MAbs are conserved in all isolates analyzed. In contrast, extreme antigenic heterogeneity is documented with regard to reactivity with 14 MAbs that, on this basis, define at least 12 epitopes involved in neutralization of FMDV of serotype C. The 31 isolates from South America were divided into 17 distinct antigenic groups and the 15 isolates from Europe into 7 groups. Lack of correspondence between antigenic composition and the origin--date and place of isolation--of the viruses was noted in several instances. Antigenic heterogeneity is shown among epidemiologically closely related FMDVs. In most--but not all--cases tested, a good correlation was found between binding of a neutralizing MAb to virions and its ability to neutralize infectivity. It is concluded that variation of epitopes involved in neutralization of FMDV is extensive among subtypes of serotype C and also among individual isolates of one subtype.