Direct evaluation of the immunodominance of a major antigenic site of foot-and-mouth disease virus in a natural host

Evaluation of immunogenicity and cross-reactive responses of vaccines prepared from two chimeric serotype O foot-and-mouth disease viruses in pigs and cattle

Foot-and-mouth disease (FMD) remains a very serious barrier to agricultural development and the international trade of animals and animal products. Recently, serotype O has been the most prevalent FMDV serotype in China, and it has evolved into four different lineages: O/SEA/Mya-98, O/ME-SA/PanAsia, O/ME-SA/Ind-2001 and O/Cathay. PanAsia-2, belonging to the O/ME-SA topotype, is prevalent in neighbouring countries and poses the risk of cross-border spread in China. This study aimed to develop a promising vaccine candidate strain that can not only provide the best protection against all serotype O FMDVs circulating in China but also be used as an emergency vaccine for the prevention and control of transboundary incursion of PanAsia-2. Here, two chimeric FMDVs (rHN/TURVP1 and rHN/NXVP1) featuring substitution of VP1 genes of the O/TUR/5/2009 vaccine strain (PanAsia-2) and O/NXYCh/CHA/2018 epidemic strain (Mya98) were constructed and evaluated. The biological properties of the two chimeric FMDVs were similar to those of the wild-type (wt) virus despite slight differences in plaque sizes observed in BHK-21 cells. The structural protein-specific antibody titres induced by the rHN/TURVP1 and wt virus vaccines in pigs and cows were higher than those induced by the rHN/NXVP1 vaccine at 28–56 dpv. The vaccines prepared from the two chimeric viruses and wt virus all induced the production of protective cross-neutralizing antibodies against the viruses of the Mya-98, PanAsia and Ind-2001 lineages in pigs and cattle at 28 dpv; however, only the animals vaccinated with the rHN/TURVP1 vaccine produced a protective immune response to the field isolate of the Cathay lineage at 28 dpv, whereas the animals receiving the wt virus and the rHN/NXVP1 vaccines did not, although the wt virus and O/GXCX/CHA/2018 both belong to the Cathay topotype. This study will provide very useful information to help develop a potential vaccine candidate for the prevention and control of serotype O FMD in China.

Diversity of SAT2 foot-and-mouth disease virus in Sudan: implication for diagnosis and control

Like other East African countries, Sudan experienced circulation of more than one topotype of SAT2 foot-and-mouth disease virus (FMDV). In Sudan, topotype XIII of SAT2 virus was recorded in 1977 and 2008 and topotype VII in 2007, 2010, 2013, 2014 and 2017. This work evaluated the impact of such diversity on diagnosis and control. After one or three doses of a vaccine derived from a Sudanese SAT2 virus of topotype VII originated in 2010, heterologous neutralizing antibody titres with Sudanese SAT2 viruses in 2008 were ≤ 1.2 log 10, not consistent with likely protection. Simultaneously, homologous titres were 1.65 (after one dose) or 1.95 and 2.55 log10 (after 3 doses). When r1 values between the vaccine virus and the SAT2 viruses isolated in 2008, whilst topotype XIII was circulating, were derived, values (≈ 0.00) suggested similarly poor antigenic relationship and unlikely cross protection. Concurrently, SAT2 positive field sera from Sudan in 2016 were not unvaryingly identified by virus neutralization tests (VNT) employing SAT2 viruses from 2010 and 2008. Proportions of positive sera by SAT2 virus from 2010 were always higher than those by viruses from 2008; consistent with the more frequent and recent circulation of topotype VII prior to 2016. Proportions by SAT2 virus from 2010 were 0.68 (± 0.1) in one location (n = 72), 0.39 (± 0.1) in another one (n = 94) and 0.52 (± 0.1) in the whole test group (n = 166). Corresponding values by viruses of 2008 were 0.53 (± 0.1), 0.27 (± 0.1) and 0.38 (± 0.1). In the whole test group, differences were statistically significant (p = .02339). Like post-vaccination sera, field sera (natural immunity) showed no considerable cross neutralization between topotype VII and presumably XIII; almost 45% (43/96) of SAT2 positive field sera were positive to one topotype but not to the other. Experimental and surveillance findings emphasized the implication of SAT2 diversity in Sudan. It is concluded that it is difficult to control SAT2 infection in Sudan using a monovalent vaccine. Beside a prophylactic vaccine from topotype VII, stockpiling of antigens from topotype XIII and enhanced virological surveillance with rapid genotyping and matching studies are necessary approaches. When more frequent circulation of more than one SAT2 topotype occurs, retrospective diagnosis by serological surveys could be problematic or imprecise.

Vaccine matching and antigenic variability of foot-and-mouth disease virus serotypes O and A from 2018 Ethiopian isolates

Foot-and-mouth disease (FMD) is highly infectious, limits live animal trade, and affects ranchers owing to the loss of animal yield. The present study was designed to perform vaccine matching for field FMD virus isolates from clinically diseased cattle and assess the antigenic properties of the field isolates against the current vaccine strains used for vaccine production at the National Veterinary Institute, Ethiopia. Both sequencing and reverse transcription-polymerase chain reactions were used for distinguishing between the viral strains. To evaluate the serological relationship of the vaccine strain with these field isolates (r1 value), in vitro cross-neutralization was performed using ETH/6/2000 and ETH/38/2005 antisera. Infectious field FMD viral samples represented serotypes A and O. Sequence analysis showed that serotype A VP1/1D possessed amino acid variability at positions 28 and 42 to 48, 138, 141, 142, 148, 156, 173, and 197 compared with the ETH/6/2000 vaccine strain, whereas serotype O possessed amino acid variability at positions 45, 48, 138, 139, 140, 141, and 197 compared with the ETH/38/2005 vaccine strain. Based on the one-dimensional virus neutralization test, serotypes A and O demonstrated antigenic matching of up to 13/17 (76.47%) with the vaccine strain, except for the isolates ETH/40/2018, ETH/48/2018, ETH/55/2018, and ETH/61/2018, which had r-values less than 0.3. Therefore, the currently used vaccine strains ETH/38/2005 for serotype O and ETH/6/2000 for serotype A protected against all and most field viruses characterized as serotypes O and A, respectively, and amino acid residue variation was observed in different FMD virus B-C loops, G-H loops, and C-termini of VP1 at sites 1 and 3 in both serotypes.

Meta-analysis of Genetic Diversity of the VP1 Gene Among the Circulating O, A, and SAT2 Serotypes and Vaccine Strains of FMD Virus in Egypt

Introduction

Three strains of the FMD virus (A, O, and SAT 2) were recognised as causes of the FMD circulating in Egypt. The aims of this study were to trace the FMDV isolates from outbreaks in Egypt to understand their epidemiology and evolution and to understand the situation of the vaccine strains compared with the circulating serotypes.

Material and Methods

A meta-analysis was carried out by using the data available for FMD outbreaks in Egypt from GenBank and the World Reference Laboratory for Foot-and-Mouth Disease (WRLFMD); a comparison was done with both data sets for the three serotypes. MEGA-X was used for the evolution analysis, through constructions of phylogenetic trees for all sequences recorded in GenBank for each serotype in different Egyptian outbreaks in different years and also within the same year. Additionally, nucleotide substitution rate, molecular clock, and mean evolutionary rates were estimated for the three serotypes to understand and compare their evolution.

Results

Absence of some records of certain serotype outbreaks from the WRLFMD database was noted as were subsequent missing appropriate vaccine programmes. Genetic variation was recorded among the virus isolates within the same years and also the vaccine strain was associated with up to 26 amino acid substitutions. The evolution rate of the SAT2 strain was the highest of the circulating strains. SAT2 had high amino acid substitution per year at an important immunogenic site (130–170), serotype A had less, and serotype O the least.

Conclusion

The need for different strategies for vaccine serotype selection is indicated.

Evidence of combined effect of amino acid substitutions within G-H and B-C loops of VP1 conferring serological heterogeneity in foot-and-mouth disease virus serotype A

Foot-and-mouth disease virus (FMDV) serotype A exhibits a higher degree of genetic and antigenic diversity resulting in frequent vaccine failure due to serological mismatch between the vaccine and heterologous strains. Currently, knowledge on the molecular basis of antigenic relationships among the FMDVs is limited; nevertheless, intratype antigenic variation due to mutation(s) is widely considered as the main hurdle to appropriate FMD vaccine development. Here, we studied genetic and antigenic variations of four FMDV serotype A isolates, BAN/GA/Sa-197/2013 (BAN-197), BAN/CH/Sa-304/2016 (BAN-304), BAN/DH/Sa-307/2016 (BAN-307) and BAN/DH/Sa-310/2017 (BAN-310) circulating in Bangladesh during 2013-2017. Initially, antigenic relationships (r₁ -values) of the field isolates were evaluated by the two-dimensional microneutralization test (2D-MNT) using the hyperimmune antisera raised in cattle against the vaccine strain, BAN-304. Interesingly, the results showed protective serological cross-reactivity (r₁ -values > 0.4) between the vaccine strain and the field isolates, BAN-307 and BAN-310, except BAN-197 that substantially mismatched (r₁  = 0.129 ± 0.043) with the BAN-304. Although VP1-based phylogeny grouped all the isolates within the same sublineage C (a subgroup of VP3^Δ59 variant) under the lineage A/ASIA/G-VII, strikingly, computational analyses of the viral capsid proteins demonstrated significant deviation at the VP1 G-H loop of BAN-197 from the vaccine strain, while VP(2-4) of both isolates were structurally conserved. To bridge the gap of how the distortion of the G-H loop and consequent antigenic hetergeneity occurred in BAN-197, we performed in silico combinatorial substitutions of the VP1 mutant amino acids (aa) of BAN-197 with the respective residues in BAN-304. Remarkably, our analyses revealed that two substitutions of distantly located aa at B-C (T48I:threonine → isoleucine) and G-H (A143V:alanine → valine) loops, in combination, distorted the VP1 G-H loop. Overall, this work contributes to understanding the molecular basis of antigenic relationships operating in serotype A FMDVs and the selection of suitable vaccine strain(s) for effective prophylaxis of FMD based on VP1-based analyses.

Enhanced efficacy of a multi-epitope vaccine for type A and O foot‑and-mouth disease virus by fusing multiple epitopes with Mycobacterium tuberculosis heparin-binding hemagglutinin (HBHA), a novel TLR4 agonist

Foot-and-mouth disease (FMD) is an acute, severe, and highly contagious disease that affects cloven-hoofed animals and can lead to serious economic losses and social effects. Therefore, a safe and effective subunit vaccine is required to prevent and control FMD. Dendritic cells (DCs) are a type of professional antigen presenting cell (APC). Immature DCs are typically stimulated by various adjuvants via immune receptors (e.g., toll-like receptor 4 [TLR4]), which activate DCs to induce their maturation. TLR4 has been well-established to induce both innate and adaptive immune responses to various external microbial or internal damage-related molecular patterns. In this study, the multi-epitope immunogen, HAO, of foot-and-mouth disease virus (FMDV) serotypes A and O was fused with the recombinant protein, heparin-binding hemagglutinin (HBHA), a novel TLR4 agonist, to obtain a new recombinant fusion protein, termed HAO-HBHA. HAO-HBHA was found to be highly efficient at activating murine DCs by the TLR4 pathway, both in vitro and in vivo. HAO-HBHA elicited strong specific humoral immune responses detected with an ELISA and virus neutralizing antibody test (VNT). HAO-HBHA also elevated the cellular immune responses, as indicated by intracellular cytokine (e.g., IFN-γ, TNF-α, IL-4, IL-6, IL-10, and IL-12p70) expression in Th1 and Th2 cells. As a TLR4 agonist, HBHA has significant advantages for enhancing the immune efficacy of a FMDV serotype A and O bivalent multi-epitope vaccine. These findings provide a novel strategy for the development of a safe and effective multi-epitope vaccine candidate against FMDV and further extends the application of TLR agonist-based vaccine platforms.

Characterization of the FMDV-serotype-O isolates collected during 1962 and 1997 discloses new topotypes, CEY-1 and WCSA-1, and six new lineages

The genetic diversity of the FMD viruses collected from the outbreaks during the second half of the 20th Century in Sri Lanka was assessed in the present study. We sequenced the VP1 genomic region of the samples collected during FMDV epidemics caused by serotype O in Sri Lanka during 1962 and 1997. For comparison, we sequenced the VP1 of the related viral isolates collected from other Asian countries. We analyzed the VP1 sequences of the viral strains using the UPGMA method with uncorrected pairwise distances. Nucleotide divergence (ND) thresholds of 15%-20% and 5%-<15% were used to differentiate topotypes and lineages, respectively. We calibrated the divergence times and lineage-specific substitution rates using Bayesian-skyline models. Based on the ND estimations and phylogenetic relationships, we identified and named two new topotypes [CEYLON 1 (CEY-1) and WEST, CENTRAL AND SOUTH ASIA 1 (WCSA-1)] and six new lineages (Syr-62, Srl-77, Tur-69, May-78, Tai-87 and Bur-77) of serotype O. We believe that the novel topotypes and lineages named may have disappeared although they have similar substitution rates for epizootic outbreaks. Because the amino acid selection analysis revealed that the two topotypes and six lineages identified were under purifying selection during the outbreaks.

Artificially designed hepatitis B virus core particles composed of multiple epitopes of type A and O foot-and-mouth disease virus as a bivalent vaccine candidate

Recently, many countries, including China, have experienced a series of type A and O foot-and-mouth disease virus (FMDV) epidemics, causing serious economic losses. Although concerns about the safety of inactivated FMD vaccines have been raised, the development of a safe and effective subunit vaccine is necessary. We constructed two chimeric virus-like particles (VLPs; rHBc/AO and rHBc/AOT VLPs) displaying tandem repeats of B cell epitopes (VP1 residue 134-161 and 200-213) derived from type A and O FMDV and one T cell epitope (3 A residue 21-35) using the truncated hepatitis B virus core (HBc) carrier. Our results indicate that the chimeric HBc can self-assemble into VLPs with these FMDV epitopes displayed on the surface. Immunization with the chimeric VLPs induced specific IgG and neutralization antibodies against type A and O FMDV in mice. Compared with the commercial type A/O FMDV bivalent inactivated vaccine, rHBc/AO and rHBc/AOT VLPs significantly stimulated the production of Th1 type cytokines (IFN-γ and IL-2), whereas Th2 cytokine production (IL-4 and IL-10) was decreased. Compared with rHBc/AO, rHBc/AOT induced increased Th2 cytokine and specific IgG production. These results demonstrate that the VLPs constructed in the current study induced both humoral and cellular immune responses and may represent potential bivalent VLP vaccines targeting both FMDV type A and O strains.

The VP1 G-H loop hypervariable epitope contributes to protective immunity against Foot and Mouth Disease Virus in swine

Foot and Mouth Disease is a highly contagious and economically important disease of livestock. While vaccination is often effective at controlling viral spread, failures can occur due to strain mismatch or viral mutation. Foot and Mouth Disease Virus (FMDV) possesses a hypervariable region within the G-H Loop of VP1, a capsid protein commonly associated with virus neutralization. Here, we investigate the effect of replacement of the G-H loop hypervariable epitope with a xenoepitope from PRRS virus on the immunogenicity and efficacy of an adenovirus vectored FMDV vaccine (Ad5-FMD). Pigs were vaccinated with Ad5-FMD, the modified Ad5-FMD_xeno, or PBS, followed by intradermal challenge with FDMV strain O₁ Manisa at 21 days post-vaccination. While overall serum antibody titers were significantly higher in Ad5-FMD_xeno vaccinated animals, neutralizing antibody titers were decreased in pigs that received Ad5-FMD_xeno, when compared to those vaccinated with Ad5-FMD, prior to viral challenge, indicative of immune redirection away from VP1 towards non-neutralizing epitopes. As expected, animals vaccinated with unmodified Ad5-FMD were protected from lesions, fever, and viremia. In contrast, animals vaccinated with Ad5-FMD_xeno developed clinical signs and viremia, but at lower levels than that observed in PBS-treated controls. No significant difference was found in nasal shedding of virions between the two Ad5-FMD vaccinated groups. This data suggests that the hypervariable epitope of the VP1 G-H loop contributes to protective immunity conferred by Ad5 vector-delivered FMD vaccines in swine, and cannot be substituted without a loss of immunogenicity.

A traditional evolutionary history of foot-and-mouth disease viruses in Southeast Asia challenged by analyses of non-structural protein coding sequences

Recombination of rapidly evolving RNA-viruses provides an important mechanism for diversification, spread, and emergence of new variants with enhanced fitness. Foot-and-mouth disease virus (FMDV) causes an important transboundary disease of livestock that is endemic to most countries in Asia and Africa. Maintenance and spread of FMDV are driven by periods of dominance of specific viral lineages. Current understanding of the molecular epidemiology of FMDV lineages is generally based on the phylogenetic relationship of the capsid-encoding genes, with less attention to the process of recombination and evolution of non-structural proteins. In this study, the putative recombination breakpoints of FMDVs endemic to Southeast Asia were determined using full-open reading frame sequences. Subsequently, the lineages’ divergence times of recombination-free genome regions were estimated. These analyses revealed a close relationship between two of the earliest endemic viral lineages that appear unrelated when only considering the phylogeny of their capsid proteins. Contrastingly, one lineage, named O/CATHAY, known for having a particular host predilection (pigs) has evolved independently. Additionally, intra-lineage recombination occurred at different breakpoints compared to the inter-lineage process. These results provide new insights about FMDV recombination patterns and the evolutionary interdependence of FMDV serotypes and lineages.

Chimeric foot-and-mouth disease virus serotype O displaying a serotype Asia1 antigenic epitope at the surface

OBJECTIVE

To determine whether the G-H loop of foot-and-mouth disease virus (FMDV) serotype O can function as a target structure to harbour and display serotype Asia1 antigenic epitope at the surface.

RESULTS

Using reverse genetics, FMDV serotype O IND R2/1975 displaying a FMDV serotype Asia1 B cell epitope at the capsid surface was constructed. The epitope-inserted recombinant chimeric virus was genetically stable up to ten serial passages in cell culture and exhibited growth properties similar to the parental serotype O virus. Furthermore, the surface-displayed Asia1 epitope able to react with serotype Asia1 specific antibodies in a competitive ELISA. Importantly, the recombinant chimeric virus showed neutralizing activity to both serotype O and Asia1 polyclonal antibodies.

CONCLUSION

The capsid protein of FMDV serotype O can effectively display potent epitope of other serotypes, making this an attractive approach for the design of new generation bi-valent FMD vaccines.

Prediction and characterization of novel epitopes of serotype A foot-and-mouth disease viruses circulating in East Africa using site-directed mutagenesis

Epitopes on the surface of the foot-and-mouth disease virus (FMDV) capsid have been identified by monoclonal antibody (mAb) escape mutant studies leading to the designation of four antigenic sites in serotype A FMDV. Previous work focused on viruses isolated mainly from Asia, Europe and Latin America. In this study we report on the prediction of epitopes in African serotype A FMDVs and testing of selected epitopes using reverse genetics. Twenty-four capsid amino acid residues were predicted to be of antigenic significance by analysing the capsid sequences (n = 56) using in silico methods, and six residues by correlating capsid sequence with serum-virus neutralization data. The predicted residues were distributed on the surface-exposed capsid regions, VP1-VP3. The significance of residue changes at eight of the predicted epitopes was tested by site-directed mutagenesis using a cDNA clone resulting in the generation of 12 mutant viruses involving seven sites. The effect of the amino acid substitutions on the antigenic nature of the virus was assessed by virus neutralization (VN) test. Mutations at four different positions, namely VP1-43, VP1-45, VP2-191 and VP3-132, led to significant reduction in VN titre (P value = 0.05, 0.05, 0.001 and 0.05, respectively). This is the first time, to our knowledge, that the antigenic regions encompassing amino acids VP1-43 to -45 (equivalent to antigenic site 3 in serotype O), VP2-191 and VP3-132 have been predicted as epitopes and evaluated serologically for serotype A FMDVs. This identifies novel capsid epitopes of recently circulating serotype A FMDVs in East Africa.

Challenges and prospects for the control of foot-and-mouth disease: an African perspective

The epidemiology of foot-and-mouth disease (FMD) in Africa is unique in the sense that six of the seven serotypes of FMD viruses (Southern African Territories [SAT] 1, SAT2, SAT3, A, O, and C), with the exception of Asia-1, have occurred in the last decade. Due to underreporting of FMD, the current strains circulating throughout sub-Saharan Africa are in many cases unknown. For SAT1, SAT2, and serotype A viruses, the genetic diversity is reflected in antigenic variation, and indications are that vaccine strains may be needed for each topotype. This has serious implications for control using vaccines and for choice of strains to include in regional antigen banks. The epidemiology is further complicated by the fact that SAT1, SAT2, and SAT3 viruses are maintained and spread by wildlife, persistently infecting African buffalo in particular. Although the precise mechanism of transmission of FMD from buffalo to cattle is not well understood, it is facilitated by direct contact between these two species. Once cattle are infected they may maintain SAT infections without the further involvement of buffalo. No single strategy for control of FMD in Africa is applicable. Decision on the most effective regional control strategy should focus on an ecosystem approach, identification of primary endemic areas, animal husbandry practices, climate, and animal movement. Within each ecosystem, human behavior could be integrated in disease control planning. Different regions in sub-Saharan Africa are at different developmental stages and are thus facing unique challenges and priorities in terms of veterinary disease control. Many science-based options targeting improved vaccinology, diagnostics, and other control measures have been described. This review therefore aims to emphasize, on one hand, the progress that has been achieved in the development of new technologies, including research towards improved tailored vaccines, appropriate vaccine strain selection, vaccine potency, and diagnostics, and how it relates to the conditions in Africa. On the other hand, we focus on the unique epidemiological, ecological, livestock farming and marketing, socioeconomic, and governance issues that constrain effective FMD control. Any such new technologies should have the availability of safe livestock products for trade as the ultimate goal.

Development of a multiplex Luminex assay for detecting swine antibodies to structural and nonstructural proteins of foot-and-mouth disease virus in Taiwan

BACKGROUND/PURPOSE(S)

Foot-and-mouth disease (FMD) and swine vesicular disease (SVD) are serious vesicular diseases that have devastated swine populations throughout the world. The aim of this study was to develop a multianalyte profiling (xMAP) Luminex assay for the differential detection of antibodies to the FMD virus of structural proteins (SP) and nonstructural proteins (NSP).

METHODS

After the xMAP was optimized, it detected antibodies to SP-VP1 and NSP-3ABC of the FMD virus in a single serum sample. These tests were also compared with 3ABC polypeptide blocking enzyme-linked immunosorbent assay (ELISA) and virus neutralization test (VNT) methods for the differential diagnosis and assessment of immune status, respectively.

RESULTS

To detect SP antibodies in 661 sera from infected naïve pigs and vaccinated pigs, the diagnostic sensitivity (DSn) and diagnostic specificity (DSp) of the xMAP were 90.0-98.7% and 93.0-96.5%, respectively. To detect NSP antibodies, the DSn was 90% and the DSp ranged from 93.3% to 99.1%. The xMAP can detect the immune response to SP and NSP as early as 4 days postinfection and 8 days postinfection, respectively. Furthermore, the SP and NSP antibodies in all 15 vaccinated but unprotected pigs were detected by xMAP. A comparison of SP and NSP antibodies detected in the sera of the infected samples indicated that the results from the xMAP had a high positive correlation with results from the VNT and a 3ABC polypeptide blocking ELISA assay. However, simultaneous quantitation detected that xMAP had no relationship with the VNT. Furthermore, the specificity was 93.3-94.9% with 3ABC polypeptide blocking ELISA for the FMDV-NSP antibody.

CONCLUSION

The results indicated that xMAP has the potential to detect antibodies to FMDV-SP-VP1 and NSP-3ABC and to distinguish FMDV-infected pigs from pigs infected with the swine vesicular disease virus.

Evaluation of a 3A-truncated foot-and-mouth disease virus in pigs for its potential as a marker vaccine

Foot-and-mouth disease (FMD) is a highly contagious and economically devastating disease of cloven-hoofed animals in the world. The disease can be effectively controlled by vaccination of susceptible animals with the conventional inactivated vaccine. However, one major concern of the inactivated FMD virus (FMDV) vaccine is that it does not allow serological discrimination between infected and vaccinated animals, and therefore interferes with serologic surveillance and the epidemiology of disease. A marker vaccine has proven to be of great value in disease eradication and control programs. In this study, we constructed a marker FMDV containing a deletion of residues 93 to 143 in the nonstructural protein 3A using a recently developed FMDV infectious cDNA clone. The marker virus, r-HN/3A_93–143, had similar growth kinetics as the wild type virus in culture cell and caused a symptomatic infection in pigs. Pigs immunized with chemically inactivated marker vaccine were fully protected from the wild type virus challenge, and the potency of this marker vaccine was 10 PD₅₀ (50% pig protective dose) per dose, indicating it could be an efficacious vaccine against FMDV. In addition, we developed a blocking ELISA targeted to the deleted epitope that could clearly differentiate animals infected with the marker virus from those infected with the wild type virus. These results indicate that a marker FMDV vaccine can be potentially developed by deleting an immunodominant epitope in NSP 3A.

Genetic heterogeneity in the leader and P1-coding regions of foot-and-mouth disease virus serotypes A and O in Africa

Genetic information regarding the leader (L) and complete capsid-coding (P1) region of FMD serotype A and O viruses prevalent on the African continent is lacking. Here, we present the complete L-P1 sequences for eight serotype A and nine serotype O viruses recovered from FMDV outbreaks in East and West Africa over the last 33 years. Phylogenetic analysis of the P1 and capsid-coding regions revealed that the African isolates grouped according to serotype, and certain clusters were indicative of transboundary as well as intra-regional spread of the virus. However, similar analysis of the L region revealed random groupings of isolates from serotypes O and A. Comparisons between the phylogenetic trees derived from the structural coding regions and the L region pointed to a possibility of genetic recombination. The intertypic nucleotide and amino acid variation of all the isolates in this study supported results from previous studies where the externally located 1D was the most variable whilst the internally located 1A was the most conserved, which likely reflects the selective pressures on these proteins. Amino acids identified previously as important for FMDV structure and functioning were found to be highly conserved. The information gained from this study will contribute to the construction of structurally designed FMDV vaccines in Africa.

Induction of partial protection against foot and mouth disease virus in guinea pigs by neutralization with the integrin β6-1 subunit

The mechanism by which the foot-and-mouth disease virus (FMDV) initiates infection of cells is thought to involve the attachment of the viral capsid to host integrins on the surface of target cells. However, the role of integrins in FMDV infection still needs to be fully understood, although it has been demonstrated that integrin αvβ6 interferes with FMDV in vitro and results in neutralization of its infectivity. In the present study, we describe the cloning and sequencing of suckling mouse integrin β6 and the subsequent expression of two segments of integrin β6 extracellular domains: β6-1 (which contains the ligand-binding domain) and β6-2. Sequencing of the mouse integrin β6 subunit revealed close homology (~90%) with its human counterpart. When recombinant integrin extracellular domains β6-1 and β6-2 formulated with adjuvant were inoculated into guinea pigs, anti-integrin antibody expression was high before FMDV challenge. Interestingly, guinea pigs (50%) inoculated with integrin β6-1 were protected from FMDV infection; in contrast, none of the animals inoculated with integrin β6-2 were protected. This result indicates that an integrin blockade may be able to interfere with FMDV infection in vivo, which raises the possibility that targeting integrin in vivo may be the basis for a new strategy to control FMDV infection.

Effects of amino acid substitutions in the VP2 B-C loop on antigenicity and pathogenicity of serotype Asia1 foot-and-mouth disease virus

Background

Foot-and-mouth disease virus (FMDV) exhibits a high degree of antigenic variability. Studies of the antigenic diversity and determination of amino acid changes involved in this diversity are important to the design of broadly protective new vaccines. Although extensive studies have been carried out to explore the molecular basis of the antigenic variation of serotype O and serotype A FMDV, there are few reports on Asia1 serotype FMDV.

Methods

Two serotype Asia1 viruses, Asia1/YS/CHA/05 and Asia1/1/YZ/CHA/06, which show differential reactivity to the neutralizing monoclonal antibody (nMAb) 1B4, were subjected to sequence comparison. Then a reverse genetics system was used to generate mutant versions of Asia1/YS/CHA/05 followed by comparative analysis of the antigenicity, growth property and pathogenicity in the suckling mice.

Results

Three amino acid differences were observed when the structural protein coding sequences of Asia1/1/YZ/CHA/06 were compared to that of Asia1/YS/CHA/05. Site-directed mutagenesis and Immunofluorescence analysis showed that the amino acid substitution in the B-C loop of the VP2 protein at position 72 is responsible for the antigenic difference between the two Asia1 FMDV strains. Furthermore, alignment of the amino acid sequences of VP2 proteins from serotype Asia1 FMDV strains deposited in GenBank revealed that most of the serotype Asia1 FMDV strains contain an Asn residue at position 72 of VP2. Therefore, we constructed a mutant virus carrying an Asp-to-Asn substitution at position 72 and named it rD72N. Our analysis shows that the Asp-to-Asn substitution inhibited the ability of the rD72N virus to react with the MAb 1B4 in immunofluorescence and neutralization assays. In addition, this substitution decreased the growth rate of the virus in BHK-21 cells and decreased the virulence of the virus in suckling mice compared with the Asia1/YS/CHA/05 parental strain.

Conclusions

These results suggest that variations in domains other than the hyper variable VP1 G-H loop (amino acid 140 to 160) are relevant to the antigenic diversity of FMDV. In addition, amino acid substitutions in the VP2 influenced replicative ability and virulence of the virus. Thus, special consideration should be given to the VP2 protein in research on structure-function relationships and in the development of an FMDV vaccine.

Insertion of type O-conserved neutralizing epitope into the foot-and-mouth disease virus type Asia1 VP1 G-H loop: effect on viral replication and neutralization phenotype

Previously, we finely mapped the neutralizing epitopes recognized by foot-and-mouth disease virus (FMDV) type Asia1-specific mAb 3E11 and FMDV type O-specific mAb 8E8. In this study, we engineered recombinant FMDVs of the serotype Asia1 (rFMDVs) displaying the type O-neutralizing epitope recognized by the mAb 8E8. These epitope-inserted viruses were genetically stable and exhibited growth properties that were similar to those of their parental virus. Importantly, the recombinant virus rFMDV-C showed neutralization sensitivity to both FMDV type Asia1 and type O mAbs, as well as to polyclonal antibodies. These results indicated that this epitope-inserted virus has the potential to induce neutralizing antibodies against both FMDV type Asia1 and type O. Our results demonstrated that the G-H loop of FMDV type Asia1 effectively displays the protective neutralizing epitopes of other FMDV serotypes, making this an attractive approach for the design of novel FMDV vaccines.

Mapping of antigenic determinants on a SAT2 foot-and-mouth disease virus using chicken single-chain antibody fragments

Recombinant single-chain variable fragments (scFvs) of antibodies make it possible to localize antigenic and immunogenic determinants, identify protective epitopes and can be exploited for the design of improved diagnostic tests and vaccines. A neutralizing epitope, as well as other potential antigenic sites of a SAT2 foot-and-mouth disease virus (FMDV) were identified using phage-displayed scFvs. Three unique ZIM/7/83-specific scFvs, designated scFv1, scFv2 and scFv3, were isolated. Further characterization of these scFvs revealed that only scFv2 was capable of neutralizing the ZIM/7/83 virus and was used to generate neutralization-resistant virus variants. Sequence analysis of the P1 region of virus escaping neutralization revealed a residue change from His to Arg at position 159 of the VP1 protein. Residue 159 is not only surface exposed but is also located at the C-terminal base of the G-H loop, a known immunogenic region of FMDV. A synthetic peptide, of which the sequence corresponded to the predicted antigenic site of the VP1 G-H loop of ZIM/7/83, inhibited binding of scFv2 to ZIM/7/83 in a concentration-dependent manner. This region can therefore be considered in the design of SAT2 vaccine seed viruses for the regional control of FMD in Africa.

Viral quasispecies evolution

Evolution of RNA viruses occurs through disequilibria of collections of closely related mutant spectra or mutant clouds termed viral quasispecies. Here we review the origin of the quasispecies concept and some biological implications of quasispecies dynamics. Two main aspects are addressed: (i) mutant clouds as reservoirs of phenotypic variants for virus adaptability and (ii) the internal interactions that are established within mutant spectra that render a virus ensemble the unit of selection. The understanding of viruses as quasispecies has led to new antiviral designs, such as lethal mutagenesis, whose aim is to drive viruses toward low fitness values with limited chances of fitness recovery. The impact of quasispecies for three salient human pathogens, human immunodeficiency virus and the hepatitis B and C viruses, is reviewed, with emphasis on antiviral treatment strategies. Finally, extensions of quasispecies to nonviral systems are briefly mentioned to emphasize the broad applicability of quasispecies theory.

Evaluation of a genetically modified foot-and-mouth disease virus vaccine candidate generated by reverse genetics

BACKGROUND

Foot-and-mouth disease (FMD) is the most economically important and highly contagious disease of cloven-hoofed animals worldwide. Control of the disease has been mainly based on large-scale vaccinations with whole-virus inactivated vaccines. In recent years, a series of outbreaks of type O FMD occurred in China (including Chinese Taipei, Chinese Hong Kong) posed a tremendous threat to Chinese animal husbandry. Its causative agent, type O FMDV, has evolved into three topotypes (East-South Asia (ME-SA), Southeast Asia (SEA), Cathay (CHY)) in these regions, which represents an important obstacle to disease control. The available FMD vaccine in China shows generally good protection against ME-SA and SEA topotype viruses infection, but affords insufficient protection against some variants of the CHY topotype. Therefore, the choice of a new vaccine strain is of fundamental importance.

RESULTS

The present study describes the generation of a full-length infectious cDNA clone of FMDV vaccine strain and a genetically modified virus with some amino acid substitutions in antigenic sites 1, 3, and 4, based on the established infectious clone. The recombinant viruses had similar growth properties to the wild O/HN/CHA/93 virus. All swine immunized with inactivated vaccine prepared from the O/HN/CHA/93 were fully protected from challenge with the viruses of ME-SA and SEA topotypes and partially protected against challenge with the virus of CHY topotype at 28 days post-immunization. In contrast, the swine inoculated with the genetically modified vaccine were completely protected from the infection of viruses of the three topotypes.

CONCLUSIONS

Some amino acid substitutions in the FMDV vaccine strain genome did not have an effect on the ability of viral replication in vitro. The vaccine prepared from genetically modified FMDV by reverse genetics significantly improved the protective efficacy to the variant of the CHY topotype, compared with the wild O/HN/CHA/93 virus. Thus, the full-length cDNA clone of FMDV can be a useful tool to develop genetically engineered FMDV vaccine candidates to help control porcinophilic FMD epidemics in China.

Foot-and-mouth disease virus epitope dominance in the antibody response of vaccinated animals

Five neutralizing antigenic sites have been identified on the surface of serotype O foot-and-mouth disease virus (FMDV). A set of mAb neutralization-escape mutant viruses was used for the first time to evaluate the relative use of known binding sites by polyclonal antibodies from three target species: cattle, sheep and pigs. Antibodies to all five neutralizing antigenic sites were detected in all three species, with most antibodies directed against antigenic site 2, followed by antigenic site 1. In 76 % of cattle, 65 % of sheep and 58 % of pigs, most antibodies were directed against site 2. Antibodies specific to antigenic sites 3, 4 and 5 were found to be minor constituents in the sera of each of the target species. This implies that antigenic site 2 is a dominant neutralization immunogenic site in serotype O FMDV and may therefore be a good candidate for designing novel vaccines.

An overview on ELISA techniques for FMD

Background

FMD is one of the major causes of economic loss of cloven-hoofed animals in the world today. The assessment of dominant genotype/lineage and prevalent trends and confirmation the presence of infection or vaccination not only provides scientific basis and first-hand information for appropriate control measure but also for disease eradication and regaining FMD free status following an outbreak. Although different biological and serological approaches are still applied to study this disease, ELISA test based on the distinct format, antigen type and specific antibody reinforce its predominance in different research areas of FMD, and this may replace the traditional methods in the near future. This review gives comprehensive insight on ELISA currently available for typing, antigenic analysis, vaccination status differentiation and surveillance vaccine purity and content at all stages of manufacture in FMDV. Besides, some viewpoint about the recent advances and trends of ELISA reagent for FMD are described here.

Methods

More than 100 studies regarding ELISA method available for FMD diagnosis, antigenic analysis and monitor were thoroughly reviewed. We investigated previous sagacious results of these tests on their sensitivity, specificity.

Results

We found that in all ELISA formats for FMD, antibody-trapping and competitive ELISAs have high specificity and RT-PCR (oligoprobing) ELISA has extra sensitivity. A panel of monoclonal antibodies to different sites or monoclonal antibody in combination of antiserum is the most suitable combination of antibodies in ELISA for FMD. Even though from its beginning, 3ABC is proven to be best performance in many studies, no single NSP can differentiate infected from vaccinated animals with complete confidence. Meanwhile, recombinant antigens and peptide derived from FMDV NPs, and NSPs have been developed for use as an alternative to the inactivated virus antigen for security.

Conclusions

There is a need of target protein, which accurately determines the susceptible animal status based on the simple, fast and reliable routine laboratory test. A further alternative based on virus-like particle (VLP, also called empty capsids) in combination of high throughput antibody technique (Phage antibody library/antibody microarray) may be the powerful ELISA diagnostic reagents in future.

Equine rhinitis A virus-like particle expressing DNA vaccine induces a virus neutralising immune response in mice

Equine rhinitis A virus (ERAV) is a respiratory pathogen of horses. Candidate vaccines to date have been hindered by low expression levels and the induction of non-neutralising antibodies. The immunodominant epitope of ERAV is conformational and is located within the quaternary structure of the capsid. This site should be retained in ERAV virus-like particles (VLPs) to stimulate the induction of neutralising antibodies. The immunogenicity of a plasmid-based DNA vaccine designed to express ERAV VLPs was assessed. The plasmid construct, pcD.P12A.3C, contained the capsid precursor (P1-2A) and the viral protease 3C, under the transcriptional control of a cytomegalovirus (CMV) promoter. Mature viral capsid proteins and VLPs were detected in vitro in transfected COS7 cells. Immunisation of BALB/c mice with pcD.P12A.3C induced virus neutralising antibodies and enhanced the virus neutralising antibody response to purified, UV-inactivated ERAV. This study further supports the use of DNA vaccines to elicit neutralising antibodies to complex antigenic proteins.

Genetic and antigenic characterization of foot-and-mouth disease viruses isolated in Taiwan between 1998 and 2009

A devastating outbreak of foot-and-mouth disease (FMD), caused by a porcinophilic serotype O virus, occurred in Taiwan in March 1997. This outbreak was brought under control by means of a stamping-out policy and vaccination. Although mandatory vaccination was conducted in Taiwan between 1997 and 2007, sporadic outbreaks of FMD occurred between 1998 and 2009; however, the viruses that caused these outbreaks remain uncharacterized. This article reports the genetic and antigenic characterization of FMD viruses isolated in Taiwan during this period. Sequence analysis of the VP1 coding region showed that the viruses isolated in Taiwan between 1998 and 2009 were most similar to viruses isolated in Taiwan in 1997 and to viruses isolated from Hong Kong and Vietnam in 1991-1996. The results of phylogenetic analysis suggested that the viruses isolated in Taiwan in 1998-2009 were derived from the viruses isolated in Taiwan in 1997. However, substantial mutations were found in the viruses isolated in 2009, and some of these changes may have resulted from vaccine pressure in the field. Serum neutralization tests confirmed that viruses isolated in 2009 showed a significant change in antigenicity. This is the first report of changes in the VP1 sequence and antigenicity of porcinophilic FMD viruses isolated from an area in which long-term mandatory vaccination against FMD was practiced.

Marker vaccine potential of a foot-and-mouth disease virus with a partial VP1 G-H loop deletion

Previous work in cattle and pigs demonstrated that protection against foot-and-mouth disease (FMD) could be achieved following vaccination with chimeric foot-and-mouth disease virus (FMDV) vaccines, in which the VP1 G-H loop had been substituted with that from another serotype. This indicated that the VP1 G-H loop may not be essential for the protection of natural hosts against FMDV. If this could be substantiated there would be potential to develop FMD marker vaccines, characterised by the absence of this region. Here, we investigate the serological responses to vaccination with a virus with a partial VP1 G-H loop deletion in order to determine the likelihood of achieving protection and the potential of this virus as a marker vaccine. Inactivated, oil adjuvanted, vaccines, consisting of chemically inactivated virus with or without a partially deleted VP1 G-H loop, were used to immunise cattle. Serum was collected on days 0, 7, 14 and 21 and antibody titres calculated using the virus neutralisation test (VNT) to estimate the likelihood of protection. We predict a good likelihood that cattle vaccinated with a vaccine characterised by a partial VP1 G-H loop would be protected against challenge with the same virus containing the VP1 G-H loop. We also present evidence on the potential of such a construct to act as a marker vaccine, when used in conjunction with a novel serological test.

Influence of the mutant spectrum in viral evolution: focused selection of antigenic variants in a reconstructed viral quasispecies

RNA viruses replicate as complex mutant distributions termed viral quasispecies. Despite this, studies on virus populations subjected to positive selection have generally been performed and analyzed as if the viral population consisted of a defined genomic nucleotide sequence; such a simplification may not reflect accurately the molecular events underlying the selection process. In the present study, we have reconstructed a foot-and-mouth disease virus quasispecies with multiple, low-frequency, genetically distinguishable mutants that can escape neutralization by a monoclonal antibody. Some of the mutants included an amino acid substitution that affected an integrin recognition motif that overlaps with the antibody-binding site, whereas other mutants included an amino acid substitution that affected antibody binding but not integrin recognition. We have monitored consensus and clonal nucleotide sequences of populations passaged either in the absence or the presence of the neutralizing antibody. In both cases, the populations focused toward a specific mutant that was surrounded by a cloud of mutants with different antigenic and cell recognition specificities. In the absence of antibody selection, an antigenic variant that maintained integrin recognition became dominant, but the mutant cloud included as one of its minority components a variant with altered integrin recognition. Conversely, in the presence of antibody selection, a variant with altered integrin recognition motif became dominant, but it was surrounded by a cloud of antigenic variants that maintained integrin recognition. The results have documented that a mutant spectrum can exert an influence on a viral population subjected to a sustained positive selection pressure and have unveiled a mechanism of antigenic flexibility in viral populations, consisting in the presence in the selected quasispecies of mutants with different antigenic and cell recognition specificities.

Mechanisms of GII.4 norovirus persistence in human populations

BACKGROUND

Noroviruses are the leading cause of viral acute gastroenteritis in humans, noted for causing epidemic outbreaks in communities, the military, cruise ships, hospitals, and assisted living communities. The evolutionary mechanisms governing the persistence and emergence of new norovirus strains in human populations are unknown. Primarily organized by sequence homology into two major human genogroups defined by multiple genoclusters, the majority of norovirus outbreaks are caused by viruses from the GII.4 genocluster, which was first recognized as the major epidemic strain in the mid-1990s. Previous studies by our laboratory and others indicate that some noroviruses readily infect individuals who carry a gene encoding a functional alpha-1,2-fucosyltransferase (FUT2) and are designated "secretor-positive" to indicate that they express ABH histo-blood group antigens (HBGAs), a highly heterogeneous group of related carbohydrates on mucosal surfaces. Individuals with defects in the FUT2 gene are termed secretor-negative, do not express the appropriate HBGA necessary for docking, and are resistant to Norwalk infection. These data argue that FUT2 and other genes encoding enzymes that regulate processing of the HBGA carbohydrates function as susceptibility alleles. However, secretor-negative individuals can be infected with other norovirus strains, and reinfection with the GII.4 strains is common in human populations. In this article, we analyze molecular mechanisms governing GII.4 epidemiology, susceptibility, and persistence in human populations.

METHODS AND FINDINGS

Phylogenetic analyses of the GII.4 capsid sequences suggested an epochal evolution over the last 20 y with periods of stasis followed by rapid evolution of novel epidemic strains. The epidemic strains show a linear relationship in time, whereby serial replacements emerge from the previous cluster. Five major evolutionary clusters were identified, and representative ORF2 capsid genes for each cluster were expressed as virus-like particles (VLPs). Using salivary and carbohydrate-binding assays, we showed that GII.4 VLP-carbohydrate ligand binding patterns have changed over time and include carbohydrates regulated by the human FUT2 and FUT3 pathways, suggesting that strain sensitivity to human susceptibility alleles will vary. Variation in surface-exposed residues and in residues that surround the fucose ligand interaction domain suggests that antigenic drift may promote GII.4 persistence in human populations. Evidence supporting antigenic drift was obtained by measuring the antigenic relatedness of GII.4 VLPs using murine and human sera and demonstrating strain-specific serologic and carbohydrate-binding blockade responses. These data suggest that the GII.4 noroviruses persist by altering their HBGA carbohydrate-binding targets over time, which not only allows for escape from highly penetrant host susceptibility alleles, but simultaneously allows for immune-driven selection in the receptor-binding region to facilitate escape from protective herd immunity.

CONCLUSIONS

Our data suggest that the surface-exposed carbohydrate ligand binding domain in the norovirus capsid is under heavy immune selection and likely evolves by antigenic drift in the face of human herd immunity. Variation in the capsid carbohydrate-binding domain is tolerated because of the large repertoire of similar, yet distinct HBGA carbohydrate receptors available on mucosal surfaces that could interface with the remodeled architecture of the capsid ligand-binding pocket. The continuing evolution of new replacement strains suggests that, as with influenza viruses, vaccines could be targeted that protect against norovirus infections, and that continued epidemiologic surveillance and reformulations of norovirus vaccines will be essential in the control of future outbreaks.

Vaccination of mice with plasmids expressing processed capsid protein of foot-and-mouth disease virus--importance of dominant and subdominant epitopes for antigenicity and protection

The capsid of foot-and-mouth disease virus (FMDV) displays several independent B cell epitopes, which stimulate the production of neutralising antibodies. Some of these epitopes are highly variable between virus strains, but dominate the immune response. The site A on VP1 is the most prominent example of a dominant and variable site. This variability is a problem when designing vaccines against this disease, because it necessitates a close match between vaccine strain and virus in an outbreak. We have introduced a series of mutations into viral capsid proteins with the aim of selectively silencing two dominant and highly variable epitopes and thereby divert immune responses toward less dominant but more conserved, protective epitopes. When mice were immunized with modified antigens, the resulting immune responses showed a higher degree of cross-reactivity towards heterologous virus as compared to mice vaccinated with wild type epitopes. Most of the modifications did not adversely affect the ability of the plasmids to induce complete protection of mice against homologous challenge.

Genetic analysis of HAV strains in Tunisia reveals two new antigenic variants

To evaluate the genetic variability of hepatitis A virus (HAV) isolates in Tunisia, serum samples were collected from 99 patients in different Tunisian areas in 2003 containing 92 cases with acute hepatitis, five with severe acute hepatitis and two with fulminant hepatitis. The entire VP1 gene was amplified and sequenced. Sequences were then aligned and a phylogenetic analysis was performed. Additionally, the amino acid (aa) sequence of the VP1 was determined. The analysis of Tunisian HAV isolates revealed that all the isolates were sub-genotype IA with 96.4%-99.8% of identity and showed the emergence of two novel antigenic variants. The Tun31-03 antigenic variant, with a 38 aa deletion containing Met156, Val171, Leu174 and Ala176 and located between 150 and 187 aa of the VP1 protein where neutralization escape mutations, was found. The second antigenic variant, Tun36-03, was isolated from a patient with fulminant hepatitis and presented a substitution of Thr by Pro at position 10 of the VP1 protein. This amino acid is located in a peptide presenting an antigenically reactive epitope of the VP1 protein. This substitution has never been described previously.

Towards a multi-site synthetic vaccine to foot-and-mouth disease: addition of discontinuous site peptide mimic increases the neutralization response in immunized animals

Synthetic replicas of both antigenic sites A and D of foot-and-mouth disease virus have been tested as a first step towards a multicomponent peptide vaccine candidate. A first evaluation has been performed by neutralization assays on cells with serum mixtures from guinea pigs immunized independently with site A (A24) and site D (D8) peptides. The addition of site D antibodies to site A antibodies has a synergistic effect on neutralization. In a second group of experiments, guinea pigs have been immunized with a dendrimeric tetravalent (MAP) presentation of site A peptide, alone or in combination with D8, using the same total peptide dose. While the first inoculation gives a preferential response to site A-only antigen, specific response to site D and global neutralization levels significantly increase after reimmunization, reflecting a synergistic effect of site D.

Isolation of foot-and-mouth disease virus specific bovine antibody fragments from phage display libraries

Foot-and-mouth disease virus (FMDV) is an important veterinary pathogen which can cause widespread epidemics. Due to the high antigenic variability of FMDV, it is important to undertake mutation analysis under immunological pressure. To study the bovine antibody response at a molecular level, phage display technology was used to produce bovine anti-FMDV Fabs. CH1-VH chains with FMDV specific binding could be isolated after selection from a library made from vaccinated cattle. Though their involvement in the bovine immune response remains to be ascertained, it is planned to express the five different selected VH domains in bacterial or insect systems as sequence homologies with integrin beta6 chain could shed light on the basis of FMDV type receptor specificities.

Foot-and-mouth disease

Foot-and-mouth disease (FMD) is a highly contagious disease of cloven-hoofed animals. The disease was initially described in the 16th century and was the first animal pathogen identified as a virus. Recent FMD outbreaks in developed countries and their significant economic impact have increased the concern of governments worldwide. This review describes the reemergence of FMD in developed countries that had been disease free for many years and the effect that this has had on disease control strategies. The etiologic agent, FMD virus (FMDV), a member of the Picornaviridae family, is examined in detail at the genetic, structural, and biochemical levels and in terms of its antigenic diversity. The virus replication cycle, including virus-receptor interactions as well as unique aspects of virus translation and shutoff of host macromolecular synthesis, is discussed. This information has been the basis for the development of improved protocols to rapidly identify disease outbreaks, to differentiate vaccinated from infected animals, and to begin to identify and test novel vaccine candidates. Furthermore, this knowledge, coupled with the ability to manipulate FMDV genomes at the molecular level, has provided the framework for examination of disease pathogenesis and the development of a more complete understanding of the virus and host factors involved.

Genetic variability of hepatitis A virus

Knowledge of the molecular biology of hepatitis A virus (HAV) has increased exponentially since its identification. HAV exploits all known mechanisms of genetic variation to ensure survival, including mutation and genetic recombination. HAV has been characterized by the emergence of different genotypes, three human antigenic variants and only one major serotype. This paper reviews the genetic variability and molecular epidemiology of HAV. Its evolutionary mechanisms are described with particular emphasis on genetic recombination and HAV mutation rate. Genotypic classification methods are also discussed.

Molecular evolution of hepatitis A virus: a new classification based on the complete VP1 protein

Hepatitis A virus (HAV) is a positive-stranded RNA virus in the genus Hepatovirus in the family Picornaviridae So far, analysis of the genetic variability of HAV has been based on two discrete regions, the VP1/2A junction and the VP1 N terminus. In this report, we determined the nucleotide and deduced amino acid sequences of the complete VP1 gene of 81 strains from France, Kosovo, Mexico, Argentina, Chile, and Uruguay and compared them with the sequences of seven strains of HAV isolated elsewhere. Overall strain variation in the complete VP1 gene was found to be as high as 23.7% at the nucleotide level and 10.5% at the amino acid level. Different phylogenetic methods revealed that HAV sequences form five distinct and well-supported genetic lineages. Within these lineages, HAV sequences clustered by geographical origin only for European strains. The analysis of the complete VP1 gene allowed insight into the mode of evolution of HAV and revealed the emergence of a novel variant with a 15-amino-acid deletion located on the VP1 region where neutralization escape mutations were found. This could be the first antigenic variant of HAV so far identified.

Effective synthetic peptide vaccine for foot-and-mouth disease in swine

We have designed a peptide-based vaccine for foot-and-mouth disease (FMD) effective in swine. The peptide immunogen has a G-H loop domain from the VP1 capsid protein of foot-and-mouth disease virus (FMDV) and a novel promiscuous T helper (Th) site for broad immunogenicity in multiple species. The G-H loop VP1 site was optimised for cross-reactivity to FMDV by the inclusion into the peptide of cyclic constraint and adjoining sequences. The incorporation of consensus residues into the hypervariable positions of the VP1 site provided for broad immunogenicity. The vaccine protected 20 out of 21 immunised pigs from infectious challenge by FMDV O1 Taiwan using peptide doses as low as 12.5 microg, and a mild adjuvant that caused no lesions. A safe chemically-defined product would have considerable advantages for vaccination against 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.

Antigenic sites of foot-and-mouth disease virus (FMDV): an analysis of the specificities of anti-FMDV antibodies after vaccination of naturally susceptible host species

Of the known neutralizing antigenic sites of foot-and-mouth disease virus (FMDV), site 1 or A, formed in part by the G-H loop of VP1, has historically been considered immunodominant because of evidence implicating its importance in the induction of a protective immune response. However, no systematic study has been done to determine the relative importance of the various specificities of antibodies against the known neutralizing antigenic sites of FMDV in the polyclonal immune response of a natural host after vaccination. In this report, we have adopted a monoclonal antibody-based competition ELISA and used antibodies specific to sites 1, 2 and 3 to provide some insight into this issue. Following vaccination of the three main target species, cattle, pigs and sheep, with an O1 serotype strain, results indicate that none of these three antigenic sites can be considered immunodominant in a polyclonal serum. Interestingly, pigs did not respond to epitopes on the carboxy terminus end of VP1 as efficiently as the ruminant species. In addition to the known sites, other as yet undefined sites might also be important in the induction of a protective immune response. Possible implications for the design of new vaccine strategies for foot-and-mouth disease are discussed.

Resistance to extinction of low fitness virus subjected to plaque-to-plaque transfers: diversification by mutation clustering

Plaque-to-plaque transfers of RNA viruses lead to accumulation of mutations and fitness decrease. To test whether continuing plaque-to-plaque transfers would lead to viral extinction, we have subjected several low fitness foot-and-mouth disease virus (FMDV) clones to up to 130 successive plaque transfers, and have analyzed the evolution of plaque titers and genomic nucleotide sequences. No case of viral extinction could be documented. Some low fitness clones that posses an internal poly(A) tract evaded extinction by modifying the length or base composition of the poly(A) tract. The comparison of entire genomic sequences of FMDV clones at increasing plaque transfer number revealed that mutations accumulated at a uniform rate, and that they were distributed unevenly along the genome. Clusters of mutations were identified at different genomic sites in two plaque transfer lineages. Mutation clustering appears to occur stochastically and could not be related to fixation of compensatory mutations. The results document resistance of viral clones to extinction, and suggest that mutation clustering may be a mechanism of genetic diversification of low fitness virus.

Duration and fitness dependence of quasispecies memory

The duration and fitness dependence of memory in viral quasispecies evolving in cell culture have been investigated using two genetic markers of foot-and-mouth disease virus (FMDV). In lineages of antigenic variant FMDV RED, which reverted to FMDV RGD, memory FMDV RED genomes were detected after 50 infectious cycles, and memory level was fitness dependent. In growth-competition experiments between a reference FMDV RGD and two different FMDV RED populations, a 7.6-fold higher fitness of the initial FMDV RED population resulted in 30 to 100-fold higher memory level. In lineages of low-fitness clones containing an elongated internal polyadenylate tract, revertants lacking excess adenylate residues became dominant by passage 20. However, genomes including a larger number of adenylate residues were detected as memory genomes after at least 150 infectious cycles. Thus, quasispecies memory can be durable and is fitness dependent, as predicted from the growth competition of two mutant forms of a genome. An understanding of factors influencing quasispecies memory levels and duration may have implications for the extended diagnosis of viruses based on the quantification of minority genomes.

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 virus lacking the VP1 G-H loop: the mutant spectrum uncovers interactions among antigenic sites for fitness gain

The Arg-Gly-Asp (RGD) triplet found in the G-H loop of capsid protein VP1 of foot-and-mouth disease virus (FMDV) is critically involved in the interaction of FMDV with integrin receptors and with neutralizing antibodies. Multiplication of FMDV C-S8c1 in baby hamster kidney 21 (BHK-21) cells selected variant viruses exploiting alternative mechanisms of cell recognition that rendered the RGD integrin-binding triplet dispensable for infectivity. By constructing chimeric viruses, we show that dispensability of the RGD in these variant FMDVs can be extended to surrounding amino acid residues. Replacement of eight amino acid residues within the G-H loop of VP1 by an unrelated FLAG marker yielded infectious virus. Evolution of FLAG-containing viruses in BHK-21 cells generated complex quasispecies in which individual mutants included amino acid replacements at other antigenic sites of FMDV. Inclusion of such replacements in the parental FLAG clone resulted in an increase of relative fitness of the viruses. These results suggest structural or functional connections between antigenic sites of FMDV and underscore the value of mutant spectrum analysis for the identification of fitness-promoting genetic modifications in viral populations. The possibility of producing viable viruses lacking antigenic site A may find application in the design of new anti-FMD vaccines.

Synthetic peptides as functional mimics of a viral discontinuous antigenic site

Functional reproduction of discontinuous antigenic site D of foot-and-mouth disease virus (FMDV) has been achieved by means of synthetic peptide constructions that integrate into a single molecule each of the three protein loops that define the antigenic site. The site D mimics are designed on the basis of the X-ray structure of FMDV type C-S8c1 with the aid of molecular dynamics, so that the five residues assumed to be involved in antigenic recognition are located on the same face of the molecule, exposed to solvent and defining a set of native-like distances and angles. The designed site D mimics are disulphide-linked heterodimers that consist of a larger unit containing VP2(71-84), followed by a polyproline module and by VP3(52-62), and a smaller unit corresponding to VP1(188-194). Guinea pig antisera to the peptides recognize the viral particle and compete with site D-specific monoclonal antibodies, while inoculation with a simple (non-covalently bound) admixture of the three VP1-VP3 sequences yields no detectable virus-specific serum conversion. Similar results have been reproduced in two cattle. Antisera to the peptides are also moderately neutralizing of FMDV in cell culture and partially protective of guinea pigs against challenge with the virus. These results demonstrate functional mimicry of the discontinuous site D by the peptides, which are therefore obvious candidates for a multicomponent peptide-based vaccine against FMDV.

Equine rhinitis A virus: structural proteins and immune response

Equine rhinitis A virus (ERAV) is a picornavirus that has been reclassified as a member of the Aphthovirus genus because of its resemblance to foot-and-mouth disease virus at the level of nucleotide sequence and overall genomic structure. The N-terminal amino acid sequence of three of the four capsid proteins of ERAV was determined and showed that the proteolytic cleavage sites within the precursor P1 polypeptide occur exactly as those predicted for an aphthovirus-like 3C protease, which generates the capsid proteins VP1 and VP3. However, the autocatalytic cleavage site between VP4 and VP2, which is independent of 3C protease cleavage, was different from that predicted previously. ERAV.393/76 antisera from horses and rabbits showed different reactivity to the viral structural proteins in both serum neutralization assays and Western blots. High neutralizing antibody titres appeared to correlate with strong reactivity to VP1 in Western blots.

Evidence for positive selection in foot-and-mouth disease virus capsid genes from field isolates

The nature of selection on capsid genes of foot-and-mouth disease virus (FMDV) was characterized by examining the ratio of nonsynonymous to synonymous substitutions in 11 data sets of sequences obtained from six different serotypes of FMDV. Using a method of analysis that assigns each codon position to one of a number of estimated values of nonsynonymous to synonymous ratio, significant evidence of positive selection was identified in 5 data sets, operating at 1-7% of codon positions. Evidence of positive selection was identified in complete capsid sequences of serotypes A and C and in VP1 sequences of serotypes SAT 1 and 2. Sequences of serotype SAT-2 recovered from a persistently infected African buffalo also revealed evidence for positive selection. Locations of codons under positive selection coincide closely with those of antigenic sites previously identified with the use of monoclonal antibody escape mutants. The vast majority of codons are under mild to strong purifying selection. However, these results suggest that arising antigenic variants benefit from a selective advantage in their interaction with the immune system, either during the course of an infection or in transmission to individuals with previous exposure to antigen. Analysis of amino acid usage at sites under positive selection indicates that this selective advantage can be conferred by amino acid substitutions that share physicochemically similar properties.