Studies on antigenic variability of C strains of foot-and-mouth disease virus by means of synthetic peptides and monoclonal antibodies

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.

New insights into the tPA-annexin A2 interaction. Is annexin A2 CYS8 the sole requirement for this association?

Annexin A2 has been described as an important receptor for tissue-type plasminogen activator in endothelium and other cell types. Interaction between tissue-type plasminogen activator and its cellular receptor is critical for many of the functions of this protease. The annexin A2 motif that mediates tissue plasminogen activator interaction has been assigned to the hexapeptide LCKLSL in the amino-terminal domain of the protein, and it has been proposed that Cys(8) of this sequence is essential for tPA binding. In an attempt to identify other amino acids critical for tPA-annexin A2 interaction, we have analyzed a set of peptides containing several modifications of the original hexapeptide, including glycine scans, alanine scans, d-amino acid scans, conservative mutations, cysteine blocking, and enantiomer and retroenantiomer sequences. Using a non-radioactive competitive binding assay, we have found that all cysteine-containing peptides, independently of their sequence, compete the interaction between tPA and annexin A2. Cysteine-containing peptides also inhibit tPA binding to the surface of cultured human umbilical vein endothelial cells (HUVEC). Mass spectrometry demonstrates that the peptides bind through a disulfide bond to a cysteine residue of annexin A2, the same mechanism that has been suggested for the inhibition mediated by homocysteine. These data call for a revision of the role of the LCKLSL sequence as the sole annexin A2 structural region required to bind tPA and indicate that further studies are necessary to better define the annexin A2-tPA interaction.

Probing degeneracy in antigen-antibody recognition at the immunodominant site of foot-and-mouth disease virus

Antigen-antibody binding is regarded as one of the most representative examples of specific molecular recognition in nature. The simplistic view of antigenic recognition in terms of a lock-and-key mechanism is obsolete, as it is evident that both antigens and antibodies are flexible and can undergo substantial mutual adaptation. This flexibility is the source of complexities such as degeneracy and nonadditivity in antigenic recognition. We have used surface plasmon resonance to study the effects of combining multiple amino acid replacements within the sequence of the antigenic GH loop of foot-and-mouth disease virus. Our aim was 2-fold: to explore the extent to which antigenic degeneracy can be extended in this particular case, and to search for potential nonadditive effects in introducing multiple amino acid replacements. Combined analysis of one such multiply substituted peptide by SPR, solution NMR and X-ray diffraction shows that antigenic degeneracy can be expected as long as residues directly interacting with the paratope are conserved and the peptide bioactive folding is unaltered.

Cleavage of the human respiratory syncytial virus fusion protein at two distinct sites is required for activation of membrane fusion

Preparations of purified full-length fusion (F) protein of human respiratory syncytial virus (HRSV) expressed in recombinant vaccinia-F infected cells, or of an anchorless mutant (F(TM(-))) lacking the C-terminal 50 amino acids secreted from vaccinia-F(TM(-))-infected cells contain a minor polypeptide that is an intermediate product of proteolytic processing of the F protein precursor F0. N-terminal sequencing of the intermediate demonstrated that it is generated by cleavage at a furin-motif, residues 106-109 of the F sequence. By contrast, the F1 N terminus derives from cleavage at residue 137 of F0 which is also C-terminal to a furin recognition site at residues 131-136. Site-directed mutagenesis indicates that processing of F0 protein involves independent cleavage at both sites. Both cleavages are required for the F protein to be active in membrane fusion as judged by syncytia formation, and they allow changes in F structure from cone- to lollipop-shaped spikes and the formation of rosettes by anchorless F.

Antigenicity modulation upon peptide cyclization: application to the GH loop of foot-and-mouth disease virus strain C1-Barcelona

Foot-and-mouth disease virus (FMDV) isolate C(1)-Barcelona (or C-S30) includes four replacements within its immunodominant site (GH loop, residues 136-150 of capsid protein VP1, YTTSTRGDLAHVTAT), relative to reference strain C-S8c1 (YTASARGDLAHLTTT). Although one of the mutations in C-S30 (147Leu-->Val) is known to be detrimental for antibody recognition, reactivity of this isolate with the neutralizing monoclonal antibody (mAb) 4C4, raised against FMDV C1-Brescia (GH loop: YTASTRGDLAHLTAT), was indistinguishable from those of strains C-S8c1 or C1-Brescia. A structural interpretation for these somewhat striking findings is available, based on the observation that 15-residue peptides reproducing the C-S30 and C-S8c1 GH loops adopt very similar, quasi-circular, conformations in crystal complexes with 4C4. Nevertheless, surface plasmon resonance (SPR) kinetic analyses of the interactions between these peptides and three anti-GH loop mAbs have now revealed that the linear C-S30 peptides were less antigenic in solution than their C-S8c1 and C1-Brescia counterparts. We have, therefore, tried to modulate peptide antigenicity in solution by cyclization. Functional SPR and structural two dimensional proton nuclear magnetic resonance (2D-1H NMR) studies of both linear and cyclic peptide antigens are discussed here. Conformation seems to have an important role in peptide antigenicity, even when continuous (i.e. linear) antigenic sites are involved.

Identification of T-cell epitopes in nonstructural proteins of foot-and-mouth disease virus

Porcine T-cell recognition of foot-and-mouth disease virus (FMDV) nonstructural proteins (NSP) was tested using in vitro lymphoproliferative responses. Lymphocytes were obtained from outbred pigs experimentally infected with FMDV. Of the different NSP, polypeptides 3A, 3B, and 3C gave the highest stimulations in the in vitro assays. The use of overlapping synthetic peptides allowed the identification of amino acid regions within these proteins that were efficiently recognized by the lymphocytes. The sequences of some of these antigenic peptides were highly conserved among different FMDV serotypes. They elicited major histocompatibility complex-restricted responses with lymphocytes from pigs infected with either a type C virus or reinfected with a heterologous FMDV. A tandem peptide containing the T-cell peptide 3A[21-35] and the B-cell antigenic site VP1[137-156] also efficiently stimulated lymphocytes from infected animals in vitro. Furthermore, this tandem peptide elicited significant levels of serotype-specific antiviral activity, a result consistent with the induction of anti-FMDV antibodies. Thus, inclusion in the peptide formulation of a T-cell epitope derived from the NSP 3A possessing the capacity to induce T helper activity can allow cooperative induction of anti-FMDV antibodies by B cells.

Molecular analysis of peptides from the GH loop of foot-and-mouth disease virus C-S30 using surface plasmon resonance: a role for kinetic rate constants

A foot-and-mouth disease virus (FMDV) field variant, isolate C-S30 (also named C(1)-Barcelona), is known to contain four changes within the main antigenic site A (GH loop of capsid protein VP1, residues 136-150), at least one of which (Leu147-->Val) involves a highly conserved position, critical for antibody recognition in the reference strain C-S8c1. However, immunoenzymatic analysis of FMDV C-S30 showed it was recognised by 4C4, a monoclonal antibody that specifically targets site A. This remarkable behaviour has led us to analyse the individual and combined contributions of the four mutations to the antigenicity of C-S30, by surface plasmon resonance (SPR) and enzyme-linked immunosorbent assay (ELISA) studies of pentadecapeptides displaying all possible combinations of the four replacements. Analysis of this family of C-S30-derived analogues shows a certain level of antibody recognition by SPR. In addition, SPR data suggest that kinetic rate constants provide an indirect measure, on the one hand, of paratope accessibility (association rate constant) and, on the other hand, of peptide fitness to the same paratope (dissociation rate constant).

Direct single-step surface plasmon resonance analysis of interactions between small peptides and immobilized monoclonal antibodies

Surface plasmon resonance (SPR) methods have been optimized to permit direct kinetic analysis of the antigenic peptide analytes interacting with immobilized monoclonal antibodies (mAbs). High reproducibility and a significant correlation between SPR and previous ELISA data on the same set of antibodies and peptides were observed. The kinetic data obtained provide further insight into the structure of the main antigenic site of foot-and-mouth disease virus (FMDV).

Cell recognition by foot-and-mouth disease virus that lacks the RGD integrin-binding motif: flexibility in aphthovirus receptor usage

Cell surface molecules that can act as virus receptors may exert an important selective pressure on RNA viral quasispecies. Large population passages of foot-and-mouth disease virus (FMDV) in cell culture select for mutant viruses that render dispensable a highly conserved Arg-Gly-Asp (RGD) motif responsible for integrin receptor recognition. Here, we provide evidence that viability of recombinant FMDVs including a Asp-143-->Gly change at the RGD motif was conditioned by a number of capsid substitutions selected upon FMDV evolution in cell culture. Multiply passaged FMDVs acquired the ability to infect human K-562 cells, which do not express integrin alpha(v)beta(3). In contrast to previously described cell culture-adapted FMDVs, the RGD-independent infection did not require binding to the surface glycosaminoglycan heparan sulfate (HS). Viruses which do not bind HS and lack the RGD integrin-binding motif replicate efficiently in BHK-21 cells. Interestingly, FMDV mutants selected from the quasispecies for the inability to bind heparin regained sensitivity to inhibition by a synthetic peptide that represents the G-H loop of VP1. Thus, a single amino acid replacement leading to loss of HS recognition can shift preferential receptor usage of FMDV from HS to integrin. These results indicate at least three different mechanisms for cell recognition by FMDV and suggest a potential for this virus to use multiple, alternative receptors for entry even into the same cell type.

Native-like cyclic peptide models of a viral antigenic site: finding a balance between rigidity and flexibility

Antigenic site A of foot-and-mouth disease virus (serotype C) has been reproduced by means of cyclic versions of peptide A15, YTASARGDLAHLTTT, corresponding to residues 136-150 of envelope protein VP1. A structural basis for the design of the cyclic peptides is provided by crystallographic data from complexes between the Fab fragments of anti-site A monoclonal antibodies and A15, in which the bound peptide is folded into a quasi-cyclic pattern. Head-to-tail cyclizations of A15 do not provide peptides of superior antigenicity. Internal disulfide cyclization, however, leads to analogs which are recognized as one to two orders of magnitude better than linear A15 in both ELISA and biosensor experiments. CD and NMR studies show that the best antigen, CTASARGDLAHLTT-Ahx-C (disulfide), is very insensitive to environment-induced conformational change, suggesting that cyclization helps to stabilize a bioactive-like structure.

Surface plasmon resonance screening of synthetic peptides mimicking the immunodominant region of C-S8c1 foot-and-mouth disease virus

The main antigenic site (site A) of foot-and-mouth disease virus (FMDV, strain C-S8c1) may be adequately reproduced by a 15-peptide with the amino acid sequence H-YTASARGDLAHLTTT-NH(2) (A15), corresponding to the residues 136-150 of the viral protein VP1. The effect of amino acid substitutions within A15 on its antigenicity towards monoclonal antibodies (MAb) raised against antigenic site A, has been studied by means of BIAcore technology, based on surface plasmon resonance (SPR). Although these antigenicities have previously been determined from enzyme-linked immunosorbent assays (ELISA), the SPR-based technique is superior in that it allows a fast and straightforward screening of antigens while simultaneously providing kinetic data of the antigen-antibody interaction. With a view to screening fairly large libraries of individual peptides, we have inverted the typical SPR experiment by immobilizing the MAb on the sensor surface and using peptides as soluble analytes. We report the validation of this approach through the screening of 44 site A peptides, with results generally in good agreement with the relative antigenicities previously determined by competition ELISA.

A comparative study of cyclization strategies applied to the synthesis of head-to-tail cyclic analogs of a viral epitope

A family of head-to-tail cyclic peptide models of the antigenic site A (G-H loop of viral protein 1) of foot-and-mouth disease virus has been designed on the basis of the three-dimensional structure adopted by the linear peptide YTASARGDLAHLTTT upon binding to neutralizing monoclonal antibodies. Three different methods of cyclization have been examined to access the peptides. Solution cyclization of a minimally protected linear precursor provided the expected products but required several purification steps that lowered the yields to approximately 10%. The two other approaches relied on side-chain anchoring of the peptide through the Asp residue and cyclization on the solid phase. A synthetic scheme combining Fmoc, tBu and OAI protections was practicable but inefficient when scaled-up. The combination of Boc, Bzl and OFm protections was more promising, but suffered from high epimerization during the initial esterification of Boc-Asp-OFm to benzyl alcohol-type resins. This problem was solved by performing the esterification via the cesium salt of Boc-Asp-OFm. With this improvement, the Boc/Bzl/OFm has become the method of choice for the preparation of cyclic head-to-tail peptides in satisfactory yields and with minimal purification.

A large-scale evaluation of peptide vaccines against foot-and-mouth disease: lack of solid protection in cattle and isolation of escape mutants

A large-scale vaccination experiment involving a total of 138 cattle was carried out to evaluate the potential of synthetic peptides as vaccines against foot-and-mouth disease. Four types of peptides representing sequences of foot-and-mouth disease virus (FMDV) C3 Argentina 85 were tested: A, which includes the G-H loop of capsid protein VP1 (site A); AT, in which a T-cell epitope has been added to site A; AC, composed of site A and the carboxy-terminal region of VP1 (site C); and ACT, in which the three previous capsid motifs are colinearly represented. Induction of neutralizing antibodies, lymphoproliferation in response to viral antigens, and protection against challenge with homologous infectious virus were examined. None of the tested peptides, at several doses and vaccination schedules, afforded protection above 40%. Protection showed limited correlation with serum neutralization activity and lymphoproliferation in response to whole virus. In 12 of 29 lesions from vaccinated cattle that were challenged with homologous virus, mutant FMDVs with amino acid substitutions at antigenic site A were identified. This finding suggests the rapid generation and selection of FMDV antigenic variants in vivo. In contrast with previous studies, this large-scale vaccination experiment with an important FMDV host reveals considerable difficulties for vaccines based on synthetic peptides to achieve the required levels of efficacy. Possible modifications of the vaccine formulations to increase protective activity are discussed.

Conformational studies of a short linear peptide corresponding to a major conserved neutralizing epitope of human respiratory syncytial virus fusion glycoprotein

The conformational properties of a 21-residue peptide, corresponding to amino acids 255 to 275 (F255-275) of the human respiratory syncytial virus fusion (F) glycoprotein have been studied by CD and nmr spectroscopy. This peptide includes residues 262, 268, and 272 of the F polypeptide that are essential for integrity of most epitopes that mapped into a major antigenic site of the F molecule. CD data indicate that F255-275 adopts a random coil conformation in aqueous solution at low peptide concentrations. However, as the concentration of peptide is increased, a higher percentage of peptide molecules adopts an organized structure. This effect can be more easily observed when trifluoroethanol (30%) is added to peptide solutions, giving rise to CD spectra that resemble those of alpha-helix structures. These conformational changes were confirmed by nmr spectroscopy. The nuclear Overhauser effects observed in 30% trifluoroethanol/ water together with the conformational H alpha chemical shift data allowed us to propose a structural model of helix-loop-helix for the peptide in solution. In addition, these helical regions contain the amino acid residues essential for epitope integrity in the native F molecule. These results give new insights into the antigenic structure of the respiratory syncytical virus F glycoprotein.

Emerging foot-and-mouth disease virus variants with antigenically critical amino acid substitutions predicted by model studies using reference viruses

One of the major obstacles to the design of effective antiviral vaccines is the frequent generation of antigenic viral variants in the field. The types of variants that will become dominant during disease outbreaks is often unpredictable. However, here we report the genetic and antigenic characterization of emerging foot-and-mouth disease virus (FMDV) variants with antigenically critical amino acid substitutions predicted by model studies using reference viruses and monoclonal antibodies. The new variants belong to serotype C and have caused a number of recent disease outbreaks in Argentina. The variants harbor antigenically drastic amino acid substitutions in each of the antigenic sites identified in FMDV. In particular, a substitution found at a major antigenic site (site A, the G-H loop of VP1) had been repeatedly selected in viruses resistant to neutralization by monoclonal and polyclonal antibodies. The association of critical amino acid replacements at predicted positions with new FMD outbreaks has a number of implications for FMD epidemiology and for the design of vaccines intended to control diseases caused by highly variable RNA viruses.

Antibody recognition of picornaviruses and escape from neutralization: a structural view

Escape of picornaviruses from neutralization by monoclonal antibodies is mediated by substitutions of very few, defined amino acid residues of the capsid, generally located on the tip of some surface-exposed loops. Substitutions at the same positions are possibly of major relevance to antigenic variation of picornaviruses in the field. Such residues tend to cluster in discrete areas, termed antigenic sites. The structure of virus-antibody and peptide-antibody complexes, determined by cryoelectron microscopy and X-ray crystallography, combined with studies using site-directed mutagenesis, are beginning to reveal new features of picornavirus epitopes. This information complements and expands the view on picornavirus antigenicity previously provided by analyses of antibody-escape mutants. In addition to amino acids found replaced in escape mutants, other surface residues which remain invariant in spite of immune pressure also participate in contacts with the antibody molecule. Some invariant residues are even critical for the antigen-antibody interaction. Escape mutations occur at the subset of antigenically critical residues which are tolerant to change because they are not essentially involved in capsid structure or function. Restrictions to variation differ among epitopes; this may contribute to explain the different number of serotypes among picornaviruses, and the frequency at which antigenically highly divergent variants occur in the field.

Molecular evolution of aphthoviruses

Aphthoviruses are an important group of animal pathogens. A combination of genetic and structural studies has revealed one of the main principles governing their evolution: severe limitations to variation imposed by functional and structural constraints, in conjunction with high mutation and recombination rates operating during genome replication. Evolution occurs by positive selection and random drift acting on complex quasispecies distributions. The mutant composition of a quasi-species (or mutant spectrum) is largely dictated by tolerance to nucleotide and amino acid substitutions in viral RNAs and proteins, which must remain functionally competent. We review recent evidence to support this proposal, and we suggest that similar concepts may apply to other RNA viruses as well.

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

The immunodominance of a major antigenic site of foot-to-mouth disease virus (FMDV) (serotype C; clone C-S8c1) in a natural host has been evaluated by serum immunoglobulin fractionation. Nineteen sera from either convalescent or vaccinated swine were fractionated by affinity chromatography using a synthetic peptide representing antigenic site A (the G-H loop of capsid protein VP1) coupled to a Sepharose matrix. Antigen-binding and neutralizing activities of serum fractions were quantitated. On average, about 57 or 27% of the virus-neutralizing activity (and about 35 or 12% of the virus-binding activity) from convalescent or vaccinated swine, respectively, corresponded to antibodies against site A. The results provide direct evidence of the important contribution of site A, and also of additional sites unrelated to site A, in the evoking of neutralizing antibodies by FMDV in a natural host. The proportion of antibodies directed to site A varied greatly among individual swine. Some animals evoked remarkably low levels of antibodies specific for site A although they were competent to raise antibodies against other antigenic sites of FMDV. Thus, the major antigenic site of FMDV shows heterogeneous dominance in a natural host. Possible implications for evolution of viral quasispecies are discussed.

A highly divergent antigenic site of foot-and-mouth disease virus retains its immunodominance

The ability of a highly divergent antigenic site of foot-and-mouth disease virus (FMDV) of serotype C to elicit neutralizing antibodies has been evaluated in mice and rabbits. The viruses compared, FMDV C-S8c1 and HR, differ in a single amino acid replacement in their capsid proteins, but represent two extreme antigenic specificities of the major antigenic site A of FMDV type C. Both, studies of cross-neutralization of homologous and heterologous virus, and fractionation of site A-specific antibodies by immunoaffinity chromatography suggest a similar immunodominance of antigenic site A in FMDV C-S8c1 and variant HR. This information is relevant to the formulation of synthetic peptide vaccines that ideally should consist of mixtures of peptides representing several antigenic specificities. These cocktail formulations may be required to control diseases caused by FMDV and, generally, by highly variable RNA viruses, since single specificity peptides may trigger selection of vaccine-escape viral mutants.

Helix formation by the phospholipase A2 38-59 fragment: influence of chain shortening and dimerization monitored by nmr chemical shifts

The solution structure of a peptide fragment corresponding to the 38-59 region of porcine phospholipase A2 has been investigated using CD, nmr chemical shifts, and nuclear overhauser effects (NOEs). This isolated fragment of phospholipase forms an alpha-helix spanning residues 38-55, very similar to the one found in the native protein, except for residues 56-58, which were helical in the crystal but found random in solution. Addition of triflouroethanol (TFE) merely increased helix population but it did not redefine helix limits. To investigate how the folding information, in particular that concerning eventual helix start and stop signals, was coded in this particular amino acid sequence, the helices formed by synthetic peptides reproducing sections of this phospholipase 38-59 fragment, namely 40-59, 42-59, 38-50, and 45-57, were characterized using NOEs and helix populations quantitatively evaluated on different peptide chain segments using nmr chemical shifts in two solvents (H2O and 30% TFE/H2O). A set of nmr spectra was also recorded and assigned under denaturing conditions (6M urea) to obtain reliable values for the chemical shifts of each peptide in the random state. Based on chemical shift data, it was concluded that the helix formed by the phospholipase 38-59 fragment was not abruptly, but progressively, destabilized all along its length by successive elimination of residues at the N end, while the removal of residues at the C end affected helix stability more locally and to a lesser extent. These results are consistent with the idea that there are not single residues responsible for helix initiation or helix stability, and they also evidence an asymmetry for contributions to helix stability by residues located at the two chain ends. The restriction of molecular mobility caused by linking with a disulphide bridge at Cys 51 two identical 38-59 peptide chains did not increase helix stability. The helix formed by the covalently formed homodimer was very similar in length and population to that formed by the monomer.

Crystallization and preliminary X-ray diffraction studies of a monoclonal antibody Fab fragment against foot-and-mouth disease virus and of its complex with the main antigenic site peptide

The Fab fragment of the neutralizing monoclonal antibody SD6 elicited against foot-and-mouth disease virus (FMDV) C-S8c1 and its complex with a peptide, corresponding to the major antigenic site of FMDV (VP1 residues 136-150, YTASARGDLAHLTTT), have been crystallized using the hanging drop vapor diffusion techniques. For the isolated Fab, crystals diffracting to 2.5 A resolution were obtained at room temperature using ammonium sulfate as precipitant. These crystals are monoclinic, space group C2, and unit cell parameters a = 109.53 A, b = 89.12 A, c = 64.04 A, and beta = 112.9 degrees and contain one Fab molecule per asymmetric unit. Crystals from the complex diffract, at least, to 2.8 A resolution and were obtained, at room temperature, using PEG as precipitant. These crystals are monoclinic, space group P2, and unit cell parameters a = 56.11 A, b = 60.67 A, c = 143.45 A, and beta = 95.4 degrees. Density packing considerations indicate that there are two Fab molecules in the asymmetric unit.

Distinct repertoire of antigenic variants of foot-and-mouth disease virus in the presence or absence of immune selection

Antigenic variants of foot-and-mouth disease virus (FMDV) were generated and frequently became dominant in clonal populations of FMDV (clone C-S8c1) grown in the absence of anti-FMDV antibodies. We have now passaged eight samples of the same FMDV clone in the presence of a limited amount of neutralizing polyclonal antibodies directed to the major antigenic site A of capsid protein VP1. Complex populations of variants showing increased resistance to polyclonal sera and to site A-specific monoclonal antibodies were selected. Some populations exhibited marked decreases in viral fitness. Multiple amino acid replacements within site A--and also elsewhere in VP1--accumulated upon passage of the virus in either the absence or the presence of neutralizing antibodies. However, antigenically critical replacements at one position in site A occurred repeatedly in FMDV passaged under antibody selection, but they were never observed in many passages carried out either in the absence of antiviral antibodies or in the presence of an irrelevant antiviral serum. Thus, even though antigenic variation of FMDV can occur in the absence or presence of immune selection, critical replacements which lead to important changes in antigenic specificity were observed only as a result of selection by neutralizing antibodies.

Use of substituted and tandem-repeated peptides to probe the relevance of the highly conserved RGD tripeptide in the immune response against foot-and-mouth disease virus

Antigenic site A of foot-and-mouth disease virus (FMDV) is an exposed, mobile loop which includes a central, highly conserved Arg-Gly-Asp tripeptide (RGD, VP1 residues 141-143 in serotype C) thought to be part of the cell attachment site. We have analyzed the contribution of RGD to the interaction of site A with antibodies by incorporating selected amino acid replacements at RGD into synthetic peptides representing site A, and analyzing the reactivity of substituted peptides with site A-specific monoclonal antibodies (MAbs). Replacement of Arg-141, Gly-142 or Asp-143 by alanine resulted in the loss of one, three and five epitopes, respectively, out of seven epitopes probed. Other replacements resulted in the loss of even larger numbers of epitopes, suggesting that the amino acids of the RGD region are either directly involved in interaction with antibodies or that they exert an important influence on the interaction of surrounding residues with antibodies. Thus, we explored the ability of tandem repeats of the RGDL sequence (corresponding to FMDV C-S8c1) to evoke neutralizing antibodies in rabbits and guinea pigs. Neutralizing activity was generally low but with a broad specificity for different FMDV serotypes and variants. Significant decreases in neutralizing titers were observed with boosting, suggesting a possible suppression of those anti-peptide antibodies which may also be directed to cellular RGD sequences. The results point to an involvement of RGD in the antigenic structure of site A, and open the possibility that broadly neutralizing antibodies might be induced by tandem repeats of the critical, conserved domain.

Cyclic disulfide model of the major antigenic site of serotype-C foot-and-mouth disease virus. Synthetic, conformational and immunochemical studies

A cyclic disulfide peptide representing antigenic site A of foot-and-mouth disease virus (FMDV) strain C-S8c1 (residues 134 to 155 of viral protein 1 (VP1) with Tyr136 and Arg153 replaced by cystine; TTCTASARGDLAHLTTTHACHL) was synthesized by solid phase methods. Formation of the cyclic disulfide was carried out by air oxidation of the fully deprotected and reduced bis-cysteine precursor, under high dilution conditions. The identity of the cyclic peptide was confirmed by both physical and enzymatic methods. A conformational study of the cyclic peptide and of its linear parent structure (YTASARGDLAHLTTTHARHLP, residues 136-156 of VP1 of FMDV C-S8c1) by circular dichroism in the presence of a structure-inducing solvent showed the cyclic disulfide analog to adopt lower levels of alpha-helix than its linear counterpart. In competitive ELISA assays both peptides reacted with similar affinity against a representative panel of neutralizing monoclonal antibodies directed towards antigenic site A. Thus, a high inherent flexibility of this loop may preclude a conformational restriction strong enough to alter recognition by anti-virus antibodies.

Chemically synthesized 182-235 segment of tau protein and analogue peptides are efficient in vitro microtubule assembly inducers of low apparent sequence specificity

A 54-amino acid peptide reproducing the first and second repeats and intervening spacer sequence of the tubulin binding motif (residues 182-235) of murine tau protein, and several congeners representing different degrees of sequence scrambling have been prepared by solid phase methods and fully characterized chemically. These double-repeat peptides have been shown to induce microtubule formation at concentrations about one order of magnitude lower than single-repeat controls, under conditions close to the critical concentration needed for tubulin self-assembly. On the other hand, partial loss of microtubule-inducing capacity was observed for peptides with primary structures increasingly disorganized with respect to the canonical peptide. These results call into question the assumption that a high degree of primary structure specificity is involved in the tau-tubulin interaction leading to in vitro microtubule formation.