Ability of linear and cyclic peptides of neutralization antigenic site 1 of poliovirus type 1 to induce virus cross-reactive and neutralizing antibodies

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.

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.

Localization of a new neutralizing epitope on the mumps virus hemagglutinin-neuraminidase protein

Four protein fragments which span the entire hemagglutinin-neuraminidase protein (HN) of mumps virus were expressed in HeLa cells and cell extracts were tested for their capability to induce neutralizing antibodies in mice. Fragment HN3 (aa 213-372) was able to induce the production of hemagglutination-inhibiting and neutralizing antibodies. When a subfragment of HN3, the synthetic peptide NSTLGVKSAREF (aa 329-340 of HN) was used for immunization, hemagglutination-inhibiting and neutralizing antibodies against mumps wild type virus but not against the Urabe Am9 vaccine virus were raised. The peptide could, therefore, contain a new epitope, which may be critical for protective host humoral immune response.

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.

Molecular design versus empirical discovery in peptide-based vaccines. Coming to terms with fuzzy recognition sites and ill-defined structure-function relationships in immunology

In view of our increased understanding of the molecular basis of immunological recognition, it is commonly believed that it should be possible to apply molecular design strategies to the development of peptide-based vaccines. The stated aim is to transform the development of a vaccine from a trial and error empirical operation into a so-called rational, structure-based process. In the present review, it is argued that it is misleading to oppose rational and empirical approaches in vaccine research since both are needed in the practice of experimental science. Many reasons are given for the view that the molecular design of synthetic vaccines is not a realistic scientific enterprise. The capacity of a peptide to induce a protective immune response depends on many extrinsic factors and regulatory mechanisms of the recipient host which are not amenable to molecular design of the peptide immunogen. It seems safe to predict that the development of peptide-based vaccines will continue to be driven by empirical discovery rather than by so-called rational design.

A hemagglutinin-derived peptide-vaccine ignored by virus-neutralizing passive antibodies, protects against murine measles encephalitis

The neutralizing and protective monoclonal antibody BH47 defines the sequential epitope H236-255 of the measles virus hemagglutinin protein (MV-H). The objective of this study was to design peptides combining this B cell epitope (BCE) with different T cell epitopes (TCE) to obtain protective immunity. Most TTB peptides based on the 15mer BCE H236-250 induced MV-crossreactive antibodies, but only certain TCE induced virus neutralizing antibodies. The shortest BCE required for MV-reactivity and -neutralization was the 8mer H243-250 containing residue R243 implicated in CD46 down-regulation. Sera obtained after immunization with the TTB peptide containing the MV-derived TCE F421-435 protected mice against a lethal challenge with a neuro-adapted MV strain. Our results further demonstrate that this TTB peptide is fully immunogenic, even in the presence of protective levels of pre-existing MV-specific antibodies, suggesting that subunit vaccines based on such peptides could potentially be used to immunize infants in the presence of persisting maternal antibodies. It is therefore interesting that neutralizing antibodies were also obtained with a TTB peptide comprising a human promiscuous TCE (tt830). However, our results also emphasize the need to test sera induced with epitope-based vaccines against different virus strains, in particular if the epitope is not fully conserved.

A cyclic peptide analogue of the loop III region of platelet-derived growth factor-BB is a synthetic antigen for the native protein

We report the synthesis and characterization of a cyclic peptide analogue of the loop III region of platelet-derived growth factor (PDGF) B-chain sequence, cyclo(73Arg-Lys-Ile-Glu-Ile-Val-Arg-Lys-Lys81-Cys), incorporating a C-terminus cysteine residue for the conjugation to a carrier protein. The synthesis involved solid-phase chemistry, utilizing Fmoc-tBu chemistry and acid labile side-chain protecting groups, followed by 'head-to-tail' cyclization using the allyl-protected glutamic acid anchored on its side chain to the solid support with HATU/HOAt as the coupling agent. Conformational differences between the cyclic and its linear counterpart PDGF peptides were determined by circular dichroism measurements in aqueous media. High titre antisera were raised to both cyclic and linear peptide immunogens. Antisera raised to the cyclic peptide cross-reacted with PDGF-BB in both Western blot and ELISA, whereas antisera raised to the linear peptide had no reactivity with PDGF-BB. The cyclic peptide (conformational design analogue) produces an immunogen which is able to antigenically mimic the secondary structure of loop III of PDGF-BB and forms a basis from which further small molecular mimetics of PDGF may be designed for use as both immunogens and also potential agonists/antagonists of PDGF. Similarly constructed immunogens may also be useful in the design of vaccines which direct responses to loop regions in other target proteins.

Structure/antigenicity relationship of cyclic and linear peptides mimicking the V3 loop of HIV2 envelope glycoprotein

We report the structure and antigenicity of the third variable region (V3) of the HIV2 envelope glycoprotein by the use of linear and cyclic peptides. To this end, a peptide mimicking this region was synthesized and purified, both as an iodoacetamidated linear peptide and a disulphide-bridged cyclic peptide. The cross-reactivity of three monoclonal antibodies (mAbs) produced against the envelope glycoprotein gp140 with the linear and cyclic peptides was tested with ELISA. The results showed that the cyclic peptide is a better ligand for the 3 mAbs 125-F, 125-J and 125-K. The avidity of the mAb/peptide interaction was further analysed by determining the concentration of linear or cyclic peptide leading to 50% inhibition of mAb-peptide complex formation (K0.5). The K0.5 value of mAb 125-F, which displayed the best reactivity with gp140, was estimated to be 5 times higher for the linear (K0.5 = 1.5 x 10(-6) M) than for the cyclic peptide (K0.5 = 3 x 10(-7) M). This indicates a higher affinity of mAb 125-F for the cyclic peptide. mAb 125-J, which exhibited a lower avidity for the gp140 compared to mAb 125-F, had a similar affinity for the cyclic and the linear peptides (K0.5 = 3 x 10(-7) M). mAb 125-K had the lowest reactivity with gp140 and its binding to adsorbed peptide could not be inhibited by the soluble linear or cyclic peptide used up to 10(-5) M. These results suggest that cyclic peptides may have a higher propensity for adopting a native-like structure for the peptide/antibody interaction. Nuclear magnetic resonance experiments at 25 degrees C in phosphate buffer pH 5.4, however, showed that neither peptide displayed a well-defined structure.

The role of structure in antibody cross-reactivity between peptides and folded proteins

Peptides have the potential for targeting vaccines against pre-specified epitopes on folded proteins. When polyclonal antibodies against native proteins are used to screen peptide libraries, most of the peptides isolated align to linear epitopes on the proteins. The mechanism of cross-reactivity is unclear; both structural mimicry by the peptide and induced fit of the epitope may occur. The most effective peptide mimics of protein epitopes are likely to be those that best mimic both the chemistry and the structure of epitopes. Our goal in this work has been to establish a strategy for characterizing epitopes on a folded protein that are candidates for structural mimicry by peptides. We investigated the chemical and structural bases of peptide-protein cross-reactivity using phage-displayed peptide libraries in combination with computational structural analysis. Polyclonal antibodies against the well-characterized antigens, hen eggwhite lysozyme and worm myohemerythrin, were used to screen a panel of phage-displayed peptide libraries. Most of the selected peptide sequences aligned to linear epitopes on the corresponding protein; the critical binding sequence of each epitope was revealed from these alignments. The structures of the critical sequences as they occur in other non-homologous proteins were analyzed using the Sequery and Superpositional Structural Assignment computer programs. These allowed us to evaluate the extent of conformational preference inherent in each sequence independent of its protein context, and thus to predict the peptides most likely to have structural preferences that match their protein epitopes. Evidence for sequences having a clear structural bias emerged for several epitopes, and synthetic peptides representing three of these epitopes bound antibody with sub-micromolar affinities. The strong preference for a type II beta-turn predicted for one peptide was confirmed by NMR and circular dichroism analyses. Our strategy for identifying conformationally biased epitope sequences provides a new approach to the design of epitope-targeted, peptide-based vaccines.