Toward a better understanding of the basis of the molecular mimicry of polysaccharide antigens by peptides: the example of Shigella flexneri 5a

Extract-Shaped Immune Repertoires as Source for Nanobody-Based Human IgE in Grass Pollen Allergy

The presence of allergen-specific IgE in serum is a biomarker for allergic disease. Specific IgE antibodies for research and diagnostics, however, remain scarce. In contrast to prototypic antibodies, camelid species have evolved single domains as moiety for antigen recognition. These so-called nanobodies represent a versatile platform for the development of diagnostic and therapeutic approaches. In this study, we aimed for generating nanobodies and derived IgE formats from an extract-shaped immune repertoire. Timothy grass pollen represents a complex, but well-defined mixture of individual allergens. Therefore, a repertoire library from a timothy grass pollen extract immunised llama was established. The selection by phage display yielded 3 nanobodies with immunoreactivity to the extract. IgE-like nanobody-based human IgE (nb-hIgE) antibodies were produced in mammalian cells and assessed in different immunoassays and commercial platforms. Immunoblotting and diagnostic ImmunoCap analysis of single timothy grass pollen allergens identified the major allergens Phl p 6 and Phl p 4 as targets. Assessment of immunoreactivity further documented significant molecular cross-reactivity with pollen extract of different grass species and variant presence of allergens within extracts of Pooideae grasses. In summary, our study shows that extract-based immunisation enables the generation of allergen-specific nanobodies and derived nb-hIgE formats linking nanobody technologies with allergological applications.

Angiotensin and Endothelin Receptor Structures With Implications for Signaling Regulation and Pharmacological Targeting

In conjunction with the endothelin (ET) type A (ETAR) and type B (ETBR) receptors, angiotensin (AT) type 1 (AT1R) and type 2 (AT2R) receptors, are peptide-binding class A G-protein-coupled receptors (GPCRs) acting in a physiologically overlapping context. Angiotensin receptors (ATRs) are involved in regulating cell proliferation, as well as cardiovascular, renal, neurological, and endothelial functions. They are important therapeutic targets for several diseases or pathological conditions, such as hypertrophy, vascular inflammation, atherosclerosis, angiogenesis, and cancer. Endothelin receptors (ETRs) are expressed primarily in blood vessels, but also in the central nervous system or epithelial cells. They regulate blood pressure and cardiovascular homeostasis. Pathogenic conditions associated with ETR dysfunctions include cancer and pulmonary hypertension. While both receptor groups are activated by their respective peptide agonists, pathogenic autoantibodies (auto-Abs) can also activate the AT1R and ETAR accompanied by respective clinical conditions. To date, the exact mechanisms and differences in binding and receptor-activation mediated by auto-Abs as opposed to endogenous ligands are not well understood. Further, several questions regarding signaling regulation in these receptors remain open. In the last decade, several receptor structures in the apo- and ligand-bound states were determined with protein X-ray crystallography using conventional synchrotrons or X-ray Free-Electron Lasers (XFEL). These inactive and active complexes provide detailed information on ligand binding, signal induction or inhibition, as well as signal transduction, which is fundamental for understanding properties of different activity states. They are also supportive in the development of pharmacological strategies against dysfunctions at the receptors or in the associated signaling axis. Here, we summarize current structural information for the AT1R, AT2R, and ETBR to provide an improved molecular understanding.

Molecular Effects of Auto-Antibodies on Angiotensin II Type 1 Receptor Signaling and Cell Proliferation

The angiotensin II (Ang II) type 1 receptor (AT1R) is involved in the regulation of blood pressure (through vasoconstriction) and water and ion homeostasis (mediated by interaction with the endogenous agonist). AT1R can also be activated by auto-antibodies (AT1R-Abs), which are associated with manifold diseases, such as obliterative vasculopathy, preeclampsia and systemic sclerosis. Knowledge of the molecular mechanisms related to AT1R-Abs binding and associated signaling cascade (dys-)regulation remains fragmentary. The goal of this study was, therefore, to investigate details of the effects of AT1R-Abs on G-protein signaling and subsequent cell proliferation, as well as the putative contribution of the three extracellular receptor loops (ELs) to Abs-AT1R signaling. AT1R-Abs induced nuclear factor of activated T-cells (NFAT) signaling, which reflects Gq/11 and Gi activation. The impact on cell proliferation was tested in different cell systems, as well as activation-triggered receptor internalization. Blockwise alanine substitutions were designed to potentially investigate the role of ELs in AT1R-Abs-mediated effects. First, we demonstrate that Ang II-mediated internalization of AT1R is impeded by binding of AT1R-Abs. Secondly, exclusive AT1R-Abs-induced Gq/11 activation is most significant for NFAT stimulation and mediates cell proliferation. Interestingly, our studies also reveal that ligand-independent, baseline AT1R activation of Gi signaling has, in turn, a negative effect on cell proliferation. Indeed, inhibition of Gi basal activity potentiates proliferation triggered by AT1R-Abs. Finally, although AT1R containing EL1 and EL3 blockwise alanine mutations were not expressed on the human embryonic kidney293T (HEK293T) cell surface, we at least confirmed that parts of EL2 are involved in interactions between AT1R and Abs. This current study thus provides extended insights into the molecular action of AT1R-Abs and associated mechanisms of interrelated pathogenesis.

Structure of intact IgE and the mechanism of ligelizumab revealed by electron microscopy

BACKGROUND

IgE is the central antibody isotype in TH2-biased immunity and allergic diseases. The structure of intact IgE and the impact of IgE-targeting molecules on IgE however remain elusive. In order to obtain insights into IgE biology and the clinical impact, we aimed for structure determination of IgE and the complex of IgE with the anti-IgE antibody ligelizumab.

METHODS

Structures of two distinct intact IgE with specificity for cross-reactive carbohydrate determinants and Der p 2 as well as complexes of ligelizumab-Fab with IgE and IgE Fc were assessed by negative stain electron microscopy and solution scattering. Inhibition of IgE binding and displacement of receptor-bound IgE were assessed using cellular assays, basophil activation testing and ELIFAB assays.

RESULTS

Our data reveal that the investigated IgE molecules share an overall rigid conformation. In contrast to the IgE Fc fragment, the IgE Fc in intact IgE is significantly less asymmetrically bent. The proximal and the distal Fabs are rigidly tethered to the Fc. Binding of ligelizumab to IgE in a 2:1 stoichiometry induces an extended and twofold symmetrical conformation of IgE, which retains a rigid Fab-Fc architecture. Analyses of effector cell activation revealed that ligelizumab inhibits IgE binding without displacing receptor-bound IgE. Together with an interference of CD23 binding, the data underline a functional activity similar to omalizumab.

CONCLUSIONS

Our data reveal the first structures of intact IgE suggesting that the IgE Fab is fixed relative to the Fc. Furthermore, we provide a structural rationale for the inhibitory mechanism of ligelizumab.

Trapping IgE in a closed conformation by mimicking CD23 binding prevents and disrupts FcεRI interaction

Anti-IgE therapeutics interfere with the ability of IgE to bind to its receptors on effector cells. Here we report the crystal structure of an anti-IgE single-domain antibody in complex with an IgE Fc fragment, revealing how the antibody inhibits interactions between IgE and the two receptors FcεRI and CD23. The epitope overlaps only slightly with the FcεRI-binding site but significantly with the CD23-binding site. Solution scattering studies of the IgE Fc reveal that antibody binding induces a half-bent conformation in between the well-known bent and extended IgE Fc conformations. The antibody acts as functional homolog of CD23 and induces a closed conformation of IgE Fc incompatible with FcεRI binding. Notably the antibody displaces IgE from both CD23 and FcεRI, and abrogates allergen-mediated basophil activation and facilitated allergen binding. The inhibitory mechanism might facilitate strategies for the future development of anti-IgE therapeutics for treatment of allergic diseases.

IgE is linked to allergic diseases and there is a great interest in developing anti-IgE therapeutics. Here the authors characterize the binding of human IgE Fc to a single domain antibody (sdab) and show that the sdab induces a closed conformation, which prevents and disrupts IgE binding to its receptor FcεRI and abrogates allergen mediated activation.

Immunogenic peptide mimotopes from an epitope of Escherichia coli O157 LPS

Escherichia coli O157:H7 is a subtype of Shiga toxin-producing E. coli that is associated with haemorrhagic colitis and haemolytic uremic syndrome (HUS). Studies of populations in endemic areas have reported that the presence of specific antibodies against the O157 lipopolysaccharide (LPS) is associated with a lower incidence of diarrhoea and HUS. Phage display and IgG anti-O157 LPS antibodies were used in the present study to select peptide mimotopes of O157 LPS expressed in protein III of the M13 phage. Synthetic peptides (SP) were designed using the derived amino acid sequences obtained from DNA nucleotides of 63 selected phagotopes. The LxP/YP/SxL motif was identified in five of the phagotope amino acid sequences. Antibody responses against the phagotopes and their corresponding SPs were evaluated. SP12, one of the designed SP, induced the production of antibodies against the homologous peptide (1:800) and O157 LPS (1:200). The specificity of anti-SP12 antiserum was confirmed by analyzing its response to SP3, an SP with a different amino acid sequence than that of SP12, as well as against an E. coli LPS different from O157. Competitive studies with SP12 and O157 LPS showed a significant decrease in anti-SP12 and anti-LPS O157 antiserum responses against SP12 and O157 LPS, respectively. Eighteen (82%) of the 22 human serum samples with positive reactivity against E coli O157 LPS reacted with SP12 SP (cut-off >0.4). These results support the idea that SP12 is an immunogenic mimotope of O157 LPS.

Non-stoichiometric O-acetylation of Shigella flexneri 2a O-specific polysaccharide: synthesis and antigenicity

Synthetic functional mimics of the O-antigen from Shigella flexneri 2a are seen as promising vaccine components against endemic shigellosis. Herein, the influence of the polysaccharide non-stoichiometric di-O-acetylation on antigenicity is addressed for the first time. Three decasaccharides, representing relevant internal mono- and di-O-acetylation profiles of the O-antigen, were synthesized from a pivotal protected decasaccharide designed to tailor late stage site-selective O-acetylation. The latter was obtained via a convergent route involving the imidate glycosylation chemistry. Binding studies to five protective mIgGs showed that none of the acetates adds significantly to broad antibody recognition. Yet, one of the five antibodies had a unique pattern of binding. With IC50 in the micromolar to submicromolar range mIgG F22-4 exemplifies a remarkable tight binding antibody against diversely O-acetylated and non-O-acetylated fragments of a neutral polysaccharide of medical importance.

Burkholderia pseudomallei Capsular Polysaccharide Recognition by a Monoclonal Antibody Reveals Key Details toward a Biodefense Vaccine and Diagnostics against Melioidosis

Burkholderia pseudomallei is the bacterium responsible for melioidosis, an infectious disease with high mortality rates. Since melioidosis is a significant public health concern in endemic regions and the organism is currently classified as a potential biothreat agent, the development of effective vaccines and rapid diagnostics is a priority. The capsular polysaccharide (CPS) expressed by B. pseudomallei is a highly conserved virulence factor and a protective antigen. Because of this, CPS is considered an attractive antigen for use in the development of both vaccines and diagnostics. In the present study, we describe the interactions of CPS with the murine monoclonal antibody (mAb) 4C4 using a multidisciplinary approach including organic synthesis, molecular biology techniques, surface plasmon resonance, and nuclear magnetic spectroscopy. Using these methods, we determined the mode of binding between mAb 4C4 and native CPS or ad hoc synthesized capsular polysaccharide fragments. Interestingly, we demonstrated that the O-acetyl moiety of CPS is essential for the interaction of the CPS epitope with mAb 4C4. Collectively, our results provide important insights into the structural features of B. pseudomallei CPS that enable antibody recognition that may help the rational design of CPS-based vaccine candidates. In addition, our findings confirm that the mAb 4C4 is suitable for use in an antibody-based detection assay for diagnosis of B. pseudomallei infections.

Chemical biology approaches to designing defined carbohydrate vaccines

Carbohydrate antigens have shown promise as important targets for developing effective vaccines and pathogen detection strategies. Modifying purified microbial glycans through synthetic routes or completely synthesizing antigenic motifs are attractive options to advance carbohydrate vaccine development. However, limited knowledge on structure-property correlates hampers the discovery of immunoprotective carbohydrate epitopes. Recent advancements in tools for glycan modification, high-throughput screening of biological samples, and 3D structural analysis may facilitate antigen discovery process. This review focuses on advances that accelerate carbohydrate-based vaccine development and various technologies that are driving these efforts. Herein we provide a critical overview of approaches and resources available for rational design of better carbohydrate antigens. Structurally defined and fully synthetic oligosaccharides, designed based on molecular understanding of antigen-antibody interactions, offer a promising alternative for developing future carbohydrate vaccines.

Carbohydrate-mimetic peptides for pan anti-tumor responses

Molecular mimicry is fundamental to biology and transcends to many disciplines ranging from immune pathology to drug design. Structural characterization of molecular partners has provided insight into the origins and relative importance of complementarity in mimicry. Chemical complementarity is easy to understand; amino acid sequence similarity between peptides, for example, can lead to cross-reactivity triggering similar reactivity from their cognate receptors. However, conformational complementarity is difficult to decipher. Molecular mimicry of carbohydrates by peptides is often considered one of those. Extensive studies of innate and adaptive immune responses suggests the existence of carbohydrate mimicry, but the structural basis for this mimicry yields confounding details; peptides mimicking carbohydrates in some cases fail to exhibit both chemical and conformational mimicry. Deconvolution of these two types of complementarity in mimicry and its relationship to biological function can nevertheless lead to new therapeutics. Here, we discuss our experience examining the immunological aspects and implications of carbohydrate-peptide mimicry. Emphasis is placed on the rationale, the lessons learned from the methodologies to identify mimics, a perspective on the limitations of structural analysis, the biological consequences of mimicking tumor-associated carbohydrate antigens, and the notion of reverse engineering to develop carbohydrate-mimetic peptides in vaccine design strategies to induce responses to glycan antigens expressed on cancer cells.

CGTase-catalysed cis-glucosylation of L-rhamnosides for the preparation of Shigella flexneri 2a and 3a haptens

We report the enzymatic synthesis of α-D-glucopyranosyl-(1→4)-α-L-rhamnopyranoside and α-D-glucopyranosyl-(1→3)-α-L-rhamnopyranoside by using a wild-type transglucosidase in combination with glucoamylase and glucose oxidase. It was shown that Bacillus circulans 251 cyclodextrin glucanotransferase (CGTase, EC 2.1.4.19) can efficiently couple an α-L-rhamnosyl acceptor to a maltodextrin molecule with an α-(1→4) linkage, albeit in mixture with the α-(1→3) regioisomer, thus giving two glucosylated acceptors in a single reaction. Optimisation of the CGTase coupling reaction with β-cyclodextrin as the donor substrate and methyl or allyl α-L-rhamnopyranoside as acceptors resulted in good conversion yields (42-70%) with adjustable glycosylation regioselectivity. Moreover, the efficient chemical conversion of the products of CGTase-mediated cis-glucosylation into protected building blocks (previously used in the synthesis of O-antigen fragments of several Shigella flexneri serotypes) was substantiated. These novel chemoenzymatic strategies towards useful, convenient intermediates in the synthesis of S. flexneri serotypes 2a and 3a oligosaccharides might find applications in developments towards synthetic carbohydrate-based vaccine candidates against bacillary dysentery.

Close-up of the immunogenic α1,3-galactose epitope as defined by a monoclonal chimeric immunoglobulin E and human serum using saturation transfer difference (STD) NMR

Anaphylaxis mediated by carbohydrate structures is a controversially discussed phenomenon. Nevertheless, IgE with specificity for the xenotransplantation antigen α1,3-Gal (α-Gal) are associated with a delayed type of anaphylaxis, providing evidence for the clinical relevance of carbohydrate epitopes in allergy. The aim of this study was to dissect immunoreactivity, interaction, and fine epitope of α-Gal-specific antibodies to obtain insights into the recognition of carbohydrate epitopes by IgE antibodies and their consequences on a molecular and cellular level. The antigen binding moiety of an α-Gal-specific murine IgM antibody was employed to construct chimeric IgE and IgG antibodies. Reactivity and specificity of the resulting antibodies were assessed by means of ELISA and receptor binding studies. Using defined carbohydrates, interaction of the IgE and human serum was assessed by mediator release assays, surface plasmon resonance (SPR), and saturation transfer difference NMR analyses. The α-Gal-specific chimeric IgE and IgG antibodies were proven functional regarding interaction with antigen and Fc receptors. SPR measurements demonstrated affinities in the micromolar range. In contrast to a reference antibody, anti-Gal IgE did not induce mediator release, potentially reflecting the delayed type of anaphylaxis. The α1,3-Gal epitope fine structures of both the recombinant IgE and affinity-purified serum were defined by saturation transfer difference NMR, revealing similar contributions of carbohydrate residues and participation of both galactose residues in interaction. The antibodies generated here constitute the principle underlying α1,3-Gal-mediated anaphylaxis. The complementary data of affinity and fine specificity may help to elucidate the recognition of carbohydrates by the adaptive immune response and the molecular requirements of carbohydrate-based anaphylaxis.

Immunization with an anti-idiotypic antibody against the broadly lipopolysaccharide-reactive antibody WN1 222-5 induces Escherichia coli R3-core-type specific antibodies in rabbits

The mouse monoclonal antibody (mAb) WN1 222-5 recognizes a carbohydrate epitope in the inner core region of LPS that is shared by all strains of Escherichia coli and Salmonella enterica and is able to neutralize their endotoxic activity in vitro and in vivo. Immunization of mice with mAb WN1 222-5 yielded several anti-idiotypic mAbs one of which (mAb S81-19) competitively inhibited binding of mAb WN1 222-5 to E. coli and Salmonella LPS. After immunization of rabbits with mAb S81-19, the serological responses towards LPS were characterized at intervals over two years. Whereas the serological response against the anti-idiotype developed as expected, the anti-anti-idiotypic response against LPS developed slowly and antibodies appeared after 200 d that bound to E. coli LPS of the R3 core-type and neutralized its TNF-α inducing capacity for human peripheral mononuclear cells. We describe the generation of a novel anti-idiotypic antibody that can induce LPS core-reactive antibodies upon immunization in rabbits and show that it is possible, in principle, to obtain LPS neutralizing antibodies by anti-idiotypic immunization against the mAb WN1 222-5. The mimicked epitope likely shares common determinants with the WN1 222-5 epitope, yet differences with respect to either affinity or specificity do exist, as binding to smaller oligosaccharides of the inner core was not observed.

Peptide inhibitors of xenoreactive antibodies mimic the interaction profile of the native carbohydrate antigens

Carbohydrate-antibody interactions mediate many cellular processes and immune responses. Carbohydrates expressed on the surface of cells serve as recognition elements for particular cell types, for example, in the ABO(H) blood group system. Antibodies that recognize host-incompatible ABO(H) system antigens exist in the bloodstream of all individuals (except AB individuals), preventing blood transfusion and organ transplantation between incompatible donors and recipients. A similar barrier exists for cross-species transplantation (xenotransplantation), in particular for pig-to-human transplantation. All humans express antibodies against the major carbohydrate xenoantigen, Galalpha (1,3)Gal (alphaGal), preventing successful xenotransplantation. Although antibody binding sites are precisely organized so as to selectively bind a specific antigen, many antibodies recognize molecules other than their native antigen. A range of peptides have been identified that can mimic carbohydrates and inhibit anti-alphaGal antibodies. However, the structural basis of how the peptides achieved this was not known. Previously, we developed an in silico method which we used to investigate carbohydrate recognition by a panel of anti-alphaGal antibodies. The method involves molecular docking of carbohydrates to antibodies and uses the docked carbohydrate poses to generate maps of th antibody binding sites in terms of prevalent hydrogen bonding and van der Waals interactions. We have applied this method to investigate peptide recognition by the anti-alphaGal antibodies. It was found that the site maps of the peptides and the carbohydrates were similar, indicating that the peptides interact with the same residues as those involved in carbohydrate recognition. This study demonstrates the potential for "design by mapping" of anti-carbohydrate antibody inhibitors.

NMR characterization and molecular modeling of fucoidan showing the importance of oligosaccharide branching in its anticomplementary activity

Fucoidan is a potent inhibitor of the human complement system whose activity is mediated through interactions with certain proteins belonging to the classical pathway, particularly the protein C4. Branched fucoidan oligosaccharides displayed a higher anticomplementary activity as compared to linear structures. Nuclear magnetic resonance (NMR) characterization of the branched oligosaccharides and saturation transfer difference-NMR experiment of the interaction with the protein C4 allowed the identification of the glycan residues in close contact with the target protein. Transferred nuclear Overhauser effect spectroscopy experiment and molecular modeling of fucoidan oligosaccharides indicated that the presence of side chains reduces the flexibility of the oligosaccharide backbone, which thus adopts a conformation which is very close to the one recognized by the protein C4. Together, these results suggest that branching of fucoidan oligosaccharides, determining their conformational state, has a major impact on their anticomplementary activity.

Inactivated and subunit vaccines to prevent shigellosis

Shigellosis remains a formidable disease globally, with children of the developing world bearing the greatest number of infections. The need for an affordable, safe and efficacious vaccine has persisted for decades. Vaccines to prevent shigellosis can be divided into living and nonliving approaches. Several nonliving Shigella vaccines are currently at different stages of development and show substantial promise. Outlined here is an overview of multiple nonliving vaccine technologies, highlighting their current status and recent advances in testing. In addition, gaps in the knowledge base regarding immune mechanisms of protection are explored.

Vaccination against shigellosis: is it the path that is difficult or is it the difficult that is the path?

Following several decades of research, there is not yet a convincing vaccine against shigellosis. It is still difficult, in spite of the breadth of strategies (i.e. live attenuated oral, killed oral, subunit parenteral) to select an optimal option. Two approaches are clearly emerging: (i) live attenuated deletion mutants based on rational selection of genes that are key in the pathogenic process, and (ii) conjugated detoxified polysaccharide parenteral vaccines, or more recently conjugated synthetic carbohydrates. Some of these approaches have already undergone phase I and II clinical trials with promising results, but important issues have also emerged, particularly the discrepancy between colonization and immunogenic potential of live attenuated vaccine candidates depending upon the population concerned (i.e. non endemic vs. endemic areas). Efforts are needed to definitely establish the proof of concept of these approaches, and thus the need for clinical trials which should also soon explore the possibility to associate different serotypes, in response to serotype specific protection against shigellosis. More basic research is also required to improve what we can still consider as first-generation vaccines, and to explore possible new paradigms including the search for cross-protective antigens.

Immunological evidence for functional rather than structural mimicry by a Shigella flexneri Y polysaccharide-mimetic peptide

An approach to vaccine design is the use of molecules that mimic the immunogenic element of interest. In this context, the interaction of MDWNMHAA, a peptide mimic of the Shigella flexneri Y O polysaccharide (PS), with an anti-carbohydrate monoclonal antibody, as studied previously by X-ray crystallography, suggested the presence of functional rather than structural mimicry and a bound peptide conformation that was not represented significantly in the free-ligand ensemble. The antibody response elicited by an MDWNMHAA-carrier protein (tetanus toxoid [TT]) conjugate has now been investigated in BALB/c mice. The mice were immunized following a homologous prime/boost strategy using MDWNMHAA-TT as the immunogen. The mice showed anti-peptide antibody (immunoglobulin G [IgG]) titers that increased after being boosted. High anti-lipopolysaccharide (LPS) (IgG) titers were observed after the last boost. A faster immune response, with cross-reactive titers, was observed with a peptide conjugate with 30% more copies of the peptide. The binding of anti-peptide polyclonal antibodies to LPS could be inhibited by LPS, PS, MDWNMHAA, and MDWNMHAA-bovine serum albumin, as assessed by inhibition enzyme-linked immunosorbent assay. Conversely, mice immunized with PS-TT showed IgG anti-peptide titers. These data demonstrate the cross-reactivity of the antibody response and support the hypothesis that functional, as opposed to structural, mimicry of the S. flexneri Y O PS by MDWNMHAA or the underrepresentation of the bound ligand conformation in the free-ligand ensemble does not compromise immunological cross-reactivity. Prime/boost strategies were performed with a heterologous boost of PS-TT or MDWNMHAA-TT. They led to high anti-LPS titers after only three injections, suggesting alternatives to improve the immunogenicity of the carbohydrate-mimetic peptide and confirming the antigenic mimicry.

A peptide inhibitor of HIV-1 neutralizing antibody 2G12 is not a structural mimic of the natural carbohydrate epitope on gp120

MAb 2G12 neutralizes HIV-1 by binding with high affinity to a cluster of high-mannose oligosaccharides on the envelope glycoprotein, gp120. Screening of phage-displayed peptide libraries with 2G12 identified peptides that bind specifically, with K(d)s ranging from 0.4 to 200 microM. The crystal structure of a 21-mer peptide ligand in complex with 2G12 Fab was determined at 2.8 A resolution. Comparison of this structure with previous structures of 2G12-carbohydrate complexes revealed striking differences in the mechanism of 2G12 binding to peptide vs. carbohydrate. The peptide occupies a site different from, but adjacent to, the primary carbohydrate-binding site on 2G12, and makes only slightly fewer contacts to the Fab than Man(9)GlcNAc(2) (51 vs. 56, respectively). However, only two antibody contacts with the peptide are hydrogen bonds in contrast to six with Man(9)GlcNAc(2), and only three of the antibody residues that interact with Man(9)GlcNAc(2) also contact the peptide. Thus, this mechanism of peptide binding to 2G12 does not support structural mimicry of the native carbohydrate epitope on gp120, since it neither replicates the oligosaccharide footprint on the antibody nor most of the contact residues. Moreover, 2G12.1 peptide is not an immunogenic mimic of the 2G12 epitope, since antisera produced against it did not bind gp120.

A rAb screening method for improving the probability of identifying peptide mimotopes of carbohydrate antigens

Peptide mimotopes have been investigated as surrogate antigens of carbohydrate (CHO) targets on pathogen and tumor cells in vaccine and therapeutic discovery. One of the main bottlenecks in peptide mimotope discovery is the inability of initial screening regimes to differentiate between true mimotopes and non-mimotopes. As a result, subsequent in vivo analysis of putative peptide mimotopes is often inefficient requiring the use of experimental animals during a lengthy in vivo immunization process. Here, we demonstrate a rapid preliminary screening method to identify putative mimotopes using a recombinant antibody (rAb) library, which may increase the probability of identifying peptides that will elicit a CHO-cross-reactive response in vivo. A human naïve rAb library was screened against both an established peptide mimotope and a non-mimotope of the Group B Streptococcus (GBS) type III polysaccharide to determine if selected antibodies cross-reacted with the original GBS polysaccharide. We were able to differentiate between these two peptides because peptide-binding Abs that cross-reacted to GBS was isolated only with the peptide mimotope. We discuss the feasibility of using this method to significantly increase the breadth of screening and reduce the discovery time for peptide mimotopes.

Recognition Characteristics of Monoclonal Antibodies That Are Cross-Reactive with Gangliosides and Lipooligosaccharide fromCampylobacter jejuniStrains Associated with Guillain-Barré and Fisher Syndromes†

The enteropathogen Campylobacter jejuni has the ability to synthesize glycan structures that are similar to mammalian gangliosides within the core component of its lipooligosaccharide (LOS). Exposure to ganglioside mimics in some individuals results in the production of autoantibodies that deleteriously attack nerve surface gangliosides, precipitating the onset of Guillain-Barré and Fisher syndromes (GBS and FS). We have characterized the interaction of four monoclonal antibodies (mAbs), established by sensitization of mice with LOS isolated from GBS- and FS-associated C. jejuni strains, with chemoenzymatically synthesized gangliooligosaccharides. Surface plasmon resonance (SPR) measurements demonstrate that three of the mAbs interact specifically with derivatives corresponding to their targeted gangliosides, with dissociation constants ranging from 10 to 20 microM. Antibody binding to the gangliooligosaccharides was probed by saturation transfer difference (STD) NMR spectroscopy. STD signals, resulting from antibody/oligosaccharide interaction, were observed for each of the four mAbs. In two cases, differential saturation transfer rates to oligosaccharide resonances enabled detailed epitope mapping. The binding of GD1a-S-Phe with GB1 is characterized by close association of the immunoglobulin with sites that are distributed over several residues of the oligosaccharide. This contrasts sharply with the profile observed for the binding of both GD3-S-Phe and GT1a-S-Phe with FS1. The close antigenic contacts in these ganglioside derivatives are confined to the N-acetylmannosaminyl portion of the terminal N-acetylneuraminic acid (NeuAc) residue of the disialosyl moiety. Our characterization of FS1 provides insight, at an atomic level, into how a single antigenic determinant presented by the LOS of C. jejuni can give rise to antibodies with binding promiscuity to [alphaNeuAc-(2-8)-alphaNeuAc]-bound epitopes and demonstrates why sera from FS patients have antibodies that are often reactive with more than one disialylated ganglioside.