Directing the immune response to carbohydrate antigens

Glucosylation endows nanoparticles with TLR4 agonist capability to trigger macrophage polarization and augment antitumor immunity

Carbohydrates have emerged as promising candidates for immunomodulation, however, how to present them to immune cells and achieve potent immunostimulatory efficacy remains challenging. Here, we proposed and established an effective way of designing unique glyconanoparticles that can amplify macrophage-mediated immune responses through structural mimicry and multiple stimulation. We demonstrate that surface modification with glucose can greatly augment the immunostimulatory efficacy of nanoparticles, comparing to mannose and galactose. In vitro studies show that glucosylation improved the pro-inflammatory efficacy of iron oxide nanoparticles (IONPs) by up to 300-fold, with the immunostimulatory activity of glucosylated IONPs even surpassing that of LPS under certain conditions. In vivo investigation show that glucosylated IONPs elicited increased antitumor immunity and achieved favorable therapeutic outcomes in multiple murine tumor models. Mechanistically, we proposed that glucosylation potentiated the immunostimulatory effect of IONPs by amplifying toll-like receptors 4 (TLR4) activation. Specifically, glucosylated IONPs directly interacted with the TLR4-MD2 complex, resulting in M1 macrophage polarization and enhanced antitumor immunity via activation of NF-κB, MAPK, and STAT1 signaling pathways. Our work provides a simple modification strategy to endow nanoparticles with potent TLR4 agonist effects, which may shed new light on the development of artificial immune modulators for cancer immunotherapy.

Harnessing Antibody Polyspecificity for Cancer Immunotherapy

Targeting the diverse glycan repertoire expressed on tumor cells is considered a viable therapeutic strategy to deal with tumor cell heterogeneity. Inherently polyspecific, natural, glycan-reactive antibodies are purported to be protective in thwarting infections and in cancer immunotherapy. Tumor-associated carbohydrate antigens (TACAs) are related to pathogen glycans, to which nascent or natural antibodies exist and IgM responses are elicited. To capture the polyspecific nature of anticarbohydrate responses, we have focused on the rational design of carbohydrate mimetic peptides (CMPs) cross-reactive with TACA reactive antibodies. In particular, we have focused on the development of CMPs that display reactivity to GD2 and Lewis Y (LeY) reactive monoclonal antibodies. They would serve as templates for pan-immunogens inducing biosimilar polyreactive antibodies. In the design, we relied on structural analyses of CMP's enhanced binding to the templates using molecular modeling. Glycan reactivity patterns of affinity CMP-purified human antibodies further refined specificity profiles in comparison with the immune response to the CMP in clinical trials. In this study, we further define the molecular characteristics for this mimicry by considering the polyspecificity of LeY and GD2 reactive antibodies binding to the lacto-ceramide core Galβ(1,4)Glcβ(1-1')Cer. Binding to this minimum building block can be capitalized on for cancer therapy and diagnostics and illustrates a new approach in designing cancer vaccines taking advantage of the latent polyspecificity of antibodies and the relevance of natural antibodies in antigen discovery and design.

Precision N-Glycoproteomic Profiling of Murine Peritoneal Macrophages After Different Stimulations

Macrophages are important immune cells that participate in both innate and adaptive immune responses, such as phagocytosis, recognition of molecular patterns, and activation of the immune response. In this study, murine peritoneal macrophages were isolated and then activated by LPS, HSV and VSV. Integrative proteomic and precision N-glycoproteomic profiling were conducted to assess the underlying macrophage activation. We identified a total of 587 glycoproteins, including 1239 glycopeptides, 526 monosaccharide components, and 8326 intact glycopeptides in glycoproteomics, as well as a total of 4496 proteins identified in proteomic analysis. These glycoproteins are widely involved in important biological processes, such as antigen presentation, cytokine production and glycosylation progression. Under the stimulation of the different pathogens, glycoproteins showed a dramatic change. We found that receptors in the Toll-like receptor pathway, such as Tlr2 and CD14, were increased under LPS and HSV stimulation. Glycosylation of those proteins was proven to influence their subcellular locations.

Unraveling the sugar code: the role of microbial extracellular glycans in plant-microbe interactions

To defend against microbial invaders but also to establish symbiotic programs, plants need to detect the presence of microbes through the perception of molecular signatures characteristic of a whole class of microbes. Among these molecular signatures, extracellular glycans represent a structurally complex and diverse group of biomolecules that has a pivotal role in the molecular dialog between plants and microbes. Secreted glycans and glycoconjugates such as symbiotic lipochitooligosaccharides or immunosuppressive cyclic β-glucans act as microbial messengers that prepare the ground for host colonization. On the other hand, microbial cell surface glycans are important indicators of microbial presence. They are conserved structures normally exposed and thus accessible for plant hydrolytic enzymes and cell surface receptor proteins. While the immunogenic potential of bacterial cell surface glycoconjugates such as lipopolysaccharides and peptidoglycan has been intensively studied in the past years, perception of cell surface glycans from filamentous microbes such as fungi or oomycetes is still largely unexplored. To date, only few studies have focused on the role of fungal-derived cell surface glycans other than chitin, highlighting a knowledge gap that needs to be addressed. The objective of this review is to give an overview on the biological functions and perception of microbial extracellular glycans, primarily focusing on their recognition and their contribution to plant-microbe interactions.

Report on the Current Inventory of the Toolbox for Plant Cell Wall Analysis: Proteinaceous and Small Molecular Probes

Plant cell walls are highly complex structures composed of diverse classes of polysaccharides, proteoglycans, and polyphenolics, which have numerous roles throughout the life of a plant. Significant research efforts aim to understand the biology of this cellular organelle and to facilitate cell-wall-based industrial applications. To accomplish this, researchers need to be provided with a variety of sensitive and specific detection methods for separate cell wall components, and their various molecular characteristics in vitro as well as in situ. Cell wall component-directed molecular detection probes (in short: cell wall probes, CWPs) are an essential asset to the plant glycobiology toolbox. To date, a relatively large set of CWPs has been produced-mainly consisting of monoclonal antibodies, carbohydrate-binding modules, synthetic antibodies produced by phage display, and small molecular probes. In this review, we summarize the state-of-the-art knowledge about these CWPs; their classification and their advantages and disadvantages in different applications. In particular, we elaborate on the recent advances in non-conventional approaches to the generation of novel CWPs, and identify the remaining gaps in terms of target recognition. This report also highlights the addition of new "compartments" to the probing toolbox, which is filled with novel chemical biology tools, such as metabolic labeling reagents and oligosaccharide conjugates. In the end, we also forecast future developments in this dynamic field.

Rational Design of an Anticalin-Type Sugar-Binding Protein Using a Genetically Encoded Boronate Side Chain

The molecular recognition of carbohydrates plays a fundamental role in many biological processes. However, the development of carbohydrate-binding reagents for biomedical research and use poses a challenge due to the generally poor affinity of proteins toward sugars in aqueous solution. Here, we describe the effective molecular recognition of pyranose monosaccharides (in particular, galactose and mannose) by a rationally designed protein receptor based on the human lipocalin scaffold (Anticalin). Complexation relies on reversible covalent cis-diol boronate diester formation with a genetically encoded l-boronophenylalanine (Bpa) residue which was incorporated as a non-natural amino acid at a sterically permissive position in the ligand pocket of the Anticalin, as confirmed by X-ray crystallography. Compared with the metal-ion and/or avidity-dependent oligovalent lectins that prevail in nature, our approach offers a novel and promising route to generate tight sugar-binding reagents both as research reagents and for biomedical applications.

Development of novel monoclonal antibodies against starch and ulvan - implications for antibody production against polysaccharides with limited immunogenicity

Monoclonal antibodies (mAbs) are widely used and powerful research tools, but the generation of mAbs against glycan epitopes is generally more problematic than against proteins. This is especially significant for research on polysaccharide-rich land plants and algae (Viridiplantae). Most antibody production is based on using single antigens, however, there are significant gaps in the current repertoire of mAbs against some glycan targets with low immunogenicity. We approached mAb production in a different way and immunised with a complex mixture of polysaccharides. The multiplexed screening capability of carbohydrate microarrays was then exploited to deconvolute the specificities of individual mAbs. Using this strategy, we generated a set of novel mAbs, including one against starch (INCh1) and one against ulvan (INCh2). These polysaccharides are important storage and structural polymers respectively, but both are generally considered as having limited immunogenicity. INCh1 and INCh2 therefore represent important new molecular probes for Viridiplantae research. Moreover, since the α-(1-4)-glucan epitope recognised by INCh1 is also a component of glycogen, this mAb can also be used in mammalian systems. We describe the detailed characterisation of INCh1 and INCh2, and discuss the potential of a non-directed mass-screening approach for mAb production against some glycan targets.

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.

Elucidating Structural Features of an Entirely Carbohydrate Cancer Vaccine Construct Employing Circular Dichroism and Fluorescent Labeling

The zwitterionic polysaccharide PS A1 from anaerobe Bacteroides fragilis ATCC 25285/NCTC 9343 is known to elicit a T-cell-dependent, major histocompatibility complex class II (MHCII) immune response through a correspondingly similar protein-antigen-based mechanism/pathway. The biological activity of PS A1 is known to arise from alternating charged motifs on adjacent monosaccharides comprising a tetrameric repeating oligomeric unit creating an alpha-helical secondary structure. However, we have learned that this alpha-helical structural characteristic may not play a role in immune activation. Paradoxically, our current knowledge of structure - activity relationships (SARs) with electrostatically charged polysaccharides has become more clearly defined, yet a lack of tools/probes for measuring dynamic structural changes hinders progress in carbohydrate-based vaccine development. Site- and region-specific structural modifications of PS A1, followed by conjugation with a known carbohydrate cancer antigen, the Thomsen-nouveau (Tn = alpha-D-GalNAc-OSer/Thr) antigen, does not alter antibody isotype switching ability and leads to specific IgG3 antibodies in C57BL/6 mice. Circular dichroism (CD) and studies using fluorescently labeled PS A1, described herein, reveal information pertaining to structure - activity relationships and the nature of Tn conjugation to chemically modified PS A1. The CD spectra of a Tn-PS A1 construct at 8.5 ≥ pH ≤ 3.5 illustrates complete loss of alpha-helical character while spectra obtained in the 3.6 ≤ pH ≥ 8.4 range denotes minimal alpha-helicity in comparison to naturally occurring PS A1. Temperatures exceeding 60 °C reveal complete loss of helical character. Two methods for Alexa Fluor488^® fluorescent labeling studies of chemically oxidized PS A1 have given rise to percent conjugation values (% loading) calculated to be on average 35 Tn molecules bound. Combined, our results argue that altering the structure of PS A1, without chemically modifying the electrostatic charge character, does not alter immune response/recognition in mice. These findings have important implications for the design of entirely carbohydrate-based vaccine constructs.

The promise of the anti-idiotype concept

A basic tenet of antibody-based immunity is their specificity to antigenic determinates from foreign pathogen products to abnormal cellular components such as in cancer. However, an antibody has the potential to bind to more than one determinate, be it an antigen or another antibody. These observations led to the idiotype network theory (INT) to explain immune regulation, which has wax and waned in enthusiasm over the years. A truer measure of the impact of the INT is in terms of the ideas that now form the mainstay of immunological research and whose roots are spawned from the promise of the anti-idiotype concept. Among the applications of the INT is understanding the structural implications of the antibody-mediated network that has the potential for innovation in terms of rational design of reagents with biological, chemical, and pharmaceutical applications that underlies concepts of reverse immunology which is highlighted herein.

Carbohydrate targets in HIV vaccine research: lessons from failures

Learning from the successes of other vaccines that enhance natural and existing protective responses to pathogens, the current effort in HIV vaccine research is directed toward inducing cytotoxic responses. Nevertheless, antibodies are fundamental players in vaccine development and are still considered in the context of passive specific immunotherapy of HIV, especially since several broadly neutralizing monoclonals are available. Special interest is directed toward antibodies binding to the glycan array on gp120 since they have the potential of broader reactivity and cross-clade neutralizing capacity. Humoral responses to carbohydrate antigens have proven effective against other pathogens, why not HIV? The variability of the epitope targets on HIV may not be the only problem to developing active or passive immunotherapeutic strategies. The dynamics of the infected immune system leads to ambiguous effects of most of the effector mechanisms calling for new approaches; some may already be available, while others are in the making.

Bridging innate and adaptive antitumor immunity targeting glycans

Effective immunotherapy for cancer depends on cellular responses to tumor antigens. The role of major histocompatibility complex (MHC) in T-cell recognition and T-cell receptor repertoire selection has become a central tenet in immunology. Structurally, this does not contradict earlier findings that T-cells can differentiate between small hapten structures like simple glycans. Understanding T-cell recognition of antigens as defined genetically by MHC and combinatorially by T cell receptors led to the “altered self” hypothesis. This notion reflects a more fundamental principle underlying immune surveillance and integrating evolutionarily and mechanistically diverse elements of the immune system. Danger associated molecular patterns, including those generated by glycan remodeling, represent an instance of altered self. A prominent example is the modification of the tumor-associated antigen MUC1. Similar examples emphasize glycan reactivity patterns of antigen receptors as a phenomenon bridging innate and adaptive but also humoral and cellular immunity and providing templates for immunotherapies.

A biologically active peptide mimetic of N-acetylgalactosamine/galactose

BACKGROUND

Glycosylated proteins and lipids are important regulatory factors whose functions can be altered by addition or removal of sugars to the glycan structure. The glycans are recognized by sugar-binding lectins that serve as receptors on the surface of many cells and facilitate initiation of an intracellular signal that changes the properties of the cells. We identified a peptide that mimics the ligand of an N-acetylgalactosamine (GalNAc)-specific lectin and asked whether the peptide would express specific biological activity.

FINDINGS

A 12-mer phage display library was screened with a GalNAc-specific lectin to identify an amino acid sequence that binds to the lectin. Phage particles that were eluted from the lectin with free GalNAc were considered to have been bound to a GalNAc-binding site. Peptides were synthesized with the selected sequence as a quadravalent structure to facilitate receptor crosslinking. Treatment of human peripheral blood mononuclear cells for 24 h with the peptide stimulated secretion of interleukin-8 (IL-8) but not of IL-1beta, IL-6, IL-10, or tumor necrosis factor-alpha (TNF-alpha). The secretion of IL-21 was stimulated as strongly with the peptide as with interferon-gamma.

CONCLUSION

The data indicate that the quadravalent peptide has biological activity with a degree of specificity. These effects occurred at concentrations in the nanomolar range, in contrast to free sugars that generally bind to proteins in the micro- to millimolar range.

Peptides mimicking GD2 ganglioside elicit cellular, humoral and tumor-protective immune responses in mice

INTRODUCTION

Because of its restricted distribution in normal tissues and its high expression on tumors of neuroectodermal origin, GD2 ganglioside is an excellent target for active specific immunotherapy. However, GD2 usually elicits low-titered IgM and no IgG or cellular immune responses, limiting its usefulness as a vaccine for cancer patients. We have previously shown that anti-idiotypic monoclonal antibody mimics of GD2 can induce antigen-specific humoral and cellular immunity in mice, but inhibition of tumor growth by the mimics could not be detected.

METHODS AND RESULTS

Here, we isolated two peptides from phage display peptide libraries by panning with GD2-specific mAb ME361. The peptides inhibited binding of the mAb to GD2. When coupled to keyhole limpet hemocyanin (KLH) or presented as multiantigenic peptides in QS21 adjuvant, the peptides induced in mice antibodies binding specifically to GD2 and delayed-type hypersensitive lymphocytes reactive specifically with GD2-positive D142.34 mouse melanoma cells. Induction of delayed-type hypersensitivity (DTH) reaction was dependent on CD4-positive lymphocytes. The immunity elicited by the peptides significantly inhibited growth of GD2-positive melanoma cells in mice.

CONCLUSION

Our study suggests that immunization with peptides mimicking GD2 ganglioside inhibits tumor growth through antibody and/or CD4-positive T cell-mediated mechanisms. Cytolytic T lymphocytes most likely do not play a role. Our results provide the basis for structural analysis of carbohydrate mimicry by peptides.

Structural and functional studies of Peptide-carbohydrate mimicry

Certain peptides act as molecular mimics of carbohydrates in that they are specifically recognizedby carbohydrate-binding proteins. Peptides that bind to anti-carbohydrate antibodies, carbohydrate-processingenzymes, and lectins have been identified. These peptides are potentially useful as vaccines andtherapeutics; for example, immunologically functional peptide molecular mimics (mimotopes) can strengthenor modify immune responses induced by carbohydrate antigens. However, peptides that bind specificallyto carbohydrate-binding proteins may not necessarily show the corresponding biological activity, andfurther selection based on biochemical studies is always required. The degree of structural mimicryrequired to generate the desired biological activity is therefore an interesting question. This reviewwill discuss recent structural studies of peptide-carbohydrate mimicry employing NMR spectroscopy,X-ray crystallography, and molecular modeling, as well as relevant biochemical data. These studiesprovide insights into the basis of mimicry at the molecular level. Comparisons with other carbohydrate-mimeticcompounds, namely proteins and glycopeptides, will be drawn. Finally, implications for the designof new therapeutic compounds will also be presented.

Multiple antigenic mimotopes of HIV carbohydrate antigens: relating structure and antigenicity

Carbohydrate mimetic peptides are designable, and they can carry T-cell epitopes and circumvent tolerance. A mimic-based human immunodeficiency virus (HIV) vaccine can be a viable alternative to carbohydrate-based antigens if the diversity of epitopes found on gp120 can be recapitulated. To improve existing mimics, an attempt was made to study the structural correlates of the observed polyspecificity of carbohydrate mimetic peptides based on the Y(P/R)Y motif in more detail. A carbohydrate mimetic peptide, D002 (RGGLCYCRYRYCVCVGR), bound a number of lectins with different specificities. Although this peptide reacted strongly with both lotus and concanavalin A (ConA) lectins, it bound to lotus stronger than ConA. By varying the central motif RYRY, five versions were produced in multiple antigen peptide format, and their avidity for lotus and ConA lectins was tested by surface plasmon resonance. Although the kinetic parameters were similar, the version based on the sequence YPYRY had an optimal affinity for both lectins as well as improved avidity for wheat germ agglutinin and phytohemagglutinin. Thus, as far as lectin specificity is concerned, YPYRY had improved multiple antigenic properties. Both RYRY and YPYRY precipitated antibodies from human IgG for intravenous use that bound to gp120 in vitro and immunoprecipitated gp120 from transfected CHO-PI cells. Thus, Y(P/R)Y motifs mimic multiple carbohydrate epitopes, many of which are found on HIV, and preimmune human IgG antibodies that bind to HIV carbohydrates cross-react to a comparable extent with both RYRY and YPYRY carbohydrate mimetic peptides.

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

Protein conjugates of oligosaccharides or peptides that mimic complex bacterial polysaccharide antigens represent alternatives to the classical polysaccharide-based conjugate vaccines developed so far. Hence, a better understanding of the molecular basis ensuring appropriate mimicry is required in order to design efficient carbohydrate mimic-based vaccines. This study focuses on the following two unrelated sets of mimics of the Shigella flexneri 5a O-specific polysaccharide (O-SP): (i) a synthetic branched pentasaccharide known to mimic the average solution conformation of S. flexneri 5a O-SP, and (ii) three nonapeptides selected upon screening of phage-displayed peptide libraries with two protective murine monoclonal antibodies (mAbs) of the A isotype specific for S. flexneri 5a O-SP. By inducing anti-O-SP antibodies upon immunization in mice when appropriately presented to the immune system, the pentasaccharide and peptides p100c and p115, but not peptide p22, were qualified as mimotopes of the native antigen. NMR studies based on transferred NOE (trNOE) experiments revealed that both kinds of mimotopes had an average conformation when bound to the mAbs that was close to that of their free form. Most interestingly, saturation transfer difference (STD) experiments showed that the characteristic turn conformations adopted by the major conformers of p100c and p115, as well as of p22, are clearly involved in mAb binding. These latter experiments also showed that the branched glucose residue of the pentasaccharide was a key part of the determinant recognized by the protective mAbs. Finally, by using NMR-derived pentasaccharide and peptide conformations coupled to STD information, models of antigen-antibody interaction were obtained. Most interestingly, only one model was found compatible with experimental data when large O-SP fragments were docked into one of the mIgA-binding sites. This newly made available system provides a new contribution to the understanding of the molecular mimicry of complex polysaccharides by peptides and short oligosaccharides.

Antigenic Properties of Peptide Mimotopes of HIV-1-associated Carbohydrate Antigens

The glycan shield of the human immunodeficiency virus (HIV) envelope protein presents many potential epitopes for vaccine development. To augment immune responses to HIV, type 1 (HIV-1), envelope-associated carbohydrate antigens, we are defining peptide mimics of HIV-associated carbohydrate antigens that function as antigen mimotopes that upon immunization will induce antibodies cross-reactive with carbohydrate antigens. We have previously defined peptides with a putative sequence tract RYRY that mimic concanavalin A-binding glycans. To imitate the multivalent binding of carbohydrates, we compared the avidity of a linear (911) and cyclic peptide (D002) reactive with concanavalin A presented in a multiple antigen peptide (MAP) format. The affinity of the MAP-D002 peptide was higher than that of the peptide MAP-911, whereas the avidity of D002 peptide was lower than that of 911. Serum from mice immunized with MAP-911 had lower titer for oligomannose-9 than those elicited by MAP-D002 under the same conditions, but both immunogens elicited antibodies that can block the binding of GP120 to dendritic cells. Antibodies that bind to the studied MAPs were found in a preparation of normal human immunoglobulin for intravenous use. Those that were purified on 911 bound back to 911 and D002, whereas anti-D002 antibodies were specific only for D002. Human antibodies reactive with both mimotopes and with a mannosyl preparation were observed to bind to envelope protein. These results suggested the potential to fine-tune the antibody response to carbohydrate antigens by modifying structural features of peptide mimotope-based immunogens.

Peptide mimics as surrogate immunogens of mosquito midgut carbohydrate malaria transmission blocking targets

Transmission blocking vaccines (TBV) against mosquito midgut carbohydrate epitopes is a promising approach to curbing the spread of malaria. However, carbohydrates as immunogens can be problematic. Via the malaria transmission blocking monoclonal antibody, MG96, we isolated dodecapeptide mimics of the conserved, nominal mosquito carbohydrate epitope from a peptide-display library. Two peptide clones, bearing a constrained, consensus motif competitively inhibited MG96 reactivity with its nominal midgut microvillar antigen. However, rabbit polyclonal antisera against these synthetic peptides recognized heterologous mosquito midgut carbohydrate and protein epitopes along the midgut basal lamina. Consequently, antisera did not block parasite development within the mosquito vector. Therefore, it is imperative that peptides not only need to be functional mimics but also complete mimotopes to effectively direct the vertebrate immune response towards the nominal, protective carbohydrate epitope on mosquito microvilli.

Induction of a protective capsular polysaccharide antibody response to a multiepitope DNA vaccine encoding a peptide mimic of meningococcal serogroup C capsular polysaccharide

Systemic infection by encapsulated organisms, such as Neisseria meningitidis, is a major cause of morbidity and mortality worldwide, especially in individuals less than 2 years of age. Antibodies directed at the capsular polysaccharide are shown to be protective against disease by inducing complement-dependent bactericidal activity. The current polysaccharide vaccine has been shown to be poorly immunogenic in high-risk groups and this is probably related to its T-independent properties. An alternative approach to eliciting a T-dependent serum immunoglobulin G (IgG) antibody response to encapsulated pathogens is DNA vaccination. We assessed the immunogenicity of a multiepitope DNA vaccine encoding a T-cell helper epitope and a peptide mimic of N. meningitidis serogroup C. The DNA construct induced a significant anti-polysaccharide antibody response that was bactericidal. Mice immunized with the DNA construct were subsequently protected against challenge with a lethal dose of N. meningitidis serogroup C.

The mannose-dependent epitope for neutralizing antibody 2G12 on human immunodeficiency virus type 1 glycoprotein gp120

We have analyzed the unique epitope for the broadly neutralizing human monoclonal antibody (MAb) 2G12 on the gp120 surface glycoprotein of human immunodeficiency virus type 1 (HIV-1). Sequence analysis, focusing on the conservation of relevant residues across multiple HIV-1 isolates, refined the epitope that was defined previously by substitutional mutagenesis (A. Trkola, M. Purtscher, T. Muster, C. Ballaun, A. Buchacher, N. Sullivan, K. Srinivasan, J. Sodroski, J. P. Moore, and H. Katinger, J. Virol. 70:1100-1108, 1996). In a biochemical study, we digested recombinant gp120 with various glycosidase enzymes of known specificities and showed that the 2G12 epitope is lost when gp120 is treated with mannosidases. Computational analyses were used to position the epitope in the context of the virion-associated envelope glycoprotein complex, to determine the variability of the surrounding surface, and to calculate the surface accessibility of possible glycan- and polypeptide-epitope components. Together, these analyses suggest that the 2G12 epitope is centered on the high-mannose and/or hybrid glycans of residues 295, 332, and 392, with peripheral glycans from 386 and 448 on either flank. The epitope is mannose dependent and composed primarily of carbohydrate, with probably no direct involvement of the gp120 polypeptide surface. It resides on a face orthogonal to the CD4 binding face, on a surface proximal to, but distinct from, that implicated in coreceptor binding. Its conservation amidst an otherwise highly variable gp120 surface suggests a functional role for the 2G12 binding site, perhaps related to the mannose-dependent attachment of HIV-1 to DC-SIGN or related lectins that facilitate virus entry into susceptible target cells.

Peptide mimotopes as surrogate antigens of carbohydrates in vaccine discovery

Carbohydrate antigens are immune targets associated with a variety of pathogens and tumor cells. Unfortunately, most carbohydrates are intrinsically T cell-independent antigens, which diminishes their efficacy as immunogens. The conversion of carbohydrate epitopes to peptide mimotopes is one means to overcome the T cell-independent nature of carbohydrate antigens because peptides have an absolute requirement for T cells. Although such conversion has great potential for the development of veterinarian and human vaccines, there are issues related to the use of peptide-based immunogens as functional surrogates. Some of these issues are fundamental, pertaining to how mimicry comes about at the molecular level, and some are application oriented, directed at elucidating important immunological mechanisms. In this article the potential and caveats of this technology regarding its application in vaccine discovery are analyzed.