Systematic Synthesis and Binding Study of HIV V3 Glycopeptides Reveal the Fine Epitopes of Several Broadly Neutralizing Antibodies

Glycoconjugates: Synthesis, Functional Studies, and Therapeutic Developments

Glycoconjugates are major constituents of mammalian cells that are formed via covalent conjugation of carbohydrates to other biomolecules like proteins and lipids and often expressed on the cell surfaces. Among the three major classes of glycoconjugates, proteoglycans and glycoproteins contain glycans linked to the protein backbone via amino acid residues such as Asn for N-linked glycans and Ser/Thr for O-linked glycans. In glycolipids, glycans are linked to a lipid component such as glycerol, polyisoprenyl pyrophosphate, fatty acid ester, or sphingolipid. Recently, glycoconjugates have become better structurally defined and biosynthetically understood, especially those associated with human diseases, and are accessible to new drug, diagnostic, and therapeutic developments. This review describes the status and new advances in the biological study and therapeutic applications of natural and synthetic glycoconjugates, including proteoglycans, glycoproteins, and glycolipids. The scope, limitations, and novel methodologies in the synthesis and clinical development of glycoconjugates including vaccines, glyco-remodeled antibodies, glycan-based adjuvants, glycan-specific receptor-mediated drug delivery platforms, etc., and their future prospectus are discussed.

Synthesis and Immunological Study of N-Glycan-Bacteriophage Qβ Conjugates Reveal Dominant Antibody Responses to the Conserved Chitobiose Core

N-Glycosylation plays an important role in many biological recognition processes. However, very few N-glycan-specific antibodies are available for functional studies and potentially for therapeutic development. In this study, we sought to synthesize bacteriophage Qβ conjugates with representative N-glycans and investigate their immunogenicity for raising N-glycan-specific antibodies. An array of Qβ glycoconjugates bearing five different human N-glycans and two different chemical linkers were synthesized, and the immunization of the N-glycan-Qβ conjugates was performed in mice. We found that the N-glycan-Qβ conjugates raised significant IgG antibodies that recognize N-glycans, but, surprisingly, most of the glycan-dependent antibodies were directed to the shared chitobiose core and were nonspecific for respective N-glycan structures. The linker chemistry was found to affect antibody specificity with adipic acid-linked N-glycan-Qβ immunogens raising antibodies capable of recognizing both the N-acetylglucosamine (GlcNAc) moieties of the chitobiose core. In contrast, antibodies raised by N-glycan-Qβ immunogens with a triazole linker preferentially recognized the innermost N-acetylglucosamine moiety at the reducing end. We also found that sialylation of the N-glycans significantly suppressed the immune response. Furthermore, the N-glycan-Qβ immunogens with an adipic acid linker elicited higher glycan-specific antibody titers than the N-glycan-triazole-Qβ immunogens. These findings delineate several challenges in eliciting mammalian N-glycan-specific antibodies through the conventional glycoconjugate vaccine design and immunization.

Peptides for Vaccine Development

This review discusses peptide epitopes used as antigens in the development of vaccines in clinical trials as well as future vaccine candidates. It covers peptides used in potential immunotherapies for infectious diseases including SARS-CoV-2, influenza, hepatitis B and C, HIV, malaria, and others. In addition, peptides for cancer vaccines that target examples of overexpressed proteins are summarized, including human epidermal growth factor receptor 2 (HER-2), mucin 1 (MUC1), folate receptor, and others. The uses of peptides to target cancers caused by infective agents, for example, cervical cancer caused by human papilloma virus (HPV), are also discussed. This review also provides an overview of model peptide epitopes used to stimulate non-specific immune responses, and of self-adjuvanting peptides, as well as the influence of other adjuvants on peptide formulations. As highlighted in this review, several peptide immunotherapies are in advanced clinical trials as vaccines, and there is great potential for future therapies due the specificity of the response that can be achieved using peptide epitopes.

Chemoenzymatic Synthesis and Antibody Binding of HIV-1 V1/V2 Glycopeptide-Bacteriophage Qβ Conjugates as a Vaccine Candidate

The broadly neutralizing antibody PG9 recognizes a unique glycopeptide epitope in the V1V2 domain of HIV-1 gp120 envelope glycoprotein. The present study describes the design, synthesis, and antibody-binding analysis of HIV-1 V1V2 glycopeptide-Qβ conjugates as a mimic of the proposed neutralizing epitope of PG9. The glycopeptides were synthesized using a highly efficient chemoenzymatic method. The alkyne-tagged glycopeptides were then conjugated to the recombinant bacteriophage (Qβ), a virus-like nanoparticle, through a click reaction. Antibody-binding analysis indicated that the synthetic glycoconjugates showed significantly enhanced affinity for antibody PG9 compared with the monomeric glycopeptides. It was also shown that the affinity of the Qβ-conjugates for antibody PG9 was dependent on the density of the glycopeptide antigen display. The glycopeptide-Qβ conjugates synthesized represent a promising candidate of HIV-1 vaccine.

Immunogenicity Evaluation of N-Glycans Recognized by HIV Broadly Neutralizing Antibodies

While the improved treatment of human immunodeficiency virus type 1 (HIV-1) infection is available, the development of an effective and safe prophylactic vaccine against HIV-1 is still an unrealized goal. Encouragingly, the discovery of broadly neutralizing antibodies (bNAbs) from HIV-1 positive patients that are capable of neutralizing a broad spectrum of HIV-1 isolates of various clades has accelerated the progress of vaccine development in the past few years. Some of these bNAbs recognize the N-glycans on the viral surface gp120 glycoprotein. We have been interested in using the glycan epitopes recognized by bNAbs for the development of vaccines to elicit bNAb-like antibodies with broadly neutralizing activities. Toward this goal, we have identified novel hybrid-type structures with subnanomolar avidity toward several bNAbs including PG16, PGT121, PGT128-3C, 2G12, VRC13, VRC-PG05, VRC26.25, VRC26.09, PGDM1400, 35O22, and 10-1074. Here, we report the immunogenicity evaluation of a novel hybrid glycan conjugated to carrier DTCRM197, a nontoxic mutant of the diphtheria toxin, for immunization in mice. Our results indicated that the IgG response was mainly against the chitobiose motif with nonspecific binding to a panel of N-glycans with reducing end GlcNAc-GlcNAc (chitobiose) printed on the glass slides. However, the IgM response was mainly toward the reducing end GlcNAc moiety. We further used the glycoconjugates of Man3GlcNAc2, Man5GlcNAc2, and Man9GlcNAc2 glycans for immunization, and a similar specificity pattern was observed. These findings suggest that the immunogenicity of chitobiose may interfere with the outcome of N-glycan-based vaccines, and modification may be necessary to increase the immunogenicity of the entire N-glycan epitope.

Carbohydrate-Based Macromolecular Biomaterials

Carbohydrates are the most abundant and one of the most important biomacromolecules in Nature. Except for energy-related compounds, carbohydrates can be roughly divided into two categories: Carbohydrates as matter and carbohydrates as information. As matter, carbohydrates are abundantly present in the extracellular matrix of animals and cell walls of various plants, bacteria, fungi, etc., serving as scaffolds. Some commonly found polysaccharides are featured as biocompatible materials with controllable rigidity and functionality, forming polymeric biomaterials which are widely used in drug delivery, tissue engineering, etc. As information, carbohydrates are usually referred to the glycans from glycoproteins, glycolipids, and proteoglycans, which bind to proteins or other carbohydrates, thereby meditating the cell-cell and cell-matrix interactions. These glycans could be simplified as synthetic glycopolymers, glycolipids, and glycoproteins, which could be afforded through polymerization, multistep synthesis, or a semisynthetic strategy. The information role of carbohydrates can be demonstrated not only as targeting reagents but also as immune antigens and adjuvants. The latter are also included in this review as they are always in a macromolecular formulation. In this review, we intend to provide a relatively comprehensive summary of carbohydrate-based macromolecular biomaterials since 2010 while emphasizing the fundamental understanding to guide the rational design of biomaterials. Carbohydrate-based macromolecules on the basis of their resources and chemical structures will be discussed, including naturally occurring polysaccharides, naturally derived synthetic polysaccharides, glycopolymers/glycodendrimers, supramolecular glycopolymers, and synthetic glycolipids/glycoproteins. Multiscale structure-function relationships in several major application areas, including delivery systems, tissue engineering, and immunology, will be detailed. We hope this review will provide valuable information for the development of carbohydrate-based macromolecular biomaterials and build a bridge between the carbohydrates as matter and the carbohydrates as information to promote new biomaterial design in the near future.

Synthetic carbohydrate-based HIV-1 vaccines

An effective prophylactic HIV-1 vaccine is essential in order to contain the HIV/AIDS global pandemic. The discovery of different broadly neutralizing antibodies (bnAbs) in the last decades has enabled the characterization of several minimal epitopes on the HIV envelope (Env) spike, including glycan-dependent fragments. Herein, we provide a brief overview of the progress made on the development of synthetic carbohydrate-based epitope mimics for the elicitation of bnAbs directed to certain regions on Env gp120 protein: the outer domain high-mannose cluster and the variable loops V1V2 and V3. We focus on the design, synthesis and biological evaluation of minimal immunogens and discuss key aspects towards the development of a successful protective vaccine against HIV-1.

Recent advances in glycan synthesis

Carbohydrates play important roles in life science, but their synthesis is always hampered by their complicated chemical structures. Scientists have never stopped trying to solve the problem of glycan synthesis from various aspects. Here a brief overview of recent progress in glycan synthesis, including chemical approaches, chemoenzymatic approaches, and automated synthesis, will be discussed, focusing on the efficiency of new glycosylation methods, the stereoselectivity of coupled products, and their applications in the assembly of complex glycan chains.

Peptide-Based Vaccines: Current Progress and Future Challenges

Vaccines have had a profound impact on the management and prevention of infectious disease. In addition, the development of vaccines against chronic diseases has attracted considerable interest as an approach to prevent, rather than treat, conditions such as cancer, Alzheimer's disease, and others. Subunit vaccines consist of nongenetic components of the infectious agent or disease-related epitope. In this Review, we discuss peptide-based vaccines and their potential in three therapeutic areas: infectious disease, Alzheimer's disease, and cancer. We discuss factors that contribute to vaccine efficacy and how these parameters may potentially be modulated by design. We examine both clinically tested vaccines as well as nascent approaches and explore current challenges and potential remedies. While peptide vaccines hold substantial promise in the prevention of human disease, many obstacles remain that have hampered their clinical use; thus, continued research efforts to address these challenges are warranted.

The Impact of Sustained Immunization Regimens on the Antibody Response to Oligomannose Glycans

The high mannose patch (HMP) of the HIV envelope protein (Env) is the structure most frequently targeted by broadly neutralizing antibodies; therefore, many researchers have attempted to use mimics of this region as a vaccine immunogen. In our previous efforts, vaccinating rabbits with evolved HMP mimic glycopeptides containing Man₉ resulted in an overall antibody response targeting the glycan core and linker rather than the full glycan or Manα1→2Man tips of Man₉ glycans. A possible reason could be processing of our immunogen by host serum mannosidases. We sought to test whether more prolonged dosing could increase the antibody response to intact glycans, possibly by increasing the availability of intact Man₉ to germinal centers. Here, we describe a study investigating the impact of immunization regimen on antibody response by testing immunogen delivery through bolus, an exponential series of mini doses, or a continuously infusing mini-osmotic pump. Our results indicate that, with our glycopeptide immunogens, standard bolus immunization elicited the strongest HIV Env-binding antibody response, even though higher overall titers to the glycopeptide were elicited by the exponential and pump regimens. Antibody selectivity for intact glycan was, if anything, slightly better in the bolus-immunized animals.

Chemoenzymatic Synthesis of HIV-1 Glycopeptide Antigens

Glycosylation is one of the most common posttranslational modifications of proteins and can exert profound effects on the inherent properties and biological functions of a given protein. Structurally well-defined homogeneous glycopeptides are highly demanded for functional studies and biomedical applications. Various chemical and chemoenzymatic methods have been reported so far for synthesizing different N- and O-glycopeptides. Among them, the chemoenzymatic method based on an endoglycosidase-catalyzed ligation of free N-glycans and GlcNAc-tagged peptides is emerging as a highly efficient method for constructing large complex N-glycopeptides. This chemoenzymatic approach consists of two key steps. The first step is to prepare the GlcNAc peptide through automated solid-phase peptide synthesis (SPPS) by incorporating an Asn-linked GlcNAc moiety at a predetermined glycosylation site; and the second step is to transfer an N-glycan from the corresponding N-glycan oxazoline en bloc to the GlcNAc peptide by an endoglycosidase or its efficient glycosynthase mutant. In this chapter, we provide detailed procedures of this chemoenzymatic method by demonstrating the synthesis of two HIV-1 V3 glycopeptide antigens carrying a high-mannose-type and a complex-type N-glycan, respectively. The described procedures should be generally applicable for the synthesis of other biologically important N-glycopeptides.

Antibody responses to the HIV-1 envelope high mannose patch

Neutralizing antibodies against human immunodeficiency virus subtype 1 (HIV-1) bind to its envelope glycoprotein (Env). Half of the molecular mass of Env is carbohydrate making it one of the most heavily glycosylated proteins known in nature. HIV-1 Env glycans are derived from the host and present a formidable challenge for host anti-glycan antibody induction. Anti-glycan antibody induction is challenging because anti-HIV-1 glycan antibodies should recognize Env antigen while not acquiring autoreactivity. Thus, the glycan network on HIV-1 Env is referred to as the glycan shield. Despite the challenges presented by immune recognition of host-derived glycans, neutralizing antibodies capable of binding the glycans on HIV-1 Env can be generated by the host immune system in the setting of HIV-1 infection. In particular, a cluster of high mannose glycans, including an N-linked glycan at position 332, form the high mannose patch and are targeted by a variety of broadly neutralizing antibodies. These high mannose patch-directed HIV-1 antibodies can be categorized into distinct categories based on their antibody paratope structure, neutralization activity, and glycan and peptide reactivity. Below we will compare and contrast each of these classes of HIV-1 glycan-dependent antibodies and describe vaccine design efforts to elicit each of these antibody types.

Glycopeptides and -Mimetics to Detect, Monitor and Inhibit Bacterial and Viral Infections: Recent Advances and Perspectives

The initial contact of pathogens with host cells is usually mediated by their adhesion to glycan structures present on the cell surface in order to enable infection. Furthermore, glycans play important roles in the modulation of the host immune responses to infection. Understanding the carbohydrate-pathogen interactions are of importance for the development of novel and efficient strategies to either prevent, or interfere with pathogenic infection. Synthetic glycopeptides and mimetics thereof are capable of imitating the multivalent display of carbohydrates at the cell surface, which have become an important objective of research over the last decade. Glycopeptide based constructs may function as vaccines or anti-adhesive agents that interfere with the ability of pathogens to adhere to the host cell glycans and thus possess the potential to improve or replace treatments that suffer from resistance. Additionally, synthetic glycopeptides are used as tools for epitope mapping of antibodies directed against structures present on various pathogens and have become important to improve serodiagnostic methods and to develop novel epitope-based vaccines. This review will provide an overview of the most recent advances in the synthesis and application of glycopeptides and glycopeptide mimetics exhibiting a peptide-like backbone in glycobiology.

Oligomannose Glycopeptide Conjugates Elicit Antibodies Targeting the Glycan Core Rather than Its Extremities

Up to ∼20% of HIV-infected individuals eventually develop broadly neutralizing antibodies (bnAbs), and many of these antibodies (∼40%) target a region of dense high-mannose glycosylation on gp120 of the HIV envelope protein, known as the "high-mannose patch" (HMP). Thus, there have been numerous attempts to develop glycoconjugate vaccine immunogens that structurally mimic the HMP and might elicit bnAbs targeting this conserved neutralization epitope. Herein, we report on the immunogenicity of glycopeptides, designed by in vitro selection, that bind tightly to anti-HMP antibody 2G12. By analyzing the fine carbohydrate specificity of rabbit antibodies elicited by these immunogens, we found that they differ from some natural human bnAbs, such as 2G12 and PGT128, in that they bind primarily to the core structures within the glycan, rather than to the Manα1 → 2Man termini (2G12) or to the whole glycan (PGT128). Antibody specificity for the glycan core may result from extensive serum mannosidase trimming of the immunogen in the vaccinated animals. This finding has broad implications for vaccine design aiming to target glycan-dependent HIV neutralizing antibodies.

Synthetic HIV V3 Glycopeptide Immunogen Carrying a N334 N-Glycan Induces Glycan-Dependent Antibodies with Promiscuous Site Recognition

The N332 high-mannose glycan on the HIV-1 gp120 V3-loop is the target of many bNAbs. About 17% HIV isolates carry the N332 to N334 mutation, but the antibody recognition of the N334 N-glycan and its immunogenicity are not well characterized. Here we report the chemoenzymatic synthesis, antigenicity, and immunogenicity of the V3 N334 glycopeptides from HIV-1 A244 gp120, a key component in the partially successful Thai clinical trials. We found that synthetic V3 glycopeptide carrying a N334 high-mannose glycan could be recognized by bNAb PGT128 and PGT126 but not by 10-1074. Rabbit immunization with the synthetic three-component A244 glycopeptide immunogen elicited substantial glycan-dependent antibodies with broad reactivity to various HIV-1 gp120/gp140 carrying N332 or N334 glycosylation sites. These results indicated that the N334 site is vulnerable and the A244 V3 glycopeptide represents a valuable immunogen for further HIV-1 vaccine studies.

Display of the HIV envelope protein at the yeast cell surface for immunogen development

As a step toward the development of variant forms of Env with enhanced immunogenic properties, we have expressed the glycoprotein in the yeast surface display system in a form that can be subjected to random mutagenesis followed by screening for forms with enhanced binding to germline antibodies. To optimize the expression and immunogenicity of the yeast-displayed Env protein, we tested different approaches for cell wall anchoring, expression of gp120 and gp140 Env from different viral strains, the effects of introducing mutations designed to stabilize Env, and the effects of procedures for altering N-linked glycosylation of Env. We find that diverse forms of HIV envelope glycoprotein can be efficiently expressed at the yeast cell surface and that gp140 forms of Env are effectively cleaved by Kex2p, the yeast furin protease homolog. Multiple yeast-displayed gp120 and gp140 proteins are capable of binding to antibodies directed against the V3-variable loop, CD4 binding site, and gp41 membrane-proximal regions, including some antibodies whose binding is known to depend on Env conformation and N-linked glycan. Based on antibody recognition and sensitivity to glycosidases, yeast glycosylation patterns partially mimic high mannose-type N-glycosylation in mammalian cells. However, yeast-displayed Env is not recognized by some anti-Env antibodies sensitive to quaternary structure, suggesting either that the displayed protein exists in a monomeric state or that for these antibodies, yeast glycosylation in certain regions hinders recognition or access. Consistent with studies in other systems, reconstructed predicted unmutated precursors to anti-Env antibodies exhibit little affinity for the yeast-displayed envelope protein.

Chemoenzymatic Methods for the Synthesis of Glycoproteins

Glycosylation is one of the most prevalent posttranslational modifications that profoundly affects the structure and functions of proteins in a wide variety of biological recognition events. However, the structural complexity and heterogeneity of glycoproteins, usually resulting from the variations of glycan components and/or the sites of glycosylation, often complicates detailed structure-function relationship studies and hampers the therapeutic applications of glycoproteins. To address these challenges, various chemical and biological strategies have been developed for producing glycan-defined homogeneous glycoproteins. This review highlights recent advances in the development of chemoenzymatic methods for synthesizing homogeneous glycoproteins, including the generation of various glycosynthases for synthetic purposes, endoglycosidase-catalyzed glycoprotein synthesis and glycan remodeling, and direct enzymatic glycosylation of polypeptides and proteins. The scope, limitation, and future directions of each method are discussed.

Multivalent Antigen Presentation Enhances the Immunogenicity of a Synthetic Three-Component HIV-1 V3 Glycopeptide Vaccine

HIV-1 envelope glycoproteins gp120 and gp41 are presented on the virus surface as a trimer of heterodimer and are the targets of broadly neutralizing antibodies (bNAbs). We describe here the synthesis and preliminary immunological evaluation of a three-component trivalent HIV-1 V3 glycopeptide immunogen aiming to raise glycopeptide epitope-specific antibodies. Click chemistry confers efficient synthesis of the lipopeptide-glycopeptide conjugate that carries three copies of HIV-1 JR-FL gp120 V3 glycopeptide with a high-mannose glycan at the N332 glycosylation site. We found that the multivalent presentation substantially enhanced the immunogenicity of the V3 glycopeptide. The antisera induced by the three-component multivalent glycopeptide immunogen exhibited stronger binding to heterologous HIV-1 gp120s and the trimeric gp140s than that from the monovalent glycopeptide immunogen. The antisera generated from this preliminary rabbit immunization did not show virus neutralization activity, probably due to the lack of somatic maturation. The ability to elicit substantial glycopeptide epitope-specific antibodies by the three-component trivalent glycopeptide immunogen suggests that it could serve as a valuable vaccine component in combination with other vaccine candidates for further immunization studies.

Microarray Analysis of Oligosaccharide-Mediated Multivalent Carbohydrate-Protein Interactions and Their Heterogeneity

Carbohydrate-protein interactions (CPIs) are involved in a wide range of biological phenomena. Hence, the characterization and presentation of carbohydrate epitopes that closely mimic the natural environment is one of the long-term goals of glycosciences. Inspired by the multivalency, heterogeneity and nature of carbohydrate ligand-mediated interactions, we constructed a combinatorial library of mannose and galactose homo- and hetero-glycodendrons to study CPIs. Microarray analysis of these glycodendrons with a wide range of biologically important plant and animal lectins revealed that oligosaccharide structures and heterogeneity interact with each other to alter binding preferences.

Synthetic Three-Component HIV-1 V3 Glycopeptide Immunogens Induce Glycan-Dependent Antibody Responses

Eliciting broadly neutralizing antibody (bNAb) responses against HIV-1 is a major goal for a prophylactic HIV-1 vaccine. One approach is to design immunogens based on known broadly neutralizing epitopes. Here we report the design and synthesis of an HIV-1 glycopeptide immunogen derived from the V3 domain. We performed glycopeptide epitope mapping to determine the minimal glycopeptide sequence as the epitope of V3-glycan-specific bNAbs PGT128 and 10-1074. We further constructed a self-adjuvant three-component immunogen that consists of a 33-mer V3 glycopeptide epitope, a universal T helper epitope P30, and a lipopeptide (Pam₃CSK₄) that serves as a ligand of Toll-like receptor 2. Rabbit immunization revealed that the synthetic self-adjuvant glycopeptide could elicit substantial glycan-dependent antibodies that exhibited broader recognition of HIV-1 gp120s than the non-glycosylated V3 peptide. These results suggest that the self-adjuvant synthetic glycopeptides can serve as an important component to elicit glycan-specific antibodies in HIV vaccine design.

Synthetic multivalent V3 glycopeptides display enhanced recognition by glycan-dependent HIV-1 broadly neutralizing antibodies

We describe here the synthesis of novel multivalent HIV V3 domain glycopeptides and their binding to broadly neutralizing antibodies PGT128 and 10-1074. Our binding data reveal a distinct mode of antigen recognition by the two antibodies and further suggest that multivalent glycopeptides could mimic the neutralizing epitopes more efficiently than the monomeric glycopeptide.

A facile synthesis of a complex type N-glycan thiazoline as an effective inhibitor against the antibody-deactivating endo-β-N-acetylglucosaminidases

Endo-β-N-acetylglucosaminidases are a class of endoglycosidases that deglycosylate N-glycans from glycoproteins. We describe here a facile synthesis of a complex type N-glycan thiazoline as a new mechanism-based inhibitor for this class of enzymes. The synthesis started with the readily available sialoglycopeptide (SGP) and its conversion into the glycan thiazoline through several enzymatic and chemical reactions. The synthetic glycan thiazoline showed potent inhibitory activity against several endoglycosidases including the two antibody-deactivating enzymes, Endo-S and Endo-S2, from human pathogen Streptococcus pyogenes, which would be useful as tools for structural and functional studies of these enzymes.