Sugar-Protein Connectivity Impacts on the Immunogenicity of Site-Selective Salmonella O-Antigen Glycoconjugate Vaccines

Exploring the variables influencing the immune response of traditional and innovative glycoconjugate vaccines

Vaccines are cost-effective tools for reducing morbidity and mortality caused by infectious diseases. The rapid evolution of pneumococcal conjugate vaccines, the introduction of tetravalent meningococcal conjugate vaccines, mass vaccination campaigns in Africa with a meningococcal A conjugate vaccine, and the recent licensure and introduction of glycoconjugates against S. Typhi underlie the continued importance of research on glycoconjugate vaccines. More innovative ways to produce carbohydrate-based vaccines have been developed over the years, including bioconjugation, Outer Membrane Vesicles (OMV) and the Multiple antigen-presenting system (MAPS). Several variables in the design of these vaccines can affect the induced immune responses. We review immunogenicity studies comparing conjugate vaccines that differ in design variables, such as saccharide chain length and conjugation chemistry, as well as carrier protein and saccharide to protein ratio. We evaluate how a better understanding of the effects of these different parameters is key to designing improved glycoconjugate vaccines.

Immunomodulatory glycomedicine: Introducing next generation cancer glycovaccines

Cancer remains a leading cause of death worldwide due to the lack of safer and more effective therapies. Cancer vaccines developed from neoantigens are an emerging strategy to promote protective and therapeutic anti-cancer immune responses. Advances in glycomics and glycoproteomics have unveiled several cancer-specific glycosignatures, holding tremendous potential to foster effective cancer glycovaccines. However, the immunosuppressive nature of tumours poses a major obstacle to vaccine-based immunotherapy. Chemical modification of tumour associated glycans, conjugation with immunogenic carriers and administration in combination with potent immune adjuvants constitute emerging strategies to address this bottleneck. Moreover, novel vaccine vehicles have been optimized to enhance immune responses against otherwise poorly immunogenic cancer epitopes. Nanovehicles have shown increased affinity for antigen presenting cells (APCs) in lymph nodes and tumours, while reducing treatment toxicity. Designs exploiting glycans recognized by APCs have further enhanced the delivery of antigenic payloads, improving glycovaccine's capacity to elicit innate and acquired immune responses. These solutions show potential to reduce tumour burden, while generating immunological memory. Building on this rationale, we provide a comprehensive overview on emerging cancer glycovaccines, emphasizing the potential of nanotechnology in this context. A roadmap towards clinical implementation is also delivered foreseeing advances in glycan-based immunomodulatory cancer medicine.

Site-Selective Unnatural Amino Acid Incorporation at Single or Multiple Positions to Control Sugar-Protein Connectivity in Glycoconjugate Vaccine Candidates

In cellulo site-specific unnatural amino acid incorporation based on amber stop codon reassignment is a powerful tool to modify proteins at defined positions. This technique is herein applied to the selective functionalization of the Pneumococcal surface adhesin A protein at three distinct positions. N^ϵ -propargyloxycarbonyl-l-lysine residues were incorporated and their alkyne groups reacted using click-chemistry with a synthetic azido-functionalized tetrasaccharide representative of one repeat unit of the Streptococcus pneumoniae serotype 14 capsular polysaccharide. Anti-PsaA antibody response induced in mice by the trivalent glycoconjugate was determined in comparison with corresponding monovalent and randomly functionalized conjugates. Our results suggest that controlled was superior to random conjugation for preserving antigenicity. In definitive, the reported strategy offers a unique opportunity to study the impact of carbohydrate antigen-carrier protein connectivity on immunogenicity.

Impact and Control of Sugar Size in Glycoconjugate Vaccines

Glycoconjugate vaccines have contributed enormously to reducing and controlling encapsulated bacterial infections for over thirty years. Glycoconjugate vaccines are based on a carbohydrate antigen that is covalently linked to a carrier protein; this is necessary to cause T cell responses for optimal immunogenicity, and to protect young children. Many interdependent parameters affect the immunogenicity of glycoconjugate vaccines, including the size of the saccharide antigen. Here, we examine and discuss the impact of glycan chain length on the efficacy of glycoconjugate vaccines and report the methods employed to size polysaccharide antigens, while highlighting the underlying reaction mechanisms. A better understanding of the impact of key parameters on the immunogenicity of glycoconjugates is critical to developing a new generation of highly effective vaccines.

Site-Specific Multi-Functionalization of the Carrier Protein CRM197 by Disulfide Rebridging for Conjugate Vaccine Development

Conjugation of an antigen to a carrier protein is widely used for vaccine development. To develop the next generation of conjugate vaccines, we describe here a method for the controlled multi‐functionalization of the widely employed carrier protein CRM₁₉₇ with a carbohydrate‐based antigen and an immune potentiator. The approach is based on the selective reduction of one of the disulfides of CRM₁₉₇ followed by disulfide rebridging employing an appropriately functionalized dibromopyridazinedione. Efficient protein modification required that the reduction and functionalization with a dibromopyridazinedione was performed as a one‐step procedure with control over the reaction temperature. Furthermore, ligations were most successful when dibromopyridazinediones were employed having a functional entity such as a TLR7/8 agonist and a cyclooctyne for further modification. Site‐specific conjugation avoids modification of T‐epitopes of the carrier protein and covalent attachment of an immune potentiator will ensure that cytokines are produced where the vaccine interacts with relevant immune cells resulting in efficient immune potentiation.

Cross reacting material (CRM197) as a carrier protein for carbohydrate conjugate vaccines targeted at bacterial and fungal pathogens

This paper gives an overview of conjugate glycovaccines which contain recombinant diphtheria toxoid CRM197 as a carrier protein. A special focus is given to synthetic methods used for preparation of neoglycoconjugates of CRM197 with oligosaccharide epitopes of cell surface carbohydrates of pathogenic bacteria and fungi. Syntheses of commercial vaccines and laboratory specimen on the basis of CRM197 are outlined briefly.

Synthetic Glycans to Improve Current Glycoconjugate Vaccines and Fight Antimicrobial Resistance

Antimicrobial resistance (AMR) is emerging as the next potential pandemic. Different microorganisms, including the bacteria Acinetobacter baumannii, Clostridioides difficile, Escherichia coli, Enterococcus faecium, Klebsiella pneumoniae, Neisseria gonorrhoeae, Pseudomonas aeruginosa, non-typhoidal Salmonella, and Staphylococcus aureus, and the fungus Candida auris, have been identified by the WHO and CDC as urgent or serious AMR threats. Others, such as group A and B Streptococci, are classified as concerning threats. Glycoconjugate vaccines have been demonstrated to be an efficacious and cost-effective measure to combat infections against Haemophilus influenzae, Neisseria meningitis, Streptococcus pneumoniae, and, more recently, Salmonella typhi. Recent times have seen enormous progress in methodologies for the assembly of complex glycans and glycoconjugates, with developments in synthetic, chemoenzymatic, and glycoengineering methodologies. This review analyzes the advancement of glycoconjugate vaccines based on synthetic carbohydrates to improve existing vaccines and identify novel candidates to combat AMR. Through this literature survey we built an overview of structure-immunogenicity relationships from available data and identify gaps and areas for further research to better exploit the peculiar role of carbohydrates as vaccine targets and create the next generation of synthetic carbohydrate-based vaccines.

Immunobiology of Carbohydrates: Implications for Novel Vaccine and Adjuvant Design Against Infectious Diseases

Carbohydrates are ubiquitous molecules expressed on the surface of nearly all living cells, and their interaction with carbohydrate-binding proteins is critical to many immunobiological processes. Carbohydrates are utilized as antigens in many licensed vaccines against bacterial pathogens. More recently, they have also been considered as adjuvants. Interestingly, unlike other types of vaccines, adjuvants have improved immune response to carbohydrate-based vaccine in humans only in a few cases. Furthermore, despite the discovery of many new adjuvants in the last years, aluminum salts, when needed, remain the only authorized adjuvant for carbohydrate-based vaccines. In this review, we highlight historical and recent advances on the use of glycans either as vaccine antigens or adjuvants, and we review the use of currently available adjuvants to improve the efficacy of carbohydrate-based vaccines. A better understanding of the mechanism of carbohydrate interaction with innate and adaptive immune cells will benefit the design of a new generation of glycan-based vaccines and of immunomodulators to fight both longstanding and emerging diseases.

Retaining the structural integrity of disulfide bonds in diphtheria toxoid carrier protein is crucial for the effectiveness of glycoconjugate vaccine candidates

The introduction of glycoconjugate vaccines marks an important point in the fight against various infectious diseases. The covalent conjugation of relevant polysaccharide antigens to immunogenic carrier proteins enables the induction of a long-lasting and robust IgG antibody response, which is not observed for pure polysaccharide vaccines. Although there has been remarkable progress in the development of glycoconjugate vaccines, many crucial parameters remain poorly understood. In particular, the influence of the conjugation site and strategy on the immunogenic properties of the final glycoconjugate vaccine is the focus of intense research. Here, we present a comparison of two cysteine selective conjugation strategies, elucidating the impact of both modifications on the structural integrity of the carrier protein, as well as on the immunogenic properties of the resulting glycoconjugate vaccine candidates. Our work suggests that conjugation chemistries impairing structurally relevant elements of the protein carrier, such as disulfide bonds, can have a dramatic effect on protein immunogenicity.

The introduction of glycoconjugate vaccines marks an important point in the fight against various infectious diseases.

Precise protein conjugation technology for the construction of homogenous glycovaccines

The introduction of vaccines for the treatment and prevention of bacterial or viral diseases in the early 19th century marked a crucial turning point in medical history. Since then, extensive immunization campaigns have eradicated smallpox and drastically reduced the number of diphtheria, tetanus, pertussis and measles cases worldwide. Although a broad selection of vaccines is available, there remains a need to develop additional vaccine candidates against a range of dangerous infectious diseases, preferably based on precise syntheses that lead to homogenous formulations. Different strategies for the construction of this type of vaccine candidates are being pursued. Glycoconjugate vaccines are successful in the fight against bacterial and viral infectious diseases. However, their exact mechanism of action remains largely unknown and the large-scale production of chemically defined constructs is challenging. In particular, the conjugation of the carbohydrate antigen to the protein carrier has proved to be crucial for the properties of these vaccines. This review highlights some of the latest findings and developments in the construction of glycoconjugate vaccines by means of site-specific chemical reactions.

Recent advances in lipopolysaccharide-based glycoconjugate vaccines

INTRODUCTION

The public health burden caused by pathogenic Gram-negative bacteria is increasingly prominent due to antimicrobial resistance. The surface carbohydrates are potential antigens for vaccines against Gram-negative bacteria. The enhanced immunogenicity of the O-specific polysaccharide (O-SP) moiety of LPS when coupled to a carrier protein may protect against bacterial pathogens. However, because of the toxic lipid A moiety and relatively high costs of O-SP isolation, LPS has not been a popular vaccine antigen until recently.

AREAS COVERED

In this review, we discuss the rationales for developing LPS-based glycoconjugate vaccines, principles of glycoconjugate-induced immunity, and highlight the recent developments and challenges faced by LPS-based glycoconjugate vaccines.

EXPERT OPINION

Advances in LPS harvesting, LPS chemical synthesis, and newer carrier proteins in the past decade have propelled LPS-based glycoconjugate vaccines toward further development, through to clinical evaluation. The development of LPS-based glycoconjugates offers a new horizon for vaccine prevention of Gram-negative bacterial infection.

Novel Simple Conjugation Chemistries for Decoration of GMMA with Heterologous Antigens

Outer Membrane Vesicles (OMV) constitute a promising platform for the development of efficient vaccines. OMV can be decorated with heterologous antigens (proteins or polysaccharides), becoming attractive novel carriers for the development of multicomponent vaccines. Chemical conjugation represents a tool for linking antigens, also from phylogenetically distant pathogens, to OMV. Here we develop two simple and widely applicable conjugation chemistries targeting proteins or lipopolysaccharides on the surface of Generalized Modules for Membrane Antigens (GMMA), OMV spontaneously released from Gram-negative bacteria mutated to increase vesicle yield and reduce potential reactogenicity. A Design of Experiment approach was used to identify optimal conditions for GMMA activation before conjugation, resulting in consistent processes and ensuring conjugation efficiency. Conjugates produced by both chemistries induced strong humoral response against the heterologous antigen and GMMA. Additionally, the use of the two orthogonal chemistries allowed to control the linkage of two different antigens on the same GMMA particle. This work supports the further advancement of this novel platform with great potential for the design of effective vaccines.

Enzymatic Methods for the Site-Specific Radiolabeling of Targeting Proteins

Site-specific conjugation of proteins is currently required to produce homogenous derivatives for medicine applications. Proteins derivatized at specific positions of the polypeptide chain can actually show higher stability, superior pharmacokinetics, and activity in vivo, as compared with conjugates modified at heterogeneous sites. Moreover, they can be better characterized regarding the composition of the derivatization sites as well as the conformational and activity properties. To this aim, several site-specific derivatization approaches have been developed. Among these, enzymes are powerful tools that efficiently allow the generation of homogenous protein-drug conjugates under physiological conditions, thus preserving their native structure and activity. This review will summarize the progress made over the last decade on the use of enzymatic-based methodologies for the production of site-specific labeled immunoconjugates of interest for nuclear medicine. Enzymes used in this field, including microbial transglutaminase, sortase, galactosyltransferase, and lipoic acid ligase, will be overviewed and their recent applications in the radiopharmaceutical field will be described. Since nuclear medicine can benefit greatly from the production of homogenous derivatives, we hope that this review will aid the use of enzymes for the development of better radio-conjugates for diagnostic and therapeutic purposes.

Recent advances on smart glycoconjugate vaccines in infections and cancer

Vaccination is one of the greatest achievements in biomedical research preventing death and morbidity in many infectious diseases through the induction of pathogen-specific humoral and cellular immune responses. Currently, no effective vaccines are available for pathogens with a highly variable antigenic load, such as the human immunodeficiency virus or to induce cellular T-cell immunity in the fight against cancer. The recent SARS-CoV-2 outbreak has reinforced the relevance of designing smart therapeutic vaccine modalities to ensure public health. Indeed, academic and private companies have ongoing joint efforts to develop novel vaccine prototypes for this virus. Many pathogens are covered by a dense glycan-coat, which form an attractive target for vaccine development. Moreover, many tumor types are characterized by altered glycosylation profiles that are known as "tumor-associated carbohydrate antigens". Unfortunately, glycans do not provoke a vigorous immune response and generally serve as T-cell-independent antigens, not eliciting protective immunoglobulin G responses nor inducing immunological memory. A close and continuous crosstalk between glycochemists and glycoimmunologists is essential for the successful development of efficient immune modulators. It is clear that this is a key point for the discovery of novel approaches, which could significantly improve our understanding of the immune system. In this review, we discuss the latest advancements in development of vaccines against glycan epitopes to gain selective immune responses and to provide an overview on the role of different immunogenic constructs in improving glycovaccine efficacy.

Molecular modeling provides insights into the loading of sialic acid-containing antigens onto CRM197: the role of chain flexibility in conjugation efficiency and glycoconjugate architecture

Vaccination is the most cost-effective way to control disease caused by encapsulated bacteria; the capsular polysaccharide (CPS) is the primary virulence factor and vaccine target. Neisseria meningitidis (Nm) serogroups B, C, Y and W all contain sialic acid, a common surface feature of human pathogens. Two protein-based vaccines against serogroup B infection are available for human use while four tetravalent conjugate vaccines including serogroups C, W and Y have been licensed. The tetravalent Menveo® conjugate vaccine is well-defined: a simple monomeric structure of oligosaccharides terminally conjugated to amino groups of the carrier protein CRM₁₉₇. However, not only is there a surprisingly low limit for antigen chain attachment to CRM₁₉₇, but different serogroup saccharides have consistently different CRM₁₉₇ loading, the reasons for which are unclear. Understanding this phenomenon is important for the long-term goal of controlling conjugation to prepare conjugate vaccines of optimal immunogenicity. Here we use molecular modeling to explore whether antigen flexibility can explain the varying antigen loading of the conjugates. Because flexibility is difficult to separate from other structural factors, we focus on sialic-acid containing CPS present in current glycoconjugate vaccines: serogroups NmC, NmW and NmY. Our simulations reveal a correlation between Nm antigen flexibility (NmW > NmC > NmY) and the number of chains attached to CRM₁₉₇, suggesting that increased flexibility enables accommodation of additional chains on the protein surface. Further, in silico models of the glycoconjugates confirm the relatively large hydrodynamic size of the saccharide chains and indicate steric constraints to further conjugation.

Immunochemical characteristics of Streptococcus pneumoniae type 3 capsular polysaccharide glycoconjugate constructs correlate with its immunogenicity in mice model

A panel of derivatives were prepared from Streptococcus pneumoniae polysaccharide type 3 (Ps3) modified with adipic acid dihydrazide (ADH). The degree of coupling between Ps3-adh derivatives and diphtheria (DTd) or tetanus (TTd) toxoids was varied by ADH linker loading. A series of Ps3 derivatives and the resultant glycoconjugates (GC) were tested for their immunochemical activity in an ELISA. Antigenic properties of components in GCs were estimated by interaction with serotype-specific and toxin-neutralizing antibodies to confirm the preservation of native protective epitopes both of Ps3 and DTd. After immunization of mice, a correlation was established between immunochemical activity and immunogenicity of these GCs. A correlation model developed for Ps3-DTd conjugates allowed to predict the immunogenicity of similar design Ps3-TTd conjugates based on ELISA testing data. The plausibility of this prediction was confirmed by the test immunization of mice with Ps3-TTds. The proposed immunochemical approach to the assessment and control of native structural and functional antigenic elements in GCs is important for the optimization of vaccine design and is an adequate alternative to extensive physicochemical characterization for assessing immunogenicity.

Rational Design of a Glycoconjugate Vaccine against Group A Streptococcus

No commercial vaccine is yet available against Group A Streptococcus (GAS), major cause of pharyngitis and impetigo, with a high frequency of serious sequelae in low- and middle-income countries. Group A Carbohydrate (GAC), conjugated to an appropriate carrier protein, has been proposed as an attractive vaccine candidate. Here, we explored the possibility to use GAS Streptolysin O (SLO), SpyCEP and SpyAD protein antigens with dual role of antigen and carrier, to enhance the efficacy of the final vaccine and reduce its complexity. All protein antigens resulted good carrier for GAC, inducing similar anti-GAC IgG response to the more traditional CRM₁₉₇ conjugate in mice. However, conjugation to the polysaccharide had a negative impact on the anti-protein responses, especially in terms of functionality as evaluated by an IL-8 cleavage assay for SpyCEP and a hemolysis assay for SLO. After selecting CRM₁₉₇ as carrier, optimal conditions for its conjugation to GAC were identified through a Design of Experiment approach, improving process robustness and yield This work supports the development of a vaccine against GAS and shows how novel statistical tools and recent advancements in the field of conjugation can lead to improved design of glycoconjugate vaccines.

Dichloroacetophenone Derivatives: A Class of Bioconjugation Reagents for Disulfide Bridging

A mild and biocompatible method for the construction of disulfide bridging in peptides using dichloroacetophenone derivatives is developed. This method is highly selective (chemo, diastereo, regio, etc.) and atom economic and works under biocompatible reaction conditions (metal-free, water, pH 7, rt, etc.).

Click chemistry compared to thiol chemistry for the synthesis of site-selective glycoconjugate vaccines using CRM197 as carrier protein

Conjugation chemistry is one of the main parameters affecting immunogenicity of glycoconjugate vaccines and a rational approach toward a deeper understanding of their mechanism of action will greatly benefit from highly-defined and well-characterized structures. Herein, different conjugation methods were investigated with the aim of controlling glycosylation site and glycosylation density on the carrier protein. S. Typhimurium lipopolysaccharide O-Antigen and CRM₁₉₇ carrier protein were used as models. In particular, thiol and click chemistry were examined, both involving the linkage of the terminal reducing sugar unit of the O-Antigen chain to different amino acids on the carrier protein. Thiol chemistry allowed O-Antigen conjugation only when the carrier protein was activated on the lysines and with a relative high number of linkers, while click chemistry allowed conjugate generation even when just one position on the protein was activated and to both lysine and tyrosine sites. The study highlights click chemistry as a leading approach for the synthesis of well-defined glycoconjugates, useful to investigate the relationship between conjugate design and immune response.

Site-specific Bioconjugation and Convergent Click Chemistry Enhances Antibody-Chromophore Conjugate Binding Efficiency

Photosensitizer (PS)-antibody conjugates (photoimmunoconjugates, PICs) enable cancer cell-targeted photodynamic therapy (PDT). Nonspecific chemical bioconjugation is widely used to synthesize PICs but gives rise to several shortcomings. The conjugates are heterogeneous, and the process is not easily reproducible. Moreover, modifications at or near the binding sites alter both binding affinity and specificity. To overcome these limitations, we introduce convergent assembly of PICs via a chemo-enzymatic site-specific approach. First, an antibody is conjugated to a clickable handle via site-specific modification of glutamine (Gln) residues catalyzed by transglutaminase (TGase, EC 2.3.2.13). Second, the modified antibody intermediate is conjugated to a compatible chromophore via click chemistry. Utilizing cetuximab, we compared this site-specific conjugation protocol to the nonspecific chemical acylation of amines using N-hydroxysuccinimide (NHS) chemistry. Both the heavy and light chains were modified via the chemical route, whereas, only a glutamine 295 in the heavy chain was modified via chemo-enzymatic conjugation. Furthermore, a 2.3-fold increase in the number of bound antibodies per cell was observed for the site-specific compared with nonspecific method, suggesting that multiple stochastic sites of modification perturb the antibody-antigen binding. Altogether, site-specific bioconjugation leads to homogenous, reproducible and well-defined PICs, conferring higher binding efficiency and probability of clinical success.

Site-Specific Conjugation for Fully Controlled Glycoconjugate Vaccine Preparation

Glycoconjugate vaccines are formed by covalently link a carbohydrate antigen to a carrier protein whose role is to achieve a long lasting immune response directed against the carbohydrate antigen. The nature of the sugar antigen, its length, its ratio per carrier protein and the conjugation chemistry impact on both structure and the immune response of a glycoconjugate vaccine. In addition it has long been assumed that the sites at which the carbohydrate antigen is attached can also have an impact. These important issue can now be addressed owing to the development of novel chemoselective ligation reactions as well as techniques such as site-selective mutagenesis, glycoengineering, or extension of the genetic code. The preparation and characterization of homogeneous bivalent pneumococcal vaccines is reported. The preparation and characterization of homogeneous bivalent pneumococcal vaccines is reported. A synthetic tetrasaccharide representative of the serotype 14 capsular polysaccharide of Streptococcus pneumoniae has been linked using the thiol/maleimide coupling chemistry to four different Pneumococcal surface adhesin A (PsaA) mutants, each harboring a single cysteine mutation at a defined position. Humoral response of these 1 to 1 carbohydrate antigen/PsaA conjugates have been assessed in mice. Our results showed that the carbohydrate antigen-PsaA connectivity impacts the anti-carrier response and raise questions about the design of glycoconjugate vaccine whereby the protein plays the dual role of immunogen and carrier.

Site-specific bioconjugation and self-assembly technologies for multi-functional biologics: on the road to the clinic

The expanding portfolio of biotherapeutics both in the research and development (R&D) and market sectors is shaping new opportunities towards multifunctional biologics (MFBs). The combination of new or pre-existing therapeutic agents into a single multifunctional format makes it possible to develop new pharmacological actions to significantly improve their efficacy and safety. In this review, I focus on novel platform technologies that are being exploited in the biotech industry to produce MFBs with potential therapeutic benefits that include half-life extension, targeted delivery, T cell engagement, and improved vaccination. In this regard, technologies of key importance are site-specific bioconjugation and self-assembly, which allow homogeneous, defined, and scalable process developments for several MFBs that are advancing towards clinical applications.

Synthesis and Reactivity of a Bis-Strained Alkyne Derived from 1,1'-Biphenyl-2,2',6,6'-tetrol

The novel "double strained alkyne" 3 has been prepared and evaluated in strain-promoted azide-alkyne cycloaddition reactions with azides. The X-ray crystallographic structure of 3, which was prepared in one step from 1,1'-biphenyl-2,2',6,6'-tetrol 4, reveals the strained nature of the alkynes. Dialkyne 3 undergoes cycloaddition reactions with a number of azides, giving mixtures of regiosiomeric products in excellent yields. The monoaddition products were not observed or isolated from the reactions, suggesting that the second cycloaddition proceeds at a faster rate than the first, and this is supported by molecular modeling studies. Dialkyne 3 was successfully employed for "peptide stapling" of a p53-based diazido peptide, whereby two azides are bridged to give a product with a stabilized conformation.

Antimicrobial glycoconjugate vaccines: an overview of classic and modern approaches for protein modification

Glycoconjugate vaccines obtained by chemical linkage of a carbohydrate antigen to a protein are part of routine vaccinations in many countries. Licensed antimicrobial glycan-protein conjugate vaccines are obtained by random conjugation of native or sized polysaccharides to lysine, aspartic or glutamic amino acid residues that are generally abundantly exposed on the protein surface. In the last few years, the structural approaches for the definition of the polysaccharide portion (epitope) responsible for the immunological activity has shown potential to aid a deeper understanding of the mode of action of glycoconjugates and to lead to the rational design of more efficacious and safer vaccines. The combination of technologies to obtain more defined carbohydrate antigens of higher purity and novel approaches for protein modification has a fundamental role. In particular, methods for site selective glycoconjugation like chemical or enzymatic modification of specific amino acid residues, incorporation of unnatural amino acids and glycoengineering, are rapidly evolving. Here we discuss the state of the art of protein engineering with carbohydrates to obtain glycococonjugates vaccines and future perspectives.

Potential targets for next generation antimicrobial glycoconjugate vaccines

Cell surface carbohydrates have been proven optimal targets for vaccine development. Conjugation of polysaccharides to a carrier protein triggers a T-cell-dependent immune response to the glycan moiety. Licensed glycoconjugate vaccines are produced by chemical conjugation of capsular polysaccharides to prevent meningitis caused by meningococcus, pneumococcus and Haemophilus influenzae type b. However, other classes of carbohydrates (O-antigens, exopolysaccharides, wall/teichoic acids) represent attractive targets for developing vaccines. Recent analysis from WHO/CHO underpins alarming concern toward antibiotic-resistant bacteria, such as the so called ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter spp.) and additional pathogens such as Clostridium difficile and Group A Streptococcus. Fungal infections are also becoming increasingly invasive for immunocompromised patients or hospitalized individuals. Other emergencies could derive from bacteria which spread during environmental calamities (Vibrio cholerae) or with potential as bioterrorism weapons (Burkholderia pseudomallei and mallei, Francisella tularensis). Vaccination could aid reducing the use of broad-spectrum antibiotics and provide protection by herd immunity also to individuals who are not vaccinated.

This review analyzes structural and functional differences of the polysaccharides exposed on the surface of emerging pathogenic bacteria, combined with medical need and technological feasibility of corresponding glycoconjugate vaccines.

Homogeneous antibody-drug conjugates via site-selective disulfide bridging

Antibody-drug conjugates (ADCs) constructed using site-selective labelling methodologies are likely to dominate the next generation of these targeted therapeutics. To this end, disulfide bridging has emerged as a leading strategy as it allows the production of highly homogeneous ADCs without the need for antibody engineering. It consists of targeting reduced interchain disulfide bonds with reagents which reconnect the resultant pairs of cysteine residues, whilst simultaneously attaching drugs. The 3 main reagent classes which have been exemplified for the construction of ADCs by disulfide bridging will be discussed in this review; bissulfones, next generation maleimides and pyridazinediones, along with others in development.

A recombinant conjugated pneumococcal vaccine that protects against murine infections with a similar efficacy to Prevnar-13

The pneumococcal conjugate vaccine (PCV) strongly protects against vaccine serotypes, but the rapid expansion of non-vaccine serotype disease and the vaccine's high expense has reduced its overall impact. We have developed Protein Glycan Coupling Technology (PGCT) as a flexible methodology for making low-cost polysaccharide/protein glycoconjugates recombinantly in Escherichia coli. We have used PGCT to make a recombinant PCV containing serotype 4 capsular polysaccharide linked to the Streptococcus pneumoniae proteins NanA, PiuA, and Sp0148. The introduction of the Campylobacter jejuni UDP-glucose 4-epimerase gene GalE (gne) into E. coli improved the yield of the resulting glycoprotein. PGCT glycoconjugate vaccination generated strong antibody responses in mice to both the capsule and the carrier protein antigens, with the PiuA/capsule glycoconjugate inducing similar anti-capsular antibody responses as the commercial PCV Prevnar-13. Antibody responses to PGCT glycoconjugates opsonised S. pneumoniae and Streptococcus mitis expressing the serotype 4 capsule and promoted neutrophil phagocytosis of S. pneumoniae to a similar level as antisera generated by vaccination with Prevnar-13. Vaccination with the PGCT glycoconjugates protected mice against meningitis and septicaemia with the same efficacy as vaccination with Prevnar-13. In addition, vaccination with the protein antigen components from PGCT glycoconjugates alone provided partial protection against septicaemia and colonisation. These data demonstrate that a vaccine made by PGCT is as effective as Prevnar-13, identifies PiuA as a carrier protein for glycoconjugate vaccines, and demonstrates that linking capsular antigen to S. pneumoniae protein antigens has additional protective benefits that could provide a degree of serotype-independent immunity.

Anomeric O-Functionalization of Carbohydrates for Chemical Conjugation to Vaccine Constructs

Carbohydrates mediate a wide range of biological interactions, and understanding these processes benefits the development of new therapeutics. Isolating sufficient quantities of glycoconjugates from biological samples remains a significant challenge. With advances in chemical and enzymatic carbohydrate synthesis, the availability of complex carbohydrates is increasing and developing methods for stereoselective conjugation these polar head groups to proteins and lipids is critically important for pharmaceutical applications. The aim of this review is to provide an overview of commonly employed strategies for installing a functionalized linker at the anomeric position as well as examples of further transformations that have successfully led to glycoconjugation to vaccine constructs for biological evaluation as carbohydrate-based therapeutics.

Recent Advances in the Synthesis of Glycoconjugates for Vaccine Development

During the last decade there has been a growing interest in glycoimmunology, a relatively new research field dealing with the specific interactions of carbohydrates with the immune system. Pathogens’ cell surfaces are covered by a thick layer of oligo- and polysaccharides that are crucial virulence factors, as they mediate receptors binding on host cells for initial adhesion and organism invasion. Since in most cases these saccharide structures are uniquely exposed on the pathogen surface, they represent attractive targets for vaccine design. Polysaccharides isolated from cell walls of microorganisms and chemically conjugated to immunogenic proteins have been used as antigens for vaccine development for a range of infectious diseases. However, several challenges are associated with carbohydrate antigens purified from natural sources, such as their difficult characterization and heterogeneous composition. Consequently, glycoconjugates with chemically well-defined structures, that are able to confer highly reproducible biological properties and a better safety profile, are at the forefront of vaccine development. Following on from our previous review on the subject, in the present account we specifically focus on the most recent advances in the synthesis and preliminary immunological evaluation of next generation glycoconjugate vaccines designed to target bacterial and fungal infections that have been reported in the literature since 2011.

Protein Carriers for Glycoconjugate Vaccines: History, Selection Criteria, Characterization and New Trends

Currently licensed glycoconjugate vaccines are composed of a carbohydrate moiety covalently linked to a protein carrier. Polysaccharides are T-cell independent antigens able to directly stimulate B cells to produce antibodies. Disease burden caused by polysaccharide-encapsulated bacteria is highest in the first year of life, where plain polysaccharides are not generally immunogenic, limiting their use as vaccines. This limitation has been overcome by covalent coupling carbohydrate antigens to proteins that provide T cell epitopes. In addition to the protein carriers currently used in licensed glycoconjugate vaccines, there is a search for new protein carriers driven by several considerations: (i) concerns that pre-exposure or co-exposure to a given carrier can lead to immune interference and reduction of the anti-carbohydrate immune response; (ii) increasing interest to explore the dual role of proteins as carrier and protective antigen; and (iii) new ways to present carbohydrates antigens to the immune system. Protein carriers can be directly coupled to activated glycans or derivatized to introduce functional groups for subsequent conjugation. Proteins can be genetically modified to pre-determine the site of glycans attachment by insertion of unnatural amino acids bearing specific functional groups, or glycosylation consensus sequences for in vivo expression of the glycoconjugate. A large portion of the new protein carriers under investigation are recombinant ones, but more complex systems such as Outer Membrane Vesicles and other nanoparticles are being investigated. Selection criteria for new protein carriers are based on several aspects including safety, manufacturability, stability, reactivity toward conjugation, and preclinical evidence of immunogenicity of corresponding glycoconjugates. Characterization panels of protein carriers include tests before conjugation, after derivatization when applicable, and after conjugation. Glycoconjugate vaccines based on non-covalent association of carrier systems to carbohydrates are being investigated with promising results in animal models. The ability of these systems to convert T-independent carbohydrate antigens into T-dependent ones, in comparison to traditional glycoconjugates, needs to be assessed in humans.

Multicomponent polysaccharide-protein bioconjugation in the development of antibacterial glycoconjugate vaccine candidates

A new synthetic strategy for the development of multivalent antibacterial glycoconjugate vaccines is described. The approach comprises the utilization of an isocyanide-based multicomponent process for the conjugation of functionalized capsular polysaccharides of S. pneumoniae and S. Typhi to carrier proteins such as diphtheria and tetanus toxoids. For the first time, oxo- and carboxylic acid-functionalized polysaccharides could be either independently or simultaneously conjugated to immunogenic proteins by means of the Ugi-multicomponent reaction, thus leading to mono- or multivalent unimolecular glycoconjugates as vaccine candidates. Despite the high molecular weight of the two or three reacting biomolecules, the multicomponent bioconjugation proved highly efficient and reproducible. The Ugi-derived glycoconjugates showed notable antigenicity and elicited good titers of functional specific antibodies. To our knowledge, this is the only bioconjugation method that enables the incorporation of two different polysaccharidic antigens to a carrier protein in a single step. Applications in the field of self-adjuvanting, eventually anticancer, multicomponent vaccines are foreseeable.

Peptide adjacent to glycosylation sites impacts immunogenicity of glycoconjugate vaccine

Glycoconjugate vaccine is composed of polysaccharides (PSs) covalently linked with carrier protein. Glycosylation site selection, as a significant factor leading to heterogeneities of glycoconjugate structure, draws more and more attentions for its impact on the immunogenicity of glycoconjugate vaccine. To elucidate the relationship between glycosylation connectivity and immunogenicity of glycoconjugate vaccine, in this study, anti-E. coli O157:H7 glycoconjugate O-PS-MBP with defined connectivity, and three selected peptide segments GS1, GS2, GS3 derived from O-PS-MBP was synthesized. Immunogenicity results showed that only peptides adjacent to the glycosylation sites (GS1 and GS2) promoted the generation of PS-specific IgG antibodies and contributed to PS-specific IgG subclass distribution. Furthermore, GS1 and GS2 had significant priming effect for eliciting PS-specific IgG antibodies. These results indicated that different locations of glycosylation sites could lead to diverse presentation of peptides and glycopeptides to APCs and influence the immunogenicity of glycoconjugate vaccine, which extend the current understanding of mechanism for adaptive immune system activation by glycoconjugate vaccine, and have implications for rational glycoconjugate vaccine design.

Synthesis and cycloaddition reactions of strained alkynes derived from 2,2′-dihydroxy-1,1′-biaryls

A series of strained alkynes, based on the 2,2′-dihydroxy-1,1′-biaryl structure, were prepared in a short sequence from readily-available starting materials.

The influence of conjugation variables on the design and immunogenicity of a glycoconjugate vaccine against Salmonella Typhi

In recent years there have been major efforts to develop glycoconjugate vaccines based on the Vi polysaccharide that will protect against Salmonella enterica Typhi infections, particularly typhoid fever, which remains a major public health concern in low-income countries. The design of glycoconjugate vaccines influences the immune responses they elicit. Here we systematically test the response in mice to Vi glycoconjugates that differ in Vi chain length (full-length and fragmented), carrier protein, conjugation chemistry, saccharide to protein ratio and size. We show that the length of Vi chains, but not the ultimate size of the conjugate, has an impact on the anti-Vi IgG immune response induced. Full-length Vi conjugates, independent of the carrier protein, induce peak IgG responses rapidly after just one immunization, and secondary immunization does not enhance the magnitude of these responses. Fragmented Vi linked to CRM197 and diphtheria toxoid, but not to tetanus toxoid, gives lower anti-Vi antibody responses after the first immunization than full-length Vi conjugates, but antibody titres are similar to those induced by full-length Vi conjugates following a second dose. The chemistry to conjugate Vi to the carrier protein, the linker used, and the saccharide to protein ratio do not significantly alter the response. We conclude that Vi length and carrier protein are the variables that influence the anti-Vi IgG response to immunization the most, while other parameters are of lesser importance.

Oxetane Grafts Installed Site-Selectively on Native Disulfides to Enhance Protein Stability and Activity In Vivo

A four‐membered oxygen ring (oxetane) can be readily grafted into native peptides and proteins through site‐selective bis‐alkylation of cysteine residues present as disulfides under mild and biocompatible conditions. The selective installation of the oxetane graft enhances stability and activity, as demonstrated for a range of biologically relevant cyclic peptides, including somatostatin, proteins, and antibodies, such as a Fab arm of the antibody Herceptin and a designed antibody DesAb‐Aβ against the human Amyloid‐β peptide. Oxetane grafting of the genetically detoxified diphtheria toxin CRM₁₉₇ improves significantly the immunogenicity of this protein in mice, which illustrates the general utility of this strategy to modulate the stability and biological activity of therapeutic proteins containing disulfides in their structures.

Dual, Site-Specific Modification of Antibodies by Using Solid-Phase Immobilized Microbial Transglutaminase

Microbial transglutaminase (MTG) was stably solid-phase immobilized on glass microbeads by using a second-generation dendronized polymer. Immobilized MTG enabled the efficient generation of site-specifically conjugated proteins, including antibody fragments, as well as whole antibodies through distinct glutamines and, unprecedentedly, also through lysines with various bifunctional substrates with defined stoichiometries. With this method, we generated dual, site-specifically modified antibodies comprising a fluorescent probe and a metal chelator for radiolabeling-a strategy anticipated to design antibodies for imaging and simultaneous therapy. Furthermore, we provide evidence that immobilized MTG features higher siteselectivity than soluble MTG.

Advancing Homogeneous Antimicrobial Glycoconjugate Vaccines

Since 2004, when the first synthetic glycoconjugate vaccine against the pneumonia and meningitis causing bacterium Haemophilus influenza type b (Hib) approved for human use in Cuba was reported, 34 million doses of the synthetic vaccine have been already distributed in several countries under the commercial name of Quimi-Hib. However, despite the success of this product, no other synthetic glycoconjugate vaccine has been licensed in the following 13 years. As well as avoiding the need to handle pathogens, synthetic glycoconjugates offer clear advantages in terms of product characterization and the possibility to understand the parameters influencing immunogenicity. Nevertheless, large scale application of synthetic sugars has been perceived as challenging because of manufacturing costs and process complexity compared to natural polysaccharides. Chemoenzymatic approaches, one-pot protocols, and automated solid-phase synthesis are rendering carbohydrate production considerably more attractive for industrialization. Here we identify three areas where chemical approaches can advance this progress: (i) chemical or enzymatic methods enabling the delivery of the minimal polysaccharide portion responsible for an effective immune response; (ii) site-selective chemical or enzymatic conjugation strategies for the exploration of the conjugation point in immune responses against carbohydrate-based vaccines, and the consistent preparation of more homogeneous products; (iii) multicomponent constructs targeting receptors responsible for immune response modulation in order to control its quality and magnitude. We discuss how synthesis of bacterial oligosaccharides is useful toward understanding the polysaccharide portion responsible for immunogenicity, and for developing robust and consistent alternatives to natural heterogeneous polysaccharides. The synthesis of sugar analogues can lead to the identification of hydrolytically more stable versions of oligosaccharide antigens. The study of bacterial polysaccharide biosynthesis aids the development of in vitro hazard-free oligosaccharide production. Novel site-selective conjugation methods contribute toward deciphering the role of conjugation sites in the immunogenicity of glycoconjugates and prove to be particularly useful when glycans are conjugated to protein serving as carrier and antigen. The orthogonal incorporation of two different carbohydrate haptens enables the reduction of vaccine components. Finally, coordinated conjugation of glycans and small molecule immunopotentiators supports simplification of vaccine formulation and localization of adjuvant. Synergistic advancement of these areas, combined with competitive manufacturing processes, will contribute to a better understanding of the features guiding the immunological activity of glycoconjugates and, ultimately, to the design of improved, safer vaccines.

Strained alkynes derived from 2,2′-dihydroxy-1,1′-biaryls; synthesis and copper-free cycloaddition with azides

A series of strained alkynes were prepared from 2,2′-dihydroxy-biaryls, and were demonstrated to react with azides without a copper catalyst.

Towards the next generation of biomedicines by site-selective conjugation

Bioconjugates represent an emerging class of medicines, which offer therapeutic opportunities overtaking those of the individual components. Many novel bioconjugates have been explored in order to address various emerging medical needs. The last decade has witnessed the exponential growth of new site-selective bioconjugation techniques, however very few methods have made the way into human clinical trials. Here we discuss various applications of site-selective conjugation in biomedicines, including half-life extension, antibody-drug conjugates, conjugate vaccines, bispecific antibodies and cell therapy. The review is intended to highlight both the progress and challenges, and identify a potential roadmap to address the gap.

Carbohydrates as T-cell antigens with implications in health and disease

Glycosylation is arguably the most ubiquitous post-translational modification on proteins in microbial and mammalian cells. During the past few years, there has been intensive research demonstrating that carbohydrates, either in pure forms or in conjunction with proteins or lipids, evoke and modulate adaptive immune responses. We now know that carbohydrates can be directly recognized by T cells or participate in T-cell stimulation as components of T-cell epitopes. T-cell recognition of carbohydrate antigens takes place via their presentation by major histocompatibility complex pathways on antigen-presenting cells. In this review, we summarize studies on carbohydrates as T-cell antigens modulating adaptive immune responses. Through discussion of glycan-containing antigens, such as glycoproteins, glycolipids, zwitterionic polysaccharides and carbohydrate-based glycoconjugate vaccines, we will illustrate the key molecular and cellular interactions between carbohydrate antigens and T cells and the implications of these interactions in health and disease.

Immunology, epidemiology and mathematical modelling towards a better understanding of invasive non-typhoidal Salmonella disease and rational vaccination approaches

INTRODUCTION

Invasive non-typhoidal Salmonella (iNTS) infections cause a high burden of lethal sepsis in young children and HIV patients, often associated with malaria, anaemia, malnutrition and sickle-cell disease. Vaccines against iNTS are urgently needed but none are licensed yet. Areas covered: This review illustrates how immunology, epidemiology and within-host pathogen behaviour affect invasive Salmonella infections and highlights how this knowledge can assist the improvement and choice of vaccines. Expert Commentary: Control of iNTS disease requires approaches that reduce transmission and improve diagnosis and treatment. These are often difficult to implement due to the fragile ecology and economies in endemic countries. Vaccines will be key tools in the fight against iNTS disease. To optimise vaccine design, we need to better define protective antigens and mechanisms of resistance to disease in susceptible populations even in those individuals where innate immunity may be impaired by widespread comorbidities.

Glycomaterials for probing host-pathogen interactions and the immune response

The initial engagement of host cells by pathogens is often mediated by glycan structures presented on the cell surface. Various components of the glycocalyx can be targeted by pathogens for adhesion to facilitate infection. Glycans also play integral roles in the modulation of the host immune response to infection. Therefore, understanding the parameters that define glycan interactions with both pathogens and the various components of the host immune system can aid in the development of strategies to prevent, interrupt, or manage infection. Glycomaterials provide a unique and powerful tool with which to interrogate the compositional and functional complexity of the glycocalyx. The objective of this review is to highlight some key contributions from this area of research in deciphering the mechanisms of pathogenesis and the associated host response.