A semisynthetic carbohydrate-lipid vaccine that protects against S. pneumoniae in mice

Ganglioside GM3-based anticancer vaccines: Reviewing the mechanism and current strategies

Ganglioside GM3 is one of the most common membrane-bound glycosphingolipids. The over-expression of GM3 on tumor cells makes it defined as a tumor-associated carbohydrate antigen (TACA). The specific expression property in cancers, especially in melanoma, make it become an important target to develop anticancer vaccines or immunotherapies. However, in the manner akin to most TACAs, GM3 is an autoantigen facing with problems of low immunogenicity and easily inducing immunotolerance, which means itself only cannot elicit a powerful enough immune response to prevent or treat cancer. With a comparative understanding of the mechanisms that how immune system responses to the carbohydrate vaccines, this review summarizes the studies on the recent efforts to development GM3-based anticancer vaccines.

A Glycolipid-Peptide-Hapten Tricomponent Conjugate Vaccine Generates Durable Antihapten Antibody Responses in Mice

Eliciting an antihapten antibody response to vaccination typically requires the use of constructs where multiple copies of the hapten are covalently attached to a larger carrier molecule. The carrier is required to elicit T cell help via presentation of peptide epitopes on major histocompatibility complex (MHC) class II molecules; as such, attachment to full-sized proteins, alone or in a complex, is generally used to account for the significant MHC diversity in humans. While such carrier-based vaccines have proven extremely successful, particularly in protecting against bacterial diseases, they can be challenging to manufacture, and repeated use can be compromised by pre-existing immunity against the carrier. One approach to reducing these complications is to recruit help from type I natural killer T (NKT) cells, which exhibit limited diversity in their antigen receptors and respond to glycolipid antigens presented by the highly conserved presenting molecule CD1d. Synthetic vaccines for universal use can, therefore, be prepared by conjugating haptens to an NKT cell agonist such as α-galactosylceramide (αGalCer, KRN7000). An additional advantage is that the quality of NKT cell help is sufficient to overcome the need for an extra immune adjuvant. However, while initial studies with αGalCer-hapten conjugate vaccines report strong and rapid antihapten antibody responses, they can fail to generate lasting memory. Here, we show that antibody responses to the hapten 4-hydoxy-3-nitrophenyl acetyl (NP) can be improved through additional attachment of a fusion peptide containing a promiscuous helper T cell epitope (Pan DR epitope, PADRE) that binds diverse MHC class II molecules. Such αGalCer-hapten-peptide tricomponent vaccines generate strong and sustained anti-NP antibody titers with increased hapten affinity compared to vaccines without the helper epitope. The tricomponent vaccine platform is therefore suitable for further exploration in the pursuit of efficacious antihapten immunotherapies.

Synthesis and immunological evaluation of TLR1/2 ligand-conjugated RBDs as self-adjuvanting vaccine candidates against SARS-CoV-2

We synthesized and evaluated Pam3CSK4-conjugated receptor binding domain (RBD)/deglycosylated RBD as potential anti-severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine candidates. Our investigation revealed the critical importance of limiting the number of introduced Pam3CSK4 molecules to the RBD in order to preserve its antigenicity. We also confirmed the harmonious integration of the adjuvant-conjugation strategy with the glycan-shield removal strategy.

Chemoenzymatic synthesis and immunological evaluation of sialyl-Thomsen-Friedenreich (sTF) antigen conjugate to CRM197

sTF (sialyl-Thomsen-Friedenreich) is a type of tumor-associated carbohydrate antigens (TACAs) and is highly expressed in various human malignancies. To validate if sTF could be a valuable molecular target for future cancer vaccine development, in this work the sTF antigen was prepared by adopting a strategy combining chemical and enzymatic methods, and then was covalently conjugated to a carrier protein, CRM197. The preliminary immunological evaluation, performed on BALB/c mice, revealed that the sTF-CRM197 conjugate elicited high titers of specific IgG antibodies. FACS experiments showed that the antisera induced by sTF-CRM197 conjugate could specifically recognize and bind to sTF-positive cancer cells T-47D. Furthermore, the conjugate mediated effective and specific antibody-mediated complement-dependent cytotoxicity (CDC).

Semisynthesis of homogeneous spike RBD glycoforms from SARS-CoV-2 for profiling the correlations between glycan composition and function

Vaccines have been the primary remedy in the global fight against coronavirus disease 2019 (COVID-19). The receptor-binding domain (RBD) of the spike protein, a critical viral immunogen, is affected by the heterogeneity of its glycan structures and relatively low immunogenicity. Here, we describe a scalable synthetic platform that enables the precise synthesis of homogeneously glycosylated RBD, facilitating the elucidation of carbohydrate structure-function relationships. Five homogeneously glycosylated RBDs bearing biantennary glycans were prepared, three of which were conjugated to T-helper epitope (Tpep) from tetanus toxoid to improve their weak immune response. Relative to natural HEK293-derived RBD, synthetic RBDs with biantennary N-glycan elicited a higher level of neutralising antibodies against SARS-CoV-2 in mice. Furthermore, RBDs containing Tpep elicited significant immune responses in transgenic mice expressing human angiotensin-converting enzyme 2. Our collective data suggest that trimming the N-glycans and Tpep conjugation of RBD could potentially serve as an effective strategy for developing subunit vaccines providing efficient protection.

Diversification of a Novel α-Galactosyl Ceramide Hotspot Boosts the Adjuvant Properties in Parenteral and Mucosal Vaccines

The development of potent adjuvants is an important step for improving the performance of subunit vaccines. CD1d agonists, such as the prototypical α-galactosyl ceramide (α-GalCer), are of special interest due to their ability to activate iNKT cells and trigger rapid dendritic cell maturation and B-cell activation. Herein, we introduce a novel derivatization hotspot at the α-GalCer skeleton, namely the N-substituent at the amide bond. The multicomponent diversification of this previously unexplored glycolipid chemotype space permitted the introduction of a variety of extra functionalities that can either potentiate the adjuvant properties or serve as handles for further conjugation to antigens toward the development of self-adjuvanting vaccines. This strategy led to the discovery of compounds eliciting enhanced antigen-specific T cell stimulation and a higher antibody response when delivered by either the parenteral or the mucosal route, as compared to a known potent CD1d agonist. Notably, various functionalized α-GalCer analogues showed a more potent adjuvant effect after intranasal immunization than a PEGylated α-GalCer analogue previously optimized for this purpose. Ultimately, this work could open multiple avenues of opportunity for the use of mucosal vaccines against microbial infections.

NKT Agonist-Antigen Conjugates as Cancer Vaccines

Natural killer T (NKT) cells are a population of innate-like T cells capable of enhancing both innate and adaptive immune responses. Co-delivering an NKT cell agonist and antigen can provide molecular signals to antigen-presenting cells, such as dendritic and B cells, that facilitate strong antigen-specific adaptive immune responses. Accordingly, there has been a significant number of developmental NKT cell-dependent vaccine therapies developed, particularly in the last decade, with many incorporating cancer antigens. In this review, we summarize studies that chemically conjugate the NKT cell agonist and antigen as an effective strategy for agonist-antigen co-delivery to drive antitumor responses.

Antigen/Adjuvant-Displaying Enveloped Viral Replica as a Self-Adjuvanting Anti-Breast-Cancer Vaccine Candidate

We report a promising cancer vaccine candidate comprising antigen/adjuvant-displaying enveloped viral replica as a novel vaccine platform. The artificial viral capsid, which consists of a self-assembled β-annulus peptide conjugated with an HER2-derived antigenic CH401 peptide, was enveloped within a lipid bilayer containing the lipidic adjuvant α-GalCer. The use of an artificial viral capsid as a scaffold enabled precise control of its size to ∼100 nm, which is generally considered to be optimal for delivery to lymph nodes. The encapsulation of the anionically charged capsid by a cationic lipid bilayer dramatically improved its stability and converted its surface charge to cationic, enhancing its uptake by dendritic cells. The developed CH401/α-GalCer-displaying enveloped viral replica exhibited remarkable antibody-production activity. This study represents a pioneering example of precise vaccine design through bottom-up construction and opens new avenues for the development of effective 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.

Innovative Vaccine Strategy: Self-Adjuvanting Conjugate Vaccines

Self-adjuvanting vaccines, covalent conjugates between antigens and adjuvants, are chemically well-defined compared with conventional vaccines formulated through mixing antigens with adjuvants. Innate immune receptor ligands effectively induce acquired immunity through the activation of innate immunity, thereby enhancing host immune responses. Thus, innate immune receptor ligands are often used as adjuvants in self-adjuvanting vaccines. In a self-adjuvanting vaccine, the covalent linkage of antigen and adjuvant enables their simultaneous uptake into immune cells where the adjuvant consequently induces antigen-specific immune responses. Importantly, self-adjuvanting vaccines do not require immobilization to carrier proteins or co-administration of additional adjuvants and thus avoid inducing undesired immune responses. Because of these excellent properties, self-adjuvanting vaccines are expected to be candidates for next-generation vaccines. Here, we take an overview of vaccine adjuvants, mainly focusing on those utilized in self-adjuvanting vaccines and then we review recent reports on self-adjuvanting conjugate vaccines.

A therapeutic vaccine strategy to prevent Pneumocystis pneumonia in an immunocompromised host in a non-human primate model of HIV and Pneumocystis co-infection

Introduction

Pneumocystis is a ubiquitous fungal pathogen that causes pneumonia (PCP) and pulmonary sequelae in HIV-infected individuals and other immunocompromised populations. With the success of anti-retroviral therapy for HIV-infected individuals the frequency of PCP in that population has decreased, however, PCP remains a significant cause of morbidity and mortality in individuals with hematologic and solid malignancies, and in individuals treated with immunosuppressive therapies for autoimmune diseases, and following bone marrow and solid organ transplantation. Despite the clinical need, there is no approved vaccine to prevent PCP in vulnerable populations. The ultimate goal of the field is to develop an effective vaccine that can overcome immune deficits in at risk populations and induce long-lasting protective immunity to Pneumocystis. Toward this goal, our laboratory has established a model of PCP co-infection in simian immunodeficiency virus (SIV)-infected non-human primates (NHP) and identified a recombinant protein sub-unit vaccine, KEX1, that induces robust anti-Pneumocystis immunity in immune-competent macaques that is durable and prevents PCP following simian immunodeficiency virus (SIV)-induced immunosuppression. Type I, or invariant natural killer T (iNKT) cells have the potential to provide B cell help under conditions of reduced CD4+ T cell help.

Methods

In the present study, we used the SIV model of HIV infection to address whether therapeutic vaccination with the iNKT cell-activating adjuvant α-galactosylceramide (α-GC) and KEX1 (α-GC+KEX1) can effectively boost anti-Pneumocystis humoral immunity following virus-induced immunosuppression.

Results

Immunization of antigen-experienced NHPs with α-GC+KEX1 during the early chronic phase of SIV-infection significantly boosted anti-Pneumocystis humoral immunity by increasing memory B cells and antibody titers, and enhanced titer durability during SIV-induced immunosuppression. This therapeutic vaccination strategy boosted anti-Pneumocystis immune responses during SIV-infection and contributed to protection against Pneumocystis co-infection in KEX1-vaccinated macaques.

Conclusion

These studies present a novel strategy for stimulating durable anti-Pneumocystis humoral immunity in the context of complex, chronic SIV-induced immunosuppression and may be further applied to immunization of other immunosuppressed populations, and toward other common recall antigens.

Use of a MAIT-Activating Ligand, 5-OP-RU, as a Mucosal Adjuvant in a Murine Model of Vibrio cholerae O1 Vaccination

Background

Mucosal-associated invariant T (MAIT) cells are innate-like T cells enriched in the mucosa with capacity for B-cell help. We hypothesize that targeting MAIT cells, using a MAIT-activating ligand as an adjuvant, could improve mucosal vaccine responses to bacterial pathogens such as Vibrio cholerae.

Methods

We utilized murine models of V. cholerae vaccination to test the adjuvant potential of the MAIT-activating ligand, 5-(2-oxopropylideneamino)-6-D-ribitylaminouracil (5-OP-RU). We measured V. cholerae-specific antibody and antibody-secreting cell responses and used flow cytometry to examine MAIT-cell and B-cell phenotype, in blood, bronchoalveolar lavage fluid (BALF), and mucosal tissues, following intranasal vaccination with live V. cholerae O1 or a V. cholerae O1 polysaccharide conjugate vaccine.

Results

We report significant expansion of MAIT cells in the lungs (P < 0.001) and BALF (P < 0.001) of 5-OP-RU treated mice, and higher mucosal (BALF, P = 0.045) but not systemic (serum, P = 0.21) V. cholerae O-specific-polysaccharide IgG responses in our conjugate vaccine model when adjuvanted with low-dose 5-OP-RU. In contrast, despite significant MAIT cell expansion, no significant differences in V. cholerae-specific humoral responses were found in our live V. cholerae vaccination model.

Conclusions

Using a murine model, we demonstrate the potential, as well as the limitations, of targeting MAIT cells to improve antibody responses to mucosal cholera vaccines. Our study highlights the need for future research optimizing MAIT-cell targeting for improving mucosal vaccines.

The influence of component structural arrangement on peptide vaccine immunogenicity

Peptide-based subunit vaccines utilise minimal immunogenic components (i.e. peptides) to generate highly specific immune responses, without triggering adverse reactions. However, strong adjuvants and/or effective delivery systems must be incorporated into such vaccines, as peptide antigens cannot induce substantial immune responses on their own. Unfortunately, many adjuvants are too weak or too toxic to be used in combination with peptide antigens. These shortcomings have been addressed by the conjugation of peptide antigens with lipidic/ hydrophobic adjuvanting moieties. The conjugates have shown promising safety profiles and improved immunogenicity without the help of traditional adjuvants and have been efficient in inducing desired immune responses following various routes of administration, including subcutaneous, oral and intranasal. However, not only conjugation per se, but also component arrangement influences vaccine efficacy. This review highlights the importance of influence of the vaccine chemical structure modification on the immune responses generated. It discusses a variety of factors that affect the immunogenicity of peptide conjugates, including: i) self-adjuvanting moiety length and number; ii) the orientation of epitopes and self-adjuvanting moieties in the conjugate; iii) the presence of spacers between conjugated components; iv) multiepitopic arrangement; and v) the effect of chirality on vaccine efficacy.

Antibiotics and Carbohydrate-Containing Drugs Targeting Bacterial Cell Envelopes: An Overview

Certain bacteria constitute a threat to humans due to their ability to escape host defenses as they easily develop drug resistance. Bacteria are classified into gram-positive and gram-negative according to the composition of the cell membrane structure. Gram-negative bacteria have an additional outer membrane (OM) that is not present in their gram-positive counterpart; the latter instead hold a thicker peptidoglycan (PG) layer. This review covers the main structural and functional properties of cell wall polysaccharides (CWPs) and PG. Drugs targeting CWPs are discussed, both noncarbohydrate-related (β-lactams, fosfomycin, and lipopeptides) and carbohydrate-related (glycopeptides and lipoglycopeptides). Bacterial resistance to these drugs continues to evolve, which calls for novel antibacterial approaches to be developed. The use of carbohydrate-based vaccines as a valid strategy to prevent bacterial infections is also addressed.

Precise immunological evaluation rationalizes the design of a self-adjuvanting vaccine composed of glycan antigen, TLR1/2 ligand, and T-helper cell epitope

Sialyl-Tn (STn), overexpressed on various tumors, has been investigated for its application in anti-cancer vaccine therapy. However, Theratope, an STn-based vaccine, failed in the phase III clinical trial due to poor immunogenicity and epitope suppression by the foreign carrier protein. We therefore developed a self-adjuvanting STn based-vaccine, a conjugate of clustered STn (triSTn) antigen, TLR1/2 ligand (Pam₃CSK₄), and T-helper (Th) cell epitope, and found that this three-component self-adjuvanting vaccine effectively resulted in the production of anti-triSTn IgG antibodies. We herein analyzed immune responses induced by this self-adjuvanting vaccine in detail. We newly synthesized two-component vaccines, i.e., Pam₃CSK₄- or Th epitope-conjugated triSTn, as references to evaluate the immune-stimulating functions of Pam₃CSK₄ and Th epitope. Immunological evaluation of the synthesized vaccine candidates revealed that Pam₃CSK₄ was essential for antibody production, indicating that the uptake of triSTn antigen by antigen-presenting cells (APCs) was promoted by the recognition of Pam₃CSK₄ by TLR1/2. The function of the Th epitope was also confirmed. Th cell activation was important for boosting antibody production and IgG subclass switching. Furthermore, flow cytometric analyses of immune cells, including T cells, B cells, dendritic cells, and other monocytes, were first employed in the evaluation of self-adjuvanting vaccines and revealed that the three-component vaccine was able to induce antigen-specific immune responses for efficient antibody production without excessive inflammatory responses. Importantly, the co-administration of Freund's adjuvants was suggested to cause excessive myeloid cell accumulation and decreased plasma cell differentiation. These results demonstrate that vaccines can be designed to achieve the desired immune responses via the bottom-up construction of each immune element.

Detailed analysis of a three-component self-adjuvanting vaccine revealed that conjugate vaccines can be designed to achieve the desired immune responses via bottom-up construction of the necessary immune elements.

Self-Adjuvanting Lipoprotein Conjugate αGalCer-RBD Induces Potent Immunity against SARS-CoV-2 and its Variants of Concern

Safe and effective vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and its variants are the best approach to successfully combat the COVID-19 pandemic. The receptor-binding domain (RBD) of the viral spike protein is a major target to develop candidate vaccines. α-Galactosylceramide (αGalCer), a potent invariant natural killer T cell (iNKT) agonist, was site-specifically conjugated to the N-terminus of the RBD to form an adjuvant-protein conjugate, which was anchored on the liposome surface. This is the first time that an iNKT cell agonist was conjugated to the protein antigen. Compared to the unconjugated RBD/αGalCer mixture, the αGalCer-RBD conjugate induced significantly stronger humoral and cellular responses. The conjugate vaccine also showed effective cross-neutralization to all variants of concern (B.1.1.7/alpha, B.1.351/beta, P.1/gamma, B.1.617.2/delta, and B.1.1.529/omicron). These results suggest that the self-adjuvanting αGalCer-RBD has great potential to be an effective COVID-19 vaccine candidate, and this strategy might be useful for designing various subunit vaccines.

Built-in adjuvants for use in vaccines

Vaccine refers to biological products that are produced using various pathogenic microorganisms for inoculation. The goal of vaccination is to induce a robust immune response against a specific antigen, thus preventing the organism from getting infected. In vaccines, adjuvants have been widely employed to enhance immunity against specific antigens. An ideal adjuvant should be stable, biodegradable, and low cost, not induce system rejection and promote an immune response. Various adjuvant components have been investigated across diverse applications. Typically, adjuvants are employed to meet the following objectives: (1) to improve the effectiveness of immunization with vaccines for specific populations, such as newborns and the elderly; (2) enhance the immunogenicity of highly purified or recombinant antigens; (3) allow immunization with a smaller dose of the vaccine, reducing drug dosage. In the present review, we primarily focus on chemically synthesized compounds that can be used as built-in adjuvants. We elaborate the classification of these compounds based on the induced immune activation mechanism and summarize their application in various vaccine types.

6″-Modifed α-GalCer-peptide conjugate vaccine candidates protect against liver-stage malaria

Self-adjuvanting vaccines consisting of peptide epitopes conjugated to immune adjuvants are a powerful way of generating antigen-specific immune responses. We previously showed that a Plasmodium-derived peptide conjugated to a rearranged form of α-galactosylceramide (α-GalCer) could stimulate liver-resident memory T (T_RM) cells that were effective killers of liver-stage Plasmodium berghei ANKA (Pba)-infected cells. To investigate if similar or even superior T_RM responses can be induced by modifying the α-GalCer adjuvant, we created new conjugate vaccine cadidates by attaching an immunogenic Plasmodium-derived peptide antigen to 6″-substituted α-GalCer analogues. Vaccine synthesis involved developing an efficient route to α-galactosylphytosphingosine (α-GalPhs), from which the prototypical iNKT cell agonist, α-GalCer, and its 6″-deoxy-6″-thio and -amino analogues were derived. Attaching a cathepsin B-cleavable linker to the 6″-modified α-GalCer created pro-adjuvants bearing a pendant ketone group available for peptide conjugation. Optimized reaction conditions were developed that allow for the efficient conjugation of peptide antigens to the pro-adjuvants via oxime ligation to create new glycolipid-peptide (GLP) conjugate vaccines. A single dose of the vaccine candidates induced acute NKT and Plasmodium-specific CD8⁺ T cell responses that generated potent hepatic T_RM responses in mice. Our findings demonstrate that attaching antigenic peptides to 6″-modifed α-GalCer generates powerful self-adjuvanting conjugate vaccine candidates that could potentially control hepatotropic infections such as liver-stage malaria.

Candidate vaccines comprised of peptide antigen conjugated to 6″-modified α-GalCer analogues generate potent hepatic T_RM responses in mice with a single dose inducing protective immunity against malaria in a Plasmodium sporozoite challenge model.

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.

Harnessing NKT cells for vaccination

Natural killer T (NKT) cells are innate-like T cells capable of enhancing both innate and adaptive immune responses. When NKT cells are stimulated in close temporal association with co-administered antigens, strong antigen-specific immune responses can be induced, prompting the study of NKT cell agonists as novel immune adjuvants. This activity has been attributed to the capacity of activated NKT cells to act as universal helper cells, with the ability to provide molecular signals to dendritic cells and B cells that facilitate T cell and antibody responses, respectively. These signals can override the requirement for conventional CD4⁺ T cell help, so that vaccines can be designed without need to consider CD4⁺ T cell repertoire and major histocompatibility complex Class II diversity. Animal studies have highlighted some drawbacks of the approach, namely, concerns around induction of NKT cell hyporesponsiveness, which may limit vaccine boosting, and potential for toxicity. Here we highlight studies that suggest these obstacles can be overcome by targeted delivery in vivo. We also feature new studies that suggest activating NKT cells can help encourage differentiation of T cells into tissue-resident memory cells that play an important role in prophylaxis against infection, and may be required in cancer therapy.

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.

Discovery of Semi- and Fully-Synthetic Carbohydrate Vaccines Against Bacterial Infections Using a Medicinal Chemistry Approach

The glycocalyx, a thick layer of carbohydrates, surrounds the cell wall of most bacterial and parasitic pathogens. Recognition of these unique glycans by the human immune system results in destruction of the invaders. To elicit a protective immune response, polysaccharides either isolated from the bacterial cell surface or conjugated with a carrier protein, for T-cell help, are administered. Conjugate vaccines based on isolated carbohydrates currently protect millions of people against Streptococcus pneumoniae, Haemophilus influenzae type b, and Neisseria meningitides infections. Active pharmaceutical ingredients (APIs) are increasingly discovered by medicinal chemistry and synthetic in origin, rather than isolated from natural sources. Converting vaccines from biologicals to pharmaceuticals requires a fundamental understanding of how the human immune system recognizes carbohydrates and could now be realized. To illustrate the chemistry-based approach to vaccine discovery, I summarize efforts focusing on synthetic glycan-based medicinal chemistry to understand the mammalian antiglycan immune response and define glycan epitopes for novel synthetic glycoconjugate vaccines against Streptococcus pneumoniae, Clostridium difficile, Klebsiella pneumoniae, and other bacteria. The chemical tools described here help us gain fundamental insights into how the human system recognizes carbohydrates and drive the discovery of carbohydrate vaccines.

Synthetic Homogeneous Glycoforms of the SARS-CoV-2 Spike Receptor-Binding Domain Reveals Different Binding Profiles of Monoclonal Antibodies

SARS‐CoV‐2 attaches to its host receptor, angiotensin‐converting enzyme 2 (ACE2), via the receptor‐binding domain (RBD) of the spike protein. The RBD glycoprotein is a critical target for the development of neutralizing antibodies and vaccines against SARS‐CoV‐2. However, the high heterogeneity of RBD glycoforms may lead to an incomplete neutralization effect and impact the immunogenic integrity of RBD‐based vaccines. Investigating the role of different carbohydrate domains is of paramount importance. Unfortunately, there is no viable method for preparing RBD glycoproteins with structurally defined glycans. Herein we describe a highly efficient and scalable strategy for the preparation of six glycosylated RBDs bearing defined structure glycoforms at T323, N331, and N343. A combination of modern oligosaccharide, peptide synthesis and recombinant protein engineering provides a robust route to decipher carbohydrate structure‐function relationships.

Synthetic Analogs of Streptococcus pneumoniae Capsular Polysaccharides and Immunogenic Activities of Glycoconjugates

Streptococcus pneumoniae is a Gram-positive bacterium (pneumococcus) that causes severe diseases in adults and children. It was established that some capsular polysaccharides of the clinically significant serotypes of S. pneumoniae in the composition of commercial pneumococcal polysaccharide or conjugate vaccines exhibit low immunogenicity. The review considers production methods and structural features of the synthetic oligosaccharides from the problematic pneumococcal serotypes that are characterized with low immunogenicity due to destruction or detrimental modification occurring in the process of their preparation and purification. Bacterial serotypes that cause severe pneumococcal diseases as well as serotypes not included in the composition of the pneumococcal conjugate vaccines are also discussed. It is demonstrated that the synthetic oligosaccharides corresponding to protective glycotopes of the capsular polysaccharides of various pneumococcal serotypes are capable of inducing formation of the protective opsonizing antibodies and immunological memory. Optimal constructs of oligosaccharides from the epidemiologically significant pneumococcal serotypes are presented that can be used for designing synthetic pneumococcal vaccines, as well as test systems for diagnosis of S. pneumoniae infections and monitoring of vaccination efficiency .

The Adjuvant of α-Galactosylceramide Presented by Gold Nanoparticles Enhances Antitumor Immune Responses of MUC1 Antigen-Based Tumor Vaccines

BACKGROUND

Therapeutic tumor vaccines are one of the most promising strategies and have attracted great attention in cancer treatment. However, most of them have shown unsatisfactory immunogenicity, there are still few available vaccines for clinical use. Therefore, there is an urgent demand to develop novel strategies to improve the immune efficacy of antitumor vaccines.

PURPOSE

This study aimed to develop novel adjuvants and carriers to enhance the immune effect of MUC1 glycopeptide antigen-based antitumor vaccines.

METHODS

An antitumor vaccine was developed, in which MUC1 glycopeptide was used as tumor-associated antigen, α-GalCer served as an immune adjuvant and AuNPs was a multivalent carrier.

RESULTS

Immunological evaluation results indicated that the constructed vaccines enabled a significant antibody response. FACS analysis and immunofluorescence assay showed that the induced antisera exhibited a specific binding with MUC1 positive MCF-7 cells. Moreover, the induced antibody can mediate CDC to kill MCF-7 cells. Besides stimulating B cells to produce MUC1-specific antibodies, the prepared vaccines also induced MUC1-specific CTLs in vitro. Furthermore, the vaccines significantly delayed tumor development in tumor-bearing mice model.

CONCLUSION

These results showed that the construction of vaccines by presenting α-GalCer adjuvant and an antigen on gold nanoparticles offers a potential strategy to improve the antitumor response in cancer immunotherapy.

Natural and synthetic carbohydrate-based vaccine adjuvants and their mechanisms of action

Modern subunit vaccines based on homogeneous antigens offer more precise targeting and improved safety compared with traditional whole-pathogen vaccines. However, they are also less immunogenic and require an adjuvant to increase the immunogenicity of the antigen and potentiate the immune response. Unfortunately, few adjuvants have sufficient potency and low enough toxicity for clinical use, highlighting the urgent need for new, potent and safe adjuvants. Notably, a number of natural and synthetic carbohydrate structures have been used as adjuvants in clinical trials, and two have recently been approved in human vaccines. However, naturally derived carbohydrate adjuvants are heterogeneous, difficult to obtain and, in some cases, unstable. In addition, their molecular mechanisms of action are generally not fully understood, partly owing to the lack of tools to elucidate their immune-potentiating effects, thus hampering the rational development of optimized adjuvants. To address these challenges, modification of the natural product structure using synthetic chemistry emerges as an attractive approach to develop well-defined, improved carbohydrate-containing adjuvants and chemical probes for mechanistic investigation. This Review describes selected examples of natural and synthetic carbohydrate-based adjuvants and their application in synthetic self-adjuvanting vaccines, while also discussing current understanding of their molecular mechanisms of action.

Recent advances in self-adjuvanting glycoconjugate vaccines

Compared to traditional vaccines that are formulated into mixtures of an adjuvant and an antigen, a self-adjuvanting vaccine consists of an antigen that is covalently conjugated to a well-defined adjuvant. In self-adjuvanting vaccines, innate immune receptor ligands are usually used as adjuvants. Innate immune receptor ligands effectively trigger acquired immunity through the activation of innate immunity to enhance host immune responses to antigens. When a self-adjuvanting vaccine is used, immune cells simultaneously uptake the antigen and the adjuvant because they are covalently linked. Consequently, the adjuvant can specifically induce immune responses against the conjugated antigen. Importantly, self-adjuvanting vaccines do not require co-administration of additional adjuvants or immobilization to carrier proteins, which enables avoidance of the use of highly toxic adjuvants or the induction of undesired immune responses. Given these excellent properties, self-adjuvanting vaccines are expected to serve as candidates for the next generation of vaccines. Herein, we review vaccine adjuvants, with a focus on the adjuvants used in self-adjuvanting vaccines, and then overview recent advances made with self-adjuvanting conjugate vaccines.

A practical and scalable synthesis of KRN7000 using glycosyl iodide as the glycosyl donor

KRN7000 is particularly useful because it is a powerful and specific CD1d agonist and has prompted intense interest in the context of immunology in the past 25 years. Its limited commercial availability and high price has led to the publication of many different syntheses. However, almost all of them focused on the methodology development rather than a scalable synthesis. Herein, we have described a practical and scalable procedure for the synthesis of KRN7000 basing on the glycosyl iodide method. This procedure involves total of eight steps to obtain the highly pure product KNR7000 on gram scale from the commercially available starting materials (d-galactose and the phytosphingosine) with only three column chromatographic purifications.

Advances in the Chemical Synthesis of Carbohydrates and Glycoconjugates

Carbohydrates are functional and structural biomolecules with structures ranging from monosaccharides to polysaccharides. They are naturally found as pure glycans or attached to lipids and proteins forming glycoconjugates. The biosynthesis of carbohydrates is not genetically controlled. The regulation takes place by the expression of enzymes that transfer and hydrolyze the glycan units, leading to glycocojugates having complex mixtures of glycan structures. Chemical synthesis emerged as the best strategy to obtain defined glycan and glycoconjugates and overcome the challenging purification processes. Here, we review the recent advances in the synthesis of oligosaccharides using manual and automated methods. The chapter covers the methods for the preparation of building blocks and control of stereoselectivity and regioselectivity during glycosylations. Finally, it also presents the strategies to obtain natural and non-natural glycoconjugates with lipids and proteins.

Immunological Evaluation of Co-Assembling a Lipidated Peptide Antigen and Lipophilic Adjuvants: Self-Adjuvanting Anti-Breast-Cancer Vaccine Candidates

Co-assembling vaccines composed of a lipidated HER2-derived antigenic CH401 peptide and either a lipophilic adjuvant, Pam₃ CSK₄ , α-GalCer, or lipid A 506, were evaluated as breast cancer vaccine candidates. This vaccine design was aimed to inherit both antigen multivalency and antigen-specific immunostimulation properties, observed in reported self-adjuvanting vaccine candidates, by using self-assembly and adjuvant-conjugated antigens. Under vaccination concentrations, respective lipophilic adjuvants underwent co-assembly with lipidated CH401, which boosted the anti-CH401 IgG and IgM production. In particular, α-GalCer was responsible for the most significant immune activation. Therefore, the newly developed vaccine design enabled the optimization of adjuvants against the antigenic CH401 peptide in a simple preparatory manner. Overall, the co-assembling vaccine design opens the door for efficient and practical self-adjuvanting vaccine development.

Self-Assembling Glycopeptide Conjugate as a Versatile Platform for Mimicking Complex Polysaccharides

Polysaccharides are a class of carbohydrates that play pivotal roles in living systems such as being chemical messengers in many vital biological pathways. However, the complexity and heterogeneity of these natural structures have posed daunting challenges on their production, characterization, evaluation, and applications. While there have been various types of synthetic skeletons that could mimic some biological aspects of polysaccharides, a safer and more easily accessed system is still desired to avoid the unnatural components and difficulties in modifying the structures. In this work, conveniently accessible self-assembling glycopeptide conjugates are developed, where the natural O-glycosidic linkages and phosphoryl modifications assist the self-assembly and concurrently reduce the risk of toxicity. The generated nanoparticles in aqueous solution offer a multivalent display of structurally controllable carbohydrates as mimics of polysaccharides, among which a mannosylated version exhibits immunostimulatory effects in both cellular assays and vaccination of mice. The obtained results demonstrate the potential of this glycopeptide conjugate-derived platform in exploiting the intriguing properties of carbohydrates in a more structurally maneuverable fashion.

Vaccines adjuvanted with an NKT cell agonist induce effective T-cell responses in models of CNS lymphoma

Aim: The efficacy of anti-lymphoma vaccines that exploit the cellular adjuvant properties of activated natural killer T (NKT) cells were examined in mouse models of CNS lymphoma. Materials & methods: Vaccines were prepared by either loading the NKT cell agonist, α-galactosylceramide onto irradiated and heat-shocked B- and T-lymphoma cells, or chemically conjugating α-galactosylceramide to MHC-binding peptides from a lymphoma-associated antigen. Vaccine efficacy was analyzed in mice bearing intracranial tumors. Results: Both forms of vaccine proved to be effective in preventing lymphoma engraftment through activity of T cells that accessed the CNS. Established lymphoma was harder to treat with responses constrained by Tregs, but this could be overcome by depleting Tregs prior to vaccination. Conclusion: Simply designed NKT cell-activating vaccines enhance T-cell responses and have the potential to protect against CNS lymphoma development or prevent CNS relapse. To be effective against established CNS lymphoma, vaccines need to be combined with Treg suppression.

Immunology of carbohydrate-based vaccines

Carbohydrates are considered as promising targets for vaccine development against infectious diseases where cell surface glycan's on many infectious agents are attributed to playing an important role in pathogenesis. Understanding the relationship between carbohydrates and immune components at a molecular level is crucial for the development of well-defined vaccines. Recently, carbohydrate immunology research has been accelerated by the development of new technologies that contribute to the design of optimum antigens, synthesis of antigens and the studies of antigen-antibody interactions, and as a result, several promising carbohydrate-based vaccine candidates have been prepared in recent years. This article briefly presents the mechanistic pathways of polysaccharide, glycoconjugate, glycolipid and zwitterionic vaccines and the interplay between carbohydrate antigen and immune response.

Carbohydrate Conjugates in Vaccine Developments

Vaccines are powerful tools that can activate the immune system for protection against various diseases. As carbohydrates can play important roles in immune recognition, they have been widely applied in vaccine development. Carbohydrate antigens have been investigated in vaccines against various pathogenic microbes and cancer. Polysaccharides such as dextran and β-glucan can serve as smart vaccine carriers for efficient antigen delivery to immune cells. Some glycolipids, such as galactosylceramide and monophosphoryl lipid A, are strong immune stimulators, which have been studied as vaccine adjuvants. In this review, we focus on the current advances in applying carbohydrates as vaccine delivery carriers and adjuvants. We will discuss the examples that involve chemical modifications of the carbohydrates for effective antigen delivery, as well as covalent antigen-carbohydrate conjugates for enhanced immune responses.

Nanoparticle formulated vaccines: opportunities and challenges

Vaccines harness the inherent properties of the immune system to prevent diseases or treat existing ones. Continuous efforts have been devoted to both gaining a mechanistic understanding of how the immune system operates and designing vaccines with high efficacies and effectiveness. Advancements in nanotechnology in recent years have generated unique opportunities to meet the daunting challenges associated with immunology and vaccine development. Firstly, nanoparticle formulated systems provide ideal model systems for studying the operation of the immune system, making it possible to systematically identify key factors and understand their roles in specific immune responses. Also, the versatile compositions/architectures of nanoparticle systems enable new strategies/novel platforms for developing vaccines with high efficacies and effectiveness. In this review, we discuss the advantages of nanoparticles and the challenges faced during vaccine development, through the framework of the immunological mechanisms of vaccination, with the aim of bridging the gap between immunology and materials science, which are both involved in vaccine design. The knowledge obtained provides general guidelines for future vaccine development.

Thermodynamic and Structural Behavior of α-Galactosylceramide and C6-Functionalized α-GalCer in 2D Layers at the Air-Liquid Interface

α-Galactosylceramide (α-GalCer; KRN7000) is a ligand for the glycoprotein CD1d that presents lipid antigens to natural killer T cells. Therefore, KRN7000 as well as some modified versions thereof have been widely investigated as part of novel immunotherapies. To examine the impact of structural modification, we investigated KRN7000 and C6-modified KRN7000 at the air-liquid interface using monolayer isotherms, BAM, IRRAS, GIXD, and TRXF. The amino group has no influence on the highly ordered sub-gel structures found at lateral pressures relevant for biological membranes. Neither lateral compression nor the protonation state of the amino group has a measurable effect on the lattice structure, which is defined by strong and rigid intermolecular hydrogen bonds. However, the first-order phase transition found for the C6-functionalized α-GalCer is connected with an extraordinary surface-inhibited nucleation. Our study demonstrates that KRN7000 can be functionalized at C6 without significantly changing the structural properties.

Modular Polymer Antigens To Optimize Immunity

Subunit vaccines can have excellent safety profiles, but their ability to give rise to robust immune responses is often compromised. For glycan-based vaccines, insufficient understanding of B and T cell epitope combinations that yield optimal immune activation hinders optimization. To determine which antigen features promote desired IgG responses, we synthesized epitope-functionalized polymers using ring-opening metathesis polymerization (ROMP) and assessed the effect of B and T cell epitope loading. The most robust responses were induced by polymers with a high valency of B and T cell epitopes. Additionally, IgG responses were greater for polymers with T cell epitopes that are readily liberated upon endosomal processing. Combining these criteria, we used ROMP to generate a nontoxic, polymeric antigen that elicited stronger antibody responses than a comparable protein conjugate. These findings highlight principles for designing synthetic antigens that elicit strong IgG responses against inherently weak immune targets such as glycans.

Leloir Glycosyltransferases in Applied Biocatalysis: A Multidisciplinary Approach

Enzymes are nature’s catalyst of choice for the highly selective and efficient coupling of carbohydrates. Enzymatic sugar coupling is a competitive technology for industrial glycosylation reactions, since chemical synthetic routes require extensive use of laborious protection group manipulations and often lack regio- and stereoselectivity. The application of Leloir glycosyltransferases has received considerable attention in recent years and offers excellent control over the reactivity and selectivity of glycosylation reactions with unprotected carbohydrates, paving the way for previously inaccessible synthetic routes. The development of nucleotide recycling cascades has allowed for the efficient production and reuse of nucleotide sugar donors in robust one-pot multi-enzyme glycosylation cascades. In this way, large glycans and glycoconjugates with complex stereochemistry can be constructed. With recent advances, LeLoir glycosyltransferases are close to being applied industrially in multi-enzyme, programmable cascade glycosylations.

Liposomal Antitumor Vaccines Targeting Mucin 1 Elicit a Lipid-Dependent Immunodominant Response

The tumor-associated antigen mucin 1 (MUC1) has been pursued as an attractive target for cancer immunotherapy, but the poor immunogenicity of the endogenous antigen hinders the development of vaccines capable of inducing effective anti-MUC1 immunodominant responses. Herein, we prepared synthetic anti-MUC1 vaccines in which the hydrophilic MUC1 antigen was N-terminally conjugated to one or two palmitoyl lipid chains (to form amphiphilic Pam-MUC1 or Pam₂ -MUC1). These amphiphilic lipid-tailed MUC1 antigens were self-assembled into liposomes containing the NKT cell agonist αGalCer as an adjuvant. The lipid-conjugated antigens reshaped the physical and morphological properties of liposomal vaccines. Promising results showed that the anti-MUC1 IgG antibody titers induced by the Pam₂ -MUC1 vaccine were more than 30- and 190-fold higher than those induced by the Pam-MUC1 vaccine and the MUC1 vaccine without lipid tails, respectively. Similarly, vaccines with the TLR1/2 agonist Pam₃ CSK₄ as an adjuvant also induced conjugated lipid-dependent immunological responses. Moreover, vaccines with the αGalCer adjuvant induced significantly higher titers of IgG antibodies than vaccines with the Pam₃ CSK₄ adjuvant. Therefore, the non-covalent assembly of the amphiphilic lipo-MUC1 antigen and the NKT cell agonist αGalCer as a glycolipid adjuvant represent a synthetically simple but immunologically effective approach for the development of anti-MUC1 cancer vaccines.

Recent advances in the production of recombinant glycoconjugate vaccines

Glycoconjugate vaccines against bacteria are one of the success stories of modern medicine and have led to a significant reduction in the global occurrence of bacterial meningitis and pneumonia. Glycoconjugate vaccines are produced by covalently linking a bacterial polysaccharide (usually capsule, or more recently O-antigen), to a carrier protein. Given the success of glycoconjugate vaccines, it is surprising that to date only vaccines against Haemophilus influenzae type b, Neisseria meningitis and Streptococcus pneumoniae have been fully licenced. This is set to change through the glycoengineering of recombinant vaccines in bacteria, such as Escherichia coli, that act as mini factories for the production of an inexhaustible and renewable supply of pure vaccine product. The recombinant process, termed Protein Glycan Coupling Technology (PGCT) or bioconjugation, offers a low-cost option for the production of pure glycoconjugate vaccines, with the in-built flexibility of adding different glycan/protein combinations for custom made vaccines. Numerous vaccine candidates have now been made using PGCT, which include those improving existing licenced vaccines (e.g., pneumococcal), entirely new vaccines for both Gram-positive and Gram-negative bacteria, and (because of the low production costs) veterinary pathogens. Given the continued threat of antimicrobial resistance and the potential peril of bioterrorist agents, the production of new glycoconjugate vaccines against old and new bacterial foes is particularly timely. In this review, we will outline the component parts of bacterial PGCT, including recent advances, the advantages and limitations of the technology, and future applications and perspectives.

Enhancing T cell responses and tumour immunity by vaccination with peptides conjugated to a weak NKT cell agonist

Activated NKT cells can stimulate antigen-presenting cells leading to enhanced peptide antigen-specific immunity. However, administration of potent NKT cell agonists like α-galactosylceramide (α-GalCer) can be associated with release of high levels of cytokines, and in some situations, hepatotoxicity. Here we show that it is possible to provoke sufficient NKT cell activity to stimulate strong antigen-specific T cell responses without these unwanted effects. This was achieved by chemically conjugating antigenic peptides to α-galactosylphytosphingosine (α-GalPhs), an NKT cell agonist with very weak activity based on structural characterisation and biological assays. Conjugation improved delivery to antigen-presenting cells in vivo, while use of a cathepsin-sensitive linker to release the α-GalPhs and peptide within the same cell promoted strong T cell activation and therapeutic anti-tumour responses in mice. The conjugates activated human NKT cells and enhanced human T cell responses to a viral peptide in vitro. Accordingly, we have demonstrated a means to safely exploit the immunostimulatory properties of NKT cells to enhance T cell activation for virus- and tumour-specific immunity.