Cognate stimulatory B-cell-T-cell interactions are critical for T-cell help recruited by glycoconjugate vaccines

Characterization of Turbo, a TLR Ligand-based Adjuvant for Glycoconjugate Vaccines

Many bacterial polysaccharide vaccines, including the typhoid Vi polysaccharide (ViPS) and tetravalent meningococcal polysaccharide conjugate (MCV4) vaccines, do not incorporate adjuvants and are not highly immunogenic, particularly in infants. I found that endotoxin, a TLR4 ligand in ViPS, contributes to the immunogenicity of typhoid vaccines. Because endotoxin is pyrogenic, and its levels are highly variable in vaccines, I developed monophosphoryl lipid A, a nontoxic TLR4 ligand-based adjuvant named Turbo. Admixing Turbo with ViPS and MCV4 vaccines improved their immunogenicity across all ages and eliminated booster requirement. To understand the characteristics of this adjuvanticity, I compared Turbo with alum. Unlike alum, which polarizes the response toward the IgG1 isotype, Turbo promoted Ab class switching to all IgG isotypes with affinity maturation; the magnitude of this IgG response is durable and accompanied by the presence of long-lived plasma cells in the mouse bone marrow. In striking contrast with the pathways employed by alum, Turbo adjuvanticity is independent of NLPR3, pyroptotic cell death effector Gasdermin D, and canonical and noncanonical inflammasome activation mediated by Caspase-1 and Caspase-11, respectively. Turbo adjuvanticity is primarily dependent on the MyD88 axis and is lost in mice deficient in costimulatory molecules CD86 and CD40, indicating that Turbo adjuvanticity includes activation of these pathways. Because Turbo formulations containing either monophosphoryl lipid A or TLR2 ligands, Pam2CysSerLys4, and Pam3CysSerLys4 help generate Ab response of all IgG isotypes, as an adjuvant Turbo can improve the immunogenicity of glycoconjugate vaccines against a wide range of bacterial pathogens whose elimination requires appropriate IgG isotypes.

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.

Profiling Germinal Center-like B Cell Responses to Conjugate Vaccines Using Synthetic Immune Organoids

Glycoengineered bacteria have emerged as a cost-effective platform for rapid and controllable biosynthesis of designer conjugate vaccines. However, little is known about the engagement of such conjugates with naïve B cells to induce the formation of germinal centers (GC), a subanatomical microenvironment that converts naïve B cells into antibody-secreting plasma cells. Using a three-dimensional biomaterials-based B-cell follicular organoid system, we demonstrate that conjugates triggered robust expression of hallmark GC markers, B cell receptor clustering, intracellular signaling, and somatic hypermutation. These responses depended on the relative immunogenicity of the conjugate and correlated with the humoral response in vivo. The occurrence of these mechanisms was exploited for the discovery of high-affinity antibodies against components of the conjugate on a time scale that was significantly shorter than for typical animal immunization-based workflows. Collectively, these findings highlight the potential of synthetic organoids for rapidly predicting conjugate vaccine efficacy as well as expediting antigen-specific antibody discovery.

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.

Polysaccharide and conjugate vaccines to Streptococcus pneumoniae generate distinct humoral responses

Streptococcus pneumoniae is a major cause of community-acquired pneumonia, bacteremia, and meningitis in older adults worldwide. Two pneumococcal vaccines containing S. pneumoniae capsular polysaccharides are in current use: the polysaccharide vaccine PPSV23 and the glycoconjugate vaccine PCV13. In clinical trials, both vaccines elicit similar opsonophagocytic killing activity. In contrast to polysaccharide vaccines, conjugate vaccines have shown consistent efficacy against nasopharyngeal carriage and noninvasive pneumonia overall and for some prevalent individual serotypes. Given these different clinical profiles, it is crucial to understand the differential immunological responses induced by these two vaccines. Here, we used a high-throughput systems serology approach to profile the biophysical and functional features of serum antibodies induced by PCV13 and PPSV23 at 1 month and 1 year. In comparison with PPSV23, PCV13 induced higher titers across antibody isotypes; more durable antibody responses across immunoglobulin G (IgG), IgA, and IgM isotypes; and increased antigenic breadth. Although titers measured in opsonophagocytic activity (OPA) assays were similar between the two groups, confirming what was observed in clinical studies, serum samples from PCV13 vaccinees could induce additional non-OPA antibody-dependent functions, including monocyte phagocytosis and natural killer cell activation. In a multivariate modeling approach, distinct humoral profiles were demonstrated in each arm. Together, these results demonstrate that the glycoconjugate PCV13 vaccine induces an antigenically broader, more durable, polyfunctional antibody response. These findings may help explain the increased protection against S. pneumoniae colonization and noninvasive pneumonia and the longer duration of protection against invasive pneumococcal disease, mediated by PCV13.

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.

Chemically Synthesized Alcaligenes Lipid A as an Adjuvant to Augment Immune Responses to Haemophilus Influenzae Type B Conjugate Vaccine

We previously identified Alcaligenes spp. as a commensal bacterium that resides in lymphoid tissues, including Peyer’s patches. We found that Alcaligenes-derived lipopolysaccharide acted as a weak agonist of Toll-like receptor four due to the unique structure of lipid A, which lies in the core of lipopolysaccharide. This feature allowed the use of chemically synthesized Alcaligenes lipid A as a safe synthetic vaccine adjuvant that induces Th17 polarization to enhance systemic IgG and respiratory IgA responses to T-cell–dependent antigens (e.g., ovalbumin and pneumococcal surface protein A) without excessive inflammation. Here, we examined the adjuvant activity of Alcaligenes lipid A on a Haemophilus influenzae B conjugate vaccine that contains capsular polysaccharide polyribosyl ribitol phosphate (PRP), a T-cell–independent antigen, conjugated with the T-cell–dependent tetanus toxoid (TT) antigen (i.e., PRP-TT). When mice were subcutaneously immunized with PRP alone or mixed with TT, Alcaligenes lipid A did not affect PRP-specific IgG production. In contrast, PRP-specific serum IgG responses were enhanced when mice were immunized with PRP-TT, but these responses were impaired in similarly immunized T-cell—deficient nude mice. Furthermore, TT-specific—but not PRP-specific—T-cell activation occurred in mice immunized with PRP-TT together with Alcaligenes lipid A. In addition, coculture with Alcaligenes lipid A promoted significant proliferation of and enhanced antibody production by B cells. Together, these findings suggest that Alcaligenes lipid A exerts an adjuvant activity on thymus-independent Hib polysaccharide antigen in the presence of a T-cell–dependent conjugate carrier antigen.

Synthesis and delivery of Streptococcus pneumoniae capsular polysaccharides by recombinant attenuated Salmonella vaccines

Pneumococcal infection-caused diseases are responsible for substantial morbidity and mortality worldwide. Traditional pneumococcal vaccines are developed based on purified capsular polysaccharides (CPS) or CPS conjugated to a protein carrier. Production processes of the traditional vaccines are laborious, and thereby increase the vaccine cost and limit their use in developing nations. A cost-effective pneumococcal vaccine using the recombinant attenuated Salmonella vaccine (RASV) was developed in this study. We cloned and expressed genes for seven serotypes of CPSs in the RASV strain. The RASV-delivered CPSs induced robust humoral and cell-mediated responses and mediated efficient protection of mice against pneumococcal infection. Our work provides an innovative strategy for mass producing low-cost bioconjugated polysaccharide vaccines for needle-free mucosal delivery against pneumococcal infections.

Streptococcus pneumoniae capsular polysaccharides (CPSs) are major determinants of bacterial pathogenicity. CPSs of different serotypes form the main components of the pneumococcal vaccines Pneumovax, Prevnar7, and Prevnar13, which substantially reduced the S. pneumoniae disease burden in developed countries. However, the laborious production processes of traditional polysaccharide-based vaccines have raised the cost of the vaccines and limited their impact in developing countries. The aim of this study is to develop a kind of low-cost live vaccine based on using the recombinant attenuated Salmonella vaccine (RASV) system to protect against pneumococcal infections. We cloned genes for seven different serotypes of CPSs to be expressed by the RASV strain. Oral immunization of mice with the RASV-CPS strains elicited robust Th1 biased adaptive immune responses. All the CPS-specific antisera mediated opsonophagocytic killing of the corresponding serotype of S. pneumoniae in vitro. The RASV-CPS2 and RASV-CPS3 strains provided efficient protection of mice against challenge infections with either S. pneumoniae strain D39 or WU2. Synthesis and delivery of S. pneumoniae CPSs using the RASV strains provide an innovative strategy for low-cost pneumococcal vaccine development, production, and use.

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.

Comparison of pneumococcal conjugate vaccine (PCV-13) cellular immune responses after primary and booster doses of vaccine

Since their widespread use, pneumococcal conjugate vaccines (PCVs) have proven effective at reducing both invasive and noninvasive pneumococcal diseases and nasopharyngeal carriage of Streptococcus pneumoniae (Spn). To establish this level of protection, a three-dose schedule with a single booster (3 + 1) was the immunization regime in the USA. Alternatively, WHO-approved schedules of 3 + 0 and 2 + 1 are now becoming adopted in many countries to reduce the cost of vaccination. Sustained protection from pneumococcal disease and carriage requires persisting levels of antibody and cellular immune memory. Although antibody responses to PCVs are well studied, less is known concerning the cellular response to the vaccine in young children. In this report, circulating PCV-13 serotype-specific B and T cell memory in paired blood samples from children before and after PCV13 dose 3 and booster immunizations was analyzed to determine changes in the adaptive immune response. No significant differences in memory B cell populations were detected comparing post dose 2 vs. post dose 3. However, the booster dose significantly increased the frequency of Spn-specific memory B cells compared to the pre-booster. Spn-specific memory T cells were not detected with the method used. These data suggest that booster vaccination increases Spn-specific memory B cells that may impact long-term protective antibody titers.

Glycoconjugate vaccines: some observations on carrier and production methods

Glycoconjugate vaccines use protein carriers to improve the immune response to polysaccharide antigens. The protein component allows the vaccine to interact with T cells, providing a stronger and longer-lasting immune response than a polysaccharide interacting with B cells alone. Whilst in theory the mere presence of a protein component in a vaccine should be sufficient to improve vaccine efficacy, the extent of improvement varies. In the present review, a comparison of the performances of vaccines developed with and without a protein carrier are presented. The usefulness of analytical tools for macromolecular integrity assays, in particular nuclear magnetic resonance, circular dichroism, analytical ultracentrifugation and SEC coupled to multi-angle light scattering (MALS) is indicated. Although we focus mainly on bacterial capsular polysaccharide-protein vaccines, some consideration is also given to research on experimental cancer vaccines using zwitterionic polysaccharides which, unusually for polysaccharides, are able to invoke T-cell responses and have been used in the development of potential all-polysaccharide-based cancer vaccines.A general trend of improved immunogenicity for glycoconjugate vaccines is described. Since the immunogenicity of a vaccine will also depend on carrier protein type and the way in which it has been linked to polysaccharide, the effects of different carrier proteins and production methods are also reviewed. We suggest that, in general, there is no single best carrier for use in glycoconjugate vaccines. This indicates that the choice of carrier protein is optimally made on a case-by-case basis, based on what generates the best immune response and can be produced safely in each individual case.Abbreviations: AUC: analytical ultracentrifugation; BSA: bovine serum albumin; CD: circular dichroism spectroscopy; CPS: capsular polysaccharide; CRM197: Cross Reactive Material 197; DT: diphtheria toxoid; Hib: Haemophilius influenzae type b; MALS: multi-angle light scattering; Men: Neisseria menigitidis; MHC-II: major histocompatibility complex class II; NMR: nuclear magnetic resonance spectroscopy; OMP: outer membrane protein; PRP: polyribosyl ribitol phosphate; PSA: Polysaccharide A1; Sa: Salmonella; St.: Streptococcus; SEC: size exclusion chromatography; Sta: Staphylococcus; TT: tetanus toxoid; ZPS: zwitterionic polysaccharide(s).

B Cell and CD4 T Cell Interactions Promote Development of Atherosclerosis

Interaction between B and CD4 T cells is crucial for their optimal responses in adaptive immunity. Immune responses augmented by their partnership promote chronic inflammation. Here we report that interaction between B and CD4 T cells augments their atherogenicity to promote lipid-induced atherosclerosis. Genetic deletion of the gene encoding immunoglobulin mu (μ) heavy chain (μMT) in ApoE^−/− mice resulted in global loss of B cells including those in atherosclerotic plaques, undetectable immunoglobulins and impaired germinal center formation. Despite unaffected numbers in the circulation and peripheral lymph nodes, CD4 T cells were also reduced in spleens as were activated and memory CD4 T cells. In hyperlipidemic μMT^−/− ApoE^−/− mice, B cell deficiency decreased atherosclerotic lesions, accompanied by absence of immunoglobulins and reduced CD4 T cell accumulation in lesions. Adoptive transfer of B cells deficient in either MHCII or co-stimulatory molecule CD40, molecules required for B and CD4 T cell interaction, into B cell-deficient μMT^−/− ApoE^−/− mice failed to increase atherosclerosis. In contrast, wildtype B cells transferred into μMT^−/− ApoE^−/− mice increased atherosclerosis and increased CD4 T cells in lesions including activated and memory CD4 T cells. Transferred B cells also increased their expression of atherogenic cytokines IL-1β, TGF-β, MCP-1, M-CSF, and MIF, with partial restoration of germinal centers and plasma immunoglobulins. Our study demonstrates that interaction between B and CD4 T cells utilizing MHCII and CD40 is essential to augment their function to increase atherosclerosis in hyperlipidemic mice. These findings suggest that targeting B cell and CD4 T cell interaction may be a therapeutic strategy to limit atherosclerosis progression.

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

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

Hepatitis E vaccine candidate harboring a non-particulate immunogen of E2 fused with CRM197 fragment A

The Hepatitis E vaccine (Hecolin, licensed in China) harbors a potent particulate immunogen, p239, designed from a 26-aa N-terminal extension of its poorly immunogenic parental protein, E2. Although an effective vaccine, we sought to design a fusion protein in a non-particulate form that could improve the delivery and immunogenicity of E2 epitopes. The non-toxic mutant of diphtheria toxin, CRM197 (Cross-Reacting Material 197) has been successfully used as a carrier protein for conjugated vaccines to enhance the immunogenicity of polysaccharides. Here, we designed a fusion non-particulate protein of E2 and the catalytic domain (fragment A) of CRM197 and evaluated its antigenicity, immunogenicity and disease prevention efficacy in primates. This fusion protein, named CRM197(A)-E2, was bacterially expressed and purified by chromatography. CRM197(A)-E2 presented as a homodimer in solution, similar to its parental E2 protein, and exhibited excellent antigenicity against representative neutralizing monoclonal antibodies, like E2 and p239. However, CRM197(A)-E2 manifested higher immunogenicity in mice compared with that achieved by the particulate p239, as indicated by the 10-times lower ED₅₀ value and 2-log higher HEV-specific antibody level that could persist for at least 28 weeks. In addition, both the 1 μg and 10 μg doses of CRM197(A)-E2 adjuvanted with aluminum could protect vaccinated monkeys against HEV challenge, matching that achieved with only the higher (10 μg) dose of the p239 vaccine. These results suggest that the CRM197 fragment A alone serves as an intra-molecular adjuvant to remarkably enhance the immunogenicity of the target of interest in a non-particulate form. These findings may pave the way for rational vaccine design, especially in cases where particulates are not accessible.

Protein glycosylation: Sweet or bitter for bacterial pathogens?

Protein glycosylation systems in many bacteria are often associated with crucial biological processes like pathogenicity, immune evasion and host-pathogen interactions, implying the significance of protein-glycan linkage. Similarly, host protein glycosylation has been implicated in antimicrobial activity as well as in promoting growth of beneficial strains. In fact, few pathogens notably modulate host glycosylation machineries to facilitate their survival. To date, diverse chemical and biological strategies have been developed for conjugate vaccine production for disease control. Bioconjugate vaccines, largely being produced by glycoengineering using PglB (the N-oligosaccharyltransferase from Campylobacter jejuni) in suitable bacterial hosts, have been highly promising with respect to their effectiveness in providing protective immunity and ease of production. Recently, a novel method of glycoconjugate vaccine production involving an O-oligosaccharyltransferase, PglL from Neisseria meningitidis, has been optimized. Nevertheless, many questions on defining antigenic determinants, glycosylation markers, species-specific differences in glycosylation machineries, etc. still remain unanswered, necessitating further exploration of the glycosylation systems of important pathogens. Hence, in this review, we will discuss the impact of bacterial protein glycosylation on its pathogenesis and the interaction of pathogens with host protein glycosylation, followed by a discussion on strategies used for bioconjugate vaccine development.

Polysaccharide structure dictates mechanism of adaptive immune response to glycoconjugate vaccines

Glycoconjugate vaccines are among the most effective interventions for preventing several serious infectious diseases. Covalent linkage of the bacterial capsular polysaccharide to a carrier protein provides CD4⁺ T cells with epitopes that facilitate a memory response to the polysaccharide. Classically, the mechanism responsible for antigen processing was thought to be similar to what was known for hapten-carrier conjugates: protease digestion of the carrier protein in the endosome and presentation of a resulting peptide to the T cell receptor on classical peptide-recognizing CD4⁺ T cells. Recently, an alternative mechanism has been shown to be responsible for the memory response to some glycoconjugates. Processing of both the protein and the polysaccharide creates glycopeptides in the endosome of antigen-presenting cells. For presentation, the peptide portion of the glycopeptide is bound to MHCII, allowing the covalently linked glycan to activate carbohydrate-specific helper CD4⁺ T cells (Tcarbs). Herein, we assessed whether this same mechanism applies to conjugates prepared from other capsular polysaccharides. All of the glycoconjugates tested induced Tcarb-dependent responses except that made with group C Neisseria meningitidis; in the latter case, only peptides generated from the carrier protein were critical for helper T cell recognition. Digestion of this acid-sensitive polysaccharide, a linear homopolymer of α(2 → 9)-linked sialic acid, to the size of the monomeric unit resulted in a dominant CD4⁺ T cell response to peptides in the context of MHCII. Our results show that different mechanisms of presentation, based on the structure of the carbohydrate, are operative in response to different glycoconjugate vaccines.

Recent Advances in Toll Like Receptor-Targeting Glycoconjugate Vaccines

Many malignant cell surface carbohydrates resulting from abnormal glycosylation patterns of certain diseases can serve as antigens for the development of vaccines against these diseases. However, carbohydrate antigens are usually poorly immunogenic by themselves, thus they need to be covalently coupled with immunologically active carrier molecules to be functional. The most well established and commonly used carriers are proteins. In recent years, the use of toll-like receptor (TLR) ligands to formulate glycoconjugate vaccines has gained significant attention because TLR ligands can serve not only as carrier molecules but also as built-in adjuvants to form fully synthetic and self-adjuvanting conjugate vaccines, which have several advantages over carbohydrate-protein conjugates and formulated mixtures with external adjuvants. This article reviews recent progresses in the development of conjugate vaccines based on TLR ligands. Two major classes of TLR ligands, lipopeptides and lipid A derivatives will be covered with more focus on monophosohoryl lipid A (MPLA) and related analogs, which are TLR4 ligands demonstrated to be able to provoke T cell-dependent, adaptive immune responses. Corresponding conjugate vaccines have shown promising application potentials to multiple diseases including cancer.

Outer membrane vesicles displaying engineered glycotopes elicit protective antibodies

The O-antigen polysaccharide (O-PS) component of lipopolysaccharides on the surface of gram-negative bacteria is both a virulence factor and a B-cell antigen. Antibodies elicited by O-PS often confer protection against infection; therefore, O-PS glycoconjugate vaccines have proven useful against a number of different pathogenic bacteria. However, conventional methods for natural extraction or chemical synthesis of O-PS are technically demanding, inefficient, and expensive. Here, we describe an alternative methodology for producing glycoconjugate vaccines whereby recombinant O-PS biosynthesis is coordinated with vesiculation in laboratory strains of Escherichia coli to yield glycosylated outer membrane vesicles (glycOMVs) decorated with pathogen-mimetic glycotopes. Using this approach, glycOMVs corresponding to eight different pathogenic bacteria were generated. For example, expression of a 17-kb O-PS gene cluster from the highly virulent Francisella tularensis subsp. tularensis (type A) strain Schu S4 in hypervesiculating E. coli cells yielded glycOMVs that displayed F. tularensis O-PS. Immunization of BALB/c mice with glycOMVs elicited significant titers of O-PS-specific serum IgG antibodies as well as vaginal and bronchoalveolar IgA antibodies. Importantly, glycOMVs significantly prolonged survival upon subsequent challenge with F. tularensis Schu S4 and provided complete protection against challenge with two different F. tularensis subsp. holarctica (type B) live vaccine strains, thereby demonstrating the vaccine potential of glycOMVs. Given the ease with which recombinant glycotopes can be expressed on OMVs, the strategy described here could be readily adapted for developing vaccines against many other bacterial pathogens.

Distinct Mechanisms Underlie Boosted Polysaccharide-Specific IgG Responses Following Secondary Challenge with Intact Gram-Negative versus Gram-Positive Extracellular Bacteria

Priming of mice with intact, heat-killed cells of Gram-negative Neisseria meningitidis, capsular serogroup C (MenC) or Gram-positive group B Streptococcus, capsular type III (GBS-III) bacteria resulted in augmented serum polysaccharide (PS)-specific IgG titers following booster immunization. Induction of memory required CD4(+) T cells during primary immunization. We determined whether PS-specific memory for IgG production was contained within the B cell and/or T cell populations, and whether augmented IgG responses following booster immunization were also dependent on CD4(+) T cells. Adoptive transfer of purified B cells from MenC- or GBS-III-primed, but not naive mice resulted in augmented PS-specific IgG responses following booster immunization. Similar responses were observed when cotransferred CD4(+) T cells were from primed or naive mice. Similarly, primary immunization with unencapsulated MenC or GBS-III, to potentially prime CD4(+) T cells, failed to enhance PS-specific IgG responses following booster immunization with their encapsulated isogenic partners. Furthermore, in contrast to GBS-III, depletion of CD4(+) T cells during secondary immunization with MenC or another Gram-negative bacteria, Acinetobacter baumannii, did not inhibit augmented PS-specific IgG booster responses of mice primed with heat-killed cells. Also, in contrast with GBS-III, booster immunization of MenC-primed mice with isolated MenC-PS, a TI Ag, or a conjugate of MenC-PS and tetanus toxoid elicited an augmented PS-specific IgG response similar to booster immunization with intact MenC. These data demonstrate that memory for augmented PS-specific IgG booster responses to Gram-negative and Gram-positive bacteria is contained solely within the B cell compartment, with a differential requirement for CD4(+) T cells for augmented IgG responses following booster immunization.

Development and characterization of Haemophilus influenzae type b conjugate vaccine prepared using different polysaccharide chain lengths

Capsular polysaccharide conjugates of Haemophilus influenzae type b (Hib) are important components of several mono- or multi-valent childhood vaccines. However, their access to the most needy people is limited due to their high cost. As a step towards developing a cost effective and more immunogenic Hib conjugate vaccine, we present a method for the preparation of Hib capsular polysaccharide (PRP)-tetanus toxoid (TT) conjugates using optimized PRP chain length and conjugation conditions. Reactive aldehyde groups were introduced into the polysaccharides by controlled periodate oxidation of the native polysaccharide, which were subsequently covalently linked to hydrazide derivatized tetanus toxoid by means of reductive amination. Native polysaccharides were reduced to average 100 or 50kDa polysaccharide and 10kDa oligosaccharides in a controlled manner. Various conjugates were prepared using Hib polysaccharide and oligosaccharide yielding conjugates with polysaccharide to protein ratios in the range of 0.25-0.5 (w/w) and free saccharide levels of less than 10%. Immunization of Sprague Dawley rats with the conjugates elicited specific antibodies to PRP. The low molecular weight PRP-TT conjugates were found to be more immunogenic as compared to their high molecular weight counterparts and the PRP-TT reference vaccine.

Intact reducing glycan promotes the specific immune response to lacto-N-neotetraose-BSA neoglycoconjugates

The mammalian immune system responds to eukaryotic glycan antigens during infections, cancer, and autoimmune disorders, but the immunological bases for such responses are unclear. Conjugate vaccines containing bacterial polysaccharides linked to carrier proteins (neoglycoconjugates) have proven successful, but these often contain repeating epitopes and the reducing end of the glycan is less important, unlike typical glycan determinants in eukaryotes, which are shorter in length and may include the reducing end. Here, we have compared the effects of two linkage methods, one that opens the ring at the reducing end of the glycan, and one that leaves the reducing end closed, on the glycan specificity of the vaccine response in rabbits and mice. We immunized rabbits and mice with bovine serum albumin (BSA) conjugates of synthetic open- and closed-ring forms (OR versus CR) of a simple tetrasaccharide lacto-N-neotetraose (LNnT, Galβ1-4GlcNAcβ1-3Galβ1-4Glc), and tested reactivity to the immunogens and several related glycans in both OR and CR versions on glycan microarrays. We found that in rabbits the immune response to the CR conjugate was directed toward the glycan, whereas the OR conjugate elicited antibodies to the reducing end of the glycan and linker region but not specifically to the glycan itself. Unexpectedly, mice did not generate a glycan-specific response to the CR conjugate. Our findings indicate that the reducing end of the sugar is crucial for generation of a glycan-specific response to some eukaryotic vaccine epitopes, and that there are species-specific differences in the ability to make a glycan-specific response to some glycoconjugates. These findings warrant further investigation with regard to rational design of glycoconjugate vaccines.

Glycans as Vaccine Antigens and Adjuvants: Immunological Considerations

Carbohydrates can be found on the cell surface of nearly every cell ranging from bacteria to fungi right up to mammalian cells. Carbohydrates and their interactions with carbohydrate-binding proteins play crucial roles in multiple biological processes including immunity, homeostasis, cellular communication, cell migration, and the regulation of serum glycoprotein levels. In the last decades, the interest in exploiting the biological activity of glycans as vaccine components has considerably increased. On the one hand, carbohydrates display epitopes to generate protective antibodies against pathogen-derived cell wall structures and on the other hand, glycans have the potential to stimulate the immune system; thus they can act as potent vaccine adjuvants.An effective vaccine consists of two major components, the vaccine antigen and an adjuvant. The vaccine antigen is an original or modified part of the pathogen that causes the disease. The immune response triggered by vaccination should induce antigen-specific plasma cells secreting protective antibodies as well as the development of memory T and B cells. Carbohydrate structures on pathogens represent an important class of antigens that can activate B cells to produce protective anti-carbohydrate antibodies in adults. A major breakthrough in vaccine development was the design of conjugate vaccines that evoke protective antibody responses against encapsulated bacteria strains such as Haemophilus influenzae, Streptococcus pneumoniae, or Neisseria meningitidis in adults, but also in young children. The first part of this chapter focuses on immune responses triggered by carbohydrate-based vaccines. The second part of the chapter discusses the immunological mechanisms of carbohydrate-based adjuvants to increase the immunogenicity of vaccines.

Vaccination against Group B streptococcus

Streptococcus agalactiae (Group B streptococcus) is an important cause of disease in infants, pregnant women, the elderly and in immunosuppressed adults. An effective vaccine is likely to prevent the majority of infant disease (both early and late onset), as well as Group B streptococcus-related stillbirths and prematurity, to avoid the current real and theoretical limitations of intrapartum antibiotic prophylaxis, and to be cost effective. The optimal time to administer such a vaccine would be in the third trimester of pregnancy. The main limitations on the production of a Group B streptococcus vaccine are not technical or scientific, but regulatory and legal. A number of candidates including capsular conjugate vaccines using traditional carrier proteins such as tetanus toxoid and mutant diphtheria toxin CRM197, as well as Group B streptococcus-specific proteins such as C5a peptidase, protein vaccines using one or more Group B streptococcus surface proteins and mucosal vaccines, have the potential to be successful vaccines. The capsular conjugate vaccines using tetanus and CRM197 carrier proteins are the most advanced candidates, having already completed Phase II human studies including use in the target population of pregnant women (tetanus toxoid conjugate), however, no definitive protein conjugates have yet been trialed. However, unless the regulatory environment is changed specifically to allow the development of a Group B streptococcus vaccine, it is unlikely that one will ever reach the market.

Enhancement of serum and mucosal immune responses to a Haemophilus influenzae Type B vaccine by intranasal delivery

Intranasal (i.n.) vaccination is potentially the most direct method for conveying upper respiratory and mucosal immunity to respiratory pathogens. However, for unclear reasons, vaccines introduced into the nasal sinuses often have lower efficacy than vaccines administered by the more frequently used parenteral routes. We examined i.n. vaccination in a mouse immune-response model with a commonly used Haemophilus influenzae type B vaccine (Hibv) composed of the polyribosylribitol phosphate (PRP) capsule antigen conjugated to tetanus toxoid. Intranasal vaccination with Hibv using a Toll-like receptor 4 (TLR4) agonist as an adjuvant significantly increased the levels of IgA specific for the PRP capsule antigen in blood serum, saliva, and mucosal secretion specimens. In contrast, control mice vaccinated transdermally (t.d.) with Hibv did not produce significant levels of PRP-specific IgA in the blood serum and saliva, and anti-PRP IgG was increased only in serum. The i.n. and t.d. vaccinations resulted in equivalent bactericidal antibody responses in blood serum, suggesting that vaccine-derived IgG is protective against infection. Elevated levels of IgG specific for the tetanus toxoid carrier protein were measured in nasal sinuses and vaginal secretions in mice vaccinated by either the t.d. or i.n. route. Tissue culture studies confirmed that the nasopharynx-associated lymphoid tissue (NALT) was at least one of the sources of PRP-specific IgA and carrier-specific IgG within the nasal sinuses. We conclude that i.n. vaccination aided by a TLR4 agonist results in robust immune responses to both the carrier protein and bacterial polysaccharide components of the Hibv.

Recent mechanistic insights on glycoconjugate vaccines and future perspectives

Vaccination is a key strategy for the control of various infectious diseases. Many pathogens, such as Streptococcus pneumoniae , Haemophilus influenzae type b (Hib), and Neisseria meningitidis produce on their surfaces dense and complex glycan structures, which represent an optimal target for eliciting carbohydrate specific antibodies able to confer protection against those bacteria. Glycoconjugates represent nowadays an important class of efficacious and safe commercial vaccines. It has been known for a long time that covalent linkage of poorly immunogenic carbohydrates to protein is fundamental to provide T cell epitopes for eliciting a memory response of the immune system against the saccharide. However, while the traditional mechanism of action of glycoconjugates has considered peptides generated from the carrier protein to be responsible of T cell help recruitment, only recently evidence of the active involvement of the carbohydrate part in determining the T cell help has been shown. In addition, zwitterionic polysaccharides have been proven to activate the adaptive immune system without further conjugation to protein. Progress in this interface area between chemistry and biology, in combination with novel synthetic and biosynthetic methods for the preparation of glycoconjugates, is opening new perspectives to clarify their mechanism of action and give new insights for the design of improved carbohydrate-based vaccines.

Carbohydrates and T cells: a sweet twosome

Carbohydrates as T cell-activating antigens have been generating significant interest. For many years, carbohydrates were thought of as T-independent antigens, however, more recent research had demonstrated that mono- or oligosaccharides glycosidically linked to peptides can be recognized by T cells. T cell recognition of these glycopeptides depends on the structure of both peptide and glycan portions of the antigen. Subsequently, it was discovered that natural killer T cells recognized glycolipids when presented by the antigen presenting molecule CD1d. A transformative insight into glycan-recognition by T cells occurred when zwitterionic polysaccharides were discovered to bind to and be presented by MHCII to CD4+ T cells. Based on this latter observation, the role that carbohydrate epitopes generated from glycoconjugate vaccines had in activating helper T cells was explored and it was found that these epitopes are presented to specific carbohydrate recognizing T cells through a unique mechanism. Here we review the key interactions between carbohydrate antigens and the adaptive immune system at the molecular, cellular and systems levels exploring the significant biological implications in health and disease.

A novel tetrameric gp350 1-470 as a potential Epstein-Barr virus vaccine

Infectious mononucleosis and B-cell transformation in response to infection with Epstein-Barr virus (EBV) is dependent upon binding of the EBV envelope glycoprotein gp350 to CD21 on B-cells. Gp350-specific antibody comprises most of the EBV neutralizing activity in the serum of infected patients, making this protein a promising target antigen for a prophylactic EBV vaccine. We describe a novel, tetrameric gp350-based vaccine that exhibits markedly enhanced immunogenicity relative to its monomeric counterpart. Plasmid DNA was constructed for synthesis, within transfected CHO cells, of a tetrameric, truncated (a.a. 1-470) gp350 protein (gp350(1-470)). Tetrameric gp350(1-470) induced ≈ 20-fold higher serum titers of gp350(1-470)-specific IgG and >19-fold enhancements in neutralizing titers at the highest dose, and was >25-fold more immunogenic on a per-weight basis than monomeric gp350(1-470). Further, epidermal immunization with plasmid DNA encoding gp350(1-470) tetramer induced 8-fold higher serum titers of gp350(1-470)-specific IgG relative to monomer. Tetrameric gp350(1-470) binding to human CD21 was >24-fold more efficient on a per-weight basis than monomer, but neither tetramer nor monomer mediated polyclonal human B-cell activation. Finally, the introduction of strong, universal tetanus toxoid (TT)-specific CD4+ T-cell epitopes into the tetrameric gp350(1-470) had no effect on the gp350(1-470)-specific IgG response in naïve mice, and resulted in suppressed gp350(1-470)-specific IgG responses in TT-primed mice. Collectively, these data suggest that tetrameric gp350(1-470) is a potentially promising candidate for testing as a prophylactic EBV vaccine, and that protein multimerization, using the approach described herein, is likely to be clinically relevant for enhancing the immunogenicity of other proteins of vaccine interest.

Novel synthetic (poly)glycerolphosphate-based antistaphylococcal conjugate vaccine

Staphylococcal infections are a major source of global morbidity and mortality. Currently there exists no antistaphylococcal vaccine in clinical use. Previous animal studies suggested a possible role for purified lipoteichoic acid as a vaccine target for eliciting protective IgG to several Gram-positive pathogens. Since the highly conserved (poly)glycerolphosphate backbone of lipoteichoic acid is a major antigenic target of the humoral immune system during staphylococcal infections, we developed a synthetic method for producing glycerol phosphoramidites to create a covalent 10-mer of (poly)glycerolphosphate for potential use in a conjugate vaccine. We initially demonstrated that intact Staphylococcus aureus elicits murine CD4(+) T cell-dependent (poly)glycerolphosphate-specific IgM and IgG responses in vivo. Naive mice immunized with a covalent conjugate of (poly)glycerolphosphate and tetanus toxoid in alum plus CpG-oligodeoxynucleotides produced high secondary titers of serum (poly)glycerolphosphate-specific IgG. Sera from immunized mice enhanced opsonophagocytic killing of live Staphylococcus aureus in vitro. Mice actively immunized with the (poly)glycerolphosphate conjugate vaccine showed rapid clearance of staphylococcal bacteremia in vivo relative to mice similarly immunized with an irrelevant conjugate vaccine. In contrast to purified, natural lipoteichoic acid, the (poly)glycerolphosphate conjugate vaccine itself exhibited no detectable inflammatory activity. These data suggest that a synthetic (poly)glycerolphosphate-based conjugate vaccine will contribute to active protection against extracellular Gram-positive pathogens expressing this highly conserved backbone structure in their membrane-associated lipoteichoic acid.

Conjugation of polysaccharide 6B from Streptococcus pneumoniae with pneumococcal surface protein A: PspA conformation and its effect on the immune response

Despite the substantial beneficial effects of incorporating the 7-valent pneumococcal conjugate vaccine (PCV7) into immunization programs, serotype replacement has been observed after its widespread use. As there are many serotypes currently documented, the use of a conjugate vaccine relying on protective pneumococcal proteins as active carriers is a promising alternative to expand PCV coverage. In this study, capsular polysaccharide serotype 6B (PS6B) and recombinant pneumococcal surface protein A (rPspA), a well-known protective antigen from Streptococcus pneumoniae, were covalently attached by two conjugation methods. The conjugation methodology developed by our laboratory, employing 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMT-MM) as an activating agent through carboxamide formation, was compared with reductive amination, a classical methodology. DMT-MM-mediated conjugation was shown to be more efficient in coupling PS6B to rPspA clade 1 (rPspA1): 55.0% of PS6B was in the conjugate fraction, whereas 24% was observed in the conjugate fraction with reductive amination. The influence of the conjugation process on the rPspA1 structure was assessed by circular dichroism. According to our results, both conjugation processes reduced the alpha-helical content of rPspA; reduction was more pronounced when the reaction between the polysaccharide capsule and rPspA1 was promoted between the carboxyl groups than the amine groups (46% and 13%, respectively). Regarding the immune response, both conjugates induced functional anti-rPspA1 and anti-PS6B antibodies. These results suggest that the secondary structure of PspA1, as well as its reactive groups (amine or carboxyl) involved in the linkage to PS6B, may not play an important role in eliciting a protective immune response to the antigens.

Inflammatory monocytes are critical for induction of a polysaccharide-specific antibody response to an intact bacterium

Although inflammatory monocytes (IM) (CD11b(+)Ly6C(hi) cells) have been shown to play important roles in cell-mediated host protection against intracellular bacteria, protozoans, and fungi, their potential impact on humoral immune responses to extracellular bacteria are unknown. IM, localized largely to the splenic marginal zone of naive CD11b-diphtheria toxin (DT) receptor bone marrow-chimeric mice were selectively depleted following treatment with DT, including no reduction of CD11b(+) peritoneal B cells. Depletion of IM resulted in a marked reduction in the polysaccharide (PS)-specific, T cell-independent IgM, and T cell-dependent IgG responses to intact, heat-killed Streptococcus pneumoniae with no effect on the associated S. pneumoniae protein-specific IgG response or on the PS- and protein-specific IgG responses to a soluble pneumococcal conjugate vaccine. IM acted largely within the first 48 h following the initiation of the immune response to S. pneumoniae to induce the subsequent production of PS-specific IgM and IgG. Adoptive transfer of highly purified IM from wild-type mice into DT-treated CD11b-DT receptor mice completely restored the defective PS-specific Ig response to S. pneumoniae. IM were phenotypically and functionally distinct from circulating CD11b(+)CD11c(low)Ly6G/C cells (immature blood dendritic cells), previously described to play a role in Ig responses to S. pneumoniae, in that they were CD11c(-) as well as Ly6C(hi) and did not internalize injected S. pneumoniae during the early phase of the response. These data are the first, to our knowledge, to establish a critical role for IM in the induction of an Ig response to an intact extracellular bacterium.

Isolation of carbohydrate-specific CD4(+) T cell clones from mice after stimulation by two model glycoconjugate vaccines

Here we describe how to isolate carbohydrate-specific T cell clones (for which we propose the designation 'Tcarbs') after stimulation by two glycoconjugate vaccines. We describe how to prepare, purify and characterize two model glycoconjugate vaccines that can be used to generate Tcarbs. These glycoconjugate vaccines (GBSIII-OVA and GBSIII-TT) are synthesized by conjugation of type III group B streptococcal polysaccharide (GBSIII) to ovalbumin (OVA) or tetanus toxoid (TT). Upon immunization of mice with GBSIII-OVA, carbohydrate epitopes are presented to and recognized by CD4(+) T cells. Subsequently, polysaccharide-recognizing CD4(+) T cells are expanded in vitro by stimulating splenic CD4(+) T cells with GBSIII-TT. The sequential use of two distinct glycoconjugate vaccines containing the same polysaccharide conjugated to heterologous carrier proteins selects for and expands carbohydrate-specific T cells. This protocol can readily be adapted to study the stimulation of the immune system by alternative glycoconjugate vaccines. This protocol takes 1-2 years to complete.

The immunogenic characteristics associated with multivalent display of Vi polysaccharide antigen using biodegradable polymer particles

Polysaccharides in their great majority are thymus-independent (TI) antigens. Anti-polysaccharide antibody responses are generally weak and characterized by lack of memory, isotype restriction and delayed ontogeny. We report here the generation of protective memory antibody response by multivalent display of polysaccharide antigens on biodegradable polymeric particles. Single dose immunization using polylactide (PLA) polymer particles entrapping Vi capsular polysaccharide antigen from Salmonella typhi promoted isotype switching and induced polysaccharide-specific memory antibody response in experimental animals. PLA nanoparticles as well as microparticles entrapping Vi polysaccharides elicited high IgG titer in comparison to soluble Vi immunization. Immunizations with particles co-entrapping both Vi polysaccharide and tetanus toxoid did not improve the anti-polysaccharide antibody responses. Lower antibody response from co-entrapped formulation was mostly due to inhibition of particle phagocytosis by the macrophages. Immunization using polylactide particles entrapping only Vi polysaccharide with higher density on surface elicited highest secondary antibody response as well as promoted isotype switching. The vaccination potential of particle based immunizations was further confirmed by the generation of quick memory antibody responses while challenging the immunized animals with live S. typhi. This approach provides a multivalent display of polysaccharide antigen using polymer particles and elicits protective memory antibody response without conjugation to a carrier protein.

Differential idiotype utilization for the in vivo type 14 capsular polysaccharide-specific Ig responses to intact Streptococcus pneumoniae versus a pneumococcal conjugate vaccine

Murine IgG responses specific for the capsular polysaccharide (pneumococcal capsular polysaccharide serotype 14; PPS14) of Streptococcus pneumoniae type 14 (Pn14), induced in response to intact Pn14 or a PPS14-protein conjugate, are both dependent on CD4(+) T cell help but appear to use marginal zone versus follicular B cells, respectively. In this study, we identify an idiotype (44.1-Id) that dominates the PPS14-specific IgG, but not IgM, responses to intact Pn14, isolated PPS14, and Group B Streptococcus (strain COH1-11) expressing capsular polysaccharide structurally identical to PPS14. The 44.1-Id, however, is not expressed in the repertoire of natural PPS14-specific Abs. In distinct contrast, PPS14-specific IgG responses to a soluble PPS14-protein conjugate exhibit minimal usage of the 44.1-Id, although significant 44.1-Id expression is elicited in response to conjugate attached to particles. The 44.1-Id elicited in response to intact Pn14 was expressed in similar proportions among all four IgG subclasses during both the primary and secondary responses. The 44.1-Id usage was linked to the Igh(a), but not Igh(b), allotype and was associated with induction of relatively high total PPS14-specific IgG responses. In contrast to PPS14-protein conjugate, avidity maturation of the 44.1-Id-dominant PPS14-specific IgG responses was limited, even during the highly boosted T cell-dependent PPS14-specific secondary responses to COH1-11. These results indicate that different antigenic forms of the same capsular polysaccharide can recruit distinct B cell clones expressing characteristic idiotypes under genetic control and suggest that the 44.1-Id is derived from marginal zone B cells.

Structurally identical capsular polysaccharide expressed by intact group B streptococcus versus Streptococcus pneumoniae elicits distinct murine polysaccharide-specific IgG responses in vivo

We previously reported distinct differences in the murine in vivo Ig polysaccharide (PS)-specific responses to intact Streptococcus pneumoniae compared with responses to Neisseria meningitidis and that in each case, the bacterial subcapsular domain markedly influences the Ig response to the associated PS. In light of potentially unique contributions of biochemically distinct capsular PS and/or their characteristic attachments to the underlying bacterium, it remains unresolved whether different bacterial subcapsular domains can exert differential effects on PS-specific Ig responses to distinct bacterial pathogens. In this report, we used a mutant strain of group B Streptococcus (Streptococcus agalactiae) type III (GBS-III) that expresses desialylated capsular polysaccharide of GBS-III, biochemically identical to capsular pneumococcal polysaccharide type 14 (PPS14) of Streptococcus pneumoniae (intact inactivated Streptococcus pneumoniae, capsular type 14, Pn14), directly to compare the in vivo PPS14-specific IgG responses to two distinct gram-positive bacteria. Although both GBS-III and Pn14 elicited relatively rapid primary PPS14-specific IgG responses dependent on CD4(+) T cells, B7-dependent costimulation, and CD40-CD40L interactions, only GBS-III induced a highly boosted ICOS-dependent PPS14-specific IgG response after secondary immunization. Of note, priming with Pn14 and boosting with GBS-III, although not isolated PPS14, elicited a similar boosted PPS14-specific IgG response that was dependent on CD4(+) T cells during secondary immunization, indicating that Pn14 primes for memory but, unlike GBS-III, fails to elicit it. The inability of Pn14 to elicit a boosted PPS14-specific IgG response was overcome by coimmunization with unencapsulated GBS-III. Collectively, these data establish that structurally identical capsular PS expressed by two distinct gram-positive extracellular bacteria can indeed elicit distinct PS-specific IgG responses in vivo.

The nature of an in vivo anti-capsular polysaccharide response is markedly influenced by the composition and/or architecture of the bacterial subcapsular domain

In vivo anti-polysaccharide Ig responses to isolated polysaccharide (PS) are T cell independent, rapid, and fail to generate memory. However, little is known regarding PS-specific Ig responses to intact gram-positive and gram-negative extracellular bacteria. We previously demonstrated that intact heat-killed Streptococcus pneumoniae, a gram-positive bacterium, elicited a rapid primary pneumococcal capsular PS (PPS) response in mice that was dependent on CD4(+) T cells, B7-dependent costimulation, and CD40-CD40L interactions. However, this response was ICOS independent and failed to generate a boosted PPS-specific secondary IgG response. In the current study, we analyzed the murine meningococcal type C PS (MCPS)-specific Ig response to i.p.-injected intact, heat-killed Neisseria meningitidis, serogroup C (MenC), a gram-negative bacterium. In contrast to S. pneumoniae, the IgG anti-MCPS response to MenC exhibited delayed primary kinetics and was highly boosted after secondary immunization, whereas the IgG anti-MCPS response to isolated MCPS was rapid, without secondary boosting, and consisted of only IgG1 and IgG3, as opposed to all four IgG isotypes in response to intact MenC. The secondary, but not primary, IgG anti-MCPS response to MenC was dependent on CD4(+) T cells, CD40L, CD28, and ICOS. The primary and secondary IgG anti-MCPS responses were lower in TLR4-defective (C3H/HeJ) but not TLR2(-/-) or MyD88(-/-) mice, but secondary boosting was still observed. Of interest, coimmunization of S. pneumoniae and MenC resulted in a boosted secondary IgG anti-PPS response to S. pneumoniae. Our data demonstrate that the nature of the in vivo anti-PS response is markedly influenced by the composition and/or architecture of the bacterial subcapsular domain.

A mechanism for glycoconjugate vaccine activation of the adaptive immune system and its implications for vaccine design

Although glycoconjugate vaccines have provided enormous health benefits globally, they have been less successful in significant high-risk populations. Exploring novel approaches to the enhancement of glycoconjugate effectiveness, we investigated molecular and cellular mechanisms governing the immune response to a prototypical glycoconjugate vaccine. In antigen-presenting cells, a carbohydrate epitope is generated upon endolysosomal processing of group B streptococcal type III polysaccharide coupled to a carrier protein. In conjunction with a carrier protein-derived peptide, this carbohydrate epitope binds to major histocompatibility class II (MHCII) and stimulates carbohydrate-specific CD4+ T-cell clones to produce interleukins 2 and 4—cytokines essential for providing T-cell help to antibody-producing B cells. An archetypical glycoconjugate vaccine constructed to maximize the presentation of carbohydrate epitopes recognized by T cells is 50–100 times more potent and significantly more protective in an animal model of infection than is a currently used vaccine construct.

Outer membrane protein complex of Meningococcus enhances the antipolysaccharide antibody response to pneumococcal polysaccharide-CRM₁₉₇ conjugate vaccine

Bacterial polysaccharides (PS) are T cell-independent antigens that do not induce immunologic memory and are poor immunogens in infants. Conjugate vaccines in which the PS is covalently linked to a carrier protein have enhanced immunogenicity that resembles that of T cell-dependent antigens. The Haemophilus influenzae type b (Hib) conjugate vaccine, which uses the outer membrane protein complex (OMPC) from meningococcus as a carrier protein, elicits protective levels of anti-capsular PS antibody (Ab) after a single dose, in contrast to other conjugate vaccines, which require multiple doses. We have previously shown that OMPC robustly engages Toll-like receptor 2 (TLR2) and enhances the early anti-Hib PS Ab titer associated with an increase in TLR2-mediated induction of cytokines. We now show that the addition of OMPC to the 7-valent pneumococcal PS-CRM₁₉₇ conjugate vaccine during immunization significantly increases the anti-PS IgG and IgM responses to most serotypes of pneumococcus contained in the vaccine. The addition of OMPC also increased the likelihood of anti-PS IgG3 production against serotypes 4, 6B, 9V, 18C, 19F, and 23F. Splenocytes from mice who had received OMPC with the pneumococcal conjugate vaccine produced significantly more interleukin-2 (IL-2), IL-4, IL-6, IL-10, tumor necrosis factor alpha (TNF-α), and gamma interferon (IFN-γ) than splenocytes from mice who received phosphate-buffered saline (PBS) plus the conjugate vaccine. We conclude that OMPC enhances the anti-PS Ab response to pneumococcal PS-CRM₁₉₇ conjugate vaccine, an effect associated with a distinct change in cytokine profile. It may be possible to reduce the number of conjugate vaccine doses required to achieve protective Ab levels by priming with adjuvants that are TLR2 ligands.

Immunological efficacy of glycoconjugates derived from Vibrio cholerae O1 serotype Ogawa detoxified LPS in mice

This study focused on changes in selected parameters of humoral and cellular immunity following vaccination of mice with unique Vibrio cholerae LPS–protein-complexed conjugates. The V. cholerae detoxified LPS (dLPS)-derived antigenic structures O-specific polysaccharide (O-SP) and de-O-acylated LPS (DeOAc-LPS) were used to prepare glycoconjugates by linking both dLPSs to glucan, the immunomodulating matrix, and then to BSA carrier. Animals were given a primary vaccination and boosted at 2-week intervals with a dose of 4.5 μg saccharide antigen. The last boost was given either subcutaneously or intraperitoneally (i.p.) to compare the boosting effect and to optimize the effective immunization route. Both conjugates (O-SP–BSA and DeOAc-LPS–BSA) induced significant levels of antigen-specific Ig isotypes, especially IgG and IgM. The i.p. booster route was more effective. A T helper 1 response was achieved only by immunization with O-SP–BSA conjugate administered i.p. Significant acceleration of phagocytic capacity and respiratory burst of neutrophils was demonstrated by both immunogenic formulations. Activation of T- and B-cell adaptive immunities was exhibited as specific changes in CD3 : CD19 and CD4 : CD8 ratios, B-cell low-affinity Fcγ II and III receptor expression and induction of CD45R antigen.

How bacterial carbohydrates influence the adaptive immune system

The capsular polysaccharides (CPSs) of most pathogenic bacteria are T cell-independent antigens whose conjugation to carrier proteins evokes a carbohydrate-specific response eliciting T cell help. However, certain bacterial CPSs, known as zwitterionic polysaccharides (ZPSs), activate the adaptive immune system through processing by antigen-presenting cells and presentation by the major histocompatibility complex class II pathway to CD4(+) T cells. This discovery was the first mechanistic insight into how carbohydrates-a class of biological molecules previously thought to be T cell independent-can in fact activate T cells. Through their ability to activate CD4(+) T cells, ZPSs direct the cellular and physical maturation of the developing immune system. In this review, we explore the still-enigmatic relations between CPSs and the adaptive immune machinery at the cellular and molecular levels, and we discuss how new insights into the biological impact of ZPSs expand our concepts of the role of carbohydrates in microbial interactions with the adaptive immune system.

Hyporesponsiveness to re-challenge dose following pneumococcal polysaccharide vaccine at 12 months of age, a randomized controlled trial

BACKGROUND

To evaluate the immunological impact of the 23-valent pneumococcal polysaccharide vaccine (23vPPS) at 12 months, for children who have received zero to three infant doses of seven-valent pneumococcal conjugate vaccine (PCV), on responses to a subsequent exposure to a small dose of 23vPPS (mPPS).

METHODS

Five hundred and fifty-two Fijian infants were stratified by ethnicity and randomized into eight groups to receive zero, one, two, or three PCV doses at 14 weeks, six and 14 weeks, or six, ten, and 14 weeks. Within each group, half received 23vPPS at 12 months and all received mPPS at 17 months. Sera were taken prior and one month post-mPPS.

FINDINGS

By 17 months, geometric mean antibody concentrations (GMC) to all 23 serotypes in 23vPPS were significantly higher in children who had received 23vPPS at 12 months compared to those who had not. Post-mPPS, children who had not received the 12 month 23vPPS had a significantly higher GMC for all PCV serotypes compared with those who had (each p<0.02). For the non-PCV serotypes, children who had not received the 12 month 23vPPS had significantly higher GMC for six of 16 non-PCV serotypes (7F, 9N, 12F, 19A, 22F, 33F) than those who did (each p<0.02). After adjusting for the pre-mPPS level, exposure to 23vPPS was associated with a lower response to mPPS for all serotypes (each p<0.001).

INTERPRETATION

Despite higher antibody concentrations at 17 months in children who had received 23vPPS at 12 months, the response to a re-challenge was poor for all 23 serotypes compared to children who had not received the 12 month 23vPPS.

Memory B and T cell responses induced by serotype 4 Streptococcus pneumoniae vaccines: longitudinal analysis comparing responses elicited by free polysaccharide, conjugate and carrier

We have conducted a 1-year longitudinal study in mice vaccinated by free serotype 4 Streptococcus pneumoniae PS (PS4), the corresponding tetanus toxoid (TT)-conjugated vaccine, or the TT carrier alone. B and T cell immunity induced by these three types of antigen, were compared by monitoring the (i) long-term persistence of specific serum antibodies, (ii) frequency of memory B cell precursors in spleen, and (iii) T cell responses against the carrier. While PS4-specific antibody response appeared later than the anti-carrier response upon primary immunization, PS4-specific B memory and serum responses were quantitatively and qualitatively similar to the ones observed against TT upon immunization by either the free carrier or the conjugate. We also observed a parallel persistent carrier-specific T cell response in the spleen. These data indicate that the nature and long-term kinetics of the anti-PS4 antibody response induced by the conjugate vaccine are similar to "classical" T-dependent response elicited by conventional protein antigens.

Antigen processing of glycoconjugate vaccines; the polysaccharide portion of the pneumococcal CRM(197) conjugate vaccine co-localizes with MHC II on the antigen processing cell surface

Pneumococcal (Pn) polysaccharides (PS) are T-independent (TI) antigens and do not induce immunological memory or antibodies in infants. Conjugation of PnPS to the carrier protein CRM(197) induces PS-specific antibody in infants, and memory similar to T-dependent (Td) antigens. Conjugates have improved immunogenicity via antigen processing and presentation of carrier protein with MHC II and recruitment of T cell help, but the fate of the PS attached to the carrier is unknown. To determine the location of the PS component of PnPS-CRM(197) in the APC, we separately labeled PS and protein and tracked their location. The PS of types 14-CRM(197) and 19F-CRM(197) was specifically labeled by Alexa Fluor 594 hydrazide (red). The CRM(197) was separately labeled red in a reaction that did not label PS. Labeled antigens were incubated with APC which were fixed, permeabilized and incubated with anti-MHC II antibody labeled green by Alexa Fluor 488, followed by confocal microscopy. Labeled CRM(197) was presented on APC surface and co-localized with MHC II (yellow). Labeled unconjugated 14 or 19F PS did not go to the APC surface, but PS labeled 14-CRM(197) and 19F-CRM(197) was internalized and co-localized with MHC II. Monoclonal antibody to type 14 PS bound to intracellular type 14 PS and PS-CRM(197). Brefeldin A and chloroquine blocked both CRM(197) and PS labeled 14-CRM(197) and 19F-CRM(197) from co-localizing with MHC II. These data suggest that the PS component of the CRM(197) glycoconjugate enters the endosome, travels with CRM(197) peptides to the APC surface and co-localizes with MHC II.