Effects of chain length on the immunogenicity in rabbits of group B Streptococcus type III oligosaccharide-tetanus toxoid conjugates

Convergent Synthesis of a Group B Streptococcus Type III Epitope Toward a Semisynthetic Carbohydrate-Based Vaccine

In this work, we synthesized an hexasaccharide derived from the capsular polysaccharide of group B Streptococcus type III capsular polysaccharide. Our convergent 3 + 3 strategy avoided the use of benzyl protecting groups allowing the installation of an azide anchoring group and providing a high yield for the final deprotection steps. Moreover, the minimal hexasaccharidic epitope was conjugated to CRM197 and BSA via copper-catalyzed azide-alkyne cycloaddition for the preparation of a semisynthetic carbohydrate-based vaccine.

Impact and Control of Sugar Size in Glycoconjugate Vaccines

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

Engineering a suite of E. coli strains for enhanced expression of bacterial polysaccharides and glycoconjugate vaccines

BACKGROUND

Glycoengineering, in the biotechnology workhorse bacterium, Escherichia coli, is a rapidly evolving field, particularly for the production of glycoconjugate vaccine candidates (bioconjugation). Efficient production of glycoconjugates requires the coordinated expression within the bacterial cell of three components: a carrier protein, a glycan antigen and a coupling enzyme, in a timely fashion. Thus, the choice of a suitable E. coli host cell is of paramount importance. Microbial chassis engineering has long been used to improve yields of chemicals and biopolymers, but its application to vaccine production is sparse.

RESULTS

In this study we have engineered a family of 11 E. coli strains by the removal and/or addition of components rationally selected for enhanced expression of Streptococcus pneumoniae capsular polysaccharides with the scope of increasing yield of pneumococcal conjugate vaccines. Importantly, all strains express a detoxified version of endotoxin, a concerning contaminant of therapeutics produced in bacterial cells. The genomic background of each strain was altered using CRISPR in an iterative fashion to generate strains without antibiotic markers or scar sequences.

CONCLUSIONS

Amongst the 11 modified strains generated in this study, E. coli Falcon, Peregrine and Sparrowhawk all showed increased production of S. pneumoniae serotype 4 capsule. Eagle (a strain without enterobacterial common antigen, containing a GalNAc epimerase and PglB expressed from the chromosome) and Sparrowhawk (a strain without enterobacterial common antigen, O-antigen ligase and chain length determinant, containing a GalNAc epimerase and chain length regulators from Streptococcus pneumoniae) respectively produced an AcrA-SP4 conjugate with 4 × and 14 × more glycan than that produced in the base strain, W3110. Beyond their application to the production of pneumococcal vaccine candidates, the bank of 11 new strains will be an invaluable resource for the glycoengineering community.

Short Vi-polysaccharide abrogates T-independent immune response and hyporesponsiveness elicited by long Vi-CRM197 conjugate vaccine

Our results suggest a rational way of designing and developing an improved typhoid conjugate vaccine and, by extension, to conjugate vaccines in general: first, modify a T-independent polysaccharide so that it no longer induces a T-independent response, then conjugate the polysaccharide to a suitable carrier protein restoring immunogenicity, thus creating a pure T-dependent antigen that induces a strongly boostable and long-lived response at an early age.

Polysaccharide-protein conjugates have been developed to overcome the T-independent response, hyporesponsiveness to repeated vaccination, and poor immunogenicity in infants of polysaccharides. To address the impact of polysaccharide length, typhoid conjugates made with short- and long-chain fractions of Vi polysaccharide with average sizes of 9.5, 22.8, 42.7, 82.0, and 165 kDa were compared. Long-chain-conjugated Vi (165 kDa) induced a response in both wild-type and T cell-deficient mice, suggesting that it maintains a T-independent response. In marked contrast, short-chain Vi (9.5 to 42.7 kDa) conjugates induced a response in wild-type mice but not in T cell-deficient mice, suggesting that the response is dependent on T cell help. Mechanistically, this was explained in neonatal mice, in which long-chain, but not short-chain, Vi conjugate induced late apoptosis of Vi-specific B cells in spleen and early depletion of Vi-specific B cells in bone marrow, resulting in hyporesponsiveness and lack of long-term persistence of Vi-specific IgG in serum and IgG⁺ antibody-secreting cells in bone marrow. We conclude that while conjugation of long-chain Vi generates T-dependent antigens, the conjugates also retain T-independent properties, leading to detrimental effects on immune responses. The data reported here may explain some inconsistencies observed in clinical trials and help guide the design of effective conjugate vaccines.

Surface Structures of Group B Streptococcus Important in Human Immunity

The surface of the Gram-positive opportunistic pathogen Streptococcus agalactiae, or group B Streptococcus (GBS), harbors several carbohydrate and protein antigens with the potential to be effective vaccines. Capsular polysaccharides of all clinically-relevant GBS serotypes coupled to immunogenic proteins of both GBS and non-GBS origin have undergone extensive testing in animals that led to advanced clinical trials in healthy adult women. In addition, GBS proteins either alone or in combination have been tested in animals; a fusion protein construct has recently advanced to human clinical studies. Given our current understanding of the antigenicity and immunogenicity of the wide array of GBS surface antigens, formulations now exist for the generation of viable vaccines against diseases caused by GBS.

Conformations of Neisseria meningitidis serogroup A and X polysaccharides: The effects of chain length and O-acetylation

Neisseria meningitidis is a major cause of bacterial meningitis worldwide especially in Africa. The capsular polysaccharide (CPS) is the main virulence factor and the target antigen for polysaccharide and conjugate vaccines. The high burden of serogroup A disease in the Meningitis Belt of sub-Saharan Africa led to the introduction of MenAfriVac^®, which has successfully reduced the number of cases of group A disease. However, several outbreaks caused by other serogroups have been reported, including those due to serogroup X. The capsular polysaccharides of serogroups A and X are both homopolymers of amino sugars (α-D-ManNAc and α-D-GlcNAc) containing phosphodiester linkages at C-6 and C-4, respectively. The similarity of the primary structures of the two polysaccharides suggests that serogroup A vaccination may provide cross-protection against serogroup X disease. Molecular dynamics simulations of a series of serogroup A and X oligosaccharides reveal that the MenA CPS behaves as a flexible random coil which becomes less conformationally defined as the length increases, whereas serogroup X forms a more stable regular helical structure. The presence of the MenX helix is supported by NMR analysis; it has four residues per turn and becomes more stable as the chain length increases. Licensed MenA vaccines are largely O-acetylated at C-3: simulations show that these O-acetyl groups are highly solvent exposed and their presence favors more extended conformations compared to the more compact conformations of MenA without O-acetylation. These findings may have implications for the design of optimal conjugate vaccines.

Chemical synthesis and biological evaluation of penta- to octa- saccharide fragments of Vi polysaccharide fromSalmonella typhi

The penta- to octa-saccharide fragments of Vi polysaccharide were synthesized efficiently, and the hexasaccharide might be the minimum epitope of Vi antigen based on ELISA analysis.

Interplay of Carbohydrate and Carrier in Antibacterial Glycoconjugate Vaccines

Bacterial infections are a serious health concern and are responsible for millions of illnesses and deaths each year in communities around the world. Vaccination is an important public health measure for reducing and eliminating this burden, and regions with comprehensive vaccination programs have achieved significant reductions in infection and mortality. This is often accomplished by immunization with bacteria-derived carbohydrates, typically in conjunction with other biomolecules, which induce immunological memory and durable protection against bacterial human pathogens. For many species, however, vaccines are currently unavailable or have suboptimal efficacy characterized by short-lived memory and incomplete protection, especially among at-risk populations. To address this challenge, new tools and techniques have emerged for engineering carbohydrates and conjugating them to carrier molecules in a tractable and scalable manner. Collectively, these approaches are yielding carbohydrate-based vaccine designs with increased immunogenicity and protective efficacy, thereby opening up new opportunities for this important class of antigens. In this chapter we detail the current understanding of how carbohydrates interact with the immune system to provide immunity; how glycoengineering, especially in the context of glycoconjugate vaccines, can be used to modify and enhance immune responses; and current trends and strategies being pursued for the rational design of next-generation glycosylated antibacterial vaccines. Graphical Abstract.

Structure of a protective epitope of group B Streptococcus type III capsular polysaccharide

Despite substantial progress in the prevention of group B Streptococcus (GBS) disease with the introduction of intrapartum antibiotic prophylaxis, this pathogen remains a leading cause of neonatal infection. Capsular polysaccharide conjugate vaccines have been tested in phase I/II clinical studies, showing promise for further development. Mapping of epitopes recognized by protective antibodies is crucial for understanding the mechanism of action of vaccines and for enabling antigen design. In this study, we report the structure of the epitope recognized by a monoclonal antibody with opsonophagocytic activity and representative of the protective response against type III GBS polysaccharide. The structure and the atomic-level interactions were determined by saturation transfer difference (STD)-NMR and X-ray crystallography using oligosaccharides obtained by synthetic and depolymerization procedures. The GBS PSIII epitope is made by six sugars. Four of them derive from two adjacent repeating units of the PSIII backbone and two of them from the branched galactose-sialic acid disaccharide contained in this sequence. The sialic acid residue establishes direct binding interactions with the functional antibody. The crystal structure provides insight into the molecular basis of antibody-carbohydrate interactions and confirms that the conformational epitope is not required for antigen recognition. Understanding the structural basis of immune recognition of capsular polysaccharide epitopes can aid in the design of novel glycoconjugate vaccines.

An Overview of Structural Features of Antibacterial Glycoconjugate Vaccines That Influence Their Immunogenicity

Bacterial cell-surface-derived or mimicked carbohydrate moieties that act as protective antigens are used in the development of antibacterial glycoconjugate vaccines. The carbohydrate antigen must have a minimum length or size to maintain the conformational structure of the antigenic epitope(s). The presence or absence of O-acetate, phosphate, glycerol phosphate and pyruvate ketal plays a vital role in defining the immunogenicity of the carbohydrate antigen. The nature of the carrier protein, spacer and conjugation pattern used to develop the glycoconjugate vaccine also defines its overall spatial orientation which in turn affects its avidity and selectivity of interaction with the desired target(s). In addition, the ratio of carbohydrate to protein in glycoconjugate vaccines also makes an important contribution in determining the optimum immunological response. This Review article presents the importance of these variables in the development of antibacterial glycoconjugate vaccines and their effects on immune efficacy.

Recombinant expression of Streptococcus pneumoniae capsular polysaccharides in Escherichia coli

Currently, Streptococcus pneumoniae is responsible for over 14 million cases of pneumonia worldwide annually, and over 1 million deaths, the majority of them children. The major determinant for pathogenesis is a polysaccharide capsule that is variable and is used to distinguish strains based on their serotype. The capsule forms the basis of the pneumococcal polysaccharide vaccine (PPV23) that contains purified capsular polysaccharide from 23 serotypes, and the pneumococcal conjugate vaccine (PCV13), containing 13 common serotypes conjugated to CRM197 (mutant diphtheria toxin). Purified capsule from S. pneumoniae is required for pneumococcal conjugate vaccine production, and costs can be prohibitively high, limiting accessibility of the vaccine in low-income countries. In this study, we demonstrate the recombinant expression of the capsule-encoding locus from four different serotypes of S. pneumoniae within Escherichia coli. Furthermore, we attempt to identify the minimum set of genes necessary to reliably and efficiently express these capsules heterologously. These E. coli strains could be used to produce a supply of S. pneumoniae serotype-specific capsules without the need to culture pathogenic bacteria. Additionally, these strains could be applied to synthetic glycobiological applications: recombinant vaccine production using E. coli outer membrane vesicles or coupling to proteins using protein glycan coupling technology.

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.

Determination of the glycation sites of Bacillus anthracis neoglycoconjugate vaccine by MALDI-TOF/TOF-CID-MS/MS and LC-ESI-QqTOF-tandem mass spectrometry

We present herein an efficient mass spectrometric method for the localization of the glycation sites of a model neoglycoconjugate vaccine formed by a construct of the tetrasaccharide side chain of the Bacillus anthracis exosporium and the protein carrier bovine serum albumin. The glycoconjugate was digested with both trypsin and GluC V8 endoproteinases, and the digests were then analyzed by MALDI-TOF/TOF-CID-MS/MS and nano-LC-ESI-QqTOF-CID-MS/MS. The sequences of the unknown peptides analyzed by MALDI-TOF/TOF-CID-MS/MS, following digestion with the GluC V8 endoproteinase, allowed us to recognize three glycopeptides whose glycation occupancies were, respectively, on Lys 235, Lys 420, and Lys 498. Similarly, the same analysis was performed on the tryptic digests, which permitted us to recognize two glycation sites on Lys 100 and Lys 374. In addition, we have also used LC-ESI-QqTOF-CID-MS/MS analysis for the identification of the tryptic digests. However, this analysis identified a higher number of glycopeptides than would be expected from a glycoconjugate composed of a carbohydrate-protein ratio of 5.4:1, which would have resulted in glycation occupancies of 18 specific sites. This discrepancy was due to the large number of glycoforms formed during the synthetic carbohydrate-spacer-carrier protein conjugation. Likewise, the LC-ESI-QqTOF-MS/MS analysis of the GluC V8 digest also identified 17 different glycation sites on the synthetic glycoconjugate.

Oxidative depolymerization of polysaccharides by reactive oxygen/nitrogen species

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are constantly produced and are tightly regulated to maintain a redox balance (or homeostasis) together with antioxidants (e.g. superoxide dismutase and glutathione) under normal physiological circumstances. These ROS/RNS have been shown to be critical for various biological events including signal transduction, aging, apoptosis, and development. Despite the known beneficial effects, an overproduction of ROS/RNS in the cases of receptor-mediated stimulation and disease-induced oxidative stress can inflict severe tissue damage. In particular, these ROS/RNS are capable of degrading macromolecules including proteins, lipids and nucleic acids as well as polysaccharides, and presumably lead to their dysfunction. The purpose of this review is to highlight (1) chemical mechanisms related to cell-free and cell-based depolymerization of polysaccharides initiated by individual oxidative species; (2) the effect of ROS/RNS-mediated depolymerization on the successive cleavage of the glycosidic linkage of polysaccharides by glycoside hydrolases; and (3) the potential biological outcome of ROS/RNS-mediated depolymerization of polysaccharides.

Neither antibody to a group B streptococcal conjugate vaccine nor the vaccine itself is teratogenic in rabbits

Group B Streptococcus (GBS) is a leading cause of human neonatal bacterial disease, resulting in pneumonia, sepsis, meningitis and sometimes, death. Supportive preclinical studies of GBS capsular polysaccharide (CPS)-protein conjugate vaccines have led to several phase 1 and phase 2 trials in healthy, non-pregnant adults, which demonstrated that the vaccines, produced at the Channing Laboratory, were safe and immunogenic. However, evaluation of the safety and immunogenicity of a GBS conjugate vaccine administered to pregnant women demanded that it be manufactured under current good manufacturing practices (cGMP) and that it undergo developmental toxicity evaluation. In this report, we describe a GBS type III CPS-tetanus toxoid (III-TT) vaccine lot 3-1-96 manufactured and vialed under cGMP and our evaluation of the effect of this vaccine and of GBS type III CPS-specific antibody on conception and early- and late-stage fetal development in rabbits. III-TT lot 3-1-96 was compositionally similar to prototype III-TT lot 91-1, produced under non-GMP, and was potent in a mouse maternal vaccination-neonatal pup challenge model of GBS disease. Four groups of 30 female rabbits each were randomized to receive III-TT lot 3-1-96 vaccine, saline-alum, or combinations of these treatments before and after insemination. The dose of conjugated CPS on a weight basis was 1 microg/kg, mimicking the anticipated actual human dose. Based on the weight of the rabbits, this was 20- to 100-fold greater than the expected human dose. Does were pre-assigned to deliver litters naturally or have their kits delivered by Caesarean-section at gestation day 29, to assess late fetal development. Sera from does and kits were collected, and the presence of type III CPS-specific IgG was confirmed by quantitative ELISA. Based on all assessments, GBS type III-TT lot 3-1-96, nor antibody to it did not affect embryo fetal viability, sex ratio, growth or cause malformations (i.e., it was non-teratogenic). In addition, that III-TT lot 3-1-96 was found to be safe and immunogenic in two clinical studies involving healthy non-pregnant adults supports a clinical evaluation of this vaccine in pregnant women.

Introduction of zwitterionic motifs into bacterial polysaccharides generates TLR2 agonists able to activate APCs

It was shown previously that bacterial polysaccharides (PS), which naturally contain both positive and negative charges, are able to activate T cells and APCs. However, the vast majority of bacterial PS are anionic and do not have these properties. In this study, we show that chemical introduction of positive charges into naturally anionic bacterial PS confers to the resulting zwitterionic PS (ZPS) the ability to activate pure human monocytes, monocyte-derived dendritic cells, and mouse bone marrow-derived dendritic cells, as do natural bacterial ZPS. Cells are induced to up-regulate MHC class II and costimulatory molecules and to produce cytokines. In mixed monocyte-T cell cocultures, ZPS induce MHC II-dependent T cell proliferation and up-regulation of activation markers. These stimulatory qualities of ZPS disappear when the positive charge is chemically removed from the molecules and thus the zwitterionic motif is destroyed. The ability of natural and chemically derived ZPS to activate APCs can be blocked by anti-TLR2 mAbs, and TLR2 transfectants show reporter gene transcription upon incubation with ZPS. In conclusion, the generation of a zwitterionic motif in bacterial PS confers the ability to activate both APCs and T cells. This finding has important implications for the design of novel polysaccharide vaccines.

Group B streptococcal type II and III conjugate vaccines: physicochemical properties that influence immunogenicity

Recent efforts toward developing vaccines against group B streptococci (GBS) have focused on increasing the immunogenicity of GBS polysaccharides by conjugation to carrier proteins. However, partial depolymerization of GBS polysaccharides for the production of vaccines is a difficult task because of their acid-labile, antigenically critical sialic acids. Here we report a method for the partial depolymerization of type II and III polysaccharides by mild deaminative cleavage to antigenic fragments with reducing-terminal 2,5-anhydro-d-mannose residues. Through the free aldehydes of their newly formed end groups, the fragments were conjugated to tetanus toxoid by reductive amination. The resulting conjugates stimulated the production in animals of high-titer type II- and III-specific antibodies which induced opsonophagocytic killing of type II and III strains of group B streptococci. For the type II conjugates, immunogenicity increased as oligosaccharide size decreased, whereas for type III conjugates, the size of the oligosaccharides did not significantly influence immunogenicity. When oligosaccharides of defined size were conjugated through sialic acid residues, the resulting cross-linkages were shown to affect immunogenicity. When oligosaccharides were conjugated through terminal aldehyde groups generated by deamination, modification of the exocyclic chain of sialic acid did not influence immunogenicity.

Versatile and efficient synthesis of protein–polysaccharide conjugate vaccines using aminooxy reagents and oxime chemistry

Applications of oxime chemistry are described for the efficient bioconjugation of proteins and polysaccharides for the preparation of conjugate vaccines. A number of approaches are described in this manuscript to functionalize proteins and polysaccharides with aminooxy (AO) groups and aldehydes which could then be covalently linked to each other via oxime formation, without the need for reduction. By using limiting numbers of active groups on each component, the extent of inter- and intramolecular crosslinking could be controlled. The approaches described are compatible and complementary to a number of chemistries currently used in conjugate vaccine synthesis. Oxime chemistry can be used to both simplify the synthesis of and increase yields of conjugate vaccines. Mice immunized with pneumococcal type 14 conjugates that were made using oxime chemistry mounted significant anti-polysaccharide immune responses. The primary immune response could be boosted, indicating that the polysaccharide conjugate had characteristics of a T cell dependent antigen.

Effect of saccharide length on the immunogenicity of neoglycoconjugates from synthetic fragments of the O-SP of Vibrio cholerae O1, serotype Ogawa

A synthetic hexasaccharide, identical to the terminal hexasaccharide of Ogawa LPS, coupled to bovine serum albumin induced protective antibodies in mice. To determine if there was a minimum saccharide length required for immunogenicity and efficacy, shorter (mono- to pentasaccharide) neoglycoconjugates (CHO-BSA) were tested in mice. The Ogawa CHO-BSA was inoculated at either a constant mass but differing moles, or equal moles but differing masses. Humoral responses were essentially the same when mice received 9 microg of the carbohydrate (0.007 mM with the pentasaccharide) in each of the neoglycoconjugates prepared from mono- through the pentasaccharide, or the same molar amount (0.007 mM), proportionally less by weight when going from the penta- to the monosaccharide. These data show that, within this dose range, the responses occurred virtually independently of the amount of immunogen. Humoral antibodies induced by these immunogens were generally not vibriocidal. Selected antisera induced by CHO-BSA immunogens were protective, but the ELISA titers of the sera were not predictive of the protective capacity. Purified, Ogawa LPS induced anti-Ogawa LPS IgM antibody titers similar to those induced by the Ogawa CHO-BSA conjugates. The anti-whole LPS sera were strongly vibriocidal, as were the previously reported sera induced by hexasaccharide conjugates. This suggests either that the shorter oligosaccharides lack a conformational epitope provided by the hexasaccharide or that the LPS has additional B cell epitopes or selects different B cells in the primary response.

Vaccines based on the cell surface carbohydrates of pathogenic bacteria

Glycoconjugate vaccines, in which a cell surface carbohydrate from a micro-organism is covalently attached to an appropriate carrier protein are proving to be the most effective means to generate protective immune responses to prevent a wide range of diseases. The technology appears to be generic and applicable to a wide range of pathogens, as long as antibodies against surface carbohydrates help protect against infection. Three such vaccines, against Haemophilus influenzae type b, Neisseria meningitidis Group C and seven serotypes of Streptococcus pneumoniae, have already been licensed and many others are in development. This article discusses the rationale for the development and use of glycoconjugate vaccines, the mechanisms by which they elicit T cell-dependent immune responses and the implications of this for vaccine development, the role of physicochemical methods in the characterisation and quality control of these vaccines, and the novel products which are under development.

Synthesis and immunological studies of glycoconjugates of Cryptococcus neoformans capsular glucuronoxylomannan oligosaccharide structures

Antibodies to the glucuronoxylomannan (GXM) component of the polysaccharide capsule of Cryptococcus neoformans are protective and GXM-protein conjugate vaccines can elicit protective immune responses. We report the synthesis of a heptasaccharide oligosaccharide representing the putative dominant motif of serotype A GXM and demonstrate that it is recognized by some monoclonal antibodies (mAbs) generated to GXM. Conjugation of the heptasaccharide to human serum albumin (HSA) resulted in an immunogenic compound that elicited high-titer IgG responses in mice when given with complete Freund's adjuvant. The antibody response elicited by the oligosaccharide conjugate vaccine had characteristics of a T-cell-dependent response. The availability of an immunogenic oligosaccharide representing a structural motif of GXM will prove useful in studies of antibody epitope specificity and represents a potential synthetic oligosaccharide vaccine against this fungal pathogen.

Size determination of bacterial capsular oligosaccharides used to prepare conjugate vaccines against Neisseria meningitidis groups Y and W135

The glycoconjugate vaccines against Neisseria meningitidis groups Y and W135 consist of pools of selected oligosaccharides conjugated to the protein carrier (CRM197). Consistent production of these vaccines requires control and thus determination of the average degree of polymerisation of the oligosaccharides used for conjugation. Acid hydrolysis generates group Y and W135 oligosaccharides with N-acetylneuraminic acid at the reducing end. A method, involving NaBH4 reduction and quantification of this terminal N-acetylneuraminic acid by use of high performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) following acid hydrolysis (2M TFA), was developed. The average degree of polymerisation is calculated from the ratio of reduced N-acetylneuraminic acid to total N-acetylneuraminic acid. The assay was qualified by application to group C, Y and W135 oligosaccharide standards characterised by liquid chromatography, mass and NMR spectroscopy.

Capsular polysaccharide-protein conjugate vaccines and intravenous immunoglobulins

Capsular polysaccharides (CPs), present on the surface of most pathogenic bacteria, have been recognised as virulence factors. Antibodies specific to these polysaccharides can mediate the killing of these bacteria by phagocytes in the presence of complement. The conjugation of polysaccharides to carrier proteins enhances their immunogenicity and renders the immune response T-cell dependent. The currently licensed capsular polysaccharide vaccines and polysaccharide-protein conjugate vaccines under development for the prevention of bacterial infections will be discussed in this review. Use of these vaccines for active vaccination and for the vaccination of healthy plasma donors to produce hyperimmune iv. immunoglobulins for the passive immunisation of appropriate patient populations is also discussed.

Construction of designer glycoconjugate vaccines with size-specific oligosaccharide antigens and site-controlled coupling

Coupling of carbohydrate antigens to protein carriers is a typical approach to enhancing the immunogenicity of carbohydrate-based vaccines. Glycoconjugates with well-defined structures are needed for studies defining the structural variables that govern antibody responses. We report a chemical strategy for preparation of an array of glycoconjugates containing saccharides of desired molecular sizes by selective depolymerization of bacterial polysaccharides and chemically controlled site-specific coupling. As an example, we synthesized and evaluated an oligosaccharide-based vaccine against type III group B Streptococcus.

Zwitterionic polysaccharides stimulate T cells by MHC class II-dependent interactions

Polysaccharides of pathogenic extracellular bacteria commonly have negatively charged groups or no charged groups at all. These molecules have been considered classic T cell-independent Ags that do not elicit cell-mediated immune responses in mice. However, bacterial polysaccharides with a zwitterionic charge motif (ZPSs), such as the capsular polysaccharides of many strains of Bacteroides fragilis, Staphylococcus aureus, and Streptococcus pneumoniae type 1 elicit potent CD4(+) T cell responses in vivo and in vitro. The cell-mediated response to ZPS depends on the presence of both positively charged and negatively charged groups on each repeating unit of the polysaccharide. In this study, we define some of the requirements for the presentation of ZPS to CD4(+) T cells. We provide evidence that direct interactions of T cells with APCs are essential for T cell activation by ZPS. Monocytes, dendritic cells, and B cells are all able to serve as APCs for ZPS-mediated T cell activation. APCs lacking MHC class II molecules do not support this activity. Furthermore, mAb to HLA-DR specifically blocks ZPS-mediated T cell activation, while mAbs to other MHC class II and class I molecules do not. Immunoprecipitation of lysates of MHC class II-expressing cells following incubation with ZPS shows binding of ZPS and HLA-DR. Electron microscopy reveals colocalization of ZPS with HLA-DR on the cell surface and in compartments of the endocytic pathway. These results indicate that MHC class II molecules expressing HLA-DR on professional APCs are required for ZPS-induced T cell activation. The implication is that binding of ZPS to HLA-DR may be required for T cell activation.

Molecular recognition of oligosaccharide epitopes by a monoclonal Fab specific for Shigella flexneri Y lipopolysaccharide: X-ray structures and thermodynamics

The antigenic recognition of Shigella flexneri O-polysaccharide, which consists of a repeating unit ABCD [-->2)-alpha-L-Rhap-(1-->2)-alpha-L-Rhap-(1-->3)-alpha-L-Rhap-(1-->3)-beta-D-GlcpNAc-(1-->], by the monoclonal antibody SYA/J6 (IgG3, kappa) has been investigated by crystallographic analysis of the Fab domain and its two complexes with two antigen segments (a pentasaccharide Rha A-Rha B-Rha C-GlcNAc D-Rha A' and a modified trisaccharide Rha B-Rha C-GlcNAc D in which Rha C* is missing a C2-OH group). These complex structures, the first for a Fab specific for a periodic linear heteropolysaccharide, reveal a binding site groove (between the V(H) and V(L) domains) that makes polar and nonpolar contacts with all the sugar residues of the pentasaccharide. Both main-chain and side-chain atoms of the Fab are used in ligand binding. The charged side chain of Glu H50 of CDR H2 forms crucial hydrogen bonds to GlcNAc of the oligosaccharides. The modified trisaccharide is more buried and fits more snugly than the pentasaccharide. It also makes as many contacts (approximately 75) with the Fab as the pentasaccharide, including the same number of hydrogen bonds (eight, with four being identical). It is further engaged in more hydrophobic interactions than the pentasaccharide. These three features favorable to trisaccharide binding are consistent with the observation of a tighter complex with the trisaccharide than the pentasaccharide. Thermodynamic data demonstrate that the native tri- to pentasaccharides have free energies of binding in the range of 6.8-7.4 kcal mol(-1), and all but one of the hydrogen bonds to individual hydroxyl groups provide no more than approximately 0.7 kcal mol(-1). They further indicate that hydrophobic interactions make significant contributions to binding and, as the native epitope becomes larger across the tri-, tetra-, pentasaccharide series, entropy contributions to the free energy become dominant.

Protection from group B streptococcal infection in neonatal mice by maternal immunization with recombinant Sip protein

The protective potential of antibodies directed against group B streptococcus (GBS) Sip surface protein was determined by using the mouse neonatal infection model. Rabbit Sip-specific antibodies administered passively to pregnant mice protected their pups against a GBS lethal challenge. In addition, active immunization with purified recombinant Sip protein of female CD-1 mice induced the production of specific antibodies that also confer protection to the newborn pups against GBS strains of serotypes Ia/c, Ib, II, III, and V. These data confirm that Sip-specific antibodies can cross the placenta and conferred protective immunity against GBS infections.

Enhanced immune response to T-independent antigen by using CpG oligodeoxynucleotides encapsulated in liposomes

Immunostimulatory CpG oligodeoxynucleotides (ODN) have been tested as immunoadjuvants for various vaccines including T-cell independent (TI) antigens. Findings from previous reports suggest that close physical association of CpG ODN to the antigen could enhance its adjuvant effect. As an alternative to chemical conjugation of CpG ODN to the antigen, the current study is aimed at determining the benefit of using liposomes as a carrier for CpG ODN to improve the immune response to biotinylated liposomes (Bx-liposomes), a model of a TI antigen. Liposomes with suboptimal concentration of hapten (1% biotin) were not immunogenic. However, when CpG ODN encapsulated in Bx-liposomes were used to immunize mice, a hapten-specific response was obtained as indicated by antibody-mediated elimination of re-administered Bx-liposomes. CpG ODN co-administered with empty Bx-liposomes could not achieve the same effect, indicating the requirement for encapsulation of the adjuvant. Using both intravenous (i.v.) and subcutaneous (s.c.) immunization methods, it was found that IgM levels, but not IgG levels were elevated. Immunization in nude mice confirmed that the immune response obtained was TI. The use of non-CpG ODN and an ODN with alternatively flanked CpG motifs showed no adjuvant effect. Incorporation of poly(ethylene)glycol (PEG)-modified lipid in liposomes enhanced the immune response even further. In conclusion, our data shows that liposomes are a useful delivery vehicle for CpG ODN as an immune adjuvant for TI antigens.

Synthetic polysaccharide type 3-related di-, tri-, and tetrasaccharide-CRM(197) conjugates induce protection against Streptococcus pneumoniae type 3 in mice

Di-, tri-, and tetrasaccharides, synthesized according to the chemical structure of pneumococcal polysaccharide type 3 (PS3), were coupled to the cross-reactive material (CRM(197)) of modified diphtheria toxin in different molar carbohydrate/protein ratios using the squarate coupling method. To study protective immunity, female BALB/c mice were subcutaneously immunized twice (with a 3-week interval) using the amount of conjugates corresponding to 2.5 microg of oligosaccharide per mouse. The conjugates evoked PS3 binding immunoglobulin G antibodies that lasted for at least 7 weeks after the booster. Immunogenicity was not influenced by the carbohydrate/protein ratio. All mice with PS3-specific antibodies survived the intraperitoneal challenge with Streptococcus pneumoniae type 3. Therefore, synthetic oligosaccharide-protein conjugates might have potential as vaccines.

The conformational epitope of type III group B Streptococcus capsular polysaccharide

In order to further characterize the conformational epitope of GBSPIII, we synthesized various oligosaccharides with the GBSPIII-related structures by a tailor-assembly synthetic scheme and a more traditional block-wise chemo-enzymatic approach. The oligosaccharides were used to probe the conformational epitope of GBSPIII using number of complementary techniques. The protective epitope of GPSPIII was further defined as length-dependent and conformational. The results of the studies confirmed that two repeating units (2RU) is the minimum binding unit and the epitope optimization mainly takes place between chain length 2RU to 7RU. Epitope optimization and multivalency were observed between 7RU and 20RU. The data support our hypothesis that the conformational epitope is an extended helical segment of the GBSPIII. GBSPIII exists mainly in the random coil form, which structurally mimics short oligosaccharide self-antigens, but it can infrequently and spontaneously form extended helices. Although not prevalent in GBSPIII the immune system preferentially selects these helical epitopes because they are unique to the polysaccharide.

T cells activated by zwitterionic molecules prevent abscesses induced by pathogenic bacteria

Immunologic paradigms classify bacterial polysaccharides as T cell-independent antigens. However, these models fail to explain how zwitterionic polysaccharides (Zps) confer protection against intraabdominal abscess formation in a T cell-dependent manner. Here, we demonstrate that Zps elicit a potent CD4+ T cell response in vitro that requires available major histocompatibility complex class II molecules on antigen-presenting cells. Specific chemical modifications to Zps show that: 1) the activity is specific for carbohydrate structure, and 2) the proliferative response depends upon free amino and carboxyl groups on the repeating units of these polysaccharides. Peptides synthesized to mimic the zwitterionic charge motif associated with Zps also exhibited these biologic properties. Lysine-aspartic acid (KD) peptides with more than 15 repeating units stimulated CD4+ T cells in vitro and conferred protection against abscesses induced by bacteria such as Bacteroides fragilis and Staphylococcus aureus. Evidence for the biologic importance of T cell activation by these zwitterionic polymers was provided when human CD4+ T cells stimulated with these molecules in vitro and adoptively transferred to rats in vivo conferred protection against intraabdominal abscesses induced by viable bacterial challenge. These studies demonstrate that bacterial polysaccharides with a distinct charge motif activate T cells and that this activity confers immunity to a distinct pathologic response to bacterial infection.

Size fractionation of bacterial capsular polysaccharides for their use in conjugate vaccines

We have developed a chromatographic method suitable for the fractionation of polysaccharides having a negatively charged group. The method permits the removal of all those polysaccharide fragments having a short sequence and which are likely unsuitable for conjugate vaccine construction. The selected polysaccharide fragments can be used to produce glycoconjugate vaccines containing a restricted saccharide polydispersion. We have applied this chromatographic method to three different antigens, Haemophilus influenzae type b and Neisseria meningitidis group A and group C polysaccharides. The method is easily adapted for manufacturing purposes.

Isolation and chemical characterization of a capsular polysaccharide antigen shared by clinical isolates of Enterococcus faecalis and vancomycin-resistant Enterococcus faecium

Enterococci are a common cause of serious infections, especially in newborns, severely immunocompromised patients, and patients requiring intensive care. To characterize enterococcal surface antigens that are targets of opsonic antibodies, rabbits were immunized with various gentamicin-killed Enterococcus faecalis strains, and immune sera were tested in an opsonophagocytic assay against a selection of clinical isolates. Serum raised against one strain killed the homologous strain (12030) at a dilution of 1:5,120 and mediated opsonic killing of 33% of all strains tested. In addition, this serum killed two (28%) of seven vancomycin-resistant Enterococcus faecium strains. Adsorption of sera with the homologous strain eliminated killing activity. The adsorbing antigens were resistant to treatment with proteinase K and to boiling for 1 h, but were susceptible to treatment with sodium periodate, indicating that the antigen inducing opsonic activity is a polysaccharide. Antibodies in immune rabbit sera reacted with a capsule-like structure visualized by electron microscopy both on the homologous E. faecalis strain and on a vancomycin-resistant E. faecium strain. The capsular polysaccharides from E. faecalis 12030 and E. faecium 838970 were purified, and chemical and structural analyses indicated they were identical glycerol teichoic acid-like molecules with a carbohydrate backbone structure of 6-alpha-D-glucose-1-2 glycerol-3-PO4 with substitution on carbon 2 of the glucose with an alpha-2-1-D-glucose residue. The purified antigen adsorbed opsonic killing activity from immune rabbit sera and elicited high titers of antibodies (when used to immunize rabbits) that both mediated opsonic killing of bacteria and bound to a capsule-like structure visualized by electron microscopy. These results indicate that approximately one-third of a sample of 15 E. faecalis strains and 7 vancomycin-resistant E. faecium strains possess shared capsular polysaccharides that are targets of opsonophagocytic antibodies and therefore are potential vaccine candidates.

Use of highly encapsulated Streptococcus pneumoniae strains in a flow-cytometric assay for assessment of the phagocytic capacity of serotype-specific antibodies

A phagocytosis assay for Streptococcus pneumoniae based on flow cytometry (FACS) with human polymorphonuclear cells and human complement was developed for the study of human vaccination antisera. Human prevaccination sera already contain high levels of C-polysaccharide (C-PS) antibodies, which are not protective in humans but which might give false positive results in a flow-cytometry-based assay. Cultures of S. pneumoniae grown to log phase on three consecutive days, followed by heat inactivation, yielded stable and highly encapsulated strains for serotypes 6A, 6B, 14, 19F, and 23F. As a result, only serotype-specific antibodies were able to facilitate phagocytosis of these strains, whereas no phagocytosis was observed with antibodies against C-PS or pneumococcal surface proteins. No, or weak, phagocytosis was observed with human prevaccination sera, whereas in general, postvaccination antisera facilitated phagocytosis. A highly significant correlation was observed between enzyme-linked immunosorbent assay titers and FACS phagocytosis titers (r = 0.98, P < 0.001) for serotype 23F pneumococci with human vaccination antisera. For all serotypes, interassay variation was below 10%. Major advantages of this assay over the classical killing assay are that (i) limited amounts of sera are required (10 microliter per titration curve), (ii) 600 samples can be processed in one day by one person, and (iii) cells can be fixed and measurement of the samples can be performed up to 1 week later.

Deletion of repeats in the alpha C protein enhances the pathogenicity of group B streptococci in immune mice

The alpha C protein is a protective surface-associated antigen of group B streptococci (GBS). The prototype alpha C protein of GBS (strain A909) contains nine identical tandem repeats, each comprising 82 amino acids, flanked by N- and C-terminal domains. Clinical isolates of GBS show variable numbers of repeats with a normal distribution and a median of 9 to 10 repeats. Here, we show that escape mutants of GBS expressing one-repeat alpha C protein were 100-fold more pathogenic than GBS expressing wild-type nine-repeat alpha C protein in neonatal mice whose dams were immunized with antiserum elicited to nine-repeat alpha C protein (50% lethal doses of 1.6 x 10(3) and 1.8 x 10(5), respectively; P = 0.0073). There was no difference in pathogenicity in nonimmune mice. Enzyme-linked immunosorbent assay inhibition showed that nine-repeat but not one-repeat alpha C protein is readily available for antibody binding on the surface of intact GBS. Immune electron microscopy studies with antibodies to the capsular polysaccharide (CPS) and to the alpha C protein demonstrated localization of the nine-repeat alpha C protein and the CPS at similar distances from the cell wall. The one-repeat alpha C protein was visualized poorly and only in close proximity to the cell wall, thus suggesting that antibody binding to the protein was hindered by CPS or other cell surface components. We concluded that deletion in the repeat region of the alpha C protein enhanced the pathogenicity of GBS in immune mice by (i) loss of a protective (conformational) epitope(s) and (ii) loss of antibody binding to the alpha C protein due to a decrease in antigen size relative to cell wall components and/or CPS.

Ozonolysis for selectively depolymerizing polysaccharides containing beta-D-aldosidic linkages

The depolymerization of polysaccharides, particularly those containing acid-sensitive components, into intact constituent repeating units can be very difficult. We describe a method using ozonolysis for depolymerizing polysaccharides containing beta-D-aldosidic linkages into short-chain polysaccharides and oligosaccharides. This method is carried out on polysaccharides that have been fully acetylated whereby beta-D-aldosidic linkages are selectively oxidized by ozone to form esters, from which the polysaccharides are subsequently cleaved with a nucleophile. Ozone oxidation of aldosidic linkages proceeds under strong stereoelectronic control, and reaction rates depend on the conformations of glycosidic linkages. Thus, beta-D-aldosidic linkages with different conformations can have very different reaction rates even in the absence of substantial chemical differences. These rate differences allowed for very high selectivity in cleaving beta-D-linkages of polysaccharides. Several polysaccharides from group B Streptococcus and other bacterial species were selectively depolymerized with this method. The repeating units of the group B Streptococcus polysaccharides all contain an acid-sensitive sialic acid residue in a terminal position on a side chain and several beta-D-residues including galactose, glucose, and N-acetylglucosamine; however, with each polysaccharide, one type of linkage was more reactive than others. Selective cleavage of the most sensitive linkage occurs randomly throughout the polymer chain, yielding fragments of controllable and narrowly distributed sizes and the same repeating-unit structure. The average size of the molecules decreases exponentially, and desired sizes can be obtained by stopping the reaction at appropriate time points. With this method the labile sialic acid residue was not affected.

Structural properties of group B streptococcal type III polysaccharide conjugate vaccines that influence immunogenicity and efficacy

In this study, we tested the hypothesis that the immunogenicity and protective efficacy of polysaccharide-protein conjugate vaccines are influenced by three variables: (i) molecular size of the conjugate, (ii) molecular size of the polysaccharide used for conjugation, and (iii) extent of polysaccharide-to-protein cross-linking. Type III group B Streptococcus capsular polysaccharide was linked by reductive amination at multiple sites to tetanus toxoid to create a polysaccharide-protein conjugate (III-TT). A single lot of III-TT was fractionated into small, medium, and large Mr pools. Whereas all three conferred protection in a maternal immunization-neonatal challenge model in mice, the smallest Mr conjugate evoked less polysaccharide-specific immunoglobulin G (IgG) than the two larger Mr conjugates. To test whether the molecular size of the polysaccharide used for conjugation also affected the immunogenicity of the conjugate, vaccines were synthesized using capsular polysaccharides with Mrs of 38,000, 105,000, and 349,000. Polysaccharide-specific IgG responses in mice increased with the Mr of the polysaccharides, and protective efficacy was lower for the smallest polysaccharide conjugate compared to the other two vaccines. Immunogenicity testing of a series of vaccines prepared with different degrees of polysaccharide-to-protein cross-linking demonstrated higher polysaccharide-specific antibody responses as the extent of cross-linking increased. However, opsonic activity was greatest in mouse antiserum raised to a moderately cross-linked conjugate, suggesting that some antibodies evoked by highly cross-linked conjugates were directed to a nonprotective epitope. We conclude that conjugate size, polysaccharide size, and degree of polysaccharide-protein cross-linking influence the immunogenicity and protective efficacy of III-TT conjugate vaccines.

Synthesis and NMR assignment of two repeating units (decasaccharide) of the type III group B Streptococcus capsular polysaccharide and its 13C-labeled and N-propionyl substituted sialic acid analogues

For the purpose of carrying out a comprehensive investigation into the nature of the conformational epitope of the type III group B Streptococcus polysaccharide, combined chemical and enzymatic methods were applied to the synthesis of three decasaccharide probes, namely beta-D-Glc-(1-->6)[alpha-NeuR-(2-->3)-beta-D-Gal-(1-->4)] -beta-D-GlcNAc-(1-->3)-beta-D-Gal-(1-->4)-beta-D- Glc-(1-->6)[alpha-NeuR-(2-->3)-beta-D-Gal-(1-->4)]-beta-D-GlcNAc-( 1-->3) -beta-D-Gal-OMe (22 NeuR = NeuAc; 23 NeuR = NeuAc with 8% 13C-labeling; 24 NeuR = NeuPr). The precursor core octasaccharide 21 was chemically synthesized from trisaccharide donor 11 and pentasaccharide acceptor 19 by block condensation. Sialylation of 21 with alpha-(2-->3)-sialyltransferase and CMP-NeuAc afforded 22. In the presence of CMP-sialic acid synthetase and alpha-(2-->3)-sialyltransferase, 21 was sialylated with sialic acid derivatives (8% 13C-labeled, or N-propionyl substituted) to give 23 and 24, respectively. Complete assignments of the 1H and 13C NMR spectra of compounds 21, 22 (23), and 24 are also presented.

Immune response to type III group B streptococcal polysaccharide-tetanus toxoid conjugate vaccine

Group B Streptococcus (GBS) is an important perinatal pathogen. Because transplacentally acquired maternal antibodies to the GBS capsular polysaccharides (CPS) confer protection, prevention of infant disease may be possible after immunization of women. Unfortunately, the purified CPS of GBS are only variably immunogenic in adults; therefore to enhance immunogenicity we have designed and developed a CPS-protein conjugate vaccine. The lability of a conformationally dependent epitope on the III CPS containing a critical sialic acid residue was important to consider in vaccine design. 100 women were randomized to receive GBS type III CPS-tetanus toxoid conjugate (III-TT) vaccine at one of three doses; unconjugated GBS type III CPS; or saline. Serum samples were obtained before immunization and 2, 4, 8, and 26 wk thereafter, and specific antibody to type III CPS was measured. Vaccines were well tolerated. In sera from recipients of the highest dose of III-TT, CPS-specific IgG levels rose from a geometric mean of 0.09 microg/ml before immunization to 4.53 microg/ml 8 wk later, whereas levels in recipients of unconjugated type III CPS rose from 0.21 microg/ml to 1.41 microg/ml. Lower doses resulted in lower antibody levels. A > or = 4-fold rise in antibody concentration was achieved in 90% of recipients of III-TT compared with 50% of those that received III CPS (P = 0.0015). Antibodies evoked by the conjugate vaccine recognized a conformationally dependent epitope of the III-CPS, promoted opsonophagocytosis and killing of GBS, and, after maternal immunization, protected neonatal mice from lethal challenge with type III GBS. We conclude that directed coupling of type III GBS polysaccharide to a carrier protein yielded a conjugate vaccine with preserved expression of a highly labile conformational epitope involving sialic acid and enhanced immunogenicity compared with uncoupled CPS.

Cell growth rate regulates expression of group B Streptococcus type III capsular polysaccharide

The capsular polysaccharide (CPS) of group B streptococci (GBS) is an important virulence factor that also serves to protect cells from nonspecific host defense mechanisms. Expression of CPS by GBS, as with other encapsulated bacterial pathogens, is not constitutive but varies during growth in vitro and in primary cultures isolated from different sites of infection. Despite this understanding, little is known about regulation of this surface-expressed carbohydrate antigen in GBS. Here we report that expression of type III CPS by GBS strain M781 grown in continuous culture with a modified chemically defined medium is regulated by growth rate. Cells in steady state at mass doubling times (tds) of 0.8, 1.4, and 1.6 h expressed an average of sixfold more cell-associated CPS than did cells held at tds of 2.3 and 11 h. Strain M781 grown at a td of 1.4 h repeatedly produced more type III CPS than those held at a td of 11.0 h, even when limited for glucose, pyridoxamine, or thiamine. In our studies, > or = 93% of the total CPS expressed by strain M781 was cell associated. Strain M781 grown at a td of 11.0 h (i.e., lowered CPS expression) was susceptible to in vitro complement-mediated opsonophagocytosis and killing by human peripheral blood leukocytes, whereas cells grown at a td of 1.4 h (i.e., higher CPS expression) were not killed unless type III CPS-specific antibody was present. Factors that allow GBS to asymptomatically colonize women yet cause invasive infection to both mother and infant are poorly understood. Our results shed new light on parameters that regulate the pathogenic potential of GBS and may also serve as a way to discern more fully the genetics and biochemistry of GBS capsule synthesis.