Antibody recognition of the type 14 pneumococcal capsule. Evidence for a conformational epitope in a neutral polysaccharide

Antibody enhanced HPLC for serotype-specific quantitation of polysaccharides in pneumococcal conjugate vaccine

Bacterial infection remains as one of the major healthcare issues, despite significant scientific and medical progress in this field. Infection by Streptococcus Pneumoniae (S. Pneumoniae) can cause pneumonia and other serious infectious diseases, such as bacteremia, sinusitis and meningitis. The pneumococcal capsular polysaccharides (CPS) that constitute the outermost layer of the bacterial cell are the main immunogens and protect the pathogen from host defense mechanisms. Over 90 pneumococcal CPS serotypes have been identified, among which more than 30 can cause invasive pneumococcal diseases that could lead to morbidity and mortality. Multivalent pneumococcal vaccines have been developed to prevent diseases caused by S. Pneumoniae. These vaccines employ either purified pneumococcal CPSs or protein conjugates of these CPSs to generate antigen-specific immune responses for patient protection. Serotype-specific quantitation of these polysaccharides (Ps) antigen species are required for vaccine clinical dosage, product release and quality control. Herein, we have developed an antibody-enhanced high-performance liquid chromatography (HPLC) assay for serotype-specific quantitation of the polysaccharide contents in multivalent pneumococcal conjugate vaccines (PCVs). A fluorescence-labeled multiplex assay format has also been developed. This work laid the foundation for a serotype-specific antigen assay format that could play an important role for future vaccine research and development.

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.

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.

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.

Glycoconjugate vaccine using a genetically modified O antigen induces protective antibodies to Francisella tularensis

Francisella tularensis is the causative agent of tularemia, a category A bioterrorism agent. The lipopolysaccharide (LPS) O antigen (OAg) of F. tularensis has been considered for use in a glycoconjugate vaccine, but conjugate vaccines tested so far have failed to confer protection necessary against aerosolized pulmonary bacterial challenge. When F. tularensis OAg was purified under standard conditions, the antigen had a small molecular size [25 kDa, low molecular weight (LMW)]. Using milder extraction conditions, we found the native OAg had a larger molecular size [80 kDa, high molecular weight (HMW)], and in a mouse model of tularemia, a glycoconjugate vaccine made with the HMW polysaccharide coupled to tetanus toxoid (HMW-TT) conferred better protection against intranasal challenge than a conjugate made with the LMW polysaccharide (LMW-TT). To further investigate the role of OAg size in protection, we created an F. tularensis live vaccine strain (LVS) mutant with a significantly increased OAg size [220 kDa, very high molecular weight (VHMW)] by expressing in F. tularensis a heterologous chain-length regulator gene (wzz) from the related species Francisella novicida Immunization with VHMW-TT provided markedly increased protection over that obtained with TT glycoconjugates made using smaller OAgs. We found that protective antibodies recognize a length-dependent epitope better expressed on HMW and VHMW antigens, which bind with higher affinity to the organism.

Complementary Role of CD4+ T Cells in Response to Pneumococcal Polysaccharide Vaccines in Humans

Bacterial pathogens expressing capsular polysaccharides are common causes of mucosal infections (pneumonia, intestinal), as well as often fatal, invasive infections (meningitis, bloodstream infections) in children and adults worldwide. These chemically simple but structurally complex carbohydrate structures on the bacterial surface confer resistance to recognition and clearance by the immune system through a range of mechanisms. Such recognition of capsular polysaccharides may be reduced by their limited ability to directly stimulate B cells and the T cells that may facilitate these humoral responses. The capsules may promote the evasion of complement deposition and activation and may sterically shield the recognition of other subjacent protein antigens by innate factors. Antibodies to capsular polysaccharides, elicited by infection and vaccines, may overcome these obstacles and facilitate bacterial agglutination at mucosal surfaces, as well as the opsonization and clearance of these organisms in tissues and the systemic compartment. However, the immunogenicity of these antigens may be limited by their lack of direct recognition by T cells (“T-independent” antigens) and their restricted ability to generate effective memory responses. In this review, we consider the mechanisms by which polysaccharides may initiate B cell responses and specific antibody responses and the role of T cells, particularly CD4+ follicular helper (TFH) cells to support this process. In addition, we also consider more recent counterintuitive data that capsular polysaccharides themselves may bind major histocompatibility antigen HLA class II to provide a more physiologic mechanism of T cell enhancement of B cell responses to capsular polysaccharides. Defining the contributions of T cells in the generation of effective humoral responses to the capsular polysaccharides will have important implications for understanding and translating this immunobiology for the development of more effective vaccines, to prevent the morbidity and mortality associated with these common mucosal and invasive pathogens in populations at risk.

Neoglycoconjugate of Tetrasaccharide Representing One Repeating Unit of the Streptococcus pneumoniae Type 14 Capsular Polysaccharide Induces the Production of Opsonizing IgG1 Antibodies and Possesses the Highest Protective Activity As Compared to Hexa- and Octasaccharide Conjugates

Identifying protective synthetic oligosaccharide (OS) epitopes of Streptococcus pneumoniae capsular polysaccharides (CPs) is an indispensable step in the development of third-generation carbohydrate pneumococcal vaccines. Synthetic tetra-, hexa-, and octasaccharide structurally related to CP of S. pneumoniae type 14 were coupled to bovine serum albumin (BSA), adjuvanted with aluminum hydroxide, and tested for their immunogenicity in mice upon intraperitoneal prime-boost immunizations. Injections of the conjugates induced production of opsonizing anti-OS IgG1 antibodies (Abs). Immunization with the tetra- and octasaccharide conjugates stimulated the highest titers of the specific Abs. Further, the tetrasaccharide ligand demonstrated the highest ability to bind OS and CP Abs. Murine immune sera developed against tetra- and octasaccharide conjugates promoted pathogen opsonization to a higher degree than antisera against conjugated hexasaccharide. For the first time, the protective activities of these glycoconjugates were demonstrated in mouse model of generalized pneumococcal infections. The tetrasaccharide conjugate possessed the highest protective activities. Conversely, the octasaccharide conjugate had lower protective activities and the lowest one showed the hexasaccharide conjugate. Sera against all of the glycoconjugates passively protected naive mice from pneumococcal infections. Given that the BSA-tetrasaccharide induced the most abundant yield of specific Abs and the best protective activity, this OS may be regarded as the most promising candidate for the development of conjugated vaccines against S. pneumoniae type 14 infections.

Preparation and immunogenicity of gold glyco-nanoparticles as antipneumococcal vaccine model

AIM

Nanotechnology-based fully synthetic carbohydrate vaccines are promising alternatives to classic polysaccharide/protein conjugate vaccines. We have prepared gold glyco-nanoparticles (GNP) bearing two synthetic carbohydrate antigens related to serotypes 19F and 14 of Streptococcus pneumoniae and evaluated their immunogenicity in vivo.

RESULTS

A tetrasaccharide fragment of serotype 14 (Tetra-14), a trisaccharide fragment of serotype 19F (Tri-19F), a T-helper peptide and d -glucose were loaded onto GNP in different ratios. Mice immunization showed that the concomitant presence of Tri-19F and Tetra-14 on the same nanoparticle critically enhanced the titers of specific IgG antibodies toward type 14 polysaccharide compared with GNP exclusively displaying Tetra-14, while no IgG antibodies against type 19F polysaccharide were elicited.

CONCLUSION

This work is a step forward toward synthetic nanosystems combining carbohydrate antigens and immunogenic peptides as potential carbohydrate-based vaccines.

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.

Antibody recognition of carbohydrate epitopes†

Carbohydrate antigens are valuable as components of vaccines for bacterial infectious agents and human immunodeficiency virus (HIV), and for generating immunotherapeutics against cancer. The crystal structures of anti-carbohydrate antibodies in complex with antigen reveal the key features of antigen recognition and provide information that can guide the design of vaccines, particularly synthetic ones. This review summarizes structural features of anti-carbohydrate antibodies to over 20 antigens, based on six categories of glyco-antigen: (i) the glycan shield of HIV glycoproteins; (ii) tumor epitopes; (iii) glycolipids and blood group A antigen; (iv) internal epitopes of bacterial lipopolysaccharides; (v) terminal epitopes on polysaccharides and oligosaccharides, including a group of antibodies to Kdo-containing Chlamydia epitopes; and (vi) linear homopolysaccharides.

Identification of pneumococcal colonization determinants in the stringent response pathway facilitated by genomic diversity

BACKGROUND

Understanding genetic determinants of a microbial phenotype generally involves creating and comparing isogenic strains differing at the locus of interest, but the naturally existing genomic and phenotypic diversity of microbial populations has rarely been exploited. Here we report use of a diverse collection of 616 carriage isolates of Streptococcus pneumoniae and their genome sequences to help identify a novel determinant of pneumococcal colonization.

RESULTS

A spontaneously arising laboratory variant (SpnYL101) of a capsule-switched TIGR4 strain (TIGR4:19F) showed reduced ability to establish mouse nasal colonization and lower resistance to non-opsonic neutrophil-mediated killing in vitro, a phenotype correlated with in vivo success. Whole genome sequencing revealed 5 single nucleotide polymorphisms (SNPs) affecting 4 genes in SpnYL101 relative to its ancestor. To evaluate the effect of variation in each gene, we performed an in silico screen of 616 previously published genome sequences to identify pairs of closely-related, serotype-matched isolates that differ at the gene of interest, and compared their resistance to neutrophil-killing. This method allowed rapid examination of multiple candidate genes and found phenotypic differences apparently associated with variation in SP_1645, a RelA/ SpoT homolog (RSH) involved in the stringent response. To establish causality, the alleles corresponding to SP_1645 were switched between the TIGR4:19F and SpnYL101. The wild-type SP_1645 conferred higher resistance to neutrophil-killing and competitiveness in mouse colonization. Using a similar strategy, variation in another RSH gene (TIGR4 locus tag SP_1097) was found to alter resistance to neutrophil-killing.

CONCLUSIONS

These results indicate that analysis of naturally existing genomic diversity complements traditional genetics approaches to accelerate genotype-phenotype analysis.

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.

Carbohydrate targets in HIV vaccine research: lessons from failures

Learning from the successes of other vaccines that enhance natural and existing protective responses to pathogens, the current effort in HIV vaccine research is directed toward inducing cytotoxic responses. Nevertheless, antibodies are fundamental players in vaccine development and are still considered in the context of passive specific immunotherapy of HIV, especially since several broadly neutralizing monoclonals are available. Special interest is directed toward antibodies binding to the glycan array on gp120 since they have the potential of broader reactivity and cross-clade neutralizing capacity. Humoral responses to carbohydrate antigens have proven effective against other pathogens, why not HIV? The variability of the epitope targets on HIV may not be the only problem to developing active or passive immunotherapeutic strategies. The dynamics of the infected immune system leads to ambiguous effects of most of the effector mechanisms calling for new approaches; some may already be available, while others are in the making.

Infections of people with complement deficiencies and patients who have undergone splenectomy

The complement system comprises several fluid-phase and membrane-associated proteins. Under physiological conditions, activation of the fluid-phase components of complement is maintained under tight control and complement activation occurs primarily on surfaces recognized as "nonself" in an attempt to minimize damage to bystander host cells. Membrane complement components act to limit complement activation on host cells or to facilitate uptake of antigens or microbes "tagged" with complement fragments. While this review focuses on the role of complement in infectious diseases, work over the past couple of decades has defined several important functions of complement distinct from that of combating infections. Activation of complement in the fluid phase can occur through the classical, lectin, or alternative pathway. Deficiencies of components of the classical pathway lead to the development of autoimmune disorders and predispose individuals to recurrent respiratory infections and infections caused by encapsulated organisms, including Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae. While no individual with complete mannan-binding lectin (MBL) deficiency has been identified, low MBL levels have been linked to predisposition to, or severity of, several diseases. It appears that MBL may play an important role in children, who have a relatively immature adaptive immune response. C3 is the point at which all complement pathways converge, and complete deficiency of C3 invariably leads to severe infections, including those caused by meningococci and pneumococci. Deficiencies of the alternative and terminal complement pathways result in an almost exclusive predisposition to invasive meningococcal disease. The spleen plays an important role in antigen processing and the production of antibodies. Splenic macrophages are critical in clearing opsonized encapsulated bacteria (such as pneumococci, meningococci, and Escherichia coli) and intraerythrocytic parasites such as those causing malaria and babesiosis, which explains the fulminant nature of these infections in persons with anatomic or functional asplenia. Paramount to the management of patients with complement deficiencies and asplenia is educating patients about their predisposition to infection and the importance of preventive immunizations and seeking prompt medical attention.

Pneumococcal capsular polysaccharide structure predicts serotype prevalence

There are 91 known capsular serotypes of Streptococcus pneumoniae. The nasopharyngeal carriage prevalence of particular serotypes is relatively stable worldwide, but the host and bacterial factors that maintain these patterns are poorly understood. Given the possibility of serotype replacement following vaccination against seven clinically important serotypes, it is increasingly important to understand these factors. We hypothesized that the biochemical structure of the capsular polysaccharides could influence the degree of encapsulation of different serotypes, their susceptibility to killing by neutrophils, and ultimately their success during nasopharyngeal carriage. We sought to measure biological differences among capsular serotypes that may account for epidemiological patterns. Using an in vitro assay with both isogenic capsule-switch variants and clinical carriage isolates, we found an association between increased carriage prevalence and resistance to non-opsonic neutrophil-mediated killing, and serotypes that were resistant to neutrophil-mediated killing tended to be more heavily encapsulated, as determined by FITC-dextran exclusion. Next, we identified a link between polysaccharide structure and carriage prevalence. Significantly, non-vaccine serotypes that have become common in vaccinated populations tend to be those with fewer carbons per repeat unit and low energy expended per repeat unit, suggesting a novel biological principle to explain patterns of serotype replacement. More prevalent serotypes are more heavily encapsulated and more resistant to neutrophil-mediated killing, and these phenotypes are associated with the structure of the capsular polysaccharide, suggesting a direct relationship between polysaccharide biochemistry and the success of a serotype during nasopharyngeal carriage and potentially providing a method for predicting serotype replacement.

Streptococcus pneumoniae, or pneumococcus, is an important pathogen worldwide and causes a wide range of diseases, mostly in young children and the elderly. There are 91 serotypes of pneumococcus, each of which produces a unique polysaccharide, called the capsule, that attaches to the bacterial surface and prevents it from being cleared by the host. The serotypes differ greatly in their prevalence in the human population. There is currently a vaccine, effective in infancy, which targets seven clinically important serotypes, but several types not covered by the vaccine are beginning to increase in carriage frequency. As a result, it is critical to understand why some serotypes are frequently carried in the human population while others are not. In this study, we find that the high-prevalence serotypes tend to be more heavily encapsulated and more resistant to killing by neutrophils. Significantly, we find that the biochemical properties of the different polysaccharides can be used to predict their carriage frequency both before and after introduction of the vaccine. These results provide a biologically plausible explanation for differences in prevalence between serotypes.

Thermodynamics and density of binding of a panel of antibodies to high-molecular-weight capsular polysaccharides

The interaction between antipolysaccharide (anti-PS) antibodies and their antigens was investigated by the use of isothermal titration calorimetry to determine the thermodynamic binding constant (K), the change in the enthalpy of binding (DeltaH), and the binding density (N) to high-molecular-weight PSs. From these values, the change in the entropy of binding (DeltaS) was calculated. The thermodynamic parameters of binding to high-molecular-weight capsular PSs are reported for two monoclonal antibodies (MAbs) with different specificities for meningococcal serogroup C PS, five MAbs specific for different pneumococcal serotypes, and the Fab fragments of two antipneumococcal MAbs. The K values were in the range of 10(6) to 10(7) M(-1), and these values were 1 to 2 orders of magnitude greater than the previously reported K values derived from antibody-oligosaccharide interactions. The DeltaH associated with binding was favorable for each MAb and Fab fragment. The DeltaS associated with binding was also generally favorable for both the MAbs and the Fab fragments, with the exception of the anti-serotype 14 MAb and its Fab fragment. N provides information regarding how densely MAbs or Fabs can bind along PS chains and, as expressed in terms of monosaccharides, was very similar for the seven MAbs, with an average of 12 monosaccharides per bound MAb. The value of N for each Fab was smaller, with five or seven monosaccharides per bound Fab. These results suggest that steric interactions between antibody molecules are a major influence on the values of N of high-affinity MAbs to capsular PSs.

Liposomal co-entrapment of CD40mAb induces enhanced IgG responses against bacterial polysaccharide and protein

Background

Antibody against CD40 is effective in enhancing immune responses to vaccines when chemically conjugated to the vaccine antigen. Unfortunately the requirement for chemical conjugation presents some difficulties in vaccine production and quality control which are compounded when multivalent vaccines are required. We explore here an alternative to chemical conjugation, involving the co-encapsulation of CD40 antibody and antigens in liposomal vehicles.

Methodology/Principal Findings

Anti-mouse CD40 mAb or isotype control mAb were co-entrapped individually in cationic liposomal vehicles with pneumococcal polysaccharides or diphtheria and tetanus toxoids. Retention of CD40 binding activity upon liposomal entrapment was assessed by ELISA and flow cytometry. After subcutaneous immunization of BALB/c female mice, anti-polysaccharide and DT/TT responses were measured by ELISA. Simple co-encapsulation of CD40 antibody allowed for the retention of CD40 binding on the liposome surface, and also produced vaccines with enhanced imunogenicity. Antibody responses against both co-entrapped protein in the form of tetanus toxoid, and Streptococcus pneumoniae capsular polysaccharide, were enhanced by co-encapsulation with CD40 antibody. Surprisingly, liposomal encapsulation also appeared to decrease the toxicity of high doses of CD40 antibody as assessed by the degree of splenomegaly induced.

Conclusions/Significance

Liposomal co-encapsulation with CD40 antibody may represent a practical means of producing more immunogenic multivalent vaccines and inducing IgG responses against polysaccharides without the need for conjugation.

Epitope specificities of the group Y and W-135 polysaccharides of Neisseria meningitidis

Previous studies have identified the length dependency of several polysaccharide (PS) protective epitopes. We have investigated whether meningococcal polysaccharides Y and W-135 possess such epitopes. Oligosaccharides (OSs) consisting of one or more disaccharide repeating units (RU) were derived from the capsular PSs of group Y and W-135 meningococci (GYMP and GWMP, respectively) by mild acid hydrolysis. The relative affinities of anticapsular antibodies binding to derivative OSs of different chain lengths were measured in inhibition enzyme-linked immunosorbent assays. As OS size increased from two to three RU, there was a notable increase in binding inhibition of rabbit anti-group Y antiserum. This pattern of antibody binding inhibition was also observed for rabbit antiserum to group W-135, though the inhibition increase was much more pronounced. In the cases of both OS species, the concentration of inhibiting antigen required to achieve 50% inhibition of rabbit immunoglobulin binding increased progressively as the inhibiting disaccharide chain length increased from 1 RU through greater than 50 RU. These data suggest that antibodies directed against both of these meningococcal PSs recognize conformational epitopes only fully expressed in higher-molecular-weight forms of these antigens.

Addition of glycerol for improved methylation linkage analysis of polysaccharides

A presolubilization procedure with the use of glycerol is shown to be applicable for the structural analysis of polysaccharides. Neutral, acidic, high-molecular-weight and low-molecular-weight polysaccharides were solubilized in glycerol prior to methylation and subsequent linkage analysis by GC-MS. All four types of polysaccharides showed significant increases in derivatization following presolubilization as measured by recovery of partially methylated alditol acetates.

Unexpected diversity in the fine specificity of monoclonal antibodies that use the same V region gene to glucuronoxylomannan of Cryptococcus neoformans

Most mAbs to the capsular polysaccharide glucuronoxylomannan (GXM) of Cryptococcus neoformans are generated from the same VH and VL gene families. Prior Ab studies have assessed protective efficacy, Id structure and binding to capsular polysaccharides, and peptide mimetics. These data have been interpreted as indicating that most mAbs to GXM have the same specificity. A new approach to Ab specificity analysis was investigated that uses genetic manipulation to generate C. neoformans variants with structurally different capsules. C. neoformans mutants expressing GXM with defective O-acetylation were isolated and complemented by the C. neoformans gene CAS1, which is necessary for the O-acetylation of GXM. The mAbs exhibited differences in their binding to the GXM from these mutant strains, indicating previously unsuspected differences in specificity. Analysis of three closely related IgMs revealed that one (mAb 12A1) bound to an epitope that did not require O-acetylation, another (mAb 21D2) was inhibited by O-acetylation, and the third (mAb 13F1) recognized an O-acetylation-dependent conformational epitope. Furthermore, an IgG Ab (mAb 18B7) in clinical development retained binding to de-O-acetylated polysaccharide; however, greater binding was observed to O-acetylated GXM. Our findings suggest that microbial genetic techniques can provide a new approach for epitope mapping of polysaccharide-binding Abs and suggest that this method may applicable for studying the antigenic complexity of polysaccharide Ags in other capsulated microorganisms.

NMR and molecular modeling of complex carbohydrates and carbohydrate-protein interactions. Applications to anti-bacteria vaccines

In order to characterize the conformational epitope of the group B meningococcal polysaccharide and of the type III group B Streptococcus capsular polysaccharide NMR measurements were done on a wide variety of native and modified polysaccharides and oligosaccharides. Since these saccharides are highly mobile and exist as random coils in solution, the analysis of the NMR data and molecular modeling was done to take into account this inherent flexibility. The conformational model of extended high-order helices being selected upon binding to a protein, although still hypothetical at this stage, has proven useful in explaining the serology for the conformational epitopes for polysaccharides of group B Neisseria meningitidis, group B Streptococcus type III and Streptococcus pneumoniae type 14.

NMR studies of carbohydrates and carbohydrate-mimetic peptides recognized by an anti-group B Streptococcus antibody

As part of a program to investigate the origins of peptide-carbohydrate mimicry, the conformational preferences of peptides that mimic the group B streptococcal type III capsular polysaccharide have been investigated by NMR spectroscopy. Detailed studies of a dodecapeptide, FDTGAFDPDWPA, a molecular mimic of the polysaccharide antigen, and two new analogs, indicated a propensity for beta-turn formation. Different beta-turn types were found to be present in the trans and cis (Trp-10-Pro-11) isomers of the peptide: the trans isomer favored a type I beta-turn from residues Asp-7-Trp-10, whereas the cis isomer exhibited a type VI beta-turn from residues Asp-9-Ala-12. The interaction of the dodecapeptide FDTGAFDPDWPA with a protective anti-group B Streptococcus monoclonal antibody has also been investigated, by transferred nuclear Overhauser effect NMR spectroscopy and saturation-transfer difference NMR spectroscopy (STD-NMR). The peptide was found to adopt a type I beta-turn conformation on binding to the antibody; the peptide residues (Asp-7-Trp-10) forming this turn are recognized by the antibody, as demonstrated by STD-NMR experiments. STD-NMR studies of the interactions of oligosaccharide fragments of the capsular polysaccharide have also been performed and provide evidence for the existence of a conformational epitope.

After chemotherapy, functional humoral response capacity is restored before complete restoration of lymphoid compartments

Chemotherapy has, besides the beneficial effects, several adverse effects. Suppression of the immune system is one of the most important problems. Infections caused by encapsulated bacteria like Streptococcus pneumoniae are responsible for a major part of infectious problems during and after treatment. The splenic marginal zone is essential in the initiation of an immune response to encapsulated bacteria. In this study, we analysed the effects of three different cytostatic agents on humoral immune responses. We found a reduced, but detectable immune response capacity at two days after treatment although the marginal zone B cell population is severely reduced at this time point. Twenty-four days after cessation of treatment, the immune response capacity was largely restored although lymphoid compartments were still not completely restored at that time point. Apparently, the presence of only few marginal zone B cells is sufficient to evoke a rise in antibody titres and although antibody titre increases are low, even small rises are most likely clinically relevant.

Recognition of bacterial capsular polysaccharides and lipopolysaccharides by the macrophage mannose receptor

The in vitro binding of the macrophage mannose receptor to a range of different bacterial polysaccharides was investigated. The receptor was shown to bind to purified capsular polysaccharides from Streptococcus pneumoniae and to the lipopolysaccharides, but not capsular polysaccharides, from Klebsiella pneumoniae. Binding was Ca(2+)-dependent and inhibitable with d-mannose. A fusion protein of the mannose receptor containing carbohydrate recognition domains 4-7 and a full-length soluble form of the mannose receptor containing all domains external to the transmembrane region both displayed very similar binding specificities toward bacterial polysaccharides, suggesting that domains 4-7 are sufficient for recognition of these structures. Surprisingly, no direct correlation could be made between polysaccharide structure and binding to the mannose receptor, suggesting that polysaccharide conformation may play an important role in recognition. The full-length soluble form of the mannose receptor was able to bind simultaneously both polysaccharide via the carbohydrate recognition domains and sulfated oligosaccharide via the cysteine-rich domain. The possible involvement of the mannose receptor, either cell surface or soluble, in the innate and adaptive immune responses to bacterial polysaccharides is discussed.

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.

Structural requirements for the adherence of Plasmodium falciparum-infected erythrocytes to chondroitin sulfate proteoglycans of human placenta

Plasmodium falciparum infection during pregnancy results in the accumulation of infected red blood cells (IRBCs) in the placenta, leading to poor pregnancy outcome. In the preceding paper (Achur, R. N., Valiyaveettil, M., Alkhalil, A., Ockenhouse, C. F., and Gowda, D. C. (2000) J. Biol. Chem. 275, 40344-40356), we reported that unusually low sulfated chondroitin sulfate proteoglycans (CSPGs) in the intervillous spaces of the placenta mediate the IRBC adherence. In this study, we report the structural requirements for the adherence and the minimum chondroitin 4-sulfate (C4S) structural motif that supports IRBC adherence. Partially sulfated C4Ss with varying sulfate contents were prepared by solvolytic desulfation of a fully sulfated C4S. These and other nonmodified C4Ss, with different proportions of 4-, 6-, and nonsulfated disaccharide repeats, were analyzed for inhibition of IRBC adherence to the placental CSPG. C4Ss containing 30-50% 4-sulfated and 50-70% nonsulfated disaccharide repeats efficiently inhibited IRBC adherence; C6S had no inhibitory activity. Oligosaccharides of varying sizes were prepared by the partial depolymerization of C4Ss containing varying levels of 4-sulfation, and their ability to inhibit the IRBC adherence was studied. Oligosaccharides with six or more disaccharide repeats inhibited IRBC adherence to the same level as that of the intact C4Ss, indicating that a dodecasaccharide is the minimum structural motif required for optimal IRBC adherence. Of the C4S dodecasaccharides, only those with two or three sulfate groups per molecule showed maximum IRBC inhibition. These data define the structural requirements for the IRBC adherence to placental CSPGs with implications for the development of therapeutics for maternal malaria.

Are the enzyme immunoassays for antibodies to pneumococcal capsular polysaccharides serotype specific?

The specificity of antibody binding to pneumococcal capsular polysaccharides (Pnc PSs) measured by enzyme immunoassay (EIA) was studied by inhibition of antibody binding by homologous and heterologous PSs. We found extensive cross-reactivity of antibody binding to type 6B, 19F, and 23F PSs but not to type 14 PS, even after treatment with cell wall PS (CPS). The cross-reactive antibody was highly prevalent in sera of infants and adults with naturally acquired antibody, but not in sera of infants and adults immunized with pneumococcal vaccines. However, a type 11A antibody response was seen after vaccination with heterologous PSs. Monoclonal antibodies prepared against a type 6B PS-tetanus toxoid conjugate recognized also other than the specific type of PS in the EIA, implying the possible existence of a cross-reactive epitope. Remarkable differences in specificity among type 6B PS preparations from different manufacturers were found. Moreover, different lots of type 11A PS from the same manufacturer showed differences in specificity. The results suggest that some Pnc PS preparations may contain cross-reactive epitopes or impurities, other than CPS, that are common to many types of Pnc PS. The specificity of antibodies, especially in sera from nonimmunized subjects, measured by EIA can be questioned.

Preparation of pneumococcal capsular polysaccharide-protein conjugate vaccines utilizing new fragmentation and conjugation technologies

There is a global urgent need for a new efficient and inexpensive vaccine to combat pneumococcal disease, which should also be affordable in developing countries. In view of this need a simple low-cost technique to prepare such a vaccine was developed. The preparation of serotype 14 and 23F pneumococcal capsular polysaccharide (PnPS)-protein conjugates to be included in a forthcoming multivalent PnPS conjugate vaccine is described. Commercial lots of PnPSs produced according to Good Manufacturing Practice from Streptococcus pneumoniae serotype 14 (PS14) and 23F (PS23F) were partially depolymerized by sonication or irradiation in an electron beam accelerator. The PnPS fragments were conjugated to tetanus toxoid (TT) using a recently developed conjugation chemistry. The application of these new simple, efficient and inexpensive fragmentation and conjugation technologies allowed the synthesis of several PnPS-protein conjugates containing PnPS fragments of preselected sizes and differing in the degree of substitution. The PS14TT and PS23FTT conjugate vaccine candidates were characterized chemically and their immunogenicity was evaluated in rabbits and mice. All PnPS conjugate vaccines, unlike the corresponding plain polysaccharides, produced high IgG titres in both animal species. The PS14TT conjugates tended to be more immunogenic than the PS23FTT conjugates. The immune response to the PS14TT conjugates, but not to the PS23FTT conjugates, was related to the size of the conjugated polysaccharide hapten. Both types of conjugates elicited strong booster effects upon secondary immunizations, resulting in high IgG1, IgG2a and IgG2b titres.

Effect of molecular size on the ability of zwitterionic polysaccharides to stimulate cellular immunity

The large-molecular-sized zwitterionic capsular polysaccharide of the anaerobe Bacteroides fragilis NCTC 9343, designated polysaccharide (PS) A, stimulates T cell proliferation in vitro and induces T cell-dependent protection against abscess formation in vivo. In the present study, we utilized a modification of a recently developed ozonolytic method for depolymerizing polysaccharides to examine the influence of the molecular size of PS A on cell-mediated immunity. Ozonolysis successfully depolymerized PS A into structurally intact fragments. PS A with average molecular sizes of 129.0 (native), 77.8, 46.9, and 17.1 kDa stimulated CD4+-cell proliferation in vitro to the same degree, whereas the 5.0-kDa fragment was much less stimulatory than the control 129.0-kDa PS A. Rats treated with 129.0-kDa, 46.9-kDa, and 17.1-kDa PS A molecules, but not those treated with the 5.0-kDa molecule, were protected against intraabdominal abscesses induced by challenge with viable B. fragilis. These results demonstrate that a zwitterionic polysaccharide as small as 22 repeating units (88 monosaccharides) elicits a T cell-dependent immune response. These findings clearly distinguish zwitterionic T cell-dependent polysaccharides from T cell-independent polysaccharides and give evidence of the existence of a novel mechanism for a polysaccharide-induced immune response.

Conformational Epitope of the Type III Group B Streptococcus Capsular Polysaccharide

The protective epitope of the type III group B streptococcal polysaccharide (GBSPIII) is length dependent and conformational. To obtain a more accurate characterization of the conformational epitope, ELISA inhibition and surface plasmon resonance studies were conducted on two GBSPIII-specific mAbs using a large panel of oligosaccharide probes. The results of the studies confirmed that 2 repeating units (RU) is the minimum binding unit and that, while increases in chain length from 2 RU to 7 RU caused further optimization of the epitope, it remained monovalent. A 3-fold increase in affinity was observed between 7 RU and 20 RU, which, by surface plasmon resonance studies on a Fab, was shown to be due to both further optimization of the individual epitope and the occurrence of multivalency of epitope. 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 Ags, 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. Contrary to a previously proposed model of GBSPIII binding in which the binding of the first Ab propagates a continuum of helical epitopes, our binding kinetics are consistent only with the helical epitope’s being discontinuous and infrequent.

Electron beam fragmentation of bacterial polysaccharides as a method of producing oligosaccharides for the preparation of conjugate vaccines

End-group mediated conjugation of bacterial polysaccharides (PSs) to carrier proteins containing T-helper cell epitopes renders such polysaccharides immunogenic also in young infants. Optimal construction of such conjugate vaccines requires fragmentation of the PS prior to the coupling reaction. In the present study a general simple and inexpensive method for the fragmentation of PSs is presented. It is based on the irradiation of isolated PSs in an electron beam accelerator. Exposure of isolated pneumococcal capsular polysaccharides (PnPSs) to ionizing radiation resulted in their partial depolymerization in a radiation dose-dependent manner. Radiation, unlike sonication, generated PnPS fragments of molecular size lower than 50 kDa and as small as 1.5 kDa when high radiation doses were used. These PnPS fragments have terminal reducing groups that can be easily used for chemical activation and subsequent coupling to any chosen carrier protein. The radiation-produced PnPS fragments retained their antigenic epitopes, when compared to native, full-size PnPSs as determined by enzyme-linked immunoassay.

Streptococcus pneumoniae type 14 polysaccharide-conjugate vaccines: length stabilization of opsonophagocytic conformational polysaccharide epitopes

A simple and convenient method was developed for the preparation of Streptococcus pneumoniae type 14 polysaccharide (Pn14PS)-tetanus toxoid (TT) conjugate vaccines, using terminally linked Pn14PS fragments of different lengths. Native Pn14PS was simultaneously depolymerized and activated for conjugation by partial N-deacetylation followed by nitrous acid deamination which yielded fragments (1.4 to 150.0 kDa) having a free aldehyde at the reducing end. These were then conjugated to TT through their terminal aldehydic groups, using the reductive amination procedure. All of the above conjugates, when injected in rabbits, induced anti-Pn14PS antibodies, whereas the native Pn14PS did not. The amounts of anti-Pn14PS antibodies elicited by these conjugates, as determined by enzyme-linked immunosorbent assay, followed a trend with conjugates containing the highest-molecular-weight Pn14PS eliciting the highest titers. The same trend was also observed in the ability of the antibodies to opsonize and kill live type 14 pneumococci, although the increase in opsonophagocytic activity was more pronounced and did not correlate linearly with increases in antibody titer. Competitive inhibition of the binding of different conjugate antisera to the native Pn14PS, using Pn14PS fragments as inhibitors, established that the conjugates induced antibodies with specificities for different lengths of Pn14PS beginning at 2 repeating units (RU). It was also established, both immunologically and antigenically, that at least 4 RU of Pn14PS were required to form an extended conformational epitope and that approximately 22 RU of Pn14PS were required to duplicate the same epitope on the same saccharide chain. The conformational epitope was found to be essential for the induction of antibodies with high opsonophagocytic activity and that augmentation of opsonophagocytic activity was also dependent on further chain extension.

NMR and molecular dynamics studies of the conformational epitope of the type III group B Streptococcus capsular polysaccharide and derivatives

The conformational epitope of the type III group B Streptococcus capsular polysaccharide (GBSP III) exhibits unique properties which can be ascribed to the presence of sialic acid in its structure and the requirement for an extended binding site. By means of NMR and molecular dynamics studies on GBSP III and its fragments, the extended epitope of GBSP III was further defined. The influence of sialic acid on the conformational properties of GBSP III was examined by performing conformational analysis on desialylated GBSP III, which is identical to the polysaccharide of Streptococcus pneumoniae type 14, and also on oxidized and reduced GBSP III. Conformational changes were gauged by 1H and 13C chemical shift analysis, NOE, 1D selective TOCSY-NOESY experiments, J(HH) and J(CH) variations, and NOE of OH resonances. Changes in mobility were examined by 13C T1 and T2 measurements. Unrestrained molecular dynamics simulations with explicit water using the AMBER force field and the GLYCAM parameter set were used to assess static and dynamic conformational models, simulate the observable NMR parameters and calculate helical parameters. GBSP III was found to be capable of forming extended helices. Hence, the length dependence of the conformational epitope could be explained by its location on extended helices within the random coil structure of GBSP III. The interaction of sialic acid with the backbone of the PS was also found to be important in defining the conformational epitope of GBSP III.

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.

Streptococcus pneumoniae: virulence factors, pathogenesis, and vaccines

Although pneumococcal conjugate vaccines are close to being licensed, a more profound knowledge of the virulence factors responsible for the morbidity and mortality caused by Streptococcus pneumoniae is necessary. This review deals with the major structures of pneumococci involved in the pathogenesis of pneumococcal disease and their interference with the defense mechanisms of the host. It is well known that protection against S. pneumoniae is the result of phagocytosis of invading pathogens. For this process, complement and anticapsular polysaccharide antibodies are required. Besides, relatively recent experimental data suggest that protection is also mediated by the removal of disintegrating pneumococci and their degradation products (cell wall, pneumolysin). These structures seem to be major contributors to illness and death caused by pneumococci. An effective conjugate vaccine should therefore preferably include the capsular polysaccharide and at least one of these inflammatory factors.

Functional capacities of clonal antibodies to Haemophilus influenzae type b polysaccharide

Haemophilus influenzae type b (Hib) is an important pathogen for young children, and children can be protected with antibodies (Abs) to Hib polysaccharide (PS) capsule, a linear polymer of ribosyl ribitol phosphate. The structure of anti-Hib-PS Abs has been well characterized at the molecular level; about two-thirds of anti-Hib-PS Abs use a V kappa gene named A2, and the remaining anti-Hib-PS Abs use one of many other VL genes. In order to understand the structural basis for the variability in the function of these Abs, we prepared 18 clonally pure Abs from adults and studied their affinity, avidity, bactericidal potency in vitro, and ability to reduce bacteremia in newborn rats. Affinities and avidities were determined as the inverse of the concentrations of short (3 repeating units) and long (20 repeating units) ligands which could bind 50% of anti-Hib-PS Ab in solution, respectively. No significant correlations between the protection of newborn rats and affinity (r = 0.02) or avidity (r = 0.16) were observed. The amount of Ab required to kill 50% of bacteria in vitro decreased with avidity (r = -0.32), as expected. However, Abs with high affinity were unexpectedly found to have less bactericidal activity (r = 0.38). This suggests that avidity may be a better predictor of Ab function than affinity. Affinity and avidity results were negatively correlated (r = 0.76, P = 0.0022), and Abs that had A2 V kappa gene products had higher avidity (P < 0.05) and lower affinity (P = 0.06) than Abs that had other VL genes. A possible explanation of these observations is that the epitope for Abs with the A2 gene is within the Hib-PS chain itself, whereas the epitope for Abs with a non-A2 gene is the terminus of Hib-PS.

Purification of preparative quantities of group B Streptococcus type III oligosaccharides

Many bacterial capsular polysaccharides are regularly repeating units of oligosaccharides. Bacterial oligosaccharides have been used in neoglycoconjugate vaccines and as reagents in the study of specific antibody binding. Unfortunately, separation methods have not been adequate for the purification of preparative quantities of bacterial oligosaccharides. Here we describe a size-exclusion procedure that resulted in the resolution of group B Streptococcus type III oligosaccharides composed of 4-25 sugars.

Evidence for the extended helical nature of polysaccharide epitopes. The 2.8 A resolution structure and thermodynamics of ligand binding of an antigen binding fragment specific for alpha-(2-->8)-polysialic acid

The antigen binding fragment from an IgG2a kappa murine monoclonal antibody with specificity for alpha-(2-->8)-linked sialic acid polymers has been prepared and crystallized in the absence of hapten. Crystals were grown by vapor diffusion equilibrium with 16-18% polyethylene glycol 4000 solutions. The structure was solved by molecular replacement methods and refined to a conventional R factor of 0.164 for data to 2.8 A. The binding site is observed to display a shape and distribution of charges that is complementary to that of the predicted conformation of the oligosaccharide epitope. A thermodynamic description of ligand binding has been compiled for oligosaccharides ranging in length from 9 to 41 residues, and the data for the largest ligand has been used in a novel way to estimate the size of the antigen binding site. A model of antigen binding is presented that satisfies this thermodynamic data, as well as a previously reported requirement of conformational specificity of the oligosaccharide. X-ray crystallographic and thermodynamic evidence are consistent with a binding site that accommodates at least eight sialic acid residues.

Haemophilus influenzae type b conjugate vaccines

In summary, all of the Hib conjugate vaccines are highly immunogenic and efficacious in children older than 12-15 months of age, and HbOC, PRP-OMPC, and PRP-T are highly immunogenic and demonstrated to be efficacious in infants as young as 2 months old. HbOC, PRP-OMPC, and PRP-T have been licensed in numerous countries for infants and are recommended for infant immunization. However, perhaps the greatest tribute one can pay to all four Hib vaccines described in this review is to note the dramatic decrease in the incidence of Hib disease that has occurred since their introduction. In fact, according to the Morbidity and Mortality Weekly Report (March 4, 1994), the incidence of Hib disease in children less than 5 years old has declined by 95% from 41 cases per 100,000 in 1987 to 2 cases per 100,000 in 1993, timing that coincides with the availability and use of the Hib conjugate vaccines (Anderson, 1994). As universal administration is achieved and the apparent vaccine-induced reduction in carriage of Hib by the population continues, Hib vaccines may follow the lead of past vaccines (such as smallpox, measles, mumps, rubella, and polio) toward eradication of disease or at least a high degree of medical control, thereby virtually eliminating the mortality and insidious morbidity associated with invasive Hib diseases.

Current status of meningococcal group B vaccine candidates: capsular or noncapsular?

Meningococcal meningitis is a severe, life-threatening infection for which no adequate vaccine exists. Current vaccines, based on the group-specific capsular polysaccharides, provide short-term protection in adults against serogroups A and C but are ineffective in infants and do not induce protection against group B strains, the predominant cause of infection in western countries, because the purified serogroup B polysaccharide fails to elicit human bactericidal antibodies. Because of the poor immunogenicity of group B capsular polysaccharide, different noncapsular antigens have been considered for inclusion in a vaccine against this serogroup: outer membrane proteins, lipooligosaccharides, iron-regulated proteins, Lip, pili, CtrA, and the immunoglobulin A proteases. Alternatively, attempts to increase the immunogenicity of the capsular polysaccharide have been made by using noncovalent complexes with outer membrane proteins, chemical modifications, and structural analogs. Here, we review the strategies employed for the development of a vaccine for Neisseria meningitidis serogroup B; the difficulties associated with the different approaches are discussed.

Epitope specificity of rabbit immunoglobulin G (IgG) elicited by pneumococcal type 23F synthetic oligosaccharide- and native polysaccharide-protein conjugate vaccines: comparison with human anti-polysaccharide 23F IgG

Streptococcus pneumoniae type 23F capsular polysaccharide (PS23F) consitss of a repeating glycerol-phosphorylated branched tetrasaccharide. The immunogenicities of the following related antigens were investigated: (i) a synthetic trisaccharide comprising the backbone of one repeating unit, (ii) a synthetic tetrasaccharide comprising the complete repeating unit, and (iii) native PS23F (all three conjugated to keyhole limpet hemocyanin [KLH]) and (iv) formalin-killed S. pneumoniae 23F. All antigens except the trisaccharide-KLH conjugate induced relatively high anti-PS23F antibody levels in rabbits. The epitope specificity of such antibodies was then studied by means of an inhibition immunoassay. The alpha(1-->2)-linked L-rhamnose branch was shown to be immunodominant for immunoglobulin G (IgG) induced by tetrasaccharide-KLH, PS23F-KLH, and killed S. pneumoniae 23F: in most sera L-rhamnose totally inhibited the binding of IgG to PS23F. Thus, there appears to be no major difference in epitope specificity between IgG induced by tetrasaccharide-KLH and that induced by antigens containing the polymeric form of PS23F. Human anti-PS23F IgG (either vaccine induced or naturally acquired) had a different epitope specificity: none of the inhibitors used, including L-rhamnose and tetrasaccharide-KLH, exhibited substantial inhibition. These observations suggest that the epitope recognized by human IgG on PS23F is larger than the epitope recognized by rabbit IgG. Both human and rabbit antisera efficiently opsonized type 23F pneumococci, as measured in a phagocytosis assay using human polymorphonuclear leukocytes.

Molecular size analysis of capsular polysaccharide preparations from Streptococcus pneumoniae

Purified capsular polysaccharide preparations from Streptococcus pneumoniae serotypes 4, 6B, 9V, 14, 18C, 19F, and 23F were analyzed by high performance size exclusion chromatography (HPSEC) with multi-angle laser light scattering (MALLS), specific viscosity (SV), and refractive index (RI) detection to determine the molecular size and molar mass of each of the pneumococcal (Pn) polysaccharides. The Mw's of the polysaccharides ranged from a low of 606 kg/mol for Pn4 to a high of 1145 kg/mol for Pn9V, and the z-average radii of gyration ranged from 59 nm for Pn14 to 72 nm for Pn18C. Estimations of molar mass of the highly anionic polysaccharides (all but Pn14) by the universal calibration approach were unsuccessful, resulting in a 27-53% overestimate of the Mw's though application of Mark-Houwink-Sakurada coefficients calculated from the HPSEC-MALLS/SV/RI data resulted in estimates of Mw that were in agreement with the MALLS estimates for all but the Pn4 preparation. These results emphasize the need for direct measurement of both molecular size and intrinsic viscosity distributions for definitive characterization of the molar mass, hydrodynamic volume, rigidity, and drainage of complex biological polymers such as the pneumococcal polysaccharides.

Identification of a genetic locus essential for capsule sialylation in type III group B streptococci

The type III capsular polysaccharide of group B streptococci (GBS) consists of a linear backbone with short side chains ending in residues of N-acetylneuraminic acid, or sialic acid. The presence of sialic acid on the surface of the organism inhibits activation of the alternative pathway of complement and is thought to be an important element in the virulence function of the capsule. We showed previously that a mutant strain of GBS that expressed a sialic acid-deficient, or asialo, form of the type III polysaccharide was avirulent, supporting a virulence function for capsular sialic acid. We now report the derivation of an asialo capsule mutant from a highly encapsulated wild-type strain of type III GBS, strain COH1, by insertional mutagenesis with transposon Tn916 delta E. In contrast to the wild-type strain, the asialo mutant strain COH1-11 was sensitive to phagocytic killing by human leukocytes in vitro and was relatively avirulent in a neonatal rat model of GBS infection. The asialo mutant accumulated free intracellular sialic acid, suggesting a defect subsequent to sialic acid synthesis in the biosynthetic pathway leading to capsule sialylation. The specific biosynthetic defect in mutant strain COH1-11 was found to be in the activation of free sialic acid to CMP-sialic acid: CMP-sialic acid synthetase activity was present in the wild-type strain COH1 but was not detected in the asialo mutant strain COH1-11. One of the two transposon insertions in the asialo mutant COH1-11 mapped to the same chromosomal location as one of the two Tn916 insertions in the previously reported asialo mutant COH31-21, identifying this site as a genetic locus necessary for expression of CMP-sialic acid synthetase activity. These studies demonstrate that the enzymatic synthesis of CMP-sialic acid by GBS is an essential step in sialylation of the type III capsular polysaccharide.

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

One method to improve the immunogenicity of polysaccharide antigens is the covalent coupling of the native polysaccharide or a derivative oligosaccharide to a carrier protein. In general, T cell-dependent properties are enhanced in conjugates of smaller saccharides, but a conformational epitope of the native polysaccharide may be better expressed in conjugates of larger saccharides. We have reported previously the synthesis and immunogenicity in animals of an oligosaccharide-tetanus toxoid conjugate vaccine against type III group B Streptococcus. In this study, we sought to determine the optimal size of group B Streptococcus type III oligosaccharide for use in a conjugate vaccine by evaluating the relative immunogenicity of conjugate vaccines containing oligosaccharides that were twofold smaller (7,000 Mr) or larger (27,000 Mr) than that reported previously (14,500 Mr). All three type III oligosaccharide conjugate vaccines were immunogenic in rabbits, in contrast to native, uncoupled group B Streptococcus type III polysaccharide. However, with respect to eliciting specific antibodies that were protective in vivo, the vaccine containing the intermediate-size oligosaccharide was superior to the smaller or larger conjugate vaccine. Analysis of opsonic activity of vaccine-induced antibodies demonstrated a predominance of IgG antibodies, thought to reflect T cell dependence, in response to shorter chain length conjugates, while the conformational epitope of the native polysaccharide was maximally expressed on longer chain length conjugates. These opposing trends may account for the optimal immunogenicity of an intermediate-size group B Streptococcus type III oligosaccharide conjugate vaccine.

Distinct pattern of antibody reactivity with oligomeric or polymeric forms of the capsular polysaccharide of Haemophilus influenzae type b

The chain length of oligosaccharides required for antibody binding has been studied by using the capsular polysaccharide from Haemophilus influenzae type b or oligosaccharides derived from it. The concentration of competing antigens required to achieve a 50% inhibition of antibody binding by human polyclonal antisera in an in vitro competition enzyme-linked immunosorbent assay decreased progressively from greater than 10(-3) to 5 x 10(-7) M as the inhibiting saccharide chain length increased from 1 to 262 repeat units. Even small oligosaccharides (one or two repeat units) are potentially capable of competing to a significant level if a high enough concentration of saccharides is used. A similar pattern of reactivity was seen with a monoclonal anti-polyribosyl ribitol phosphate antibody, suggesting that the differences in the avidity of the antibody subpopulations in the polyclonal antisera do not contribute to the binding patterns observed. The binding reaction was specific as evaluated with pneumococcal saccharides. Furthermore, an oligosaccharide-protein conjugate binds antibody better than the free oligosaccharides do. Such a difference in binding was not observed between the polysaccharide and a polysaccharide-protein conjugate. Overall, the data suggest that identical epitopes are expressed by oligomeric and polymeric forms of the antigen and that a particularly more stable conformation in polysaccharides is preferred by antibodies. Covalent coupling of oligomers to protein increases the expression of stable conformation of epitopes. The data further suggest that this kind of antigenic analysis may be important for the design and synthesis of glycoconjugate vaccines.