Preparation, characterization, and immunogenicity of Haemophilus influenzae type b polysaccharide-protein conjugates

History of vaccination

Vaccines have a history that started late in the 18th century. From the late 19th century, vaccines could be developed in the laboratory. However, in the 20th century, it became possible to develop vaccines based on immunologic markers. In the 21st century, molecular biology permits vaccine development that was not possible before.

The incidence of pediatric invasive Haemophilus influenzae and pneumococcal disease in Chiba prefecture, Japan before and after the introduction of conjugate vaccines

The Haemophilus influenzae type b (Hib) vaccine and the heptavalent pneumococcal conjugate vaccine (PCV7) were introduced in Japan in 2008 and 2010, respectively. In 2011, immunization with these two vaccines was encouraged throughout Japan through a governmental program. Children treated in Chiba prefecture for culture-proven invasive H. influenzae disease (IHiD) and invasive Streptococcus pneumoniae disease (IPD) were identified in a prefectural surveillance study from 2008 to 2013. The incidence rate ratio (IRR) and its confidence interval (CI) were calculated to compare the 3 years before and after governmental financial support for vaccination. The average number of IHiD and IPD cases among children <5 years of age in 2011-2013 decreased 84% (IRR: 0.16, 95% CI: 0.09-0.26, p<0.0001) and 51% (IRR: 0.49, 95% CI: 0.37-0.63, p<0.0001) compared with those occurring in 2008-2010. The most common non-PCV7 serotype encountered in 2011 and 2013 was 19A. After governmental subsidization of Hib and PCV7 vaccination, IHiD and IPD decreased in Chiba prefecture, Japan. Continuous surveillance is necessary to determine the effectiveness of these two vaccines and for detection of emerging invasive serotypes.

Recognition of flavin mononucleotide, Haemophilus influenzae type b and its capsular polysaccharide vaccines by antibodies specific to D-ribitol-5-phosphate

D-Ribitol-5-phosphate (Rbt-5-P) is an important metabolite in the pentose phosphate pathway and an integral part of bacterial cell wall polysaccharides, specifically as polyribosyl ribitol phosphate (PRP) in Haemophilus influenzae type b (Hib). The major objective of this study was to investigate whether an antibody specific to Rbt-5-P can recognize the PRP of Hib. D-Ribose-5-phosphate was reacted with proteins in the presence of sodium cyanoborohydride to obtain Rbt-5-P epitopes; 120 h reaction resulted in conjugation of ~30 and ~17 moles of Rbt-5-P/mole of BSA and OVA, respectively, based on decrease in amino groups, MALDI-TOF analyses, an increase in apparent molecular weight (SDS-PAGE) and glycoprotein staining. Immunization of rabbits with Rbt-5-P-BSA conjugate generated antibodies to Rbt-5-P as demonstrated by dot immunoblot and non-competitive ELISA. Homogeneous Rbt-5-P-specific antibody was purified from Rbt-5-P-BSA antiserum subjected to caprylic acid precipitation followed by hapten-affinity chromatography; its affinity constant is 7.1 × 10(8) M(-1). Rbt-5-P antibody showed 100 % specificity to Rbt-5-P, ~230 %, 10 % and 3.4 % cross-reactivity to FMN, riboflavin and FAD, respectively; the antibody showed ~4 % cross-reactivity to D-ribitol and <3 % to other sugars/sugar alcohols. Rbt-5-P-specific antibody recognized Hib conjugate vaccines containing PRP which was inhibited specifically by Rbt-5-P, and also detected Hib cell-surface capsular polysaccharides by immunofluorescence. In conclusion, Rbt-5-P-protein conjugate used as an immunogen elicited antibodies binding to an epitope also present in PRP and Hib bacteria. Rbt-5-P-specific antibody has potential applications in the detection and quantification of free/bound Rbt-5-P and FMN as well as immunological recognition of Hib bacteria and its capsular polysaccharide.

Lessons learned during the development and transfer of technology related to a new Hib conjugate vaccine to emerging vaccine manufacturers

The incidence of Haemophilus Influenzae type b (Hib) disease in developed countries has decreased since the introduction of Hib conjugate vaccines in their National Immunization Programs (NIP). In countries where Hib vaccination is not applied routinely, due to limited availability and high cost of the vaccines, invasive Hib disease is still a cause of mortality. Through the development of a production process for a Hib conjugate vaccine and related quality control tests and the transfer of this technology to emerging vaccine manufacturers in developing countries, a substantial contribution was made to the availability and affordability of Hib conjugate vaccines in these countries. Technology transfer is considered to be one of the fastest ways to get access to the technology needed for the production of vaccines. The first Hib conjugate vaccine based on the transferred technology was licensed in 2007, since then more Hib vaccines based on this technology were licensed. This paper describes the successful development and transfer of Hib conjugate vaccine technology to vaccine manufacturers in India, China and Indonesia. By describing the lessons learned in this process, it is hoped that other technology transfer projects can benefit from the knowledge and experience gained.

Leaning in to the power of the possible: the crucial role of women scientists on preventing Haemophilus influenzae type b disease

Beginning in an era when female scientists were a lonely minority, women have made major contributions to our understanding of Haemophilus influenzae type b (Hib) as a pathogen, its treatment and its prevention. The individual scientific and public health contributions, and their collective impact, are reviewed in the context of the development and successful implementation of highly efficacious Hib vaccines that are now being deployed to nearly every country worldwide for the prevention of life-threatening pediatric Hib disease.

Carrier priming or suppression: understanding carrier priming enhancement of anti-polysaccharide antibody response to conjugate vaccines

INTRODUCTION

With the availability of newer conjugate vaccines, immunization schedules have become increasingly complex due to the potential for unpredictable immunologic interference such as 'carrier priming' and 'carrier induced epitopic suppression'. Carrier priming refers to an augmented antibody response to a carbohydrate portion of a glycoconjugate vaccine in an individual previously primed with the carrier protein. This review aims to provide a critical evaluation of the available data on carrier priming (and suppression) and conceptualize ways by which this phenomenon can be utilized to strengthen vaccination schedules.

METHODS

We conducted this literature review by searching well-known databases to date to identify relevant studies, then extracted and synthesized the data on carrier priming of widely used conjugate polysaccharide vaccines, such as, pneumococcal conjugate vaccine (PCV), meningococcal conjugate vaccine (MenCV) and Haemophilus influenzae type b conjugate vaccines (HibV).

RESULTS

We found evidence of carrier priming with some conjugate vaccines, particularly HibV and PCV, in both animal and human models but controversy surrounds MenCV. This has implications for the immunogenicity of conjugate polysaccharide vaccines following the administration of tetanus-toxoid or diphtheria-toxoid containing vaccine (such as DTP).

CONCLUSION

Available evidence supports a promising role for carrier priming in terms of maximizing the immunogenicity of conjugate vaccines and enhancing immunization schedule by making it more efficient and cost effective.

Designing vaccines for the twenty-first century society

The history of vaccination clearly demonstrates that vaccines have been highly successful in preventing infectious diseases, reducing significantly the incidence of childhood diseases and mortality. However, many infections are still not preventable with the currently available vaccines and they represent a major cause of mortality worldwide. In the twenty-first century, the innovation brought by novel technologies in antigen discovery and formulation together with a deeper knowledge of the human immune responses are paving the way for the development of new vaccines. Final goal will be to rationally design effective vaccines where conventional approaches have failed.

Recent mechanistic insights on glycoconjugate vaccines and future perspectives

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

Protein carriers of conjugate vaccines: characteristics, development, and clinical trials

The immunogenicity of polysaccharides as human vaccines was enhanced by coupling to protein carriers. Conjugation transformed the T cell-independent polysaccharide vaccines of the past to T cell-dependent antigenic vaccines that were much more immunogenic and launched a renaissance in vaccinology. This review discusses the conjugate vaccines for prevention of infections caused by Hemophilus influenzae type b, Streptococcus pneumoniae, and Neisseria meningitidis. Specifically, the characteristics of the proteins used in the construction of the vaccines including CRM, tetanus toxoid, diphtheria toxoid, Neisseria meningitidis outer membrane complex, and Hemophilus influenzae protein D are discussed. The studies that established differences among and key features of conjugate vaccines including immunologic memory induction, reduction of nasopharyngeal colonization and herd immunity, and antibody avidity and avidity maturation are presented. Studies of dose, schedule, response to boosters, of single protein carriers with single and multiple polysaccharides, of multiple protein carriers with multiple polysaccharides and conjugate vaccines administered concurrently with other vaccines are discussed along with undesirable consequences of conjugate vaccines. The clear benefits of conjugate vaccines in improving the protective responses of the immature immune systems of young infants and the senescent immune systems of the elderly have been made clear and opened the way to development of additional vaccines using this technology for future vaccine products.

Noncovalent association of protein and capsular polysaccharide on bacteria-sized latex beads as a model for polysaccharide-specific humoral immunity to intact gram-positive extracellular bacteria

Intact Streptococcus pneumoniae expressing type 14 capsular polysaccharide (PPS14) and type III S. agalactiae containing a PPS14 core capsule identical to PPS14 exhibit noncovalent associations of PPS14 and bacterial protein, in contrast to soluble covalent conjugates of these respective Ags. Both bacteria and conjugates induce murine PPS14-specific IgG responses dependent on CD4⁺ T cells. Further, secondary immunization with conjugate and S. agalactiae, although not S. pneumoniae, results in a boosted response. However, in contrast to conjugate, PPS14-specific IgG responses to bacteria lack affinity maturation use the 44.1-idiotype and are dependent on marginal zone B cells. To better understand the mechanism underlying this dichotomy, we developed a minimal model of intact bacteria in which PPS14 and pneumococcal surface protein A (PspA) were stably attached to 1 μm (bacteria-sized) latex beads, but not directly linked to each other, in contrast to PPS14-PspA conjugate. Beads coated simultaneously with PPS14+[PspA], similar to conjugate, induced in mice boosted PPS14-specific IgG secondary responses, dependent on T cells and ICOS-dependent costimulation, and in which priming could be achieved with PspA alone. In contrast to conjugate, but similar to intact bacteria, the primary PPS14-specific IgG response to beads coated simultaneously with PPS14+[PspA] peaked rapidly, with the secondary response highly enriched for the 44.1-idiotype and lacking affinity maturation. These results demonstrate that noncovalent association in a particle, of polysaccharide and protein, recapitulates essential immunologic characteristics of intact bacteria that are distinct from soluble covalent conjugates of these respective Ags.

Synthetically defined glycoprotein vaccines: current status and future directions

Primary examples in vaccine design have shown good levels of carbohydrate-specific antibody generation when raised using extracted or fully synthetic capsular polysaccharide glycans covalently coupled to a protein carrier. Herein, we cover recent clinical developments of carbohydrate-based vaccines and describe how novel cutting-edge methodology for the total synthesis of oligosaccharides and for the precise placement of carbohydrates at pre-determined sites within a protein may be used to further improve the safety and efficacy of glycovaccines.

The impact of protein-conjugate polysaccharide vaccines: an endgame for meningitis?

The development and implementation of conjugate polysaccharide vaccines against invasive bacterial diseases, specifically those caused by the encapsulated bacteria Neisseria meningitidis, Haemophilus influenzae and Streptococcus pneumoniae, has been one of the most effective public health innovations of the last 25 years. These vaccines have resulted in significant reductions in childhood morbidity and mortality worldwide, with their effectiveness due in large part to their ability to induce long-lasting immunity in a range of age groups. At the population level this immunity reduces carriage and interrupts transmission resulting in herd immunity; however, these beneficial effects can be counterbalanced by the selection pressures that immunity against carriage can impose, potentially promoting the emergence and spread of virulent vaccine escape variants. Studies following the implementation of meningococcal serogroup C vaccines improved our understanding of these effects in relation to the biology of accidental pathogens such as the meningococcus. This understanding has enabled the refinement of the implementation of conjugate polysaccharide vaccines against meningitis-associated bacteria, and will be crucial in maintaining and improving vaccine control of these infections. To date there is little evidence for the spread of virulent vaccine escape variants of the meningococcus and H. influenzae, although this has been reported in pneumococci.

Accelerating next-generation vaccine development for global disease prevention

Vaccines are among the greatest successes in the history of public health. However, past strategies for vaccine development are unlikely to succeed in the future against major global diseases such as AIDS, tuberculosis, and malaria. For such diseases, the correlates of protection are poorly defined and the pathogens evade immune detection and/or exhibit extensive genetic variability. Recent advances have heralded in a new era of vaccine discovery. However, translation of these advances into vaccines remains impeded by lack of understanding of key vaccinology principles in humans. We review these advances toward vaccine discovery and suggest that for accelerating successful vaccine development, new human immunology-based clinical research initiatives be implemented with the goal of elucidating and more effectively generating vaccine-induced protective immune responses.

Carbohydrates and T cells: a sweet twosome

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

Novel synthetic (poly)glycerolphosphate-based antistaphylococcal conjugate vaccine

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

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

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

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

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

Passive protection of diabetic rats with antisera specific for the polysaccharide portion of the lipopolysaccharide isolated from Pseudomonas pseudomallei

Polyclonal and monoclonal antisera raised to tetanus toxoid-conjugated polysaccharide of lipopolysaccharide (lps) and purified lps of Pseudomonas pseudomallei that reacted with a collection of 41 strains of this bacterium from 23 patients are described. The common antigen recognized by these sera was within the polysaccharide component of the lps of the cells. The sera were specific for P pseudomallei in that none of 37 strains of other bacteria, including 20 Gram-negative and three Gram-positive species, were recognized, although cross-reaction occurred using the anticonjugate serum with some strains of Pseudomonas cepacia serotype A, a closely related bacterium. Passive protection studies using a diabetic rat model of P pseudomallei infection showed that partially purified rabbit polyclonal and mouse monoclonal antisera were protective when the median lethal dose was raised by four to five orders of magnitude. The wide distribution of the polysaccharide antigen among isolates of P pseudomallei used in this study and the protective role of antibody to the conjugated polysaccharide antigen suggest potential as a vaccine.

Technology transfer in human vaccinology: a retrospective review on public sector contributions in a privatizing science field

As health intervention, vaccination has had a tremendous impact on reducing mortality and morbidity caused by infectious diseases. Traditionally vaccines were developed and made in the western, industrialised world and from there on gradually and with considerable delay became available for developing countries. Today that is beginning to change. Most vaccine doses are now produced in emerging economies, although industrialised countries still have a lead in vaccine development and in manufacturing innovative vaccines. Technology transfer has been an important mechanism for this increase in production capacity in emerging economies. This review looks back on various technology transfer initiatives and outlines the role of WHO and other public and private partners. It goes into a more detailed description of the role of the National Institute of Public Health and the Environment (RIVM) in Bilthoven, the Netherlands. For many decades RIVM has been providing access to vaccine technology by capacity building and technology transfer initiatives not only through multilateral frameworks, but also on a bilateral basis including a major project in China in the 90 s of the previous century. Looking forward it is expected that, in a globalizing world, the ambition of BRICS countries to play a role in global health will lead to an increase of south-south technology transfers. Further, it is argued that push approaches including technology transfer from the public domain, connecting innovative enabling platforms with competent developing country vaccine manufacturers (DCVM), will be critical to ensure a sustainable supply of affordable and quality vaccines to national immunization programmes in developing countries.

A century of pneumococcal vaccination research in humans

Sir Almroth Wright coordinated the first trial of a whole-cell pneumococcal vaccine in South Africa from 1911 to 1912. Wright started a chain of events that delivered pneumococcal vaccines of increasing clinical and public-health value, as medicine advanced from a vague understanding of the germ theory of disease to today's rational vaccine design. Early whole-cell pneumococcal vaccines mimicked early typhoid vaccines, as early pneumococcal antisera mimicked the first diphtheria antitoxins. Pneumococcal typing systems developed by Franz Neufeld and others led to serotype-specific whole-cell vaccines. Pivotally, Alphonse Dochez and Oswald Avery isolated pneumococcal capsular polysaccharides in 1916-17. Serial refinements permitted Colin MacLeod and Michael Heidelberger to conduct a 1944-45 clinical trial of quadrivalent pneumococcal polysaccharide vaccine (PPV), demonstrating a high degree of efficacy in soldiers against pneumococcal pneumonia. Two hexavalent PPVs were licensed in 1947, but were little used as clinicians preferred therapy with new antibiotics, rather than pneumococcal disease prevention. Robert Austrian's recognition of high pneumococcal case-fatality rates, even with antibiotic therapy, led to additional trials in South Africa, the USA and Papua New Guinea, with 14-valent and 23-valent PPVs licensed in 1977 and 1983 for adults and older children. Conjugation of polysaccharides to proteins led to several pneumococcal conjugate vaccines licensed since 2000, enabling immunization of infants and young children and resultant herd protection for all ages. Today, emergence of disease caused by pneumococcal serotypes not included in various vaccine formulations fuels research into conserved proteins or other means to maximize protection against more than 90 known pneumococcal serotypes.

Synthetic oligosaccharides as tools to demonstrate cross-reactivity between polysaccharide antigens

Escherichia coli O148 is a nonencapsulated enterotoxigenic (ETEC) Gram negative bacterium that can cause diarrhea, hemorrhagic colitis, and hemolytic uremic syndrome in humans. The surface-exposed O-specific polysaccharide (O-SP) of the lipopolysaccharide of this bacterium is considered both a virulence factor and a protective antigen. It is built up of the linear tetrasaccharide repeating unit [3)-α-L-Rhap-(1→2)-α-D-Glcp-(1→3)-α-D-GlcNAcp-(1→3)-α-L-Rhap-(1→] differing from that of the O-SP of Shigella dysenteriae type 1 (SD) only in that the latter contains a D-Galp residue in place of the glucose moiety of the former. The close similarity of the O-SPs of these bacteria indicated a possible cross-reactivity. To answer this question we synthesized several oligosaccharide fragments of E. coli O148 O-SP, up to a dodecasaccharide, as well as their bovine serum albumin or recombinant diphtheria toxin conjugates. Immunization of mice with these conjugates induced anti-O-SP-specific serum IgG antibody responses. The antisera reacted equally well with the LPSs of both bacteria, indicating cross-reactivity between the SD and E. coli O148 O-SPs that was further supported by Western-blot and dot-blot analyses, as well as by inhibition of binding between the antisera and the O-SPs of both bacteria.

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.

Serotype-independent pneumococcal experimental vaccines that induce cellular as well as humoral immunity

For prevention of Streptococcus pneumoniae (pneumococcus) infections in infancy, protein-conjugated capsular polysaccharide vaccines provide serotype-specific, antibody-mediated immunity but do not cover all of the 90+ capsule serotypes. Therefore, microbiologists have sought protective noncapsular antigens common to all strains. Alternatively, we investigated killed cells of a noncapsulated strain, which expose many such common antigens. Given to mice intranasally, this vaccine elicits antibody-independent, CD4+ T lymphocyte-dependent accelerated clearance of pneumococci of various serotypes from the nasopharynx mediated by the cytokine IL-17A. Such immunity may reproduce the natural resistance that develops in infants before capsular antibodies arise. Given by injection, the killed cell vaccine induces bifunctional immunity: plasma antibodies protective against fatal pneumonia challenge, as well as IL-17A-mediated nasopharyngeal clearance. Human testing of this inexpensive candidate vaccine by intramuscular injection is planned. Bacterial cellular vaccines are complex--a challenge for reproducibility. However, when several known protective antigens were deleted, the killed pneumococcal vaccine was still protective. This antigenic redundancy may prevent vaccine escape variants by recombinational loss, which is frequent in pneumococcus. Biochemically defined immunogens with bifunctional activity have also been devised. These immunogens are three-component conjugates in which cell wall teichoic acid (a common antigen capable of T cell activation) is coupled to a genetic fusion of two common pneumococcal proteins: a protective surface antigen and a derivative of pneumolysin, which provides TLR4 agonist activity and induces antitoxic immunity. Such constructs induce accelerated clearance when given intranasally and induce both immune mechanisms when injected. The defined composition permits analysis of structure-function activity.

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

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

The development of vaccines: how the past led to the future

The history of vaccine development has seen many accomplishments, but there are still many diseases that are difficult to target, and new technologies are being brought to bear on them. Past successes have been largely due to elicitation of protective antibodies based on predictions made from the study of animal models, natural infections and seroepidemiology. Those predictions have often been correct, as indicated by the decline of many infections for which vaccines have been made over the past 200 years.

The design of semi-synthetic and synthetic glycoconjugate vaccines

INTRODUCTION

Glycoconjugate vaccines are among the safest and most efficacious vaccines developed during the last 30 years. They are a potent tool for prevention of life-threatening bacterial infectious diseases like meningitis and pneumonia. The concept of hapten-carrier conjugation is now being extended to other disease areas.

AREAS COVERED

This is an overview of the history and current status of glycoconjugate vaccines. The authors discuss the approaches for their preparation and quality control as well as those variables which might affect their product profile. The authors also look at the potential to develop fully synthetic conjugate vaccines based on the progress of organic chemistry. Additionally, new applications of conjugate vaccines technology in the field of non-infectious diseases are discussed. Through this review, the reader will have an insight regarding the issues and complexities involved in the preparation and characterization of conjugate vaccines, the variables that might affect their immunogenicity and the potential for future applications.

EXPERT OPINION

The immunogenicity of weak T-independent antigens can be increased in quantity and quality by conjugation to protein carriers, which provide T-cell help. Glycoconjugate vaccines are among the safest and most efficacious vaccines developed so far. Various conjugation procedures and carrier proteins can be used. Many variables impact on the immunogenicity of conjugate vaccines and a tight control through physicochemical tests is important to ensure manufacturing and clinical consistency. New and challenging targets for conjugate vaccines are represented by cancer and other non-infectious diseases.

Experimental design to optimize an Haemophilus influenzae type b conjugate vaccine made with hydrazide-derivatized tetanus toxoid

The introduction of type b Haemophilus influenzae conjugate vaccines into routine vaccination schedules has significantly reduced the burden of this disease; however, widespread use in developing countries is constrained by vaccine costs, and there is a need for a simple and high-yielding manufacturing process. The vaccine is composed of purified capsular polysaccharide conjugated to an immunogenic carrier protein. To improve the yield and rate of the reductive amination conjugation reaction used to make this vaccine, some of the carboxyl groups of the carrier protein, tetanus toxoid, were modified to hydrazides, which are more reactive than the ε -amine of lysine. Other reaction parameters, including the ratio of the reactants, the size of the polysaccharide, the temperature and the salt concentration, were also investigated. Experimental design was used to minimize the number of experiments required to optimize all these parameters to obtain conjugate in high yield with target characteristics. It was found that increasing the reactant ratio and decreasing the size of the polysaccharide increased the polysaccharide:protein mass ratio in the product. Temperature and salt concentration did not improve this ratio. These results are consistent with a diffusion controlled rate limiting step in the conjugation reaction. Excessive modification of tetanus toxoid with hydrazide was correlated with reduced yield and lower free polysaccharide. This was attributed to a greater tendency for precipitation, possibly due to changes in the isoelectric point. Experimental design and multiple regression helped identify key parameters to control and thereby optimize this conjugation reaction.

The potential role for protein-conjugate pneumococcal vaccine in adults: what is the supporting evidence?

Vaccination with protein-conjugate pneumococcal vaccine (PCV) provides children with extraordinary protection against pneumococcal disease, although the protective effect may be blunted by the emergence of replacement strains. Studies in adults have compared PCV with pneumococcal polysaccharide vaccine (PPV) using surrogate markers of protection, namely, serum anticapsular IgG antibody and opsonic activity. Results suggest that PCV is at least as effective as PPV for the strains covered, but a definitive and consistent advantage has not been demonstrated. Unfortunately, persons who are most in need of vaccine do not respond as well as otherwise healthy adults to either vaccine. Newer formulations of PCV will protect against the most prevalent of the current replacement strains, but replacement strains will create a moving target for PCVs. Unless an ongoing trial comparing 13-valent PCV with placebo (not to PPV) demonstrates a clearly better effect than that seen in the past with PPV, cost-effectiveness considerations are likely to prevent widespread use of PCV in adults.

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

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

Oligosaccharide conjugates of Bordetella pertussis and bronchiseptica induce bactericidal antibodies, an addition to pertussis vaccine

Pertussis is a highly contagious respiratory disease that is especially dangerous for infants and children. Despite mass vaccination, reported pertussis cases have increased in the United States and other parts of the world, probably because of increased awareness, improved diagnostic means, and waning vaccine-induced immunity among adolescents and adults. Licensed vaccines do not kill the organism directly; the addition of a component inducing bactericidal antibodies would improve vaccine efficacy. We investigated Bordetella pertussis and Bordetella bronchiseptica LPS-derived core oligosaccharide (OS) protein conjugates for their immunogenicity in mice. B. pertussis and B. bronchiseptica core OS were bound to aminooxylated BSA via their terminal Kdo residues. The two conjugates induced similar anti-B. pertussis LPS IgG levels in mice. B. bronchiseptica was investigated because it is easier to grow than B. pertussis. Using B. bronchiseptica genetically modified strains deficient in the O-specific polysaccharide, we isolated fractions of core OS with one to five repeats of the terminal trisaccharide, having at the nonreducing end a GlcNAc or GalNAc, and bound them to BSA at different densities. The highest antibody levels in mice were elicited by conjugates containing an average of 8-17 OS chains per protein and with one repeat of the terminal trisaccharide. Conjugate-induced antisera were bactericidal against B. pertussis, and the titers correlated with ELISA-measured antibody levels (r = 0.74). Such conjugates are easy to prepare and standardize; added to a recombinant pertussis toxoid, they may induce antibacterial and antitoxin immunity.

From vaccine practice to vaccine science: the contribution of human immunology to the prevention of infectious disease

Over the past 50 years, the practice of vaccination has reached the important goal of reducing many of the diseases that afflicted humanity in past centuries. A better understanding of immunological mechanisms underlying the induction of immune protection and the advent of new technology led to improved vaccine preparations based on purified microbial antigens and new adjuvants able to boost both humoral and cellular immune responses. Despite these tremendous advances, much remains to be done. The emergence of new pathogens, the spread of strains resistant to antibiotics and the enormous increase in latent infections are urgently demanding more and more effective vaccines. Understanding the immunological mechanisms that mediate resistance against infections would certainly provide valuable information for the design of new candidate vaccines.

Immunochemical studies of Shigella flexneri 2a and 6, and Shigella dysenteriae type 1 O-specific polysaccharide-core fragments and their protein conjugates as vaccine candidates

There is no licensed vaccine for the prevention of shigellosis. Our approach to the development of a Shigella vaccines is based on inducing serum IgG antibodies to the O-specific polysaccharide (O-SP) domain of their lipopolysaccharides (LPS). We have shown that low molecular mass O-SP-core (O-SPC) fragments isolated from Shigella sonnei LPS conjugated to proteins induced significantly higher antibody levels in mice than the full length O-SP conjugates. This finding is now extended to the O-SPC of Shigella flexneri 2a and 6, and Shigella dysenteriae type 1. The structures of O-SPC, containing core plus 1-4 O-SP repeat units (RUs), were analyzed by NMR and mass spectroscopy. The first RUs attached to the cores of S. flexneri 2a and 6 LPS were different from the following RUs in their O-acetylation and/or glucosylation. Conjugates of core plus more than 1 RU were necessary to induce LPS antibodies in mice. The resulting antibody levels were comparable to those induced by the full length O-SP conjugates. In S. dysenteriae type 1, the first RU was identical to the following RUs, with the exception that the GlcNAc was bound to the core in the beta-configuration, while in all other RUs the GlcNAc was present in the alpha-configuration. In spite of this difference, conjugates of S. dysenteriae type 1 core with 1, 2, or 3 RUs induced LPS antibodies in mice with levels statistically higher than those of the full size O-SP conjugates. O-SPC conjugates are easy to prepare, characterize, and standardize, and their clinical evaluation is planned.

How bacterial carbohydrates influence the adaptive immune system

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

Physical and chemical characterization and immunologic properties of Salmonella enterica serovar typhi capsular polysaccharide-diphtheria toxoid conjugates

Typhoid fever remains a serious public health problem in developing countries, especially among young children. Recent studies showed more than 50% of typhoid cases are in children under 5 years old. Licensed vaccines, such as Salmonella enterica serovar Typhi capsular Vi, did not confer protection against typhoid fever for this age group. Vi conjugate, prepared by binding Vi to Pseudomonas aeruginosa recombinant exoprotein A (rEPA), induces protective levels of antibody at as young as 2 years old. Because of the lack of regulatory precedent for rEPA in licensing vaccines, we employed diphtheria toxoid (DT) as the carrier protein to accommodate accessibility in developing countries. Five lots of Vi-DT conjugates were prepared using adipic acid dihydrazide (ADH) as the linker. All 5 lots showed consistency in their physical and chemical characteristics and final yields. These Vi-DT conjugates elicited levels of IgG anti-Vi in young mice significantly higher than those in mice injected with Vi alone and induced a booster response upon reinjection. This booster effect was absent if the Vi replaced one of the two conjugate injections. Vi-DT was stable under repeated freeze-thaw (20 cycles). We plan to perform clinical evaluation of the safety and immunogenicity of Vi-DT when added to the infant combination vaccines.