Immunology of bacterial polysaccharide antigens

Assessing the Impact of a Vi-polysaccharide Conjugate Vaccine in Preventing Typhoid Infections Among Nepalese Children: A Protocol for a Phase III, Randomized Control Trial

Background

Enteric fever is estimated to affect 11–20 million people worldwide each year. Morbidity and mortality from enteric fever primarily occur in lower-income countries, with children under 5 years of age experiencing a significant portion of the burden. Over the last few decades, the control of enteric fever has focused primarily on improved water and sanitation, with the available vaccines unsuitable for children and primarily used by travelers. A new typhoid conjugate vaccine (Vi-TCV), prequalified by the World Health Organization (WHO) and highly immunogenic in children under 5, has the potential to reduce the typhoid burden in endemic countries.

Methods

This study is a double-blinded, randomized, controlled trial with a 2-year follow-up to assess the protective impact of the Vi-TCV vaccine, compared with a control vaccine, in children from 9 months to 16 years of age. The primary outcome of interest is the reduction in the number of culture-confirmed typhoid cases attributable to Vi-TCV. Approximately 20 000 children living in the Lalitpur district, within the Kathmandu valley, will be enrolled in the study and followed to measure both safety and efficacy data, which will include adverse events, hospitalizations, antibiotic use, and fever frequency.

Results

Both the intervention and control vaccines are WHO prequalified vaccines, which provide a health benefit to all participants. Children have been chosen to participate because they bear a substantial burden of both typhoid morbidity and mortality in this population. The results of this study will be disseminated through a series of published articles. The findings will also be made available to the participants and the broader community, as well as local stakeholders, within Nepal.

Conclusions

This is the first large-scale, individually randomized, controlled trial of Vi-TCV in children in an endemic setting, and will provide new data on Vi-TCV field efficacy. With Vi-TCV introduction being considered in high-burden countries, this study will support important policy decisions.

Clinical Trials Registration

The trial is registered on the ISRCTN registry (for details, see https://doi.org/10.1186/ISRCTN43385161; registry number: ISRCTN 43385161).

Assessing the Impact of a Vi-polysaccharide Conjugate Vaccine in Preventing Typhoid Infection Among Bangladeshi Children: A Protocol for a Phase IIIb Trial

Background

Typhoid fever illnesses are responsible for more than 100 000 deaths worldwide each year. In Bangladesh, typhoid fever is endemic, with incidence rates between 292–395 per 100 000 people annually. While considerable effort has been made to improve access to clean water and sanitation services in the country, there is still a significant annual typhoid burden, which particularly affects children. A typhoid conjugate vaccine (Vi-TCV) was recently prequalified by the World Health Organization and recommended for use, and offers the potential to greatly reduce the typhoid burden in Bangladesh.

Methods

This study is a double-blind, cluster-randomized, controlled trial of Vi-TCV in a geographically defined area in Dhaka, Bangladesh. At least 32 500 children from 9 months to <16 years of age will be vaccinated and followed for 2 years to assess the effectiveness and safety of Vi-TCV in a real-world setting. All cluster residents will also be followed to measure the indirect effect of Vi-TCV in this community.

Ethics and Dissemination

This protocol has been approved by the International Centre for Diarrhoeal Disease Research, Bangladesh; a University of Oxford research review; and both ethical review committees. Informed written consent and assent will be obtained before enrollment. Vi-TCV has been shown to be safe and effective in previous, smaller-scale studies. The results of this study will be shared through a series of peer-reviewed journal articles. The findings will also be disseminated to the local government, stakeholders within the community, and the population within which the study was conducted.

Conclusions

This trial is the largest and only cluster-randomized control trial of Vi-TCV ever conducted, and will describe the effectiveness of Vi-TCV in an endemic population. The results of this trial may provide important evidence to support the introduction of TCVs in countries with a high burden of typhoid.

Clinical Trials Registration

ISRCTN11643110.

Progress in the overall understanding of typhoid fever: implications for vaccine development

INTRODUCTION

Typhoid fever continues to have a substantial impact on human health, especially in Asia and sub-Saharan Africa. Access to safe water, and adequate sanitation and hygiene remain the cornerstone of prevention, but these are not widely available in many impoverished settings. The emergence of antibiotic resistance affects typhoid treatment and adds urgency to typhoid control efforts. Vaccines provide opportunities to prevent and control typhoid fever in endemic settings.

AREAS COVERED

Literature search was performed looking for evidence concerning the global burden of typhoid and strategies for the prevention and treatment of typhoid fever. Cost of illness, available typhoid and paratyphoid vaccines and cost-effectiveness were also reviewed. The objective was to provide a critical overview of typhoid fever, in order to assess the current understanding and potential future directions for typhoid treatment and control.

EXPERT COMMENTARY

Our understanding of typhoid burden and methods of prevention has grown over recent years. However, typhoid fever still has a significant impact on health in low and middle-income countries. Introduction of typhoid conjugate vaccines to the immunization schedule is expected to make a major contribution to control of typhoid fever in endemic countries, although vaccination alone is unlikely to eliminate the disease.

Ubiquitous waterborne pathogens

Water is one of the most important substances on earth and without it life cannot exist. However, poor water quality in many parts of the world has increased the number of water-related diseases, making it the leading cause of disease and death globally for both young and old. Waterborne pathogens cause diseases in humans through two major exposure pathways: drinking water and recreational waters. This chapter on waterborne pathogens will be starting with an introduction, followed by descriptions on classical waterborne pathogens; bacteria, viruses, protozoans, and helminths placing emphasis on the World Health Organization guidelines. Further to conventional waterborne pathogens, fresh organisms and new strains from already known pathogens are being identified and that present important additional challenges to both the water and public health sectors. Hence later part of the chapter focuses on the potential waterborne pathogens and will conclude with a summary of the content.

Antibody recognition of bacterial surfaces and extracellular polysaccharides

Because of the ongoing increase in antibiotic-resistant microbes, new strategies such as therapeutic antibodies and effective vaccines are required. Bacterial carbohydrates are known to be particularly antigenic, and several monoclonal antibodies that target bacterial polysaccharides have been generated, with more in current development. This review examines the known 3D crystal structures of anti-bacterial antibodies and the structural basis for carbohydrate recognition and explores the potential mechanisms for antibody-dependent bacterial cell death. Understanding the key interactions between an antibody and its polysaccharide target on the surface of bacteria or in biofilms can provide essential information for the development of more specific and effective antibody therapeutics as well as carbohydrate-based vaccines.

Vaccine Protection against Multidrug-Resistant Klebsiella pneumoniae in a Nonhuman Primate Model of Severe Lower Respiratory Tract Infection

Klebsiella pneumoniae is a human gut communal organism and notorious opportunistic pathogen. The relative high burden of asymptomatic colonization by K. pneumoniae is often compounded by multidrug resistance-a potential problem for individuals with significant comorbidities or other risk factors for infection. A carbapenem-resistant K. pneumoniae strain classified as multilocus sequence type 258 (ST258) is widespread in the United States and is usually multidrug resistant. Thus, treatment of ST258 infections is often difficult. Inasmuch as new preventive and/or therapeutic measures are needed for treatment of such infections, we developed an ST258 pneumonia model in cynomolgus macaques and tested the ability of an ST258 capsule polysaccharide type 2 (CPS2) vaccine to moderate disease severity. Compared with sham-vaccinated animals, those vaccinated with ST258 CPS2 had significantly less disease as assessed by radiography 24 h after intrabronchial installation of 108 CFU of ST258. All macaques vaccinated with CPS2 ultimately developed ST258-specific antibodies that significantly enhanced serum bactericidal activity and killing of ST258 by macaque neutrophils ex vivo Consistent with a protective immune response to CPS2, transcripts encoding inflammatory mediators were increased in infected lung tissues obtained from CPS-vaccinated animals compared with control, sham-vaccinated macaques. Taken together, our data provide support for the idea that vaccination with ST258 CPS can be used to prevent or moderate infections caused by ST258. As with studies performed decades earlier, we propose that this prime-boost vaccination approach can be extended to include multiple capsule types.IMPORTANCE Multidrug-resistant bacteria continue to be a major problem worldwide, especially among individuals with significant comorbidities and other risk factors for infection. K. pneumoniae is among the leading causes of health care-associated infections, and the organism is often resistant to multiple classes of antibiotics. A carbapenem-resistant K. pneumoniae strain known as multilocus sequence type 258 (ST258) is the predominant carbapenem-resistant Enterobacteriaceae in the health care setting in the United States. Infections caused by ST258 are often difficult to treat and new prophylactic measures and therapeutic approaches are needed. To that end, we developed a lower respiratory tract infection model in cynomolgus macaques in which to test the ability of ST258 CPS to protect against severe ST258 infection.

ELLSA based profiling of surface glycosylation in microorganisms reveals that ß-glucan rich yeasts' surfaces are selectively recognized with recombinant banana lectin

The surface of microorganisms is covered with polysaccharide structures which are in immediate contact with receptor structures on host's cells and antibodies. The interaction between microorganisms and their host is dependent on surface glycosylation and in this study we have tested the interaction of plant lectins with different microorganisms. Enzyme-linked lectin sorbent assay - ELLSA was used to test the binding of recombinant Musa acuminata lectin - BL to 27 selected microorganisms and 7 other lectins were used for comparison: Soy bean agglutinin - SBA, Lens culinaris lectin - LCA, Wheat germ agglutinin - WGA, RCA₁₂₀ - Ricinus communis agglutinin, Con A - from Canavalia ensiformis, Sambucus nigra agglutinin - SNA I and Maackia amurensis agglutinin - MAA. The goal was to define the microorganisms' surface glycosylation by means of interaction with the selected plant lectins and to make a comparison with BL. Among the tested lectins most selective binding was observed for RCA₁₂₀ which preferentially bound Lactobacillus casei DG. Recombinant banana lectin showed specific binding to all of the tested fungal species. The binding of BL to Candida albicans was further tested with fluorescence microscopy and flow cytometry and it was concluded that this lectin can differentiate ß-glucan rich surfaces. The binding of BL to S. boulardii could be inhibited with ß-glucan from yeast with IC₅₀ 1.81 μg mL^-1 and to P. roqueforti with 1.10 μg mL^-1. This unique specificity of BL could be exploited for screening purposes and potentially for the detection of ß-glucan in solutions.

IgA and IgG1 Specific to Vi Polysaccharide of Salmonella Typhi Correlate With Protection Status in a Typhoid Fever Controlled Human Infection Model

Vaccination against Salmonella Typhi using the Vi capsular polysaccharide, a T-cell independent antigen, can protect from the development of typhoid fever. This implies that antibodies to Vi alone can protect in the absence of a T cell-mediated immune response; however, protective Vi antibodies have not been well-characterized. We hypothesized that variability in the biophysical properties of vaccine-elicited antibodies, including subclass distribution and avidity, may impact protective outcomes. To interrogate the relationship between antibody properties and protection against typhoid fever, we analyzed humoral responses from participants in a vaccine efficacy (VE) trial using a controlled human infection model (CHIM) who received either a purified Vi polysaccharide (Vi-PS) or Vi tetanus toxoid conjugate (Vi-TT) vaccine followed by oral challenge with live S. Typhi. We determined the avidity, overall magnitude, and vaccine-induced fold-change in magnitude from before immunization to day of challenge of Vi IgA and IgG subclass antibodies. Amongst those who received the Vi-PS vaccine, Vi IgA magnitude (FDR p = 0.01) and fold-change (FDR p = 0.02) were significantly higher in protected individuals compared with those individuals who developed disease ("diagnosed"). In the Vi-TT vaccine group, the responses of protected individuals had higher fold-change in Vi IgA (FDR p = 0.06) and higher Vi IgG1 avidity (FDR p = 0.058) than the diagnosed Vi-TT vaccinees, though these findings were not significant at p < 0.05. Overall, protective antibody signatures differed between the Vi-PS and Vi-TT vaccines, thus, we conclude that although the Vi-PS and Vi-TT vaccines were observed to have similar efficacies, these vaccines may protect through different mechanisms. These data will inform studies on mechanisms of protection against typhoid fever, including identification of antibody effector functions, as well as informing future vaccination strategies.

Biosynthesis and regulation mechanisms of the Pasteurella multocida capsule

Pasteurella multocida possesses a polysaccharide capsule composed of a viscous surface layer that acts as a critical structural component and virulence factor. Capsular polysaccharides are structurally similar to vertebrate glycosaminoglycans, providing an immunological mechanism for bacterial molecular mimicry, resistance to phagocytosis, and immune evasion during the infection process. In recent years, a series of important research advances have been made in understanding the biosynthesis and regulatory aspects of the P. multocida capsule. This review systematically examines the serogroups, polysaccharide composition and structures, biosynthetic loci and functions, biosynthesis pathways, and expression regulation mechanisms of the P. multocida capsule, supplying a theoretical basis for the molecular pathogenesis of the P. multocida capsule and the future development of capsular polysaccharide vaccines.

Lipoarabinomannan in Active and Passive Protection Against Tuberculosis

Glycolipids of the cell wall of Mycobacterium tuberculosis (Mtb) are important immunomodulators in tuberculosis. In particular, lipoarabinomannan (LAM) has a profound effect on the innate immune response. LAM and its structural variants can be recognized by and activate human CD1b-restricted T cells, and emerging evidence indicates that B cells and antibodies against LAM can modulate the immune response to Mtb. Anti-LAM antibodies are induced during Mtb infection and after bacille Calmette-Guerin (BCG) vaccination, and monoclonal antibodies against LAM have been shown to confer protection by passive administration in mice and guinea pigs. In this review, we describe the immune response against LAM and the potential use of the mannose-capped arabinan moiety of LAM in the construction of vaccine candidates against tuberculosis.

Development of a new 15-valent pneumococcal conjugate vaccine (PCV15) and evaluation of its immunogenicity

A new 15-valent pneumococcal conjugate vaccine (PCV15) against serotypes 1, 3, 4, 5, 6A, 6B, 7F, 9V, 11A, 14, 18C, 19A, 19F, 22F, and 23F has been developed using aluminum phosphate as an adjuvant. Using the rabbit model, immunogenicity of each serotype was evaluated by measuring antigen specific antibodies and functional antibody titers and comparing them to a control vaccine, Prevnar13®. Among the shared serotypes in both PCV15 and Prevnar13®, Type 3 and 23F in PCV15 exhibited a lower opsonic index than Prevnar13®. Conversely, the other types showed greater or nearly the same immunogenic effects. Type 11A and 22F are two additional serotypes included in PCV15, and only 22F showed a reasonable opsonic index compared with other types. Type 11A exhibited a basal level fold-increase in OPA; thus, we further optimized 11A as well as 3 and 23F by controlling the polysaccharide-to-protein conjugation ratio as a variable. Antibody levels and functional antibody activities were evaluated by ELISA and OPA, and improved levels of immunogenic activities were observed for all three serotypes. In this study, we propose a new PCV15 candidate, in which the common 13 serotypes and a licensed control vaccine have equivalent efficacy while two additional serotypes showed adequate immunogenicity in the rabbit model.

Detection and characterization of bacterial polysaccharides in drug-resistant enterococci

Enterococcus faecium (E. faecium) has emerged as one of today's leading causes of health care-associated infections that is difficult to treat with the available antibiotics. These pathogens produce capsular polysaccharides on the cell surface which play a significant role in adhesion, virulence and evasion. Therefore, we aimed at the identification and characterization of bacterial polysaccharide antigens which are central for the development of vaccine-based prophylactic approaches. The crude cell wall-associated polysaccharides from E. faecium, its mutant and complemented strains were purified and analyzed by a primary antibody raised against lipoteichoic acid (LTA) and diheteroglycan (DHG). The resistant E. faecium strains presumably possess novel capsular polysaccharides that allow them to avoid the evasion from opsonic killing. The E. faecium U0317 strain was very well opsonized by anti-U0317 (~95%), an antibody against the whole bacterial cell. The deletion mutant showed a significantly increased susceptibility to opsonophagocytic killing (90-95%) against the penicillin binding protein (anti-PBP-5). By comparison, in a mouse urinary tract and rat endocarditis infection model, respectively, there were no significant differences in virulence. In this study we explored the biological role of the capsule of E. faecium. Our findings showed that the U0317 strain is not only sensitive to anti-LTA but also to antibodies against other enterococcal surface proteins. Our findings demonstrate that polysaccharides capsule mediated-resistance to opsonophagocytosis. We also found that the capsular polysaccharides do not play an important role in bacterial virulence in urinary tract and infective endocarditis in vivo models.

AIE-based theranostic systems for detection and killing of pathogens

Pathogenic bacteria, fungi and viruses pose serious threats to the human health under appropriate conditions. There are many rapid and sensitive approaches have been developed for identification and quantification of specific pathogens, but many challenges still exist. Culture/colony counting and polymerase chain reaction are the classical methods used for pathogen detection, but their operations are time-consuming and laborious. On the other hand, the emergence and rapid spread of multidrug-resistant pathogens is another global threat. It is thus of utmost urgency to develop new therapeutic agents or strategies. Luminogens with aggregation-induced emission (AIEgens) and their derived supramolecular systems with unique optical properties have been developed as fluorescent probes for turn-on sensing of pathogens with high sensitivity and specificity. In addition, AIE-based supramolecular nanostructures exhibit excellent photodynamic inactivation (PDI) activity in aggregate, offering great potential for not only light-up diagnosis of pathogen, but also image-guided PDI therapy for pathogenic infection.

Antibodies to Conserved Surface Polysaccharides Protect Mice Against Bacterial Conjunctivitis

Purpose

Bacterial conjunctivitis is a major problem in ocular health. Little is known about protective immune effectors in the conjunctiva. We evaluated whether opsonic antibody to the conserved surface/capsular polysaccharide poly-N-acetyl glucosamine (PNAG) expressed by Streptococcus pneumoniae and Staphylococcus aureus was protective against bacterial conjunctivitis, as well as an antibody to the Pseudomonas aeruginosa surface polysaccharide alginate.

Methods

Bacteria were injected directly into the conjunctivae of either A/J mice or into conjunctivae of wild type C57Bl/6 mice for comparisons to responses of recombination activating gene 1-knock out (RAG 1 KO) or germ-free mice in the C57Bl/6 genetic background. Human IgG1 monoclonal antibodies (MAb) to either PNAG or alginate were administered as follows: direct injection of 10 μg into the conjunctivae or topical application onto the cornea 4, 24, and 32 hours post infection; or intraperitoneal injection of 200 μg 18 hours prior to and then 4, 24, and 32-hours postinfection. After 48 hours, eyes were scored for pathology, mice were euthanized, and CFU/conjunctiva was determined.

Results

All methods of antibody administration reduced S. pneumoniae, S. aureus, or P. aeruginosa pathology and bacterial levels in the conjunctivae. Histopathologic analysis showed severe inflammatory cell infiltrates in conjunctivae of mice treated with control MAb, whereas immune mice showed only very mild cellular infiltration. The protective effect of MAb to PNAG was abolished in RAG 1 KO and germ-free mice.

Conclusions

Antibodies to both PNAG and alginate demonstrated therapeutic efficacy in models of S. pneumoniae, S. aureus, and P. aeruginosa conjunctivitis, validating the protective capacity of antibodies to surface polysaccharides in distinct ocular tissues.

Carbohydrate Specificity and Isotypes of Monoclonal and Polyclonal Antibodies to Conjugated Tetrasaccharide, a Synthetic Analogue of Repeating Unit of Capsular Polysaccharide of Streptococcus Pneumoniae Serotype 14

We studied carbohydrate specificity and isotypes of antibodies to BSA-conjugated tetrasaccharide, a repeating unit of the capsular polysaccharide of Streptococcus pneumoniae serotype 14, in mouse polyclonal sera and hybridoma-synthesized products. Natural IgM antibodies to the tetrasaccharide containing epitopes similar to surface carbohydrate structures of mammalian and human cells in low titers were determined in native mouse serum by ELISA using biotinylated tetrasaccharide and synthetic capsular polysaccharide as the solid-phase antigens. Polyclonal sera to the conjugated tetrasaccharide contained IgM and all subclasses of IgG antibodies, which were detected in a higher titer when the biotinylated tetrasaccharide was used as a solid phase antigen compared to synthetic capsular polysaccharide. Monoclonal antibodies to S. pneumoniae serotype 14 tetrasaccharide were identified in an equivalent titer using either biotinylated tetrasaccharide or synthetic capsular polysaccharide. Monoclonal antibodies obtained in vitro belonged to IgM isotype and cross-reacted with secondary full-size IgG antibodies. In the serum of mice inoculated with hybridoma, IgM and IgG2a antibodies recognizing the tetrasaccharide epitope in the structure of synthetic capsular polysaccharide were simultaneously determined.

Polysaccharide structure dictates mechanism of adaptive immune response to glycoconjugate vaccines

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

Engineering a Next-Generation Glycoconjugate-Like Streptococcus pneumoniae Vaccine

We detail the development of a next-generation Streptococcus pneumoniae liposomal encapsulation of polysaccharides (LEPS) vaccine, with design characteristics geared toward best-in-class efficacy. The first generation LEPS vaccine, which contained 20 encapsulated pneumococcal capsular polysaccharides (CPSs) and two surface-displayed virulence-associated proteins (GlpO and PncO), enabling prophylactic potency against 70+ serotypes of Streptococcus pneumoniae (the causative agent of pneumococcal disease), was rationally redesigned for advanced clinical readiness and best-in-class coverage. In doing so, the virulent-specific GlpO protein antigen was removed from the final formulation due to off-target immunogenicity toward bacterial species within the human microbiome, while directed protection was maintained by increasing the dose of PncO from 17 to 68 μg. LEPS formulation parameters also readily facilitated an increase in CPS valency (to a total of 24) and systematic variation in protein-liposome attachment mechanisms in anticipation of clinical translation. An additional safety assessment study demonstrated that LEPS does not exhibit appreciable toxicological effects even when administered at ten times the effective dose. In summary, this new design offers the broadest, safest, and most-complete protection while maintaining desirable glycoconjugate-like features, positioning the LEPS vaccine platform for clinical success and a global health impact.

Direct Dehydrative Glycosylation of C1-Alcohols

Due to the central role played by carbohydrates in a multitude of biological processes, there has been a sustained interest in developing effective glycosylation methods to enable more thorough investigation of their essential functions. Among the myriad technologies available for stereoselective glycoside bond formation, dehydrative glycosylation possesses a distinct advantage given the unique properties of C1-alcohols such as straightforward preparation, stability, and a general reactivity compatible with a diverse set of reaction conditions. In this Focus Review, a survey of direct dehydrative glycosylations of C1-alcohols is provided with an emphasis on recent achievements, pervading limitations, mechanistic insights, and applications in total synthesis.

Synthesis of the tetrasaccharide related to the repeating unit of the O-antigen from Azospirillum brasilense Jm125A2 in the form of its 2-aminoethyl glycoside

Total chemical synthesis of the linear tetrasaccharide repeating unit β-D-Glc-(1 → 2)-α-L-Rha-(1 → 3)-α-L-Rha-(1 → 2)-α-L-Rha-CH₂CH₂NH₂ of the O-antigen from Azospirillum brasilense Jm125A2 is accomplished through rational protecting group manipulations of commercially available monosaccharides and stereoselective glycosylations. The target tetrasaccharide in the form of its 2-aminoethyl glycoside is obtained in ∼24% yield over 10 steps following a linear strategy. The structure is particularly suitable for further glycoconjugate formation through the terminal free amine without hampering the reducing end stereochemistry.

The mechanism behind the biphasic pulsatile drug release from physically mixed poly(dl-lactic(-co-glycolic) acid)-based compacts

Successful immunization often requires a primer, and after a certain lag time, a booster administration of the antigen. To improve the vaccinees' comfort and compliance, a single-injection vaccine formulation with a biphasic pulsatile release would be preferable. Previous work has shown that such a release profile can be obtained with compacts prepared from physical mixtures of various poly(dl-lactic(-co-glycolic) acid) types (Murakami et al., 2000). However, the mechanism behind this release profile is not fully understood. In the present study, the mechanism that leads to this biphasic pulsatile release was investigated by studying the effect of the glass transition temperature (Tg) of the polymer, the temperature of compaction, the compression force, the temperature of the release medium, and the molecular weight of the incorporated drug on the release behavior. Compaction resulted in a porous compact. Once immersed into release medium with a temperature above the Tg of the polymer, the drug was released by diffusion through the pores. Simultaneously, the polymer underwent a transition from the glassy state into the rubbery state. The pores were gradually closed by viscous flow of the polymer and further release was inhibited. After a certain period of time, the polymer matrix ruptured, possibly due to a build-up in osmotic pressure, resulting in a pulsatile release of the remaining amount of drug. The compression force and the molecular weight of the incorporated drug did not influence the release profile. Understanding this mechanism could contribute to further develop single-injection vaccines.

Detecting live bacteria instantly utilizing AIE strategies

A new class of biosensor molecules evoking fluorescent emission by rotation-restricted binding with bacteria was examined for its applicability in detecting live bacteria instantly. The fluorogens possessed multiple tetraphenylethene (TPE)-cored boronic acids to oligomerize through complexation with cis-diols on bacterial surfaces, resulting in aggregation-induced emission (AIE). The fluorogen having two boronic acid units discriminated between live and dead bacteria by showing AIE activity only with the latter. Live bacteria were instantly detected by consequent treatment with reagents of three and four di-boronates (which showed AIE activity with both live and dead bacteria). This phenomenon may lead to a practical method for live bacteria detection.

Protein Carriers for Glycoconjugate Vaccines: History, Selection Criteria, Characterization and New Trends

Currently licensed glycoconjugate vaccines are composed of a carbohydrate moiety covalently linked to a protein carrier. Polysaccharides are T-cell independent antigens able to directly stimulate B cells to produce antibodies. Disease burden caused by polysaccharide-encapsulated bacteria is highest in the first year of life, where plain polysaccharides are not generally immunogenic, limiting their use as vaccines. This limitation has been overcome by covalent coupling carbohydrate antigens to proteins that provide T cell epitopes. In addition to the protein carriers currently used in licensed glycoconjugate vaccines, there is a search for new protein carriers driven by several considerations: (i) concerns that pre-exposure or co-exposure to a given carrier can lead to immune interference and reduction of the anti-carbohydrate immune response; (ii) increasing interest to explore the dual role of proteins as carrier and protective antigen; and (iii) new ways to present carbohydrates antigens to the immune system. Protein carriers can be directly coupled to activated glycans or derivatized to introduce functional groups for subsequent conjugation. Proteins can be genetically modified to pre-determine the site of glycans attachment by insertion of unnatural amino acids bearing specific functional groups, or glycosylation consensus sequences for in vivo expression of the glycoconjugate. A large portion of the new protein carriers under investigation are recombinant ones, but more complex systems such as Outer Membrane Vesicles and other nanoparticles are being investigated. Selection criteria for new protein carriers are based on several aspects including safety, manufacturability, stability, reactivity toward conjugation, and preclinical evidence of immunogenicity of corresponding glycoconjugates. Characterization panels of protein carriers include tests before conjugation, after derivatization when applicable, and after conjugation. Glycoconjugate vaccines based on non-covalent association of carrier systems to carbohydrates are being investigated with promising results in animal models. The ability of these systems to convert T-independent carbohydrate antigens into T-dependent ones, in comparison to traditional glycoconjugates, needs to be assessed in humans.

The spleen as a neuroimmune interface after spinal cord injury

Traumatic spinal cord injury (SCI) causes widespread damage to neurons, glia and endothelia located throughout the spinal parenchyma. In response to the injury, resident and blood-derived leukocytes orchestrate an intraspinal inflammatory response that propagates secondary neuropathology and also promotes tissue repair. SCI also negatively affects autonomic control over peripheral immune organs, notably the spleen. The spleen is the largest secondary lymphoid organ in mammals, with major roles in blood filtration and host defense. Splenic function is carefully regulated by neuroendocrine mechanisms that ensure that the immune responses to infection or injury are proportionate to the initiating stimulus, and can be terminated when the stimulus is cleared. After SCI, control over the viscera, including endocrine and lymphoid tissues is lost due to damage to spinal autonomic (sympathetic) circuitry. This review begins by examining the normal structure and function of the spleen including patterns of innervation and the role played by the nervous system in regulating spleen function. We then describe how after SCI, loss of proper neural control over splenic function leads to systems-wide neuropathology, immune suppression and autoimmunity. We conclude by discussing opportunities for targeting the spleen to restore immune homeostasis, reduce morbidity and mortality, and improve functional recovery after SCI.

Development of Subunit Vaccines That Provide High-Level Protection and Sterilizing Immunity against Acute Inhalational Melioidosis

Burkholderia pseudomallei, the etiologic agent of melioidosis, causes severe disease in humans and animals. Diagnosis and treatment of melioidosis can be challenging, and no licensed vaccines currently exist. Several studies have shown that this pathogen expresses a variety of structurally conserved protective antigens that include cell surface polysaccharides and cell-associated and cell-secreted proteins. Based on those findings, such antigens have become important components of the subunit vaccine candidates that we are currently developing. In the present study, the 6-deoxyheptan capsular polysaccharide (CPS) from B. pseudomallei was purified, chemically activated, and covalently linked to recombinant CRM197 diphtheria toxin mutant (CRM197) to produce CPS-CRM197. Additionally, tandem nickel-cobalt affinity chromatography was used to prepare highly purified recombinant B. pseudomallei Hcp1 and TssM proteins. Immunization of C57BL/6 mice with CPS-CRM197 produced high-titer IgG and opsonizing antibody responses against the CPS component of the glycoconjugate, while immunization with Hcp1 and TssM produced high-titer IgG and robust gamma interferon-secreting T cell responses against the proteins. Extending upon these studies, we found that when mice were vaccinated with a combination of CPS-CRM197 and Hcp1, 100% of the mice survived a lethal inhalational challenge with B. pseudomallei. Remarkably, 70% of the survivors had no culturable bacteria in their lungs, livers, or spleens, indicating that the vaccine formulation had generated sterilizing immune responses. Collectively, these studies help to better establish surrogates of antigen-induced immunity against B. pseudomallei as well as provide valuable insights toward the development of a safe, affordable, and effective melioidosis vaccine.

A Semi-Synthetic Glycoconjugate Vaccine Candidate for Carbapenem-Resistant Klebsiella pneumoniae

Hospital-acquired infections are an increasingly serious health concern. Infections caused by carpabenem-resistant Klebsiella pneumoniae (CR-Kp) are especially problematic, with a 50 % average survival rate. CR-Kp are isolated from patients with ever greater frequency, 7 % within the EU but 62 % in Greece. At a time when antibiotics are becoming less effective, no vaccines to protect from this severe bacterial infection exist. Herein, we describe the convergent [3+3] synthesis of the hexasaccharide repeating unit from its capsular polysaccharide and related sequences. Immunization with the synthetic hexasaccharide 1 glycoconjugate resulted in high titers of cross-reactive antibodies against CR-Kp CPS in mice and rabbits. Whole-cell ELISA was used to establish the surface staining of CR-Kp strains. The antibodies raised were found to promote phagocytosis. Thus, this semi-synthetic glycoconjugate is a lead for the development of a vaccine against a rapidly progressing, deadly bacterium.

Shotgun Glycomics Identifies Tumor-Associated Glycan Ligands Bound by an Ovarian Carcinoma-Specific Monoclonal Antibody

Cancers display distinctive carbohydrate molecules (glycans) on their surface proteins and lipids. mAb A4, an in-house generated monoclonal IgM antibody, is capable of distinguishing malignant ovarian carcinoma cells from benign ovarian epithelia by binding specifically to cancer cell-associated glycans. However, the structural details of the glycan targets of mAb A4 have been elusive. Here we developed a novel approach of isolating and fractionating glycan molecules released from glycoproteins in cancer cell lysates using HILIC-UPLC, and used them as probes on a microarray for affinity-based identification of the binding targets, allowing full-size, difficult to synthesize, cancer-associated glycans to be directly studied. As a result of this "shotgun" glycomics approach, we corroborate the previously assigned specificity of mAb A4 by showing that mAb A4 binds primarily to large (>15 glucose units), sialylated N-glycans containing the H-type 1 antigen (Fuc-α1,2-Gal-β1,3-GlcNAc). Although mAb A4 was also capable of directly binding to type 1 N-acetyl-lactosamine, this epitope was mostly shielded by sialylation and thus relatively inaccessible to binding. Knowledge of the structure of mAb A4 antigen will facilitate its clinical development as well as its use as a diagnostic biomarker.

Mechanisms of humoral immune response against Pseudomonas aeruginosa biofilm infection in cystic fibrosis

P. aeruginosa chronic lung infection is the major cause of morbidity and mortality in patients with cystic fibrosis (CF), and is characterized by a biofilm mode of growth, increased levels of specific IgG antibodies and immune complex formation. However, despite being designed to combat this infection, such elevated humoral response is not associated with clinical improvement, pointing to a lack of anti-pseudomonas effectiveness. The mode of action of specific antibodies, as well as their structural features, and even the background involving B-cell production, stimulation and differentiation into antibody-producing cells in the CF airways are poorly understood. Thus, the aim of this review is to discuss studies that have addressed the intrinsic features of the humoral immune response and provide new insights regarding its insufficiency in the CF context.

IgG avidity to Pseudomonas aeruginosa over the course of chronic lung biofilm infection in cystic fibrosis

BACKGROUND AND OBJECTIVES

The mechanisms leading to low effectiveness of the humoral immune response against P. aeruginosa in cystic fibrosis (CF) are poorly understood. The aim of the present study was to assess the avidity maturation of specific antipseudomonal IgG before and during the development of chronic lung infection in a cohort of Danish CF patients.

METHODS

Avidity maturation was assessed against a pooled P. aeruginosa antigen (St-Ag) and against P. aeruginosa alginate in 10 CF patients who developed chronic lung infection and 10 patients who developed intermittent lung colonization, using an ELISA technique with the thiocyanate elution method. Avidity was quantitatively determined by calculating the avidity Constant (Kav).

RESULTS

IgG avidity to St-Ag significantly increased at the onset (Median Kav=2.47) and one year after the onset of chronic infection (Median Kav=3.27), but did not significantly changed in patients who developed intermittent colonization. IgG avidity against alginate did not significantly change over the years neither in patients who developed chronic lung infection (Median Kav=3.84 at the onset of chronic infection), nor in patients who developed intermittent colonization.

CONCLUSION

IgG avidity to P. aeruginosa alginate does not significantly enhance as chronic lung infection progresses. This probably plays a role in the difficulty to mount an effective opsonophagocytic killing to clear mucoid P. aeruginosa infection in CF.

The Role of BAFF System Molecules in Host Response to Pathogens

The two ligands B cell-activating factor of the tumor necrosis factor family (BAFF) and a proliferation-inducing ligand (APRIL) and the three receptors BAFF receptor (BAFF-R), transmembrane activator and calcium-modulating cyclophilin ligand interactor (TACI), and B cell maturation antigen (BCMA) are members of the "BAFF system molecules." BAFF system molecules are primarily involved in B cell homeostasis. The relevance of BAFF system molecules in host responses to microbial assaults has been investigated in clinical studies and in mice deficient for each of these molecules. Many microbial products modulate the expression of these molecules. Data from clinical studies suggest a correlation between increased expression levels of BAFF system molecules and elevated B cell responses. Depending on the pathogen, heightened B cell responses may strengthen the host response or promote susceptibility. Whereas pathogen-mediated increases in the expression levels of the ligands and/or the receptors appear to promote microbial clearance, certain pathogens have evolved to ablate B cell responses by suppressing the expression of TACI and/or BAFF-R on B cells. Other than its well-established role in B cell responses, the TACI-mediated activation of macrophages is also implicated in resistance to intracellular pathogens. An improved understanding of the role that BAFF system molecules play in infection may assist in devising novel strategies for vaccine development.

Structural and immunochemical relatedness suggests a conserved pathogenicity motif for secondary cell wall polysaccharides in Bacillus anthracis and infection-associated Bacillus cereus

Bacillus anthracis (Ba) and human infection-associated Bacillus cereus (Bc) strains Bc G9241 and Bc 03BB87 have secondary cell wall polysaccharides (SCWPs) comprising an aminoglycosyl trisaccharide repeat: →4)-β-d-ManpNAc-(1→4)-β-d-GlcpNAc-(1→6)-α-d-GlcpNAc-(1→, substituted at GlcNAc residues with both α- and β-Galp. In Bc G9241 and Bc 03BB87, an additional α-Galp is attached to O-3 of ManNAc. Using NMR spectroscopy, mass spectrometry and immunochemical methods, we compared these structures to SCWPs from Bc biovar anthracis strains isolated from great apes displaying “anthrax-like” symptoms in Cameroon (Bc CA) and Côte d’Ivoire (Bc CI). The SCWPs of Bc CA/CI contained the identical HexNAc trisaccharide backbone and Gal modifications found in Ba, together with the α-Gal-(1→3) substitution observed previously at ManNAc residues only in Bc G9241/03BB87. Interestingly, the great ape derived strains displayed a unique α-Gal-(1→3)-α-Gal-(1→3) disaccharide substitution at some ManNAc residues, a modification not found in any previously examined Ba or Bc strain. Immuno-analysis with specific polyclonal anti-Ba SCWP antiserum demonstrated a reactivity hierarchy: high reactivity with SCWPs from Ba 7702 and Ba Sterne 34F2, and Bc G9241 and Bc 03BB87; intermediate reactivity with SCWPs from Bc CI/CA; and low reactivity with the SCWPs from structurally distinct Ba CDC684 (a unique strain producing an SCWP lacking all Gal substitutions) and non-infection-associated Bc ATCC10987 and Bc 14579 SCWPs. Ba-specific monoclonal antibody EAII-6G6-2-3 demonstrated a 10–20 fold reduced reactivity to Bc G9241 and Bc 03BB87 SCWPs compared to Ba 7702/34F2, and low/undetectable reactivity to SCWPs from Bc CI, Bc CA, Ba CDC684, and non-infection-associated Bc strains. Our data indicate that the HexNAc motif is conserved among infection-associated Ba and Bc isolates (regardless of human or great ape origin), and that the number, positions and structures of Gal substitutions confer unique antigenic properties. The conservation of this structural motif could open a new diagnostic route in detection of pathogenic Bc strains.

Parasite Carbohydrate Vaccines

Vaccination is an efficient means of combating infectious disease burden globally. However, routine vaccines for the world's major human parasitic diseases do not yet exist. Vaccines based on carbohydrate antigens are a viable option for parasite vaccine development, given the proven success of carbohydrate vaccines to combat bacterial infections. We will review the key components of carbohydrate vaccines that have remained largely consistent since their inception, and the success of bacterial carbohydrate vaccines. We will then explore the latest developments for both traditional and non-traditional carbohydrate vaccine approaches for three of the world's major protozoan parasitic diseases-malaria, toxoplasmosis, and leishmaniasis. The traditional prophylactic carbohydrate vaccine strategy is being explored for malaria. However, given that parasite disease biology is complex and often arises from host immune responses to parasite antigens, carbohydrate vaccines against deleterious immune responses in host-parasite interactions are also being explored. In particular, the highly abundant glycosylphosphatidylinositol molecules specific for Plasmodium, Toxoplasma, and Leishmania spp. are considered exploitable antigens for this non-traditional vaccine approach. Discussion will revolve around the application of these protozoan carbohydrate antigens for vaccines currently in preclinical development.

Polysaccharides in Human Health Care

Polysaccharides are abundant natural polymers found in plants, animals and microorganisms with exceptional properties and essential roles to sustain life. They are well known for their high nutritive value and the positive effects on our immune and digestive functions and detoxification system. The knowledge and recognition of the important role they play for promoting and maintaining human health and wellbeing is continuously increasing. This review describes some important polysaccharides (e.g. mucilages and gums, glycosamine glycans and chitin/chitosan) and their medical, cosmetic and pharmaceutical applications, with emphasis on the relationship between structure and function. Next, the use of polysaccharides as nutraceuticals and vaccines is discussed in more detail. An analysis of the trends and challenges in polysaccharide research concludes the paper.

T Cell-Mediated Humoral Immune Responses to Type 3 Capsular Polysaccharide of Streptococcus pneumoniae

Most pathogenic bacteria express surface carbohydrates called capsular polysaccharides (CPSs). CPSs are important vaccine targets because they are easily accessible and recognizable by the immune system. However, CPS-specific adaptive humoral immune responses can only be achieved by the covalent conjugation of CPSs with carrier proteins to produce glycoconjugate vaccines. We previously described a mechanism by which a model glycoconjugate vaccine can activate the adaptive immune system and demonstrated that the mammalian CD4⁺ T cell repertoire contains a population of carbohydrate-specific T cells. In this study, we use glycoconjugates of type 3 Streptococcus pneumoniae CPS (Pn3P) to assess whether the carbohydrate-specific adaptive immune response exemplified in our previous study can be applied to the conjugates of this lethal pathogen. In this article, we provide evidence for the functional roles of Pn3P-specific CD4⁺ T cells utilizing mouse immunization schemes that induce Pn3P-specific IgG responses in a carbohydrate-specific T cell-dependent manner.

Synthesis and immunogenicity of PG-tb1 monovalent glycoconjugate

A PG-tb1 hapten from the West Beijing strains of Mycobacterium tuberculosis cell wall has been efficiently synthesized and conjugated to CRM₁₉₇ in a simple way as linker-equipped carbohydrate by applying squaric acid chemistry for an original neoglycoprotein, creating a potent T-dependent conjugate vaccine. The intermediate monoester can be easily purified and the degree of incorporation can be monitored by MALDI-TOF mass spectrometry. After administered systemically in mice without any adjuvant, the conjugate induced high antigen-specific IgG levels in serum. Furthermore, following the third immunization, significant antibody titers frequently exceeding 0.8 million were observed in the sera of mice vaccinated with PG-CRM₁₉₇ conjugate which showed the potential for preparation of TB vaccine.

T cells control the generation of nanomolar-affinity anti-glycan antibodies

Vaccines targeting glycan structures at the surface of pathogenic microbes must overcome the inherent T cell-independent nature of immune responses against glycans. Carbohydrate conjugate vaccines achieve this by coupling bacterial polysaccharides to a carrier protein that recruits heterologous CD4 T cells to help B cell maturation. Yet they most often produce low- to medium-affinity immune responses of limited duration in immunologically fit individuals and disappointing results in the elderly and immunocompromised patients. Here, we hypothesized that these limitations result from suboptimal T cell help. To produce the next generation of more efficacious conjugate vaccines, we have explored a synthetic design aimed at focusing both B cell and T cell recognition to a single short glycan displayed at the surface of a virus-like particle. We tested and established the proof of concept of this approach for 2 serotypes of Streptococcus pneumoniae. In both cases, these vaccines elicited serotype-specific, protective, and long-lasting IgG antibodies of nanomolar affinity against the target glycans in mice. We further identified a requirement for CD4 T cells in the anti-glycan antibody response. Our findings establish the design principles for improved glycan conjugate vaccines. We surmise that the same approach can be used for any microbial glycan of interest.

Enhanced control of Mycobacterium tuberculosis extrapulmonary dissemination in mice by an arabinomannan-protein conjugate vaccine

Currently there are a dozen or so of new vaccine candidates in clinical trials for prevention of tuberculosis (TB) and each formulation attempts to elicit protection by enhancement of cell-mediated immunity (CMI). In contrast, most approved vaccines against other bacterial pathogens are believed to mediate protection by eliciting antibody responses. However, it has been difficult to apply this formula to TB because of the difficulty in reliably eliciting protective antibodies. Here, we developed capsular polysaccharide conjugates by linking mycobacterial capsular arabinomannan (AM) to either Mtb Ag85b or B. anthracis protective antigen (PA). Further, we studied their immunogenicity by ELISA and AM glycan microarrays and protection efficacy in mice. Immunization with either Abg85b-AM or PA-AM conjugates elicited an AM-specific antibody response in mice. AM binding antibodies stimulated transcriptional changes in Mtb. Sera from AM conjugate immunized mice reacted against a broad spectrum of AM structural variants and specifically recognized arabinan fragments. Conjugate vaccine immunized mice infected with Mtb had lower bacterial numbers in lungs and spleen, and lived longer than control mice. These findings provide additional evidence that humoral immunity can contribute to protection against Mtb.

Vaccine design in the TB field has been driven by the imperative of attempting to elicit strong cell-mediated responses. However, in recent decades evidence has accumulated that humoral immunity can protect against many intracellular pathogens through numerous mechanisms. In this work, we demonstrate that immunization with mycobacterial capsular arabinomannan (AM) conjugates elicited responses that contributed to protection against Mtb infection. We developed two different conjugates including capsular AM linked to the Mtb related protein Ag85b or the Mtb unrelated PA from B. anthracis and found that immunization with AM conjugates elicited antibody populations with different specificities. These surface-specific antibodies could directly modify the transcriptional profile and metabolism of mycobacteria. In addition, we observed a prolonged survival and a reduction in bacterial numbers in lungs and spleen in mice immunized with Ag85b-AM conjugates after infection with Mtb and that the presence of AM-binding antibodies was associated with modest prolongation in survival and a marked reduction in mycobacterial dissemination. Finally, we show that AM is antigenically variable and could potentially form the basis for a serological characterization of mycobacteria based on serotypes.

Immunogenicity and safety of a tetravalent E. coli O-antigen bioconjugate vaccine in animal models

BACKGROUND

Extra-intestinal pathogenic Escherichia coli (ExPEC) are major human pathogens; however, no protective vaccine is currently available. We assessed in animal models the immunogenicity and safety of a 4-valent E. coli conjugate vaccine (ExPEC-4V, serotypes O1, O2, O6 and O25 conjugated to Exotoxin A from Pseudomonas aeruginosa (EPA)) produced using a novel in vivo bioconjugation method.

METHODS

Three doses of ExPEC-4V (with or without aluminum hydroxide) were administered to rabbits (2μg or 20μg per O-antigen, subcutaneously), mice (0.2μg or 2μg per O-antigen, subcutaneously) and rats (0.4μg or 4μg per O-antigen, intramuscularly). Antibody persistence and boostability were evaluated in rats using O6-EPA monovalent conjugate (0.4μg O-antigen/dose, intramuscularly). Toxicity was assessed in rats (16μg total polysaccharide, intramuscularly). Serum IgG and IgM antibodies were measured by ELISA.

RESULTS

Robust antigen-specific IgG responses were observed in all animal models, with increased responses in rabbits when administered with adjuvant. O antigen-specific antibody responses persisted up to 168days post-priming. Booster immunization induced a rapid recall response. Toxicity of ExPEC-4V when administered to rats was considered to be at the no observed adverse effect level.

CONCLUSIONS

ExPEC-4V conjugate vaccine showed good immunogenicity and tolerability in animal models supporting progression to clinical evaluation.

Outer membrane vesicles displaying engineered glycotopes elicit protective antibodies

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

Evaluation of the Immunogenicity of Diphtheria Toxoid Conjugated to Salmonella Typhimurium-Derived OPS in a Mouse Model: A Potential Vaccine Candidate Against Salmonellosis

Background

Salmonella enterica serovar Typhimurium (S. Typhimurium) causes gastroenteritis in humans and paratyphoid disease in some animals. Given the emergence of antibiotic resistance, vaccines are more effective than chemotherapy in disease control.

Objectives

The aim of this experimental study was to evaluate the immunogenicity of diphtheria toxoid (DT) conjugated with S. Typhimurium -derived OPS (O side chain isolation) in mice to determine its potential as a vaccine candidate against salmonellosis.

Materials and Methods

Lipopolysaccharide (LPS) was extracted from the bacterial strain. After isolation of the O side chain of LPS, detoxification, and conjugation of the detoxified OPS samples with DT, pyrogenicity, toxicity, and sterility tests were performed. To vaccination, four groups of female Balb/c mice were used in an immunization test. Antibody responses were measured by the ELISA method. Challenging processes were performed to analyze the efficacy of the OPS-DT compound.

Results

Two weeks after the first vaccination dose, there was no significant difference in the antibody titers of the OPS and OPS-DT groups. However, after the second and third doses, the antibody titers of the OPS-DT group increased significantly compared with those of the control groups (P < 0.001). The induction of anti-OPS antibodies was as follows: OPS-DT>OPS. The most anti-OPS IgG antibody was IgG1. Challenging procedure showed successful protective characteristics in clinical examinations.

Conclusions

The results indicated that DT increased anti-OPS antibodies against the OPS-DT compound. The antibody response to OPS-DT was greater than that to OPS alone. We conclude that OPS-DT is an appropriate and acceptable vaccine candidate against salmonellosis.