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

Testing a Recombinant Form of Tetanus Toxoid as a Carrier Protein for Glycoconjugate Vaccines

Glycoconjugate vaccines play a major role in the prevention of infectious diseases worldwide, with significant impact on global health, enabling the polysaccharides to induce immunogenicity in infants and immunological memory. Tetanus toxoid (TT), a chemically detoxified bacterial toxin, is among the few carrier proteins used in licensed glycoconjugate vaccines. The recombinant full-length 8MTT was engineered in E. coli with eight individual amino acid mutations to inactivate three toxin functions. Previous studies in mice showed that 8MTT elicits a strong IgG response, confers protection, and can be used as a carrier protein. Here, we compared 8MTT to traditional carrier proteins TT and cross-reactive material 197 (CRM197), using different polysaccharides as models: Group A Streptococcus cell-wall carbohydrate (GAC), Salmonella Typhi Vi, and Neisseria meningitidis serogroups A, C, W, and Y. The persistency of the antibodies induced, the ability of the glycoconjugates to elicit booster response after re-injection at a later time point, the eventual carrier-induced epitopic suppression, and immune interference in multicomponent formulations were also evaluated. Overall, immunogenicity responses obtained with 8MTT glycoconjugates were compared to those obtained with corresponding TT and, in some cases, were higher than those induced by CRM197 glycoconjugates. Our results support the use of 8MTT as a good alternative carrier protein for glycoconjugate vaccines, with advantages in terms of manufacturability compared to TT.

Exploring the variables influencing the immune response of traditional and innovative glycoconjugate vaccines

Vaccines are cost-effective tools for reducing morbidity and mortality caused by infectious diseases. The rapid evolution of pneumococcal conjugate vaccines, the introduction of tetravalent meningococcal conjugate vaccines, mass vaccination campaigns in Africa with a meningococcal A conjugate vaccine, and the recent licensure and introduction of glycoconjugates against S. Typhi underlie the continued importance of research on glycoconjugate vaccines. More innovative ways to produce carbohydrate-based vaccines have been developed over the years, including bioconjugation, Outer Membrane Vesicles (OMV) and the Multiple antigen-presenting system (MAPS). Several variables in the design of these vaccines can affect the induced immune responses. We review immunogenicity studies comparing conjugate vaccines that differ in design variables, such as saccharide chain length and conjugation chemistry, as well as carrier protein and saccharide to protein ratio. We evaluate how a better understanding of the effects of these different parameters is key to designing improved glycoconjugate vaccines.

Immune persistence and response to booster dose of Vi-DT vaccine at 27.5 months post-first dose

Vaccination with typhoid conjugate vaccines (TCV) is a major part of typhoid prevention. However, little is known about long-term immune persistence following vaccination with TCVs. In this phase-2, randomized double-blind trial (NCT03527355), 285 children aged 6–23 months were randomized to one of three groups: (1) the group that received a first dose of Vi polysaccharide conjugated to diphtheria-toxoid (Vi-DT) vaccine followed by an “early booster” at 24 weeks, (2) the group that which received a first dose of Vi-DT followed by a “late booster” at 96 or 110 weeks, and (3) comparator group. Safety and immunogenicity of anti-Vi IgG GMTs were assessed at weeks 0, 4, 24, 28, 60, 96, 110, and 114 since the first dose. Here, we describe persistence of immune responses at weeks 60, 96, 110, and 114 post first dose. The anti-Vi IgG seroconversion rate after 27.5 months of follow-up was 88.16% (95% CI: 79.00, 93.64) in late-booster and 94.76% (95% CI: 86.91, 97.88) in early booster Vi-DT groups (p = 0.081). Whereas anti-Vi IgG GMTs were significantly higher in the early booster group (11.95 [95% CI: 9.65, 14.81]) than prebooster GMTs in the late booster group (5.50 [95% CI: 4.44, 6.80], p < 0.0001). GMT in the late booster group significantly increased to 351.76 (95% CI: 265.01, 466.93) (p < 0.0001) when measured 4 weeks after they received their “late-booster” shot. In conclusion, late booster dosing with Vi-DT at 27.5 months post first dose was safe and elicited robust anti-Vi IgG immune responses. Anti-Vi IgG seroconversion rates were persistently comparable in early and late-booster Vi-DT groups.

Rotavirus spike protein ΔVP8* as a novel carrier protein for conjugate vaccine platform with demonstrated antigenic potential for use as bivalent vaccine

Conjugate vaccine platform is a promising strategy to overcome the poor immunogenicity of bacterial polysaccharide antigens in infants and children. A carrier protein in conjugate vaccines works not only as an immune stimulator to polysaccharide, but also as an immunogen; with the latter generally not considered as a measured outcome in real world. Here, we probed the potential of a conjugate vaccine platform to induce enhanced immunogenicity of a truncated rotavirus spike protein ΔVP8*. ΔVP8* was covalently conjugated to Vi capsular polysaccharide (Vi) of Salmonella Typhi to develop a bivalent vaccine, termed Vi-ΔVP8*. Our results demonstrated that the Vi-ΔVP8* vaccine can induce specific immune responses against both antigens in immunized mice. The conjugate vaccine elicits high antibody titers and functional antibodies against S. Typhi and Rotavirus (RV) when compared to immunization with a single antigen. Together, these results indicate that Vi-ΔVP8* is a potent and immunogenic vaccine candidate, thus strengthening the potential of conjugate vaccine platform with enhanced immune responses to carrier protein, including ΔVP8*.

Evidence of Extended Thermo-Stability of Typhoid Polysaccharide Conjugate Vaccines

Typhoid conjugate vaccines (TCV) are effective in preventing enteric fever caused by Salmonella enterica serovar Typhi in Southeast Asia and Africa. To facilitate vaccination with the Vi capsular polysaccharide–tetanus toxoid conjugate vaccine, Typbar TCV, and allow it to be transported and stored outside a cold chain just prior to administration, an extended controlled-temperature conditions (ECTC) study was performed to confirm the quality of the vaccine at 40 °C for 3 days at the end of its shelf-life (36 months at 2–8 °C). Studies performed in parallel by the vaccine manufacturer, Bharat Biotech International Limited, and an independent national control laboratory (NIBSC) monitored its stability-indicating parameters: O-acetylation of the Vi polysaccharide, integrity of the polysaccharide–protein conjugate, and its molecular size and pH. ECTC samples stored at 40 °C and 45 °C in comparison with control samples stored at 4 °C and 55 or 56 °C, were shown to have stable O-acetylation and pH; only very slight increases in the percentage of free saccharide and corresponding decreases in molecular size were observed. The deoxycholate method for precipitating conjugated polysaccharide was very sensitive to small incremental increases in percentage of free saccharide, in line with storage temperature and duration. This extended ECTC study demonstrated minimal structural changes to the Vi polysaccharide and conjugate vaccine and a stable formulation following extended exposure to elevated temperatures for the desired durations. This outcome supports the manufacturer’s ECTC claim for the vaccine to be allowed to be taken outside the cold chain before its administration.

Transcutaneous Vaccination with Conjugate Typhoid Vaccine Vi-DT Induces Systemic, Mucosal, and Memory Anti-Polysaccharide Responses

Transcutaneous vaccination can induce both mucosal and systemic immune responses. However, there are few data on anti-polysaccharide responses following transcutaneous vaccination of polysaccharides, despite the role that anti-polysaccharide responses play in protecting against intestinal mucosal and respiratory pathogens. Whether transcutaneous vaccination with a conjugate polysaccharide vaccine would be able to induce memory responses is also unknown. To address this, we transcutaneously vaccinated mice with virulence antigen (Vi) polysaccharide of Salmonella enterica serovar Typhi (the cause of typhoid fever), either in unconjugated or conjugated form (the latter as a Vi-DT conjugate). We also assessed the ability of the immunoadjuvant cholera toxin to impact responses following vaccination. We found that presenting Vi in a conjugate versus nonconjugate form transcutaneously resulted in comparable serum IgG responses but higher serum and lamina propria lymphocyte IgA anti-Vi responses, as well as increased IgG memory responses. The addition of immunoadjuvant did not further increase these responses; however, it boosted fecal IgA and serum IgG anti-Vi responses. Our results suggest that transcutaneous vaccination of a conjugate vaccine can induce systemic as well as enhanced mucosal and memory B-cell anti-polysaccharide responses.

Review on the Recent Advances on Typhoid Vaccine Development and Challenges Ahead

Control of Salmonella enterica serovar typhi (S. typhi), the agent of typhoid fever, continues to be a challenge in many low- and middle-income countries. The major transmission route of S. typhi is fecal-oral, through contaminated food and water; thus, the ultimate measures for typhoid fever prevention and control include the provision of safe water, improved sanitation, and hygiene. Considering the increasing evidence of the global burden of typhoid, particularly among young children, and the long-term horizon for sustained, effective water and sanitation improvements in low-income settings, a growing consensus is to emphasize preventive vaccination. This review provides an overview of the licensed typhoid vaccines and vaccine candidates under development, and the challenges ahead for introduction.

Six-month follow up of a randomized clinical trial-phase I study in Indonesian adults and children: Safety and immunogenicity of Salmonella typhi polysaccharide-diphtheria toxoid (Vi-DT) conjugate vaccine

Introduction

There is a high global incidence of typhoid fever, with an annual mortality rate of 200,000 deaths. Typhoid fever also affects younger children, particularly in resource-limited settings in endemic countries. Typhoid vaccination is an important prevention tool against typhoid fever. However, the available polysaccharide typhoid vaccines are not recommended for children under 2 years of age. A new typhoid conjugate Vi-diphtheria toxoid (Vi-DT) vaccine has been developed for infant immunization. We aimed to define the safety and immunogenicity of the Vi-DT vaccine among adults and children in Indonesia.

Methods

An observational, blinded, comparative, randomized, phase I safety study in two age de-escalating cohorts was conducted in East Jakarta, Indonesia, from April 2017 to February 2018. We enrolled 100 healthy subjects in 2 age groups: adults and children (18–40 and 2–5 years old). These groups were randomized into study groups (Vi-DT vaccine), and comparator groups (Vi-polysaccharide (Vi-PS) vaccine and another additional vaccine) which was administered in 4 weeks apart. Subjects were followed up to six months.

Result

One hundred healthy adults and children subjects completed the study. The Vi-DT and Vi-PS vaccines showed no difference in terms of intensity of any immediate local and systemic events within 30 minutes post-vaccination. Overall, pain was the most common local reaction, and muscle pain was the most common systemic reaction in the first 72 hours. No serious adverse events were deemed related to vaccine administration. The first and second doses of the Vi-DT vaccine induced seroconversion and higher geometric mean titers (GMT) in all subjects compared to that of baseline. However, in terms of GMT, the second dose of Vi-DT did not induce a booster response.

Conclusion

The Vi-DT vaccine is safe and immunogenic in adults and children older than two years. A single dose of the vaccine is able to produce seroconversion and high GMT in all individuals.

IL-7 Enables Antibody Responses to Bacterial Polysaccharides by Promoting B Cell Receptor Diversity

Polysaccharide vaccines such as the Vi polysaccharide (ViPS) of Salmonella enterica serovar Typhi induce efficient Ab responses in adults but not in young children. The reasons for this difference are not understood. IL-7 dependency in B cell development increases progressively with age. IL-7Rα-mediated signals are required for the expression of many VH gene segments that are distal to DH-JH in the IgH locus and for the complete diversification of the BCR repertoire. Therefore, we hypothesized that B cells generated in the absence of IL-7 do not recognize a wide range of Ags because of a restricted BCR repertoire. Compared with adult wildtype mice, young wildtype mice and IL-7-deficient adult mice generated a significantly reduced Ab response to ViPS. Additionally, ViPS-binding B cells in adult wildtype mice predominantly used distal VH gene segments. Transgenic expression of either IL-7 or a BCR encoded by a distal VH gene segment permitted young mice to respond efficiently to bacterial polysaccharides. These results indicate that restricted VH gene usage early in life results in a paucity of Ag-specific B cell precursors, thus limiting antipolysaccharide responses.

Evaluation of alum-based adjuvant on the immunogenicity of salmonella enterica serovar typhi conjugates vaccines

The function of adjuvant in maintaining the long-term immune response to Typhoid conjugate vaccine (TCV) was evaluated in. Two TCV products, Vi-DT and Vi-TT, were formulated in either aluminum phosphate (AlPO₄) or aluminum hydroxide (AlOH) as adjuvants and TCV formulated in phosphate buffer saline were used as controls. In each case, a group of Balb/c mice was injected intramuscularly with two doses of the formulated vaccine at two-week intervals. The anti-Vi IgG responses were monitored by Enzyme-Linked Immunosorbent Assay and the levels of CD4⁺ T-cells expressing cytokine were characterized using intracellular cytokine staining. All mice immunized by TCV formulated in adjuvant elicited anti-Vi response to a higher level than the group receiving TCV formulated in PBS. The extent of adsorption of TCV in AlOH was greater than that in AlPO₄, and this finding correlated well with the observation that the mice immunized with two doses of Vi-DT(AlOH) elicited anti-Vi IgG to a level higher than that seen with Vi-DT(AlPO₄). The mice primed with Vi-TT(AlOH) produced lower anti-Vi IgG (25.901 GM) compared to those receiving Vi-TT(AlPO₄) (49.219 GM). However, after the second injection, the former raised the antibody level significantly to 137.008 GM while the latter provided a value of only 104.966 GM. The groups of mice vaccinated by TCV formulated in AlOH expressed IL4 at higher levels than the other groups, which correlated positively with the high Anti-Vi IgG in these animals. In conclusion, AlOH could be recommended as an effective adjuvant for TCV to provide a long-term immune response.

The influence of conjugation variables on the design and immunogenicity of a glycoconjugate vaccine against Salmonella Typhi

In recent years there have been major efforts to develop glycoconjugate vaccines based on the Vi polysaccharide that will protect against Salmonella enterica Typhi infections, particularly typhoid fever, which remains a major public health concern in low-income countries. The design of glycoconjugate vaccines influences the immune responses they elicit. Here we systematically test the response in mice to Vi glycoconjugates that differ in Vi chain length (full-length and fragmented), carrier protein, conjugation chemistry, saccharide to protein ratio and size. We show that the length of Vi chains, but not the ultimate size of the conjugate, has an impact on the anti-Vi IgG immune response induced. Full-length Vi conjugates, independent of the carrier protein, induce peak IgG responses rapidly after just one immunization, and secondary immunization does not enhance the magnitude of these responses. Fragmented Vi linked to CRM197 and diphtheria toxoid, but not to tetanus toxoid, gives lower anti-Vi antibody responses after the first immunization than full-length Vi conjugates, but antibody titres are similar to those induced by full-length Vi conjugates following a second dose. The chemistry to conjugate Vi to the carrier protein, the linker used, and the saccharide to protein ratio do not significantly alter the response. We conclude that Vi length and carrier protein are the variables that influence the anti-Vi IgG response to immunization the most, while other parameters are of lesser importance.

First characterization of immunogenic conjugates of Vi negative Salmonella Typhi O-specific polysaccharides with rEPA protein for vaccine development

Efficacious typhoid vaccines for young children will significantly reduce the disease burden in developing world. The Vi polysaccharide based conjugate vaccines (Vi-rEPA) against Salmonella Typhi Vi positive strains has shown high efficacy but may be ineffective against Vi negative S. Typhi. In this study, for the first time, we report the synthesis and evaluation of polysaccharide-protein conjugates of Vi negative S. Typhi as potential vaccine candidates. Four different conjugates were synthesized using recombinant exoprotein A of Pseudomonas aeruginosa (rEPA) and human serum albumin (HSA) as the carrier proteins, using either direct reductive amination or an intermediate linker molecule, adipic acid dihydrazide (ADH). Upon injection into mice, a significantly higher antibody titer was observed in mice administrated with conjugate-1 (OSP-HSA) (P=0.0001) and conjugate 2 (OSP-rEPA) (P≤0.0001) as compared to OSP alone. In contrast, the antibody titer elicited by conjugate 3 (OSPADH-HSA) and conjugate 4 (OSPADH-rEPA) were insignificant (P=0.1684 and P=0.3794, respectively). We conclude that reductive amination is the superior method to prepare the S. Typhi OSP glycoconjugate. Moreover, rEPA was a better carrier protein than HSA. Thus OSP-rEPA conjugate seems to be efficacious typhoid vaccines candidate, it may be evaluated further and recommended for the clinical trials.

A recombinant protein of Salmonella Typhi induces humoral and cell-mediated immune responses including memory responses

Gram negative enteric bacteria, Salmonella enterica serovar Typhi (S. Typhi), the etiological agent of typhoid fever is a major public health problem in developing countries. While a permanent solution to the problem would require improved sanitation, food and water hygiene, controlling the infection by vaccination is urgently required due to the emergence of multidrug resistant strains in multiple countries. The currently licensed vaccines are moderately efficacious with limited applicability, and no recommended vaccines exist for younger children. We had previously reported that a candidate vaccine based on recombinant outer membrane protein (rT2544) of S. Typhi is highly immunogenic and protective in mice. Here we show that rT2544-specific antiserum is capable of mediating bacterial lysis by the splenocytes through Antibody-Dependent Cellular Cytotoxicity (ADCC). Increased populations of rT2544-specific IgA and IgG secreting plasma cells are found in the spleen, mesenteric lymph nodes and peyer's patches. Cell-Mediated Immune Responses (CMIR) induced by rT2544 consist of Th1 cell differentiation and generation of cytotoxic T lymphocytes (CTL), which produce IFN-γ and are capable of destroying cells displaying T2544-derived antigens. rT2544 elicits pro-inflammatory cytokines (TNF-α, IL-6) from Bone Marrow-Derived Dendritic cells (BMDCs), while in vitro re-stimulation of rT2544-primed CD4+ T cells induces cell proliferation and generates higher amounts of Th1 cytokines, such as IFN-gamma, TNF-α and IL-2. Finally, the candidate vaccine induces immunological memory in the form of memory B and T lymphocytes. Taken together, the study further supports the potential of rT2544 as a novel and improved vaccine candidate against S. Typhi.

Molecular dynamics approach to probe the antigenicity of PagN - an outer membrane protein of Salmonella Typhi

PagN is a highly immunogenic 27-kDa outer membrane adhesin present in Salmonella Typhi. It plays a major role in the pathogenesis of typhoid fever and has emerged as a strong vaccine candidate. In this report, we predict the three-dimensional structure of PagN and describe the conformational dynamics associated with its four extracellular loops based on two 100-ns molecular dynamics simulations at 300 and 310 K. The formation and deformation of the secondary structures on these loops were also investigated during the simulations which revealed loops L1 and L2 to be highly flexible, whereas the relative flexibility of loops L3 and L4 was minimal. Essential dynamics and principal component analysis deciphered more realistic dynamic behaviours of the loops, particularly at 310 K. Moreover, our epitope predictions suggest that the antigenic peptides for B-cell recognition are located within the loops L1 and L2, while those for T-cell recognition are located within the loops L3 and L4. The binding specificities of the antigenic peptides towards specific human MHC-I and MHC-II HLA alleles closely resembled the stability of the loops L3 and L4 inferred from the simulations. Finally, we identified potential antigenic peptides in the flexible (L1 and L2) as well as stable (L3 and L4) regions of PagN for both B- and T-cell recognitions, which can help in developing effective sub-unit vaccines.

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.

Glycoconjugate Vaccines: The Regulatory Framework

Most vaccines, including the currently available glycoconjugate vaccines, are administered to healthy infants, to prevent future disease. The safety of a prospective vaccine is a key prerequisite for approval. Undesired side effects would not only have the potential to damage the individual infant but also lead to a loss of confidence in the respective vaccine-or vaccines in general-on a population level. Thus, regulatory requirements, particularly with regard to safety, are extremely rigorous. This chapter highlights regulatory aspects on carbohydrate-based vaccines with an emphasis on analytical approaches to ensure the consistent quality of successive manufacturing lots.

Salmonella chronic carriage: epidemiology, diagnosis, and gallbladder persistence

Typhoid (enteric fever) remains a major cause of morbidity and mortality worldwide, causing over 21 million new infections annually, with the majority of deaths occurring in young children. Because typhoid fever-causing Salmonella have no known environmental reservoir, the chronic, asymptomatic carrier state is thought to be a key feature of continued maintenance of the bacterium within human populations. Despite the importance of this disease to public health, our understanding of the molecular mechanisms that catalyze carriage, as well as our ability to reliably identify and treat the Salmonella carrier state, have only recently begun to advance.

Complex adaptive immunity to enteric fevers in humans: lessons learned and the path forward

Salmonella enterica serovar Typhi (S. Typhi), the causative agent of typhoid fever, and S. Paratyphi A and B, causative agents of paratyphoid fever, are major public health threats throughout the world. Although two licensed typhoid vaccines are currently available, they are only moderately protective and immunogenic necessitating the development of novel vaccines. A major obstacle in the development of improved typhoid, as well as paratyphoid vaccines is the lack of known immunological correlates of protection in humans. Considerable progress has been made in recent years in understanding the complex adaptive host responses against S. Typhi. Although the induction of S. Typhi-specific antibodies (including their functional properties) and memory B cells, as well as their cross-reactivity with S. Paratyphi A and S. Paratyphi B has been shown, the role of humoral immunity in protection remains undefined. Cell mediated immunity (CMI) is likely to play a dominant role in protection against enteric fever pathogens. Detailed measurements of CMI performed in volunteers immunized with attenuated strains of S. Typhi have shown, among others, the induction of lymphoproliferation, multifunctional type 1 cytokine production, and CD8(+) cytotoxic T-cell responses. In addition to systemic responses, the local microenvironment of the gut is likely to be of paramount importance in protection from these infections. In this review, we will critically assess current knowledge regarding the role of CMI and humoral immunity following natural S. Typhi and S. Paratyphi infections, experimental challenge, and immunization in humans. We will also address recent advances regarding cross-talk between the host's gut microbiota and immunization with attenuated S. Typhi, mechanisms of systemic immune responses, and the homing potential of S. Typhi-specific B- and T-cells to the gut and other tissues.

Preparation and testing of a Vi conjugate vaccine using pneumococcal surface protein A (PspA) from Streptococcus pneumoniae as the carrier protein

In the current study pneumococcal surface protein A (PspA) was conjugated to Vi capsular polysaccharide from Salmonella Typhi to make available a vaccine against typhoid fever that has the potential to also provide broad protection from Streptococcus pneumoniae. High yielding production processes were developed for the purification of PspAs from families 1 and 2. The purified PspAs were conjugated to Vi with high recovery of both Vi and PspA. The processes developed especially for PspA family 2 could readily be adapted for large scale production under cGMP conditions. Previously we have shown that conjugation of diphtheria toxoid (DT) to Vi polysaccharide improves the immune response to Vi but can also enhance the response to DT. In this study it was shown that conjugation of PspA to Vi enhanced the anti-PspA response and that PspA was a suitable carrier protein as demonstrated by the characteristics of a T-cell dependent response to the Vi. We propose that a bivalent vaccine consisting of PspA from families 1 and 2 bound to Vi polysaccharide would protect against typhoid fever and has the potential to also protect against pneumococcal disease and should be considered for use in developing countries.

Vaccines against invasive Salmonella disease: current status and future directions

Though primarily enteric pathogens, Salmonellae are responsible for a considerable yet under-appreciated global burden of invasive disease. In South and South-East Asia, this manifests as enteric fever caused by serovars Typhi and Paratyphi A. In sub-Saharan Africa, a similar disease burden results from invasive nontyphoidal Salmonellae, principally serovars Typhimurium and Enteritidis. The existing Ty21a live-attenuated and Vi capsular polysaccharide vaccines target S. Typhi and are not effective in young children where the burden of invasive Salmonella disease is highest. After years of lack of investment in new Salmonella vaccines, recent times have seen increased interest in the area led by emerging-market manufacturers, global health vaccine institutes and academic partners. New glycoconjugate vaccines against S. Typhi are becoming available with similar vaccines against other invasive serovars in development. With other new vaccines under investigation, including live-attenuated, protein-based and GMMA vaccines, now is an exciting time for the Salmonella vaccine field.

Increasing rates of Salmonella Paratyphi A and the current status of its vaccine development

Enteric fever caused by Salmonella enterica serovar Typhi and Salmonella enterica serovar Paratyphi is still a major disease burden mainly in developing countries. Previously, S. Typhi was believed to be the major cause of enteric fever. The real situation is now becoming clear with reports emerging from many Asian countries of S. Paratyphi, mostly S. Paratyphi A, causing a substantial number of cases of enteric fever. Although there have been advances in the use of the currently available typhoid vaccines and in the development of newer typhoid vaccines, paratyphoid vaccine development is lagging behind. Since the disease caused by S. Typhi and S. Paratyphi are clinically indistinguishable and are commonly termed 'enteric' fever, it will be necessary to have a vaccine available against both S. Typhi and S. Paratyphi A as a bivalent 'enteric fever vaccine'.

Development of pneumococcal polysaccharide conjugate vaccine with long spacer arm

Streptococcus pneumoniae is a serious Gram-positive pathogen responsible for several life-threatening pneumococcal diseases. Pneumococcal capsular polysaccharide (CPS) is a key virulence determinant of S. pneumoniae and its immunogenicity can be improved by conjugation with a carrier protein. Reductive amination, the most widely used approach for pneumococcal CPS conjugate vaccine (PCV), suffers from low conjugation efficiency and the problem of steric hindrance. Here, copper-catalyzed azide-alkyne cycloaddition was used for development of PCV with long spacer arm (L-PCV). Tetanus toxoid (TT) was used as the carrier protein. The long spacer arm in L-PCV can minimize the problem of steric hindrance between CPS and TT, thereby improving the CPS-specific antibody titers in the mice model. L-PCV can also induce high avidity functional antibody and elicit immunological memory in response to the native CPS.

Identification of an outer membrane protein of Salmonella enterica serovar Typhimurium as a potential vaccine candidate for Salmonellosis in mice

We report our investigation of the functions of PagN in Salmonella pathogenesis and its potential as a vaccine candidate. Further investigation conducted in this study indicates that the outer membrane protein PagN is important for Salmonella adhesion/invasion of epithelial cells as well as bacterial virulence. When pagN was deleted from Salmonella enterica serovar Typhimurium (S. Typhimurium), the adhesion and invasion of HT-29 epithelial cells was significantly decreased compared with the wild type strain. Mice infected with the pagN mutant strain exhibited less pathological signs in the intestine and survived longer than the wild-type-infected mice. PagN is widely distributed and conserved among clinical isolates of different Salmonella serovars, making PagN a potential vaccine candidate for Salmonella infection. To elucidate the potential of PagN as a vaccine, we expressed and purified recombinant PagN (rPagN). When rPagN was tested in mice, it provided significant protection against Salmonella infection in vivo. In vitro, anti-PagN serum enhanced clearance of Salmonella, indicating a contribution of PagN-specific antibodies to the killing process. This correlates well with the observed protection of mice immunized with rPagN. Our preliminary results indicate more functions of PagN in S. Typhimurium virulence as well as its potential as a protective vaccine.

A human IgG anti-Vi reference for Salmonella typhi with weight-based antibody units assigned

Recent data showing the high incidence of typhoid fever in young children, the demonstration of safety and efficacy of a Vi conjugate for this age group, the safety and similar immunogenicity in infants when administrated concurrently with EPI vaccines, together with the interests of manufacturers and investigators in studying such conjugate vaccines prompted us to prepare a human IgG anti-Vi standard to facilitate this work. Volunteers were injected with an investigational Vi-recombinant Pseudomonas aeruginosa exoprotein A (Vi-rEPA) conjugate vaccine. Plasmas with the highest levels of IgG anti-Vi were pooled. The IgG anti-Vi content of this preparation, assayed by precipitin analysis with purified Vi, was 33 μg/ml. Accordingly, the estimated IgG anti-Vi protective level of 3.5 ELISA unit/ml, derived from our efficacy trial of Vi-rEPA in 2-5 years old children, is equivalent to 4.3 μg/ml. This reagent is suitable for comparison of immune response of Vi conjugate vaccines or for other purposes requiring anti-Vi measurement.

Vaccines for typhoid fever and other salmonelloses

PURPOSE OF REVIEW

This review summarizes the recent advances in vaccination against Salmonella enterica serovar Typhi and highlights the data supporting the development of next generation vaccines to address paratyphoid fever and invasive nontyphoidal Salmonella (iNTS) disease.

RECENT FINDINGS

There has been increasing awareness of the disease burden caused by S. Typhi particularly in Africa and greater recognition of S. Paratyphi A's contribution to enteric fever episodes throughout Asia. Groups have been working to improve the existing typhoid vaccines and provide comprehensive data on the feasibility of their implementation in endemic settings. These data have resulted in modifications to the recommendations for typhoid vaccination in traveller markets and endemic settings, and has also led to the development of S. Paratyphi A vaccine components that can be combined with existing typhoid vaccines to generate bivalent formulations against enteric fever. The epidemiology of iNTS serovars as cause of appreciable morbidity and mortality in Africa, and the need for vaccines, has also become more widely appreciated.

SUMMARY

Current typhoid vaccines, although moderately effective for short periods of time, cannot be used in all age groups and only target one of the clinically relevant Salmonella serovars. Greater effort must be placed on the development and implementation of improved vaccines for the disease burden resulting from Typhi, Paratyphi A or iNTS infections.

Vi antigen of Salmonella enetrica serovar Typhi — biosynthesis, regulation and its use as vaccine candidate

Vi capsular polysaccharide (Vi antigen) was first identified as the virulence antigen of Salmonella enterica serovar Typhi (S. Typhi), the causative agent of typhoid fever in humans. The presence of Vi antigen differentiates S. Typhi from other serovars of Salmonella. Vi antigen is a linear polymer consisting of α-1,4-linked-N-acetyl-galactosaminuronate, whose expression is controlled by three chromosomal loci, namely viaA, viaB and ompB. Both viaA and viaB region are present on Salmonella Pathogenicity Island-7, a large, mosaic, genetic island. The viaA region encodes a positive regulator and the viaB locus is composed of 11 genes designated tviA-tviE (for Vi biosyhthesis), vexA-vexE (for Vi antigen export) and ORF 11. Vi polysaccharide is synthesized from UDP-N-acetyl glucosamine in a series of steps requiring TviB, TviC, and TviE, and regulation of Vi polysaccharide synthesis is controlled by two regulatory systems, rscB-rscC (viaA locus) and ompR-envZ (ompB locus), which respond to changes in osmolarity. This antigen is highly immunogenic and has been used for the formulation of one of the currently available vaccines against typhoid. Despite advancement in the area of vaccinology, its pace of progress needs to be accelerated and effective control programmes will be needed for proper disease management.

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

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

A bivalent vaccine to protect against Streptococcus pneumoniae and Salmonella typhi

Pneumococcal and Salmonella typhi infections are two major diseases for children in developing countries. For typhoid fever, licensed Vi polysaccharide vaccines are ineffective in children <2-year old. While investigational Vi conjugate vaccines have been shown effective in clinical trials, they are currently only available to restricted areas. Pneumococcal capsular polysaccharide conjugate vaccines are highly effective in children, but suffer from some limitations including cost and limited serotype coverage. We have previously shown that a fusion conjugate vaccine, consisting of pneumococcal fusion protein PsaA and pneumolysoid (PdT) conjugated to a polysaccharide, results in enhanced antibody and CD4+ Th17 cell responses as well as protection against pneumococcal colonization and disease in mice. Here we applied this approach to develop a bivalent vaccine against pneumococcus and S. typhi. Two species-conserved pneumococcal antigens (SP1572 or SP2070) were fused to the nonhemolytic pneumolysoid PdT. SP1572-PdT was then conjugated to Vi polysaccharide and SP2070-PdT was conjugated to the pneumococcal cell wall polysaccharide (CWPS; also conserved). Mice immunized with this bivalent conjugate were protected against pneumococcal colonization and sepsis challenges, and made anti-Vi antibody concentrations higher by 40-fold compared to mice that received equimolar mixtures of the antigens. An enhanced killing of Vi-bearing Salmonellae in vitro was demonstrated from plasma of mice that received the fusion conjugate but not the mixture of antigens. Our results support further evaluation of this bivalent immunogen for the prevention of pneumococcal colonization and disease, and of typhoid fever.

Glycoconjugate vaccine strategies for protection against invasive Salmonella infections

Salmonella enterica serovars Typhi and Paratyphi A and B and certain non-typhoidal Salmonella enterica (NTS) serovars are important causes of invasive Salmonella disease worldwide. NTS serovars Typhimurium and Enteritidis typically cause gastroenteritis in healthy children and adults in industrialized countries but in certain hosts (e.g., young infants, the elderly, immunocompromised individuals) they also cause invasive infections. These two serovars also cause invasive disease in infants and young children in sub-Saharan Africa. Whereas Salmonella surface polysaccharides are poor immunogens in animal models and do not generate immunologic memory, conjugation with carrier proteins overcomes these limitations. S. Typhi expresses a Vi polysaccharide capsule; Vi either alone or as a glycoconjugate protects humans from typhoid fever. In contrast, S. Paratyphi A and B and NTS (with rare exceptions) do not express capsular polysaccharides. Rather, their surface polysaccharides are the O polysaccharide (OPS) of lipopolysaccharide. In animal studies, immunization with Salmonella COPS (core polysaccharide-OPS) conjugated with carrier proteins generates functional immunity and protects against fatal Salmonella challenge. Conjugating to Salmonella proteins (flagellin, porins) may extend immune responses to another relevant target for antibody generation and enhance the glyconjugate's efficacy.

Preparation and evaluation of immunogenic conjugates of Salmonella enterica serovar Typhi O-specific polysaccharides with diphtheria toxoid

Typhoid fever, caused by Salmonella enterica serovar Typhi (S. Typhi), is a major health problem particularly in developing countries. The available vaccines have certain limitations regarding their efficacy, and inability to induce an immune response especially in individuals under 2 years of age. Conjugate vaccines which consist of a bacteria-specific polysaccharide chemically bound to a carrier protein overcome these problems by inducing a T-cell dependent immune response characterized by enhanced immunogenicity in all ages. In this study, O-specific polysaccharides (OSP) of S. Typhi were conjugated to diphtheria toxoid (DT) using adipic acid dihydrazide (ADH) as a linker. These conjugates (OSP-AH-DT) were then evaluated for their immunogenicity using mice as a model and showed significantly higher levels of IgG ELISA titers (P = 0.0241 and 0.0245) than lipopolysaccharides alone. Different immunization  schedules were compared and it was found that schedule-B (three injections with 4-weeks interval) induced higher immune responses than schedule-A (three injections with 2-weeks interval). We showed that diphtheria toxoid can be successfully employed as a carrier protein for conjugation with Salmonella OSP and play an important role in facilitating adequate immune response.

Immune suppression induced by Vi capsular polysaccharide is overcome by Vi-DT conjugate vaccine

The influence pre-exposure of mice to Vi capsular polysaccharide, purified from Salmonella enterica Serovar Typhi, on the subsequent immune response induced by a Vi-diphtheria toxoid (Vi-DT) conjugate was evaluated. Vi induced low anti Vi IgG titers with the dominant subclass being IgG3. The Vi-DT conjugate induced high titers of anti Vi IgG with the dominant subclass being IgG1 but with considerable quantities of IgG2a, IgG2b and IgG3. Priming of mice with Vi suppressed the response to a subsequent dose of conjugate and the suppression was overcome by a second dose of conjugate. Priming with conjugate prevented suppression of the anti Vi response and subsequent dosing with Vi raised titers back to previous levels but did not boost to new higher levels. The anti DT IgG response to one dose of conjugate was relatively strong and protracted and continued to rise for 12 weeks, compared to the response to one dose of DT which was poor and peaked at two weeks. The prolonged anti DT response was most likely due to the slow release of DT from the conjugate lattice as it degrades within the mouse resulting in a continuous stimulation of the immune response. The presence of increasing amounts of un-conjugated Vi, up to 50%, administered with the conjugate resulted in increasingly higher levels of both anti Vi and anti DT. Larger amounts of un-conjugated Vi inhibited the anti Vi response. These findings have implications for vaccine quality and a limit for un-conjugated polysaccharide should not exceed 50% and from a vaccine program perspective if the results presented here translate to humans then a Vi conjugate, once it becomes available, should replace Vi polysaccharide vaccines.

Production of a conjugate vaccine for Salmonella enterica serovar Typhi from Citrobacter Vi

A conjugate vaccine for Salmonella enterica serovar Typhi was produced by chemically linking Vi, purified from Citrobacter, to the non-toxic mutant diphtheria toxin CRM(197) via an adipic dihydrazide spacer using N-(3-Dimethylaminopropyl)-N'-ethylcarbodiimide coupling chemistry. The polysaccharide purification process was developed based on Vi precipitation from culture supernatant with cetyl trimethylammonium bromide (CTAB), solubilization of the CTA-polysaccharide salt with ethanol followed by exchange of the CTA(+) counter ion with Na(+). The purified Vi polysaccharide was fully O-acetylated and with high purity. The conjugation process was optimized to obtain a scalable process that has been used for GMP production at pilot scale of vaccine currently in clinical trials.

Salmonella enterica serovar enteritidis core O polysaccharide conjugated to H:g,m flagellin as a candidate vaccine for protection against invasive infection with S. enteritidis

Nontyphoidal Salmonella enterica serovars Enteritidis and Typhimurium are a common cause of gastroenteritis but also cause invasive infections and enteric fever in certain hosts (young children in sub-Saharan Africa, the elderly, and immunocompromised individuals). Salmonella O polysaccharides (OPS) and flagellar proteins are virulence factors and protective antigens. The surface polysaccharides of Salmonella are poorly immunogenic and do not confer immunologic memory, limitations overcome by covalently attaching them to carrier proteins. We conjugated core polysaccharide-OPS (COPS) of Salmonella Enteritidis lipopolysaccharide (LPS) to flagellin protein from the homologous strain. COPS and flagellin were purified from a genetically attenuated (ΔguaBA) "reagent strain" (derived from an isolate from a patient with clinical bacteremia) engineered for increased flagellin production (ΔclpPX). Conjugates were constructed by linking flagellin monomers or polymers at random COPS hydroxyls with various polysaccharide/protein ratios by 1-cyano-4-dimethylaminopyridinium tetrafluoroborate (CDAP) or at the 3-deoxy-d-manno-octulosonic acid (KDO) terminus by thioether chemistry. Mice immunized on days 0, 28, and 56 with COPS-flagellin conjugates mounted higher anti-LPS IgG levels than mice receiving unconjugated COPS and exhibited high antiflagellin IgG; anti-LPS and antiflagellin IgG levels increased following booster doses. Antibodies generated by COPS-flagellin conjugates mediated opsonophagocytosis of S. Enteritidis cells into mouse macrophages. Mice immunized with flagellin alone, COPS-CRM₁₉₇, or COPS-flagellin conjugates were significantly protected from lethal challenge with wild-type S. Enteritidis (80 to 100% vaccine efficacy).

An adhesion protein of Salmonella enterica serovar Typhi is required for pathogenesis and potential target for vaccine development

More than half of all Salmonella enterica serovar Typhi genes still remain unannotated. Although pathogenesis of S. Typhi is incompletely understood, treatment of typhoid fever is complicated by the emergence of drug resistance. Effectiveness of the currently available vaccines is also limited. In search of novel virulence proteins, we have identified several putative adhesins of S. Typhi through computational approaches. Our experiment shows that a 27-kDa outer membrane protein (T2544) plays a major role in bacterial adhesion to the host through high-affinity binding to laminin. Its role in bacterial pathogenesis is underscored by reduced systemic invasion and a 10-fold higher LD(50) of the mutant bacteria in mice. T2544 is strongly immunogenic as revealed by the detection of sustained high titers of serum IgG and intestinal secretory IgA in the immunized mice. In vitro, T2544 antiserum enhanced uptake and clearance of Salmonella by macrophages and augmented complement-mediated lysis, indicating a contribution of T2544-specific antibodies to the killing process. This correlates well with the observed protection of mice immunized with recombinant T2544 or passively immunized with T2544 antiserum against subsequent bacterial challenge, suggesting that T2544-specific antibodies are involved in protection. The present study describes an adhesion protein of S. Typhi that contributes to bacterial pathogenesis. Protective antibodies in mice, rapid seroconversion of naturally infected individuals with increasing titers of anti-T2544 IgG from acute to convalescent sera suggesting antibody response in humans, and wide distribution and conservation of the cell-surface adhesin in the clinical isolates of different Salmonella serovars make T2544 a potential vaccine candidate.

Vi-CRM 197 as a new conjugate vaccine against Salmonella Typhi

An efficacious, low cost vaccine against typhoid fever, especially for young children, would make a major impact on disease burden in developing countries. The virulence capsular polysaccharide of Salmonella Typhi (Vi) coupled to recombinant mutant Pseudomonas aeruginosa exoprotein A (Vi-rEPA) has been shown to be highly efficacious. We investigated the use of carrier proteins included in infant vaccines, standardized the conjugation process and developed key assays required for routine lot release at production scale. Vi from a BSL1 organism, Citrobacter freundii, strain WR7011, was used as an alternative to Vi from S. Typhi. We showed that Vi conjugated to CRM(197), a non-toxic mutant of diphtheria toxin, widely used in commercial vaccines, was produced at high yield. Vi-CRM(197) proved immunogenic in animal studies, even without adjuvant. Thus, Vi-CRM(197) appears to be a suitable candidate for the development of a commercially viable, effective typhoid vaccine for developing countries.