Vi-specific serological correlates of protection for typhoid fever

A TLR4 ligand-based adjuvant for promoting the immunogenicity of typhoid subunit vaccines

None of the typhoid Vi Polysaccharide (ViPS) subunit vaccines incorporate adjuvants, and the immunogenicity of ViPS vaccines (e.g. Typbar TCV® and Typhim Vi®) is in part due to associated TLR4 ligands such as endotoxin present in these vaccines. Since endotoxin content in vaccines is variable and kept very low due to inherent toxicity, it was hypothesized that incorporating a defined amount of a non-toxic TLR4-ligand such as monophosphoryl lipid A in ViPS vaccines would improve their immunogenicity. To test this hypothesis, a monophosphoryl lipid A-based adjuvant formulation named Turbo was developed. Admixing Turbo with Typbar TCV® (ViPS-conjugated to tetanus toxoid) increased the levels of anti-ViPS IgM, IgG1, IgG2b, IgG2a/c, and IgG3 in inbred and outbred mice. In infant mice, a single immunization with Turbo adjuvanted Typbar TCV® resulted in a significantly increased and durable IgG response and improved the control of bacterial burden compared to mice immunized without Turbo. Similarly, when adjuvanted with Turbo, the antibody response and control of bacteremia were also improved in mice immunized with Typhim Vi®, an unconjugated vaccine. The immunogenicity of unconjugated ViPS is inefficient in young mice and is lost in adult mice when immunostimulatory ligands in ViPS are removed. Nevertheless, when adjuvanted with Turbo, poorly immunogenic ViPS induced a robust IgG response in young and adult mice, and this was observed even under antigen-limiting conditions. These data suggest that incorporation of Turbo as an adjuvant will make typhoid vaccines more immunogenic regardless of their intrinsic immunogenicity or conjugation status and maximize the efficacy across all ages.

The safety and immunogenicity of a bivalent conjugate vaccine against Salmonella enterica Typhi and Paratyphi A in healthy Indian adults: a phase 1, randomised, active-controlled, double-blind trial

BACKGROUND

Enteric fever caused by Salmonella enterica Typhi and Salmonella Paratyphi A is an important public health problem, especially in low-income and middle-income countries with limited access to safe water and sanitation. We present results from, to our knowledge, the first ever human study of a bivalent paratyphoid A-typhoid conjugate vaccine (Sii-PTCV).

METHODS

In this double-blind phase 1 study, 60 healthy Indian adults were randomly assigned (1:1) to receive a single intramuscular dose of either Sii-PTCV or typhoid conjugate vaccine (Typbar-TCV). Safety was assessed by observing solicited adverse events for 1 week, unsolicited events for 1 month, and serious adverse events (SAEs) over 6 months. Immunogenicity at 1 month and 6 months was assessed by measuring anti-capsular polysaccharide antigen Vi (anti-Vi) IgG and IgA against Salmonella Typhi and anti-lipopolysaccharide (LPS) IgG against Salmonella Paratyphi A by ELISA, and functional antibodies using serum bactericidal assay (SBA) against Salmonella Paratyphi A. This study is registered with Clinical Trial Registry-India (CTRI/2022/06/043608) and is completed.

FINDINGS

60 participants were enrolled. Of these 60 participants, 57 (95%) participants were male and three (5%) participants were female. Solicited adverse events were observed in 27 (90%) of 30 participants who received Sii-PTCV and 26 (87%) of 30 participants who received Typbar-TCV. The most common local solicited event was pain in 27 (90%) participants who received Sii-PTCV and in 23 (77%) participants who received Typbar-TCV. The most common solicited systemic event was myalgia in five (17%) participants who received Sii-PTCV, whereas four (13%) participants who received Typbar-TCV had myalgia and four (13%) had headache. No vaccine-related unsolicited adverse events or SAEs were reported. The seroconversion rates on day 29 were 96·7% (95% CI 82·8-99·9) with Sii-PTCV and 100·0% (88·4-100·0) with Typbar-TCV for anti-Vi IgG; 93·3% (77·9-99·2) with Sii-PTCV and 100·0% (88·4-100·0) with Typbar-TCV for anti-Vi IgA; 100·0% (88·4-100·0) with Sii-PTCV and 3·3% (0·1-17·2) with Typbar-TCV for anti-LPS (paratyphoid); and 93·3% (77·9-99·2) with Sii-PTCV and 0% (0·0-11·6) with Typbar-TCV for SBA titres (paratyphoid). Paratyphoid anti-LPS immune responses were sustained at day 181.

INTERPRETATION

Sii-PTCV was safe and immunogenic for both typhoid and paratyphoid antigens indicating its potential for providing comprehensive protection against enteric fever.

FUNDING

Serum Institute of India.

Typhoid Conjugate Vaccine: A Boon for Endemic Regions

Typhoid fever has the highest disease burden in countries in low- and middle-income countries, primarily located in Asia and Sub-Saharan Africa. Previous typhoid vaccines such as the live attenuated typhoid (Ty21a) vaccine and Vi (virulence) capsular polysaccharide vaccine had the limitation that they could not be administered with other standard childhood immunizations and were ineffective in children under two years of age. To address these shortcomings of the previous vaccines, typhoid conjugate vaccines (TCVs) were developed and prequalified by the World Health Organization. Cross-reacting material and tetanus toxoid are widely used as carrier proteins in TCVs. According to various studies, TCV has higher efficacy, has a more extended protection period, and is safe and immunogenic in infants as young as six months. This review article aims to comprehensively appraise the data available on TCVs' efficacy, duration of protection, safety, and immunogenicity in endemic regions.

TLR4 Ligands in Typhoid Vi Polysaccharide Subunit Vaccines Contribute to Immunogenicity

Activation of B cells and T cells requires the engagement of costimulatory signaling pathways in addition to Ag receptor signaling for efficient immune responses. None of the typhoid Vi polysaccharide (ViPS) subunit vaccines contains adjuvants that could activate costimulatory signaling pathways, yet these vaccines are very immunogenic. I hypothesized that residual TLR ligands present in the ViPS preparation used for making typhoid subunit vaccines account for the robust immune response generated by these vaccines. I show the presence of endotoxin, a potent agonist of TLR4, in ViPS preparations and ViPS vaccines. Furthermore, I found that ViPS obtained from various sources induces the production of proinflammatory cytokines such as IL-6 from mouse peritoneal exudate cells. Unconjugated and tetanus toxoid-conjugated ViPS vaccines activate human and mouse TLR4. Mice deficient in TLR4 or the signaling adaptors MyD88 and Trif (Toll/IL-1R domain-containing adapter inducing IFN-β) are severely impaired in generating anti-ViPS responses to these vaccines. Elimination of the TLR4 agonist in ViPS preparation resulted in the loss of immunogenicity, and addition of lipid A, a known TLR4 agonist, restored the immunogenicity. These data highlight the importance of associated TLR ligands in the immunogenicity of ViPS subunit vaccines.

Typhoid fever

Typhoid fever is an invasive bacterial disease associated with bloodstream infection that causes a high burden of disease in Africa and Asia. Typhoid primarily affects individuals ranging from infants through to young adults. The causative organism, Salmonella enterica subsp. enterica serovar Typhi is transmitted via the faecal-oral route, crossing the intestinal epithelium and disseminating to systemic and intracellular sites, causing an undifferentiated febrile illness. Blood culture remains the practical reference standard for diagnosis of typhoid fever, where culture testing is available, but novel diagnostic modalities are an important priority under investigation. Since 2017, remarkable progress has been made in defining the global burden of both typhoid fever and antimicrobial resistance; in understanding disease pathogenesis and immunological protection through the use of controlled human infection; and in advancing effective vaccination programmes through strategic multipartner collaboration and targeted clinical trials in multiple high-incidence priority settings. This Primer thus offers a timely update of progress and perspective on future priorities for the global scientific community.

Strategic and scientific contributions of human challenge trials for vaccine development: facts versus fantasy

The unprecedented speed of delivery of SARS-CoV-2 pandemic vaccines has redefined the limits for all vaccine development. Beyond the aspirational 100-day timeline for tomorrow's hypothetical pandemic vaccines, there is a sense of optimism that development of other high priority vaccines can be accelerated. Early in the COVID-19 pandemic, an intense and polarised academic and public discourse arose concerning the role of human challenge trials for vaccine development. A case was made for human challenge trials as a powerful tool to establish early proof-of-concept of vaccine efficacy in humans, inform vaccine down selection, and address crucial knowledge gaps regarding transmission, pathogenesis, and immune protection. We review the track record of human challenge trials contributing to the development of vaccines for 19 different pathogens and discuss relevant limitations, barriers, and pitfalls. This Review also highlights opportunities for efforts to broaden the scope and boost the effects of human challenge trials, to accelerate all vaccine development.

Vaccine value profile for Shigella

Shigella is the leading bacterial cause of diarrhoea and the second leading cause of diarrhoeal mortality among all ages. It also exhibits increasing levels of antibiotic resistance. The greatest burden is among children under five in low- and middle-income countries (LMICs). As such, a priority strategic goal of the World Health Organization (WHO) is the development of a safe, effective and affordable vaccine to reduce morbidity and mortality from Shigella-attributable dysentery and diarrhea, including long term outcomes associated with chronic inflammation and growth faltering, in children under 5 years of age in LMICs. In addition, a safe and effective Shigella vaccine is of potential interest to travellers and military both to prevent acute disease and rarer, long-term sequelae. An effective Shigella vaccine is also anticipated to reduce antibiotic use and thereby help diminish further emergence of enteric pathogens resistant to antimicrobials. The most advanced vaccine candidates are multivalent, parenteral formulations in Phase 2 and Phase 3 clinical studies. They rely on O-antigen-polysaccharide protein conjugate technologies or, alternatively, outer membrane vesicles expressing penta-acylated lipopolysaccharide that has been detoxified. Other parenteral and oral formulations, many delivering a broader array of Shigella antigens, are at earlier stages of clinical development. These formulations are being assessed in alignment with the WHO Preferred Product Characteristics, which call for a 1 to 2 dose primary immunization series given during the first 12 months of life, ideally starting at 6 months of age. This 'Vaccine Value Profile' (VVP) for Shigella is intended to provide a high-level, holistic assessment of the information and data that are currently available to inform the potential public health, economic and societal value of pipeline vaccines and vaccine-like products. This VVP was developed by a working group of subject matter experts from academia, non-profit organizations, government agencies and multi-lateral organizations. All contributors have extensive expertise on various elements of the Shigella VVP and collectively aimed to identify current research and knowledge gaps. The VVP was developed using only existing and publicly available information.

Early transcriptional responses to human enteric fever challenge

Enteric fever, caused by oral infection with typhoidal Salmonella serovars, presents as a non-specific febrile illness preceded by an incubation period of 5 days or more. The enteric fever human challenge model provides a unique opportunity to investigate the innate immune response during this incubation period, and how this response is altered by vaccination with the Vi polysaccharide or conjugate vaccine. We find that on the same day as ingestion of typhoidal Salmonella, there is already evidence of an immune response, with 199 genes upregulated in the peripheral blood transcriptome 12 hours post-challenge (false discovery rate <0.05). Gene sets relating to neutrophils, monocytes, and innate immunity were over-represented (false discovery rate <0.05). Estimating cell proportions from gene expression data suggested a possible increase in activated monocytes 12 hours post-challenge (P = 0.036, paired Wilcoxon signed-rank test). Furthermore, plasma TNF-α rose following exposure (P = 0.011, paired Wilcoxon signed-rank test). There were no significant differences in gene expression (false discovery rate <0.05) in the 12 hours response between those who did and did not subsequently develop clinical or blood culture confirmed enteric fever or between vaccination groups. Together, these results demonstrate early perturbation of the peripheral blood transcriptome after enteric fever challenge and provide initial insight into early mechanisms of protection.

Durable Anti-Vi IgG and IgA Antibody Responses in 15-Month-Old Children Vaccinated With Typhoid Conjugate Vaccine in Burkina Faso

We assessed anti-Vi IgG/IgA responses to typhoid conjugate vaccine (TCV) in children enrolled in a double-blind randomized controlled, phase 2 trial in Burkina Faso. Anti-Vi IgG seroconversion and anti-Vi IgA titers were higher in TCV than control recipients at 30-35 months post-vaccination. TCV induces durable immunity in Burkinabe children vaccinated at 15 months.

Bacterial ghost cell based bivalent candidate vaccine against Salmonella Typhi and Salmonella Paratyphi A: A prophylactic study in BALB/c mice

Typhoid and emerging paratyphoid fever are a severe enteric disease worldwide with high morbidity and mortality. Licensed typhoid vaccines are in the market, but no paratyphoid vaccine is currently available. In the present study we developed a bivalent vaccine against Salmonella Typhi and Paratyphi A using a bacterial ghost platform. Bacterial ghost cells (BGs) are bacteria-derived cell membranes without cytoplasmic contents that retain their cellular morphology, including all cell surface features. Furthermore, BGs have inherent adjuvant properties that promote an enhanced humoral and cellular immune reaction to the target antigen. Sodium hydroxide was used to prepare ghost cells of Salmonella Typhi and Paratyphi A. The bacterial ghost cells were characterised using electron microscopy. Then BALB/c mice were immunized three times (0th, 14th and 28th day) with the bivalent typhoidal bacterial ghost cells. Haematological study of adult mice throughout immunization period reflected that the immunogen was safe to administer and does not affect the animals' health. After complete immunization, we found significant serum antibody titter against whole cell lysate, outer membrane protein and lipopolysaccharide of both bacteria, and cell-mediated immunity was observed in an ex-vivo experiment. CD4+, CD8a+ and CD19+ splenic cell populations were increased in immunized animals. Bivalent Typhoidal ghost cell immunized mice showed better survival, less bacterial colonization in systemic organs, and less inflammation and/or destruction of tissue in histopathological analysis than non-immunized control mice.Serum antibodies of immunized animals can significantly inhibit bacterial motility and mucin penetration ability with better killing properties against Salmonella Typhi and Paratyphi A. Furthermore, significant passive protection was observed through the adoptive transfer of serum antibody and lymphocytes of immunized animals to naïve animals after bacterial infection. In summary, Bivalent Typhoidal Bacterial Ghost cells (BTBGs) enhances immunogenic properties and serves as a safe and effective prevention strategy against Salmonella Typhi and Paratyphi A.

The Challenge Non-Typhoidal Salmonella (CHANTS) Consortium: Development of a non-typhoidal Salmonella controlled human infection model: Report from a consultation group workshop, 05 July 2022, London, UK

Invasive non-typhoidal Salmonella disease (iNTS) is a major cause of morbidity and mortality globally, particularly as a cause of bloodstream infection in children and immunocompromised adults in sub-Saharan Africa. Vaccines to prevent non-typhoidal Salmonella (NTS) would represent a valuable public health tool in this setting to avert cases and prevent expansion of antimicrobial resistance. Several NTS and combination typhoidal-NTS vaccine candidates are in early-stage development, although the pathway to licensure is unclear due to challenges in conducting large phase III field trials. Controlled human infection models (CHIM) present an opportunity to accelerate vaccine development for a range of enteric pathogens. Several recent typhoidal Salmonella CHIMs have been conducted safely and have played pivotal roles in progressing vaccine candidates to pre-qualification and licensure. The Challenge Non-Typhoidal Salmonella (CHANTS) consortium has been formed with funding from the Wellcome Trust, to deliver the first NTS CHIM, which can act as a platform for future vaccine evaluation. This paper reports the conclusions of a consultation group workshop convened with key stakeholders. The aims of this meeting were to: (1) define the rationale for an NTS CHIM (2) map the NTS vaccine pipeline (3) refine study design and (4) establish potential future use cases.

The Challenge Non-Typhoidal Salmonella (CHANTS) Consortium: Development of a non-typhoidal Salmonella controlled human infection model: Report from a consultation group workshop, 05 July 2022, London, UK

Invasive non-typhoidal Salmonella disease (iNTS) is a major cause of morbidity and mortality globally, particularly as a cause of bloodstream infection in children and immunocompromised adults in sub-Saharan Africa. Vaccines to prevent non-typhoidal Salmonella (NTS) would represent a valuable public health tool in this setting to avert cases and prevent expansion of antimicrobial resistance. Several NTS and combination typhoidal-NTS vaccine candidates are in early-stage development, although the pathway to licensure is unclear due to challenges in conducting large phase III field trials. Controlled human infection models (CHIM) present an opportunity to accelerate vaccine development for a range of enteric pathogens. Several recent typhoidal Salmonella CHIMs have been conducted safely and have played pivotal roles in progressing vaccine candidates to pre-qualification and licensure. The Challenge Non-Typhoidal Salmonella (CHANTS) consortium has been formed with funding from the Wellcome Trust, to deliver the first NTS CHIM, which can act as a platform for future vaccine evaluation. This paper reports the conclusions of a consultation group workshop convened with key stakeholders. The aims of this meeting were to: (1) define the rationale for an NTS CHIM (2) map the NTS vaccine pipeline (3) refine study design and (4) establish potential future use cases.

Molecular correlates of vaccine-induced protection against typhoid fever

BACKGROUNDTyphoid fever is caused by the Gram-negative bacterium Salmonella enterica serovar Typhi and poses a substantial public health burden worldwide. Vaccines have been developed based on the surface Vi-capsular polysaccharide of S. Typhi; these include a plain-polysaccharide-based vaccine, ViPS, and a glycoconjugate vaccine, ViTT. To understand immune responses to these vaccines and their vaccine-induced immunological protection, molecular signatures were analyzed using bioinformatic approaches.METHODSBulk RNA-Seq data were generated from blood samples obtained from adult human volunteers enrolled in a vaccine trial, who were then challenged with S. Typhi in a controlled human infection model (CHIM). These data were used to conduct differential gene expression analyses, gene set and modular analyses, B cell repertoire analyses, and time-course analyses at various post-vaccination and post-challenge time points between participants receiving ViTT, ViPS, or a control meningococcal vaccine.RESULTSTranscriptomic responses revealed strong differential molecular signatures between the 2 typhoid vaccines, mostly driven by the upregulation in humoral immune signatures, including selective usage of immunoglobulin heavy chain variable region (IGHV) genes and more polarized clonal expansions. We describe several molecular correlates of protection against S. Typhi infection, including clusters of B cell receptor (BCR) clonotypes associated with protection, with known binders of Vi-polysaccharide among these.CONCLUSIONThe study reports a series of contemporary analyses that reveal the transcriptomic signatures after vaccination and infectious challenge, while identifying molecular correlates of protection that may inform future vaccine design and assessment.TRIAL REGISTRATIONClinicalTrials.gov NCT02324751.

Polyfunctional antibodies: a path towards precision vaccines for vulnerable populations

Vaccine efficacy determined within the controlled environment of a clinical trial is usually substantially greater than real-world vaccine effectiveness. Typically, this results from reduced protection of immunologically vulnerable populations, such as children, elderly individuals and people with chronic comorbidities. Consequently, these high-risk groups are frequently recommended tailored immunisation schedules to boost responses. In addition, diverse groups of healthy adults may also be variably protected by the same vaccine regimen. Current population-based vaccination strategies that consider basic clinical parameters offer a glimpse into what may be achievable if more nuanced aspects of the immune response are considered in vaccine design. To date, vaccine development has been largely empirical. However, next-generation approaches require more rational strategies. We foresee a generation of precision vaccines that consider the mechanistic basis of vaccine response variations associated with both immunogenetic and baseline health differences. Recent efforts have highlighted the importance of balanced and diverse extra-neutralising antibody functions for vaccine-induced protection. However, in immunologically vulnerable populations, significant modulation of polyfunctional antibody responses that mediate both neutralisation and effector functions has been observed. Here, we review the current understanding of key genetic and inflammatory modulators of antibody polyfunctionality that affect vaccination outcomes and consider how this knowledge may be harnessed to tailor vaccine design for improved public health.

Accelerating clinical development of a live attenuated vaccine against Salmonella Paratyphi A (VASP): study protocol for an observer-participant-blind randomised control trial of a novel oral vaccine using a human challenge model of Salmonella Paratyphi A infection in healthy adult volunteers

INTRODUCTION

This is the first efficacy study of an oral live attenuated vaccine against Salmonella Paratyphi A using a human challenge model of paratyphoid infection. S. Paratyphi A is responsible for 3.3 million cases of enteric fever every year, with over 19 000 deaths. Although improvements to sanitation and access to clean water are vital to reduce the burden of this condition, vaccination offers a cost-effective, medium-term solution. Efficacy trials of potential S. Paratyphi vaccine candidates in the field are unlikely to be feasible given the large number of participants required. Human challenge models therefore offer a unique, cost-effective solution to test efficacy of such vaccines.

METHODS AND ANALYSIS

This is an observer-blind, randomised, placebo-controlled trial phase I/II of the oral live-attenuated vaccine against S. Paratyphi A, CVD 1902. Volunteers will be randomised 1:1 to receive two doses of CVD 1902 or placebo, 14 days apart. One month following second vaccination all volunteers will ingest S. Paratyphi A bacteria with a bicarbonate buffer solution. They will be reviewed daily in the following 14 days and diagnosed with paratyphoid infection if the predefined microbiological or clinical diagnostic criteria are met. All participants will be treated with antibiotics on diagnosis, or at day 14 postchallenge if not diagnosed. The vaccine efficacy will be determined by comparing the relative attack rate, that is, the proportion of those diagnosed with paratyphoid infection, in the vaccine and placebo groups.

ETHICS AND DISSEMINATION

Ethical approval for this study has been obtained from the Berkshire Medical Research Ethics Committee (REC ref 21/SC/0330). The results will be disseminated via publication in a peer-reviewed journal and presentation at international conferences.

TRIAL REGISTRATION NUMBER

ISRCTN15485902.

A systems serology approach to the investigation of infection-induced antibody responses and protection in trachoma

Background

Ocular infections with Chlamydia trachomatis serovars A-C cause the neglected tropical disease trachoma. As infection does not confer complete immunity, repeated infections are common, leading to long-term sequelae such as scarring and blindness. Here, we apply a systems serology approach to investigate whether systemic antibody features are associated with susceptibility to infection.

Methods

Sera from children in five trachoma endemic villages in the Gambia were assayed for 23 antibody features: IgG responses towards two C. trachomatis antigens and three serovars [elementary bodies and major outer membrane protein (MOMP), serovars A-C], IgG responses towards five MOMP peptides (serovars A-C), neutralization, and antibody-dependent phagocytosis. Participants were considered resistant if they subsequently developed infection only when over 70% of other children in the same compound were infected.

Results

The antibody features assayed were not associated with resistance to infection (false discovery rate < 0.05). Anti-MOMP SvA IgG and neutralization titer were higher in susceptible individuals (p < 0.05 before multiple testing adjustment). Classification using partial least squares performed only slightly better than chance in distinguishing between susceptible and resistant participants based on systemic antibody profile (specificity 71%, sensitivity 36%).

Conclusions

Systemic infection-induced IgG and functional antibody responses do not appear to be protective against subsequent infection. Ocular responses, IgA, avidity, or cell-mediated responses may play a greater role in protective immunity than systemic IgG.

Reduced immunogenicity of a live Salmonella enterica serovar Typhimurium vaccine in aged mice

Introduction

Non-typhoidal Salmonella (NTS) is responsible for a high burden of foodborne infections and deaths worldwide. In the United States, NTS infections are the leading cause of hospitalizations and deaths due to foodborne illnesses, and older adults (≥65 years) are disproportionately affected by Salmonella infections. Due to this public health concern, we have developed a live attenuated vaccine, CVD 1926 (I77 ΔguaBA ΔclpP ΔpipA ΔhtrA), against Salmonella enterica serovar Typhimurium, a common serovar of NTS. Little is known about the effect of age on oral vaccine responses, and due to the decline in immune function with age, it is critical to evaluate vaccine candidates in older age groups during early product development.

Methods

In this study, adult (six-to-eight-week-old) and aged (18-month-old) C57BL/6 mice received two doses of CVD 1926 (109 CFU/dose) or PBS perorally, and animals were evaluated for antibody and cell-mediated immune responses. A separate set of mice were immunized and then pre-treated with streptomycin and challenged orally with 108 CFU of wild-type S. Typhimurium SL1344 at 4 weeks postimmunization.

Results

Compared to PBS-immunized mice, adult mice immunized with CVD 1926 had significantly lower S. Typhimurium counts in the spleen, liver, and small intestine upon challenge. In contrast, there were no differences in bacterial loads in the tissues of vaccinated versus PBS aged mice. Aged mice exhibited reduced Salmonella-specific antibody titers in the serum and feces following immunization with CVD 1926 compared to adult mice. In terms of T cell responses (T-CMI), immunized adult mice showed an increase in the frequency of IFN-γ- and IL-2-producing splenic CD4 T cells, IFN-γ- and TNF-α-producing Peyer's Patch (PP)-derived CD4 T cells, and IFN-γ- and TNF-α-producing splenic CD8 T cells compared to adult mice administered PBS. In contrast, in aged mice, T-CMI responses were similar in vaccinated versus PBS mice. CVD 1926 elicited significantly more PP-derived multifunctional T cells in adult compared to aged mice.

Conclusion

These data suggest that our candidate live attenuated S. Typhimurium vaccine, CVD 1926, may not be sufficiently protective or immunogenic in older humans and that mucosal responses to live-attenuated vaccines decrease with increasing age.

Vi polysaccharide and conjugated vaccines afford similar early, IgM or IgG-independent control of infection but boosting with conjugated Vi vaccines sustains the efficacy of immune responses

Introduction

Vaccination with Vi capsular polysaccharide (Vi-PS) or protein-Vi typhoid conjugate vaccine (TCV) can protect adults against Salmonella Typhi infections. TCVs offer better protection than Vi-PS in infants and may offer better protection in adults. Potential reasons for why TCV may be superior in adults are not fully understood.

Methods and results

Here, we immunized wild-type (WT) mice and mice deficient in IgG or IgM with Vi-PS or TCVs (Vi conjugated to tetanus toxoid or CRM197) for up to seven months, with and without subsequent challenge with Vi-expressing Salmonella Typhimurium. Unexpectedly, IgM or IgG alone were similarly able to reduce bacterial burdens in tissues, and this was observed in response to conjugated or unconjugated Vi vaccines and was independent of antibody being of high affinity. Only in the longer-term after immunization (>5 months) were differences observed in tissue bacterial burdens of mice immunized with Vi-PS or TCV. These differences related to the maintenance of antibody responses at higher levels in mice boosted with TCV, with the rate of fall in IgG titres induced to Vi-PS being greater than for TCV.

Discussion

Therefore, Vi-specific IgM or IgG are independently capable of protecting from infection and any superior protection from vaccination with TCV in adults may relate to responses being able to persist better rather than from differences in the antibody isotypes induced. These findings suggest that enhancing our understanding of how responses to vaccines are maintained may inform on how to maximize protection afforded by conjugate vaccines against encapsulated pathogens such as S. Typhi.

Correlates of immunity to Group A Streptococcus: a pathway to vaccine development

Understanding immunity in humans to Group A Streptococcus (Strep A) is critical for the development of successful vaccines to prevent the morbidity and mortality attributed to Strep A infections. Despite decades of effort, no licensed vaccine against Strep A exists and immune correlates of protection are lacking; a major impediment to vaccine development. In the absence of a vaccine, we can take cues from the development of natural immunity to Strep A in humans to identify immune correlates of protection. The age stratification of incidence of acute Strep A infections, peaking in young children and waning in early adulthood, coincides with the development of specific immune responses. Therefore, understanding the immune mechanisms involved in natural protection from acute Strep A infection is critical to identifying immune correlates to inform vaccine development. This perspective summarises the findings from natural infection studies, existing assays of immunity to Strep A, and highlights the gaps in knowledge to guide the development of Strep A vaccines and associated correlates of protection.

Identification of collaborative cross mouse strains permissive to Salmonella enterica serovar Typhi infection

Salmonella enterica serovar Typhi is the causative agent of typhoid fever restricted to humans and does not replicate in commonly used inbred mice. Genetic variation in humans is far greater and more complex than that in a single inbred strain of mice. The Collaborative Cross (CC) is a large panel of recombinant inbred strains which has a wider range of genetic diversity than laboratory inbred mouse strains. We found that the CC003/Unc and CC053/Unc strains are permissive to intraperitoneal but not oral route of S. Typhi infection and show histopathological changes characteristic of human typhoid. These CC strains are immunocompetent, and immunization induces antigen-specific responses that can kill S. Typhi in vitro and control S. Typhi in vivo. Our results indicate that CC003/Unc and CC053/Unc strains can help identify the genetic basis for typhoid susceptibility, S. Typhi virulence mechanism(s) in vivo, and serve as a preclinical mammalian model system to identify effective vaccines and therapeutics strategies.

Complementary measurement of nontyphoidal Salmonella-specific IgG and IgA antibodies in oral fluid and serum

Objectives

Immuno-epidemiological studies of orally acquired, enteric pathogens such as nontyphoidal Salmonella (NTS) often focus on serological measures of immunity, ignoring potentially relevant oral mucosal responses. In this study we sought to assess the levels and detectability of both oral fluid and serum IgG and IgA to NTS antigens, in endemic and non-endemic populations.

Methods

IgG and IgA antibodies specific for Salmonella Typhimurium and Salmonella Enteritidis O antigen and phase 1 flagellin were assessed using Enzyme Linked Immunosorbent Assay (ELISA). Paired oral fluid and serum samples were collected from groups of 50 UK adults, Kenyan adults and Kenyan infants. Additionally, oral fluid alone was collected from 304 Kenyan individuals across a range of ages.

Results

Antigen-specific IgG and IgA was detectable in the oral fluid of both adults and infants. Oral fluid antibody increased with age, peaking in adulthood for both IgG and IgA but a separate peak was also observed for IgA in infants. Oral fluid and serum responses correlated for IgG but not IgA. Despite standardised collection the relationship between oral fluid volume and antibody levels varied with age and country of origin.

Conclusions

Measurement of NTS-specific oral fluid antibody can be used to complement measurement of serum antibody. For IgA in particular, oral fluid may offer insights into how protective immunity to NTS changes as individuals transition with age, from maternal to acquired systemic and mucosal immunity. This may prove useful in helping to guide future vaccine design.

Antagonizing FcαR1 (CD89) as treatment in IgA-mediated chronic inflammation and autoimmunity

Introduction

Immunoglobulin A (IgA) is mostly considered as a non-inflammatory regulator at mucosal areas. However, previous work of our group showed that IgA can also be involved in disease pathology, because it provides a potent stimulus to activate neutrophils after crosslinking of surface CD89 (FcaRI), resulting in chronic inflammation and tissue damage. IgA (auto)antibodies and neutrophils are key players in various diseases, including blistering skin diseases and rheumatoid arthritis. Therefore, we generated an array of anti-CD89 monoclonal antibodies (mAbs) for therapeutic targeting of CD89. The biological activity of newly developed anti-human CD89 mAbs and their potential therapeutic capacity were investigated.

Methods

Human neutrophils were isolated from heparinized healthy donor blood. The ability of anti-CD89 mAbs to bind human neutrophils was investigated by flow cytometry. Furthermore, the capacity of these anti-CD89 mAbs to inhibit IgA-mediated phagocytosis, neutrophil extracellular trap (NET) release and migration was studied. To this end, neutrophils were pre-incubated with/without anti-CD89 mAbs after which they were stimulated with IgA-coated beads. The amount of phagocytosed beads, NET release and migrated neutrophils were subsequently analysed. In parallel, chemoattractant leukotriene B4 and lactoferrin (as a measure for degranulation) release were determined. Finally, the therapeutic potential of our prototypic anti-CD89 mAb clone 10E7 was in vivo tested in anti-mouse collagen XVII human IgA-treated transgenic CD89 mice, a preclinical model for autoimmune linear IgA bullous disease (LABD).

Results

Our results show that all generated anti-CD89 mAbs bound surface CD89 on neutrophils. Although these anti-CD89 mAbs bind to different epitopes on EC1 of CD89, they all have the capacity to inhibit IgA-mediated phagocytosis, neutrophil extracellular trap (NET) release and neutrophil migration. Moreover, IgA mediated leukotriene B4 and lactoferrin release are decreased in supernatant from anti-CD89 mAbs-treated neutrophils. Finally, anti-CD89 mAb clone 10E7, that was selected based on its selective binding profile on tissue micro arrays, reduced anti-mouse collagen XVII hIgA-induced neutrophil influx in an in vivo linear IgA bullous disease (LABD) mice model.

Conclusion

This study clearly indicates that our newly developed anti-CD89 mAbs inhibited IgA-induced neutrophil activation and reduced anti-autoantigen IgA-induced neutrophil influx in vivo, supporting further clinical development for the treatment of LABD.

The Lack of Natural IgM Increases Susceptibility and Impairs Anti-Vi Polysaccharide IgG Responses in a Mouse Model of Typhoid

Circulating IgM present in the body prior to any apparent Ag exposure is referred to as natural IgM. Natural IgM provides protective immunity against a variety of pathogens. Salmonella enterica serovar Typhi (S. Typhi) is the causative agent of typhoid fever in humans. Because mice are not permissive to S. Typhi infection, we employed a murine model of typhoid using S. enterica serovar Typhimurium expressing the Vi polysaccharide (ViPS) of S. Typhi (S. Typhimurium strain RC60) to evaluate the role of natural IgM in pathogenesis. We found that natural mouse IgM binds to S. Typhi and S. Typhimurium. The severity of S. Typhimurium infection in mice is dependent on presence of the natural resistance-associated macrophage protein 1 (Nramp1) allele; therefore, we infected mice deficient in secreted form of IgM (sIgM) on either a Nramp1-resistant (129S) or -susceptible (C57BL/6J) background. We found that the lack of natural IgM results in a significantly increased susceptibility and an exaggerated liver pathology regardless of the route of infection or the Nramp1 allele. Reconstitution of sIgM-/- mice with normal mouse serum or purified polyclonal IgM restored the resistance to that of sIgM+/+ mice. Furthermore, immunization of sIgM-/- mice with heat-killed S. Typhi induced a significantly reduced anti-ViPS IgG and complement-dependent bactericidal activity against S. Typhi in vitro, compared with that of sIgM+/+ mice. These findings indicate that natural IgM is an important factor in reducing the typhoid severity and inducing an optimal anti-ViPS IgG response to vaccination.

The next frontier in vaccine design: blending immune correlates of protection into rational vaccine design

Despite the extraordinary speed and success in SARS-Cov-2 vaccine development, the emergence of variants of concern perplexed the vaccine development community. Neutralizing antibodies waned antibodies waned and were evaded by viral variants, despite the preservation of protection against severe disease and death across vaccinated populations. Similar to other vaccine design efforts, the lack of mechanistic correlates of immunity against Coronavirus Disease 2019, raised questions related to the need for vaccine redesign and boosting. Hence, our limited understanding of mechanistic correlates of immunity - across pathogens - remains a major obstacle in vaccine development. The identification and incorporation of mechanistic correlates of immunity are key to the accelerated design of highly impactful globally relevant vaccines. Systems-biology tools can be applied strategically to define a complete understanding of mechanistic correlates of immunity. Embedding immunological dissection and target immune profile identification, beyond canonical antibody binding and neutralization, may accelerate the design and success of durable protective vaccines.

Controlled human infectious models, a path forward in uncovering immunological correlates of protection: Lessons from enteric fevers studies

Enteric infectious diseases account for more than a billion disease episodes yearly worldwide resulting in approximately 2 million deaths, with children under 5 years old and the elderly being disproportionally affected. Enteric pathogens comprise viruses, parasites, and bacteria; the latter including pathogens such as Salmonella [typhoidal (TS) and non-typhoidal (nTS)], cholera, Shigella and multiple pathotypes of Escherichia coli (E. coli). In addition, multi-drug resistant and extensively drug-resistant (XDR) strains (e.g., S. Typhi H58 strain) of enteric bacteria are emerging; thus, renewed efforts to tackle enteric diseases are required. Many of these entero-pathogens could be controlled by oral or parenteral vaccines; however, development of new, effective vaccines has been hampered by lack of known immunological correlates of protection (CoP) and limited knowledge of the factors contributing to protective responses. To fully comprehend the human response to enteric infections, an invaluable tool that has recently re-emerged is the use of controlled human infection models (CHIMs) in which participants are challenged with virulent wild-type (wt) organisms. CHIMs have the potential to uncover immune mechanisms and identify CoP to enteric pathogens, as well as to evaluate the efficacy of therapeutics and vaccines in humans. CHIMs have been used to provide invaluable insights in the pathogenesis, host-pathogen interaction and evaluation of vaccines. Recently, several Oxford typhoid CHIM studies have been performed to assess the role of multiple cell types (B cells, CD8+ T, Tregs, MAIT, Monocytes and DC) during S. Typhi infection. One of the key messages that emerged from these studies is that baseline antigen-specific responses are important in that they can correlate with clinical outcomes. Additionally, volunteers who develop typhoid disease (TD) exhibit higher levels and more activated cell types (e.g., DC and monocytes) which are nevertheless defective in discrete signaling pathways. Future critical aspects of this research will involve the study of immune responses to enteric infections at the site of entry, i.e., the intestinal mucosa. This review will describe our current knowledge of immunity to enteric fevers caused byS. Typhi and S. Paratyphi A, with emphasis on the contributions of CHIMs to uncover the complex immunological responses to these organisms and provide insights into the determinants of protective immunity.

Maternal Fc-mediated non-neutralizing antibody responses correlate with protection against congenital human cytomegalovirus infection

Human cytomegalovirus (HCMV) is the most common congenital infection and a leading cause of stillbirth, neurodevelopmental impairment, and pediatric hearing loss worldwide. Development of a maternal vaccine or therapeutic to prevent congenital HCMV has been hindered by limited knowledge of the immune responses that protect against HCMV transmission in utero. To identify protective antibody responses, we measured HCMV-specific IgG binding and antiviral functions in paired maternal and cord blood sera from HCMV-seropositive transmitting (n = 41) and non-transmitting (n = 40) mother-infant dyads identified via a large, US-based, public cord blood bank. We found that high-avidity IgG binding to HCMV and antibody-dependent cellular phagocytosis (ADCP) were associated with reduced risk of congenital HCMV infection. We also determined that HCMV-specific IgG activation of FcγRI and FcγRII was enhanced in non-transmitting dyads and that increased ADCP responses were mediated through both FcγRI and FcγRIIA expressed on human monocytes. These findings suggest that engagement of FcγRI/FcγRIIA and Fc effector functions including ADCP may protect against congenital HCMV infection. Taken together, these data can guide future prospective studies on immune correlates against congenital HCMV transmission and inform HCMV vaccine and immunotherapeutic development.

The omics strategy: the use of systems vaccinology to characterise immune responses to childhood immunisation

INTRODUCTION

Vaccines have had a transformative impact on child health. Despite this impact the immunological processes involved in protective responses are not entirely understood and vaccine development has been largely empirical. Recent technological advances offer the opportunity to reveal the immunology underlying vaccine response at an unprecedented resolution. These data could revolutionise the way vaccines are developed and tested and further augment their role in securing the health of children around the world.

AREAS COVERED

Systems level information and the tools are now being deployed by vaccinologists at all stages of the vaccine development pathway; however, this review will specifically describe some of the key findings that have be gleaned from multi-omics datasets collected in the context of childhood immunisation.

EXPERT OPINION

Despite the success of vaccines there remains hard-to-target pathogens, refractory to current vaccination strategies. Moreover, zoonotic diseases with pandemic potential are a threat to global health, as recently illustrated by COVID-19. Systems vaccinology holds a great deal of promise in revealing a greater understanding of vaccine responses and consequently modernising vaccinology. However, there is a need for future studies -particularly in vulnerable populations that are targets for vaccination programmes - if this potential is to be fulfilled.

A Phase 3, Multicenter, Randomized, Controlled Trial to Evaluate Immune Equivalence and Safety of Multidose and Single-dose Formulations of Vi-DT Typhoid Conjugate Vaccine in Healthy Filipino Individuals 6 Months to 45 Years of Age

Trial Design

Phase 3, randomized, controlled, multicenter, equivalence trial.

Methods

Recruitment of participants occurred between 04Februray2020 and 15July2020 at four centers in the Philippines: University of the East - Ramon Magsaysay Memorial Medical Center Inc., Quezon City; University of Philippines Manila - National Institute of Health, Ermita Manila; Asian Hospital and Medical Center, Metro Manila, Philippines Study; and Medical Research Unit, Tropical Disease Foundation, Makati City, Metro Manila, Philippines.

Participants

1800 adults and children 6-months to 45-years of age.

Interventions

Participants received a single injection of multidose (MD) or single dose (SD) Vi-DT as test vaccines or meningococcal conjugate vaccine as a comparator.

Objective

To evaluate immune equivalence of SD and MD formulations of Vi-DT, and to assess the safety of both formulations compared with comparator vaccine.

Outcome Measurement

Blood draw for immunogenicity was performed at baseline prior to vaccine receipt and at four weeks after vaccination for a subset of participants to determine anti-Vi IgG geometric mean titers (GMT) and seroconversion rates. The primary outcome was comparison of anti Vi-IgG seroconversion and GMT between the two formulations of Vi-DT at 4 weeks following vaccine administration. Immune equivalence of MD and SD formulations was confirmed when the two-tailed 95% confidence interval (CI) of the GMT ratio is within [0.67, 1.5] at a two-sided significance level of 0.05. All participants were followed for safety events for six months after vaccine administration.

Randomization

Participants were randomized to receive SD Vi-DT, MD Vi-DT, or meningococcal conjugate vaccines in 2.5:2.5:1 allocation ratio.

Blinding

Study participants and observers were blinded to treatment assignment.

Findings

Immune equivalence of SD (n=252) and MD (n=247) formulations was confirmed by anti-Vi IgG GMT ratio of 1.14 (95%CI: 0.91, 1.43) with respective GMTs in the MD and SD groups of 640.62 IU/mL (95%CI: 546.39, 751.11) and 562.57 IU/mL (95%CI: 478.80, 661.00) (p=0.259). Similarly, anti-Vi IgG seroconversion rate difference between the two formulations of ‒0.43% (95%CI: -4.42, 3.56) confirmed immune equivalence with corresponding seroconversion rates of 98.38% (95%CI: 95.91, 99.37) and 98.81% (95%CI: 96.56, 99.59) in MD and SD Vi-DT formulations, respectively (p=0.722). Both formulations of Vi-DT had a satisfactory safety profile - all five serious adverse events reported during the study were unrelated to the investigational product.

Interpretation

The MD and SD formulations of Vi-DT elicited robust and equivalent immune responses following one dose vaccination, and both formulations demonstrated a favorable safety profile.

Trial Registration

ClinicalTrials.gov: NCT04204096.

Funding

This study was funded by the Bill & Melinda Gates Foundation (OPP 1115556).

Shigella-Controlled Human Infection Models: Current and Future Perspectives

Shigella-controlled human infection models (CHIMs) are an invaluable tool utilized by the vaccine community to combat one of the leading global causes of infectious diarrhea, which affects infants, children and adults regardless of socioeconomic status. The impact of shigellosis disproportionately affects children in low- and middle-income countries (LMICs) resulting in cognitive and physical stunting, perpetuating a cycle that must be halted. Shigella-CHIMs not only facilitate the early evaluation of enteric countermeasures and up-selection of the most promising products but also provide insight into mechanisms of infection and immunity that are not possible utilizing animal models or in vitro systems. The greater understanding of shigellosis obtained in CHIMs builds and empowers the development of new generation solutions to global health issues which are unattainable in the conventional laboratory and clinical settings. Therefore, refining, mining and expansion of safe and reproducible infection models hold the potential to create effective means to end diarrheal disease and associated co-morbidities associated with Shigella infection.

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.

Capsules and Extracellular Polysaccharides in Escherichia coli and Salmonella

Escherichia coli and Salmonella isolates produce a range of different polysaccharide structures that play important roles in their biology. E. coli isolates often possess capsular polysaccharides (K antigens), which form a surface structural layer. These possess a wide range of repeat-unit structures. In contrast, only one capsular polymer (Vi antigen) is found in Salmonella, and it is confined to typhoidal serovars. In both genera, capsules are vital virulence determinants and are associated with the avoidance of host immune defenses. Some isolates of these species also produce a largely secreted exopolysaccharide called colanic acid as part of their complex Rcs-regulated phenotypes, but the precise function of this polysaccharide in microbial cell biology is not fully understood. E. coli isolates produce two additional secreted polysaccharides, bacterial cellulose and poly-N-acetylglucosamine, which play important roles in biofilm formation. Cellulose is also produced by Salmonella isolates, but the genes for poly-N-acetylglucosamine synthesis appear to have been lost during its evolution toward enhanced virulence. Here, we discuss the structures, functions, relationships, and sophisticated assembly mechanisms for these important biopolymers.

Building a better antibody through the Fc: advances and challenges in harnessing antibody Fc effector functions for antiviral protection

Antibodies can provide antiviral protection through neutralization and recruitment of innate effector functions through the Fc domain. While neutralization has long been appreciated for its role in antibody-mediated protection, a growing body of work indicates that the antibody Fc domain also significantly contributes to antiviral protection. Recruitment of innate immune cells such as natural killer cells, neutrophils, monocytes, macrophages, dendritic cells and the complement system by antibodies can lead to direct restriction of viral infection as well as promoting long-term antiviral immunity. Monoclonal antibody therapeutics against viruses are increasingly incorporating Fc-enhancing features to take advantage of the Fc domain, uncovering a surprising breadth of mechanisms through which antibodies can control viral infection. Here, we review the recent advances in our understanding of antibody-mediated innate immune effector functions in protection from viral infection and review the current approaches and challenges to effectively leverage innate immune cells via antibodies.

Salmonella Typhi Vi capsule prime-boost vaccination induces convergent and functional antibody responses

Vaccine development to prevent Salmonella Typhi infections has accelerated over the past decade, resulting in licensure of new vaccines, which use the Vi polysaccharide (Vi PS) of the bacterium conjugated to an unrelated carrier protein as the active component. Antibodies elicited by these vaccines are important for mediating protection against typhoid fever. However, the characteristics of protective and functional Vi antibodies are unknown. In this study, we investigated the human antibody repertoire, avidity maturation, epitope specificity, and function after immunization with a single dose of Vi-tetanus toxoid conjugate vaccine (Vi-TT) and after a booster with plain Vi PS (Vi-PS). The Vi-TT prime induced an IgG1-dominant response, whereas the Vi-TT prime followed by the Vi-PS boost induced IgG1 and IgG2 antibody production. B cells from recipients who received both prime and boost showed evidence of convergence, with shared V gene usage and CDR3 characteristics. The detected Vi antibodies showed heterogeneous avidity ranging from 10 μM to 500 pM, with no evidence of affinity maturation after the boost. Vi-specific antibodies mediated Fc effector functions, which correlated with antibody dissociation kinetics but not with association kinetics. We identified antibodies induced by prime and boost vaccines that recognized subdominant epitopes, indicated by binding to the de–O-acetylated Vi backbone. These antibodies also mediated Fc-dependent functions, such as complement deposition and monocyte phagocytosis. Defining strategies on how to broaden epitope targeting for S. Typhi Vi and enriching for antibody Fc functions that protect against typhoid fever will advance the design of high-efficacy Vi vaccines for protection across diverse populations.

The Controlled Human Infection Model for Enterotoxigenic Escherichia coli

The controlled human infection model (CHIM) for enterotoxigenic Escherichia coli (ETEC) has been instrumental in defining ETEC as a causative agent of acute watery diarrhea, providing insights into disease pathogenesis and resistance to illness, and enabling preliminary efficacy evaluations for numerous products including vaccines, immunoprophylactics, and drugs. Over a dozen strains have been evaluated to date, with a spectrum of clinical signs and symptoms that appear to replicate the clinical illness seen with naturally occurring ETEC. Recent advancements in the ETEC CHIM have enhanced the characterization of clinical, immunological, and microbiological outcomes. It is anticipated that omics-based technologies applied to ETEC CHIMs will continue to broaden our understanding of host-pathogen interactions and facilitate the development of primary and secondary prevention strategies.

Persistence of Antibody After a Vi-Tetanus Toxoid Conjugate Vaccine and Effect of Boosting With a Plain Polysaccharide Vaccine on Vi Antibody and Antigen-Specific B Cells

Background

Salmonella enterica serovar Typhi is estimated to cause 9 to 13 million cases of typhoid fever annually. Typhoid conjugate vaccines represent a promising prophylactic measure to prevent disease, but there are few data assessing persistence of immunity. The effect of a Vi polysaccharide booster vaccine in individuals previously vaccinated with the Vi-tetanus toxoid typhoid conjugate vaccine has not been assessed previously.

Methods

Thirty five healthy adult volunteers received a single dose of the Vi conjugate vaccine (Vi-TT) and 37 received a single dose of Vi polysaccharide vaccine (Vi-PS) prior to oral challenge with live S. Typhi bacteria as part of a randomised controlled, phase 2b study. In addition to data previously published showing persistence of Vi IgG and IgA antibodies for 7 months after Vi vaccination, titres were measured at intervals until 13 months post-vaccination. Ten participants who received Vi-TT (both challenged and unchallenged) were re-vaccinated with Vi-PS at an interval of 19-23 months post-prime. Anti-Vi IgG and IgA titres, and Vi-specific antibody secreting cells and memory B cells were measured at seven days and one month post-boost.

Findings

Vi IgG and IgA antibody titres remained significantly elevated above baseline levels 13 months after priming with Vi-TT, with a 4-fold rise retained in 90% and 88% of recipients (Vi IgG and IgA, respectively). Anti-Vi IgG and IgA antibody titres were found to persist at higher levels in participants who received a single dose of Vi-TT than in those who received Vi-PS. No significant boost in Vi-antibody titre was observed in response to oral challenge with S. Typhi bacteria, one month after vaccination. Following a Vi-PS booster vaccination in those previously vaccinated with Vi-TT, anti-Vi IgG and IgA titres were significantly elevated, with similar titres observed at one month post-boost compared with one month after primary vaccination. The frequency of Vi-specific IgA antibody secreting cells increased significantly 7 days post-boost compared with pre-boost. No memory B cell response was observed following Vi-PS booster vaccination.

Interpretation

Strong persistence of anti-Vi IgG and IgA following Vi-TT vaccination suggests that the conjugate vaccine may offer durable protection, supporting its use in endemic settings.

Association of Antibody-Dependent Neutrophil Phagocytosis With Distinct Antibody Glycosylation Profiles Following Typhoid Vaccination

Typhoid Vi-conjugate vaccines (Vi-TCV) have been developed to control typhoid fever in children in endemic regions. Previously, in a human challenge model of typhoid, Vi-TCV was administered prior to deliberate ingestion of Salmonella Typhi by healthy adult volunteers in the UK. Vi-specific antibody-dependent neutrophil phagocytosis (ADNP) was associated with protection against enteric fever in this model, but it is not known if ADNP is induced by vaccination of children. We measured ADNP in a cohort of Nepalese children receiving a Vi-TCV in a field study to investigate whether functional antibody responses were also present in children in an endemic setting. Furthermore, we investigated relationships between the functional antibody measures and other properties of the antibody response, including Vi-IgG and IgA titres, and Fc region glycosylation. Antibody-dependent neutrophil phagocytosis significantly increased in children aged 9 months to 15 years between the day of vaccination and 28 days following administration of Vi-TCV (D28). The magnitude of ADNP was also comparable with the levels of ADNP induced by plasma from vaccinated UK adults. Neither IgG nor IgA antibody titres significantly correlated with ADNP scores at D28; however, increased vaccine-induced ADNP was associated with decreased levels of IgG1 sialylation. These data suggest that vaccination with Vi-TCV produces functional antibody responses in children, which associate with specific glycosylation patterns of the Fc region.

Immunogenicity and efficacy of live-attenuated Salmonella Typhimurium vaccine candidate CVD 1926 in a rhesus macaque model of gastroenteritis

Salmonella Typhimurium are a common cause of food-borne gastroenteritis, and a less frequent but important cause of invasive disease, especially in developing countries. In our previous work, we showed that a live-attenuated S. Typhimurium vaccine (CVD 1921) was safe and immunogenic in rhesus macaques, although shed for an unacceptably long period (10 days) post-immunization. Consequently, we engineered a new strain, CVD 1926, which was shown to be safe and immunogenic in mice, as well as less reactogenic in mice and human cell-derived organoids than CVD 1921. In this study, we assessed reactogenicity and efficacy of CVD 1926 in rhesus macaques. Animals were given two doses of either CVD 1926 or saline perorally. The vaccine was well-tolerated, with shedding in stool limited to a mean of 5 days. All CVD 1926 immunized animals made both a serological and a T cell response to vaccination. At four weeks post-immunization, animals were challenged with wild-type S. Typhimurium I77. Unvaccinated (saline) animals had severe diarrhea, with two animals succumbing to infection. Animals receiving CVD 1926 were largely protected, with only one animal having moderate diarrhea. Vaccine efficacy in this gastroenteritis model was 80%. S. Typhimurium vaccine strain CVD 1926 was safe and effective in rhesus macaques and shed for a shorter period than other previously tested live-attenuated vaccine strains. This strain could be combined with other live-attenuated Salmonella vaccine strains to create a pan-Salmonella vaccine.

A Salmonella Typhi Controlled Human Infection Study for Assessing Correlation between Bactericidal Antibodies and Protection against Infection Induced by Typhoid Vaccination

Vi-polysaccharide conjugate vaccines are efficacious against typhoid fever in children living in endemic settings, their recent deployment is a promising step in the control of typhoid fever. However, there is currently no accepted correlate of protection. IgG and IgA antibodies generated in response to Vi conjugate or Vi plain polysaccharide vaccination are important but there are no definitive protective titre thresholds. We adapted a luminescence-based serum bactericidal activity (SBA) for use with S. Typhi and assessed whether bactericidal antibodies induced by either Vi tetanus toxoid conjugate (Vi-TT) or Vi plain polysaccharide (Vi-PS) were associated with protection in a controlled human infection model of typhoid fever. Both Vi-PS and Vi-TT induced significant increase in SBA titre after 28 days (Vi-PS; p < 0.0001, Vi-TT; p = 0.003), however higher SBA titre at the point of challenge did not correlate with protection from infection or reduced symptom severity. We cannot eliminate the role of SBA as part of a multifactorial immune response which protects against infection, however, our results do not support a strong role for SBA as a mechanism of Vi vaccine mediated protection in the CHIM setting.

Host restriction, pathogenesis and chronic carriage of typhoidal Salmonella

While conjugate vaccines against typhoid fever have recently been recommended by the World Health Organization for deployment, the lack of a vaccine against paratyphoid, multidrug resistance and chronic carriage all present challenges for the elimination of enteric fever. In the past decade, the development of in vitro and human challenge models has resulted in major advances in our understanding of enteric fever pathogenesis. In this review, we summarise these advances, outlining mechanisms of host restriction, intestinal invasion, interactions with innate immunity and chronic carriage, and discuss how this knowledge may progress future vaccines and antimicrobials.