Induction of long term mucosal immunological memory in humans by an oral inactivated multivalent enterotoxigenic Escherichia coli vaccine

Colonization factors of human and animal-specific enterotoxigenic Escherichia coli (ETEC)

Colonization factors (CFs) are major virulence factors of enterotoxigenic Escherichia coli (ETEC). This pathogen is among the most common causes of bacterial diarrhea in children in low- and middle-income countries, travelers, and livestock. CFs are major candidate antigens in vaccines under development as preventive measures against ETEC infections in humans and livestock. Recent molecular studies have indicated that newly identified CFs on human ETEC are closely related to animal ETEC CFs. Increased knowledge of pathogenic mechanisms, immunogenicity, regulation, and expression of ETEC CFs, as well as the possible spread of animal ETEC to humans, may facilitate the future development of ETEC vaccines for humans and animals. Here, we present an updated review of CFs in ETEC.

A Perspective on the Strategy for Advancing ETVAX®, An Anti-ETEC Diarrheal Disease Vaccine, into a Field Efficacy Trial in Gambian Children: Rationale, Challenges, Lessons Learned, and Future Directions

For the first time in over 20 years, an Enterotoxigenic Escherichia coli (ETEC) vaccine candidate, ETVAX®, has advanced into a phase 2b field efficacy trial for children 6-18 months of age in a low-income country. ETVAX® is an inactivated whole cell vaccine that has gone through a series of clinical trials to provide a rationale for the design elements of the Phase 2b trial. This trial is now underway in The Gambia and will be a precursor to an upcoming pivotal phase 3 trial. To reach this point, numerous findings were brought together to define factors such as safe and immunogenic doses for children, and the possible benefit of a mucosal adjuvant, double mutant labile toxin (dmLT). Considering the promising but still underexplored potential of inactivated whole cells in oral vaccination, we present a perspective compiling key observations from past ETVAX® trials that informed The Gambian trial design. This report will update the trial's status and explore future directions for ETEC vaccine trials. Our aim is to provide not only an update on the most advanced ETEC vaccine candidate but also to offer insights beneficial for the development of other much-needed oral whole-cell vaccines against enteric and other pathogens.

Safety and immunogenicity of ETVAX®, an oral inactivated vaccine against enterotoxigenic Escherichia coli diarrhoea: a double-blinded, randomized, placebo-controlled trial amongst Finnish travellers to Benin, West Africa

BACKGROUND

No licensed human vaccines are available against enterotoxigenic Escherichia coli (ETEC), a major diarrhoeal pathogen affecting children in low- and middle-income countries and foreign travellers alike. ETVAX®, a multivalent oral whole-cell vaccine containing four inactivated ETEC strains and the heat-labile enterotoxin B subunit (LTB), has proved promising in Phase 1 and Phase 1/ 2 studies.

METHODS

We conducted a Phase 2b double-blinded, randomized, placebo-controlled trial amongst Finnish travellers to Benin, West Africa. This report presents study design and safety and immunogenicity data. Volunteers aged 18-65 years were randomized 1:1 to receive ETVAX® or placebo. They visited Benin for 12 days, provided stool and blood samples and completed adverse event (AE) forms. IgA and IgG antibodies to LTB and O78 lipopolysaccharide (LPS) were measured by electrochemiluminescence.

RESULTS

The AEs did not differ significantly between vaccine (n = 374) and placebo (n = 375) recipients. Of the solicited AEs, loose stools/diarrhoea (26.7/25.9%) and stomach ache (23.0/20.0%) were reported most commonly. Of all possibly/probably vaccine-related AEs, the most frequent were gastrointestinal symptoms (54.0/48.8%) and nervous system disorders (20.3/25.1%). Serious AEs were recorded for 4.3/5.6%, all unlikely to be vaccine related. Amongst the ETVAX® recipients, LTB-specific IgA antibodies increased 22-fold. For the 370/372 vaccine/placebo recipients, the frequency of ≥2-fold increases against LTB was 81/2.4%, and against O78 LPS 69/2.7%. The majority of ETVAX® recipients (93%) responded to either LTB or O78.

CONCLUSIONS

This Phase 2b trial is the largest on ETVAX® undertaken amongst travellers to date. ETVAX® showed an excellent safety profile and proved strongly immunogenic, which encourages the further development of this vaccine.

Developments in oral enterotoxigenic Escherichia coli vaccines

Enterotoxigenic Escherichia coli (ETEC) is a leading cause of diarrhea in children in developing countries and in travelers. WHO has affirmed ETEC as a priority vaccine target, but there is no licensed ETEC vaccine available yet. We here describe recent, promising developments of different live, inactivated, and subunit ETEC candidate vaccines expressing or containing nontoxic enterotoxin and/or colonization factor antigens with a focus on oral vaccines. Many of the ETEC candidate vaccines have been tested in clinical trials for safety and immunogenicity and some of them also for protective efficacy in field trials or in challenge studies.

A Shigella flexneri 2a synthetic glycan-based vaccine induces a long-lasting immune response in adults

Shigella is a leading cause of moderate to severe diarrhea worldwide and of diarrhea-associated deaths in children under 5 years of age in low-and middle-income countries. A vaccine against shigellosis is in high demand. SF2a-TT15, a synthetic carbohydrate-based conjugate vaccine candidate against Shigella flexneri 2a (SF2a) was found safe and strongly immunogenic in adult volunteers. Here, SF2a-TT15 at 10 µg oligosaccharide (OS) vaccine dose is shown to induce a sustained immune response in magnitude and functionality in the majority of volunteers followed up 2 and 3 years post-vaccination. High levels of either one of the humoral parameters as well as the number of specific-IgG memory B-cells determined 3 months after vaccination were good predictors of the durability of the immune response. This study is the first to examine the long-term durability of antibody functionality and memory B-cell response induced by a Shigella vaccine candidate.

Route and antigen shape immunity to dmLT-adjuvanted vaccines to a greater extent than biochemical stress or formulation excipients

A key aspect to vaccine efficacy is formulation stability. Biochemical evaluations provide information on optimal compositions or thermal stability but are routinely validated by ex vivo analysis and not efficacy in animal models. Here we assessed formulations identified to improve or reduce stability of the mucosal adjuvant dmLT being investigated in polio and enterotoxigenic E. coli (ETEC) clinical vaccines. We observed biochemical changes to dmLT protein with formulation or thermal stress, including aggregation or subunit dissociation or alternatively resistance against these changes with specific buffer compositions. However, upon injection or mucosal vaccination with ETEC fimbriae adhesin proteins or inactivated polio virus, experimental findings indicated immunization route and co-administered antigen impacted vaccine immunogenicity more so than dmLT formulation stability (or instability). These results indicate the importance of both biochemical and vaccine-derived immunity assessment in formulation optimization. In addition, these studies have implications for use of dmLT in clinical settings and for delivery in resource poor settings.

Oral inactivated whole cell vaccine for mucosal immunization: ETVAX case study

Oral immunization is an effective strategy for inducing protective immunity against mucosal enteric pathogens. Although live-attenuated as well as subunit approaches have been explored for vaccination against enteric pathogens, inactivated whole bacterial cells may also be effective in introducing protective immunity. Successfully accomplishing this goal with inactivated whole bacterial cells will require that a complex antigenic repertoire be presented in controlled immunogenic amounts, in a safe and relatively simple and self-contained delivery format. The benefit from immunization with whole cell vaccines can be further enhanced through genetic engineering to over-express selected antigens and also by the use of mucosal adjuvants to direct a more robust immunologic response. These steps are being taken for the development of ETVAX, the most clinically advanced vaccine candidate against the major enteric pathogen, enterotoxigenic Escherichia coli (ETEC) with significant positive impact.

An Age-stratified, Randomized Immunogenicity Trial of Killed Oral Cholera Vaccine with Delayed Second Dose in Cameroon

The recommended schedule for killed oral cholera vaccine (OCV) is two doses, 2 weeks apart. However, during vaccine campaigns, the second round is often delayed by several months. Because more information is needed to document antibody responses when the second dose is delayed, we conducted an open-label, phase 2, noninferiority clinical trial of OCV. One hundred eighty-six participants were randomized into three dose-interval groups (DIGs) to receive the second dose 2 weeks, 6 months, or 11.5 months after the first dose. The DIGs were stratified into three age strata: 1 to 4, 5 to 14, and > 14 years. Inaba and Ogawa vibriocidal titers were assessed before and after vaccination. The primary analysis was geometric mean titer (GMT) 2 weeks after the second dose. Data for primary analysis was available from 147 participants (54, 44, and 49 participants from the three DIGs respectively). Relative to the 2-week interval, groups receiving a delayed second dose had significantly higher GMTs after the second dose. Two weeks after the second dose, Inaba GMTs were 55.1 190.3, and 289.8 and Ogawa GMTs were 70.4, 134.5, and 302.4 for the three DIGs respectively. The elevated titers were brief, returning to lower levels within 3 months. We conclude that when the second dose of killed oral cholera vaccine was given after 6 or 11.5 months, vibriocidal titers were higher than when given after the standard period of 2 weeks. This provides reassurance that a delayed second dose does not compromise, but rather enhances, the serological response to the vaccine.

Immune Response after Skin Delivery of a Recombinant Heat-Labile Enterotoxin B Subunit of Enterotoxigenic Escherichia coli in Mice

Enterotoxigenic Escherichia coli (ETEC) infections have been identified as a major cause of acute diarrhoea in children in developing countries, associated with substantial morbidity and mortality rates. Additionally, ETEC remains the most common cause of acute diarrhea of international travellers to endemic areas. The heat-labile toxin (LT) is a major virulence factor of ETEC, with a significant correlation between the presence of antibodies against LT and protection in infected patients. In the present work, we constructed a recombinant LTB unit (rLTB) and studied the capacity of this toxoid incorporated in microneedles (rLTB-MN) to induce a specific immune response in mice. MN were prepared from aqueous blends of the polymer Gantrez AN^® [poly (methyl vinyl ether-co-maleic anhydride)], which is not cytotoxic and has been shown to possess immunoadjuvant properties. The mechanical and dissolution properties of rLTB-MNs were evaluated in an in vitro Parafilm M^® model and in mice and pig skin ex vivo models. The needle insertion ranged between 378 µm and 504 µm in Parafilm layers, and MNs fully dissolved within 15 min of application inside porcine skin. Moreover, female and male BALB/c mice were immunized through ear skin with one single dose of 5 μg·rLTB in MNs, eliciting significant fecal anti-LT IgA antibodies, higher in female than in male mice. Moreover, we observed an enhanced production of IL-17A by spleen cells in the immunized female mice, indicating a mucosal non-inflammatory and neutralizing mediated response. Further experiments will now be required to validate the protective capacity of this new rLTB-MN formulation against this deadly non-vaccine-preventable disease.

Induction of mucosal and systemic immune responses against the common O78 antigen of an oral inactivated ETEC vaccine in Bangladeshi children and infants

We tested an oral enterotoxigenic Escherichia coli (ETEC) vaccine, ETVAX, consisting of inactivated E. coli overexpressing the most prevalent ETEC colonization factors (CFs) and a toxoid (LCTBA), in Bangladeshi children for capacity to induce mucosal and plasma immune responses against O78 lipopolysaccharide (LPS) expressed on the vaccine strains. The vaccine was given ± double-mutant heat-labile toxin (dmLT) adjuvant. We evaluated the impact of dmLT on anti-O78 LPS immune responses and whether such responses can predict responses against the CFs as a marker for vaccine "take". Two fractionated doses of ETVAX ± different amounts of dmLT were administered biweekly to groups of children 24-59 (n = 125), 12-23 (n = 97) and 6-11 (n = 158) months of age. Immune responses were evaluated in antibody in lymphocyte supernatants (ALS), fecal extracts and plasma. ALS IgA responses against O78 LPS were induced in 44-49% of the children aged 12-59 months. The magnitudes of the ALS responses were significantly higher in children receiving a half-dose (5 × 10¹⁰ bacteria) of ETVAX ± dmLT than in placebo recipients. <10% of the vaccinees aged 6-11 months mounted ALS responses against O78 LPS. However, 49% of the infants developed fecal secretory IgA responses which were significantly more frequent in those receiving a quarter-dose (2.5 × 10¹⁰ bacteria) of vaccine + dmLT (62%) compared to a quarter-dose alone (36%). Plasma IgA antibody responses were induced in 80% of older children and 36% of infants. The frequencies of O78 LPS responses in plasma and feces were comparable or higher than against the vaccine CFs in infants. Our findings show that ETVAX induced mucosal and systemic immune responses against O78 LPS in all age groups and that dmLT improved intestinal immune responses among infants. These observations may have implications for more successful use of other oral vaccines based on O antigens in children.

Confronting challenges to enterotoxigenic Escherichia coli vaccine development

The enterotoxigenic Escherichia coli (ETEC) are a diverse and genetically plastic pathologic variant (pathovar) of E. coli defined by their production of heat-labile (LT) and heat-stable (ST) enterotoxins. These pathogens, which came to recognition more than four decades ago in patients presenting with severe cholera-like diarrhea, are now known to cause hundreds of millions of cases of symptomatic infection annually. Children in low-middle income regions of the world lacking access to clean water and basic sanitation are disproportionately affected by ETEC. In addition to acute diarrheal morbidity, these pathogens remain a significant cause of mortality in children under the age of five years and have also been linked repeatedly to sequelae of childhood malnutrition and growth stunting. Vaccines that could prevent ETEC infections therefore remain a high priority. Despite several decades of effort, a licensed vaccine that protects against the breadth of these pathogens remains an aspirational goal, and the underlying genetic plasticity of E. coli has posed a fundamental challenge to development of a vaccine that can encompass the complete antigenic spectrum of ETEC. Nevertheless, novel strategies that include toxoids, a more complete understanding of ETEC molecular pathogenesis, structural details of target immunogens, and the discovery of more highly conserved antigens essential for virulence should accelerate progress and make a broadly protective vaccine feasible.

Mucosal immune responses against an oral ETEC vaccine evaluated in clinical trials

Enterotoxigenic Escherichia coli (ETEC) is a leading cause of mortality and morbidity in children in low-income countries. We have tested an oral ETEC vaccine, ETVAX, consisting of inactivated E coli overexpressing the most prevalent colonization factors and a toxoid, LCTBA, administered together with a mucosal adjuvant, double-mutant heat-labile toxin (dmLT), for capacity to induce mucosal immune responses and immunological memory against the primary vaccine antigens, ie, colonization factors, heat-labile toxin B-subunit and O antigen. The studies show that ETVAX could induce strong intestine-derived and/or fecal immune responses in a majority of vaccinated Swedish adults and in different age groups, including infants, in Bangladesh.

Long-term antibody response and protective effect induced by attenuated scorpion toxins: Involvement of memory plasma cells

In Algeria, Androctonus australis hector scorpion envenomation remains a major problem of public health because of non-efficient therapy. The development of safe vaccine against scorpion venom could be one key strategy for the envenomation prevention. The irradiation of venom by γ-rays develops suitable immunogens which produced effective antivenom and safe vaccine. In this study, we investigated the ability of the irradiated toxic fraction (γ-FtoxG50) to induce long-term memory humoral response in immunized animals (mice and rabbits), by involving the long-lived plasma cells to prevent efficiently the lethality of scorpion envenomation. For this purpose, an appropriate immunization schedule was established in mice and rabbits using three (3) similar doses of γ-FtoxG50 associated with Alum adjuvant. Obtained results indicate that the long-term immunogenicity of γ-FtoxG50 is able to induce the long-term memory humoral response with a high level of specific antibodies. The long-term persistence of antibody levels could depend on bone marrow memory plasma cells. These cells produce continuously antibodies without antigen stimulus. Furthermore, an enhanced memory response was obtained post-repeated envenomation with toxic native venom that leads to improved protection of animals. Together, pre-existing protective antibodies and the activation of memory B-cells could induce a rapid neutralization of scorpion toxins and long-term protection against scorpion envenomation.

Vaccines for Protecting Infants from Bacterial Causes of Diarrheal Disease

The global diarrheal disease burden for Shigella, enterotoxigenic Escherichia coli (ETEC), and Campylobacter is estimated to be 88M, 75M, and 75M cases annually, respectively. A vaccine against this target trio of enteric pathogens could address about one-third of diarrhea cases in children. All three of these pathogens contribute to growth stunting and have demonstrated increasing resistance to antimicrobial agents. Several combinations of antigens are now recognized that could be effective for inducing protective immunity against each of the three target pathogens in a single vaccine for oral administration or parenteral injection. The vaccine combinations proposed here would result in a final product consistent with the World Health Organization's (WHO) preferred product characteristics for ETEC and Shigella vaccines, and improve the vaccine prospects for support from Gavi, the Vaccine Alliance, and widespread uptake by low- and middle-income countries' (LMIC) public health stakeholders. Broadly protective antigens will enable multi-pathogen vaccines to be efficiently developed and cost-effective. This review describes how emerging discoveries for each pathogen component of the target trio could be used to make vaccines, which could help reduce a major cause of poor health, reduced cognitive development, lost economic productivity, and poverty in many parts of the world.

A Systems Biology Approach Identifies B Cell Maturation Antigen (BCMA) as a Biomarker Reflecting Oral Vaccine Induced IgA Antibody Responses in Humans

Vaccines against enteric diseases could improve global health. Despite this, only a few oral vaccines are currently available for human use. One way to facilitate such vaccine development could be to identify a practical and relatively low cost biomarker assay to assess oral vaccine induced primary and memory IgA immune responses in humans. Such an IgA biomarker assay could complement antigen-specific immune response measurements, enabling more oral vaccine candidates to be tested, whilst also reducing the work and costs associated with early oral vaccine development. With this in mind, we take a holistic systems biology approach to compare the transcriptional signatures of peripheral blood mononuclear cells isolated from volunteers, who following two oral priming doses with the oral cholera vaccine Dukoral®, had either strong or no vaccine specific IgA responses. Using this bioinformatical method, we identify TNFRSF17, a gene encoding the B cell maturation antigen (BCMA), as a candidate biomarker of oral vaccine induced IgA immune responses. We then assess the ability of BCMA to reflect oral vaccine induced primary and memory IgA responses using an ELISA BCMA assay on a larger number of samples collected in clinical trials with Dukoral® and the oral enterotoxigenic Escherichia coli vaccine candidate ETVAX. We find significant correlations between levels of BCMA and vaccine antigen-specific IgA in antibodies in lymphocyte secretion (ALS) specimens, as well as with proportions of circulating plasmablasts detected by flow cytometry. Importantly, our results suggest that levels of BCMA detected early after primary mucosal vaccination may be a biomarker for induction of long-lived vaccine specific memory B cell responses, which are otherwise difficult to measure in clinical vaccine trials. In addition, we find that ALS-BCMA responses in individuals vaccinated with ETVAX plus the adjuvant double mutant heat-labile toxin (dmLT) are significantly higher than in subjects given ETVAX only. We therefore propose that as ALS-BCMA responses may reflect the total vaccine induced IgA responses to oral vaccination, this BCMA ELISA assay could also be used to estimate the total adjuvant effect on vaccine induced-antibody responses, independently of antigen specificity, further supporting the usefulness of the assay.

The Intriguing Interaction of Escherichia coli with the Host Environment and Innovative Strategies To Interfere with Colonization: a Summary of the 2019 E. coli and the Mucosal Immune System Meeting

The third E. coli and the Mucosal Immune System (ECMIS) meeting was held at Ghent University in Belgium from 2 to 5 June 2019. It brought together an international group of scientists interested in mechanisms of colonization, host response, and vaccine development. ECMIS distinguishes itself from related meetings on these enteropathogens by providing a greater emphasis on animal health and disease and covering a broad range of pathotypes, including enterohemorrhagic, enteropathogenic, enterotoxigenic, enteroaggregative, and extraintestinal pathogenic Escherichia coli As it is well established that the genus Shigella represents a subspecies of E. coli, these organisms along with related enteroinvasive E. coli are also included. In addition, Tannerella forsythia, a periodontal pathogen, was presented as an example of a pathogen which uses its surface glycans for mucosal interaction. This review summarizes several highlights from the 2019 meeting and major advances to our understanding of the biology of these pathogens and their impact on the host.

Immunogenicity Evaluation of Chimeric Subunit Vaccine Comprising Adhesion Coli Surface Antigens from Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is the most common agent of diarrhea morbidity in developing countries. ETEC adheres to host intestinal epithelial cells via various colonization factors. The CooD and CotD proteins play a significant role in bacteria binding to the intestinal epithelial cells as adhesin tip subunits of CS1 and CS2 pili. The purpose here was to design a new construction containing cooD and cotD genes and use several types of bioinformatics software to predict the structural and immunological properties of the designed antigen. The fusion gene was synthesized with codon bias of E. coli in order to increase the expression level of the protein. The amino acid sequences, protein structure, and immunogenicity properties of potential antigens were analyzed in silico. The chimeric protein was expressed in E. coliBL21 (DE3). The antigenicity of the recombinant proteins was verified by Western blotting and ELISA. In order to assess the induced immunity, the immunized mice were challenged with wild-type ETEC by an intraperitoneal route. Immunological analyses showed the production of a high titer of IgG serum with no sign of serum-mucosal IgA antibody response. The result of the challenge assay showed that 30% of immunized mice survived. The results of this study showed that CooD-CotD recombinant protein can stimulate immunity against ETEC. The designed chimera could be a prototype for the subunit vaccine, which is worthy of further consideration.

U-Omp19 from Brucella abortus increases dmLT immunogenicity and improves protection against Escherichia coli heat-labile toxin (LT) oral challenge

•

Oral co-administration of dmLT with U-Omp19 increases dmLT immunogenicity.

•

U-Omp19 oral co-delivery with dmLT induces anti-LT antibody responses.

•

U-Omp19 co-administered with dmLT protects against oral challenge with LT.

•

U-Omp19 can allow antigen dose sparing by oral route.

•

U-Omp19 can be used as adjuvant in an oral vaccine formulation against ETEC.

Acute diarrhea disease caused by bacterial infections is a major global health problem. Enterotoxigenic Escherichia coli (ETEC) is one of the top causes of diarrhea-associated morbidity and mortality in young children and travelers to low-income countries. There are currently no licensed vaccines for ETEC. Induction of immunity at the site of entry of the bacteria is key to prevent infection. Current approaches to ETEC vaccines include a less toxic mutant form of E. coli heat-labile toxin (double-mutant heat-labile enterotoxin -dmLT-) with both antigenic and immunostimulatory properties. U-Omp19 is a protease inhibitor from Brucella spp. with immunostimulatory properties that has been used as oral adjuvant. In this work, we use U-Omp19 as adjuvant in an oral vaccine formulation against ETEC containing dmLT in outbred and inbred mice. To evaluate antigen dose sparing by U-Omp19 three different immunization protocols with three different doses of dmLT were evaluated. We demonstrated that U-Omp19 co-delivery increases anti-LT IgA in feces using a mid-dose of dmLT following a prime-boost protocol (after one or two boosts). Oral immunization with U-Omp19 induced protection against LT challenge when co-formulated with dmLT in CD-1 and BALB/c mice. Indeed, there was a significant increase in anti-LT IgG and IgA avidity after a single oral administration of dmLT plus U-Omp19 in comparison with dmLT delivered alone. Interestingly, sera from dmLT plus U-Omp19 vaccinated mice significantly neutralize LT effect on intestine inflammation in vivo compared with sera from the group immunized with dmLT alone. These results demonstrate the adjuvant capacity of U-Omp19 to increase dmLT immunogenicity by the oral route and support its use in an oral subunit vaccine formulation against ETEC.

Safety and immunogenicity of the oral, inactivated, enterotoxigenic Escherichia coli vaccine ETVAX in Bangladeshi children and infants: a double-blind, randomised, placebo-controlled phase 1/2 trial

Background

Enterotoxigenic Escherichia coli causes diarrhoea, leading to substantial mortality and morbidity in children, but no specific vaccine exists. This trial tested an oral, inactivated, enterotoxigenic E coli vaccine (ETVAX), which has been previously shown to be safe and highly immuongenic in Swedish and Bangladeshi adults. We tested the safety and immunogenicity of ETVAX, consisting of four E coli strains overexpressing the most prevalent colonisation factors (CFA/I, CS3, CS5, and CS6) and a toxoid (LCTBA) administered with or without a double-mutant heat-labile enterotoxin (dmLT) as an adjuvant, in Bangladeshi children.

Methods

We did a randomised, double-blind, placebo-controlled, dose-escalation, age-descending, phase 1/2 trial in Dhaka, Bangladesh. Healthy children in one of three age groups (24–59 months, 12–23 months, and 6–11 months) were eligible. Children were randomly assigned with block randomisation to receive either ETVAX, with or without dmLT, or placebo. ETVAX (half [5·5 × 10¹⁰ cells], quarter [2·5 × 10¹⁰ cells], or eighth [1·25 × 10¹⁰ cells] adult dose), with or without dmLT adjuvant (2·5 μg, 5·0 μg, or 10·0 μg), or placebo were administered orally in two doses 2 weeks apart. Investigators and participants were masked to treatment allocation. The primary endpoint was safety and tolerability, assessed in all children who received at least one dose of vaccine. Antibody responses to vaccine antigens, defined as at least a two-times increase in antibody levels between baseline and post-immunisation, were assessed as secondary endpoints. This trial is registered with ClinicalTrials.gov, NCT02531802.

Findings

Between Dec 7, 2015, and Jan 10, 2017, we screened 1500 children across the three age groups, of whom 430 were enrolled and randomly assigned to the different treatment groups (130 aged 24–59 months, 100 aged 12–23 months, and 200 aged 6–11 months). All participants received at least one dose of vaccine. No solicited adverse events occurred that were greater than moderate in severity, and most were mild. The most common solicited event was vomiting (ten [8%] of 130 patients aged 24–59 months, 13 [13%] of 100 aged 12–23 months, and 29 [15%] of 200 aged 6–11 months; mostly of mild severity), which appeared related to dose and age. The addition of dmLT did not modify the safety profile. Three serious adverse events occurred but they were not considered related to the study drug. Mucosal IgA antibody responses in lymphocyte secretions were detected against all primary vaccine antigens (CFA/I, CS3, CS5, CS6, and the LCTBA toxoid) in most participants in the two older age groups, whereas such responses to four of the five antigens were less frequent and of lower magnitude in infants aged 6–11 months than in older children. Faecal secretory IgA immune responses were recorded against all vaccine antigens in infants aged 6–11 months. 78 (56%) of 139 infants aged 6–11 months who were vaccinated developed mucosal responses against at least three of the vaccine antigens versus 14 (29%) of 49 of the infants given placebo. Addition of the adjuvant dmLT enhanced the magnitude, breadth, and kinetics (based on number of responders after the first dose of vaccine) of immune responses in infants.

Interpretation

The encouraging safety and immunogenicity of ETVAX and benefit of dmLT adjuvant in young children support its further assessment for protective efficacy in children in enterotoxigenic E coli-endemic areas.

Funding

PATH (Bill & Melinda Gates Foundation and the UK's Department for International Development), the Swedish Research Council, and The Swedish Foundation for Strategic Research.

Role of antigen specific T and B cells in systemic and mucosal immune responses in ETEC and Shigella infections, and their potential to serve as correlates of protection in vaccine development

The generation of robust systemic and mucosal antibody and cell-mediated immune (CMI) responses that are protective, long-lasting, and can quickly be recalled upon subsequent re-exposure to the cognate antigen is the key to the development of effective vaccine candidates. These responses, whether they represent mechanistic or non-mechanistic immunological correlates of protection, usually entail the activation of T cell memory and effector subsets (T-CMI) and induction of long-lasting memory B cells. However, for ETEC and Shigella, the precise role of these key immune cells in primary and secondary (anamnestic) immune responses remains ill-defined. A workshop to address immune correlates for ETEC and Shigella, in general, and to elucidate the mechanistic role of T-cell subsets and B-cells, both systemically and in the mucosal microenvironment, in the development of durable protective immunity against ETEC and Shigella was held at the recent 2nd Vaccines against Shigella and ETEC (VASE) conference in June 2018. This report is a summary of the presentations and the discussion that ensued at the workshop.

Heat-Stable Enterotoxins of Enterotoxigenic Escherichia coli and Their Impact on Host Immunity

Enterotoxigenic Escherichia coli (ETEC) are an important diarrhea-causing pathogen and are regarded as a global threat for humans and farm animals. ETEC possess several virulence factors to infect its host, including colonization factors and enterotoxins. Production of heat-stable enterotoxins (STs) by most ETEC plays an essential role in triggering diarrhea and ETEC pathogenesis. In this review, we summarize the heat-stable enterotoxins of ETEC strains from different species as well as the molecular mechanisms used by these heat-stable enterotoxins to trigger diarrhea. As recently described, intestinal epithelial cells are important modulators of the intestinal immune system. Thus, we also discuss the impact of the heat-stable enterotoxins on this role of the intestinal epithelium and how these enterotoxins might affect intestinal immune cells. Finally, the latest developments in vaccination strategies to protect against infections with ST secreting ETEC strains are discussed. This review might inform and guide future research on heat-stable enterotoxins to further unravel their molecular pathogenesis, as well as to accelerate vaccine design.

Different frequencies of memory B-cells induced by tetanus, botulinum, and heat-labile toxin binding domains

Along with robust immunogenicity, an ideal vaccine candidate should be able to produce a long lasting protection. In this regard, the frequency of memory B-cells is possibly an important factor in memory B-cell persistency and duration of immunological memory. On this basis, binding domains of tetanus toxin (HcT), botulinum type A1 toxin (HcA), and heat-labile toxin (LTB) were selected as antigen models that induced long-term, midterm and short-term immune memory, respectively. In the present study, the frequency of total memory B-cells after immunization with HcT, HcA and LTB antigens after 90 and 180 days, and also after one booster, in 190 days, was evaluated. The results showed a significant correlation between frequency of total memory B-cells and duration of humoral immunity. Compared to other antigens, the HcT antibody titers and HcT total memory B-cell populations were greater and persistent even after 6 months. At 6 months after the final immunization, all HcT- and HcA-immunized mice survived against tetanus and botulinum toxins, and also LT toxin binding to GM1 ganglioside was blocked in LTB-immunized mice. We conclude the frequency of memory B-cells and their duration are likely a key factor for vaccine memory duration.

Evaluation of the safety and immunogenicity of the oral inactivated multivalent enterotoxigenic Escherichia coli vaccine ETVAX in Bangladeshi adults in a double-blind, randomized, placebo-controlled Phase I trial using electrochemiluminescence and ELISA assays for immunogenicity analyses

The safety and immunogenicity of the second generation oral enterotoxigenic Escherichia coli (ETEC) vaccine ETVAX, consisting of inactivated recombinant E. coli strains over-expressing the colonization factors (CFs) CFA/I, CS3, CS5 and CS6 and the heat labile toxoid LCTBA, were evaluated in Bangladeshi volunteers. To enable analysis of antibody responses against multiple vaccine antigens for subsequent use in small sample volumes from children, a sensitive electrochemiluminescence (ECL) assay for analysis of intestine-derived antibody-secreting cell responses using the antibodies in lymphocyte secretions (ALS) assay was established using Meso Scale Discovery technology. Three groups of Bangladeshi adults (n = 15 per group) received two oral doses of ETVAX with or without double mutant LT (dmLT) adjuvant or placebo in the initial part of a randomized, double-blind, placebo-controlled, age-descending, dose-escalation trial. CF- and LTB-specific ALS and plasma IgA responses were analyzed by ECL and/or ELISA. ETVAX was safe and well tolerated in the adults. Magnitudes of IgA ALS responses determined by ECL and ELISA correlated well (r = 0.85 to 0.98 for the five primary antigens, P < 0.001) and ECL was selected as the ALS readout method. ALS IgA responses against each of the primary antigens were detected in 87-100% of vaccinees after the first and in 100% after the second vaccine dose. Plasma IgA responses against different CFs and LTB were observed in 62-93% and 100% of vaccinees, respectively. No statistically significant adjuvant effect of dmLT on antibody responses to any antigen was detected, but the overall antigenic breadth of the plasma IgA response tended to favor the adjuvanted vaccine when responses to 4 or more or 5 vaccine antigens were considered. Responses in placebo recipients were infrequent and mainly detected against single antigens. The promising results in adults supported testing ETVAX in descending age groups of children. ClinicalTrials.gov Identifier: NCT02531802.

Pathophysiology of environmental enteric dysfunction and its impact on oral vaccine efficacy

Environmental enteric dysfunction (EED) refers to a subclinical disorder of intestinal function common in tropical countries and in settings of poverty and economic disadvantage. The enteropathy that underlies this syndrome is characterized by mucosal inflammation and villus blunting mediated by T cell activation. Epithelial cell disruption and microbial translocation drive systemic inflammation. EED in young children is associated geographically with growth failure, malnutrition, and greatly impaired responses to oral vaccines, notably rotavirus and poliovirus vaccines. In this review, we describe the pathophysiology of EED and examine the evidence linking EED and oral vaccine failure. This evidence is far from conclusive. Although our understanding of EED is still sketchy, there is limited evidence of disturbed innate immunity, B cell disturbances including aggregation into lymphoid follicles, and autoantibody generation. Pathways of T cell activation and the possibility of dendritic cell anergy, which could help explain oral vaccine failure, require further work.

Vaccines as alternatives to antibiotics for food producing animals. Part 2: new approaches and potential solutions

Vaccines and other alternative products are central to the future success of animal agriculture because they can help minimize the need for antibiotics by preventing and controlling infectious diseases in animal populations. To assess scientific advancements related to alternatives to antibiotics and provide actionable strategies to support their development, the United States Department of Agriculture, with support from the World Organisation for Animal Health, organized the second International Symposium on Alternatives to Antibiotics. It focused on six key areas: vaccines; microbial-derived products; non-nutritive phytochemicals; immune-related products; chemicals, enzymes, and innovative drugs; and regulatory pathways to enable the development and licensure of alternatives to antibiotics. This article, the second part in a two-part series, highlights new approaches and potential solutions for the development of vaccines as alternatives to antibiotics in food producing animals; opportunities, challenges and needs for the development of such vaccines are discussed in the first part of this series. As discussed in part 1 of this manuscript, many current vaccines fall short of ideal vaccines in one or more respects. Promising breakthroughs to overcome these limitations include new biotechnology techniques, new oral vaccine approaches, novel adjuvants, new delivery strategies based on bacterial spores, and live recombinant vectors; they also include new vaccination strategies in-ovo, and strategies that simultaneously protect against multiple pathogens. However, translating this research into commercial vaccines that effectively reduce the need for antibiotics will require close collaboration among stakeholders, for instance through public–private partnerships. Targeted research and development investments and concerted efforts by all affected are needed to realize the potential of vaccines to improve animal health, safeguard agricultural productivity, and reduce antibiotic consumption and resulting resistance risks.

Activated T follicular helper-like cells are released into blood after oral vaccination and correlate with vaccine specific mucosal B-cell memory

T follicular helper (Tfh)-like cells with potent B-cell helping ability are mobilized into human circulation after parenteral vaccination and are generally held to reflect ongoing germinal center reactions. However, whether mucosal vaccination induces systemic Tfh responses and how such responses may relate to IgA production are unknown. We investigated the frequencies, phenotype and function of circulating Tfh-like CD4⁺CXCR5⁺ T cells (cTfh) in adults receiving an oral inactivated enterotoxigenic Escherichia coli vaccine. Subjects were classified as vaccine responders or weak/non-responders based on their intestine-derived antibody-secreting cell (ASC) IgA responses to major vaccine antigens. Oral immunization induced significantly increased proportions of cTfh cells expressing the cTfh activation marker inducible costimulator (ICOS) in ASC responders, but not in weak/non-responders. Vaccination also enhanced the expression of IL-21, Th17 markers and integrin β7 by activated cTfh cells, supporting functionality and gut homing potential. cTfh cells promoted total and vaccine specific IgA production from cocultured B cells. Magnitudes of cTfh responses assessed within a week after primary vaccinations correlated with memory intestine-derived vaccine specific IgA responses 1-2 years later. We conclude that activated ICOS⁺ Tfh-like cells are mobilized into blood after oral vaccination and may be used as biomarkers of vaccine specific mucosal memory in humans.

A Chimeric protein of CFA/I, CS6 subunits and LTB/STa toxoid protects immunized mice against enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia Coli (ETEC) strains are the commonest bacteria causing diarrhea in children in developing countries and travelers to these areas. Colonization factors (CFs) and enterotoxins are the main virulence determinants in ETEC pathogenesis. Heterogeneity of CFs is commonly considered the bottleneck to developing an effective vaccine. It is believed that broad spectrum protection against ETEC would be achieved by induced anti-CF and anti-enterotoxin immunity simultaneously. Here, a fusion antigen strategy was used to construct a quadrivalent recombinant protein called 3CL and composed of CfaB, a structural subunit of CFA/I, and CS6 structural subunits, LTB and STa toxoid of ETEC. Its anti-CF and antitoxin immunogenicity was then assessed. To achieve high-level expression, the 3CL gene was synthesized using E. coli codon bias. Female BALB/C mice were immunized with purified recombinant 3CL. Immunized mice developed antibodies that were capable of detecting each recombinant subunit in addition to native CS6 protein and also protected the mice against ETEC challenge. Moreover, sera from immunized mice also neutralized STa toxin in a suckling mouse assay. These results indicate that 3CL can induce anti-CF and neutralizing antitoxin antibodies along with introducing CFA/I as a platform for epitope insertion.

Cross-reactivity and avidity of antibody responses induced in humans by the oral inactivated multivalent enterotoxigenicEscherichia coli (ETEC) vaccine ETVAX

We investigated whether the oral inactivated, multivalent enterotoxigenic Escherichia coli (ETEC) vaccine ETVAX, consisting of four E. coli strains over-expressing the colonisation factors (CFs) CFA/I, CS3, CS5 and CS6, combined with the toxoid LCTBA, could induce cross-reactive antibodies to CFs related to the CFA/I and CS5 families. We also evaluated the avidity of vaccine induced antibodies against the toxoid and CFs. Cross-reactivity was analysed in mucosal (faecal and antibodies in lymphocyte supernatants, ALS) samples, and antibody avidity in serum and ALS samples, from two phase I trials: a primary vaccination study, where two oral doses of ETVAX were given±the double mutant heat labile toxin (dmLT) adjuvant at a 2-week interval, and a booster vaccination study, where a single booster dose of ETVAX was given 13-23months after primary vaccinations. We found that 65-90% of subjects who had responded to CFA/I in ALS or faecal specimens also developed cross-reactive antibodies to the related CFs tested, i.e. CS1, CS14 and CS17, and that approximately 80% of those responding to CS5 also responded to the closely related CS7. For subjects who had developed cross-reactive antibodies, the magnitudes of responses against vaccine CFs and related non-vaccine CFs were comparable. Using both a simple method of antibody avidity determination based on limiting antigen dilution, as well as a chaotropic ELISA method, we found that the avidity of serum and ALS antibodies to key vaccine antigens increased after a late booster dose compared to after primary vaccination. Our results suggest that the cross-reactive antibody responses against multiple CFs may result in expanded ETEC strain coverage of ETVAX and that repeated vaccinations induce vaccine-specific antibodies with increased binding capacity.

Changes in microbiome during and after travellers' diarrhea: what we know and what we do not

The study of bacterial pathogens has advanced from culture on plates to basic biochemical studies; to sequencing reference genomes and attempting to ascribe those qualities to the whole species; to evaluating complete communities based on the universal marker of the 16S rRNA gene; to now sequencing all available nucleic acids in any sample to characterize the community as a whole. These types of scientific advances highlight that there are multiple layers of information within any sample, and to some extent we are only now beginning to be able to identify, categorize and understand the complex interactions of the host, pathogen and microbiome. In most cases, we can only now generate the data to examine these samples, but the detailed understanding of the significance of the data is lacking. We anticipate that in the future these microbiota and community data will be able to be modelled, understood and potentially used as either markers of disease, or diagnostics. This review provides a brief overview of the state of the use of microbiome studies using Travellers Diarrhea samples and what those studies have told us. More importantly these studies highlight what is remaining to be elucidated.

An engineered food-grade Lactococcus lactis strain for production and delivery of heat-labile enterotoxin B subunit to mucosal sites

Background

Recent researches have been focusing on mucosal immune adjuvants, which play the key roles in mucosal immunization and have become the limitation for non-injected vaccine development. Escherichia coli heat-labile enterotoxin B subunit (LTB) was regarded as a promising mucosal adjuvant for its nontoxicity and potent activity. LTB preparation issues have always been recurring, in part owing to that the recombinant LTB expressed by E. coli does not act as its native form.

Results

We constructed an engineered Lactococcus lactis strain using a food-grade expression system. The LTB secreted by the engineered strain was detected in the culture supernatant, constituting 10.3% of the supernatant proteins, and recognized by mouse anti-LTB antibodies. The engineered strain, co-administered orally to SPF BALB/c mice with a H. pylori vaccine candidate expressing Lpp20 antigen, could significantly enhance the Lpp20-induced mucosal SIgA antibody responses against H. pylori.

Conclusions

This is the first report that LTB was efficiently produced and delivered via using a food-grade lactococcal expression system, which offers a novel production and utilization mode of this crucial mucosal adjuvant. The engineered L. lactis strain secreting LTB has considerable potential for oral vaccine formulation owing to its outstanding safety, adjuvant activity and high-level production.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-017-0345-6) contains supplementary material, which is available to authorized users.