Development and preclinical evaluation of safety and immunogenicity of an oral ETEC vaccine containing inactivated E. coli bacteria overexpressing colonization factors CFA/I, CS3, CS5 and CS6 combined with a hybrid LT/CT B subunit antigen, administered alone and together with dmLT adjuvant. Vaccine. 2013;31:2457-2464. [PMID: 23541621 DOI: 10.1016/j.vaccine.2013.03.027]

Potential for a Combined Oral Inactivated Whole-Cell Vaccine Against ETEC and Shigella: Preclinical Studies Supporting Feasibility

BACKGROUND

Enteric disease caused by Shigella, Campylobacter, and enterotoxigenic Escherichia coli (ETEC) represents a significant global health burden, particularly among children in low-resource settings. However, no licensed vaccines are currently available for these bacterial pathogens. Given the wide range of enteric pathogens and the constraints posed by an increasingly crowded infant immunization schedule, the development of combination vaccines or combined administration of individual oral vaccines may offer a practical approach to address this unmet need.

OBJECTIVES

In this study, we evaluated the combined administration of two multicomponent oral vaccine candidates: ETVAX, targeting ETEC, and a trivalent whole-cell vaccine targeting Shigella.

METHODS

The vaccine candidates were administered orally in mice, both individually and in combination, with and without the inclusion of the double-mutant heat-labile toxin (dmLT) adjuvant.

RESULTS

The results demonstrated systemic and intestinal-mucosal immune responses to the key protective antigens following both individual and combined vaccine administration. Importantly, the combination of the two vaccines did not compromise the elicitation of specific antibody responses. The inclusion of dmLT as an adjuvant significantly enhanced immune responses to several antigens, highlighting its potential to improve vaccine efficacy.

CONCLUSIONS

These findings underscore the feasibility of combining ETEC and Shigella vaccine candidates into a single formulation without compromising immunogenicity. This combined approach has the potential to provide broad protective coverage, thereby mitigating the global impact of enteric diseases and streamlining vaccine delivery within existing childhood immunization programs. Our results support further development of this combination vaccine strategy as a promising tool in combating enteric infections and improving health outcomes, particularly among young children in endemic regions who are vulnerable to enteric disease.

Development of protective egg yolk immunoglobulins (IgY) targeting CfaB, LTB, and EtpA recombinant proteins of Enterotoxigenic Escherichia coli (ETEC) for inhibiting toxin activity and bacterial adherence

Enterotoxigenic Escherichia coli (ETEC) stands as a prevalent bacterial cause of global diarrheal incidents. ETEC's primary virulence factors encompass the B subunit of the Heat Labile Enterotoxin, along with the adhesion factors CfaB and EtpA. In this study, we isolated IgY antibodies against the three virulence factors individually, in pairs, and as triple cocktails. The in vitro efficacy of these IgY antibodies was examined, focusing on inhibiting heat-labile enterotoxin (LT) toxin cytotoxicity and impeding ETEC adherence to HT29 cells. Assessing the impact of IgY-treated bacteria on intestinal epithelial cells utilized the standard ileal loop method. Results demonstrated that the anti-LTB IgY antibody at 125 µg/ml and IgY antibodies from double and tertiary cocktails at 200 µg/ml effectively inhibited LT toxin attachment to the Y1 cell line. Pre-incubation of HT29 intestinal cells with specific IgYs reduced bacterial attachment by 59.7%. In the ileal loop test, toxin neutralization with specific IgYs curtailed the toxin's function in the intestine, leading to a 74.8% reduction in fluid accumulation compared to control loops. These findings suggest that egg yolk immunoglobulins against recombinant proteins LTB, CfaB, and EtpA, either individually or in combination, hold promise as prophylactic antibodies to impede the functioning of ETEC bacteria.

Targeting Enterotoxins: Advancing Vaccine Development for Enterotoxigenic Escherichia coli ETEC

Enterotoxigenic Escherichia coli (ETEC) is a major cause of diarrheal disease worldwide, particularly in children in low- and middle-income countries. Its ability to rapidly colonize the intestinal tract through diverse colonization factors and toxins underpins its significant public health impact. Despite extensive research and several vaccine candidates reaching clinical trials, no licensed vaccine exists for ETEC. This review explores the temporal and spatial coordination of ETEC virulence factors, focusing on the interplay between adherence mechanisms and toxin production as critical targets for therapeutic intervention. Advancements in molecular biology and host-pathogen interaction studies have uncovered species-specific variations and cross-reactivity between human and animal strains. In particular, the heat-labile (LT) and heat-stable (ST) toxins have provided crucial insights into molecular mechanisms and intestinal disruption. Additional exotoxins, such as EAST-1 and hemolysins, further highlight the multifactorial nature of ETEC pathogenicity. Innovative vaccine strategies, including multiepitope fusion antigens (MEFAs), mRNA-based approaches, and glycoconjugates, aim to enhance broad-spectrum immunity. Novel delivery methods, like intradermal immunization, show promise in eliciting robust immune responses. Successful vaccination against ETEC will offer an effective and affordable solution with the potential to greatly reduce mortality and prevent stunting, representing a highly impactful and cost-efficient solution to a critical global health challenge.

Protein Expression Platforms and the Challenges of Viral Antigen Production

Several protein expression platforms exist for a wide variety of biopharmaceutical needs. A substantial proportion of research and development into protein expression platforms and their optimization since the mid-1900s is a result of the production of viral antigens for use in subunit vaccine research. This review discusses the seven most popular forms of expression systems used in the past decade-bacterial, insect, mammalian, yeast, algal, plant and cell-free systems-in terms of advantages, uses and limitations for viral antigen production in the context of subunit vaccine research. Post-translational modifications, immunogenicity, efficacy, complexity, scalability and the cost of production are major points discussed. Examples of licenced and experimental vaccines are included along with images which summarize the processes involved.

A Review of Current Achievements and Recent Challenges in Bacterial Medium-Chain-Length Polyhydroxyalkanoates: Production and Potential Applications

Using petroleum-derived plastics has contributed significantly to environmental issues, such as greenhouse gas emissions and the accumulation of plastic waste in ecosystems. Researchers have focused on developing ecofriendly polymers as alternatives to traditional plastics to address these concerns. This review provides a comprehensive overview of medium-chain-length polyhydroxyalkanoates (mcl-PHAs), biodegradable biopolymers produced by microorganisms that show promise in replacing conventional plastics. The review discusses the classification, properties, and potential substrates of less studied mcl-PHAs, highlighting their greater ductility and flexibility compared to poly(3-hydroxybutyrate), a well-known but brittle PHA. The authors summarize existing research to emphasize the potential applications of mcl-PHAs in biomedicine, packaging, biocomposites, water treatment, and energy. Future research should focus on improving production techniques, ensuring economic viability, and addressing challenges associated with industrial implementation. Investigating the biodegradability, stability, mechanical properties, durability, and cost-effectiveness of mcl-PHA-based products compared to petroleum-based counterparts is crucial. The future of mcl-PHAs looks promising, with continued research expected to optimize production techniques, enhance material properties, and expand applications. Interdisciplinary collaborations among microbiologists, engineers, chemists, and materials scientists will drive progress in this field. In conclusion, this review serves as a valuable resource to understand mcl-PHAs as sustainable alternatives to conventional plastics. However, further research is needed to optimize production methods, evaluate long-term ecological impacts, and assess the feasibility and viability in various industries.

The colonization factor CS6 of enterotoxigenic Escherichia coli contributes to host cell invasion

Enterotoxigenic Escherichia coli (ETEC) is one of the main causes of diarrhea in children and travelers in low-income regions. The virulence of ETEC is attributed to its heat-labile and heat-stable enterotoxins, as well as its colonization factors (CFs). CFs are essential for ETEC adherence to the intestinal epithelium. However, its invasive capability remains unelucidated. In this study, we demonstrated that the CS6-positive ETEC strain 4266 can invade mammalian epithelial cells. The invasive capability was reduced in the 4266 ΔCS6 mutant but reintroduction of CS6 into this mutant restored the invasiveness. Additionally, the laboratory E. coli strain Top 10, which lacks the invasive capability, was able to invade Caco-2 cells after gaining the CS6-expressing plasmid pCS6. Cytochalasin D inhibited cell invasion in both 4266 and Top10 pCS6 cells, and F-actin accumulation was observed near the bacteria on the cell membrane, indicating that CS6-positive bacteria were internalized via actin polymerization. Other cell signal transduction inhibitors, such as genistein, wortmannin, LY294002, PP1, and Ro 32-0432, inhibited the CS6-mediated invasion of Caco-2 cells. The internalized bacteria of both 4266 and Top10 pCS6 strains were able to survive for up to 48 h, and 4266 cells were able to replicate within Caco-2 cells. Immunofluorescence microscopy revealed that the internalized 4266 cells were present in bacteria-containing vacuoles, which underwent a maturation process indicated by the recruitment of the early endosomal marker EEA-1 and late endosomal marker LAMP-1 throughout the infection process. The autophagy marker LC3 was also observed near these vacuoles, indicating the initiation of LC-3-associated phagocytosis (LAP). However, intracellular bacteria continued to replicate, even after the initiation of LAP. Moreover, intracellular filamentation was observed in 4266 cells at 24 h after infection. Overall, this study shows that CS6, in addition to being a major CF, mediates cell invasion. This demonstrates that once internalized, CS6-positive ETEC is capable of surviving and replicating within host cells. This capability may be a key factor in the extended and recurrent nature of ETEC infections in humans, thus highlighting the critical role of CS6.

MecVax supplemented with CFA MEFA-II induces functional antibodies against 12 adhesins (CFA/I, CS1-CS7, CS12, CS14, CS17, and CS21) and 2 toxins (STa, LT) of enterotoxigenic Escherichia coli (ETEC)

Enterotoxigenic Escherichia coli (ETEC) strains that produce various adhesins and one or two enterotoxins are the leading causes of children's diarrhea and travelers' diarrhea. MecVax, a multivalent ETEC vaccine candidate, consists of two proteins, an adhesin multiepitope fusion antigen (MEFA) that stimulates antibodies to the seven most important ETEC adhesins (CFA/I and CS1-CS6) and a toxoid fusion antigen which stimulates antibodies against ETEC enterotoxins (heat-labile toxin and heat-stable toxin). CFA MEFA-II, another polyvalent MEFA protein, has been demonstrated to stimulate antibodies to another five important ETEC adhesins (CS7, CS12, CS14, CS17, and CS21). We hypothesize that MecVax coverage and efficacy can be expanded if MecVax could stimulate antibodies to all 12 adhesins. In this study, we supplemented MecVax with CFA MEFA-II, examined broad immunity to the 12 targeted ETEC adhesins and 2 ETEC toxins (STa, LT) in mice, and assessed mouse antibody functions for inhibiting the adherence of the 12 adhesins and neutralizing the enterotoxicity of 2 toxins, thus assessing the potential application of a broadly protective pan-ETEC vaccine. Mice intramuscularly immunized with MecVax and CFA MEFA-II developed robust antibody responses to the 12 ETEC adhesins and 2 toxins; furthermore, mouse serum antibodies showed functional activities against the adherence from each of the targeted adhesins and the enterotoxicity of either toxin. Data also indicated that CFA MEFA-II was antigenically compatible with MecVax. These results demonstrated that the inclusion of CFA MEFA-II further expands MecVax broad immunogenicity and protection coverage, suggesting the feasibility of developing a vaccine against all important diarrheal ETEC strains.IMPORTANCEThere are no vaccines licensed for Enterotoxigenic Escherichia coli (ETEC), a leading cause of children's diarrhea and the most common cause of travelers' diarrhea. Since ETEC strains produce over 25 adhesins and 2 distinctive enterotoxins, heterogeneity is a key obstacle to vaccine development. MecVax, a multivalent ETEC vaccine candidate, induces protective antibodies against the seven most important adhesins (CFA/I and CS1-CS6) associated with two-thirds of ETEC clinical cases. However, ETEC prevalence shifts chronically and geographically, and other adhesins are also associated with clinical cases. MecVax would become a pan-ETEC vaccine if it also protects against the remaining important adhesins. This study demonstrated that MecVax supplemented with adhesin protein CFA MEFA-II induces functional antibodies against 12 important ETEC adhesins (CFA/I, CS1-CS7, CS12, CS14, CS17, and CS21), enabling the development of a more broadly protective ETEC vaccine and further validating the application of the MEFA vaccinology platform for multivalent vaccine development.

Effects of prime-boost strategies on the protective efficacy and immunogenicity of a PLGA (85:15)-encapsulated Chlamydia recombinant MOMP nanovaccine

To begin to optimize the immunization routes for our reported PLGA-rMOMP nanovaccine [PLGA-encapsulated Chlamydia muridarum (Cm) recombinant major outer membrane protein (rMOMP)], we compared two prime-boost immunization strategies [subcutaneous (SC) and intramuscular (IM-p) prime routes followed by two SC-boosts)] to evaluate the nanovaccine-induced protective efficacy and immunogenicity in female BALB/c mice. Our results showed that mice immunized via the SC and IM-p routes were protected against a Cm genital challenge by a reduction in bacterial burden and with fewer bacteria in the SC mice. Protection of mice correlated with rMOMP-specific Th1 (IL-2 and IFN-γ) and not Th2 (IL-4, IL-9, and IL-13) cytokines, and CD4+ memory (CD44highCD62Lhigh) T-cells, especially in the SC mice. We also observed higher levels of IL-1α, IL-6, IL-17, CCL-2, and G-CSF in SC-immunized mice. Notably, an increase of cytokines/chemokines was seen after the challenge in the SC, IM-p, and control mice (rMOMP and PBS), suggesting a Cm stimulation. In parallel, rMOMP-specific Th1 (IgG2a and IgG2b) and Th2 (IgG1) serum, mucosal, serum avidity, and neutralizing antibodies were more elevated in SC than in IM-p mice. Overall, the homologous SC prime-boost immunization of mice induced enhanced cellular and antibody responses with better protection against a genital challenge compared to the heterologous IM-p.

Vaccines against extraintestinal pathogenic Escherichia coli (ExPEC): progress and challenges

The emergence of antimicrobial resistance (AMR) is a principal global health crisis projected to cause 10 million deaths annually worldwide by 2050. While the Gram-negative bacteria Escherichia coli is commonly found as a commensal microbe in the human gut, some strains are dangerously pathogenic, contributing to the highest AMR-associated mortality. Strains of E. coli that can translocate from the gastrointestinal tract to distal sites, called extraintestinal E. coli (ExPEC), are particularly problematic and predominantly afflict women, the elderly, and immunocompromised populations. Despite nearly 40 years of clinical trials, there is still no vaccine against ExPEC. One reason for this is the remarkable diversity in the ExPEC pangenome across pathotypes, clades, and strains, with hundreds of genes associated with pathogenesis including toxins, adhesins, and nutrient acquisition systems. Further, ExPEC is intimately associated with human mucosal surfaces and has evolved creative strategies to avoid the immune system. This review summarizes previous and ongoing preclinical and clinical ExPEC vaccine research efforts to help identify key gaps in knowledge and remaining challenges.

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.

Safety, tolerability, and immunogenicity of an oral inactivated ETEC vaccine (ETVAX®) with dmLT adjuvant in healthy adults and children in Zambia: An age descending randomised, placebo-controlled trial

BACKGROUND

Enterotoxigenic Escherichia coli (ETEC) is an important cause of moderate to severe diarrhoea in children for which there is no licensed vaccine. We evaluated ETVAX®, an oral, inactivated ETEC vaccine containing four E. coli strains over-expressing the major colonization factors CFA/I, CS3, CS5, and CS6, a toxoid (LCTBA) and double mutant heat-labile enterotoxin (dmLT) adjuvant for safety, tolerability, and immunogenicity.

METHODS

A double-blind, placebo-controlled, age-descending, dose-finding trial was undertaken in 40 adults, 60 children aged 10-23 months, and 146 aged 6-9 months. Adults received one full dose of ETVAX® and children received 3 doses of either 1/4 or 1/8 dose. Safety was evaluated as solicited and unsolicited events for 7 days following vaccination. Immunogenicity was assessed by evaluation of plasma IgA antibody responses to CFA/I, CS3, CS5, CS6, and LTB, and IgG responses to LTB.

RESULTS

Solicited adverse events were mostly mild or moderate with only 2 severe fever reports which were unrelated to the vaccine. The most common events were abdominal pain in adults (26.7 % in vaccinees vs 20 % in placebos), and fever in children aged 6-9 months (44 % vs 54  %). Dosage, number of vaccinations and decreasing age had no influence on severity or frequency of adverse events. The vaccine induced plasma IgA and IgG responses against LTB in 100 % of the adults and 80-90 % of the children. In the 6-23 months cohort, IgA responses to more than 3 vaccine antigens after 3 doses determined as ≥2-fold rise was significantly higher for 1/4 dose compared to placebo (56.7 % vs 27.2 %, p = 0.01). In the 6-9 months cohort, responses to the 1/4 dose were significantly higher than 1/8 dose after 3 rather than 2 doses.

CONCLUSION

ETVAX® was safe, tolerable, and immunogenic in Zambian adults and children. The 1/4 dose induced significantly stronger IgA responses and is recommended for evaluation of protection in children.

CLINICAL TRIALS REGISTRATION

The trial is registered with the Pan African Clinical Trials Registry (PACTR Ref. 201905764389804) and a description of this clinical trial is available on: https://pactr.samrc.ac.za/Trial Design.

The role of the minor colonization factor CS14 in adherence to intestinal cell models by geographically diverse ETEC isolates

Enterotoxigenic Escherichia coli (ETEC) is a primary causative agent of diarrhea in travelers and young children in low- to middle-income countries. ETEC adheres to small intestinal epithelia via colonization factors (CFs) and secretes heat-stable toxin and/or heat-labile toxin, causing dysregulated ion transport and water secretion. There are over 30 CFs identified, including major CFs associated with moderate-to-severe diarrhea (MSD) and minor CFs for which a role in pathogenesis is less clear. The Global Enteric Multicenter Study identified CS14, a class 5a fimbriae, as the only minor CF significantly associated with MSD and was recommended for inclusion in ETEC vaccines. Despite detection of CS14 in ETEC isolates, the sequence conservation of the CS14 operon, its role in adherence, and functional cross-reactivity to other class 5a fimbriae like CFA/I and CS4 are not understood. Sequence analysis determined that the CS14 operon is >99.9% identical among seven geographically diverse isolates with expanded sequence analysis demonstrating SNPs exclusively in the gene encoding the tip adhesin CsuD. Western blots and electron microscopy demonstrated that CS14 expression required the growth of isolates on CFA agar with the iron chelator deferoxamine mesylate. CS14 expression resulted in significantly increased adherence to cultured intestinal cells and human enteroids. Anti-CS14 antibodies and anti-CS4 antibodies, but not anti-CFA/I antibodies, inhibited the adherence of a subset of ETEC isolates, demonstrating CS14-specific inhibition with partial cross-reactivity within the class 5a fimbrial family. These data provide support for CS14 as an important fimbrial CF and its consideration as a vaccine antigen in future strategies. IMPORTANCE Enterotoxigenic Escherichia coli (ETEC) infection causes profuse watery diarrhea in adults and children in low- to middle-income countries and is a leading cause of traveler's diarrhea. Despite increased use of rehydration therapies, young children especially can suffer long-term effects including gastrointestinal dysfunction as well as stunting and malnutrition. As there is no licensed vaccine for ETEC, there remains a need to identify and understand specific antigens for inclusion in vaccine strategies. This study investigated one adhesin named CS14. This adhesin is expressed on the bacterial surface of ETEC isolates and was recently recognized for its significant association with diarrheal disease. We demonstrated that CS14 plays a role in bacterial adhesion to human target cells, a critical first step in the disease process, and that adherence could be blocked by CS14-specific antibodies. This work will significantly impact the ETEC field by supporting inclusion of CS14 as an antigen for ETEC vaccines.

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.

T helper cell responses in adult diarrheal patients following natural infection with enterotoxigenic Escherichia coli are primarily of the Th17 type

Background

Infection with enterotoxigenic Escherichia coli (ETEC) gives rise to IgA antibodies against both the heat labile toxin (LT) and colonization factors (CFs), which are considered to synergistically protect against ETEC diarrhea. Since the development of ETEC-specific long lived plasma cells and memory B cells is likely to be dependent on T helper (Th) cells, we investigated if natural ETEC diarrhea elicits ETEC-specific Th cells and their relation to IgA responses.

Methods

Th cell subsets were analyzed in adult Bangladeshi patients hospitalized due to ETEC diarrhea by flow cytometric analysis of peripheral blood mononuclear cells (PBMCs) isolated from blood collected day 2, 7, 30 and 90 after hospitalization as well as in healthy controls. The LT- and CF-specific Th responses were determined by analysis of IL-17A and IFN-γ in antigen stimulated PBMC cultures using ELISA. ETEC-specific IgA secreted by circulating antibody secreting cells (plasmablasts) were analyzed by using the antibodies in lymphocyte supernatants (ALS) ELISA-based method and plasma IgA was also measured by ELISA.

Results

ETEC patients mounted significant ALS and plasma IgA responses against LTB and CFs on day 7 after hospitalization. ETEC patients had significantly elevated proportions of memory Th cells with a Th17 phenotype (CCR6+CXCR3-) in blood compared to controls, while frequencies of Th1 (CCR6-CXCR3+) or Th2 (CCR6-CXCR3-) cells were not increased. Antigen stimulation of PBMCs revealed IL-17A responses to LT, most clearly observed after stimulation with double mutant heat labile toxin (dmLT), but also with LT B subunit (LTB), and to CS6 in samples from patients with LT+ or CS6+ ETEC bacteria. Some individuals also mounted IFN-γ responses to dmLT and LTB. Levels of LTB specific IgA antibodies in ALS, but not plasma samples correlated with both IL-17A (r=0.5, p=0.02) and IFN-γ (r=0.6, p=0.01) responses to dmLT.

Conclusions

Our results show that ETEC diarrhea induces T cell responses, which are predominantly of the Th17 type. The correlations between IL-17A and IFN-g and intestine-derived plasmablast responses support that Th responses may contribute to the development of protective IgA responses against ETEC infection. These observations provide important insights into T cell responses that need to be considered in the evaluation of advanced ETEC vaccine candidates.

The Immunogenicity and Properties of a Whole-Cell ETEC Vaccine Inactivated with Psoralen and UVA Light in Comparison to Formalin

Inactivated whole-cell vaccines present a full repertoire of antigens to the immune system. Formalin treatment, a standard method for microbial inactivation, can modify or destroy protein antigenic epitopes. We tested the hypothesis that photochemical inactivation with psoralen and UVA light (PUVA), which targets nucleic acid, would improve the immunogenicity of an Enterotoxigenic E. coli (ETEC) vaccine relative to a formalin-inactivated counterpart. Exposure of ETEC H10407 to PUVA using the psoralen drug 4'-Aminomethyltrioxsalen hydrochloride (AMT) yielded replication-incompetent bacteria that retained their metabolic activity. CFA/I-mediated mannose-resistant hemagglutination (MRHA) was equivalent for PUVA-inactivated and live ETEC, but was severely reduced for formalin-ETEC, indicating that PUVA preserved fimbrial protein functional integrity. The immunogenicity of PUVA-ETEC and formalin-ETEC was compared in mice ± double mutant heat-labile enterotoxin (dmLT) adjuvant. Two weeks after an intramuscular prime/boost, serum anti-ETEC IgG titers were similar for the two vaccines and were increased by dmLT. However, the IgG responses raised against several conserved ETEC proteins were greater after vaccination with PUVA-ETEC. In addition, PUVA-ETEC generated IgG specific for heat-labile toxin (LT) in the absence of dmLT, which was not a property of formalin-ETEC. These data are consistent with PUVA preserving ETEC protein antigens in their native-like form and justify the further testing of PUVA as a vaccine platform for ETEC using murine challenge models.

Establishment of isotype-switched, antigen-specific B cells in multiple mucosal tissues using non-mucosal immunization

Although most pathogens infect the human body via mucosal surfaces, very few injectable vaccines can specifically target immune cells to these tissues where their effector functions would be most desirable. We have previously shown that certain adjuvants can program vaccine-specific helper T cells to migrate to the gut, even when the vaccine is delivered non-mucosally. It is not known whether this is true for antigen-specific B cell responses. Here we show that a single intradermal vaccination with the adjuvant double mutant heat-labile toxin (dmLT) induces a robust endogenous, vaccine-specific, isotype-switched B cell response. When the vaccine was intradermally boosted, we detected non-circulating vaccine-specific B cell responses in the lamina propria of the large intestines, Peyer's patches, and lungs. When compared to the TLR9 ligand adjuvant CpG, only dmLT was able to drive the establishment of isotype-switched resident B cells in these mucosal tissues, even when the dmLT-adjuvanted vaccine was administered non-mucosally. Further, we found that the transcription factor Batf3 was important for the full germinal center reaction, isotype switching, and Peyer's patch migration of these B cells. Collectively, these data indicate that specific adjuvants can promote mucosal homing and the establishment of activated, antigen-specific B cells in mucosal tissues, even when these adjuvants are delivered by a non-mucosal route. These findings could fundamentally change the way future vaccines are formulated and delivered.

Production of monoclonal antibody of heat-labile toxin A subunit to identify enterotoxigenic Escherichia coli by epitope mapping using synthetic peptides

Background

Enterotoxigenic Escherichia coli (ETEC) is a major cause of diarrhea through two enterotoxins, a heat-labile toxin and a heat-stable toxin. These toxins alter the cellular signaling pathways, ultimately triggering an increase in chloride secretion and watery diarrhea.

Objective

For the development of an ETEC vaccine, we attempted to construct a peptide-specific monoclonal antibody library against heat-labile enterotoxin A subunit (LT-A) by epitope mapping using synthetic peptides.

Methods

Sera produced by five mice immunized with recombinant LT-A protein were examined for specific recognition with synthetic 15-mer and 34-mer peptides of LT-A proteins using enzyme-linked immunosorbent assay. The analysis revealed that the synthetic peptides number 8, 16, 24, 33, 36, 38, and 39 reacted with an anti-LT-A polyclonal antibody. For the possible prediction of LT-A epitopes, each full-length protein sequence was subjected to BCPreds analysis and three-dimensional protein structure analysis. The data showed that three peptides (synthetic peptide numbers: 33, 36, and 38-39) have identical antigenic specificities with LT-A protein, suggesting the usefulness of these linear peptide epitopes.

Results

Based on these peptides, we produced monoclonal antibodies to improve the specificity of LT-A detection. Monoclonal antibodies produced from two peptides (numbers 33 and 36) showed affinity for an LT-A recombinant antigen. Moreover, peptide epitope prediction analysis showed that the sites of the three peptides were identical to those exhibiting actual antigenicity. Also, it was confirmed that the amino acid sequence that actually showed antigenicity was included in the peptide predicted only by ETEC-LT-A-33. Also, the specificity of the antibody for ETEC-LT-A-33 was validated using bacterial cells, and the neutralizing effect of the antibody was determined by assessing cytokine release in infected HCT-8 cells.

Conclusion

The monoclonal antibodies produced in this study are useful toolsfor vaccine production against ETEC and can be used to identify peptide antigencandidates.

Research progress in the development of natural-product-based mucosal vaccine adjuvants

Mucosal vaccines have great potential and advantages in preventing infection caused by multiple pathogens. In developing mucosal vaccines, the biggest challenge comes from finding safe and effective adjuvants and drug delivery systems. Great progress has been made in the generation of mucosal adjuvants using detoxified bacterial toxin derivatives, pathogen-related molecules, cytokines, and various vaccine delivery systems. However, many problems, relating to the safety and efficacy of mucosal vaccine adjuvants, remain. Certain natural substances can boost the immune response and thus could be used as adjuvants in vaccination. These natural-product-based immune adjuvants have certain advantages over conventional adjuvants, such as low toxicity, high stability, and low cost of production. In this review, we summarize the latest natural-product-based immune adjuvants, and discuss their properties and clinical applications.

Three structural solutions for bacterial adhesion pilus stability and superelasticity

Bacterial adhesion pili are key virulence factors that mediate host-pathogen interactions in diverse epithelial environments. Deploying a multimodal approach, we probed the structural basis underpinning the biophysical properties of pili originating from enterotoxigenic (ETEC) and uropathogenic bacteria. Using cryo-electron microscopy we solved the structures of three vaccine target pili from ETEC bacteria, CFA/I, CS17, and CS20. Pairing these and previous pilus structures with force spectroscopy and steered molecular dynamics simulations, we find a strong correlation between subunit-subunit interaction energies and the force required for pilus unwinding, irrespective of genetic similarity. Pili integrate three structural solutions for stabilizing their assemblies: layer-to-layer interactions, N-terminal interactions to distant subunits, and extended loop interactions from adjacent subunits. Tuning of these structural solutions alters the biophysical properties of pili and promotes the superelastic behavior that is essential for sustained bacterial attachment.

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.

Current Progress and Challenges in the Study of Adjuvants for Oral Vaccines

Over the past 20 years, a variety of potential adjuvants have been studied to enhance the effect of oral vaccines in the intestinal mucosal immune system; however, no licensed adjuvant for clinical application in oral vaccines is available. In this review, we systematically updated the research progress of oral vaccine adjuvants over the past 2 decades, including biogenic adjuvants, non-biogenic adjuvants, and their multi-type composite adjuvant materials, and introduced their immune mechanisms of adjuvanticity, aiming at providing theoretical basis for developing feasible and effective adjuvants for oral vaccines. Based on these insights, we briefly discussed the challenges in the development of oral vaccine adjuvants and prospects for their future development.

Genomics and pathotypes of the many faces of Escherichia coli

Escherichia coli is the most researched microbial organism in the world. Its varied impact on human health, consisting of commensalism, gastrointestinal disease, or extraintestinal pathologies, has generated a separation of the species into at least eleven pathotypes (also known as pathovars). These are broadly split into two groups, intestinal pathogenic E. coli (InPEC) and extraintestinal pathogenic E. coli (ExPEC). However, components of E. coli's infinite open accessory genome are horizontally transferred with substantial frequency, creating pathogenic hybrid strains that defy a clear pathotype designation. Here, we take a birds-eye view of the E. coli species, characterizing it from historical, clinical, and genetic perspectives. We examine the wide spectrum of human disease caused by E. coli, the genome content of the bacterium, and its propensity to acquire, exchange, and maintain antibiotic resistance genes and virulence traits. Our portrayal of the species also discusses elements that have shaped its overall population structure and summarizes the current state of vaccine development targeted at the most frequent E. coli pathovars. In our conclusions, we advocate streamlining efforts for clinical reporting of ExPEC, and emphasize the pathogenic potential that exists throughout the entire species.

Whole Genome Sequencing and CRISPR/Cas9 Gene Editing of Enterotoxigenic Escherichia coli BE311 for Fluorescence Labeling and Enterotoxin Analyses

Some prevention strategies, including vaccines and antibiotic alternatives, have been developed to reduce enterotoxigenic Escherichia coli proliferation in animal production. In this study, a wild-type strain of BE311 with a virulent heat-stable enterotoxin gene identical to E. coli K99 was isolated for its high potential for gene expression ability. The whole genome of E. coli BE311 was sequenced for gene analyses and editing. Subsequently, the fluorescent gene mCherry was successfully knocked into the genome of E. coli BE311 by CRISPR/Cas9. The E. coli BE311−mCherry strain was precisely quantified through the fluorescence intensity and red colony counting. The inflammatory factors in different intestinal tissues all increased significantly after an E. coli BE311−mCherry challenge in Sprague−Dawley rats (p < 0.05). The heat-stable enterotoxin gene of E. coli BE311 was knocked out, and an attenuated vaccine host E. coli BE311-STKO was constructed. Flow cytometry showed apoptotic cell numbers were lower following a challenge of IPEC-J2 cells with E. coli BE311-STKO than with E. coli BE311. Therefore, the E. coli BE311−mCherry and E. coli BE311-STKO strains that were successfully constructed based on the gene knock-in and knock-out technology could be used as ideal candidates in ETEC challenge models and for the development of attenuated vaccines.

Recent strategies driving oral biologic administration

INTRODUCTION

High patient compliance, noninvasiveness, and self-administration are the leading features of vaccine delivery through the oral route. The implementation of swift mass vaccination campaigns in pandemic outbreaks fascinates the use of oral vaccination. This approach can elicit both mucosal and systemic immune responses to protect against infection at the surface of the mucosa.

AREA COVERED

As pathogen entry and spread mainly occurs through the gastrointestinal tract (GIT) mucosal surfaces, oral vaccination may protect and limit disease spread. Oral vaccines target various potential mucosal inductive sites in the GIT, such as the oral cavity, gastric area, and small intestine. Orally delivered vaccines having subunit and nucleic acid pass through various GIT-associated risks, such as the biodegradation of biologics and their reduced absorption. This article presents a summarized review of the existing technologies and prospects for oral vaccination.

EXPERT OPINION

The intestinal mucosa focuses on current approaches, while future strategies target new mucosal sites, i.e. oral cavity and stomach. Recent developments in biologic delivery through the oral route and their potential use in future oral vaccination are mainly considered.

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.

Designing and determining immunogenicity of a recombinant protein due to producing a new vaccine against Enterotoxigenic Escherichia coli containing CfaE and CotD subunits

Enterotoxigenic Escherichia coli (ETEC) is one of the most common bacterial causes of mortalities in developing countries due to diarrhea. Since mucosal immune responses to CFs can prevent the disease, a chimeric protein containing ETEC's CFA/I (CfaE) tip subunits and CS2 (CotD) sub-structural units is developed to produce effective vaccine. Using bioinformatics tools, the chimeric construct was analyzed and then the optimized gene was synthesized and expressed in E. coli. The recombinant protein was expressed and purified by the Ni-NTA chromatography column and confirmed by anti-his tag antibody by western blotting. Mice were immunized with recombinant protein, and the IgG and IgA antibodies' titrations of the sera were analyzed by ELISA. In addition, the immunogenicity and protective efficacy against the live ETEC bacteria in the challenge test were determined. Western blot analysis verified the chimeric protein expression of CotD-CfaE. The outcome of ELISA was a substantial improvement in the IgG antibody titer in immunized mice. In a live ETEC challenge, the survival percentage of 30% was shown for immunized mice. The developed recombinant chimeric protein could be suggested as an effective component in producing an efficient vaccine against Enterotoxigenic E. coli with other crucial subunits, different immunization route, and other factors.

Enterotoxigenic Escherichia coli (ETEC) vaccines: Priority activities to enable product development, licensure, and global access

Diarrhoeal disease attributable to enterotoxigenic Escherichia coli (ETEC) causes substantial morbidity and mortality predominantly in paediatric populations in low- and middle-income countries. In addition to acute illness, there is an increasing appreciation of the long-term consequences of enteric infections, including ETEC, on childhood growth and development. Provision of potable water and sanitation and appropriate clinical care for acute illness are critical to reduce the ETEC burden. However, these interventions are not always practical and may not achieve equitable and sustainable coverage. Vaccination may be the most cost-effective and equitable means of primary prevention; however, additional data are needed to accelerate the investment and guide the decision-making process for ETEC vaccines. First, to understand and quantify the ETEC disease burden, additional data are needed on the association between ETEC infection and physical and cognitive stunting as well as delayed educational attainment. Furthermore, the role of inappropriate or inadequate antibiotic treatment of ETEC-attributable diarrhoea may contribute to the development of antimicrobial resistance (AMR) and needs further elucidation. An ETEC vaccine that mitigates acute diarrhoeal illness and minimizes the longer-term disease manifestations could have significant public health impact and be a cost-effective countermeasure. Herein we review the ETEC vaccine pipeline, led by candidates compatible with the general parameters of the Preferred Product Characteristics (PPC) recently developed by the World Health Organization. Additionally, we have developed an ETEC Vaccine Development Strategy to provide a framework to underpin priority activities for researchers, funders and vaccine manufacturers, with the goal of addressing globally unmet data needs in the areas of research, product development, and policy, as well as commercialization and delivery. The strategy also aims to guide prioritization and co-ordination of the priority activities needed to minimize the timeline to licensure and use of ETEC vaccines, especially in in low- and middle-income countries, where they are most urgently needed.

Preclinical Characterization of Immunogenicity and Efficacy against Diarrhea from MecVax, a Multivalent Enterotoxigenic E. coli Vaccine Candidate

There are no vaccines licensed for enterotoxigenic Escherichia coli (ETEC), a leading cause of diarrhea for children in developing countries and international travelers. Virulence heterogeneity among strains and difficulties identifying safe antigens for protective antibodies against STa, a potent but poorly immunogenic heat-stable toxin which plays a key role in ETEC diarrhea, are challenges in ETEC vaccine development. To overcome these challenges, we applied a toxoid fusion strategy and a novel epitope- and structure-based multiepitope fusion antigen (MEFA) vaccinology platform to construct two chimeric multivalent proteins, toxoid fusion 3xSTa_N12S-mnLT_R192G/L211A and adhesin CFA/I/II/IV MEFA, and demonstrated that the proteins induced protective antibodies against STa and heat-labile toxin (LT) produced by all ETEC strains or the seven most important ETEC adhesins (CFA/I and CS1 to CS6) expressed by the ETEC strains causing 60 to 70% of diarrheal cases and moderate to severe cases. Combining two proteins, we prepared a protein-based multivalent ETEC vaccine, MecVax. MecVax was broadly immunogenic; mice and pigs intramuscularly immunized with MecVax developed no apparent adverse effects but had robust antibody responses to the target toxins and adhesins. Importantly, MecVax-induced antibodies were broadly protective, demonstrated by significant adherence inhibition against E. coli bacteria producing any of the seven adhesins and neutralization of STa and cholera toxin (CT) enterotoxicity. Moreover, MecVax protected against watery diarrhea and provided over 70% and 90% protection against any diarrhea from an STa-positive or an LT-positive ETEC strain in a pig challenge model. These results indicated that MecVax induces broadly protective antibodies and prevents diarrhea preclinically, signifying that MecVax is potentially an effective injectable vaccine for ETEC. IMPORTANCE Enterotoxigenic Escherichia coli (ETEC) bacteria are a top cause of children's diarrhea and travelers' diarrhea and are responsible for over 220 million diarrheal cases and more than 100,000 deaths annually. A safe and effective ETEC vaccine can significantly improve public health, particularly in developing countries. Data from this preclinical study showed that MecVax induces broadly protective antiadhesin and antitoxin antibodies, becoming the first ETEC vaccine candidate to induce protective antibodies inhibiting adherence of the seven most important ETEC adhesins and neutralizing the enterotoxicity of not only LT but also STa toxin. More importantly, MecVax is shown to protect against clinical diarrhea from STa-positive or LT-positive ETEC infection in a pig challenge model, recording protection from antibodies induced by the protein-based, injectable, subunit vaccine MecVax against ETEC diarrhea and perhaps the possibility of intramuscularly administered protein vaccines for protection against intestinal mucosal infection.

dmLT Adjuvant Enhances Cytokine Responses to T Cell Stimuli, Whole Cell Vaccine Antigens and Lipopolysaccharide in Both Adults and Infants

Enhancement of mucosal immune responses in children and infants using novel adjuvants such as double mutant heat labile toxin (dmLT) is an important goal in the enteric vaccine field. dmLT has been shown to enhance mucosal IgA responses to the oral inactivated enterotoxigenic Escherichia coli (ETEC) vaccine ETVAX. dmLT can enhance IL-17A production from adult T cells, which may increase the production and secretion of mucosal IgA antibodies. However, the adjuvant mechanism remains to be fully elucidated and might differ between infants and adults due to age-related differences in the development of the immune system. The main objective of this study was to determine how dmLT influences antigen presenting cells and T cells from infants compared to adults, and the role of IL-1β for mediating the adjuvant activity. Peripheral blood mononuclear cells (PBMCs) from Bangladeshi infants (6-11 months) and adults (18-40 years) were stimulated with the mitogen phytohaemagglutinin (PHA), the superantigen Staphylococcal enterotoxin B (SEB), ETVAX whole cell component (WCC) or E. coli lipopolysaccharide (LPS) ± dmLT, and cytokine production was measured using ELISA and electrochemiluminescence assays. The adjuvant dmLT significantly enhanced SEB- and PHA-induced IL-17A, but not IFN-γ responses, in PBMCs from both infants and adults. Blocking experiments using an IL-1 receptor antagonist demonstrated the importance of IL-1 signaling for the adjuvant effect. dmLT, ETVAX WCC and LPS induced dose-dependent IL-1β responses of comparable magnitudes in infant and adult cells. Depletion experiments suggested that IL-1β was mainly produced by monocytes. dmLT enhanced IL-1β responses to low doses of WCC and LPS, and the adjuvant effect appeared over a wider dose-range of WCC in infants. dmLT and WCC also induced IL-6, IL-23 and IL-12p70 production in both age groups and dmLT tended to particularly enhance IL-23 responses to WCC. Our results show that dmLT can induce IL-1β as well as other cytokines, which in turn may enhance IL-17A and potentially modulate other immunological responses in both infants and adults. Thus, dmLT may have an important function in promoting immune responses to the ETVAX vaccine, as well as other whole cell- or LPS-based vaccines in infants in low- and middle-income countries.

Killed whole-genome reduced-bacteria surface-expressed coronavirus fusion peptide vaccines protect against disease in a porcine model

As the coronavirus disease 2019 (COVID-19) pandemic rages on, it is important to explore new evolution-resistant vaccine antigens and new vaccine platforms that can produce readily scalable, inexpensive vaccines with easier storage and transport. We report here a synthetic biology-based vaccine platform that employs an expression vector with an inducible gram-negative autotransporter to express vaccine antigens on the surface of genome-reduced bacteria to enhance interaction of vaccine antigen with the immune system. As a proof-of-principle, we utilized genome-reduced Escherichia coli to express SARS-CoV-2 and porcine epidemic diarrhea virus (PEDV) fusion peptide (FP) on the cell surface, and evaluated their use as killed whole-cell vaccines. The FP sequence is highly conserved across coronaviruses; the six FP core amino acid residues, along with the four adjacent residues upstream and the three residues downstream from the core, are identical between SARS-CoV-2 and PEDV. We tested the efficacy of PEDV FP and SARS-CoV-2 FP vaccines in a PEDV challenge pig model. We demonstrated that both vaccines induced potent anamnestic responses upon virus challenge, potentiated interferon-γ responses, reduced viral RNA loads in jejunum tissue, and provided significant protection against clinical disease. However, neither vaccines elicited sterilizing immunity. Since SARS-CoV-2 FP and PEDV FP vaccines provided similar clinical protection, the coronavirus FP could be a target for a broadly protective vaccine using any platform. Importantly, the genome-reduced bacterial surface-expressed vaccine platform, when using a vaccine-appropriate bacterial vector, has potential utility as an inexpensive, readily manufactured, and rapid vaccine platform for other pathogens.

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.

Oral Biologic Delivery: Advances Toward Oral Subunit, DNA, and mRNA Vaccines and the Potential for Mass Vaccination During Pandemics

Oral vaccination enables pain-free and self-administrable vaccine delivery for rapid mass vaccination during pandemic outbreaks. Furthermore, it elicits systemic and mucosal immune responses. This protects against infection at mucosal surfaces, which may further enhance protection and minimize the spread of disease. The gastrointestinal (GI) tract presents a number of prospective mucosal inductive sites for vaccine targeting, including the oral cavity, stomach, and small intestine. However, currently available oral vaccines are effectively limited to live-attenuated and inactivated vaccines against enteric diseases. The GI tract poses a number of challenges,including degradative processes that digest biologics and mucosal barriers that limit their absorption. This review summarizes the approaches currently under development and future opportunities for oral vaccine delivery to established (intestinal) and relatively new (oral cavity, stomach) mucosal targets. Special consideration is given to recent advances in oral biologic delivery that offer promise as future platforms for the administration of oral vaccines.

Investigation of a monoclonal antibody against enterotoxigenic Escherichia coli, expressed as secretory IgA1 and IgA2 in plants

Passive immunization with antibodies is a promising approach against enterotoxigenic Escherichia coli diarrhea, a prevalent disease in LMICs. The objective of this study was to investigate expression of a monoclonal anti-ETEC CfaE secretory IgA antibody in N. benthamiana plants, with a view to facilitating access to ETEC passive immunotherapy. SIgA1 and SIgA2 forms of mAb 68-81 were produced by co-expressing the light and engineered heavy chains with J chain and secretory component in N. benthamiana. Antibody expression and assembly were compared with CHO-derived antibodies by SDS-PAGE, western blotting, size-exclusion chromatography and LC-MS peptide mapping. N-linked glycosylation was assessed by rapid fluorescence/mass spectrometry and LC-ESI-MS. Susceptibility to gastric digestion was assessed in an in vitro model. Antibody function was compared for antigen binding, a Caco-2 cell-based ETEC adhesion assay, an ETEC hemagglutination inhibition assay and a murine in vivo challenge study. SIgA1 assembly appeared superior to SIgA2 in plants. Both sub-classes exhibited resistance to degradation by simulated gastric fluid, comparable to CHO-produced 68-61 SIgA1. The plant expressed SIgAs had more homogeneous N-glycosylation than CHO-derived SIgAs, but no alteration of in vitro functional activity was observed, including antibodies expressed in a plant line engineered for mammalian-like N glycosylation. The plant-derived SIgA2 mAb demonstrated protection against diarrhea in a murine infection model. Although antibody yield and purification need to be optimized, anti-ETEC SIgA antibodies produced in a low-cost plant platform are functionally equivalent to CHO antibodies, and provide promise for passive immunotherapy in LMICs.

Vaccines against gastroenteritis, current progress and challenges

Enteric viral and bacterial infections continue to be a leading cause of mortality and morbidity in young children in low-income and middle-income countries, the elderly, and immunocompromised individuals. Vaccines are considered an effective and practical preventive approach against the predominantly fecal-to-oral transmitted gastroenteritis particularly in the resource-limited countries or regions where implementation of sanitation systems and supply of safe drinking water are not quickly achievable. While vaccines are available for a few enteric pathogens including rotavirus and cholera, there are no vaccines licensed for many other enteric viral and bacterial pathogens. Challenges in enteric vaccine development include immunological heterogeneity among pathogen strains or isolates, a lack of animal challenge models to evaluate vaccine candidacy, undefined host immune correlates to protection, and a low protective efficacy among young children in endemic regions. In this article, we briefly updated the progress and challenges in vaccines and vaccine development for the leading enteric viral and bacterial pathogens including rotavirus, human calicivirus, Shigella, enterotoxigenic Escherichia coli (ETEC), cholera, nontyphoidal Salmonella, and Campylobacter, and introduced a novel epitope- and structure-based vaccinology platform known as MEFA (multiepitope fusion antigen) and the application of MEFA for developing broadly protective multivalent vaccines against heterogenous pathogens.

Potential of the Escherichia coli LT(R192G/L211A) toxoid as a mucosal adjuvant for rainbow trout (Oncorhynchus mykiss)

Oral vaccines are highly demanded by aquaculture sector that requires alternatives to injectable vaccines, involving fish handling, stress-related immunosuppression and mortalities. However, most previous attempts to obtain effective oral vaccines have failed due to a restricted tolerance mechanisms in intestine, whose mucosa is at the frontline of antigen encounter and has to balance the equilibrium between tolerance and immunity in a microbe-rich environment. Thus, the search for oral adjuvants that could augment immune responses triggered by antigens allowing them to circumvent intestinal tolerance is of great relevance. The present work focuses on the adjuvant potential of the Escherichia coli LT(R192G/L211A) toxoid (dmLT). To undertake an initial screening of the potential that dmLT has as an oral adjuvant in rainbow trout (Oncorhynchus mykiss), we have analyzed its transcriptional effects alone or in combination with Aeromonas salmonicida subsp. salmonicida or viral hemorrhagic septicemia virus (VHSV) on rainbow trout intestinal epithelial cell line RTgutGC and gut explants. Our results show that although dmLT provoked no significant effects by itself, it increased the transcription of pro-inflammatory cytokines and antimicrobial genes induced by the bacteria. In contrast, when combined with VHSV, dmLT only increased the transcription of Mx and the intracellular adhesion molecule 1 (ICAM1). Therefore, the protocol designed is an effective method to initially evaluate the effects of potential oral adjuvants, and points to dmLT as an effective adjuvant for oral antibacterial vaccines.

Clinical aspects of heat-labile and heat-stable toxin-producing enterotoxigenic Escherichia coli: A prospective study among Finnish travellers

BACKGROUND

Enterotoxigenic Escherichia coli (ETEC) is a major pathogen causing travellers' diarrhoea (TD) among visitors to low- and middle-income countries (LMIC). Scant data are available on rates of travel-acquired ETEC producing heat-labile (LT) and/or heat-stable (ST) toxin or its subtypes, STh (human) and STp (porcine) in various geographic regions, and on clinical pictures associated with each toxin.

METHODS

Using qPCR, we analysed LT, STh, and STp in stools positive for ETEC in a prospective study among 103 Finnish travellers visiting LMIC. They filled in questionnaires and provided stool samples before and after travel. We scrutinized geographic distribution of LT, STh, and STp ETEC findings, and association between these different ETEC subtypes and moderate/severe TD.

RESULTS

Among the 103 stool samples positive for ETEC toxins, the rate for LT was 76%, for STh 26%, and STp 41%. The rate for LT-only was 44%, for STh-only 6%, STp-only 16%, LT + STh 10%, LT + STp 15%, STh + STp 3%, and LT + STh + STp 8%. Findings varied by destination; the rates of LT, STh, and STp were 79%, 21%, and 57%, respectively, in Southern Asia (n = 14); 85%, 10%, and 20% in South-eastern Asia (n = 20); 84%, 13%, and 29% in Eastern Africa (n = 31); and 56%, 50%, and 63% in Western Africa (n = 32), respectively. Of travellers positive for LT, STh, and STp, 83%, 100%, and 88%, encountered TD; 35%, 55%, and 41% reported moderate/severe TD. STh was associated with moderate/severe TD.

CONCLUSIONS

Toxin findings varied by destination; multiple toxins were commonly detected. Moderate/severe TD was reported most frequently by subjects with STh-ETEC.

Technological Approaches for Improving Vaccination Compliance and Coverage

Vaccination has been well recognised as a critically important tool in preventing infectious disease, yet incomplete immunisation coverage remains a major obstacle to achieving disease control and eradication. As medical products for global access, vaccines need to be safe, effective and inexpensive. In line with these goals, continuous improvements of vaccine delivery strategies are necessary to achieve the full potential of immunisation. Novel technologies related to vaccine delivery and route of administration, use of advanced adjuvants and controlled antigen release (single-dose immunisation) approaches are expected to contribute to improved coverage and patient compliance. This review discusses the application of micro- and nano-technologies in the alternative routes of vaccine administration (mucosal and cutaneous vaccination), oral vaccine delivery as well as vaccine encapsulation with the aim of controlled antigen release for single-dose vaccination.

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

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

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

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

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

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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.

Evaluation of the reactogenicity, adjuvanticity and antigenicity of LT(R192G) and LT(R192G/L211A) by intradermal immunization in mice

The development of an effective subunit vaccine is frequently complicated by the difficulty of eliciting protective immune responses, often requiring the co-administration of an adjuvant. Heat-labile toxin (LT), an enterotoxin expressed by enterotoxigenic E. coli (ETEC) with an AB5 structure similar to cholera toxin, is a strong adjuvant. While the mucosa represents the natural route of exposure to LT and related toxins, the clinical utility of LT and similar adjuvants given by mucosal routes has been limited by toxicity, as well as the association between intranasal delivery of LT and Bell's palsy. Single and double amino acid mutants of LT, LT(R192G)/mLT and LT(R192G/L211A)/dmLT respectively, have been proposed as alternatives to reduce the toxicity associated with the holotoxin. In the present study, we compared mLT and dmLT given via a non-mucosal route (i.e. intradermally) to investigate their adjuvanticity when co-administrated with an enterotoxigenic E. coli vaccine candidate, CfaEB. Antigenicity (i.e. ability to elicit response against LT) and reactogenicity at the injection site were also evaluated. BALB/c mice were immunized by the intradermal route with CfaEB plus increasing doses of either mLT or dmLT (0.01 to 2.5 μg). Both adjuvants induced dose-dependent skin reactogenicity, with dmLT being less reactogenic than mLT. Both adjuvants significantly boosted the anti-CfaE IgG and functional hemagglutination inhibiting (HAI) antibody responses, compared to the antigen alone. In addition to inducing anti-LT responses, even at the lowest dose tested (0.01 μg), the adjuvants also prompted in vitro cytokine responses (IFN-γ, IL-4, IL-5, IL-10 and IL-17) that followed different patterns, depending on the protein used for stimulation (CfaE or LTB) and/or the dose used for immunization. The two LT mutants evaluated here, mLT and dmLT, are potent adjuvants for intradermal immunization and should be further investigated for the intradermal delivery of subunit ETEC vaccines.

Intradermal or Sublingual Delivery and Heat-Labile Enterotoxin Proteins Shape Immunologic Responses to a CFA/I Fimbria-Derived Subunit Antigen Vaccine against Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is a major cause of infectious diarrhea in children, travelers, and deployed military personnel. As such, development of a vaccine would be advantageous for public health. One strategy is to use subunits of colonization factors combined with antigen/adjuvant toxoids as an ETEC vaccine. Here, we investigated the intradermal (i.d.) or sublingual (s.l.) delivery of CFA/I fimbrial antigens, including CfaEB and a CfaE-heat-labile toxin B subunit (LTB) chimera admixed with double mutant heat-labile toxin (LT) LT-R192G/L211A (dmLT). In addition, we compared dmLT with other LT proteins to better understand the generation of adjuvanted fimbrial and toxoid immunity as well as the influence on any local skin reactogenicity. We demonstrate that immunization with dmLT admixed with CfaEB induces robust serum and fecal antibody responses to CFA/I fimbriae and LT but that i.d. formulations are not optimal for s.l. delivery. Improved s.l. vaccination outcomes were observed when higher doses of dmLT (1 to 5 μg) were admixed with CfaEB or, even better, when a CfaE-LTB chimera antigen was used instead. Serum anti-CFA/I total antibodies, detected by enzyme-linked immunosorbent assay, were the best predictor of functional antibodies, based on the inhibition of red blood cell agglutination by ETEC. Immunization with other LT proteins or formulations with altered B-subunit binding during i.d. immunization (e.g., by addition of 5% lactose, LTA1, or LT-G33D) minimally altered the development of antibody responses and cytokine recall responses but reduced skin reactogenicity at the injection site. These results reveal how formulations and delivery parameters shape the adaptive immune responses to a toxoid and fimbria-derived subunit vaccine against ETEC.

Adjuvant effect of enterotoxigenic Escherichia coli (ETEC) double-mutant heat-labile toxin (dmLT) on systemic immunogenicity induced by the CFA/I/II/IV MEFA ETEC vaccine: Dose-related enhancement of antibody responses to seven ETEC adhesins (CFA/I, CS1-CS6)

Double-mutant heat-labile toxin (dmLT, LT_R192G/L211A) of enterotoxigenic Escherichia coli (ETEC) is an effective mucosal adjuvant. Recent studies have shown that dmLT also exhibits adjuvanticity for antigens administered parenterally. In this study, we subcutaneously (SC) immunized mice with the ETEC adhesin-based vaccine, CFA/I/II/IV MEFA (multiepitope fusion antigen), adjuvanted with dmLT and examined the impact of dmLT on antibody responses specific to the seven adhesins in the vaccine construction [CFA/I, CFA/II (CS1, CS2, CS3) and CFA/IV (CS4, CS5, CS6)]. Mice were immunized with a fixed dose of CFA/I/II/IV MEFA and ascending doses of dmLT adjuvant (0, 0.05, 0.1, 0.5 or 1.0 µg) to assess the potential dmLT dose response relationship. Data showed that dmLT enhanced systemic antibody responses to all seven antigens (CFA/I, CS1-CS6) targeted by MEFA in a dose-dependent way. The adjuvant effect of dmLT on the MEFA construct plateaued at a dose of 0.1 µg. Results also indicated that dmLT is an effective parenteral adjuvant when given by the SC route with the ETEC adhesin MEFA vaccine and that antibody enhancement was achieved with relatively low doses. These observations suggest the potential usefulness of dmLT for parenteral ETEC vaccine candidates and also perhaps for vaccines against other pathogens.

Enterotoxigenic Escherichia coli Infections

PURPOSE OF REVIEW

Review recent developments pertaining to the epidemiology, molecular pathogenesis, and sequelae of enterotoxigenic Escherichia coli (ETEC) infections in addition to discussion of challenges for vaccinology.

RECENT FINDINGS

ETEC are a major cause of diarrheal illness in resource poor areas of the world where they contribute to unacceptable morbidity and continued mortality particularly among young children; yet, precise epidemiologic estimates of their contribution to death and chronic disease have been difficult to obtain. Although most pathogenesis studies, and consequently vaccine development have focused intensively on canonical antigens, more recently identified molecules unique to the ETEC pathovar may inform our understanding of ETEC virulence, and the approach to broadly protective vaccines. ETEC undeniably continue to have a substantial impact on global health; however, further studies are needed to clarify the true impact of these infections, particularly in regions where access to care may be limited. Likewise, our present understanding of the relationship of ETEC infection to non-diarrheal sequelae is presently limited, and additional effort will be required to achieve a mechanistic understanding of these diseases and to fulfill Koch's postulates on a molecular level. Precise elucidation of the role played by novel virulence factors, the global burden of acute illness, and the contribution of these pathogens and/or their toxins to non-diarrheal morbidity remain important imperatives.

Live attenuated enterotoxigenic Escherichia coli (ETEC) vaccine with dmLT adjuvant protects human volunteers against virulent experimental ETEC challenge

Background

There is no licensed vaccine against enterotoxigenic Escherichia coli (ETEC), a major cause of diarrhea-associated morbidity and mortality among infants and children in low-income countries and travelers. The results of this vaccination/challenge study demonstrate strong protection by an attenuated ETEC vaccine candidate, ACE527, when co-administered with a mucosal adjuvant, the double-mutant heat-labile toxin (dmLT) of ETEC.

Methods

Sixty healthy adults participated in a randomized, placebo-controlled, double-blind study with three doses of lyophilized ACE527 (∼3 × 10⁹ of each strain per dose) administered orally with or without dmLT adjuvant (25 µg/dose). Six months later, 36 of these volunteers and a control group of 21 unvaccinated volunteers were challenged with virulent ETEC strain H10407. The primary outcome was severe diarrhea, defined as passing >800 g of unformed stools during the inpatient period following challenge.

Findings

The vaccine was well tolerated and induced robust immune responses to key antigens. The protective efficacy (PE) against the primary outcome of severe diarrhea was 65.9% (95% confidence interval [CI] 5.4–87.7, p = 0.003). Among subjects receiving the adjuvanted vaccine, the attack rate of severe diarrhea was 23.1, while in unimmunized controls it was 67.7%. The PE against diarrhea of any severity was 58.5% (95% CI 3.8– 82.1, p = 0.016). There was a strong inverse correlation between shedding of the vaccine strain after either of the first two doses and absence of severe diarrhea upon challenge (RR = 0.29, 95% CI 0.08–1.05, p = 0.041). Challenge strain shedding was 10-fold lower in those receiving the adjuvant than in those receiving vaccine alone. The unadjuvanted vaccine was not protective (PE = 23.1%).

Interpretation

The results of this study support further development of ACE527 + dmLT as a vaccine for children in endemic countries and travelers. This is the first clinical demonstration that dmLT can contribute significantly to vaccine efficacy and may warrant testing with other oral vaccines.

(ClinicalTrials.gov registration: NCT01739231).

Specificity of Escherichia coli Heat-Labile Enterotoxin Investigated by Single-Site Mutagenesis and Crystallography

Diarrhea caused by enterotoxigenic Escherichia coli (ETEC) is one of the leading causes of mortality in children under five years of age and is a great burden on developing countries. The major virulence factor of the bacterium is the heat-labile enterotoxin (LT), a close homologue of the cholera toxin. The toxins bind to carbohydrate receptors in the gastrointestinal tract, leading to toxin uptake and, ultimately, to severe diarrhea. Previously, LT from human- and porcine-infecting ETEC (hLT and pLT, respectively) were shown to have different carbohydrate-binding specificities, in particular with respect to N-acetyllactosamine-terminating glycosphingolipids. Here, we probed 11 single-residue variants of the heat-labile enterotoxin with surface plasmon resonance spectroscopy and compared the data to the parent toxins. In addition we present a 1.45 Å crystal structure of pLTB in complex with branched lacto-N-neohexaose (Galβ4GlcNAcβ6[Galβ4GlcNAcβ3]Galβ4Glc). The largest difference in binding specificity is caused by mutation of residue 94, which links the primary and secondary binding sites of the toxins. Residue 95 (and to a smaller extent also residues 7 and 18) also contribute, whereas residue 4 shows no effect on monovalent binding of the ligand and may rather be important for multivalent binding and avidity.

Alpha-galactosylceramide enhances mucosal immunity to oral whole-cell cholera vaccines

Cholera is a severe diarrheal disease caused by the bacterium Vibrio cholerae (V. cholerae) that results in 3-4 million cases globally with 100,000-150,000 deaths reported annually. Mostly confined to developing nations, current strategies to control the spread of cholera include the provision of safe drinking water and improved sanitation and hygiene, ideally in conjunction with oral vaccination. However, difficulties associated with the costs and logistics of these strategies have hampered their widespread implementation. Specific challenges pertaining to oral cholera vaccines (OCVs) include a lack of safe and effective adjuvants to further enhance gut immune responses, the complex and costly multicomponent vaccine manufacturing, limitations of conventional liquid formulation and the lack of an integrated delivery platform. Herein we describe the use of the orally active adjuvant α-Galactosylceramide (α-GalCer) to strongly enhance intestinal bacterium- and toxin-specific IgA responses to the OCV, Dukoral^® in C57BL/6 and BALB/c mice. We further demonstrate the mucosal immunogenicity of a novel multi-antigen, single-component whole-cell killed V. cholerae strain and the enhancement of its immunogenicity by adding α-GalCer. Finally, we report that combining these components and recombinant cholera toxin B subunit in the SmPill^® minisphere delivery system induced strong intestinal and systemic antigen-specific antibody responses.

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.