Establishment, Validation, and Application of a New World Primate Model of Enterotoxigenic Escherichia coli Disease for Vaccine Development

Conserved antigens for enteric vaccines

Enterotoxigenic Escherichia coli (ETEC), Shigella, and Campylobacter have been identified as major causes of diarrheal diseases worldwide. In addition to overt disease and death, they are responsible for stunting in children with the risk of lifelong consequences on health and economic opportunities. All three of these bacterial pathogens, which collectively account for approximately 30 % of the cases of diarrheal diseases, are recognized as antimicrobial resistance (AMR) threats. In spite of the dangers these pathogens represent for both children and adults, there is as yet no licensed vaccine available for any of them. Fortunately, much has been accomplished to identify conserved antigens against each of these pathogens so that now relatively simple vaccines have the potential to be developed into multi-pathogen vaccines which could have a major impact on reduction of diarrheal diseases. Conserved antigens may be used even more efficiently if consolidated and expressed on a cellular vector or as part of a conjugate vaccine. A new mucosal adjuvant, double mutant heat-labile toxin (dmLT), has been shown to not only be among the conserved antigens against ETEC, but to also have properties which drive robust mucosal and systemic immune responses for antigens given orally or intramuscularly. Conserved antigens and the strategies for their use such as co-administration with dmLT will be presented in this review.

One Health Networks for Infectious Diseases Surveillance and Pandemic Preparedness in Central and South America

The SARS-CoV-2 2019 pandemic prompted the emergence of collaborative initiatives within South America and the Caribbean, to tackle common challenges. Many initiatives included local government, international entities, military, academia, and research institutions, united to face the challenges brought by the pandemic. Some collaborations were new, but most were built on top of existing networks developed to prevent and control challenges like zoonotic diseases. In the last 40 years, the U.S. Naval Medical Research Unit (NAMRU) SOUTH has helped ensure the readiness and health of U.S. service members, Peruvian partners, and civilian population through research, surveillance, and global health, covering One Health interconnectedness of human, animal, and environmental health to address zoonotic diseases, antimicrobial resistance, and vector-borne diseases. This article puts together the different communications, data sharing, and initiatives developed throughout South America towards One Health surveillance, focusing on zoonotic pathogens, and to describe the best practices for these networks.

Landscape of Bacterial Enteric Disease and Traveler´s Diarrhea in South America

Diarrheal disease is a global public health threat. Cases of enteric disease caused by bacterial enteropathogens result in a significant health burden and can lead to antimicrobial resistance patterns that increase and evolve over time and geography, posing serious health challenges for local populations as well as civilian travelers and military personnel deployed to endemic regions. This review describes the prevalence of the most common bacterial agents of diarrheal disease in South America, as well as the distribution patterns of antibiotic resistance and predominant strains that are present in the region. Furthermore, the development of relevant prevention and treatment strategies are described, and United States Naval Medical Research Unit SOUTH disease surveillance and enteric disease research efforts in this regional context are highlighted.

Efficacy Evaluation of an Intradermally Delivered Enterotoxigenic Escherichia coli CF Antigen I Fimbrial Tip Adhesin Vaccine Coadministered with Heat-Labile Enterotoxin with LT(R192G) against Experimental Challenge with Enterotoxigenic E. coli H10407 in Healthy Adult Volunteers

BACKGROUND

Enterotoxigenic E. coli (ETEC) is a principal cause of diarrhea in travelers, deployed military personnel, and children living in low to middle-income countries. ETEC expresses a variety of virulence factors including colonization factors (CF) that facilitate adherence to the intestinal mucosa. We assessed the protective efficacy of a tip-localized subunit of CF antigen I (CFA/I), CfaE, delivered intradermally with the mutant E. coli heat-labile enterotoxin, LTR192G, in a controlled human infection model (CHIM).

METHODS

Three cohorts of healthy adult subjects were enrolled and given three doses of 25 μg CfaE + 100 ng LTR192G vaccine intradermally at 3-week intervals. Approximately 28 days after the last vaccination, vaccinated and unvaccinated subjects were admitted as inpatients and challenged with approximately 2 × 107 cfu of CFA/I+ ETEC strain H10407 following an overnight fast. Subjects were assessed for moderate-to-severe diarrhea for 5 days post-challenge.

RESULTS

A total of 52 volunteers received all three vaccinations; 41 vaccinated and 43 unvaccinated subjects were challenged and assessed for moderate-to-severe diarrhea. Naïve attack rates varied from 45.5% to 64.7% across the cohorts yielding an overall efficacy estimate of 27.8% (95% confidence intervals: -7.5-51.6%). In addition to reducing moderate-severe diarrhea rates, the vaccine significantly reduced loose stool output and overall ETEC disease severity.

CONCLUSIONS

This is the first study to demonstrate protection against ETEC challenge after intradermal vaccination with an ETEC adhesin. Further examination of the challenge methodology is necessary to address the variability in naïve attack rate observed among the three cohorts in the present study.

Shigella-Controlled Human Infection Models: Current and Future Perspectives

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

Bioactivity and efficacy of a hyperimmune bovine colostrum product- Travelan, against shigellosis in a non-Human primate model (Macaca mulatta)

Infectious diarrhea is a World Health Organization public health priority area due to the lack of effective vaccines and an accelerating global antimicrobial resistance crisis. New strategies are urgently needed such as immunoprophylactic for prevention of diarrheal diseases. Hyperimmune bovine colostrum (HBC) is an established and effective prophylactic for infectious diarrhea. The commercial HBC product, Travelan® (Immuron Ltd, Australia) targets multiple strains of enterotoxigenic Escherichia coli (ETEC) is highly effective in preventing diarrhea in human clinical studies. Although Travelan® targets ETEC, preliminary studies suggested cross-reactivity with other Gram-negative enteric pathogens including Shigella and Salmonella species. For this study we selected an invasive diarrheal/dysentery-causing enteric pathogen, Shigella, to evaluate the effectiveness of Travelan®, both in vitro and in vivo. Here we demonstrate broad cross-reactivity of Travelan® with all four Shigella spp. (S. flexneri, S. sonnei, S. dysenteriae and S. boydii) and important virulence factor Shigella antigens. Naïve juvenile rhesus macaques (NJRM) were randomized, 8 dosed with Travelan® and 4 with a placebo intragastrically twice daily over 6 days. All NJRM were challenged with S. flexneri 2a strain 2457T on the 4th day of treatment and monitored for diarrheal symptoms. All placebo-treated NJRM displayed acute dysentery symptoms within 24-36 hours of challenge. Two Travelan®-treated NJRM displayed dysentery symptoms and six animals remained healthy and symptom-free post challenge; resulting in 75% efficacy of prevention of shigellosis (p = 0.014). These results strongly indicate that Travelan® is functionally cross-reactive and an effective prophylactic for shigellosis. This has positive implications for the prophylactic use of Travelan® for protection against both ETEC and Shigella spp. diarrheal infections. Future refinement and expansion of pathogens recognized by HBC including Travelan® could revolutionize current management of gastrointestinal infections and outbreaks in travelers' including military, peacekeepers, humanitarian workers and in populations living in endemic regions of the world.

Antibody induction in mice by liposome-displayed recombinant enterotoxigenic Escherichia coli (ETEC) colonization antigens

BACKGROUND

Enterotoxigenic Escherichia coli (ETEC) strains cause infectious diarrhea and colonize host intestine epithelia via surface-expressed colonization factors. Colonization factor antigen I (CFA/I), a prevalent ETEC colonization factor, is a vaccine target since antibodies directed to this fimbria can block ETEC adherence and prevent diarrhea.

METHODS

Two recombinant antigens derived from CFA/I were investigated with a vaccine adjuvant system that displays soluble antigens on the surface of immunogenic liposomes. The first antigen, CfaEB, is a chimeric fusion protein comprising the minor (CfaE) and major (CfaB) subunits of CFA/I. The second, CfaEad, is the adhesin domain of CfaE.

RESULTS

Owing to their His-tag, recombinant CfaEB and CfaEad, spontaneously bound upon admixture with nanoliposomes containing cobalt-porphyrin phospholipid (CoPoP), as well as a synthetic monophosphoryl lipid A (PHAD) adjuvant. Intramuscular immunization of mice with sub-microgram doses CfaEB or CfaEad admixed with CoPoP/PHAD liposomes elicited serum IgG and intestinal IgA antibodies. The smaller CfaEad antigen benefitted more from liposome display. Serum and intestine antibodies from mice immunized with liposome-displayed CfaEB or CfaEad recognized native CFA/I fimbria as evidenced by immunofluorescence and hemagglutination inhibition assays using the CFA/I-expressing H10407 ETEC strain.

CONCLUSION

These data show that colonization factor-derived recombinant ETEC antigens exhibit immunogenicity when delivered in immunogenic particle-based formulations.

Vaccine value profile for enterotoxigenic Escherichia coli (ETEC)

Enterotoxigenic Escherichia coli (ETEC) is one of the leading bacterial causes of diarrhoea, especially among children in low-resource settings, and travellers and military personnel from high-income countries. WHO's primary strategic goal for ETEC vaccine development is to develop a safe, effective, and affordable ETEC vaccine that reduces mortality and morbidity due to moderate-to-severe diarrhoeal disease in infants and children under 5 years of age in LMICs, as well as the long-term negative health impact on infant physical and cognitive development resulting from infection with this enteric pathogen. An effective ETEC vaccine will also likely reduce the need for antibiotic treatment and help limit the further emergence of antimicrobial resistance bacterial pathogens. The lead ETEC vaccine candidate, ETVAX, has shown field efficacy in travellers and has moved into field efficacy testing in LMIC infants and children. A Phase 3 efficacy study in LMIC infants is projected to start in 2024 and plans for a Phase 3 trial in travellers are under discussion with the U.S. FDA. Licensing for both travel and LMIC indications is projected to be feasible in the next 5-8 years. Given increasing recognition of its negative impact on child health and development in LMICs and predominance as the leading etiology of travellers' diarrhoea (TD), a standalone vaccine for ETEC is more cost-effective than vaccines targeting other TD pathogens, and a viable commercial market also exists. In contrast, combination of an ETEC vaccine with other vaccines for childhood pathogens in LMICs would maximize protection in a more cost-effective manner than a series of stand-alone vaccines. This 'Vaccine Value Profile' (VVP) for ETEC is intended to provide a high-level, holistic assessment of available data to inform the potential public health, economic and societal value of pipeline vaccines and vaccine-like products. This VVP was developed by a working group of subject matter experts from academia, non-profit organizations, public private partnerships, and multi-lateral organizations. All contributors have extensive expertise on various elements of the ETEC VVP and collectively aimed to identify current research and knowledge gaps. The VVP was developed using only existing and publicly available information.

A First in Human Clinical Trial Assessing the Safety and Immunogenicity of Two Intradermally Delivered Enterotoxigenic Escherichia coli CFA/I Fimbrial Tip Adhesin Antigens with and without Heat-Labile Enterotoxin with Mutation LT(R192G)

INTRODUCTION

Enterotoxigenic E. coli (ETEC) is a leading cause of diarrhea in travelers as well as for children living in low- to middle-income countries. ETEC adhere to intestinal epithelium via colonization factors (CFs). CFA/I, a common CF, is composed of a polymeric stalk and a tip-localized minor adhesive subunit, CfaE. Vaccine delivery by the transcutaneous immunization of dscCfaE was safe but was poorly immunogenic in a phase 1 trial when administered to volunteers with LTR(192G) and mLT. To potentially enhance the immunogenicity of CfaE while still delivering via a cutaneous route, we evaluated the safety and immunogenicity of two CfaE constructs administered intradermally (ID) with or without mLT.

METHODS

CfaE was evaluated as a donor strand-complemented construct (dscCfaE) and as a chimeric construct (Chimera) in which dscCfaE replaces the A1 domain of the cholera toxin A subunit and assembles non-covalently with the pentamer of heat-labile toxin B (LTB). Subjects received three ID vaccinations three weeks apart with either dscCfaE (1, 5, and 25 µg) or Chimera (2.6 and 12.9 µg) with and without 0.1 µg of mLT. Subjects were monitored for local and systemic adverse events. Immunogenicity was evaluated by serum and antibody-secreting cell (ASC) responses.

RESULTS

The vaccine was well-tolerated with predominantly mild and moderate local vaccine site reactions characterized by erythema, induration and post-inflammatory hyperpigmentation. High rates of serologic and ASC responses were seen across study groups with the most robust responses observed in subjects receiving 25 µg of dscCfaE with 0.1 mcg of LT(R192G).

CONCLUSION

Both ETEC adhesin vaccine prototypes were safe and immunogenic when co-administered with mLT by the ID route. The observed immune responses induced with the high dose of dscCfaE and mLT warrant further assessment in a controlled human infection model.

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.

Detection of early myocardial cell death in owl monkeys (Aotus nancymai) using complement component C9 immunohistochemistry in formalin-fixed paraffin-embedded heart tissues: A retrospective study

BACKGROUND

Owl monkeys are commonly used in biomedical research which is affected by the high incidence of cardiomyopathy in this species. Occasionally, owl monkeys with no clinical signs of heart disease are found dead and at necropsy show no, or very mild, cardiomyopathy. A possible explanation for sudden death is acute myocardial infarction; however, early myocardial changes may be difficult to assess by conventional stains and light microscopy.

METHODS

Complement component C9 immunohistochemistry was performed in paraffin-embedded heart tissue samples from owl monkeys who died suddenly, or were euthanized due to sickness, to determine whether these animals suffered from acute myocardial infarcts.

RESULTS AND CONCLUSION

C9 deposits were found in the myocardium of 19 out of 20 (95%) animals. The findings in this study suggest owl monkeys suffer from acute myocardial infarcts, and complement component C9 immunohistochemistry may be a useful diagnostic tool.

Safety and immunogenicity of intramuscularly administered CS6 subunit vaccine with a modified heat-labile enterotoxin from enterotoxigenic Escherichia coli

INTRODUCTION

Enterotoxigenic Escherichia coli (ETEC) is a common cause of infectious diarrhoea and a leading cause of morbidity and mortality in children living in resource-limited settings. It is also the leading cause of travellers' diarrhoea among civilian and military travellers. Its dual importance in global public health and travel medicine highlights the need for an effective vaccine. ETEC express colonization factors (CFs) that mediate adherence to the small intestine. An epidemiologically prevalent CF is coli surface antigen 6 (CS6). We assessed the safety and immunogenicity of a CS6-targeted candidate vaccine, CssBA, co-administered intramuscularly with the double-mutant heat-labile enterotoxin, dmLT [LT(R192G/L211A)].

METHODS

This was an open-label trial. Fifty subjects received three intramuscular injections (Days 1, 22 and 43) of CssBA alone (5 µg), dmLT alone (0.1 µg) or CssBA (5, 15, 45 µg) + dmLT (0.1 and 0.5 µg). Subjects were actively monitored for adverse events for 28 days following the third vaccination. Antibody responses (IgG and IgA) were characterized in the serum and from lymphocyte supernatants (ALS) to CS6 and the native ETEC heat labile enterotoxin, LT.

RESULTS

Across all dose cohorts, the vaccine was safe and well-tolerated with no vaccine-related severe or serious adverse events. Among vaccine-related adverse events, a majority (98%) were mild with 79% being short-lived vaccine site reactions. Robust antibody responses were induced in a dose-dependent manner with a clear dmLT adjuvant effect. Response rates in subjects receiving 45 µg CssBA and 0.5 µg dmLT ranged from 50 to 100% across assays.

CONCLUSION

This is the first study to demonstrate the safety and immunogenicity of CssBA and/or dmLT administered intramuscularly. Co-administration of the two components induced robust immune responses to CS6 and LT, paving the way for future studies to evaluate the efficacy of this vaccine target and development of a multivalent, subunit ETEC vaccine.

Development and Comparison of a Panel of Modified CS17 Fimbrial Tip Adhesin Proteins as Components for an Adhesin-Based Vaccine against Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is a leading cause of diarrhea in travelers and children in resource-limited countries. ETEC colonization factors, fimbrial tip adhesins and enterotoxins are key virulence factors, and thus have been studied as vaccine candidates. Some prevalent colonization factors, including CFA/I and CS17, belong to the class 5 family. We previously found that passive oral administration of hyperimmune bovine colostral IgG (bIgG) raised against dscCfaE (donor strand complemented CFA/I tip adhesin) protected volunteers against CFA/I⁺ ETEC challenge, while anti-dscCsbD bIgG (CS17 tip adhesin) did not confer protection. These findings led us to develop and optimize a panel of alternative CsbD-based vaccine candidates based on allele matching and in silico protein engineering. Physicochemical characterizations revealed that an optimized vaccine candidate dscCsbD_LSN139(P218A/G3) had the greatest thermal stability among the six tested dscCsbD adhesins, whereas the overall secondary structures and solubility of these adhesins had no obvious differences. Importantly, dscCsbD_LSN139(P218A/G3) elicited significantly higher CS17⁺ ETEC hemagglutination inhibition titers in sera from mice intranasally immunized with the panel of dscCsbD adhesins, while no significant difference was observed among heterologous neutralizing titers. Our results strongly advocate for the incorporation of these modifications into a new generation of CsbD-based ETEC vaccine candidates.

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.

Immunomodulatory regulation by heat-labile enterotoxins and potential therapeutic applications

Introduction: Heat-labile enterotoxins (HLTs) and their cognate ganglioside receptors have been extensively studied because of their therapeutic potential. Gangliosides play arole in modulating effector cells of the immune system, and HLTs provide a novel means for stimulating ganglioside-mediated responses in immunocompetent cells.Areas covered: To evaluate the mechanisms of HLT adjuvanticity, a systemic literature review was performed using relevant keyword searches of the PubMed database, accessing literature published as recently as late 2020. Since HLTs bind to specific ganglioside receptors on immunocytes, they can act as regulators via stimulation or tapering of immune responses from associated signal transduction events. Binding of HLTs to gangliosides can increase proliferation of T-cells, increase cytokine release, augment mucosal/systemic antibody responses, and increase the effectiveness of antigen presenting cells. Subunit components also independently stimulate certain immune responses. Mutant forms of HLTs have potent immunomodulatory effects without the toxicity associated with holotoxins.Expert opinion: HLTs have been the subject of abundant research exploring their use as vaccine adjuvants, in the treatment of autoimmune conditions, in cancer therapy, and for weight loss, proving that these molecules are promising tools in the field of immunotherapy.

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.

Functional analysis of colonization factor antigen I positive enterotoxigenic Escherichia coli identifies genes implicated in survival in water and host colonization

Enterotoxigenic Escherichia coli (ETEC) expressing the colonization pili CFA/I are common causes of diarrhoeal infections in humans. Here, we use a combination of transposon mutagenesis and transcriptomic analysis to identify genes and pathways that contribute to ETEC persistence in water environments and colonization of a mammalian host. ETEC persisting in water exhibit a distinct RNA expression profile from those growing in richer media. Multiple pathways were identified that contribute to water survival, including lipopolysaccharide biosynthesis and stress response regulons. The analysis also indicated that ETEC growing in vivo in mice encounter a bottleneck driving down the diversity of colonizing ETEC populations.

Establishment and Validation of Pathogenic CS17+ and CS19+ Enterotoxigenic Escherichia coli Challenge Models in the New World Primate Aotus nancymaae

Enterotoxigenic Escherichia coli (ETEC) is a common cause of diarrheal illness in the military, travelers, and children living in low- to middle-income countries. Increased antibiotic resistance, the absence of a licensed vaccine, and the lack of broadly practical therapeutics perpetuate the significant health and financial burden resulting from ETEC infection. A critical step in the evaluation of vaccines and therapeutics is preclinical screening in a relevant animal disease model that closely replicates human disease. We previously developed a diarrheal model of class 5a colonization factor (CF) CFA/I-expressing ETEC in the New World owl monkey species Aotus nancymaae using ETEC strain H10407. In order to broaden the use of the model, we report here on the development of A. nancymaae models of ETEC expressing the class 5b CFs CS17 and CS19 with strains LSN03-016011/A and WS0115A, respectively. For both models, we observed diarrheal attack rates of ≥80% after oral inoculation with 5 × 10¹¹ CFU of bacteria. These models will aid in assessing the efficacy of future ETEC vaccine candidates and therapeutics.

A One Health Perspective for Defining and Deciphering Escherichia coli Pathogenic Potential in Multiple Hosts

E. coli is one of the most common species of bacteria colonizing humans and animals. The singularity of E. coli 's genus and species underestimates its multifaceted nature, which is represented by different strains, each with different combinations of distinct virulence factors. In fact, several E. coli pathotypes, or hybrid strains, may be associated with both subclinical infection and a range of clinical conditions, including enteric, urinary, and systemic infections. E. coli may also express DNA-damaging toxins that could impact cancer development. This review summarizes the different E. coli pathotypes in the context of their history, hosts, clinical signs, epidemiology, and control. The pathotypic characterization of E. coli in the context of disease in different animals, including humans, provides comparative and One Health perspectives that will guide future clinical and research investigations of E. coli infections.

Evaluation of the immunogenicity and protective efficacy of a recombinant CS6-based ETEC vaccine in an Aotus nancymaae CS6 + ETEC challenge model

Colonization factors or Coli surface antigens (CFs or CS) are important virulence factors of Enterotoxigenic E. coli (ETEC) that mediate intestinal colonization and accordingly are targets of vaccine development efforts. CS6 is a highly prevalent CF associated with symptomatic ETEC infection both in endemic populations and amongst travelers. In this study, we used an Aotus nancymaae non-human primate ETEC challenge model with a CS6 + ETEC strain, B7A, to test the immunogenicity and protective efficacy (PE) of a recombinant CS6-based subunit vaccine. Specifically, we determined the ability of dscCssBA, the donor strand complemented recombinant stabilized fusion of the two subunits of the CS6 fimbriae, CssA and CssB, to elicit protection against CS6 + ETEC mediated diarrhea when given intradermally (ID) with the genetically attenuated double mutant heat-labile enterotoxin LT(R192G/L211A) (dmLT). ID vaccination with dscCssBA + dmLT induced strong serum antibody responses against CS6 and LT. Importantly, vaccination with dscCssBA + dmLT resulted in no observed diarrheal disease (PE = 100%, p = 0.03) following B7A challenge as compared to PBS immunized animals, with an attack rate of 62.5%. These data demonstrate the potential role that CS6 may play in ETEC infection and that recombinant dscCssBA antigen can provide protection against challenge with the homologous CS6 + ETEC strain, B7A, in the Aotus nancymaae diarrheal challenge model. Combined, these data indicate that CS6, and more specifically, a recombinant engineered derivative should be considered for further clinical development.

Cross-Reactivity, Epitope Mapping, and Potency of Monoclonal Antibodies to Class 5 Fimbrial Tip Adhesins of Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is a leading diarrheagenic bacterial pathogen among travelers and children in resource-limited regions. Adherence to host intestinal cells mediated by ETEC fimbriae is believed to be a critical first step in ETEC pathogenesis. These fimbriae are categorized into related classes based on sequence similarity, with members of the class 5 fimbrial family being the best characterized. The eight related members of the ETEC class 5 fimbrial family are subdivided into three subclasses (5a, 5b, and 5c) that share similar structural arrangements, including a fimbrial tip adhesin.

Enterotoxigenic Escherichia coli (ETEC) is a leading diarrheagenic bacterial pathogen among travelers and children in resource-limited regions. Adherence to host intestinal cells mediated by ETEC fimbriae is believed to be a critical first step in ETEC pathogenesis. These fimbriae are categorized into related classes based on sequence similarity, with members of the class 5 fimbrial family being the best characterized. The eight related members of the ETEC class 5 fimbrial family are subdivided into three subclasses (5a, 5b, and 5c) that share similar structural arrangements, including a fimbrial tip adhesin. However, sequence variability among the class 5 adhesins may hinder the generation of cross-protective antibodies. To better understand functional epitopes of the class 5 adhesins and their ability to induce intraclass antibody responses, we produced 28 antiadhesin monoclonal antibodies (MAbs) to representative adhesins CfaE, CsbD, and CotD, respectively. We determined the MAb cross-reactivities, localized the epitopes, and measured functional activities as potency in inhibition of hemagglutination induced by class 5 fimbria-bearing ETEC. The MAbs’ reactivities to a panel of class 5 adhesins in enzyme-linked immunosorbent assays (ELISAs) revealed several reactivity patterns, including individual adhesin specificity, intrasubclass specificity, intersubclass specificity, and class-wide cross-reactivity, suggesting that some conserved epitopes, including two conserved arginines, are shared by the class 5 adhesins. However, the cross-reactive MAbs had functional activities limited to strains expressing colonization factor antigen I (CFA/I), coli surface antigen 17 (CS17), or CS1, suggesting that the breadth of functional activities of the MAbs was more restricted than the repertoire of cross-reactivities measured by ELISA. The results imply that multivalent adhesin-based ETEC vaccines or prophylactics need more than one active component to reach broad protection.

Evaluation of the Immunogenicity and Protective Efficacy of an Enterotoxigenic Escherichia coli CFA/I Adhesin-Heat-Labile Toxin Chimera

Recent efforts to develop an enterotoxigenic Escherichia coli (ETEC) vaccine have focused on the antigenically conserved tip adhesins of colonization factors. We showed previously that intranasal immunization with dsc₁₉CfaE, a soluble variant of the in cis donor strand-complemented tip adhesin of a colonization factor of the class 5 family (CFA/I) fimbria, is highly immunogenic and protects against oral challenge with CFA/I-positive (CFA/I⁺) ETEC strain H10407 in the Aotus nancymaae nonhuman primate. We also reported a cholera toxin (CT)-like chimera (called dsc₁₉CfaE-CTA2/CTB) in which the CTA1 domain of CT was replaced by dsc₁₉CfaE that was strongly immunogenic when administered intranasally or orogastrically in mice. Here, we evaluate the immunogenicity and protective efficacy (PE) of a refined and more stable chimera comprised of a pentameric B subunit of ETEC heat-labile toxin (LTB) in lieu of the CTB pentamer and a donor strand truncation (dsc₁₄) of CfaE. The refined chimera, dsc₁₄CfaE-sCTA2/LTB, was highly immunogenic in mice when administered intranasally or intradermally, eliciting serum and fecal antibody responses against CfaE and LTB, as well as strong hemagglutination inhibition titers, a surrogate for neutralization of intestinal adhesion mediated by CfaE. Moreover, the chimera was safe and highly immunogenic when administered intradermally to guinea pigs. In A. nancymaae, intradermal (i.d.) immunization with chimera plus single-mutant heat-labile toxin [LT(R192G)] elicited strong serum anti-CfaE and anti-LTB antibody responses and conferred significant reduction of diarrhea compared to phosphate-buffered saline (PBS) controls (PE = 84.1%; P < 0.02). These data support the further evaluation of dsc₁₄CfaE-sCTA2/LTB as an ETEC vaccine in humans.

Preclinical optimization of an enterotoxigenic Escherichia coli adjuvanted subunit vaccine using response surface design of experiments

Enterotoxigenic E. coli (ETEC) is a leading cause of moderate-to-severe diarrhoea. ETEC colonizes the intestine through fimbrial tip adhesin colonization factors and produces heat-stable and/or heat-labile (LT) toxins, stimulating fluid and electrolyte release leading to watery diarrhoea. We reported that a vaccine containing recombinant colonization factor antigen (CfaEB) targeting fimbrial tip adhesin of the colonization factor antigen I (CFA/I) and an attenuated LT toxoid (dmLT) elicited mucosal and systemic immune responses against both targets. Additionally, the toll-like receptor 4 ligand second-generation lipid adjuvant (TLR4-SLA) induced a potent mucosal response, dependent on adjuvant formulation. However, a combination of vaccine components at their respective individual optimal doses may not achieve the optimal immune profile. We studied a subunit ETEC vaccine prototype in mice using a response surface design of experiments (DoE), consisting of 64 vaccine dose-combinations of CfaEB, dmLT and SLA in four formulations (aqueous, aluminium oxyhydroxide, squalene-in-water stable nanoemulsion [SE] or liposomes containing the saponin Quillaja saponaria-21 [LSQ]). Nine readouts focusing on antibody functionality and plasma cell response were selected to profile the immune response of parenterally administered ETEC vaccine prototype. The data were integrated in a model to identify the optimal dosage of each vaccine component and best formulation. Compared to maximal doses used in mouse models (10 µg CfaEB, 1 µg dmLT and 5 µg SLA), a reduction in the vaccine components up to 37%, 60% and 88% for CfaEB, dmLT and SLA, respectively, maintained or even maximized immune responses, with SE and LSQ the best formulations. The DoE approach can help determine the best vaccine composition with a limited number of experiments and may accelerate development of multi-antigen/component ETEC vaccines.

Biochemical and immunological characterization of an ETEC CFA/I adhesin cholera toxin B subunit chimera

Surface-expressed colonization factors and their subunits are promising candidates for inclusion into a multivalent vaccine targeting enterotoxigenic Escherichia coli (ETEC), a leading cause of acute bacterial diarrhea in developing regions. However, soluble antigens are often poorly immunogenic in the absence of an adjuvant. We show here that the serum immune response to CfaE, the adhesin of the ETEC colonization factor CFA/I, can be enhanced in BALB/c mice by immunization with a chimeric antigen containing CfaE and pentameric cholera toxin B subunit (CTB) of cholera toxin from Vibrio cholerae. We constructed this antigen by replacing the coding sequence for the A1 domain of the cholera toxin A subunit (CTA) with the sequence of donor strand complemented CfaE (dscCfaE) within the cholera toxin operon, resulting in a dscCfaE-CTA2 fusion. After expression, via non-covalent interactions between CTA2 and CTB, the fusion and CTB polypeptides assemble into a complex containing a single dscCfaE-CTA2 protein bound to pentameric CTB (dscCfaE-CTA2/CTB). This holotoxin-like chimera retained the GM1 ganglioside binding activity of CTB, as well as the ability of CfaE to mediate the agglutination of bovine red blood cells when adsorbed to polystyrene beads. When administered intranasally to mice, the presence of CTB in the chimera significantly increased the serum immune response to CfaE compared to dscCfaE alone, stimulating a response similar to that obtained with a matched admixture of dscCfaE and CTB. However, by the orogastric route, immunization with the chimera elicited a superior functional immune response compared to an equivalent admixture of dscCfaE and CTB, supporting further investigation of the chimera as an ETEC vaccine candidate.

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.

Evaluation of transcutaneous immunization as a delivery route for an enterotoxigenic E. coli adhesin-based vaccine with CfaE, the colonization factor antigen 1 (CFA/I) tip adhesin

dscCfaE is a recombinant form of the CFA/I tip adhesin CfaE, expressed by a large proportion of enterotoxigenic E. coli (ETEC). It is highly immunogenic by the intranasal route in mice and Aotus nancymaae, protective against challenge with CFA/I+ ETEC in an A. nancymaae challenge model, and antibodies to dscCfaE passively protect against CFA/I+ ETEC challenge in human volunteers. Here, we show that transcutaneous immunization (TCI) with dscCfaE in mice resulted in strong anti-CfaE IgG serum responses, with a clear dose-response effect. Co-administration with heat-labile enterotoxin (LT) resulted in enhanced immune responses over those elicited by dscCfaE alone and strong anti-LT antibody responses. The highest dose of dscCfaE administered transcutaneously with LT elicited strong HAI titers, a surrogate for the neutralization of intestinal adhesion. Fecal anti-adhesin IgG and IgA antibody responses were also induced. These findings support the feasibility of TCI for the application of an adhesin-toxin based ETEC vaccine.