Comparison of the antibody in lymphocyte supernatant (ALS) and ELISPOT assays for detection of mucosal immune responses to antigens of enterotoxigenic Escherichia coli in challenged and vaccinated volunteers

Protective antibodies against enterotoxigenic Escherichia coli are generated from heat-labile toxoid vaccination and exhibit subject- and vaccine-specific diversity

Heat-labile toxin (LT) from enterotoxigenic Escherichia coli (ETEC) is an important pathogenic protein. Anti-LT antibodies (Abs) induced by vaccination can neutralize the toxin and potentially prevent diarrheal secretion from ~ 60% of ETEC strains expressing LT. However, only superficial investigation of the anti-toxin response is usually conducted in clinical trials. Here, we utilized human serum samples from two clinical trials performed to assess safety, immunogenicity and protection in a controlled human infection model with a LT + ST + CFA/I + H10407 ETEC strain. These Phase 1 and Phase 2b clinical trials explored a prototype ETEC adhesin (CfaE) and a chimeric adhesin-toxoid protein (dscCfaE-CTA2/LTB5) delivered intradermally or transcutaneously with a mutated form of LT (mLT) as an adjuvant. Serum samples were tested for antigen-specific IgG or IgA Abs by immunoblot, enzyme-linked immunosorbent assay (ELISA), or functional neutralizing Abs using LT holotoxin, LTA or LTB subunits. Abs to both LT subunits were present, but the response to each was altered by vaccine formulation, dose, and delivery routes as well as subject. The anti-LT IgG response correlated best to neutralizing antibodies and protection from H10407 controlled challenge when compared to other measures including serum IgA or anti-fimbriae (CfaE) Abs. In addition, our results helped to explain cohort attack rate differences in naïve unvaccinated participants and we found higher anti-LTA IgG post-challenge significantly related to ETEC severity score. Thus, strategies generating and measuring immunity to the complete AB5 structure of LT and subunits are better determinant of assessing protective immunity against LT + or LT + ST + ETEC diarrheal secretion in humans.

Safety and immunogenicity in humans of enterotoxigenic Escherichia coli double mutant heat-labile toxin administered intradermally

Enterotoxigenic Escherichia coli (ETEC) diarrhea is associated with a high burden of disease globally, for which no licensed vaccine is available. A Phase 1, double-blind, dose-escalation (0.1-2.0 µg) study was conducted to evaluate the safety and immunogenicity of double mutant heat-labile toxin LTR192G/L211A (dmLT) delivered intradermally (ID) to healthy adults. Subjects received up to three immunizations at three-week intervals. The vaccine was safe, although it induced mild local and some gastrointestinal adverse events, as well as frequent hyperpigmentation at the injection site. High levels of serum IgG and IgA, LT neutralizing antibodies, and IgG and IgA antibodies in lymphocyte supernatant were elicited post-vaccination, most prominently at the largest dose (2.0 μg). Rates of responses were the highest in subjects who received the largest dose (2.0 μg) and multiple immunizations. The ETEC dmLT vaccine was safe and highly immunogenic, inducing long-lasting systemic and mucosal responses when administered by the ID route. Trial registration Clinical Trials NCT02531685.

Proportions of IgA antibodies targeting glycosylated epitopes of secreted Escherichia coli mucinase YghJ in initial plasmablast response differ from salivary and intestinally secreted IgA

Mucosal infections normally cause an immune response including activation of antigen-specific B cells in regional mucosa-associated lymphoid tissue. After recirculation of plasmablasts, and maturation at mucosal surfaces or bone marrow, plasma cells produce secretory or systemic IgA. It remains uncertain to what extent secretory and systemic IgA share the same target specificities. For vaccine candidate optimization, it is important to know whether IgA targeting of glycosylated epitopes of a protein antigen vary between mucosal and systemic sites. We evaluated glycosylated epitope specificity of systemic and mucosally secreted IgA against YghJ, a potential vaccine candidate antigen secreted by most pathogenic Escherichia coli. IgA from intestinal lavage, saliva, serum, and blood-derived antibody in lymphocyte supernatants (ALS) were collected from 21 volunteers following experimental infection with enterotoxigenic E. coli. Methods for preparing IgA from saliva and ALS were developed, and multiplex bead flow cytometric immunoassays were used to determine levels of IgA targeting natively glycosylated YghJ and estimating what proportion of these antibodies specifically targeted glycosylated epitopes. Following infection, anti-YghJ IgA levels increased substantially for most volunteers across all four specimen types. Target specificity of ALS IgA correlated well with serum IgA, but not with mucosally secreted IgA. Furthermore, glycosylation-specific proportion of salivary IgA was higher than, and did not correlate with, intestinally secreted IgA. These results indicate a new degree of complexity to our understanding of epitope-targeting and tissue specificity of mucosal antibody responses. Our findings also suggest that all features of an intestinal IgA response may not be well reflected in serum, saliva, or ALS, which are commonly used proxy specimens for evaluating intestinal immune responses.

The Controlled Human Infection Model for Enterotoxigenic Escherichia coli

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

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.

Antibody in Lymphocyte Supernatant (ALS) responses after oral vaccination with live Shigella sonnei vaccine candidates WRSs2 and WRSs3 and correlation with serum antibodies, ASCs, fecal IgA and shedding

The levels of antigen-specific Antibodies in Lymphocyte Supernatant (ALS) using an ELISA are being used to evaluate mucosal immune responses as an alternate to measuring the number of Antibody Secreting Cells (ASCs) using an ELISpot assay. A recently completed trial of two novel S. sonnei live oral vaccine candidates WRSs2 and WRSs3 established that both candidates were safe, well tolerated and immunogenic in a vaccine dose-dependent manner. Previously, mucosal immune responses were measured by assaying IgA- and IgG-ASC in peripheral blood mononuclear cells (PBMCs). In this report, the magnitude of the S. sonnei antigen-specific IgA- and IgG-ALS responses was measured and correlated with previously described ASCs, serum antibodies, fecal IgA and vaccine shedding. Overall, the magnitude of S. sonnei anti-Invaplex50 ALS was higher than that of LPS or IpaB, and both vaccines demonstrated a more robust IgA-ALS response than IgG; however, compared to WRSs3, the magnitude and percentage of responders were higher among WRSs2 recipients for IgA- or IgG-ALS. All WRSs2 vaccinees at the two highest doses responded for LPS and Invaplex50-specific IgA-ALS and 63-100% for WRSs3 vaccinees responded. Regardless of the vaccine candidate, vaccine dose or detecting antigen, the kinetics of ALS responses were similar peaking on days 7 to 9 and returning to baseline by day 14. The ALS responses were vaccine-specific since no responses were detected among placebo recipients at any time. A strong correlation and agreement between responders/non-responders were noted between ALS and other mucosal (ASC and fecal IgA) and systemic (serum antibody) immune responses. These data indicate that the ALS assay can be a useful tool to evaluate mucosal responses to oral vaccination, an observation noted with trials of other bacterial diarrheal pathogens. Furthermore, this data will guide the list of immunological assays to be conducted for efficacy trials in different populations. It is hoped that an antigen-specific-ALS titer may be a key mucosal correlate of protection, a feature not currently available for any Shigella vaccines candidates. https://clinicaltrials.gov/show/NCT01336699.

Systemic and intestinal porcine epidemic diarrhea virus-specific antibody response and distribution of antibody-secreting cells in experimentally infected conventional pigs

Porcine epidemic diarrhea (PED) is a coronavirus disease characterized by the rapid spread of severe diarrhea among pigs. PED virus (PEDV) infects and replicates mainly in the epithelial cells of the duodenum, jejunum, ileum and colon. Serum or mucosal IgA antibody levels have been used to predict both vaccine efficacy and the level of protective immunity to enteric infectious diseases in individuals or herds. Details of the B-cell immune response upon PEDV infection, such as the systemic and mucosal PEDV IgA antibody response, the distribution of IgA antibody-secreting cells (ASCs), and their role in virus clearance are not yet clear. In this experimental infection study, we observed similar fluctuations in PEDV IgA antibody levels in serum and intestinal contents of the upper and lower jejunum and ileum, but not fecal samples, over the 4-week experimental course. ASCs that actively secrete PEDV IgA antibody without in vitro stimulation were distributed mainly in the upper jejunum, whereas memory B cells that showed enhanced PEDV IgA antibody production upon in vitro stimulation were observed in mesenteric lymph nodes and the ileum. Our findings will contribute to the development of effective vaccines and diagnostic methods for PEDV.

Human Mucosal IgA Immune Responses against Enterotoxigenic Escherichia coli

Infection with enterotoxigenic Escherichia coli (ETEC) is a major contributor to diarrheal illness in children in low- and middle-income countries and travelers to these areas. There is an ongoing effort to develop vaccines against ETEC, and the most reliable immune correlate of protection against ETEC is considered to be the small intestinal secretory IgA response that targets ETEC-specific virulence factors. Since isolating IgA from small intestinal mucosa is technically and ethically challenging, requiring the use of invasive medical procedures, several other indirect methods are used as a proxy for gauging the small intestinal IgA responses. In this review, we summarize the literature reporting on anti-ETEC human IgA responses observed in blood, activated lymphocyte assayss, intestinal lavage/duodenal aspirates, and saliva from human volunteers being experimentally infected with ETEC. We describe the IgA response kinetics and responder ratios against classical and noncanonical ETEC antigens in the different sample types and discuss the implications that the results may have on vaccine development and testing.

A roadmap for enterotoxigenic Escherichia coli vaccine development based on volunteer challenge studies

Enterotoxigenic Escherichia coli (ETEC) is a major cause of travelers’ diarrhea and of diarrhea among young children in developing countries. Experimental challenge studies in adult volunteers have played a pivotal role in establishing ETEC as an enteric pathogen, elucidating its pathogenesis by identifying specific virulence attributes, characterizing the human immune response to clinical and sub-clinical ETEC infection and assessing preliminarily the clinical acceptability, immunogenicity and efficacy of prototype ETEC vaccines. This review provides a historical perspective of experimental challenge studies with ETEC. It summarizes pioneering early studies carried out by investigators at the University of Maryland School of Medicine to show how those studies provided key information that influenced the directions taken by many research groups to develop vaccines to prevent ETEC. In addition, key experimental challenge studies undertaken at other institutions will also be cited.

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.

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

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

The Antibody-Secreting Cell Response to Infection: Kinetics and Clinical Applications

Despite the availability of advances in molecular diagnostic testing for infectious disease, there is still a need for tools that advance clinical care and public health. Current methods focus on pathogen detection with unprecedented precision, but often lack specificity. In contrast, the host immune response is highly specific for the infecting pathogen. Serological studies are rarely helpful in clinical settings, as they require acute and convalescent antibody testing. However, the B cell response is much more rapid and short-lived, making it an optimal target for determining disease aetiology in patients with infections. The performance of tests that aim to detect circulating antigen-specific antibody-secreting cells (ASCs) has previously been unclear. Test performance is reliant on detecting the presence of ASCs in the peripheral blood. As such, the kinetics of the ASC response to infection, the antigen specificity of the ASC response, and the methods of ASC detection are all critical. In this review, we summarize previous studies that have used techniques to enumerate ASCs during infection. We describe the emergence, peak, and waning of these cells in peripheral blood during infection with a number of bacterial and viral pathogens, as well as malaria infection. We find that the timing of antigen-specific ASC appearance and disappearance is highly conserved across pathogens, with a peak response between day 7 and day 8 of illness and largely absent following day 14 since onset of symptoms. Data show a sensitivity of ~90% and specificity >80% for pathogen detection using ASC-based methods. Overall, the summarised work indicates that ASC-based methods may be very sensitive and highly specific for determining the etiology of infection and have some advantages over current methods. Important areas of research remain, including more accurate definition of the timing of the ASC response to infection, the biological mechanisms underlying variability in its magnitude and the evolution and the B cell receptor in response to immune challenge. Nonetheless, there is potential of the ASC response to infection to be exploited as the basis for novel diagnostic tests to inform clinical care and public health priorities.

Clinical Trial of an Oral Live Shigella sonnei Vaccine Candidate, WRSS1, in Thai Adults

Live attenuated Shigella sonnei vaccine candidate WRSS1, previously tested in U.S. and Israeli volunteers, was evaluated in a population of adult Thai volunteers in which the organism is endemic. In a randomized placebo-controlled, double-blind design, inpatient participants received a single oral dose of 1.6 × 10(4) CFU of WRSS1. The vaccine was generally well tolerated, with equal numbers of vaccinees and placebo controls showing mild symptoms. Only 3 of 13 vaccinees (23%) had culture-positive stools, while a total of 9 vaccinees were positive by PCR. Lack of vaccine shedding in volunteers correlated with lack of clinical symptoms and immune responses, just as the duration of fecal shedding correlated directly with stronger immune responses. Two months following immunization, 10 vaccinees and 10 newly recruited naive controls received a challenge dose of 1,670 CFU of virulent S. sonnei strain 53G. This dose had previously demonstrated a 75% attack rate for dysentery in Thai volunteers. However, in this study the attack rate for dysentery in naive controls after challenge was 20%. Based on clinical record summaries, 3 vaccinees and 5 naive controls experienced clinically relevant illness (diarrhea/dysentery/fever/shigellosis), and a 40% vaccine efficacy was calculated. When these data are compared to those for the performance of this vaccine candidate in more naive populations, it is clear that a single oral dose of WRSS1 at 10(4) CFU failed to achieve its full potential in a population in which the organism is endemic. Higher doses and/or repeated immunizations may contribute to improved vaccine shedding and consequent elevation of protective immune responses in a population in which the organism is endemic. (The study has been registered at ClinicalTrials.gov under registration no. NCT01080716.).

Circulating Gut-Homing (α4β7+) Plasmablast Responses against Shigella Surface Protein Antigens among Hospitalized Patients with Diarrhea

Developing countries are burdened with Shigella diarrhea. Understanding mucosal immune responses associated with natural Shigella infection is important to identify potential correlates of protection and, as such, to design effective vaccines. We performed a comparative analysis of circulating mucosal plasmablasts producing specific antibodies against highly conserved invasive plasmid antigens (IpaC, IpaD20, and IpaD120) and two recently identified surface protein antigens, pan-Shigella surface protein antigen 1 (PSSP1) and PSSP2, common to all virulent Shigella strains. We examined blood and stool specimens from 37 diarrheal patients admitted to the Infectious Diseases & Beliaghata General Hospital, Kolkata, India. The etiological agent of diarrhea was investigated in stool specimens by microbiological methods and real-time PCR. Gut-homing (α4β7 (+)) antibody-secreting cells (ASCs) were isolated from patient blood by means of combined magnetic cell sorting and two-color enzyme-linked immunosorbent spot (ELISPOT) assay. Overall, 57% (21 of 37) and 65% (24 of 37) of the patients were positive for Shigella infection by microbiological and real-time PCR assays, respectively. The frequency of α4β7 (+) IgG ASC responders against Ipas was higher than that observed against PSSP1 or PSSP2, regardless of the Shigella serotype isolated from these patients. Thus, α4β7 (+) ASC responses to Ipas may be considered an indirect marker of Shigella infection. The apparent weakness of ASC responses to PSSP1 is consistent with the lack of cross-protection induced by natural Shigella infection. The finding that ASC responses to IpaD develop in patients with recent-onset shigellosis indicates that such responses may not be protective or may wane too rapidly and/or be of insufficient magnitude.

Salivary IgA from the sublingual compartment as a novel noninvasive proxy for intestinal immune induction

Whole-saliva IgA appears like an attractive noninvasive readout for intestinal immune induction after enteric infection or vaccination, but has failed to show consistent correlation with established invasive markers and IgA in feces or intestinal lavage. For reference, we measured antibodies in samples from 30 healthy volunteers who were orally infected with wild-type enterotoxigenic Escherichia coli. The response against these bacteria in serum, lavage, and lymphocyte supernatants (antibody-in-lymphocyte-supernatant, ALS) was compared with that in targeted parotid and sublingual/submandibular secretions. Strong correlation occurred between IgA antibody levels against the challenge bacteria in sublingual/submandibular secretions and in lavage (r=0.69, P<0.0001) and ALS (r=0.70, P<0.0001). In sublingual/submandibular secretions, 93% responded with more than a twofold increase in IgA antibodies against the challenge strain, whereas the corresponding response in parotid secretions was only 67% (P=0.039). With >twofold ALS as a reference, the sensitivity of a >twofold response for IgA in sublingual/submandibular secretion was 96%, whereas it was only 67% in the parotid fluid. To exclude that flow rate variations influenced the results, we used albumin as a marker. Our data suggested that IgA in sublingual/submandibular secretions, rather than whole saliva with its variable content of parotid fluid, is a preferential noninvasive proxy for intestinal immune induction.

An Evidenced-Based Scale of Disease Severity following Human Challenge with Enteroxigenic Escherichia coli

Background

Experimental human challenge models have played a major role in enhancing our understanding of infectious diseases. Primary outcomes have typically utilized overly simplistic outcomes that fail to entirely account for complex illness syndromes. We sought to characterize clinical outcomes associated with experimental infection with enterotoxigenic Escherichia coli (ETEC) and to develop a disease score.

Methods

Data were obtained from prior controlled human ETEC infection studies. Correlation and univariate regression across sign and symptom severity was performed. A multiple correspondence analysis was conducted. A 3-parameter disease score with construct validity was developed in an iterative fashion, compared to standard outcome definitions and applied to prior vaccine challenge trials.

Results

Data on 264 subjects receiving seven ETEC strains at doses from 1x10⁵ to 1x10¹⁰ cfu were used to construct a standardized dataset. The strongest observed correlation was between vomiting and nausea (r = 0.65); however, stool output was poorly correlated with subjective activity-impacting outcomes. Multiple correspondence analyses showed covariability in multiple signs and symptoms, with severity being the strongest factor corresponding across outcomes. The developed disease score performed well compared to standard outcome definitions and differentiated disease in vaccinated and unvaccinated subjects.

Conclusion

Frequency and volumetric definitions of diarrhea severity poorly characterize ETEC disease. These data support a disease severity score accounting for stool output and other clinical signs and symptoms. Such a score could serve as the basis for better field trial outcomes and gives an additional outcome measure to help select future vaccines that warrant expanded testing in pivotal pre-licensure trials.

Evaluating the A-Subunit of the Heat-Labile Toxin (LT) As an Immunogen and a Protective Antigen Against Enterotoxigenic Escherichia coli (ETEC)

Diarrheal illness contributes to malnutrition, stunted growth, impaired cognitive development, and high morbidity rates in children worldwide. Enterotoxigenic Escherichia coli (ETEC) is a major contributor to this diarrheal disease burden. ETEC cause disease in the small intestine by means of colonization factors and by production of a heat-labile enterotoxin (LT) and/or a small non-immunogenic heat-stable enterotoxin (ST). Overall, the majority of ETEC produce both ST and LT. LT induces secretion via an enzymatically active A-subunit (LT-A) and a pentameric, cell-binding B-subunit (LT-B). The importance of anti-LT antibodies has been demonstrated in multiple clinical and epidemiological studies, and a number of potential ETEC vaccine candidates have included LT-B as an important immunogen. However, there is limited information about the potential contribution of LT-A to development of protective immunity. In the current study, we evaluate the immune response against the A-subunit of LT as well as the A-subunit’s potential as a protective antigen when administered alone or in combination with the B-subunit of LT. We evaluated human sera from individuals challenged with a prototypic wild-type ETEC strain as well as sera from individuals living in an ETEC endemic area for the presence of anti-LT, anti-LT-A and anti-LT-B antibodies. In both cases, a significant number of individuals intentionally or endemically infected with ETEC developed antibodies against both LT subunits. In addition, animals immunized with the recombinant proteins developed robust antibody responses that were able to neutralize the enterotoxic and cytotoxic effects of native LT by blocking binding and entry into cells (anti-LT-B) or the intracellular enzymatic activity of the toxin (anti-LT-A). Moreover, antibodies to both LT subunits acted synergistically to neutralize the holotoxin when combined. Taken together, these data support the inclusion of both LT-A and LT-B in prospective vaccines against ETEC.

Vaccines for preventing enterotoxigenic Escherichia coli (ETEC) diarrhoea

BACKGROUND

Infection with enterotoxigenic Escherichia coli (ETEC) bacteria is a common cause of diarrhoea in adults and children in developing countries and is a major cause of 'travellers' diarrhoea' in people visiting or returning from endemic regions. A killed whole cell vaccine (Dukoral®), primarily designed and licensed to prevent cholera, has been recommended by some groups to prevent travellers' diarrhoea in people visiting endemic regions. This vaccine contains a recombinant B subunit of the cholera toxin that is antigenically similar to the heat labile toxin of ETEC. This review aims to evaluate the clinical efficacy of this vaccine and other vaccines designed specifically to protect people against diarrhoea caused by ETEC infection.

OBJECTIVES

To evaluate the efficacy, safety, and immunogenicity of vaccines for preventing ETEC diarrhoea.

SEARCH METHODS

We searched the Cochrane Infectious Disease Group Specialized Register, the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, EMBASE, LILACS, and http://clinicaltrials.gov up to December 2012.

SELECTION CRITERIA

Randomized controlled trials (RCTs) and quasi-RCTs comparing use of vaccines to prevent ETEC with use of no intervention, a control vaccine (either an inert vaccine or a vaccine normally given to prevent an unrelated infection), an alternative ETEC vaccine, or a different dose or schedule of the same ETEC vaccine in healthy adults and children living in endemic regions, intending to travel to endemic regions, or volunteering to receive an artificial challenge of ETEC bacteria.

DATA COLLECTION AND ANALYSIS

Two authors independently assessed each trial for eligibility and risk of bias. Two independent reviewers extracted data from the included studies and analyzed the data using Review Manager (RevMan) software. We reported outcomes as risk ratios (RR) with 95% confidence intervals (CI). We assessed the quality of the evidence using the GRADE approach.

MAIN RESULTS

Twenty-four RCTs, including 53,247 participants, met the inclusion criteria. Four studies assessed the protective efficacy of oral cholera vaccines when used to prevent diarrhoea due to ETEC and seven studies assessed the protective efficacy of ETEC-specific vaccines. Of these 11 studies, seven studies presented efficacy data from field trials and four studies presented efficacy data from artificial challenge studies. An additional 13 trials contributed safety and immunological data only. Cholera vaccinesThe currently available, oral cholera killed whole cell vaccine (Dukoral®) was evaluated for protection of people against 'travellers' diarrhoea' in a single RCT in people arriving in Mexico from the USA. We did not identify any statistically significant effects on ETEC diarrhoea or all-cause diarrhoea (one trial, 502 participants, low quality evidence).Two earlier trials, one undertaken in an endemic population in Bangladesh and one undertaken in people travelling from Finland to Morocco, evaluated a precursor of this vaccine containing purified cholera toxin B subunit rather than the recombinant subunit in Dukoral®. Short term protective efficacy against ETEC diarrhoea was demonstrated, lasting for around three months (RR 0.43, 95% CI 0.26 to 0.71; two trials, 50,227 participants). This vaccine is no longer available. ETEC vaccinesAn ETEC-specific, killed whole cell vaccine, which also contains the recombinant cholera toxin B-subunit, was evaluated in people travelling from the USA to Mexico or Guatemala, and from Austria to Latin America, Africa, or Asia. We did not identify any statistically significant differences in ETEC-specific diarrhoea or all-cause diarrhoea (two trials, 799 participants), and the vaccine was associated with increased vomiting (RR 2.0, 95% CI 1.16 to 3.45; nine trials, 1528 participants). The other ETEC-specific vaccines in development have not yet demonstrated clinically important benefits.

AUTHORS' CONCLUSIONS

There is currently insufficient evidence from RCTs to support the use of the oral cholera vaccine Dukoral® for protecting travellers against ETEC diarrhoea. Further research is needed to develop safe and effective vaccines to provide both short and long-term protection against ETEC diarrhoea.

Different kinetics of circulating antibody-secreting cell responses after primary and booster oral immunizations: a tool for assessing immunological memory

We show that the kinetics of circulating IgA as well as IgG antibody-secreting cell (ASC) responses differs considerably after primary and booster vaccination with the oral cholera vaccine Dukoral(®), as determined by the antibody in lymphocyte supernatant (ALS) as well as ELISPOT methods. Thus, whereas the antitoxin ASC responses did not peak until 7-9 days after primary vaccination, peak responses to a second dose given after two weeks, or a single booster dose given 6 months to 14 years later, were recorded already after 4-5 days and then rapidly declined. Our results indicate that many previous studies reporting ASC results 7-10 days after repeated immunization may have substantially underestimated the magnitudes of the responses. The results also suggest that detection of peak ASC responses at an early time point after booster immunization can be used as a simple tool to assess immunological memory.

Enzyme-linked immunospot assays for direct ex vivo measurement of vaccine-induced human humoral immune responses in blood

The enzyme-linked immunospot (ELISPOT) assay was originally developed to enumerate antigen-specific antibody-secreting cells (ASCs), and has subsequently been adapted for various applications, including the detection cytokine-secreting cells. Owing to its exceptionally high sensitivity, the ELISPOT has proven to be especially useful for detecting discrete populations of active cells (e.g., antigen-specific cells). Because of its versatility, the ELISPOT assay is used for a wide range of applications, including clonal analyses of immune responses after vaccination or after immunotherapy. Here we describe standard protocols for the detection of human ASCs specific to virtually any vaccine antigen after enrichment of circulating plasmablasts. In addition, a protocol is described for the measurement of mucosal ASC responses after prior immunomagnetic enrichment of mucosally derived blood lymphocytes. The protocols described allow rapid (~6-8 h) detection of specific ASCs in small (1-2 ml) samples of blood and can be performed in resource-poor settings.

The oral, live attenuated enterotoxigenic Escherichia coli vaccine ACE527 reduces the incidence and severity of diarrhea in a human challenge model of diarrheal disease

An oral, live attenuated, three-strain recombinant bacterial vaccine, ACE527, was demonstrated to generate strong immune responses to colonization factor and toxin antigens of enterotoxigenic Escherichia coli (ETEC) in human volunteers. The vaccine was safe and well tolerated at doses of up to 10(11) CFU, administered in each of two doses given 21 days apart. These observations have now been extended in a phase 2b study with a total of 70 subjects. Fifty-six of these subjects were challenged 28 days after the second dose of vaccine with the highly virulent ETEC strain H10407 to obtain preliminary indicators of efficacy against disease and to support further development of the vaccine for both travelers and infants in countries where ETEC is endemic. The vaccine had a significant impact on intestinal colonization by the challenge strain, as measured by quantitative fecal culture 2 days after challenge, demonstrating the induction of a functional immune response to the CFA/I antigen. The incidence and severity of diarrhea were also reduced in vaccinees as measured by a number of secondary and ad hoc endpoints, although the 27% reduction seen in the primary endpoint, moderate to severe diarrhea, was not statistically significant. Together, these observations support the hypothesis that the ACE527 vaccine has a dual mode of action, targeting both colonization factors and the heat-labile enterotoxin (LT), and suggest that it should be further developed for more advanced trials to evaluate its impact on the burden of ETEC disease in field settings.

Recent progress toward an enterotoxigenic Escherichia coli vaccine

Enterotoxigenic Escherichia coli(ETEC) is the most common cause of bacterial diarrhea in children in Africa, Asia and Latin America and in travelers to these regions. Despite this, no effective vaccine for ETEC is available. ETEC causes disease by colonizing the small intestine with colonization factors, most of which are fimbriae, and production of heat-labile and/or heat-stable enterotoxins. Antibodies against heat-labile enterotoxin and the colonization factors have been shown to be protective, and local immunity in the gut seems to be of prime importance for protection. Hence, several inactivated and live candidate ETEC vaccines consisting of toxin antigens, alone or together with colonization factors, have been evaluated in clinical trials. In this review, the authors describe ETEC vaccine development in progress and the rationale for constructing different types of vaccines. They also discuss possibilities of enhancing immune responses to candidate ETEC vaccines, particularly in children.

A combination vaccine consisting of three live attenuated enterotoxigenic Escherichia coli strains expressing a range of colonization factors and heat-labile toxin subunit B is well tolerated and immunogenic in a placebo-controlled double-blind phase I trial in healthy adults

Immune responses against colonization factors (CFs) and the nontoxic B component of the enterotoxigenic Escherichia coli (ETEC) heat-labile toxin (LTB) are considered to be important for immunity against diarrhea caused by ETEC. Individual live attenuated ETEC derivatives that have had their toxin genes removed and whose aroC, ompC, and ompF genes are deleted have shown promise as vaccines against ETEC. The development of such strains has culminated in the testing of a three-strain-combination live attenuated vaccine known as ACE527, comprised of strains ACAM2025 expressing colonization factor antigen I (CFA/I) and LTB; ACAM2022, expressing CS5, CS6, and LTB; and ACAM2027, expressing CS1, CS2, CS3, and LTB. The recombinant CF and LTB genes expressed in the three strains were inserted into the bacterial chromosome to ensure their stable inheritance and expression without the requirement for any selection. ACE527 has been tested in a randomized placebo-controlled, double-blind, phase I safety and immunogenicity study in healthy adult volunteers and proved to be well tolerated and immunogenic at dose levels of 10(10) and 10(11) total CFU. There was no indication of strain interference on the basis of fecal shedding patterns, with all three being detected in the feces of 50% and 83% of low- and high-dose vaccine recipients, respectively. Similarly, strong immune responses to LTB and to CFs expressed on all three constituent strains were induced, with at least 50% of subjects in the high-dose group responding to LTB, CFA/I, CS3, and CS6.

Comparative evaluation of the antibody in lymphocyte supernatant (ALS) and enzyme-linked immunospot (ELISPOT) assays for measuring mucosal immune responses to Shigella antigens

Accurately assessing mucosal immune responses to candidate vaccines remains a technical challenge. ELISPOT is widely used as a surrogate of mucosal immune response by directly enumerating circulating antibody secreting cells (ASCs), while antibody in lymphocyte supernatant (ALS) titers the total amount of antibody secreted by ASC ex vivo using ELISA. ALS is more practical than ELISPOT because the ASC supernatant is frozen for ELISA that can be conducted at any time, with any antigen, and in any laboratory. We compared IgA and IgG responses to serotype-specific Shigella LPS using ELISPOT and ALS in subjects following vaccination or infection with Shigella. ALS results correlated well with ELISPOT results, and the ALS method was both sensitive and specific for the detection of antibody responses against Shigella LPS. Based on these observations, the ALS assay is a practical and flexible alternative to ELISPOT for measuring mucosal IgA responses to Shigella LPS antigen.

The pangenome structure of Escherichia coli: comparative genomic analysis of E. coli commensal and pathogenic isolates

Whole-genome sequencing has been skewed toward bacterial pathogens as a consequence of the prioritization of medical and veterinary diseases. However, it is becoming clear that in order to accurately measure genetic variation within and between pathogenic groups, multiple isolates, as well as commensal species, must be sequenced. This study examined the pangenomic content of Escherichia coli. Six distinct E. coli pathovars can be distinguished using molecular or phenotypic markers, but only two of the six pathovars have been subjected to any genome sequencing previously. Thus, this report provides a seminal description of the genomic contents and unique features of three unsequenced pathovars, enterotoxigenic E. coli, enteropathogenic E. coli, and enteroaggregative E. coli. We also determined the first genome sequence of a human commensal E. coli isolate, E. coli HS, which will undoubtedly provide a new baseline from which workers can examine the evolution of pathogenic E. coli. Comparison of 17 E. coli genomes, 8 of which are new, resulted in identification of approximately 2,200 genes conserved in all isolates. We were also able to identify genes that were isolate and pathovar specific. Fewer pathovar-specific genes were identified than anticipated, suggesting that each isolate may have independently developed virulence capabilities. Pangenome calculations indicate that E. coli genomic diversity represents an open pangenome model containing a reservoir of more than 13,000 genes, many of which may be uncharacterized but important virulence factors. This comparative study of the species E. coli, while descriptive, should provide the basis for future functional work on this important group of pathogens.

Comparative safety and immunogenicity of two attenuated enterotoxigenic Escherichia coli vaccine strains in healthy adults

A vaccine against enterotoxigenic Escherichia coli (ETEC) is needed to prevent diarrheal illness among children in developing countries and at-risk travelers. Two live attenuated ETEC strains, PTL002 and PTL003, which express the ETEC colonization factor CFA/II, were evaluated for safety and immunogenicity. In a randomized, double-blind, placebo-controlled trial, 19 subjects ingested one dose, and 21 subjects ingested two doses (days 0 and 10) of PTL-002 or PTL-003 at 2 x 10(9) CFU/dose. Anti-CFA/II mucosal immune responses were determined from the number of antibody-secreting cells (ASC) in blood measured by enzyme-linked immunospot assay, the antibody in lymphocyte supernatants (ALS) measured by enzyme-linked immunosorbent assay (ELISA), and fecal immunoglobulin A (IgA) levels determined by ELISA. Time-resolved fluorescence (TRF) ELISA was more sensitive than standard colorimetric ELISA for measuring serum antibody responses to CFA/II and its components, CS1 and CS3. Both constructs were well tolerated. Mild diarrhea occurred after 2 of 31 doses (6%) of PTL-003. PTL-003 produced more sustained intestinal colonization than PTL-002 and better IgA response rates: 90% versus 55% (P = 0.01) for anti-CFA/II IgA-ASCs, 55% versus 30% (P = 0.11) for serum anti-CS1 IgA by TRF, and 65% versus 25% (P = 0.03) for serum anti-CS3 IgA by TRF. Serum IgG response rates to CS1 or CS3 were 55% in PTL-003 recipients and 15% in PTL-002 recipients (P = 0.02). Two doses of either strain were not significantly more immunogenic than one. Based on its superior immunogenicity, which was comparable to that of a virulent ETEC strain and other ETEC vaccine candidates, PTL-003 will be developed further as a component of a live, oral attenuated ETEC vaccine.