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).

Human Experimental Challenge With Enterotoxigenic Escherichia coli Elicits Immune Responses to Canonical and Novel Antigens Relevant to Vaccine Development

Background

Enterotoxigenic Escherichia coli (ETEC) is a major cause of diarrheal illness in the developing world. Enterotoxigenic E coli vaccinology has been challenged by genetic diversity and heterogeneity of canonical antigens. Examination of the antigenic breadth of immune responses associated with protective immunity could afford new avenues for vaccine development.

Methods

Antibody lymphocyte supernatants (ALS) and sera from 20 naive human volunteers challenged with ETEC strain H10407 and from 10 volunteers rechallenged 4-6 weeks later with the same strain (9 of whom were completely protected on rechallenge) were tested against ETEC proteome microarrays containing 957 antigens.

Results

Enterotoxigenic E coli challenge stimulated robust serum and mucosal (ALS) responses to canonical vaccine antigens (CFA/I, and the B subunit of LT) as well as a small number of antigens not presently targeted in ETEC vaccines. These included pathovar-specific secreted proteins (EtpA, EatA) as well as highly conserved E coli antigens including YghJ, flagellin, and pertactin-like autotransporter proteins, all of which have previously afforded protection against ETEC infection in preclinical studies.

Conclusions

Taken together, studies reported here suggest that immune responses after ETEC infection involve traditional vaccine targets as well as a select number of more recently identified protein antigens that could offer additional avenues for vaccine development for these pathogens.

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.

Stabilization of a plasmid coding for a heterologous antigen in Salmonella enterica serotype typhi vaccine strain CVD908-htrA by using site-specific recombination

A gene cassette incorporating the crs-rsd site-specific recombination system from the Salmonella enterica subsp. enterica serovar Dublin virulence plasmid improved the inheritance in S. enterica serotype Typhi strain CVD908-htrA of a multicopy plasmid expression vector. Use of this recombination cassette may improve expression of heterologous antigens from multicopy plasmid expression vectors in attenuated bacterial vaccine strains.

Construction and phase I clinical evaluation of the safety and immunogenicity of a candidate enterotoxigenic Escherichia coli vaccine strain expressing colonization factor antigen CFA/I

Oral delivery of toxin-negative derivatives of enterotoxigenic Escherichia coli (ETEC) that express colonization factor antigens (CFA) with deletions of the aroC, ompC, ompF, and toxin genes may be an effective approach to vaccination against ETEC-associated diarrhea. We describe the creation and characterization of an attenuated CFA/I-expressing ETEC vaccine candidate, ACAM2010, from a virulent isolate in which the heat-stable enterotoxin (ST) and CFA/I genes were closely linked and on the same virulence plasmid as the enteroaggregative E. coli heat-stable toxin (EAST1) gene. A new suicide vector (pJCB12) was constructed and used to delete the ST and EAST1 genes and to introduce defined deletion mutations into the aroC, ompC, and ompF chromosomal genes. A phase I trial, consisting of an open-label dose escalation phase in 18 adult outpatient volunteers followed by a placebo-controlled double-blind phase in an additional 31 volunteers, was conducted. The vaccine was administered in two formulations, fresh culture and frozen suspension. These were both well tolerated, with no evidence of significant adverse events related to vaccination. Immunoglobulin A (IgA) and IgG antibody-secreting cells specific for CFA/I were assayed by ELISPOT. Positive responses (greater than twofold increase) were seen in 27 of 37 (73%) subjects who received the highest dose level of vaccine (nominally 5 x 10(9) CFU). Twenty-nine of these volunteers were secreting culturable vaccine organisms at day 3 following vaccination; five were still positive on day 7, with a single isolation on day 13. This live attenuated bacterial vaccine is safe and immunogenic in healthy adult volunteers.

Attenuated Salmonella enterica serovar Typhi expressing urease effectively immunizes mice against Helicobacter pylori challenge as part of a heterologous mucosal priming-parenteral boosting vaccination regimen

Recombinant vaccine strains of Salmonella enterica serovar Typhi capable of expressing Helicobacter pylori urease were generated by transforming strains CVD908 and CVD908-htrA with a plasmid harboring the ureAB genes under the control of an in vivo-inducible promoter. The plasmid did not interfere with the ability of either strain to replicate and persist in human monocytic cells or with their transient colonization of mouse lungs. When administered to mice intranasally, both recombinant strains elicited antiurease immune responses skewed towards a Th1 phenotype. Vaccinated mice exhibited strong immunoglobulin G2a (IgG2a)-biased antiurease antibody responses as well as splenocyte populations capable of proliferation and gamma interferon (IFNgamma) secretion in response to urease stimulation. Boosting of mice with subcutaneous injection of urease plus alum enhanced immune responses and led them to a more balanced Th1/Th2 phenotype. Following parenteral boost, IgG1 and IgG2a antiurease antibody titers were raised significantly, and strong urease-specific splenocyte proliferative responses, accompanied by IFNgamma as well as interleukin-4 (IL-4), IL-5, and IL-10 secretion, were detected. Neither immunization with urease-expressing S. enterica serovar Typhi alone nor immunization with urease plus alum alone conferred protection against challenge with a mouse-adapted strain of H. pylori; however, a vaccination protocol combining both immunization regimens was protective. This is the first report of effective vaccination against H. pylori with a combined mucosal prime-parenteral boost regimen in which serovar Typhi vaccine strains are used as antigen carriers. The significance of these findings with regard to development of a human vaccine against H. pylori and modulation of immune responses by heterologous prime-boost immunization regimens is discussed.

Construction and characterization of genetically defined aro omp mutants of enterotoxigenic Escherichia coli and preliminary studies of safety and immunogenicity in humans

Enterotoxigenic Escherichia coli (ETEC) is a leading cause of diarrhea in travelers to countries where the disease is endemic and causes a major disease burden in the indigenous population, particularly children. We describe here the generation and preclinical characterization of candidate strains of ETEC which are intended to provide the basis of a live attenuated oral vaccine to prevent this disease. It has been shown previously that a spontaneously arising toxin-negative variant ETEC strain, E1392/75-2A, could confer 75% protection against challenge when administered to volunteers. Unfortunately this strain induced mild diarrhea in 15% of recipients. To eliminate the unacceptable reactogenicity of strain E1392/75-2A, it was further attenuated by introducing three different combinations of defined deletion mutations into the chromosome. A mouse intranasal model of immunization was developed and used to show that all of the strains were immunogenic. Immune responses against colonization factor antigens (CFAs) were particularly strong when the bacterial inocula were grown on "CFA agar," which induces strong expression of these antigens. Two of the strains were selected for a phase I dose escalation safety study with healthy adult volunteers. Freshly grown organisms were harvested from CFA agar plates and administered to volunteers as a suspension containing from 5 x 10(7) to 5 x 10(9) CFU. The vaccine was well tolerated at all doses and induced significant immune responses in all recipients at the highest dose of either strain. The results provide the basis for further clinical evaluation of these vaccine candidates.

Spectrophotometric quantitation of silver in X-ray film images and silver-stained electrophoresis gel bands

This paper describes a novel spectrophotometric method for quantitating the silver in silver-stained electrophoresis gel bands and X-ray film images. Silver in excised areas is quantitated by (i) oxidation/solubilization of Ag0 to Ag+ with nitric acid, (ii) neutralization followed by extraction into an organic solution of dithizone (diphenylthiocarbazone), and (iii) visible absorbance measurement at 600 nm. Spectrophotometric quantitation is possible because dithizone undergoes a dramatic color change from blue-green to yellow upon binding Ag+ (delta epsilon 600 = 3.28 x 10(4) M-1 cm-1). Experiments with silver-stained gels and with X-ray film images obtained by chemiluminescence and autoradiography demonstrated that the color change relates to the amount of silver present and hence to the amount of material represented by the band or image. This method is a convenient, inexpensive alternative to the use of a densitometer to quantitate silver-containing images.

Aspects of antibody-catalyzed primary amide hydrolysis

Because there are many known C-terminally amidated peptides of biological importance, there is great potential in medicine and organic synthesis for antibodies that catalyze primary amide bond hydrolysis or formation. We characterized a catalytic antibody, 13D11, raised to a phosphinate hapten, that hydrolyzed the primary amide of a dansyl-alkylated derivative of (R)-phenylalaninamide (DNS-(R)F-NH2). At pH 9.0, 13D11 hydrolyzed DNS-(R)F-NH2 with a kcat of 1.65 x 10(-7) s-1 (kcat/kuncat = 132) and a Km of 432 microM, and was stereospecifically hapten-inhibited (Ki = 14.0 microM). Control experiments indicated that the catalytic activity was not the result of a contaminating protease. In accordance with the hapten being a transition-state analog of base hydrolysis, the rate of DNS-(R)F-NH2 hydrolysis increased with hydroxide concentration to an optimum pH of 9.5. Above pH 9.5, activity declined rapidly suggesting the antibody was inactivated during the long incubation period. This work demonstrates the feasibility of generating catalytic antibodies to hydrolyze unactivated amide bonds without cofactor assistance.

Isoabzymes: structurally and mechanistically similar catalytic antibodies from the same immunization

Mechanistic and structural comparisons of five catalytic monoclonal antibodies generated from the same hybridoma fusion indicated that all five hydrolyze phenyl acetate by subtle variations of the same mechanism. All of the antibodies showed a pre-steady-state multi-turnover burst in which kcat and Km declined but kcat/Km did not change. The burst of one of the antibodies, 20G9, has previously been found to result from inhibition by the product, phenol. Although all of the antibodies showed the burst, their individual values for kcat, Km, and hapten Ki differed substantially. Three of the antibodies that were investigated for the effect of pH on kcat showed an acid limb pK of 9.5-9.6. Substrate inhibition was seen in four of the five antibodies. Variable region nucleotide sequencing of the heavy and light chains confirmed that all five antibodies were structurally similar and also revealed several potentially critical tyrosines. Despite their structural similarities, analysis of their sequences suggested that the antibodies are products of distinct, independent rearrangements of immunoglobulin gene segments that took place in different progenitor B cells. A plot of Ki for hapten inhibition vs Km/kcat for substrate hydrolysis for the mechanistically related antibodies ("isoabzymes") gave a linear relationship suggesting a catalytic role for transition-state complementarity. Taken together with previous work [Martin et al. (1991) Biochemistry 30, 9757-9761], the data conform to a mechanism in which the antibodies exploit both transition-state complementarity and an acyl-tyrosyl intermediate during phenyl acetate hydrolysis.

New family of exon-shuffled recombinant genes reveals extensive interdomain interactions in class I histocompatibility antigens and identifies residues involved

The specificities of an extensive panel of anti-H-2Dd monoclonal antibodies, which had been previously characterized using exon-shuffled H-2Dd/H-2Ld molecules and a number of anti-H-2DP antibodies, were examined using H-2Dd/H-2DP recombinants. The use of this new family of recombinant antigens revealed extensive interaction between the membrane-distal (N and Cl) domains of class I molecules. 20 out of 48 mAbs recognize complex epitopes formed by the interaction of these two domains. These antibodies exhibit a number of distinct patterns of crossreactivity with other class I proteins, revealing the presence of multiple epitopes within the region of domain interaction. Comparison of the data presented here with those from previous work allowed the identification of a small number of residues in the Cl domain that participate in the generation of complex epitopes involving both the N and Cl domains. The results are discussed in terms of the structural information available for these two domains.

Modelling of the combining sites of three anti-lysozyme monoclonal antibodies and of the complex between one of the antibodies and its epitope

Models of the antigen combining sites of three monoclonal antibodies, which recognise different but overlapping epitopes within the 'loop' region of hen egg lysozyme (HEL), have been generated from the cDNA sequences of their Fv regions (the VL and VH domains) and the known crystal structures of immunoglobulin fragments. The alpha-carbon backbone of the structurally conserved framework region has been derived from the IgG myeloma protein NEW, and models for the hypervariable loop regions have been selected on the basis of length and maximum sequence homology. The model structures have been refined by energy minimisation. Both the size and chemical nature of the predicted combining site models correlate broadly with the epitope boundaries previously determined by affinity studies. A model of the complex formed between one antibody and the corresponding lysozyme epitope is described, and contact residues are identified for subsequent testing by oligonucleotide-directed site-specific mutagenesis.

Three distinct epitopes within the loop region of hen egg lysozyme defined with monoclonal antibodies

Five monoclonal antibodies specific for the loop region of hen egg lysozyme were prepared by immunisation with a synthetic conjugate of a proteolytic fragment of lysozyme coupled to bovine serum albumin. Their fine specificities were investigated using a panel of variant lysozymes and peptide fragments of lysozyme in a quantitative radio-immunoassay procedure. Knowledge of the structure of hen lysozyme to high resolution and the use of computer graphics enables the localisation of the epitopes recognised by the antibodies with some precision. The antibodies were shown to define three distinct, overlapping epitopes within what was previously considered to be a single antigenic site. These results are discussed in relation to current ideas of the antigenic nature of proteins and other recent studies in which anti-protein antibodies have been elicited by immunisation with small peptides.