The Mucosal Vaccine Adjuvant LT(R192G/L211A) or dmLT

Targeting a bacterial DNABII protein with a chimeric peptide immunogen or humanised monoclonal antibody to prevent or treat recalcitrant biofilm-mediated infections

Background:

Chronic and recurrent bacterial diseases are recalcitrant to treatment due to the ability of the causative agents to establish biofilms, thus development of means to prevent or resolve these structures are greatly needed. Our approach targets the DNABII family of bacterial DNA-binding proteins, which serve as critical structural components within the extracellular DNA scaffold of biofilms formed by all bacterial species tested to date. DNABII-directed antibodies rapidly disrupt biofilms and release the resident bacteria which promote their subsequent clearance by either host immune effectors or antibiotics that are now effective at a notably reduced concentration.

Methods:

First, as a therapeutic approach, we used intact IgG or Fab fragments against a chimeric peptide immunogen designed to target protective epitopes within the DNA-binding tip domains of integration host factor to disrupt established biofilms in vitro and to mediate resolution of existing disease in vivo. Second, we performed preventative active immunisation with the chimeric peptide to induce the formation of antibody that blocks biofilm formation and disease development in a model of viral-bacterial superinfection. Further, toward the path for clinical use, we humanised a monoclonal antibody against the chimeric peptide immunogen, then characterised and validated that it maintained therapeutic efficacy.

Findings:

We demonstrated efficacy of each approach in two well-established pre-clinical models of otitis media induced by the prevalent respiratory tract pathogen nontypeable Haemophilus influenzae, a common biofilm disease.

Interpretation:

Collectively, our data revealed two approaches with substantive efficacy and potential for broad application to combat diseases with a biofilm component.

Funding

Supported by R01 DC011818 to LOB and SDG.

Technological Approaches for Improving Vaccination Compliance and Coverage

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

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

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

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

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

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

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

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

Oral immunization of BALB/c mice with recombinant Helicobacter pylori antigens and double mutant heat-labile toxin (dmLT) induces prophylactic protective immunity against H. pylori infection

Helicobacter pylori infection and associated diseases remain a major public health problem worldwide. Much effort has been made over the last several decades in vaccine development, but there is no licensed vaccine on the market. We have previously reported that oral immunization with H. pylori lysates and double mutant heat-labile toxin (dmLT) affords prophylactic protection against H. pylori infection in mice. In the present study, we investigated the effects of oral immunization with recombinant H. pylori protein antigens (NAP/UreA/UreB) and dmLT on H. pylori challenge in BALB/c mice. We found that oral immunization with candidate antigens and dmLT significantly reduced the gastric colonization of H. pylori 6 weeks after challenge, as compared to unimmunized mice. Moreover, the subunit vaccine appeared to provide a better protection than the bacterial lysate vaccine. The immunized mice showed enhanced antigen-specific lymphocyte proliferation, and serum IgG and mucosal IgA responses. Furthermore, the immunization increased the proportion of CD4⁺ IL-17+ lymphocytes in spleen and mesenteric lymph nodes, and enhanced the production of IL-17, IL-16, IL-6 and TNF-α in lymphocyte culture supernatants. Taken together, our results suggest that oral vaccination with recombinant H. pylori antigens (NAP/UreA/UreB) and dmLT confers more effective prophylactic protection against H. pylori infection than whole bacterial lysates in BALB/c mice. The reduction of H. pylori colonization was associated with the induction of antigen-specific Th17 and local mucosal IgA immune responses.

Optimization of an Experimental Vaccine To Prevent Escherichia coli Urinary Tract Infection

Urinary tract infections (UTI) affect half of all women at least once during their lifetime. The rise in the numbers of extended-spectrum beta-lactamase-producing strains and the potential for carbapenem resistance within uropathogenic Escherichia coli (UPEC), the most common causative agent of UTI, create an urgent need for vaccine development. Intranasal immunization of mice with UPEC outer membrane iron receptors FyuA, Hma, IreA, and IutA, conjugated to cholera toxin, provides protection in the bladder or kidneys under conditions of challenge with UPEC strain CFT073 or strain 536. On the basis of these data, we sought to optimize the vaccination route (intramuscular, intranasal, or subcutaneous) in combination with adjuvants suitable for human use, including aluminum hydroxide gel (alum), monophosphoryl lipid A (MPLA), unmethylated CpG synthetic oligodeoxynucleotides (CpG), polyinosinic:polycytidylic acid (polyIC), and mutated heat-labile E. coli enterotoxin (dmLT). Mice intranasally vaccinated with dmLT-IutA and dmLT-Hma displayed significant reductions in bladder colonization (86-fold and 32-fold, respectively), with 40% to 42% of mice having no detectable CFU. Intranasal vaccination of mice with CpG-IutA and polyIC-IutA significantly reduced kidney colonization (131-fold) and urine CFU (22-fold), respectively. dmLT generated the most consistently robust antibody response in intranasally immunized mice, while MPLA and alum produced greater concentrations of antigen-specific serum IgG with intramuscular immunization. On the basis of these results, we conclude that intranasal administration of Hma or IutA formulated with dmLT adjuvant provides the greatest protection from UPEC UTI. This report advances our progress toward a vaccine against uncomplicated UTI, which will significantly improve the quality of life for women burdened by recurrent UTI and enable better antibiotic stewardship.IMPORTANCE Urinary tract infections (UTI) are among the most common bacterial infection in humans, affecting half of all women at least once during their lifetimes. The rise in antibiotic resistance and health care costs emphasizes the need to develop a vaccine against the most common UTI pathogen, Escherichia coli Vaccinating mice intranasally with a detoxified heat-labile enterotoxin and two surface-exposed receptors, Hma or IutA, significantly reduced bacterial burden in the bladder. This work highlights progress in the development of a UTI vaccine formulated with adjuvants suitable for human use and antigens that encode outer membrane iron receptors required for infection in the iron-limited urinary tract.

Immunization with a Biofilm-Disrupting Nontypeable Haemophilus influenzae Vaccine Antigen Did Not Alter the Gut Microbiome in Chinchillas, Unlike Oral Delivery of a Broad-Spectrum Antibiotic Commonly Used for Otitis Media

The prevalence of chronic and recurrent diseases, combined with the overuse/abuse of antibiotics that has led to the sobering emergence of bacteria resistant to multiple antibiotics, has mandated that we develop novel approaches to better manage these diseases or, ideally, prevent them. Biofilms play a key role in the pathogenesis of chronic and recurrent bacterial diseases but are difficult, if not impossible, to eradicate with antibiotics. We developed a vaccine antigen designed to mediate biofilm disruption; however, it is also important that delivery of this vaccine does not induce collateral damage to the microbiome. The studies described here validated a vaccine approach that targets biofilms without the consequences of an altered gut microbiome. While delivery of the antibiotic most commonly given to children with ear infections did indeed alter the gut microbiome, as expected, immunization via traditional injection or by noninvasive delivery to the skin did not result in changes to the chinchilla gut microbiome.

The use of broad-spectrum antibiotics to treat diseases, such as the highly prevalent pediatric disease otitis media (OM), contributes significantly to the worldwide emergence of multiple-antibiotic-resistant microbes, and gut dysbiosis with diarrhea is a common adverse sequela. Moreover, for many diseases, like OM, biofilms contribute significantly to chronicity and recurrence, yet biofilm-resident bacteria are characteristically highly resistant to antibiotics. The most cost-effective way to both prevent and resolve diseases like OM, as well as begin to address the problem of growing antibiotic resistance, would be via the development of novel approaches to eradicate bacterial biofilms. Toward this goal, we designed a vaccine antigen that induces the formation of antibodies that prevent biofilm formation and, thereby, experimental OM in the middle ears of chinchillas by the predominant Gram-negative pathogen responsible for this disease, nontypeable Haemophilus influenzae. These antibodies also significantly disrupt preexisting biofilms formed by diverse pathogens. Whereas preclinical data strongly support the continued development of this vaccine antigen, which targets an essential structural element of bacterial biofilms, a concern has been whether active immunization would also lead to unintended collateral damage in the form of an altered gut microbiome. To address this concern, we assessed changes in the microbiome of the chinchilla gut over time after the delivery of either amoxicillin-clavulanate, the standard of care for OM, or after immunization with our biofilm-targeted vaccine antigen either via a traditional subcutaneous route or via a novel noninvasive transcutaneous route. We show that differences in the abundance of specific taxa were found only in the stools of antibiotic-treated animals.

IMPORTANCE The prevalence of chronic and recurrent diseases, combined with the overuse/abuse of antibiotics that has led to the sobering emergence of bacteria resistant to multiple antibiotics, has mandated that we develop novel approaches to better manage these diseases or, ideally, prevent them. Biofilms play a key role in the pathogenesis of chronic and recurrent bacterial diseases but are difficult, if not impossible, to eradicate with antibiotics. We developed a vaccine antigen designed to mediate biofilm disruption; however, it is also important that delivery of this vaccine does not induce collateral damage to the microbiome. The studies described here validated a vaccine approach that targets biofilms without the consequences of an altered gut microbiome. While delivery of the antibiotic most commonly given to children with ear infections did indeed alter the gut microbiome, as expected, immunization via traditional injection or by noninvasive delivery to the skin did not result in changes to the chinchilla gut microbiome.

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.

Transcutaneous immunization with a nontypeable Haemophilus influenzae dual adhesin-directed immunogen induces durable and boostable immunity

Otitis media (OM) is a very common pediatric disease and nontypeable Haemophilus influenzae (NTHI) is the predominant causative agent. We've developed a chimeric immunogen, chimV4, that simultaneously targets two NTHI adhesins, OMP P5 and the type IV pilus. Transcutaneous immunization (TCI) via bandaid with chimV4 plus the adjuvant dmLT provides significant protection against experimental NTHI-induced OM in chinchilla models. Herein, we now examined the durability and boostability of the induced immune response. Bandaid immunization with chimV4+dmLT followed by two sequential middle ear challenges with NTHI resulted in rapid bacterial clearance and significantly accelerated disease resolution. Moreover, TCI with chimV4+dmLT significantly increased mature B-cell phenotypes and antibody-secreting cells within nasal-associated lymphoid tissues, a response that was further augmented upon TCI two months later. Thus, bandaid immunization induced durable and boostable immunity. The simplicity and non-invasive nature of TCI with chimV4+dmLT supports its utility as a highly effective additional immunization strategy for NTHI-induced OM.

LTA1 and dmLT enterotoxin-based proteins activate antigen-presenting cells independent of PKA and despite distinct cell entry mechanisms

Enterotoxin-based proteins are powerful manipulators of mucosal immunity. The A1 domain of heat-labile enterotoxin from E. coli, or LTA1, is a newer adjuvant from this family under investigation for intranasal vaccines. Although LTA1 has been examined in mouse vaccination studies, its ability to directly stimulate immune cells compared to related adjuvant proteins has not been well explored. Here, we perform the first rigorous examination of LTA1’s effect on antigen presenting cells (APC) using a human monocyte cell line THP-1. To better understand LTA1’s stimulatory effects, we compared it to dmLT, or LT-R192G/L211A, a related AB₅ adjuvant in clinical trials for oral or parenteral vaccines. LTA1 and dmLT both activated APCs to upregulate MHC-II (HLA-DR), CD86, cytokine secretion (e.g., IL-1β) and inflammasome activation. The effect of LTA1 on surface marker changes (e.g., MHC-II) was highly dose-dependent whereas dmLT exhibited high MHC-II expression regardless of dose. In contrast, cytokine secretion profiles were similar and dose-dependent after both LTA1 and dmLT treatment. Cellular activation by LTA1 was independent of ganglioside binding, as pre-treatment with purified GM1 blocked the effect of dmLT but not LTA1. Unexpectedly, while activation of the inflammasome and cytokine secretion by LTA1 or dmLT was blocked by the protein kinase A inhibitor H89 (similar to previous reports), these responses were not inhibited by a more specific PKA peptide inhibitor or antagonist; thus Indicating that a novel and unknown mechanism is responsible for inflammasome activation and cytokine secretion by LT proteins. Lastly, LTA1 stimulated a similar cytokine profile in primary human monocytes as it did in THP1 cells, including IL-1β, IL-6, IL-8, MIP-1α, MIP-1β, and TNFα. Thus, we report that LTA1 protein programs a dendritic cell-like phenotype in APCs similar to dmLT in a mechanism that is independent of PKA activation and GM1 binding and entry.

Immunomodulation as a Novel Strategy for Prevention and Treatment of Bordetella spp. Infections

Well-adapted pathogens have evolved to survive the many challenges of a robust immune response. Defending against all host antimicrobials simultaneously would be exceedingly difficult, if not impossible, so many co-evolved organisms utilize immunomodulatory tools to subvert, distract, and/or evade the host immune response. Bordetella spp. present many examples of the diversity of immunomodulators and an exceptional experimental system in which to study them. Recent advances in this experimental system suggest strategies for interventions that tweak immunity to disrupt bacterial immunomodulation, engaging more effective host immunity to better prevent and treat infections. Here we review advances in the understanding of respiratory pathogens, with special focus on Bordetella spp., and prospects for the use of immune-stimulatory interventions in the prevention and treatment of infection.

Highly Pathogenic Avian Influenza H5 Hemagglutinin Fused with the A Subunit of Type IIb Escherichia coli Heat Labile Enterotoxin Elicited Protective Immunity and Neutralization by Intranasal Immunization in Mouse and Chicken Models

Highly pathogenic avian influenza viruses are classified by the World Organization for Animal Health (OIE) as causes of devastating avian diseases. This study aimed to develop type IIb Escherichiacoli heat-labile enterotoxin (LTIIb) as novel mucosal adjuvants for mucosal vaccine development. The fusion protein of H5 and LTIIb-A subunit was expressed and purified for mouse and chicken intranasal immunizations. Intranasal immunization with the H5-LTIIb-A fusion protein in mice elicited potent neutralizing antibodies in sera and bronchoalveolar lavage fluids, induced stronger Th1 and Th17 cellular responses in spleen and cervical lymph nodes, and improved protection against H5N1 influenza virus challenge. More interestingly, intranasal immunization with the H5-LTIIb-A fusion protein in chickens elicited high titers of IgY, IgA, hemagglutinin inhibition (HAI), and neutralizing antibodies in their antisera. This study employed the novel adjuvants of LTIIb for the development of a new generation of mucosal vaccines against highly pathogenic avian influenza viruses.

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

Background

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

Methods

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

Findings

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

Interpretation

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

Funding

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

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

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

A new human challenge model for testing heat-stable toxin-based vaccine candidates for enterotoxigenic Escherichia coli diarrhea - dose optimization, clinical outcomes, and CD4+ T cell responses

Enterotoxigenic Escherichia coli (ETEC) are a common cause of diarrheal illness in young children and travelers. There is yet no licensed broadly protective vaccine against ETEC. One promising vaccine development strategy is to target strains expressing the heat-stable toxin (ST), particularly the human ST (STh), since infections with these strains are among the leading causes of diarrhea in children in low-and-middle income countries. A human challenge model based on an STh-only ETEC strain will be useful to evaluate the protective efficacy of new ST-based vaccine candidates. To develop this model, we experimentally infected 21 healthy adult volunteers with the epidemiologically relevant STh-only ETEC strain TW10722, identified a suitable dose, assessed safety, and characterized clinical outcomes and immune responses caused by the infection. Doses of 1×10¹⁰ colony-forming units (CFU) of TW10722 gave a suitable attack risk of 67% for moderate or severe diarrhea and an overall diarrhea attack risk of 78%. Non-diarrheal symptoms were mostly mild or moderate, and there were no serious adverse events. During the first month after ingesting the challenge strain, we measured significant increases in both activated CD4+ T cells and levels of serum IgG and IgA antibodies targeting coli surface antigen 5 (CS5) and 6 (CS6), as well as the E. coli mucinase YghJ. The CS5-specific CD4+ T cell and antibody responses were still significantly elevated one year after experimental infection. In conclusion, we have developed a safe STh-only ETEC-based human challenge model which can be efficiently used in Phase 2B trials to evaluate the protective efficacy of new ST-based vaccine candidates.

Enterotoxigenic Escherichia coli (ETEC) is a common cause of diarrheal illness in young children living in low- and middle-income countries and in travelers to these countries. Several ETEC vaccine candidates are currently being developed, but so far, no broadly protective vaccines have been licensed. Since most moderate and severe ETEC diarrheal episodes are caused by strains that express the heat-stable enterotoxin (ST), ST represents a promising vaccine target. Here we present a human challenge model that can be used to estimate the protective efficacy of ST-based vaccine candidates in clinical vaccine trials. The model is based on the epidemiologically relevant ST-only ETEC strain TW10722, which we show is safe to ingest by volunteers and readily induce diarrhea.

LTA1 is a safe, intranasal enterotoxin-based adjuvant that improves vaccine protection against influenza in young, old and B-cell-depleted (μMT) mice

Enterotoxin-based adjuvants including cholera toxin and heat-labile toxin (LT) are powerful manipulators of mucosal immunity; however, past clinical trials identified unacceptable neurological toxicity when LT or mutant AB5 adjuvant proteins were added to intranasal vaccines. Here, we examined the isolated enzymatic A1 domain of LT (LTA1) for intranasal safety and efficacy in combination with influenza (flu) vaccination. LTA1-treated mice exhibited no neurotoxicity, as measured by olfactory system testing and H&E staining of nasal tissue in contrast with cholera toxin. In vaccination studies, intranasal LTA1 enhanced immune responses to inactivated virus antigen and subsequent protection against H1N1 flu challenge in mice (8-week or 24-months). In addition, lung H1N1 viral titers post-challenge correlated to serum antibody responses; however, enhanced protection was also observed in μMT mice lacking B-cells while activation and recruitment of CD4 T-cells into the lung was apparent. Thus, we report that LTA1 protein is a novel, safe and effective enterotoxin adjuvant that improves protection of an intranasal flu vaccination by a mechanism that does not appear to require B-cells.

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

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

Redirecting the immune response towards immunoprotective domains of a DNABII protein resolves experimental otitis media

The chronicity and recurrence of many bacterial diseases is largely attributable to the presence of a biofilm, and eradication of these structures is confounded by an extracellular DNA-rich matrix. DNABII proteins, including integration host factor (IHF), are critical components of the matrix formed by all human pathogens tested to date. Whereas the natural adaptive immune response to IHF is against non-protective epitopes within the carboxyl-terminal region, antibodies against the DNA-binding “tips” induce biofilm collapse. We designed a “tip-chimer” immunogen to mimic the DNA-binding regions within the α-subunit and β-subunit of IHF from nontypeable Haemophilus influenzae (IHF_NTHi). Re-direction of the natural adaptive immune response toward immunoprotective domains disrupted NTHi biofilms in vitro and in an experimental model of otitis media. Our data support the rational design of a powerful therapeutic approach, and also that of a DNABII-directed vaccine antigen that would avoid augmentation of any pre-existing natural, but nonprotective, immune response.

Bacterial biofilms are characterized by the presence of a protective extracellular polymeric substance (EPS) that incorporates both eDNA and members of the DNABII family of bacterial DNA-binding proteins. Antibodies against the “tips” of these DNA binding-domains can cause biofilm collapse, but these epitopes are masked from the host adaptive immune system when bound to eDNA, making biofilm eradication difficult. Here, the team led by Lauren Bakaletz used a chimeric peptide to generate tip-specific antibodies against nontypeable Haemophilus influenzae to treat biofilms in vitro and in vivo. The “tip-chimer” contained the immunoprotective domains from the DNA-binding tips of a DNABII protein, integration host factor (IHF), expressed by nontypeable Haemophilus influenzae. The consequent antibodies disrupted H. influenzae biofilms in vitro and were used to treat a chinchilla model of experimental otitis media when inoculated directly into the middle ear, resulting in reduced bacterial load and clearance of already established mucosal biofilms. These findings suggest that redirecting the host adaptive immune response towards the immunoprotective tips of DNABII proteins could provide a strategy to eradicate biofilms caused by various pathogens that produce these proteins.

Review of Newly Identified Functions Associated With the Heat-Labile Toxin of Enterotoxigenic Escherichia coli

Heat-labile toxin (LT) is a well-characterized powerful enterotoxin produced by enterotoxigenic Escherichia coli (ETEC). This toxin is known to contribute to diarrhea in young children in developing countries, international travelers, as well as many different species of young animals. Interestingly, it has also been revealed that LT is involved in other activities in addition to its role in enterotoxicity. Recent studies have indicated that LT toxin enhances enteric pathogen adherence and subsequent intestinal colonization. LT has also been shown to act as a powerful adjuvant capable of upregulating vaccine antigenicity; it also serves as a protein or antigenic peptide display platform for new vaccine development, and can be used as a naturally derived cell targeting and protein delivery tool. This review summarizes the epidemiology, secretion, delivery, and mechanisms of action of LT, while also highlighting new functions revealed by recent studies.

The TLR4 agonist adjuvant SLA-SE promotes functional mucosal antibodies against a parenterally delivered ETEC vaccine

Many pathogens establish infection at mucosal surfaces such as the enteric pathogen Enterotoxigenic E. coli (ETEC). Thus, there is a pressing need for effective vaccination strategies that promote protective immunity at mucosal surfaces. Toll-like receptor (TLR) ligands have been extensively developed as vaccine adjuvants to promote systemic immunity, whereas attenuated bacterial toxins including cholera toxin and heat-labile toxin (LT) have initially been developed to promote mucosal immunity. Here we evaluate the ability of the TLR4 agonist second-generation lipid adjuvant formulated in a stable emulsion (SLA-SE) to augment functional mucosal antibodies elicited by intramuscular immunization with a recombinant ETEC vaccine antigen. We find that, in mice, parenterally delivered SLA-SE is at least as effective as the double-mutant LT (LTR192G/L211A, dmLT) adjuvant in promoting functional antibodies and eliciting intestinal IgA responses to the vaccine antigen. In addition, SLA-SE enhanced both the IgG2a response in the mucosa and serum, and the production of LT neutralizing serum antibodies elicited by dmLT four to eightfold. These results reveal unexpected mucosal adjuvant properties of this TLR4 agonist adjuvant when delivered intramuscularly. This may have a substantial impact on the development of vaccines against enteric and other mucosal pathogens.

Although offering great potential for generating intestinal immunity, vaccination by the oral route suffers from several barriers such as the breakdown of protein vaccines in the stomach and/or the induction of oral tolerance. To investigate whether these barriers can be circumvented, Mark T. Orr and colleagues at the Infectious Disease Research Institute use a parenteral (intramuscular) vaccination protocol in mice. Intramuscular immunization with an enterotoxigenic E. coli (ETEC) vaccine plus a Toll-like receptor 4 adjuvant in stable emulsion (SLA-SE) elicits a functional antibody response in both the gut and serum. Importantly, this intramuscular vaccination triggers robust production of IgA in the gut. These findings suggest that with the right adjuvant combination it might possible to generate potent protective mucosal immunity following parenteral immunization.

Identification of Conserved Candidate Vaccine Antigens in the Surface Proteome of Giardia lamblia

Giardia lamblia, one of the most common protozoal infections of the human intestine, is an important worldwide cause of diarrheal disease, malabsorption, malnutrition, delayed cognitive development in children, and protracted postinfectious syndromes. Despite its medical importance, no human vaccine is available against giardiasis. A crude veterinary vaccine has been developed, and experimental vaccines based on expression of multiple variant-specific surface proteins have been reported, but poorly defined vaccine components and excessive antigen variability are problematic for pharmaceutical vaccine production. To expand the repertoire of antigen candidates for vaccines, we reasoned that surface proteins may provide an enriched source of such antigens since key host effectors, such as secretory IgA, can directly bind to such antigens in the intestinal lumen and interfere with epithelial attachment. Here, we have applied a proteomics approach to identify 23 novel surface antigens of G. lamblia that show >90% amino acid sequence identity between the two human-pathogenic genetic assemblages (A and B) of the parasite. Surface localization of a representative subset of these proteins was confirmed by immunostaining. Four selected proteins, uridine phosphorylase-like protein-1, protein 21.1 (GL50803_27925), α1-giardin, and α11-giardin, were subsequently produced in recombinant form and shown to be immunogenic in mice and G. lamblia-infected humans and confer protection against G. lamblia infection upon intranasal immunization in rodent models of giardiasis. These results demonstrate that identification of conserved surface antigens provides a powerful approach for overcoming a key rate-limiting step in the design and construction of an effective vaccine against giardiasis.

Parenteral Vaccination With a Tuberculosis Subunit Vaccine in Presence of Retinoic Acid Provides Early but Transient Protection to M. Tuberculosis Infection

Most microbes invading through mucosal surfaces cause disease and therefore strategies to induce mucosal immune responses are strongly needed. Vitamin A metabolites, such as retinoic acid (RA), play crucial roles in programming T and B cells to home to mucosal compartments, therefore we evaluated the capacity of RA to elicit mucosal immune responses against tuberculosis (TB) after parenteral vaccination. We found that mice immunized through subcutaneous injections with the TB subunit vaccine (CAF01+H56) in presence of RA show enhanced mucosal H56-specific IgA responses and enhanced Ag-specific CD4⁺ T lymphocytes homing to the lung as compared with control mice. Immunization with CAF01+H56 in presence of RA resulted in lower bacterial loads in the lungs of mice 14 days after challenge with virulent Mycobacterium tuberculosis (Mtb) as compared to mice immunized in the absence of RA or vaccinated with BCG. Higher amounts of IFNγ and IL-17 pro-inflammatory cytokines were found in lung homogenates of mice immunized with CAF01+H56 and RA 24 h after Mtb infection. However, 6 weeks after infection the protection was comparable in vaccinated mice with or without RA even though treatment with RA during immunization is able to better contain the inflammatory response by the host. Furthermore, at later stage of the infection a higher percentage of Mtb specific CD4⁺PD1⁺ T lymphocytes were found in the lungs of mice immunized with CAF01+H56 and RA. These data show that an enhanced mucosal immune response is generated during parenteral vaccination in presence of RA. Furthermore, RA treatment contained the bacterial growth at an early stage of the infection and limited the inflammatory response in the lung at later time points.

Enterotoxigenic Escherichia coli Infections

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

HIV-1 vaccination by needle-free oral injection induces strong mucosal immunity and protects against SHIV challenge

The oral mucosa is an attractive site for mucosal vaccination, however the thick squamous epithelium limits antigen uptake. Here we utilize a modified needle-free injector to deliver immunizations to the sublingual and buccal (SL/B) tissue of rhesus macaques. Needle-free SL/B vaccination with modified vaccinia Ankara (MVA) and a recombinant trimeric gp120 protein generates strong vaccine-specific IgG responses in serum as well as vaginal, rectal and salivary secretions. Vaccine-induced IgG responses show a remarkable breadth against gp70-V1V2 sequences from multiple clades of HIV-1. In contrast, topical SL/B immunizations generates minimal IgG responses. Following six intrarectal pathogenic SHIV-SF162P3 challenges, needle-free but not topical immunization results in a significant delay of acquisition of infection. Delay of infection correlates with non-neutralizing antibody effector function, Env-specific CD4+ T-cell responses, and gp120 V2 loop specific antibodies. These results demonstrate needle-free MVA/gp120 oral vaccination as a practical and effective route to induce protective immunity against HIV-1.