Genetic fusions of heat-labile toxoid (LT) and heat-stable toxin b (STb) of porcine enterotoxigenic Escherichia coli elicit protective anti-LT and anti-STb antibodies

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.

A polyvalent multiepitope protein cross-protects against Vibrio cholerae infection in rabbit colonization and passive protection models

Using epitope- and structure-based multiepitope fusion antigen vaccinology platform, we constructed a polyvalent protein immunogen that presents antigenic domains (epitopes) of Vibrio cholerae toxin-coregulated pilus A, cholera toxin (CT), sialidase, hemolysin A, flagellins (B, C, and D), and peptides mimicking lipopolysaccharide O-antigen on a flagellin B backbone. Mice and rabbits immunized intramuscularly with this polyvalent protein immunogen developed antibodies to all of the virulence factors targeted by the immunogen except lipopolysaccharide. Mouse and rabbit antibodies exhibited functional activities against CT enterotoxicity, CT binding to GM₁ ganglioside, bacterial motility, and in vitro adherence of V. cholerae O1, O139, and non-O1/non-O139 serogroup strains. When challenged orogastrically with V. cholerae O1 El Tor N16961 or a non-O1/non-O139 strain, rabbits IM immunized with the immunogen showed a 2-log (99%) reduction in V. cholerae colonization of small intestines. Moreover, infant rabbits born to the mother immunized with the protein immunogen acquired antibodies passively and were protected from bacterial intestinal colonization (>2-log reduction), severe diarrhea (100%), and mild diarrhea (88%) after infection with V. cholerae O1 El Tor (N16961), O1 classical (O395), O139 (Bengal), or a non-O1/non-O139 strain. This study demonstrated that this polyvalent cholera protein is broadly immunogenic and cross-protective, and an adult rabbit colonization model and an infant rabbit passive protection model fill a gap in preclinical efficacy assessment in cholera vaccine development.

Pig vaccination strategies based on enterotoxigenic Escherichia coli toxins

Enterotoxigenic Escherichia coli (ETEC) are responsible for diarrhea in humans as well as in farm animals. ETEC infections in newborn, suckling, and especially in post-weaning piglets are associated with reduced growth rate, morbidity, and mortality. ETEC express virulence factors as adhesin and enterotoxins that play a central role in the pathogenic process. Adhesins associated with pigs are of diverse type being either fimbrial or non-fimbrial. Enterotoxins belong to two groups: heat-labile (LT) and heat-stable (ST). Heterogeneity of ETEC strains encompass expression of various fimbriae (F4, F5, F6, F18, and F41) and enterotoxins (LT, STa, STb, and EAST1). In the late years, attempts to immunize animals against neonatal and post-weaning diarrhea were focused on the development of anti-adhesin strategies as this is the initial step of ETEC pathogenesis. Although those vaccines demonstrated some protection against ETEC infections, as enterotoxins are pivotal to the virulence of ETEC, a new generation of vaccinal molecules, which include adhesin and one or more enterotoxins, were recently tested. Some of these newly developed chimeric fusion proteins are intended to control as well human diarrhea as enterotoxins are more or less common with the ones found in pigs. As these could not be tested in the natural host (human), either a mouse or pig model was substituted to evaluate the protection efficacy. For the advancement of pig vaccine, mice were sometimes used for preliminary testing. This review summarizes advances in the anti-enterotoxin immunization strategies considered in the last 10 years.

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

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

Application of a Novel Epitope- and Structure-Based Vaccinology-Assisted Fimbria-Toxin Multiepitope Fusion Antigen of Enterotoxigenic Escherichia coli for Development of Multivalent Vaccines against Porcine Postweaning Diarrhea

Enterotoxigenic Escherichia coli (ETEC) strains producing K88 (F4) or F18 fimbriae and enterotoxins are the predominant cause of pig postweaning diarrhea (PWD). We recently identified neutralizing epitopes of fimbriae K88 and F18, heat-labile toxin (LT), heat-stable toxins type I (STa) and type II (STb), and Shiga toxin 2e (Stx2e). In this study, we explored a novel epitope- and structure-based vaccinology platform, multiepitope fusion antigen (MEFA), for PWD vaccine development. By using an epitope substitution LT toxoid, which lacks enterotoxicity but retains immunogenicity, as the backbone to present neutralizing epitopes of two ETEC fimbriae and four toxins, we generated PWD fimbria-toxin MEFA to mimic epitope native antigenicity. We then examined MEFA protein immunogenicity and evaluated MEFA application in PWD vaccine development. Mice subcutaneously immunized with PWD MEFA protein developed strong IgG responses to K88, F18, LT, and STb and moderate responses to the toxins Stx2e and STa. Importantly, MEFA-induced antibodies inhibited adherence of K88 or F18 fimbrial bacteria to pig intestinal cells and also neutralized LT, STa, STb, and Stx2e toxicity. These results indicated that PWD fimbria-toxin MEFA induced neutralizing antibodies against an unprecedent two fimbriae and four toxins and strongly suggested a potential application of this MEFA protein in developing a broadly protective PWD vaccine.IMPORTANCE ETEC-associated postweaning diarrhea (PWD) causes significant economic losses to swine producers worldwide. Currently, there is no effective prevention against PWD. A vaccine that blocks ETEC fimbriae (K88 and F18) from attaching to host receptors and prevents enterotoxins from stimulating water hypersecretion in pig small intestinal epithelial cells can effectively protect against PWD and significantly improves pig health and well-being. The fimbria-toxin MEFA generated from this study induced neutralizing antibodies against both ETEC fimbriae and all four ETEC toxins, suggesting a great potential of this fimbria-toxin MEFA in PWD vaccine development and further supporting the general application of this novel MEFA vaccinology platform for multivalent vaccine development.

Mapping the Neutralizing Epitopes of Enterotoxigenic Escherichia coli K88 (F4) Fimbrial Adhesin and Major Subunit FaeG

Enterotoxigenic Escherichia coli (ETEC) strains that produce immunologically heterogeneous fimbriae and enterotoxins are the primary cause of neonatal diarrhea and postweaning diarrhea in young pigs. A multivalent vaccine inducing protective immunity against ideally all ETEC fimbriae and enterotoxins could be effective against diarrhea in young pigs. However, developing a vaccine to broadly protect against various ETEC virulence determinants has proven challenging. Recently developed structure- and epitope-based multiepitope fusion antigen (MEFA) technology that presents neutralizing epitopes of various virulence determinants at a backbone immunogen and that mimics epitope native immunogenicity suggests the feasibility of developing multivalent vaccines. With neutralizing epitopes from ETEC fimbria F18 and enterotoxins being identified, it becomes urgent to identify protective epitopes of K88 (F4) fimbriae, which play a major role in pig neonatal and postweaning diarrhea. In this study, we identified B-cell immunodominant epitopes in silico from the K88ac fimbrial major subunit (also adhesin) FaeG and embedded each epitope in a heterogeneous carrier for epitope fusions. We then immunized mice with each epitope fusion protein and examined epitope antigenicity and also neutralizing activities of epitope-induced antibodies. Data showed that while all nine FaeG epitope fusions induced antibodies to K88ac fimbria, anti-K88 IgG antibodies derived from epitopes MTGDFNGSVD (ep1), LNDLTNGGTK (ep2), GRTKEAFATP (ep3), ELRKPDGGTN (ep4), PMKNAGGTKVGAVKVN (ep5), and RENMEYTDGT (ep8) significantly inhibited adherence of K88ac fimbrial bacteria to porcine intestinal cell line IPEC-J2, indicating that these peptides were the neutralizing epitopes of K88ac fimbrial major subunit FaeG and suggesting the future application of FaeG epitopes in ETEC vaccine development.IMPORTANCE Enterotoxigenic Escherichia coli (ETEC) strains producing K88ac fimbriae and enterotoxins are a major cause of porcine neonatal diarrhea and postweaning diarrhea in the United States. Currently, there is no vaccine to induce broadly protective antiadhesin and antitoxin immunity against ETEC-associated diarrhea. To develop a broadly effective ETEC vaccine, we need to target the most important if not all ETEC virulence determinants. While conventional vaccinology approaches encounter difficulties at integrating or including heterogeneous ETEC fimbria and toxin antigens into a vaccine product, multiepitope fusion antigen (MEFA) structural vaccinology provides a new platform to combine neutralizing antigenic elements or epitopes from various heterogeneous virulence factors for broad immunity and protection. Identification of the neutralizing epitopes of K88ac fimbria from this study added the last antigens to an MEFA-based multivalent vaccine against ETEC-associated diarrhea in pigs. An effective vaccine against pig diarrhea can significantly improve swine health and well-being and reduce economic losses to the swine industry worldwide.

Development of an enterotoxigenic Escherichia coli vaccine based on the heat-stable toxin

Infection with enterotoxigenic Escherichia coli (ETEC) is an important cause of diarrhea-related illness and death among children under 5 years of age in low– and middle-income countries (LMIC). Recent studies have found that it is the ETEC subtypes that produce the heat-stable enterotoxin (ST), irrespective of whether they also secrete the heat-labile enterotoxin (LT), which contribute most importantly to the disease burden in children from LMIC. Therefore, adding an ST toxoid would importantly complement ongoing ETEC vaccine development efforts. The ST’s potent toxicity, its structural similarity to the endogenous peptides guanylin and uroguanylin, and its poor immunogenicity have all complicated the advancement of ST-based vaccine development. Recent remarkable progress, however, including the unprecedented screening for optimal ST mutants, mapping of cross-reacting ST epitopes and improved ST-carrier coupling strategies (bioconjugation and genetic fusion), enables the rational design of safe, immunogenic, and well-defined ST-based vaccine candidates.

Phylogenetic analysis, biofilm production, and antimicrobial resistance profile of Escherichia coli isolated from slaughtered pigs

ABSTRACT Enteric diseases of bacterial origin are frequent in the pig industry, of particular notoriety are the colibacillosis that mainly affect piglets and cause great damage to the swine industry worldwide. The aim of the study was to analyze phylogenetics, to detect biofilm production, and to determine antimicrobial resistance profile in 126 strains of Escherichia coli isolated from swabs obtained from fragments of the small intestines of 235 healthy pigs killed in slaughterhouses in Pernambuco (Brazil) using polymerase chain reaction (PCR), adherence to microplates test and disc diffusion technique. Of the analyzed samples, 88.10% (111/126) were classified in phylogenetic group B1; 4.76% (6/126) in group D; 3.97% (5/126) in group B2 and, 3.17% (4/126) in group A. Antimicrobial resistance rates observed were: lincomycin 100% (126/126), erythromycin 100% (126/126), chlortetracycline 94.44% (119/126), cephalothin 51.59% (65/126), ampicillin 38.89% (49/126), sulfamethoxazole + trimethoprim 37.3% (47/126), ciprofloxacin 19.84% (25/126), norfloxacin 14.29% (18/126), gentamicin 8.73% (11/126) and, chloramphenicol 5.55% (7/126). Multiple antibiotic resistance (MAR) ranged from 0.2 to 0.9. Of the strains tested 46.03% (58/126) produced biofilm, and 99.21% (125/126) of the strains exhibited multi-resistance. Further studies are required to elucidate the importance of each phylogenetic group in pigs and to prevent the propagation of multi-resistant E. coli strains.

A Chimeric protein of CFA/I, CS6 subunits and LTB/STa toxoid protects immunized mice against enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia Coli (ETEC) strains are the commonest bacteria causing diarrhea in children in developing countries and travelers to these areas. Colonization factors (CFs) and enterotoxins are the main virulence determinants in ETEC pathogenesis. Heterogeneity of CFs is commonly considered the bottleneck to developing an effective vaccine. It is believed that broad spectrum protection against ETEC would be achieved by induced anti-CF and anti-enterotoxin immunity simultaneously. Here, a fusion antigen strategy was used to construct a quadrivalent recombinant protein called 3CL and composed of CfaB, a structural subunit of CFA/I, and CS6 structural subunits, LTB and STa toxoid of ETEC. Its anti-CF and antitoxin immunogenicity was then assessed. To achieve high-level expression, the 3CL gene was synthesized using E. coli codon bias. Female BALB/C mice were immunized with purified recombinant 3CL. Immunized mice developed antibodies that were capable of detecting each recombinant subunit in addition to native CS6 protein and also protected the mice against ETEC challenge. Moreover, sera from immunized mice also neutralized STa toxin in a suckling mouse assay. These results indicate that 3CL can induce anti-CF and neutralizing antitoxin antibodies along with introducing CFA/I as a platform for epitope insertion.

Animal Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is the most common cause of E. coli diarrhea in farm animals. ETEC are characterized by the ability to produce two types of virulence factors: adhesins that promote binding to specific enterocyte receptors for intestinal colonization and enterotoxins responsible for fluid secretion. The best-characterized adhesins are expressed in the context of fimbriae, such as the F4 (also designated K88), F5 (K99), F6 (987P), F17, and F18 fimbriae. Once established in the animal small intestine, ETEC produce enterotoxin(s) that lead to diarrhea. The enterotoxins belong to two major classes: heat-labile toxins that consist of one active and five binding subunits (LT), and heat-stable toxins that are small polypeptides (STa, STb, and EAST1). This review describes the disease and pathogenesis of animal ETEC, the corresponding virulence genes and protein products of these bacteria, their regulation and targets in animal hosts, as well as mechanisms of action. Furthermore, vaccines, inhibitors, probiotics, and the identification of potential new targets by genomics are presented in the context of animal ETEC.

Immunogenicity and protective efficacy of a recombinant adenoviral based vaccine expressing heat-stable enterotoxin (STa) and K99 adhesion antigen of enterotoxigenic Escherichia coli in mice

The diarrheal disease of domestic animals or in humans caused by enterotoxigenic Escherichia coli (ETEC) infections remains a major issue for public health in developing countries. Unfortunately, there is no effective vaccine available for preventing from an ETEC infection. Therefore, the development of a safe and effective vaccine against ETEC is urgently needed. In the present study, A recombinant adenoviral vector Ad5-STa-K99 that capable of expressing a fusion protein of heat-stable enterotoxin (STa) and K99 adhesion antigen of ETEC was generated and its immunogenicity was evaluated in a murine model. The intestinal mucosal secretory IgA(sIgA), serum anti-STa-K99 antibody responses, antigen-specific CD4(+) and CD8(+) T cells frequencies, as well as T-cell proliferation of mice immunized with the viral vector were determined as immunological indexes. The results demonstrated that Ad5-STa-K99 was able to enhance humoral responses with a dramatically augmented antigen-specific serum IgG antibody, and an elevated production of intestinal sIgA in immunized mice, suggesting the elicitation of both of humoral and mucosal immune responses. In addition, this adenoviral vector could significantly promote splenic T cell proliferation and increase the frequencies of CD4(+) and CD8(+) T cell populations in mice, indicative of a capacity to activate T cell responses. More importantly, vaccination of the Ad5-STa-K99 showed a potential to evoke a protective effect from ETEC challenge in mice. These data indicate that the Ad5-STa-K99 is a highly immunogenic vector able to induce a broad range of antigen-specific immune responses in vivo, and evoke a protective immune response against ETEC infections, implying that it may be a novel vaccine candidate warranted for further investigation.

Current Progress in Developing Subunit Vaccines against Enterotoxigenic Escherichia coli-Associated Diarrhea

Diarrhea continues to be a leading cause of death in children <5 years of age, and enterotoxigenic Escherichia coli (ETEC) is the most common bacterial cause of children's diarrhea. Currently, there are no available vaccines against ETEC-associated diarrhea. Whole-cell vaccine candidates have been under development but require further improvements because they provide inadequate protection and produce unwanted adverse effects. Meanwhile, a newer approach using polypeptide or subunit vaccine candidates focusing on ETEC colonization factor antigens (CFAs) and enterotoxins, the major virulence determinants of ETEC diarrhea, shows substantial promise. A conservative CFA/I adhesin tip antigen and a CFA MEFA (multiepitope fusion antigen) were shown to induce cross-reactive antiadhesin antibodies that protected against adherence by multiple important CFAs. Genetic fusion of toxoids derived from ETEC heat-labile toxin (LT) and heat-stable toxin (STa) induced antibodies neutralizing both enterotoxins. Moreover, CFA-toxoid MEFA polypeptides, generated by fusing CFA MEFA to an STa-LT toxoid fusion, induced antiadhesin antibodies that broadly inhibited adherence of the seven most important ETEC CFAs associated with about 80% of the diarrhea cases caused by ETEC strains with known CFAs. This same antigen preparation also induced antitoxin antibodies that neutralized both toxins that are associated with all cases of ETEC diarrhea. Results from these studies suggest that polypeptide or subunit vaccines have the potential to effectively protect against ETEC diarrhea. In addition, novel adhesins and mucin proteases have been investigated as potential alternatives or, more likely, additional antigens for ETEC subunit vaccine development.

Analysis and application of a neutralizing linear epitope on liable toxin B of enterotoxin Escherichia coli

Heat-labile enterotoxin (LT) of enterotoxigenic Escherichia coli (ETEC) is one of the major virulence factors for causing diarrhea in piglets, and LT is a strong immunogen. Thus, LT represents an important target for development of vaccines and diagnostic tests. In this study, bioinformatic tools were used to predict six antigenic B cell epitopes in the B subunit of LT protein (LTB) of ETEC strains. Then, seven antigenic B cell epitopes of LTB were identified by polyclonal antisera (polyclonal antibody (PAb)) using a set of LTB-derived peptides expressed as maltose-binding protein (MBP) fusion protein. In addition, one LTB-specific monoclonal antibody (MAb) was generated and defined its corresponding epitope as mentioned above. This MAb was able to specifically bind with native LT toxin and has no cross-reaction with LT-II (type II heat-labile enterotoxin), Stx1 (Shiga toxin I), Stx2 (Shiga toxin II), STa (heat-stable enterotoxin I), and STb (heat-stable enterotoxin II) toxins. Further, this MAb was able to interrupt LT toxin specific binding to GM1 receptor, indicating that the corresponding epitope is the specific binding region to GM1 receptor. Moreover, in vitro and in vivo assay showed that the MAb was able to neutralize the native LT toxin. Diarrheal suckling pigs challenged with LT-positive ETEC strain recovered when an enema with this purified MAb was administered. This study will provide the foundation for further studies about the interaction between LT toxin and GM1 receptor and about the developing of epitope-based vaccines and specific therapeutic agent.

Construction and evaluation of a fluorescence-based live attenuated Escherichia coli delivery system for generating oral vaccine candidate

Enter toxigenic Escherichia coli (ETEC) is a major pathogen of swine industry that can have a substantial impact on morbidity and mortality. Therefore, it is necessary to develop effective vaccines for the prevention of ETEC infection. Live attenuated bacteria delivery system are effective tools for mucosal immunization. The purpose of this study was to construct a novel delivery system that can present the LTR192G-STb fusion protein as oral vaccine candidate. Firstly, the PRPL-mKate2 fluorescent cassette was inserted into the genome (yaiT pseudogene) of an attenuated E. coli by homologous recombination methods to construct the delivery system O142(yaiT::PRPL-mKate2). Secondly, the oral vaccine O142(yaiT:: LT192-STb) (ER-B) was derived for replacing the PRPL-mKate2 by LT192-STb fusion gene, and then it was tested for its feasibility as oral vaccine candidate. Subsequently, BALB/c mice were orogastrically immunized with ER-B. Results showed that mice orally immunized with ER-B produced high levels of specific IgA and IgG antibodies. The induced antibodies demonstrated neutralizing effects to enter toxins LT and STb. In addition, results of cellular immune responses showed that stimulation index values of immunized mice were significantly higher than the control group (P < 0.05) and with a marked shift towards Th 2 immunity. These data indicated that the recombinant E. coli ER-B could be a valuable candidate of future vaccines against ETEC infection.

Longitudinal Characterization of Escherichia coli in Healthy Captive Non-Human Primates

The gastrointestinal (GI) tracts of non-human primates (NHPs) are well known to harbor Escherichia coli, a known commensal of human beings and animals. While E. coli is a normal inhabitant of the mammalian gut, it also exists in a number of pathogenic forms or pathotypes, including those with predisposition for the GI tract as well as the urogenital tract. Diarrhea in captive NHPs has long been a problem in both zoo settings and research colonies, including the Como Zoo. It is an animal welfare concern, as well as a public health concern. E. coli has not been extensively studied; therefore, a study was performed during the summer of 2009 in collaboration with a zoo in Saint Paul, MN, which was previously experiencing an increased incidence and severity of diarrhea among their NHP collection. Fresh fecal samples were collected weekly from each member of the primate collection, between June and August of 2009, and E. coli were isolated. A total of 33 individuals were included in the study, representing eight species. E. coli isolates were examined for their genetic relatedness, phylogenetic relationships, plasmid replicon types, virulence gene profiles, and antimicrobial susceptibility profiles. A number of isolates were identified containing virulence genes commonly found in several different E. coli pathotypes, and there was evidence of clonal transmission of isolates between animals and over time. Overall, the manifestation of chronic diarrhea in the Como Zoo primate collection is a complex problem whose solution will require regular screening for microbial agents and consideration of environmental causes. This study provides some insight toward the sharing of enteric bacteria between such animals.

Designing vaccines to neutralize effective toxin delivery by enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) are a leading cause of diarrheal illness in developing countries. Despite the discovery of these pathogens as a cause of cholera-like diarrhea over 40 years ago, and decades of vaccine development effort, there remains no broadly protective ETEC vaccine. The discovery of new virulence proteins and an improved appreciation of the complexity of the molecular events required for effective toxin delivery may provide additional avenues to pursue in development of an effective vaccine to prevent severe diarrhea caused by these important pathogens.

Genes de enterotoxinas e perfil antimicrobiano de Escherichia coli isoladas de suínos hígidos no Distrito Federal

Um total de 127 cepas de Escherichia coli foi isolado de suínos no Distrito Federal, testado para a presença de genes de enterotoxinas (STa, LT-I, LT-II, Stx1 e Stx2) e para resistência antimicrobiana. Das cepas isoladas, oito (6,3%) possuíam genes para enterotoxinas, sendo quatro (3,2%) positivas somente para LT-I, três (2,4%) somente para STa e uma (0,8%) positiva para STa e LT-I. Nenhuma das cepas isoladas apresentou genes para LT-II, Stx1 ou Stx2. Quanto ao perfil de resistência antimicrobiano, os antibióticos com maiores porcentagens de resistência foram lincomicina (100%), sulfonamidas (74,8%) e tetraciclina (70,1%), enquanto os maiores índices de sensibilidade foram observados na norfloxacina (82,7%), gentamicina (75,6%) e sulfametoxazol + trimetoprim (63%). Esses resultados demonstraram a presença de genes de enterotoxinas e altas taxas de resistência antimicrobiana em E. coli isoladas de suínos hígidos no DF.

Construction and immunogenicity evaluation of an epitope-based antigen against swine influenza A virus using Escherichia coli heat-labile toxin B subunit as a carrier-adjuvant

Influenza A virus causes a highly contagious respiratory disease in a variety of avian and mammalian hosts, including humans and pigs. The primary means for preventing influenza epidemics is vaccination. Epitope-based vaccine represents a new approach to achieve protective immunity. The objective of this study was to construct and evaluate the immunogenicity of an epitope-based antigen for its potential application in future influenza vaccine development. The antigen, comprised of a set of consensus influenza A virus epitopes (IAVe), was genetically linked to a subunit of the bacterial heat-labile enterotoxin (LTB) as an adjuvant. Immunogenicity of this LTB-IAVe antigen was evaluated in a pig model. Despite an inability to detect neutralizing antibodies directed toward the whole virus, humoral immunity against the IAVe was demonstrated in both serum (IgA and IgG) and mucosal secretions (IgG) of immunized pigs. Specific cellular immunity was also induced after LTB-IAVe immunization, as evidenced by up-regulating of IL-1β, IL-8, and IL-4 expression in peripheral blood mononuclear cells (PBMCs) of vaccinated pigs. In comparison to the non-immunized pigs, pigs immunized with the LTB-IAVe showed improved protection against a pathogenic H1N1 swine influenza virus challenge, with about 50% decrease of pneumonic lesions and 10-fold reduction of the viral load in lung and nasal secretion at five days post challenge. This study establishes a platform for future construction of epitope-based vaccines against influenza A virus infection.

Oral immunization of a live attenuated Escherichia coli strain expressing a holotoxin-structured adhesin-toxoid fusion (1FaeG-FedF-LTA₂:5LTB) protected young pigs against enterotoxigenic E. coli (ETEC) infection

ETEC strains expressing K88 (F4) or F18 fimbriae and enterotoxins are the predominant cause of porcine post-weaning diarrhea (PWD). PWD continues causing significant economic losses to swine producers worldwide. Vaccines effectively protecting against PWD are needed. Our recent study revealed that a tripartite adhesin-toxin monomer (FaeG-FedF-LT(A2-B)) elicited protective antibodies. In this study, we constructed a new adhesin-toxoid fusion, expressed it as a 1A:5B holotoxin-structured antigen (1FaeG-FedF-LT(192A2):5LT(B)) in an avirulent Escherichia coli strain, and evaluated its vaccine potential in pig challenge studies. Piglets orally inoculated with this live strain showed no adverse effects but developed systemic and mucosal antibodies that neutralized cholera toxin and inhibited adherence of K88 and F18 fimbriae in vitro. Moreover, the immunized piglets, when were challenged with ETEC strain 3030-2 (K88ac/LT/STb), had significant fewer bacteria colonized at small intestines and did not develop diarrhea; whereas the control piglets developed severe diarrhea and died. These results indicated the 1FaeG-FedF-LT(192A2):5LT(B) fusion antigen induced protective antiadhesin and antitoxin immunity in pigs, and suggested a live attenuated vaccine can be potentially developed against porcine ETEC diarrhea. Additionally, presenting antigens in a holotoxin structure to target host local mucosal immunity can be used in vaccine development against other enteric diseases.

Progress and hurdles in the development of vaccines against enterotoxigenic Escherichia coli in humans

Diarrhea is the second leading cause of death in children younger than 5 years. Enterotoxigenic Escherichia coli (ETEC) strains are the most common bacterial cause of diarrhea in young children living in endemic countries and children and adults traveling to these areas. Pathogenesis of ETEC diarrhea has been well studied, and the key virulence factors are bacterial colonization factor antigens and enterotoxins produced by ETEC strains. Colonization factor antigens mediate bacteria attachment to host small intestinal epithelial cells and subsequent colonization, whereas enterotoxins including heat-labile and heat-stable toxins disrupt fluid homeostasis in host epithelial cells, which leads to fluid and electrolyte hypersecretion and diarrhea. Vaccines stimulating host anti-adhesin immunity to block ETEC attachment and colonization and also antitoxin immunity to neutralize enterotoxicity are considered optimal for prevention of ETEC diarrhea. Vaccines under development have been designed to stimulate local intestinal immunity and are either oral vaccines or transcutaneous vaccines. A cholera vaccine (Dukoral®) does stimulate anti-heat-labile toxin immunity and is licensed for short-term protection of ETEC diarrhea in travelers in some countries. Newer experimental ETEC vaccine candidates are being developed with hope to provide long-lasting and more broad-based protection against ETEC. Some have shown promising results in safety and immunogenicity studies and are approaching field trials for efficacy. A key problem is the development of a vaccine that is both practical and inexpensive so that it can be affordable for use in poor countries where it is needed.

Avirulent K88 (F4)+ Escherichia coli strains constructed to express modified enterotoxins protect young piglets from challenge with a virulent enterotoxigenic Escherichia coli strain that expresses the same adhesion and enterotoxins

Virulence of enterotoxigenic Escherichia coli (ETEC) is associated with fimbrial adhesins and enterotoxins such as heat-labile (LT) and/or heat-stable (ST) enterotoxins. Previous studies using a cell culture model suggest that exclusion of ETEC from attachment to epithelial cells requires expression of both an adhesin such as K88 (F4) fimbriae, and LT. To test the ability of non-pathogenic E. coli constructs to exclude virulent ETEC sufficiently to prevent clinical disease, we utilized a piglet ETEC challenge model. Thirty-nine 5-day-old piglets were inoculated with a placebo (control), or with either of the three K88(+)E. coli strains isogenic with regard to modified LT expression: 8017 (pBR322 plasmid vector control), non-toxigenic mutant 8221 (LT(R192G)) in pBR322, or 8488, with the LT gene fused to the STb gene in pBR322 (LT(R192G)-STb). Piglets were challenged with virulent ETEC Strain 3030-2 (K88(+)/LT/STb) 24h post-inoculation. K88ac receptor-positive piglets in the control group developed diarrhea and became dehydrated 12-24h post-challenge. Piglets inoculated with 8221 or 8488 did not exhibit clinical signs of ETEC disease; most piglets inoculated with 8017 showed diarrhea. Control pigs exhibited significant weight loss, increased blood total protein, and higher numbers of colony-forming units of 3030-2 E. coli in washed ileum and jejunum than treated pigs. This study shows for the first time that pre-inoculation with an avirulent strain expressing adhesive fimbriae and a non-toxic form of LT provides significant short term protection from challenge with a virulent ETEC strain that expresses the same fimbrial adhesion and enterotoxin.

Modified heat-stable toxins (hSTa) of enterotoxigenic Escherichia coli lose toxicity but display antigenicity after being genetically fused to heat-labile toxoid LT(R192G)

Enterotoxigenic Escherichia coli (ETEC) strains are a major cause of diarrhea in humans and animals. Heat-stable (STa) and heat-labile (LT) enterotoxins produced by ETEC disrupt fluid homeostasis in host small intestinal epithelial cells and cause fluid and electrolyte hyper-secretion that leads to diarrhea. ETEC strains producing STa or LT are sufficiently virulent to cause diarrhea, therefore STa and LT antigens must be included in ETEC vaccines. However, potent toxicity and poor immunogenicity (of STa) prevent them from being directly applied as vaccine components. While LT toxoids, especially LT(R192G), being used in vaccine development, STa toxoids have not been included. A recent study (IAI, 78:316-325) demonstrated porcine-type STa toxoids [pSTa(P12F) and pSTa(A13Q)] elicited protective anti-STa antibodies after being fused to a porcine-type LT toxoid [pLT(R192G)]. In this study, we substituted the 8th, 9th, 16th, or the 17th amino acid of a human-type STa (hSTa) and generated 28 modified STa peptides. We tested each STa peptide for toxicity and structure integrity, and found nearly all modified STa proteins showed structure alteration and toxicity reduction. Based on structure similarity and toxic activity, three modified STa peptides: STa(E8A), STa(T16Q) and STa(G17S), were selected to construct LT₁₉₂-STa_-toxoid fusions. Constructed fusions were used to immunize mice, and immunized mice developed anti-STa antibodies. Results from this study provide useful information in developing toxoid vaccines against ETEC diarrhea.

A tripartite fusion, FaeG-FedF-LT(192)A2:B, of enterotoxigenic Escherichia coli (ETEC) elicits antibodies that neutralize cholera toxin, inhibit adherence of K88 (F4) and F18 fimbriae, and protect pigs against K88ac/heat-labile toxin infection

Enterotoxigenic Escherichia coli (ETEC) strains expressing K88 (F4) or F18 fimbriae and heat-labile (LT) and/or heat-stable (ST) toxins are the major cause of diarrhea in young pigs. Effective vaccines inducing antiadhesin (anti-K88 and anti-F18) and antitoxin (anti-LT and anti-ST) immunity would provide broad protection to young pigs against ETEC. In this study, we genetically fused nucleotides coding for peptides from K88ac major subunit FaeG, F18 minor subunit FedF, and LT toxoid (LT(192)) A2 and B subunits for a tripartite adhesin-adhesin-toxoid fusion (FaeG-FedF-LT(192)A2:B). This fusion was used for immunizations in mice and pigs to assess the induction of antiadhesin and antitoxin antibodies. In addition, protection by the elicited antiadhesin and antitoxin antibodies against a porcine ETEC strain was evaluated in a gnotobiotic piglet challenge model. The data showed that this FaeG-FedF-LT(192)A2:B fusion elicited anti-K88, anti-F18, and anti-LT antibodies in immunized mice and pigs. In addition, the anti-porcine antibodies elicited neutralized cholera toxin and inhibited adherence against both K88 and F18 fimbriae. Moreover, immunized piglets were protected when challenged with ETEC strain 30302 (K88ac/LT/STb) and did not develop clinical disease. In contrast, all control nonvaccinated piglets developed severe diarrhea and dehydration after being challenged with the same ETEC strain. This study clearly demonstrated that this FaeG-FedF-LT(192)A2:B fusion antigen elicited antibodies that neutralized LT toxin and inhibited the adherence of K88 and F18 fimbrial E. coli strains and that this fusion could serve as an antigen for vaccines against porcine ETEC diarrhea. In addition, the adhesin-toxoid fusion approach used in this study may provide important information for developing effective vaccines against human ETEC diarrhea.

Immunogenicity of nuclear-encoded LTB:ST fusion protein from Escherichia coli expressed in tobacco plants

Enterotoxigenic Escherichia coli (ETEC) is one of the main causative agents of diarrhea in infants and for travelers. Inclusion of a heat-stable (ST) toxin into vaccine formulations is mandatory as most ETEC strains can produce both heat-labile (LT) and ST enterotoxins. In this study, a genetic fusion gene encoding for an LTB:ST protein has been constructed and transferred into tobacco via Agrobacterium tumefaciens-mediated transformation. Transgenic tobacco plants carrying the LTB:ST gene are then subjected to GM1-ELISA revealing that the LTB:ST has assembled into pentamers and displays antigenic determinants from both LTB and ST. Protein accumulation of up to 0.05% total soluble protein is detected. Subsequently, mucosal and systemic humoral responses are elicited in mice orally dosed with transgenic tobacco leaves. This has suggested that the plant-derived LTB:ST is immunogenic via the oral route. These findings are critical for the development of a plant-based vaccine capable of eliciting broader protection against ETEC and targeting both LTB and ST. Features of this platform in comparison to transplastomic approaches are discussed.

Escherichia coli K88ac fimbriae expressing heat-labile and heat-stable (STa) toxin epitopes elicit antibodies that neutralize cholera toxin and STa toxin and inhibit adherence of K88ac fimbrial E. coli

Enterotoxigenic Escherichia coli (ETEC) strains are a major cause of diarrheal disease in humans and animals. Bacterial adhesins and heat-labile (LT) and heat-stable (ST) enterotoxins are the virulence determinants in ETEC diarrhea. It is believed that vaccines inducing anti-adhesin immunity to inhibit bacterial adherence and anti-toxin immunity to eliminate toxin activity would provide broad-spectrum protection against ETEC. In this study, an ETEC fimbrial adhesin was used as a platform to express LT and STa for adhesin-toxin fusion antigens to induce anti-toxin and anti-adhesin immunity. An epitope from the B subunit of LT toxin (LTP1, (8)LCSEYRNTQIYTIN(21)) and an STa toxoid epitope ((5)CCELCCNPQCAGCY(18)) were embedded in the FaeG major subunit of E. coli K88ac fimbriae. Constructed K88ac-toxin chimeric fimbriae were harvested and used for rabbit immunization. Immunized rabbits developed anti-K88ac, anti-LT, and anti-STa antibodies. Moreover, induced antibodies not only inhibited adherence of K88ac fimbrial E. coli to porcine small intestinal enterocytes but also neutralized cholera toxin and STa toxin. Data from this study demonstrated that K88ac fimbriae expressing LT and STa epitope antigens elicited neutralizing anti-toxin antibodies and anti-adhesin antibodies and suggested that E. coli fimbriae could serve as a platform for the development of broad-spectrum vaccines against ETEC.