Monoclonal antibodies against enterotoxigenic Escherichia coli colonization factor antigen I (CFA/I) that cross-react immunologically with heterologous CFAs

Colonization factor CS30 from enterotoxigenic Escherichia coli binds to sulfatide in human and porcine small intestine

The ability to adhere via colonization factors to specific receptors located on the intestinal mucosa is a key virulence factor in enterotoxigenic Escherichia coli (ETEC) pathogenesis. Here, the potential glycosphingolipid receptors of the novel human ETEC colonization factor CS30 were examined by binding of CS30-expressing bacteria to glycosphingolipids on thin-layer chromatograms. We thereby found a highly specific binding of CS30-expressing bacteria to a fast-migrating acid glycosphingolipid of human and porcine small intestine, while no binding was obtained with a mutant ETEC strain unable to express CS30 fimbriae. The CS30 binding glycosphingolipid from human small intestine was isolated and characterized by mass spectrometry as sulfatide (SO₃-3Galβ1Cer). Comparative binding studies using sulfatides with different ceramide compositions gave a preferential binding of CS30 to sulfatide with d18:1-h24:0 ceramide. This ceramide species of sulfatide was also isolated from human small intestine and characterized by mass spectrometry and antibody binding. These studies implicate sulfatide as candidate receptor for mediating attachment of CS30-fimbriated ETEC to human and porcine small intestinal cells. Our findings may be a basis for designing receptor saccharide analogues for inhibition of the intestinal adhesion of CS30-expressing E. coli.

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

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

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

Glyco-engineered cell line and computational docking studies reveals enterotoxigenic Escherichia coli CFA/I fimbriae bind to Lewis a glycans

We have previously reported clinical data to suggest that colonization factor I (CFA/I) fimbriae of enterotoxigenic Escherichia coli (ETEC) can bind to Lewis a (Le^a), a glycan epitope ubiquitous in the small intestinal mucosa of young children (<2 years of age), and individuals with a genetic mutation of FUT2. To further elucidate the physiological binding properties of this interaction, we engineered Chinese Hamster Ovary (CHO-K1) cells to express Le^a or Le^b determinants on both N- and O-glycans. We used our glyco-engineered CHO-K1 cell lines to demonstrate that CfaB, the major subunit of ETEC CFA/I fimbriae, as well as four related ETEC fimbriae, bind more to our CHO-K1 cell-line expressing Le^a, compared to cells carrying Le^b or the CHO-K1 wild-type glycan phenotype. Furthermore, using in-silico docking analysis, we predict up to three amino acids (Glu²⁵, Asn²⁷, Thr²⁹) found in the immunoglobulin (Ig)-like groove region of CfaB of CFA/I and related fimbriae, could be important for the preferential and higher affinity binding of CFA/I fimbriae to the potentially structurally flexible Le^a glycan. These findings may lead to a better molecular understanding of ETEC pathogenesis, aiding in the development of vaccines and/or anti-infection therapeutics.

Identification and Characterization of Human Monoclonal Antibodies for Immunoprophylaxis against Enterotoxigenic Escherichia coli Infection

Enterotoxigenic Escherichia coli (ETEC) causes diarrheal illness in infants in the developing world and travelers to countries where the disease is endemic, including military personnel. ETEC infection of the host involves colonization of the small intestinal epithelium and toxin secretion, leading to watery diarrhea.

Enterotoxigenic Escherichia coli (ETEC) causes diarrheal illness in infants in the developing world and travelers to countries where the disease is endemic, including military personnel. ETEC infection of the host involves colonization of the small intestinal epithelium and toxin secretion, leading to watery diarrhea. There is currently no vaccine licensed to prevent ETEC infection. CFA/I is one of the most common colonization factor antigens (CFAs). The CFA/I adhesin subunit, CfaE, is required for ETEC adhesion to host intestinal cells. Human antibodies against CfaE have the potential to block colonization of ETEC and serve as an immunoprophylactic against ETEC-related diarrhea. Mice transgenic for human immunoglobulin genes were immunized with CfaE to generate a panel of human monoclonal IgG1 antibodies (HuMAbs). The most potent IgG1 antibodies identified in the in vitro functional assays were selected and isotype switched to secretory IgA (sIgA) and tested in animal colonization assays via oral administration. Over 300 unique anti-CfaE IgG1 HuMAbs were identified. The lead IgG1 anti-CfaE HuMAbs completely inhibited hemagglutination and blocked adhesion of ETEC to Caco-2 cells. Epitope mapping studies revealed that HuMAbs recognized epitopes in the N-terminal domain of CfaE near the putative receptor binding site. Oral administration of anti-CfaE antibodies in either IgG or sIgA isotypes inhibited intestinal colonization in mice challenged with ETEC. A 2- to 4-log decrease in CFU was observed in comparison to mice challenged with irrelevant isotype controls. We identified fully human monoclonal antibodies against the CfaE adhesion domain that can be potentially employed as an immunoprophylactic to prevent ETEC-related diarrhea.

Assessing antigen specific HLA-DR+ antibody secreting cell (DR+ASC) responses in whole blood in enteric infections using an ELISPOT technique

Antibody secreting cells (ASCs) generate antibodies in an antigen-specific manner as part of the adaptive immune response to infections, and these cells increase their surface expression of HLA-DR. We have studied this parameter (HLA-DR+ ASC) in patients with recent diarrheal infection using immuno-magnetic cell sorting and an enzyme linked immunospot (ELISPOT) technique that requires only one milliliter of blood. We validated this approach in adult patients with cholera (n = 15) or ETEC diarrhea (n = 30) on days 2, 7 and 30 after showing clinical symptom at the International Centre for Diarrhoeal Disease Research, Bangladesh (icddr,b) hospital in Dhaka, and we compared responses to age-matched healthy controls (n = 7). We found that HLA-DR+ ASC (DR+ASC) responses specific both for T cell-dependent (cholera toxin B subunit), and T cell-independent (lipopolysaccharide) antigens were elevated at day 7 after showing clinical cholera symptom. Similarly, DR+ASCs were elevated against both heat-labile toxin and colonization factors following ETEC infection. We observed significant correlations between antigen-specific DR+ASC responses and antigen-specific, gut homing ASC and plasma antibody responses. This study demonstrates that a simple ELISPOT procedure allows determination of antigen-specific ASC responses using a small volume of whole blood following diarrhea. This technique may be particularly useful in studying DR+ASC responses in young children and infants, either following infection or vaccination.

Cross-reactivity and avidity of antibody responses induced in humans by the oral inactivated multivalent enterotoxigenicEscherichia coli (ETEC) vaccine ETVAX

We investigated whether the oral inactivated, multivalent enterotoxigenic Escherichia coli (ETEC) vaccine ETVAX, consisting of four E. coli strains over-expressing the colonisation factors (CFs) CFA/I, CS3, CS5 and CS6, combined with the toxoid LCTBA, could induce cross-reactive antibodies to CFs related to the CFA/I and CS5 families. We also evaluated the avidity of vaccine induced antibodies against the toxoid and CFs. Cross-reactivity was analysed in mucosal (faecal and antibodies in lymphocyte supernatants, ALS) samples, and antibody avidity in serum and ALS samples, from two phase I trials: a primary vaccination study, where two oral doses of ETVAX were given±the double mutant heat labile toxin (dmLT) adjuvant at a 2-week interval, and a booster vaccination study, where a single booster dose of ETVAX was given 13-23months after primary vaccinations. We found that 65-90% of subjects who had responded to CFA/I in ALS or faecal specimens also developed cross-reactive antibodies to the related CFs tested, i.e. CS1, CS14 and CS17, and that approximately 80% of those responding to CS5 also responded to the closely related CS7. For subjects who had developed cross-reactive antibodies, the magnitudes of responses against vaccine CFs and related non-vaccine CFs were comparable. Using both a simple method of antibody avidity determination based on limiting antigen dilution, as well as a chaotropic ELISA method, we found that the avidity of serum and ALS antibodies to key vaccine antigens increased after a late booster dose compared to after primary vaccination. Our results suggest that the cross-reactive antibody responses against multiple CFs may result in expanded ETEC strain coverage of ETVAX and that repeated vaccinations induce vaccine-specific antibodies with increased binding capacity.

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.

Antigen-specific memory B-cell responses to enterotoxigenic Escherichia coli infection in Bangladeshi adults

Background

Multiple infections with diverse enterotoxigenic E. coli (ETEC) strains lead to broad spectrum protection against ETEC diarrhea. However, the precise mechanism of protection against ETEC infection is still unknown. Therefore, memory B cell responses and affinity maturation of antibodies to the specific ETEC antigens might be important to understand the mechanism of protection.

Methodology

In this study, we investigated the heat labile toxin B subunit (LTB) and colonization factor antigens (CFA/I and CS6) specific IgA and IgG memory B cell responses in Bangladeshi adults (n = 52) who were infected with ETEC. We also investigated the avidity of IgA and IgG antibodies that developed after infection to these antigens.

Principal Findings

Patients infected with ETEC expressing LT or LT+heat stable toxin (ST) and CFA/I group or CS6 colonization factors developed LTB, CFA/I or CS6 specific memory B cell responses at day 30 after infection. Similarly, these patients developed high avidity IgA and IgG antibodies to LTB, CFA/I or CS6 at day 7 that remained significantly elevated at day 30 when compared to the avidity of these specific antibodies at the acute stage of infection (day 2). The memory B cell responses, antibody avidity and other immune responses to CFA/I not only developed in patients infected with ETEC expressing CFA/I but also in those infected with ETEC expressing CFA/I cross-reacting epitopes. We also detected a significant positive correlation of LTB, CFA/I and CS6 specific memory B cell responses with the corresponding increase in antibody avidity.

Conclusion

This study demonstrates that natural infection with ETEC induces memory B cells and high avidity antibodies to LTB and colonization factor CFA/I and CS6 antigens that could mediate anamnestic responses on re-exposure to ETEC and may help in understanding the requirements to design an effective vaccination strategies.

Enterotoxigenic Escherichia coli (ETEC) is a non-invasive pathogen causing diarrhea in children as well as in adults and travelers in developing countries. After colonizing the intestine using colonization factors, the organisms secrete heat-stable (ST) and/or heat-labile (LT) enterotoxin to cause watery diarrhea. Natural infection with ETEC provides protection against subsequent infection; however, the precise mechanism is unknown. In this study, we have shown that adult patients with diarrhea infected with ETEC develop toxin (LTB) and colonization factor (CFA/I and CS6) specific memory B cell responses as well as highly avid antigen-specific antibodies. The antibody avidity indices were shown to be positively associated with memory B cell responses, suggesting that these processes may occur in concert. This study encourages further evaluation of such responses in children as well as in vaccinees.

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.

Preexisting antibodies to homologous colonization factors and heat-labile toxin in serum, and the risk to develop enterotoxigenic Escherichia coli-associated diarrhea

Preexisting serum immunoglobulin (Ig) A and IgG titers against colonization factors and heat-labile toxin of enterotoxigenic Escherichia coli (ETEC) were examined in young adults who subsequently developed ETEC-associated diarrhea and in healthy matched controls. The data suggest an inverse association between the antibody titers against colonization factors, but not heat-labile toxin, and development of ETEC-associated diarrhea.

Development of a monoclonal antibody-based co-agglutination test to detect enterotoxigenic Escherichia coli isolated from diarrheic neonatal calves

Escherichia coli (E. coli) strains were collected from young diarrheic calves in farms and field. Strains that expressed the K99 (F5) antigen were identified by agglutination tests using reference antibodies to K99 antigen and electron microscopy. The K99 antigen from a selected field strain (SAR-14) was heat-extracted and fractionated on a Sepharose CL-4B column. Further purification was carried out by sodium deoxycholate treatment and/or ion-exchange chromatography. Monoclonal antibodies to purified K99 antigen were produced by the hybridoma technique, and a specific clone, NEK99-5.6.12, was selected for propagation in tissue culture. The antibodies, thus obtained, were affinity-purified, characterized and coated onto Giemsa-stained Cowan-I strain of Staphylococcus aureus (S. aureus). The antibody-coated S. aureus were used in a co-agglutination test to detect K99+ E. coli isolated from feces of diarrheic calves. The specificity of the test was validated against reference monoclonal antibodies used in co-agglutination tests, as well as in ELISA. Specificity of the monoclonal antibodies was also tested against various Gram negative bacteria. The developed antibodies specifically detected purified K99 antigen in immunoblots, as well as K99+ E. coli in ELISA and co-agglutination tests. The co-agglutination test was specific and convenient for large-scale screening of K99+ E. coli isolates.

Homologous and cross-reactive immune responses to enterotoxigenic Escherichia coli colonization factors in Bangladeshi children

We have studied homologous (HoM) and cross-reacting (CR) immunoglobulin A (IgA) antibody responses to colonization factors (CFs) in Bangladeshi children with diarrhea due to enterotoxigenic E. coli (ETEC) strains of the CF antigen I (CFA/I) group (CFA/I, n = 25; coli surface antigen 4 [CS4], n = 8; CS14, n = 11) and the CS5 group (CS5, n = 15; CS7, n = 8), respectively. The responses to the HoM, CR, and heterologous (HeT) CF antigens in each group of patient were studied and compared to that seen in healthy children (n = 20). In the CFA/I group (CFA/I and CS14), patients responded with antibody-secreting cell (ASC) responses to HoM CFs (geometric mean, 156 to 329 ASCs/10(6) peripheral blood mononuclear cells [PBMCs]) and to CR CFs ( approximately 15 to 38 ASCs/10(6) PBMCs) but least of all to the HeT CS5 antigen (2 to 4 ASCs/10(6) PBMCs). For the CS5 group of patients with ETEC (CS5 and CS7), likewise, responses to HoM CFs (230 to 372 ASCs/10(6) PBMCs) and CR CFs (27 to 676 ASCs/10(6) PBMCs) were seen, along with lower responses to the HeT CFA/I antigen (9 to 38 ASCs/10(6) PBMCs). Both groups of patients responded with CF-specific IgA antibodies to HoM and CR antigens in plasma but responded less to the HeT CFs. The responses in patients were seen very soon after the onset of diarrhea and peaked around 1 week after onset. Vaccinees who had received two doses of the oral, killed whole-cell ETEC vaccine (CF-BS-ETEC) responded with plasma IgA antibodies to CFA/I, a component of the vaccine, but also to the CR CS14 antigen, which was not included in the vaccine, showing that antibody responses can be stimulated by a CFA/I-containing ETEC vaccine to a CR-reacting antigen in individuals in countries where ETEC is endemic.

Enterotoxigenic Escherichia coli in developing countries: epidemiology, microbiology, clinical features, treatment, and prevention

ETEC is an underrecognized but extremely important cause of diarrhea in the developing world where there is inadequate clean water and poor sanitation. It is the most frequent bacterial cause of diarrhea in children and adults living in these areas and also the most common cause of traveler's diarrhea. ETEC diarrhea is most frequently seen in children, suggesting that a protective immune response occurs with age. The pathogenesis of ETEC-induced diarrhea is similar to that of cholera and includes the production of enterotoxins and colonization factors. The clinical symptoms of ETEC infection can range from mild diarrhea to a severe cholera-like syndrome. The effective treatment of ETEC diarrhea by rehydration is similar to treatment for cholera, but antibiotics are not used routinely for treatment except in traveler's diarrhea. The frequency and characterization of ETEC on a worldwide scale are inadequate because of the difficulty in recognizing the organisms; no simple diagnostic tests are presently available. Protection strategies, as for other enteric infections, include improvements in hygiene and development of effective vaccines. Increases in antimicrobial resistance will dictate the drugs used for the treatment of traveler's diarrhea. Efforts need to be made to improve our understanding of the worldwide importance of ETEC.

Phenotypic diversity of enterotoxigenic Escherichia coli (ETEC) isolated from cases of travelers' diarrhea in Kenya

BACKGROUND

The aim of this study was to characterize phenotypically enterotoxins, colonization factors (CFs) and the antibiotic susceptibility of enterotoxigenic Escherichia coli (ETEC) strains isolated from cases of acute diarrhea that occurred in Europeans traveling to resorts in Mombasa, Kenya; this information is critical for the development of vaccines and empirical treatment.

METHODS

Over a 1-year period from 1996 to 1997, five E. coli-like colonies were obtained from each of 463 cases with acute diarrhea. These strains were characterized for enterotoxins using GM-1 ELISA, for CFs using a dot-blot assay, and for antibiotic susceptibility using antibiotic disks.

RESULTS

Of 164 strains characterized for ETEC phenotype, 30 (18%) expressed heat-labile toxin (LT) only, 83 (51%) heat-stable toxin (ST) only, and 51 (31%) both LT and ST. Analysis for CF expression demonstrated that 107 (65%) of the strains were positive for CFs, including CFA/IV (46%), CFA/II (35%), and CFA/I (5%), while less than 4% expressed less common CFs. All ETEC strains tested were resistant to erythromycin and sensitive to ceftriaxone. Over one-third of the strains were resistant to sulfamethoxazole-trimethoprim or tetracycline. Six strains were resistant to nalidixic acid; none of these were resistant to ciprofloxacin.

CONCLUSIONS

Cumulatively, our findings indicate that ETEC in this region comprises a highly diverse group of bacterial enteropathogens, and that the development of prophylactic agents against ETEC faces major challenges because of this diversity.

Genetic characterization and immunogenicity of coli surface antigen 4 from enterotoxigenic Escherichia coli when it is expressed in a Shigella live-vector strain

The genes that encode the enterotoxigenic Escherichia coli (ETEC) CS4 fimbriae, csaA, -B, -C, -E, and -D', were isolated from strain E11881A. The csa operon encodes a 17-kDa major fimbrial subunit (CsaB), a 40-kDa tip-associated protein (CsaE), a 27-kDa chaperone-like protein (CsaA), a 97-kDa usher-like protein (CsaC), and a deleted regulatory protein (CsaD'). The predicted amino acid sequences of the CS4 proteins are highly homologous to structural and assembly proteins of other ETEC fimbriae, including CS1 and CS2, and to CFA/I in particular. The csaA, -B, -C, -E operon was cloned on a stabilized plasmid downstream from an osomotically regulated ompC promoter. pGA2-CS4 directs production of CS4 fimbriae in both E. coli DH5alpha and Shigella flexneri 2a vaccine strain CVD 1204, as detected by Western blot analysis and bacterial agglutination with anti-CS4 immune sera. Electron-microscopic examination of Shigella expressing CS4 confirmed the presence of fimbriae on the bacterial surface. Guinea pigs immunized with CVD 1204(pGA2-CS4) showed serum and mucosal antibody responses to both the Shigella vector and the ETEC fimbria CS4. Among the seven most prevalent fimbrial antigens of human ETEC, CS4 is the last to be cloned and sequenced. These findings pave the way for CS4 to be included in multivalent ETEC vaccines, including an attenuated Shigella live-vector-based ETEC vaccine.

Antigenic variation within the subunit protein of members of the colonization factor antigen I group of fimbrial proteins in human enterotoxigenic Escherichia coli

Colonization factor antigens (CFAs) of human enterotoxigenic Escherichia coli can be divided in groups based on the N-terminal amino acid sequence of their major subunit protein. One of the groups that has been distinguished in this way, is the CFA/I group of fimbriae. The sequence of the fimbrial subunit genes in the operons encoding the antigens CS4, CS14 and CS17, all members of this group, was determined. A duplication of the fimbrial subunit gene (csuA) was found in the CS14 operon, both genes encoding very similar proteins. Purified CS14 fimbriae consist of two proteins with different molecular masses (15.5 and 17.0 kDa) but identical N-terminal amino acid sequences, which strongly suggests that both csuA genes are transcribed. A phylogenetic tree derived from the amino acid sequences of the CFA/I, CS1, CS2, CS4, CS14, CS17 and CS19 subunit proteins shows that CS1, CS17 and CS19 belong to the same subgroup. CFA/I, CS4 and CS14 belong to a second subgroup, while CS2 is distinct within the CFA/I group of fimbriae. The genetic similarity between CS1, CS17 and CS19 is reflected in the substantial immunological cross-reactivity observed, both between their protein subunits and intact fimbriae.

DNA immunisation against the CFA/I fimbriae of enterotoxigenic Escherichia coli (ETEC)

The CFA/I fimbria promotes the attachment of enterotoxigenic Escherichia coli (ETEC) to the surface of human enterocytes. The generation of a protective immune response requires the induction of antibodies able to block the CFA/I-mediated binding of ETEC to receptors located on the small intestine epithelium or on the surface of human red blood cells, in hemagglutination tests. An eukaryotic expression plasmid, pBLCFA, encoding the CFA/I gene under the control of the human cytomegalovirus major immediate-early promoter was constructed as a prototype DNA vaccine against ETEC. pBLCFA-tranfected BHK-21 cells secreted a peptide cross-reacting with a monoclonal antibody raised against CFA/I subunits. BALB/c mice immunized intramuscularly with one or two doses of purified pBLCFA developed CFA/I-specific serum antibodies for at least 52 weeks, composed predominantly of the IgG1 subclass. pBLCFA-induced antibodies bind mainly to epitopes exposed on the surface of intact CFA/I fimbriae and do not react with immune recessive epitopes found in other ETEC fimbra sharing amino acid homologies with CFA/I. Furthermore, pBLCFA-induced antibodies were able to block the adhesive properties of the CFA/I fimbriae, as evaluated by the ability to inhibit the hemagglutination promoted by CFA/I-expressing ETEC cells. These results suggest that secretion of CFA/I encoded by pBLCFA preserves important conformational epitopes required for the generation of protective antibodies against the adhesive properties of the CFA/I fimbriae and open new perspectives for the development of DNA vaccines against enteric bacterial pathogens.

Salmonella flagellin fused with a linear epitope of colonization factor antigen I (CFA/I) can prime antibody responses against homologous and heterologous fimbriae of enterotoxigenic Escherichia coli

In this work, a 15-amino-acid-long peptide derived from the enterotoxigenic Escherichia coli CFA/I fimbria (11VDPVIDLLQADGNAL25) was genetically fused to the Salmonella flagellin and used to prime and boost serum antibody responses (IgG) against homologous (CFA/I) and heterologous (CS1) colonization factors (CFs) in BALB/c mice. Antibodies raised against the hybrid flagellin (Fla II) cross-reacted with CFA/I, CS1, CS2, and PCFO166 but not with CS4. Parenteral administration of Fla II primed antibody responses against both CFA/I and CS1 but boosted IgG responses only against CFA/I. These findings confirm that linear epitopes derived from the CFA/I fimbria can prime antibody responses against homologous and heterologous CFs and indicate that Salmonella flagellin represents a potential carrier for the development of broad-range peptide-based anti-colonization ETEC vaccines.

New tools in an old trade: CS1 pilus morphogenesis

CS1 pili serve as the prototype for a large class of serologically distinct pili associated with enterotoxigenic Escherichia coli that cause diarrhoea in humans. The four genes essential for CS1 pilus morphogenesis, cooB, A, C and D, are arranged in an operon and encode structural and assembly proteins unlike those of other pilus systems commonly associated with Gram-negative bacteria. CS1 pili are composed primarily of the major pilin subunit, CooA, which determines the serological type of the pilus. The major pilin subunit is assembled into pili by the proteins CooB, CooC and CooD. CooD is both a minor component found at the pilus tip and an essential assembly protein, whereas CooC is an outer membrane protein thought to be involved in pilin transport. CooB is a novel periplasmic chaperone-like protein that forms intermolecular complexes with and stabilizes the major and minor pilins. Unlike other pilin chaperones, CooB also stabilizes the outer membrane component of the assembly system, CooC. The proteins of CS1 pili have no significant homology to those of the well-characterized Pap (pyelonephritis-associated) pili and related systems, although most of the features of pilus morphogenesis are similar. Therefore, these appear to be among the rare cases of convergent evolution. Thus, for CS1 pili, enterotoxigenic E. coli use new protein 'tools' in the old 'trade' of forming functional pili.

Epitope specificity of a monoclonal antibody generated against the dissociated CFA/I fimbriae of enterotoxigenic Escherichia coli

A monoclonal antibody (MAb 84) raised against the dissociated CFA/I fimbriae of enterotoxigenic Escherichia coli was characterized with regard to antigen binding and epitope specificity. Enzyme-linked immunosorbent assay (ELISA) showed that MAb 84 had higher affinity to CFA/I subunits than to intact CFA/I fimbriae and recognized a Salmonella flagellin carrying an insert corresponding to amino acids 32 to 45 of the CFA/I subunit. Fine epitope mapping based on the Pepscan technique showed that the peptide 39TFESY43, derived from the sequence of the mature CFA/I subunit, was specifically recognized by MAb 84. The 39TFESY43 sequence is probably not accessible on the surface of the native CFA/I fimbriae since MAb 84 did not bind to intact fimbriae as evaluated in inhibition ELISA tests. Moreover, MAb 84 did not agglutinate fimbriated ETEC cells nor inhibit CFA/I-mediated hemagglutination or the adhesion to Caco-2 cells.

Identification of two genes, cpsX and cpsY, with putative regulatory function on capsule expression in group B streptococci

Two divergently transcribed open reading frames: cpsX and cpsY separated by a common regulatory region was identified upstream of the cpsA-D genes involved in polysaccharide capsule biosynthesis in group B streptococci (GBS). We suggest that these genes are involved in the regulation of capsule expression in GBS, since the CpsX protein shares sequence similarities with LytR of Bacillus subtilis, an attenuator of transcription while CpsY has similarity to a wide variety of members of the LysR family of transcriptional regulators. No deletions, insertions, DNA rearrangements, or apparent differences were discovered in the postulated regulatory genes when the gene region was compared in GBS with different capsule phenotypes. Thus, other yet unidentified gene loci may control capsule phase variation in GBS.

Antibody-secreting cells in the stomachs of symptomatic and asymptomatic Helicobacter pylori-infected subjects

In this study we analyzed whether infection with Helicobacter pylori gives rise to specific B-cell responses against a number of putative virulence factors of H. pylori, e.g., urease, flagellin, and different bacterial surface antigens, locally in the gastric mucosa. This was studied in antrum and corpus biopsies collected from 11 H. pylori-infected patients with duodenal ulcers, 11 asymptomatic H. pylori carriers, and 13 noninfected, healthy controls. Mononuclear cells were isolated from the biopsies and assayed for frequencies of total and H. pylori-specific antibody-secreting cells (ASCs) by means of the enzyme-linked immunospot technique. The H. pylori-infected subjects had remarkably higher frequencies of total immunoglobulin A (IgA)- and IgM-secreting cells than the noninfected subjects, while the frequencies of IgG-secreting cells were virtually the same in the different groups. In addition, most of the infected subjects had IgA ASCs reacting with H. pylori membrane proteins, flagellin, and urease, while none of the noninfected subjects had any detectable H. pylori-reactive ASCs. Furthermore, half of the infected subjects also had ASCs reacting with a Helicobacter-specific 26-kDa protein, while only a few of them had ASCs reacting with neutrophil-activating protein, the neuraminyllactose-binding hemagglutinin HpaA, or lipopolysaccharides purified from different H. pylori strains. The frequencies of H. pylori-specific ASCs in the antrum and corpus were almost identical, and no differences in either antigen specificity or magnitude of the B-cell response in the stomach could be detected between the ulcer patients and the asymptomatic H. pylori carriers. This study demonstrates that H. pylori infection induces strong antibody responses in the human gastric mucosa, both in asymptomatic carriers and in duodenal ulcer patients. However, the outcome of infection could not be explained by differences in the local B-cell response to any of the antigens used in this study.

Immunization against the colonization factor antigen I of enterotoxigenic Escherichia coli by administration of a bivalent Salmonella typhimurium aroA strain

An expression plasmid (pCFA-1) carrying the cfaB gene that codes for the enterotoxigenic Escherichia coli (ETEC) fimbrial adhesin colonization factor antigen I (CFA/I) subunit was constructed and used to transform a derivative of the attenuated Salmonella typhimurium aroA vaccine strain SL3261 carrying an F'lacIq. Treatment of the transformed strain with isopropyl-beta-D-thiogalactopyranoside (IPTG) resulted in elevated in vitro expression of the CFA/I subunit. Although flagellar function and lipopolysaccharide (LPS) synthesis were similar in both the parental and the recombinant strains, spleen colonization was reduced in the recombinant strain. All BALB/c mice parenterally inoculated with the recombinant strain developed significant anti-CFA/I and anti-LPS serum antibody titers (P < 0.05). Moreover, 2 of 5 mice orally inoculated with the engineered Salmonella strain developed anti-CFA/I intestinal IgA (P > 0.05) while 4/5 of the same mice developed anti-LPS IgA (P < 0.05). The results indicate that the vaccine strain elicited an antibody response against the bacterial host both after oral and intravenous immunization while the response against the CFA/I antigen was significant only after inoculation by the intravenous route.

Breastfeeding and the risk of life-threatening enterotoxigenic Escherichia coli diarrhea in Bangladeshi infants and children

OBJECTIVE

To assess the relationship between breastfeeding and the risk of life-threatening enterotoxigenic Escherichia coli (ETEC) diarrhea among Bangladeshi infants and young children <36 months of age.

DESIGN

Case-control study.

SETTING

A rural Bangladesh community.

PARTICIPANTS

A total of 168 cases with clinically severe ETEC diarrhea detected in a treatment center-based surveillance system during 1985 to 1986 and 3679 controls selected in three surveys of the same community during the same calendar interval.

OUTCOMES

Cases and controls were compared for the frequency of antecedent breastfeeding patterns.

RESULTS

Compared with other feeding modes, exclusive breastfeeding of infants was associated with significant protection against severe ETEC diarrhea (relative risk [RR] = 0.51; 95% confidence interval [CI]: 0.28,0.96). However, during the second and third years of life, the risk of this outcome was similar in both breastfed and nonbreastfed children (RR = 0.98; 95% CI: 0.45,2.12), and no significant overall protective association between breastfeeding and severe ETEC diarrhea was evident during the first 3 years of life (RR = 0.86; 95% CI: 0.43,1. 74).

CONCLUSIONS

Exclusive breastfeeding appeared to protect infants against severe ETEC diarrhea, but breastfeeding was not associated with protection after infancy, nor was it associated with a major overall reduction of severe ETEC disease during the first 3 years of life. Although not diminishing the importance of breastfeeding, our findings suggest that other interventions, such as immunization and education about proper food hygiene, may also be required in efforts to prevent this major pediatric disease.

Binding of enterotoxigenic Escherichia coli to isolated enterocytes and intestinal mucus

Binding of human enterotoxigenic Escherichia coli (ETEC) to the small intestine is a prerequisite for colonization and is mediated by colonization factor (CF) antigens. Coli surface antigen 6 (CS6) is considered a CF but binding to isolated enterocytes has not been established. In this study bacteria expressing CS6 were analysed for binding to enterocytes from human and rabbit small intestine, isolated using either an EDTA-containing buffer or a buffer devoid of EDTA. We found that the bacteria bound to enterocytes from rabbit ileum and human duodenum, but only when the cells had been isolated in the absence of EDTA. Pretreatment of rabbit enterocytes with meta-periodate resulted in a decreased proportion of cells with bound bacteria. Purified CS6, and for comparison other ETEC CFs, were also tested for binding to different human and rabbit mucus fractions. These analyses showed that purified CS6 bound to mucus from rabbit duodenum and ileum as well as from human duodenum, jejunum and ileum and that this binding was abolished by pretreatment of the mucus material with meta-periodate or Proteinase K. CFA/I, CS1 to CS5, CS7, CS17, putative CF (PCF) O159 (CS12), PCFO166 (CS14), and CFA/III (CS8) also bound to the rabbit mucus material although with different patterns; the binding of CS2 and CS5 was abolished by meta-periodate treatment. Thus, ETEC bacteria expressing CS6 might bind to carbohydrate-containing structure(s) in the apical membrane of isolated rabbit ileal and human duodenal enterocytes that could probably be released by EDTA treatment. In addition, CS6 and other ETEC CFs bind to component(s), in some instances protein-associated carbohydrate structures, in mucus fractions from small intestine.

Peptides 14VIDLL18 and 96FEAAAL101 defined as epitopes of antibodies raised against amino acid sequences of enterotoxigenic Escherichia coli colonization factor antigen I fused to Salmonella flagellin

Antibodies raised against four hybrid Salmonella flagellins carrying amino acid sequences derived from the fimbrial subunit of the colonization factor I antigen (CFA/I) of enterotoxigenic Escherichia coli (ETEC), i.e. hybrid flagellins Fla I (aa 1-15), Fla II (aa 11-25), Fla III (aa 32-45) and Fla IV (aa 88-102), were not able to inhibit the in vitro binding of CFA/I-expressing ETEC bacteria to enterocyte-like Caco-2 cells. However, one of the hybrid flagellins (Fla II) was recognized by a previously described anti-CFA/I subunit mAb (S-CFA/I 17:8) which was able to block adhesion of CFA/I-expressing bacteria to Caco-2 cells and to bind to the amino acid sequences 15IDLLQ19 of the CFA/I fimbrial subunit. Pepscan analysis of antibodies raised against the hybrid flagellins Fla II and Fla IV showed that they were specific for the sequences 14VIDLL18 and 96FEAAAL101, respectively, of the CFA/I fimbrial subunit. Thus, the discrepancy in the abilities of the anti-Fla II serum and the mAb S-CFA/I 17:8 to block binding might be ascribed to their slightly different fine specificity for epitopes.

Detection and characterization of the coli surface antigen 6 of enterotoxigenic Escherichia coli strains by using monoclonal antibodies

We describe, for the first time, the production of monoclonal antibodies (MAbs) against coli surface antigen 6 (CS6) of enterotoxigenic Escherichia coli (ETEC) and their use for characterization and diagnosis of CS6. Two MAbs, MAbs CS6-20:11:9 and CS6-2A:14, were produced by immunizing mice with purified CS6 or CS6-containing bacterial extracts. The MAb specificity was demonstrated by enzyme-linked immunosorbent assay (ELISA), immunoblotting, and immunoelectron microscopy, which showed that the MAbs bound to CS6-expressing bacteria as well as to purified CS6 and CS6 structural subunits but not to CS6-negative bacteria or other purified ETEC colonization factors. By using bacterial recombinants, i.e., strains with a complete CS6 operon or parts thereof, it was found that both MAbs were specific for CssB, one of the two structural subunits of CS6. Although the MAbs bound specifically to the entire surface of CS6-expressing bacteria, no structure of CS6 could be identified. The usefulness of the MAbs for the detection of CS6 was evaluated in an inhibition ELISA and in a dot blot test. Ninety-two ETEC strains with known colonization factors were analyzed, and all CS6-positive strains were identified by either assay with MAb CS6-2A:14, whereas MAb CS6-20:11:9 failed to identify two CS6-positive strains; in no instance was any CS6-negative strain identified by either of the MAbs. Parallel analyses of 48 strains with a gene probe specific for the other structural subunit of CS6, i.e., CssA, and the MAb-based assays gave identical results, suggesting the simultaneous presence of both subunits.

Cloning and expression of colonization factor antigen I (CFA/I) epitopes of enterotoxigenic Escherichia coli (ETEC) in Salmonella flagellin

Oligonucleotides coding for linear epitopes of the fimbrial colonization factor antigen I (CFA/I) of enterotoxigenic Escherichia coli (ETEC) were cloned and expressed in a deleted form of the Salmonella muenchen flagellin fliC (H1-d) gene. Four synthetic oligonucleotide pairs coding for regions corresponding to amino acids 1 to 15 (region I), amino acids 11 to 25 (region II), amino acids 32 to 45 (region III) and amino acids 88 to 102 (region IV) were synthesized and cloned in the Salmonella flagellin-coding gene. All four hybrid flagellins were exported to the bacterial surface where they produced flagella, but only three constructs were fully motile. Sera recovered from mice immunized with intraperitoneal injections of purified flagella containing region II (FlaII) or region IV (FlaIV) showed high titres against dissociated solid-phase-bound CFA/I subunits. Hybrid flagellins containing region I (FlaI) or region III (FlaIII) elicited a weak immune response as measured in enzyme-linked immunosorbent assay (ELISA) with dissociated CFA/I subunits. None of the sera prepared with purified hybrid flagella were able to agglutinate or inhibit haemagglutination promoted by CFA/I-positive strains. Moreover, inhibition ELISA tests indicated that antisera directed against region I, II, III or IV cloned in flagellin were not able to recognize surface-exposed regions on the intact CFA/I fimbriae.

Identification of a cross-reactive continuous B-cell epitope in enterotoxigenic Escherichia coli colonization factor antigen I

Enterotoxigenic Escherichia coli (ETEC) colonizes the intestine by means of several antigenically distinct colonization factors (CFs). Several of these CFs have very significant amino acid sequence similarity or identity, particularly in the N-terminal end. We have previously shown that a monoclonal antibody (MAb) raised against the subunits of colonization factor antigen I (CFA/I) fimbriae, which reacts with a peptide corresponding to the 25 N-terminal amino acids of such subunits, can inhibit attachment to intestinal cells of ETEC expressing heterologous as well as homologous CFs, with related amino acid sequences. In this study we have, by means of Pepscan analysis, determined the sequence of the MAb-specific linear epitope to be 15IDLLQ19. Parenteral immunization of rabbits with an N-terminal 25-mer synthetic peptide of CFA/I fimbrial subunit, either covalently coupled to bovine serum albumin or uncoupled, induced high titers of specific antibodies against this peptide as well as against CFA/I fimbriae. Increased titers against several heterologous CF fimbriae with a related N-terminal sequence were also induced, whereas no increase was seen against fimbriae with an unrelated sequence. Neither antisera against the coupled peptide nor antisera against the uncoupled peptide inhibited binding of CF-expressing bacteria to the human intestinal cell line Caco-2 in spite of high titers. The difference in the inhibitory capabilities of the antipeptide sera and the MAb might be due to slightly different epitope specificities. Thus, whereas the antipeptide sera bound to several continuous epitopes in the N-terminal end, none of them reacted specifically with the epitope 15IDLLQ19.

Colonization factors of diarrheagenic E. coli and their intestinal receptors

While Escherichia coli is common as a commensal organism in the distal ileum and colon, the presence of colonization factors (CF) on pathogenic strains of E. coli facilitates attachment of the organism to intestinal receptor molecules in a species- and tissue-specific fashion. After the initial adherence, colonization occurs, and the involvement of additional virulence determinants leads to illness. Enterotoxigenic E. coli (ETEC) is the most extensively studied of the five categories of E. coli that cause diarrheal disease, and has the greatest impact on health worldwide. ETEC can be isolated from domestic animals and humans. The biochemistry, genetics, epidemiology, antigenic characteristics, and cell and receptor binding properties of ETEC have been extensively described. Another major category, enteropathogenic E. coli (EPEC), has virulence mechanisms, primarily effacement and cytoskeletal rearrangement of intestinal brush borders, that are distinct from ETEC. An EPEC CF receptor has been purified and characterized as a sialidated transmembrane glycoprotein complex directly attached to actin, thereby associating CF-binding with host-cell response. Three additional categories of E. coli diarrheal disease, their colonization factors and their host cell receptors, are discussed. It appears that biofilms exist in the intestine in a manner similar to oral bacterial biofilms, and that E. coli is part of these biofilms as both commensals and pathogens.