Antigenic specificity of neutralizing antibody to cholera toxin

Cholera

Cholera was first described in the areas around the Bay of Bengal and spread globally, resulting in seven pandemics during the past two centuries. It is caused by toxigenic Vibrio cholerae O1 or O139 bacteria. Cholera is characterised by mild to potentially fatal acute watery diarrhoeal disease. Prompt rehydration therapy is the cornerstone of management. We present an overview of cholera and its pathogenesis, natural history, bacteriology, and epidemiology, while highlighting advances over the past 10 years in molecular epidemiology, immunology, and vaccine development and deployment. Since 2014, the Global Task Force on Cholera Control, a WHO coordinated network of partners, has been working with several countries to develop national cholera control strategies. The global roadmap for cholera control focuses on stopping transmission in cholera hotspots through vaccination and improved water, sanitation, and hygiene, with the aim to reduce cholera deaths by 90% and eliminate local transmission in at least 20 countries by 2030.

Vibrio cholerae at the Intersection of Immunity and the Microbiome

Vibrio cholerae is a noninvasive pathogen that colonizes the small intestine and produces cholera toxin, causing severe secretory diarrhea. Cholera results in long lasting immunity, and recent studies have improved our understanding of the antigenic repertoire of V. cholerae Interactions between the host, V. cholerae, and the intestinal microbiome are now recognized as factors which impact susceptibility to cholera and the ability to mount a successful immune response to vaccination. Here, we review recent data and corresponding models to describe immune responses to V. cholerae infection and explain how the host microbiome may impact the pathogenesis of V. cholerae In the ongoing battle against cholera, the intestinal microbiome represents a frontier for new approaches to intervention and prevention.

Single-Cell Analysis of the Plasmablast Response to Vibrio cholerae Demonstrates Expansion of Cross-Reactive Memory B Cells

We characterized the acute B cell response in adults with cholera by analyzing the repertoire, specificity, and functional characteristics of 138 monoclonal antibodies (MAbs) generated from single-cell-sorted plasmablasts. We found that the cholera-induced responses were characterized by high levels of somatic hypermutation and large clonal expansions. A majority of the expansions targeted cholera toxin (CT) or lipopolysaccharide (LPS). Using a novel proteomics approach, we were able to identify sialidase as another major antigen targeted by the antibody response to Vibrio cholerae infection. Antitoxin MAbs targeted both the A and B subunits, and most were also potent neutralizers of enterotoxigenic Escherichia coli heat-labile toxin. LPS-specific MAbs uniformly targeted the O-specific polysaccharide, with no detectable responses to either the core or the lipid moiety of LPS. Interestingly, the LPS-specific antibodies varied widely in serotype specificity and functional characteristics. One participant infected with the Ogawa serotype produced highly mutated LPS-specific antibodies that preferentially bound the previously circulating Inaba serotype. This demonstrates durable memory against a polysaccharide antigen presented at the mucosal surface and provides a mechanism for the long-term, partial heterotypic immunity seen following cholera.

Cholera is a diarrheal disease that results in significant mortality. While oral cholera vaccines are beneficial, they do not achieve equivalent protection compared to infection with Vibrio cholerae. Although antibodies likely mediate protection, the mechanisms of immunity following cholera are poorly understood, and a detailed understanding of antibody responses to cholera is of significance for human health. In this study, we characterized the human response to cholera at the single-plasmablast, monoclonal antibody level. Although this approach has not been widely applied to the study of human bacterial infection, we were able to uncover the basis of cross-reactivity between different V. cholerae serotypes and the likely impact of prior enterotoxigenic Escherichia coli exposure on the response to cholera, as well as identify novel antigenic targets. In addition to improving our understanding of the repertoire and function of the antibody response to cholera in humans, this study has implications for future cholera vaccination efforts.

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

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

Rapid and scalable plant-based production of a cholera toxin B subunit variant to aid in mass vaccination against cholera outbreaks

INTRODUCTION

Cholera toxin B subunit (CTB) is a component of an internationally licensed oral cholera vaccine. The protein induces neutralizing antibodies against the holotoxin, the virulence factor responsible for severe diarrhea. A field clinical trial has suggested that the addition of CTB to killed whole-cell bacteria provides superior short-term protection to whole-cell-only vaccines; however, challenges in CTB biomanufacturing (i.e., cost and scale) hamper its implementation to mass vaccination in developing countries. To provide a potential solution to this issue, we developed a rapid, robust, and scalable CTB production system in plants.

METHODOLOGY/PRINCIPAL FINDINGS

In a preliminary study of expressing original CTB in transgenic Nicotiana benthamiana, the protein was N-glycosylated with plant-specific glycans. Thus, an aglycosylated CTB variant (pCTB) was created and overexpressed via a plant virus vector. Upon additional transgene engineering for retention in the endoplasmic reticulum and optimization of a secretory signal, the yield of pCTB was dramatically improved, reaching >1 g per kg of fresh leaf material. The protein was efficiently purified by simple two-step chromatography. The GM1-ganglioside binding capacity and conformational stability of pCTB were virtually identical to the bacteria-derived original B subunit, as demonstrated in competitive enzyme-linked immunosorbent assay, surface plasmon resonance, and fluorescence-based thermal shift assay. Mammalian cell surface-binding was corroborated by immunofluorescence and flow cytometry. pCTB exhibited strong oral immunogenicity in mice, inducing significant levels of CTB-specific intestinal antibodies that persisted over 6 months. Moreover, these antibodies effectively neutralized the cholera holotoxin in vitro.

CONCLUSIONS/SIGNIFICANCE

Taken together, these results demonstrated that pCTB has robust producibility in Nicotiana plants and retains most, if not all, of major biological activities of the original protein. This rapid and easily scalable system may enable the implementation of pCTB to mass vaccination against outbreaks, thereby providing better protection of high-risk populations in developing countries.

Engineered single-domain antibodies with high protease resistance and thermal stability

The extreme pH and protease-rich environment of the upper gastrointestinal tract is a major obstacle facing orally-administered protein therapeutics, including antibodies. Through protein engineering, several Clostridium difficile toxin A-specific heavy chain antibody variable domains (V_HHs) were expressed with an additional disulfide bond by introducing Ala/Gly54Cys and Ile78Cys mutations. Mutant antibodies were compared to their wild-type counterparts with respect to expression yield, non-aggregation status, affinity for toxin A, circular dichroism (CD) structural signatures, thermal stability, protease resistance, and toxin A-neutralizing capacity. The mutant V_HHs were found to be well expressed, although with lower yields compared to wild-type counterparts, were non-aggregating monomers, retained low nM affinity for toxin A, albeit the majority showed somewhat reduced affinity compared to wild-type counterparts, and were capable of in vitro toxin A neutralization in cell-based assays. Far-UV and near-UV CD spectroscopy consistently showed shifts in peak intensity and selective peak minima for wild-type and mutant V_HH pairs; however, the overall CD profile remained very similar. A significant increase in the thermal unfolding midpoint temperature was observed for all mutants at both neutral and acidic pH. Digestion of the V_HHs with the major gastrointestinal proteases, at biologically relevant concentrations, revealed a significant increase in pepsin resistance for all mutants and an increase in chymotrypsin resistance for the majority of mutants. Mutant V_HH trypsin resistance was similar to that of wild-type V_HHs, although the trypsin resistance of one V_HH mutant was significantly reduced. Therefore, the introduction of a second disulfide bond in the hydrophobic core not only increases V_HH thermal stability at neutral pH, as previously shown, but also represents a generic strategy to increase V_HH stability at low pH and impart protease resistance, with only minor perturbations in target binding affinities. These are all desirable characteristics for the design of protein-based oral therapeutics.

Mucosal immunogenicity and adjuvant activity of the recombinant A subunit of the Escherichia coli heat-labile enterotoxin

The Escherichia coli heat-labile enterotoxin (LT) is an exceptionally effective mucosal immunogen and mucosal immunoadjuvant towards coadministered antigens. Although, in general, the molecular basis of these properties is poorly understood, both the toxic ADP-ribosylation activity of the LTA subunit and the cellular toxin receptor, ganglioside, GM1-binding properties of the LTB-pentamer have been suggested to be involved. In recent studies we found that GM1-binding is not essential for the adjuvanticity of LT, suggesting an important role for the LTA subunit in immune stimulation. We now describe the immunomodulatory properties of recombinant LTA molecules with or without ADP-ribosylation activity, LTA(His)10 and LTA-E112K(His)10, respectively. These molecules were expressed as fusion proteins with an N-terminal His-tag to allow simple purification on nickel-chelate columns. Their immunogenic and immunoadjuvant properties were assessed upon intranasal administration to mice, and antigen-specific serum immunoglobulin-isotype and -subtype responses and mucosal secretory immunoglobulin A (IgA) responses were monitored using enzyme-linked immunosorbent assay. With respect to immunogenicity, both LTA(His)10 and LTA-E112K(His)10 failed to induce antibody responses. On the other hand, immunization with both LT and the non-toxic LT-E112K mutant not only induced brisk LTB-specific, but also LTA-specific serum and mucosal antibody responses. Therefore, we conclude that linkage of LTA to the LTB pentamer is essential for the induction of LTA-specific responses. With respect to adjuvanticity, both LTA(His)10 and LTA-E112K(His)10 were found to stimulate serum and mucosal antibody responses towards coadministered influenza subunit antigen. Remarkably, responses obtained with LTA(His)10 were comparable in both magnitude and serum immunoglobulin isotype and subtype distributions to those observed after coimmunization with LT, LT-E112K, or recombinant LTB. We conclude that LTA, by itself, can act as a potent adjuvant for intranasally administered antigens in a fashion independent of ADP-ribosylation activity and association with the LTB pentamer.

Mucosal immunogenicity of genetically detoxified derivatives of heat labile toxin from Escherichia coli

Using a fixed dose of antigen, the immune response to detoxified mutants of LT-WT following intranasal (i.n.), subcutaneous (s.c.) and oral (i.g.) immunisation has been studied. When given i.n., both LT-WT and mutant toxin, K63, generated significant levels of toxin-specific IgG in the serum, and the levels of IgA in nasal and lung lavages were greater than those induced by rLT-B. In comparison, i.g. immunisation of mice with a similar quantity of either LT-WT or K63 toxin induced barely detectable levels of IgG in the sera. However, if the amount of protein used for i.g. immunisation was increased tenfold, relatively good levels of toxin-specific IgG were induced in the sera by both LT-WT or K63. Low levels of toxin-specific IgA were also observed in intestinal washes from these mice. Western blotting of the sera, using the native toxin as an antigen, demonstrated the presence of both anti-A and anti-B subunit antibodies. Most significantly, toxin-neutralising antibodies were induced in the serum, with the strongest activity being induced by the LT-WT, an intermediate activity induced by mutant K63 and a lower response by rLT-B. Together, these data show that ADP-ribosyltransferase is not necessary for mucosal immunogenicity of these proteins, and that the i.n. route of immunisation is more effective than the i.g. route of immunisation for the generation of both systemic (IgG) and mucosal (IgA) immune responses.

The role of ADP-ribosylation and G(M1)-binding activity in the mucosal immunogenicity and adjuvanticity of the Escherichia coli heat-labile enterotoxin and Vibrio cholerae cholera toxin

The mucosal route of vaccination has attracted a great deal of attention recently. Not only is mucosal application of vaccines, for example, orally or intranasally, particularly convenient, it also offers the possibility to induce locally produced and secreted S-IgA antibodies in addition to systemic IgG antibodies. These IgA antibodies are known to play a key role in protection against pathogens that invade the host through mucosal surfaces. Induction of such responses is not readily achieved by currently used vaccination strategies, which generally involve intramuscular or subcutaneous injection with inactivated pathogens or antigens thereof. For the induction of a mucosal immune response, the vaccine needs to be applied locally. However, local vaccination with non-replicating antigens is usually ineffective and may result in tolerance unless a mucosal immunoadjuvant is included. The most potent mucosal immunoadjuvants known to date are probably cholera toxin (CT) and the closely related Escherichia coli heat-labile enterotoxin (LT). Although CT and LT have become standard adjuvants for experimental mucosal vaccines, the intrinsic toxicity has thus far precluded their use as adjuvants for human vaccine formulations. In the present review, the mucosal immunogenic and adjuvant properties of LT and CT are described, with special emphasis on the functional role of the individual subunits on their immune-stimulatory properties.

Mutants of the Escherichia coli heat-labile enterotoxin with reduced ADP-ribosylation activity or no activity retain the immunogenic properties of the native holotoxin

The Escherichia coli heat-labile enterotoxin (LT) is a potent inducer of mucosal immune responses. In a previous study (L. DeHaan, W. R. Verweij, M. Holtrop, E. Agsteribbe, and J. Wilschut, Vaccine 14:620-626, 1996), we have shown that efficient induction of an LTB-specific mucosal immune response by LT requires the presence of the LTA chain, suggesting a possible role of the enzymatic activity of LTA in the induction of these responses. In the present study, we generated LT mutants with altered ADP-ribosylation activities and evaluated their immunogenicity upon intranasal administration to mice. The results demonstrate that the mucosal immunogenicity of LT is not dependent on its ADP-ribosylation activity.

Expression of fragment C of tetanus toxin fused to a carboxyl-terminal fragment of diphtheria toxin in Salmonella typhi CVD 908 vaccine strain

We report the expression of fragment C of tetanus toxin (FC) fused to the eukaryotic cell binding domain (the carboxyl-terminus) of diphtheria toxin (FC-bDt fusion) in attenuated Salmonella typhi live vector vaccine strain CVD 908. The FC-bDt protein fusion was constructed using plasmid pTETnir15 which carries the gene encoding FC under control of the nirB promoter (nirBP). The open reading frame for FC was modified to incorporate an in-frame glycine-proline hinge region and a set of four restriction sites at the 3' end of the FC gene. A 482 bp DNA fragment encoding the eukaryotic cell binding domain of diphtheria toxin was then inserted at the 3' end of the modified FC gene to create an in-frame FC-bDt fusion gene. The resulting plasmid, pOG215, was able to express the FC-bDt fusion protein in both Escherichia coli DH5a and S. typhi CVD 908, as evidenced by Western immunoblots using anti-FC and anti-C-terminal diphtheria toxin monoclonal antibodies. Maximum expression of the FC-bDt fusion protein was achieved by growing CVD 908(pOG215) at the low oxidation-reduction potential of thioglycollate broth, i.e. in conditions that activate nirBP and drive transcription of the FC-bDt fusion gene. Whereas maximum expression of FC alone was also observed using thioglycollate broth, expression of bDt alone was unsuccessful using a variety of growth conditions. FC fusions constitute one strategy to "rescue" expression of proteins which are otherwise difficult to express.

Construction of nontoxic derivatives of cholera toxin and characterization of the immunological response against the A subunit

Using computer modelling, we have identified some of the residues of the A subunit of cholera toxin (CT) and heat-labile toxin that are involved in NAD binding, catalysis, and toxicity. Here we describe the site-directed mutagenesis of the CT gene and the construction of CT mutants. Nine mutations of the A subunit gene were generated. Six of them encoded proteins that were fully assembled in the AB5 structure and were nontoxic; these proteins were CT-D53 (Val-53-->Asp), CT-K63 (Ser-63-->Lys), CT-K97 (Val-97-->Lys), CT-K104 (Tyr-104-->Lys), CT-S106 (Pro-106-->Ser), and the double mutant CT-D53/K63 (Val-53-->Asp, Ser-63-->Lys). Two of the mutations encoded proteins that were assembled into the AB5 structure but were still toxic; these proteins were CT-H54 (Arg-54-->His) and CT-N107 (His-107-->Asn). Finally, one of the mutant proteins, CT-E114 (Ser-114-->Glu), was unable to assemble the A and the B subunits and produced only the B oligomer. The six nontoxic mutants were purified from the culture supernatants of recombinant Vibrio cholerae strains and further characterized. The CT-K63 mutant, which was the most efficient in assembly of the AB5 structure, was used to immunize rabbits and was shown to be able to induce neutralizing antibodies against both the A and B subunits. This molecule may be useful for the construction of improved vaccines against cholera.

Cholera

Despite more than a century of study, cholera still presents challenges and surprises to us. Throughout most of the 20th century, cholera was caused by Vibrio cholerae of the O1 serogroup and the disease was largely confined to Asia and Africa. However, the last decade of the 20th century has witnessed two major developments in the history of this disease. In 1991, a massive outbreak of cholera started in South America, the one continent previously untouched by cholera in this century. In 1992, an apparently new pandemic caused by a previously unknown serogroup of V. cholerae (O139) began in India and Bangladesh. The O139 epidemic has been occurring in populations assumed to be largely immune to V. cholerae O1 and has rapidly spread to many countries including the United States. In this review, we discuss all aspects of cholera, including the clinical microbiology, epidemiology, pathogenesis, and clinical features of the disease. Special attention will be paid to the extraordinary advances that have been made in recent years in unravelling the molecular pathogenesis of this infection and in the development of new generations of vaccines to prevent it.

A genetically detoxified derivative of heat-labile Escherichia coli enterotoxin induces neutralizing antibodies against the A subunit

Escherichia coli enterotoxin (LT) and the homologous cholera toxin (CT) are A-B toxins that cause travelers' diarrhea and cholera, respectively. So far, experimental live and killed vaccines against these diseases have been developed using only the nontoxic B portion of these toxins. The enzymatically active A subunit has not been used because it is responsible for the toxicity and it is reported to induce a negligible titer of toxin neutralizing antibodies. We used site-directed mutagenesis to inactivate the ADP-ribosyltransferase activity of the A subunit and obtained nontoxic derivatives of LT that elicited a good titer of neutralizing antibodies recognizing the A subunit. These LT mutants and equivalent mutants of CT may be used to improve live and killed vaccines against cholera and enterotoxinogenic E. coli.

Cholera toxin and its subunits as potential oral adjuvants

Cholera toxin has been shown to have adjuvant effects in multiple different systems. The dose, timing and genetic background of the recipient all seem to be important variables. The role of the two subunits in both the immunogenicity and the adjuvanticity of this molecule remain unclear. The mechanisms of the adjuvant effect likely involves effects on regulatory T cells; there is evidence that the adjuvant effect is due at least in part to inhibition of suppressor T cells. When KLH is used as a model antigen, the adjuvanticity of cholera toxin appears to be related to its immunogenicity in that both properties occur mainly in mouse strains that are high responders to cholera toxin. The genetic engineering of chimeric neoantigens consisting of cholera toxin subunits coupled to antigens of interest has been shown to be technically possible and is an attractive future approach for the generation of effective oral vaccines.

Excretion of cholera toxin from Escherichia coli: a potential oral vaccine for cholera

Escherichia coli strain N100 has been mutagenized by transposon mutagenesis and mutants with a cell surface leaky phenotype have been isolated. The mutant designated as E. coli N100::Tn5 excreted periplasmic proteins like ribonuclease and alkaline phosphatase. When this mutant strain was transformed with plasmids containing cloned cholera toxin genes, the toxin protein synthesized in the cells were excreted. The potentiality of this strain as a live oral vaccine for cholera has been discussed.

Cholera toxin neutralization: a comparison of purified serum IgG and biliary secretory IgA antibodies

Rats were immunized three times with cholera toxin via the intraintestinal or intravenous route, and their respective biliary secretory IgA (sIgA) or serum IgG antibodies were affinity-purified on a cholera toxin immunoabsorbent. On a molar basis, the sIgA antibodies were roughly seven-fold more efficient than IgG antibodies in neutralizing cholera toxin in the ligated intestinal loop assay. Various explanations for this difference in neutralizing capacity are proposed.

Cloning and expression of the Salmonella enterotoxin gene

This report examines the genetic basis for Salmonella typhimurium Q1 enterotoxin production. A 918-base-pair XbaI-HincII fragment of plasmid pJM17, composed of cholera toxin (CT) coding sequences (ctxAB), was used as a gene probe. With this probe, the S. typhimurium enterotoxin was identified on a 6.3-kilobase EcoRI-PstI fragment of chromosomal DNA from plasmidless strain Q1. We cloned this 6.3-kilobase fragment into Escherichia coli RR1. The genetic map of the cloned Salmonella enterotoxin (stx) gene was similar but not identical to the CT and E. coli heat-labile enterotoxin genes. By using synthetic oligonucleotides derived from the sequences of CT subunits A (ctxA) and B (ctxB), it was revealed that there were some conserved regions of DNA encoding the enterotoxins of strain Q1 and Vibrio cholerae. Expression of the cloned stx gene in minicells and subsequent Western blot (immunoblot) analysis with CT antitoxin demonstrated that the Salmonella enterotoxin had two or more subunits with molecular sizes of 45, 26, and 12 kilodaltons. Crude cell lysates of E. coli RR1(pCHP4), containing the cloned Salmonella enterotoxin gene, elicited fluid secretion in ligated rabbit intestinal loops and firm induration in rabbit skin. Both of these enterotoxic responses were neutralized by antisera specific for CT. Mucosal tissue from positive intestinal loops contained elevated levels of cyclic AMP. These data suggest some evolutionary relatedness between the enterotoxin genes of S. typhimurium and V. cholerae.

Oral immunization of mice with attenuated Salmonella enteritidis containing a recombinant plasmid which codes for production of the B subunit of heat-labile Escherichia coli enterotoxin

We used Salmonella enteritidis serotype dublin strain SL1438, a nonreverting, aromatic-dependent, histidine-requiring mutant, as a recipient for a recombinant plasmid coding for production of the nontoxic B subunit of the heat-labile Escherichia coli enterotoxin. The S. enteritidis derivative EL23 produced heat-labile enterotoxin subunit B that was indistinguishable from heat-labile enterotoxin subunit B produced by strains of E. coli or Salmonella typhi harboring the same plasmid. Mice immunized orally with strain EL23 developed progressively increasing mucosal and serum antibody responses to both heat-labile enterotoxin subunit B and to the lipopolysaccharide of the vaccine strain. The mucosal antibody response was shown to be immunoglobulin A specific and to be capable of neutralizing the biological activities of both E. coli heat-labile enterotoxin and cholera enterotoxin in vitro.

Construction of a potential live oral bivalent vaccine for typhoid fever and cholera-Escherichia coli-related diarrheas

We used the Salmonella typhi galactose epimerase (galE) mutant strain Ty21a, shown to be a safe, effective, living, attenuated oral typhoid vaccine, as a recipient for a recombinant plasmid containing the gene for production of the nontoxic B subunit of the heat-labile enterotoxin of Escherichia coli. The S. typhi derivative, strain SE12, produced heat-labile enterotoxin subunit B that was structurally and immunologically indistinguishable from heat-labile enterotoxin subunit B produced by strains of E. coli harboring the same plasmid. Tests in mice and guinea pigs showed that strain SE12 was safe when given orally and was capable of inducing a significant antitoxic antibody response when injected parenterally. Moreover, it retained the galactose sensitivity of the parent strain, preserving its utility as a typhoid vaccine. This strain may prove to be a useful live oral bivalent vaccine strain for typhoid fever and cholera-E. coli-related diarrheas.

Characterization of monoclonal antibodies to heat-labile enterotoxin encoded by a plasmid from a clinical isolate of Escherichia coli

Eight selected hybridoma cell lines that produced monoclonal antibodies against heat-labile enterotoxin from an Escherichia coli strain of human origin (LTh) were characterized. Antibodies produced by these cell lines were tested for binding specificity in a series of solid-phase radioimmunoassays and Western blots by using as test antigens LTh, the A, A1, A2, and B polypeptides of LTh, the heat-labile enterotoxin from an E. coli strain of porcine origin, and cholera toxin. The monoclonal antibodies were also tested for isotype and ability to neutralize LTh. Two of the anti-LTh monoclonal antibodies cross-reacted with cholera toxin, and six were specific for determinants of LTh that were not present on cholera toxin. One was specific for a unique epitope of LTh that was not shared by the heat-labile enterotoxin from an E. coli strain of porcine origin or cholera toxin. Four antibodies specific for epitopes on the B subunit of LTh (LTh-B) reacted with pentameric LTh-B but did not react in Western blots with monomeric LTh-B. The remaining four antibodies were specific for epitopes on LTh-A; two of these antibodies bound to A1, one reacted with A2, and one recognized only intact LTh-A. Only one monoclonal antibody had detectable neutralizing activity, and it was specific for LTh-A.

Local and systemic antibody responses and immunological memory in humans after immunization with cholera B subunit by different routes

A single oral or intramuscular immunization with purified cholera B subunit induced an intestinal secretory immunoglobulin A (IgA) antitoxin response in, respectively, 10 out of 11 and 9 out of 12 Bangladeshi volunteers. The IgA titre rise in intestinal lavage fluid was similar by either route of immunization, but the duration of the response was usually longer after the oral dose. A second immunization by either route, given 25 days after the first, and a third dose (oral only), given 15 months later, resulted in intestinal immune responses which did not differ in magnitude from that induced by the initial immunization but were observed significantly earlier, usually by day 3. Both the first oral and intramuscular immunizations induced significant antitoxin titre rises, mainly IgG, in the serum in most vaccinees but the magnitude of the response was considerably higher after the intramuscular dose. Significant IgA antitoxin titre rises in saliva and breast milk were seen after both oral and intramuscular immunization.

New knowledge on pathogenesis of bacterial enteric infections as applied to vaccine development

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Monoclonal antibodies to cholera toxin with special reference to cross-reactions with Escherichia coli heat-labile enterotoxin

Seventy monoclonal antibodies to cholera toxin were prepared and characterized. All were of immunoglobulin G (IgG) isotypes (39 IgG1, 29 IgG2, and 2 IgG3). A total of 61 clones produced antibody directed against the B subunit, and 9 clones produced antibodies with specificity for the cholera toxin A subunit. Among both the anti-B and anti-A antibodies, there were representatives which showed full cross-reactivity with the heat-labile enterotoxin of Escherichia coli (14 clones), others which gave partial cross-reactions (12), and still others (44) which did not cross-react. Although 24 of 25 tested anti-B monoclonal antibodies could neutralize cholera toxin, none of the 9 anti-A clones had any detectable neutralizing ability. Among the anti-B antibodies, those which cross-reacted completely with E. coli heat-labile enterotoxin all had strong cholera toxin-neutralizing capacity, whereas those with lesser or no degree of cross-reactivity varied more in their neutralizing potency. The isolation of monoclonal antibodies that distinguish between enterotoxins of different bacterial origin suggests the possibility of developing immunodiagnostic methods allowing species-specific enterotoxin detection in stools of patients with diarrheal disease.

Isolation of hybridoma cell lines and characterization of monoclonal antibodies against cholera enterotoxin and its subunits

Hybridoma cell lines which produced monoclonal antibodies against cholera toxin were isolated. These cell lines were detected with an enzyme-linked immunosorbent assay screening procedure with purified cholera toxin or subunit A of cholera toxin. Seven cell lines were characterized with respect to their reactivity with cholera toxin subunits by Western blot analysis. Five clones produced antibodies which were directed against subunit A, and two clones produced antibodies which reacted with subunit B. These antibodies were also characterized by Western blot analysis for reactivity with the heat-labile enterotoxin produced by porcine and human enterotoxinogenic strains of Escherichia coli. Monoclonal antibodies which reacted with subunit A of cholera toxin also reacted with subunit A of both porcine and human heat-labile enterotoxins. In contrast, monoclonal antibodies to subunit B of cholera toxin did not react with subunit B of the heat-labile enterotoxin. Antibodies directed against subunit B neutralized the biological activity of cholera toxin in vitro in the S49 mouse lymphosarcoma assay. In contrast to polyclonal anti-subunit A antisera, monoclonal anti-subunit A from four of five clones had small but measurable neutralizing capacities in vitro.

Actions of cholera toxin and the prevention and treatment of cholera

The drastic intestinal secretion of fluid and electrolytes that is characteristic of cholera is the result of reasonably well understood cellular and biochemical actions of the toxin secreted by Vibrio cholerae. Based on this understanding it is possible to devise new techniques for the treatment and prophylaxis of cholera to complement those based on fluid replacement therapy and sanitation.

Lincomycin increases synthetic rate and periplasmic pool size for cholera toxin

Increased enterotoxigenicity of Vibrio cholerae 569B grown with low concentrations of lincomycin, previously described in terms of increased extracellular biological activity (capillary permeability factor and fluid accumulation in ligated rabbit ileal loops), was further characterized. Polyacrylamide gel electrophoresis and single radial immunodiffusion showed that lincomycin-stimulated cells produced increased molar quantities of cholera toxin (CT) both extra- and intracellularly. The intracellular CT was released in comparable amounts by sonication, deoxycholate extraction, and polymyxin B treatment. Polymyxin B release of CT was nearly complete under conditions wherein only 6% of total cellular beta-galactosidase was released, implying a periplasmic pool of CT in stimulated cells. No intracellular choleragenoid (CT subunit B) was found in stimulated cells by polymyxin B release. No proteolysis of 14C-labeled CT was detected after prolonged incubation with sonicated nonstimulated cultures or sonicated concentrated cells. These data support the conclusion that the stimulatory effect of lincomycin involves an increase in the rate of synthesis of the CT molecule, and argue against alternative models involving inhibition of putative normal degradation of CT, increased release of otherwise cell-bound CT, or activation of inactive, or less active, forms of CT.

Influence of route of administration on immediate and extended protection in rats immunized with Escherichia coli heart-labile enterotoxin

The effect of route of administration, dosage, and number of boosts employed during immunization with the polymyxin-release form of Escherichia coli heat-labile (LT) enterotoxin on the degree and duration of protection afforded was evaluated in rats which were challenged by the ligated loop technique. Increasing the boosting dosage by fivefold from 50 to 250 mug resulted in a marked increase in protection against challenge with toxin in rats immunized either just by the parenteral route (i.p./i.p.) or by a parenteral prime, followed by peroral boosts (i.p./p.o.) in rats pretreated with cimetidine to ablate gastric secretions; such was not the case, however, even with a 50-fold increase in dosage in rats immunized just by the peroral route (p.o./p.o.). Four weekly peroral boosts were required to achieve the strongest degree of protection. Increasing the boosting dosage also increased the degree of protection against challenge with viable LT(+)/ST(-) and LT(+)/ST(+) strains (ST indicates heat-stable enterotoxin) in rats immunized by the i.p./p.o., but not by the i.p./i.p., route; no protection was evident against an LT(-)/ST(+) strain. Protection was lost within 3 weeks after immunization in rats immunized by the i.p./i.p. route. In contrast, protection was extended over the 3-month observation period in those immunized by the i.p./p.o. route; the degree of protection was enhanced in rats which received an additional boost at 2 months. These observations establish the fact that immunization with LT is similar to that with cholera toxin in that arousal of the local immune intestinal response by means of peroral immunization provides maximal extended protection.

Protection against experimental cholera by oral or parenteral immunization

Comparisons were made between the antigenic potency and protective capacity of several cholera toxin derivatives. Rabbits were immunized parenterally with 50 microgram of cholera toxin, A subunit, B subunit, procholeragenoid, or Wyeth glutaraldehyde toxoid 20101. Examination of the antibody response curves revealed that cholera toxin elicited serum antitoxin responses that rose more quickly than in the subunit-immunized animals; however, antitoxin levels were of the same magnitude after 10 weeks. Parenteral immunization with procholeragenoid evoked antibody titers that were similar to the toxin, whereas Wyeth toxoid yielded only one-tenth the level of antitoxin. Oral immunization with procholeragenoid as well as Wyeth toxoid resulted in lower serum antitoxin titers than that achieved with parenteral immunization, despite the oral administration of 10 times the parenteral dose. Analysis of protection against live-cell challenge revealed that parenteral administration of procholeragenoid provided the best protection against fluid accumulation. Oral immunization with procholeragenoid also was very effective, whereas oral immunization with B subunit or Wyeth toxoid resulted in minimal protection. Also, the A subunit provided surprisingly more protection than did cholera toxin.