Immunogenicity of Vibrio cholerae O1 toxin-coregulated pili in experimental and clinical cholera

Outer membrane proteins and vesicles as promising vaccine candidates against Vibrio spp. infections

Indiscriminate use of antibiotics to treat bacterial infections has brought unmanageable antibiotic-resistant strains into existence. Vibrio spp. represents one such gram-negative enteric pathogenic group with more than 100 species, infecting humans and fish. The Vibrio spp. is demarcated into two groups, one that causes cholera and the other producing non-cholera or vibriosis infections. People who encounter contaminated water are at risk, but young children and pregnant women are the most vulnerable. Though controllable, Vibrio infection still necessitates the development of preventative measures, such as vaccinations, that can lessen the severity of the infection and reduce reliance on antibiotic use. With emerging multi-drug resistant strains, efforts are needed to develop newer vaccines, such as subunit-based or outer membrane vesicle-based. Thus, this review strives to bring together available information about Vibrio spp. outer membrane proteins and vesicles, encompassing their structure, function, and immunoprotective role.

Human Challenge Studies for Cholera

The human challenge model permits an estimate of the vaccine protection against moderate and severe cholera. It eliminates the difficulty in setting up a vaccine study in endemic area including uncertainties about the incidence of cholera and the logistic arrangements for capturing those who do/do not become ill. Valuable information from small groups of subjects can be obtained in a short period. Under proper precautions and study design, the challenge model is safe and efficient. Although the model has evolved since it was introduced over 50 years ago, it has been used extensively to test vaccine efficacy. Vaccine licensure has resulted from data obtained using the human challenge model. In addition, the model has been used to: (1) Establish and validate a standardized inoculum, (2) Identify immune markers and immune responses, (3) Determine natural immunity (in re-challenge studies), (4) Identify the role of the gastric acid barrier in preventing cholera infection, (5) Show homologous and heterologous infection-derived immunity, and (6) Test the efficacy of anti-diarrheal/anti-secretory small molecules. The aim of this chapter is to present an overview on the state of the art for human challenge models used to study cholera and new medical interventions against it.

The Live Attenuated Cholera Vaccine CVD 103-HgR Primes Responses to the Toxin-Coregulated Pilus Antigen TcpA in Subjects Challenged with Wild-Type Vibrio cholerae

One potential advantage of live attenuated bacterial vaccines is the ability to stimulate responses to antigens which are only expressed during the course of infection. To determine whether the live attenuated cholera vaccine CVD 103-HgR (Vaxchora) results in antibody responses to the in vivo-induced toxin-coregulated pilus antigen TcpA, we measured IgA and IgG responses to Vibrio cholerae O1 El Tor TcpA in a subset of participants in a recently reported experimental challenge study. Participants were challenged with V. cholerae O1 El Tor Inaba N16961 either 10 days or 90 days after receiving the vaccine or a placebo. Neither vaccination nor experimental infection with V. cholerae alone resulted in a robust TcpA IgG or IgA response, but each did elicit a strong response to cholera toxin. However, compared to placebo recipients, vaccinees had a marked increase in IgG TcpA antibodies following the 90-day challenge, suggesting that vaccination with CVD 103-HgR resulted in priming for a subsequent response to TcpA. No such difference between vaccine and placebo recipients was observed for volunteers challenged 10 days after vaccination, indicating that this was insufficient time for vaccine-induced priming of the TcpA response. The priming of the response to TcpA and potentially other antigens expressed in vivo by attenuated V. cholerae may have relevance to the maintenance of immunity in areas where cholera is endemic.

Recombinant toxin-coregulated pilus A (TcpA) as a candidate subunit cholera vaccine

BACKGROUND AND OBJECTIVES

The toxin co-regulated pilus A (TcpA) has been described as a critical pathogenicity factor of Vibrio cholerae. TcpA is a candidate for making subunit vaccine against cholera. The aim of this study was to produce a candidate vaccine by expressing recombinant TcpA in E. coli.

MATERIALS AND METHODS

In this study, the toxin co-regulated pilus A gene from EL-Tor, V. cholerae subspecies, was amplified by PCR and sub-cloned into prokaryotic expression vector pGEX4T1. E. coli BL21 (DE3) was transformed with pGEX4T1- TcpA and gene expression was induced by IPTG and purified by GST resin. The integrity of the product was confirmed by Western blot analysis using a standard rabbit anti-V. cholerae antibody. Sera reactivity of infected individuals was further analyzed against the recombinant TcpA protein.

RESULTS

The concentration of purified recombinant protein was calculated to be 8 mg/L of initial culture. The integrity of product was confirmed by Western blot analysis using a standard rabbit anti V. cholerae antibody. Sera reactivity of infected individual was further analyzed against the recombinant TcpA protein. The obtained data indicated that recombinant TcpA protein from V. cholerae was recognized by patient serum and animal sera.

CONCLUSION

These results show that the recombinant TcpA is antigenic and could be used in a carrier host as an oral vaccine against cholera.

Postgenomic approaches to cholera vaccine development

Cholera remains an important public health threat. A cholera vaccine that provides durable protection at the mucosal surface, especially among children in endemic settings, is urgently needed. The availability of the complete genome sequence of a clinical isolate of Vibrio cholerae O1 El Tor has allowed for comparative and functional genomic approaches in the study of cholera. This work holds promise for the identification of bacterial targets of protective human immune responses and may contribute to the development of a new generation of cholera vaccines.

Transcutaneous immunization with toxin-coregulated pilin A induces protective immunity against Vibrio cholerae O1 El Tor challenge in mice

Toxin-coregulated pilin A (TcpA) is the main structural subunit of a type IV bundle-forming pilus of Vibrio cholerae, the cause of cholera. Toxin-coregulated pilus is involved in formation of microcolonies of V. cholerae at the intestinal surface, and strains of V. cholerae deficient in TcpA are attenuated and unable to colonize intestinal surfaces. Anti-TcpA immunity is common in humans recovering from cholera in Bangladesh, and immunization against TcpA is protective in murine V. cholerae models. To evaluate whether transcutaneously applied TcpA is immunogenic, we transcutaneously immunized mice with 100 mug of TcpA or TcpA with an immunoadjuvant (cholera toxin [CT], 50 mug) on days 0, 19, and 40. Mice immunized with TcpA alone did not develop anti-TcpA responses. Mice that received transcutaneously applied TcpA and CT developed prominent anti-TcpA immunoglobulin G (IgG) serum responses but minimal anti-TcpA IgA. Transcutaneous immunization with CT induced prominent IgG and IgA anti-CT serum responses. In an infant mouse model, offspring born to dams transcutaneously immunized either with TcpA and CT or with CT alone were challenged with 10(6) CFU (one 50% lethal dose) wild-type V. cholerae O1 El Tor strain N16961. At 48 h, mice born to females transcutaneously immunized with CT alone had 36% +/- 10% (mean +/- standard error of the mean) survival, while mice born to females transcutaneously immunized with TcpA and CT had 69% +/- 6% survival (P < 0.001). Our results suggest that transcutaneous immunization with TcpA and an immunoadjuvant induces protective anti-TcpA immune responses. Anti-TcpA responses may contribute to an optimal cholera vaccine.

Transcriptional profiling of Vibrio cholerae recovered directly from patient specimens during early and late stages of human infection

Understanding gene expression by bacteria during the actual course of human infection may provide important insights into microbial pathogenesis. In this study, we evaluated the transcriptional profile of Vibrio cholerae, the causative agent of cholera, in clinical specimens from cholera patients. We collected samples of human stool and vomitus that were positive by dark-field microscopy for abundant vibrios and used a microarray to compare gene expression in organisms recovered directly from specimens collected during the early and late stages of human infection. Our results reveal that V. cholerae gene expression within the human host environment differs from patterns defined in in vitro models of pathogenesis. tcpA, the major subunit of the essential V. cholerae colonization factor, was significantly more highly expressed in early than in late stages of infection; however, the genes encoding cholera toxin were not highly expressed in either phase of human infection. Furthermore, expression of the virulence regulators toxRS and tcpPH was uncoupled. Interestingly, the pattern of gene expression indicates that the human upper intestine may be a uniquely suitable environment for the transfer of genetic elements that are important in the evolution of pathogenic strains of V. cholerae. These findings provide a more detailed assessment of the transcriptome of V. cholerae in the human host than previous studies of organisms in stool alone and have implications for cholera control and the design of improved vaccines.

The major subunit of the toxin-coregulated pilus TcpA induces mucosal and systemic immunoglobulin A immune responses in patients with cholera caused by Vibrio cholerae O1 and O139

Diarrhea caused by Vibrio cholerae is known to give long-lasting protection against subsequent life-threatening illness. The serum vibriocidal antibody response has been well studied and has been shown to correlate with protection. However, this systemic antibody response may be a surrogate marker for mucosal immune responses to key colonization factors of this organism, such as the toxin-coregulated pilus (TCP) and other factors. Information regarding immune responses to TCP, particularly mucosal immune responses, is lacking, particularly for patients infected with the El Tor biotype of V. cholerae O1 or V. cholerae O139 since highly purified TcpA from these strains has not been available previously for use in immune assays. We studied the immune responses to El Tor TcpA in cholera patients in Bangladesh. Patients had substantial and significant increases in TcpA-specific antibody-secreting cells in the circulation on day 7 after the onset of illness, as well as similar mucosal responses as determined by an alternate technique, the assay for antibody in lymphocyte supernatant. Significant increases in antibodies to TcpA were also seen in sera and feces of patients on days 7 and 21 after the onset of infection. Overall, 93% of the patients showed a TcpA-specific response in at least one of the specimens compared with the results obtained on day 2 and with healthy controls. These results demonstrate that TcpA is immunogenic following natural V. cholerae infection and suggest that immune responses to this antigen should be evaluated for potential protection against subsequent life-threatening illness.

Detection of antibodies to toxin-coregulated pili in sera from cholera patients

Monoclonal antibodies (MAbs) were prepared against toxin-coregulated pili (TCP) isolated from Vibrio cholerae O1 El Tor. Despite their limited bactericidal potential, two MAbs were able to mediate biotype-specific protection against experimental cholera in infant mice. These MAbs were used in immunoblotting studies to assess seroconversion to El Tor TCP following cholera. Clear anti-pilus responses were observed in five of nine patients.

Evaluation of the protective efficacy of Vibrio cholerae ghost (VCG) candidate vaccines in rabbits

An effective Vibrio cholerae vaccine is needed to reduce the morbidity and mortality caused by this pathogen. Despite the availability of current oral vaccines with measurable efficacy, there is need for more effective vaccines with broad-spectrum efficacy in target populations. Recent studies have shown that bacterial ghosts, produced by the expression of cloned lysis gene E, possess adjuvant properties and are immunogenic. In this study, ghosts were prepared from V. cholerae O1 or O139 and evaluated as vaccines in the reversible intestinal tie adult rabbit diarrhea (RITARD) model. Rabbits were orally immunized with different doses of V. cholerae ghost (VCG) formulations. The vaccine formulations elicited high levels of serum vibriocidal titers against indicator strains. The magnitude of the response was measured as the geometric mean titer (GMT) increase for all rabbits in relation to prevaccination titers. The induction of cross protection was evidenced by the ability of serum from VCG-immunized rabbits to mediate complement-dependent killing of both the homologous and the heterologous strains. Immunized rabbits were protected against intraduodenal challenge 30 days after primary immunization. Protective immunity against challenge appeared to be dose dependent and was associated with marked inhibition of colonization. These results indicate that VCGs represent a novel approach to cholera vaccine development and constitute an effective vaccine delivery vehicle.

Use of in vivo-induced antigen technology (IVIAT) to identify genes uniquely expressed during human infection with Vibrio cholerae

In vivo-induced antigen technology is a method to identify proteins expressed by pathogenic bacteria during human infection. Sera from 10 patients convalescing from cholera infection in Bangladesh were pooled, adsorbed against in vitro-grown El Tor Vibrio cholerae O1, and used to probe a genomic expression library in Escherichia coli constructed from El Tor V. cholerae O1 strain N16961. We identified 38 positive clones in the screen, encoding pili (PilA and TcpA), cell membrane proteins (PilQ, MshO, MshP, and CapK), methyl-accepting chemotaxis proteins, chemotaxis and motility proteins (CheA and CheR), a quorum-sensing protein (LuxP), and four hypothetical proteins. Analysis of immune responses to purified PilA and TcpA in individual patients demonstrated that the majority seroconverted to these proteins, confirming results with pooled sera. These results suggest that PilA and its outer membrane secretin, PilQ, are expressed during human infection and may be involved in colonization of the gastrointestinal tract. These results also demonstrate substantial immune responses to TcpA in patients infected with El Tor V. cholerae O1. In vivo-induced antigen technology provides a simple method for identifying microbial proteins expressed during human infection, but not during in vitro growth.

Immune response genes modulate serologic responses to Vibrio cholerae TcpA pilin peptides

Cholera is an enteric disease caused by Vibrio cholerae. Toxin-coregulated pilus (TCP), a type 4 pilus expressed by V. cholerae, is a cholera virulence factor that is required for host colonization. The TCP polymer is composed of subunits of TcpA pilin. Antibodies directed against TcpA are protective in animal models of cholera. While natural or recombinant forms of TcpA are difficult to purify to homogeneity, it is anticipated that synthesized TcpA peptides might serve as immunogens in a subunit vaccine. We wanted to assess the potential for effects of the immune response (Ir) gene that could complicate a peptide-based vaccine. Using a panel of mice congenic at the H-2 locus we tested the immunogenicity of TcpA peptide sequences (peptides 4 to 6) found in the carboxyl termini of both the classical (Cl) and El Tor (ET) biotypes of TCP. Cl peptides have been shown to be immunogenic in CD-1 mice. Our data clearly establish that there are effects of the Ir gene associated with both biotypes of TcpA. These effects are dynamic and dependent on the biotype of TcpA and the haplotypes of the host. In addition to the effects of the classic class II Ir gene, class I (D, L) or nonclassical class I (Qa-2) may also affect immune responses to TcpA peptides. To overcome the effects of the class II Ir gene, multiple TcpA peptides similar to peptides 4, 5, and 6 could be used in a subunit vaccine formulation. Identification of the most protective B-cell epitopes of TcpA within a particular peptide and conjugation to a universal carrier may be the most effective method to eliminate the effects of the class II and class I Ir genes.

Anti-class II monoclonal antibody-targeted Vibrio cholerae TcpA pilin: modulation of serologic response, epitope specificity, and isotype

Toxin-coregulated pilus (TCP) is a colonization factor required for cholera infection. It is not a strong immunogen when delivered in the context of whole cells, yet pilus subunits or TcpA derivative synthetic peptides induce protective responses. We examined the efficacy of immunizing mice with TCP conjugated to anti-class II monoclonal antibodies (MAb) with or without the addition of cholera toxin (CT) or anti-CD40 MAb to determine if the serologic response to TcpA could be manipulated. Anti-class II MAb-targeted TCP influenced the anti-TCP peptide serologic response with respect to titer and isotype. Responses to TcpA peptide 4 were induced with class II MAb-targeted TCP and not with nontargeted TCP. Class II MAb-targeting TcpA reduced the response to peptide 6 compared to the nontargeted TCP response. Class II MAb-targeted TcpA, if delivered with CT, enhanced the serologic response to TcpA peptides. The effectiveness of the combination of targeted TCP and CT was reduced if anti-CD40 MAb were included in the primary immunization. These data establish the need to understand the role of TCP presentation in the generation of B-cell epitopes in order to optimize TcpA-based cholera vaccines.

Characterization and immunogenicity of Vibrio cholerae ghosts expressing toxin-coregulated pili

Bacterial ghosts are attractive for use as non-living vaccines and as carriers of heterologous antigens of vaccine relevance. Ghosts were prepared from Vibrio cholerae strains of O1 or O139 serogroup after growth under culture conditions, which favor or repress the production of toxin-coregulated pili (TCP). Immunoblotting confirmed the TCP status of these V. cholerae ghosts (VCG), which retained the cellular morphology and envelope sub-component profile of viable bacteria. Rabbits were immunized with VCGs prepared from O139 bacteria with TCP-positive or TCP-negative phenotypes and the resulting sera assayed for antibodies to lipopolysaccharide (LPS) and to TCP. Regardless of the TCP status of the VCG preparations used for immunization, all animals produced antibodies to LPS as demonstrated in bactericidal assays. These antibodies were probably responsible for the capacity of the antisera to confer passive immunity to challenge with the homologous O139 strain in the infant mouse cholera model (IMCM). Only following immunization with TCP-positive VCG, however, were antibodies to TCP generated, as judged by the potential of antisera to mediate protection against a challenge strain of heterologous serogroup.

Passive protection of serum from volunteers inoculated with attenuated strain 638 of Vibrio cholerae O1 in animal models

As part of the studies to obtain an oral vaccine against cholera disease, the protective effect of serum from volunteers inoculated in a controlled trial with a candidate live attenuated vaccine of Vibrio cholerae O1, El Tor Ogawa (638; CTX&phi; mutant, hap::celA), was tested. It was confirmed that the serum, as well as the purified IgG and IgA from the volunteers had a protective effect in both of the animal models used, although the purified antibodies needed the presence of complement to be protective. These results emphasize the expectations about the protective potential of the candidate in challenge studies in humans to be conducted very soon.

Pathogenic and vaccine significance of toxin-coregulated pili of Vibrio cholerae E1 Tor

Vibrio cholerae O1 strains are classified into one of two biotypes, classical and E1 Tor, the latter being primarily responsible for cholera cases worldwide since 1961. Recent studies in our laboratory have focused upon the pathogenic and vaccine significance of the toxin-coregulated pili (TCP) produced by strains of E1 Tor biotype. Mutants in which the tcpA gene (encoding the pilin subunit protein) has been inactivated are dramatically attenuated in the infant mouse cholera model, showing markedly reduced colonisation potential in mixed-infection competition experiments. Significantly, in the vaccine context, antibodies to TCP are sufficient to prevent experimental infection, although our data suggest that this protective effect might be limited to strains of homologous biotype. Since we have shown that tcpA sequences are conserved within a biotype but differ between biotypes, this latter observation suggests that the biotype-restricted pilin epitopes might have greater vaccine significance. Similar studies indicate that TCP also play a critical role in colonisation by strains of the recently-recognised O139 serogroup, which is thought to have evolved from an O1 E1 Tor strain. In contrast to the effect of introducing mutations in the tcpA gene, strains carrying inactivated mshA genes (encoding the subunit of the mannose-sensitive haemagglutinin pilus) show unaltered in vivo behaviour. Consistent with this finding is our inability to demonstrate any protective effect associated with antibodies to MSHA. Ongoing approaches to vaccine development are variously aimed at improving the immunogenicity of the current inactivated whole-cell vaccine, or assessing the field efficacy of a promising live attenuated strain. The possible implications of our findings are discussed in relation to both of these options.

Investigation of the roles of toxin-coregulated pili and mannose-sensitive hemagglutinin pili in the pathogenesis of Vibrio cholerae O139 infection

In this study, adult volunteers were fed tcpA and mshA deletion mutants of V. cholerae O139 strain CVD 112 to determine the role of toxin-coregulated pili (TCP) and mannose-sensitive hemagglutinin (MSHA) in intestinal colonization. Eight of 10 volunteers who received CVD 112 or CVD 112 delta mshA shed the vaccine strains in their stools; the geometric mean peak excretion for both groups was 1.4 x 10(5) CFU/g of stool. In contrast, only one of nine recipients of CVD 112 delta tcpA shed vibrios in his stool (P < 0.01); during the first 24 h after inoculation, 3 x 10(2) CFU/g was recovered from this volunteer. All recipients of CVD 112 and 8 (80%) of the recipients of CVD 112 delta mshA developed at least a fourfold rise in vibriocidal titer after immunization. In contrast, only one (11%) of the nine recipients of CVD 112 delta tcpA developed a fourfold rise in vibriocidal titer (P < 0.01). We conclude that TCP are an important colonization factor of V. cholerae O139 and probably of El Tor V. cholerae O1. In contrast, MSHA does not appear to promote intestinal colonization in humans.

Induction of fimbriated Vibrio cholerae O139

Several fimbriated phases of Vibrio cholerae O139 strains were selectively induced and compared immunologically and biochemically with those of V. cholerae O1. Fimbrial antigens were detected on the surfaces of vibrio cells colonizing the epithelial cells of a rabbit small intestine. Convalescent-phase sera from six individuals infected with V. cholerae O139 revealed the development of antibody against the fimbrillin. These findings suggest that the fimbriae of V. cholerae O1 and O139 are expressed in vivo during infection and that consideration must be given to the use of fimbrial antigens as components of 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.

The maltose regulon of Vibrio cholerae affects production and secretion of virulence factors

The effects of maltose on production and secretion of virulence factors of Vibrio cholerae in strain X28214, classical biotype, and in maltose-defective transposon mutants constructed from this strain were characterized. Maltose was found to inhibit secretion of cholera toxin and to reduce production of the mannose-sensitive hemagglutinin and the soluble hemagglutinin-protease. In contrast, the amount of toxin-coregulated pilus was increased in the presence of maltose. The maltose effect was apparently mediated by genes of the maltose regulon, since inactivation of the malQ or malF gene of V. cholerae by transposon insertion was found to affect production and secretion of the same virulence factors that were responsive to maltose. The malQ and malF mutants showed, in addition, reduced virulence in an infant-mouse model. These results suggest that maltose may have a significant regulatory role in the production of virulence factors and that an intact maltose regulon is needed for full virulence of V. cholerae.

Vibrio cholerae O139 Bengal

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Vibrio cholerae non-O1 serogroup associated with cholera gravis genetically and physiologically resembles O1 E1 Tor cholera strains

Until recently, only Vibrio cholerae strains of the O1 serogroup have been associated with epidemic cholera. In December 1992, an outbreak of cholera gravis in Vellore, India, was attributed to a new serogroup of V. cholerae recently designated O139. Serogroup O139 cholera has since spread to 13 countries and has reached pandemic proportions. Serogroup O139 cholera evades immunity to O1 cholera and is not detected by the standard O1 antigen test. Understanding the origins of O139 cholera and determining the relatedness of O139 to O1 cholera are necessary to device strategies for detecting, reporting, and controlling this new pandemic. In order to determine the origins of this novel cholera serogroup, O139 was analyzed for virulence genes, for virulence proteins and their regulation, and for its genomic background. We found that O139 and O1 V. cholera strains of the E1 Tor biotype possess highly homologous virulence genes encoding cholera toxin and toxin-coregulated pili and that the regulation of virulence protein expression likewise was indistinguishable between O139 and O1. Pulsed-field gel electrophoresis (PFGE) revealed the restriction digest pattern of O139 strains to be closely related to that of O1 serogroup E1 Tor biotype cholera strains from the Indian subcontinent. However, PFGE showed minor differences among individual O139 cholera isolates, suggesting that O139 V. cholerae is evolving.

TcpA pilin sequences and colonization requirements for O1 and O139 vibrio cholerae

The distribution, characterization and function of the tcpA gene was investigated in Vibrio cholerae O1 strains of the El Tor biotype and in a newly emergent non-O1 strain classified as serogroup O139. The V. cholerae tcpA gene from the classical biotype strain O395 was used as a probe to identify a clone carrying the tcpA gene from the El Tor biotype strain E7946. The sequence of the E7946 tcpA gene revealed that the mature El Tor TcpA pilin has the same number of residues as, and is 82% identical to, TcpA of classical biotype strain O395. The majority of differences in primary structure are either conservative or clustered in a manner such that compensatory changes retain regional amino acid size, polarity and charge. In a functional analysis, the cloned gene was used to construct an El Tor mutant strain containing an insertion in tcpA. This strain exhibited a colonization defect in the infant mouse cholera model similar in magnitude to that previously described for classical biotype tcpA mutants, thus establishing an equivalent role for TCP in intestinal colonization by El Tor biotype strains. The tcpA analysis was further extended to both a prototype El Tor strain from the Peru epidemic and to the first non-O1 strain known to cause epidemic cholera, an O139 V. cholerae isolate from the current widespread Asian epidemic. These strains were shown to carry tcpA with a sequence identical to E7946. These results provide further evidence that the newly emergent non-O1 serogroup O139 strain represents a derivative of an El Tor biotype strain and, despite its different LPS structure, shares common TCP-associated antigens.(ABSTRACT TRUNCATED AT 250 WORDS)

Major outer membrane proteins of Vibrio cholerae and their role in induction of protective immunity through inhibition of intestinal colonization

Vibrio cholerae O1 organisms belonging to different biotypes and serotypes were shown to express major outer membrane proteins (MOMPs) with subunit molecular masses of 48 to 50, 40 to 43, 35 to 36, 27 to 28, and 20 kDa. Antisera raised against individual MOMPs of a V. cholerae O1 strain recognized MOMPs of corresponding molecular masses in other O1 and non-O1 strains. Serological data also suggested possible differences in the cell surface exposition of these MOMPs. However, no marked differences between V. cholerae cells grown in vitro and in vivo could be noted in respect to the expression or surface exposition of these MOMPs. Of five MOMPs studied in this work, 40- to 43- and 20-kDa cell surface proteins were shown to be of considerable importance, as antisera to these proteins induced significant protection against V. cholerae challenge in the suckling mouse model. Similar protection, although to a lesser extent, was demonstrable with the antiserum to the 27- to 28-kDa protein. These results were corroborated with the Fab (immunoglobulin G) [Fab(IgG)] fragments of the antisera, thereby suggesting that the observed protection induced by anti-MOMP antibodies did not arise as a result of bacterial clumping. Subsequent studies demonstrated that these antisera as well as their Fab (IgG) fragments induced significant inhibition of intestinal colonization of V. cholerae. The 40- to 43- and 27- to 28-kDa proteins appeared to be porinlike, while the 20-kDa protein was found to be antigenically related to TcpA (subunit A of toxin-coregulated pilus). All these results demonstrate the involvement of more than one cell surface antigen of V. cholerae in the induction of protective immunity through inhibition of intestinal colonization of vibrios.

Analysis of expression of toxin-coregulated pili in classical and El Tor Vibrio cholerae O1 in vitro and in vivo

The expression of toxin-coregulated pili (TCP) and their structural subunit TcpA was compared in 20 strains of Vibrio cholerae of the classical and El Tor biotypes. Bacteria were isolated from the intestines of rabbits with experimental cholera and compared with the same strains grown under optimal TCP expression conditions in vitro. Immunoblotting revealed that TcpA production was induced in both biotypes after vibrios entered the intestinal milieu; TcpA-negative inocula gave rise to TcpA-positive vibrios after multiplication in the gut. The levels of TcpA expressed during growth in the intestine were, for most strains, comparable to those attained under optimal growth conditions in vitro. Of 11 classical strains tested, 10 expressed TCP antigen on the bacterial surface at levels comparable to or exceeding those seen after growth in vitro as determined by an inhibition enzyme-linked immunosorbent assay. In contrast, only one of the nine El Tor strains studied produced detectable amounts of TCP surface antigen in vivo and no fimbriae or surface antigen reacting with anti-TCP serum was found on El Tor vibrios from human cholera stools. Distinct TCP fimbriae were observed by immunoelectron microscopy on classical-biotype vibrios grown either in rabbit intestines or in vitro but were not detected on El Tor vibrios. The results show that TCP is expressed on V. cholerae O1 of the classical biotype but not on V. cholerae O1 of the El Tor biotype in the intestines of rabbits with experimental cholera infection.