Initial clinical studies of CVD 112 Vibrio cholerae O139 live oral vaccine: safety and efficacy against experimental challenge

Combatting infectious diarrhea: innovations in treatment and vaccination strategies

INTRODUCTION

The escalating prevalence of infectious diseases is an important cause of concern in society. Particularly in several developing countries, infectious diarrhea poses a major problem, with a high fatality rate, especially among young children. The condition is divided into four classes, namely, acute diarrhea, invasive diarrhea, acute bloody diarrhea, and chronic diarrhea. Various pathogenic agents, such as bacteria, viruses, protozoans, and helminths, contribute to the onset of this condition.

AREAS COVERED

The review discusses the scenario of infectious diarrhea, the prevalent types, as well as approaches to management including preventive, therapeutic, and vaccination strategies. The vaccination techniques are extensively discussed including the available vaccines, their advantages as well as limitations.

EXPERT OPINION

There are several approaches available to develop new-improved vaccines. In addition, route of immunization is important and aerosols/nasal sprays, oral route, skin patches, powders, and liquid jets to minimize needles can be used. Plant-based vaccines, such as rice, might save packing and refrigeration costs by being long-lasting, non-refrigerable, and immunogenic. Future research should utilize predetermined PCR testing intervals and symptom monitoring to identify persistent pathogens after therapy and symptom remission.

Controlled Human Infection Models To Accelerate Vaccine Development

The timelines for developing vaccines against infectious diseases are lengthy, and often vaccines that reach the stage of large phase 3 field trials fail to provide the desired level of protective efficacy. The application of controlled human challenge models of infection and disease at the appropriate stages of development could accelerate development of candidate vaccines and, in fact, has done so successfully in some limited cases. Human challenge models could potentially be used to gather critical information on pathogenesis, inform strain selection for vaccines, explore cross-protective immunity, identify immune correlates of protection and mechanisms of protection induced by infection or evoked by candidate vaccines, guide decisions on appropriate trial endpoints, and evaluate vaccine efficacy. We prepared this report to motivate fellow scientists to exploit the potential capacity of controlled human challenge experiments to advance vaccine development. In this review, we considered available challenge models for 17 infectious diseases in the context of the public health importance of each disease, the diversity and pathogenesis of the causative organisms, the vaccine candidates under development, and each model's capacity to evaluate them and identify correlates of protective immunity. Our broad assessment indicated that human challenge models have not yet reached their full potential to support the development of vaccines against infectious diseases. On the basis of our review, however, we believe that describing an ideal challenge model is possible, as is further developing existing and future challenge models.

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.

Defining Polysaccharide-Specific Antibody Targets against Vibrio cholerae O139 in Humans following O139 Cholera and following Vaccination with a Commercial Bivalent Oral Cholera Vaccine, and Evaluation of Conjugate Vaccines Targeting O139

Cholera caused by Vibrio cholerae O139 could reemerge, and proactive development of an effective O139 vaccine would be prudent. To define immunoreactive and potentially immunogenic carbohydrate targets of Vibrio cholerae O139, we assessed immunoreactivities of various O-specific polysaccharide (OSP)-related saccharides with plasma from humans hospitalized with cholera caused by O139, comparing responses to those induced in recipients of a commercial oral whole-cell killed bivalent (O1 and O139) cholera vaccine (WC-O1/O139). We also assessed conjugate vaccines containing selected subsets of these saccharides for their ability to induce protective immunity using a mouse model of cholera. We found that patients with wild-type O139 cholera develop IgM, IgA, and IgG immune responses against O139 OSP and many of its fragments, but we were able to detect only a moderate IgM response to purified O139 OSP-core, and none to its fragments, in immunologically naive recipients of WC-O1/O139. We found that immunoreactivity of O139-specific polysaccharides with antibodies elicited by wild-type infection markedly increase when saccharides contain colitose and phosphate residues, that a synthetic terminal tetrasaccharide fragment of OSP is more immunoreactive and protectively immunogenic than complete OSP, that native OSP-core is a better protective immunogen than the synthetic OSP lacking core, and that functional vibriocidal activity of antibodies predicts in vivo protection in our model but depends on capsule thickness. Our results suggest that O139 OSP-specific responses are not prominent following vaccination with a currently available oral cholera vaccine in immunologically naive humans and that vaccines targeting V. cholerae O139 should be based on native OSP-core or terminal tetrasaccharide. IMPORTANCE Cholera is a severe dehydrating illness of humans caused by Vibrio cholerae serogroup O1 or O139. Protection against cholera is serogroup specific, and serogroup specificity is defined by O-specific polysaccharide (OSP). Little is known about immunity to O139 OSP. In this study, we used synthetic fragments of the O139 OSP to define immune responses to OSP in humans recovering from cholera caused by V. cholerae O139, compared these responses to those induced by the available O139 vaccine, and evaluated O139 fragments in next-generation conjugate vaccines. We found that the terminal tetrasaccharide of O139 is a primary immune target but that the currently available bivalent cholera vaccine poorly induces an anti-O139 OSP response in immunologically naive individuals.

Immunogenicity and protective efficacy of a live, oral cholera vaccine formulation stored outside-the-cold-chain for 140 days

Background

Cholera, an acute watery diarrhoeal disease caused by Vibrio cholerae serogroup O1 and O139 across the continents. Replacing the existing WHO licensed killed multiple-dose oral cholera vaccines that demand ‘cold chain supply’ at 2–8 °C with a live, single-dose and cold chain-free vaccine would relieve the significant bottlenecks and cost determinants in cholera vaccination campaigns. In this direction, a prototype cold chain-free live attenuated cholera vaccine formulation (LACV) was developed against the toxigenic wild-type (WT) V. cholerae O139 serogroup. LACV was found stable and retained its viability (5 × 10⁶ CFU/mL), purity and potency at room temperature (25 °C ± 2 °C, and 60% ± 5% relative humidity) for 140 days in contrast to all the existing WHO licensed cold-chain supply (2–8 °C) dependent killed oral cholera vaccines.

Results

The LACV was evaluated for its colonization potential, reactogenicity, immunogenicity and protective efficacy in animal models after its storage at room temperature for 140 days. In suckling mice colonization assay, the LACV recorded the highest recovery of (7.2 × 10⁷ CFU/mL) compared to those of unformulated VCUSM14P (5.6 × 10⁷ CFU/mL) and the WT O139 strain (3.5 × 10⁷ CFU/mL). The LACV showed no reactogenicity even at an inoculation dose of 10⁴–10⁶ CFU/mL in a rabbit ileal loop model. The rabbits vaccinated with the LACV or unformulated VCUSM14P survived a challenge with WT O139 and showed no signs of diarrhoea or death in the reversible intestinal tie adult rabbit diarrhoea (RITARD) model. Vaccinated rabbits recorded a 275-fold increase in anti-CT IgG and a 15-fold increase in anti-CT IgA antibodies compared to those of rabbits vaccinated with unformulated VCUSM14P. Vibriocidal antibodies were increased by 31-fold with the LACV and 14-fold with unformulated VCUSM14P.

Conclusion

The vaccine formulation mimics a natural infection, is non-reactogenic and highly immunogenic in vivo and protects animals from lethal wild-type V. cholerae O139 challenge. The single dose LACV formulation was found to be stable at room temperature (25 ± 2 °C) for 140 days and it would result in significant cost savings during mass cholera vaccination campaigns.

Vibrio cholerae filamentation promotes chitin surface attachment at the expense of competition in biofilms

The human pathogen Vibrio cholerae, when not inside of a host, grows in cell clusters (biofilms) on pieces of detritus in aquatic environments. Here we discovered that some isolates of V. cholerae can change their shape from small comma-shaped cells to long filaments in seawater. This altered cell shape allows cells to make new types of biofilms, and provides an advantage in quickly colonizing particles in seawater, at the expense of longer-term competitive ability. The filamentous cell-shape strategy is particularly effective at competing in environments with quick turnover of chitin particles. This result showcases how bacterial cell shape can be coupled to environmental success during surface occupation, competition within biofilms, and dispersal to new resource patches.

Collective behavior in spatially structured groups, or biofilms, is the norm among microbes in their natural environments. Though biofilm formation has been studied for decades, tracing the mechanistic and ecological links between individual cell morphologies and the emergent features of cell groups is still in its infancy. Here we use single-cell–resolution confocal microscopy to explore biofilms of the human pathogen Vibrio cholerae in conditions mimicking its marine habitat. Prior reports have noted the occurrence of cellular filamentation in V. cholerae, with variable propensity to filament among both toxigenic and nontoxigenic strains. Using a filamenting strain of V. cholerae O139, we show that cells with this morphotype gain a profound competitive advantage in colonizing and spreading on particles of chitin, the material many marine Vibrio species depend on for growth in seawater. Furthermore, filamentous cells can produce biofilms that are independent of primary secreted components of the V. cholerae biofilm matrix; instead, filamentous biofilm architectural strength appears to derive at least in part from the entangled mesh of cells themselves. The advantage gained by filamentous cells in early chitin colonization and growth is countered in long-term competition experiments with matrix-secreting V. cholerae variants, whose densely packed biofilm structures displace competitors from surfaces. Overall, our results reveal an alternative mode of biofilm architecture that is dependent on filamentous cell morphology and advantageous in environments with rapid chitin particle turnover. This insight provides an environmentally relevant example of how cell morphology can impact bacterial fitness.

Cholera

Cholera is an acute, watery diarrhoeal disease caused by Vibrio cholerae of the O1 or O139 serogroups. In the past two centuries, cholera has emerged and spread from the Ganges Delta six times and from Indonesia once to cause global pandemics. Rational approaches to the case management of cholera with oral and intravenous rehydration therapy have reduced the case fatality of cholera from more than 50% to much less than 1%. Despite improvements in water quality, sanitation, and hygiene, as well as in the clinical treatment of cholera, the disease is still estimated to cause about 100 000 deaths every year. Most deaths occur in cholera-endemic settings, and virtually all deaths occur in developing countries. Contemporary understanding of immune protection against cholera, which results from local intestinal immunity, has yielded safe and protective orally administered cholera vaccines that are now globally stockpiled for use in the control of both epidemic and endemic cholera.

Quantifying Vibrio cholerae Enterotoxicity in a Zebrafish Infection Model

Vibrio cholerae is the etiological agent of cholera, an acute intestinal infection in humans characterized by voluminous watery diarrhea. Cholera is spread through ingestion of contaminated food or water, primarily in developing countries that lack the proper infrastructure for proper water and sewage treatment. Vibrio cholerae is an aquatic bacterium that inhabits coastal and estuarine areas, and it is known to have several environmental reservoirs, including fish. Our laboratory has recently described the use of the zebrafish as a new animal model for the study of V. cholerae intestinal colonization, pathogenesis, and transmission. As early as 6 h after exposure to V. cholerae, zebrafish develop diarrhea. Prior work in our laboratory has shown that this is not due to the action of cholera toxin. We hypothesize that accessory toxins produced by V. cholerae are the cause of diarrhea in infected zebrafish. In order to assess the effects of accessory toxins in the zebrafish, it was necessary to develop a method of quantifying diarrheal volume as a measure of pathogenesis. Here, we have adapted cell density, protein, and mucin assays, along with enumeration of V. cholerae in the zebrafish intestinal tract and in the infection water, to achieve this goal. Combined, these assays should help us determine which toxins have the greatest diarrheagenic effect in fish and, consequently, which toxins may play a role in environmental transmission.IMPORTANCE Identification of the accessory toxins that cause diarrhea in zebrafish can help us understand more about the role of fish in the wild as aquatic reservoirs for V. cholerae It is plausible that accessory toxins can act to prolong colonization and subsequent shedding of V. cholerae back into the environment, thus perpetuating and facilitating transmission during an outbreak. It is also possible that accessory toxins help to maintain low levels of intestinal colonization in fish, giving V. cholerae an advantage when environmental conditions are not optimal for survival in the water. Studies such as this one are critical because fish could be an overlooked source of cholera transmission in the environment.

Vaccines for viral and bacterial pathogens causing acute gastroenteritis: Part I: Overview, vaccines for enteric viruses and Vibrio cholerae

Efforts to develop vaccines for prevention of acute diarrhea have been going on for more than 40 y with partial success. The myriad of pathogens, more than 20, that have been identified as a cause of acute diarrhea throughout the years pose a significant challenge for selecting and further developing the most relevant vaccine candidates. Based on pathogen distribution as identified in epidemiological studies performed mostly in low-resource countries, rotavirus, Cryptosporidium, Shigella, diarrheogenic E. coli and V. cholerae are predominant, and thus the main targets for vaccine development and implementation. Vaccination against norovirus is most relevant in middle/high-income countries and possibly in resource-deprived countries, pending a more precise characterization of disease impact. Only a few licensed vaccines are currently available, of which rotavirus vaccines have been the most outstanding in demonstrating a significant impact in a short time period. This is a comprehensive review, divided into 2 articles, of nearly 50 vaccine candidates against the most relevant viral and bacterial pathogens that cause acute gastroenteritis. In order to facilitate reading, sections for each pathogen are organized as follows: i) a discussion of the main epidemiological and pathogenic features; and ii) a discussion of vaccines based on their stage of development, moving from current licensed vaccines to vaccines in advanced stage of development (in phase IIb or III trials) to vaccines in early stages of clinical development (in phase I/II) or preclinical development in animal models. In this first article we discuss rotavirus, norovirus and Vibrio cholerae. In the following article we will discuss Shigella, Salmonella (non-typhoidal), diarrheogenic E. coli (enterotoxigenic and enterohemorragic), and Campylobacter jejuni.

Immunizing adult female mice with a TcpA-A2-CTB chimera provides a high level of protection for their pups in the infant mouse model of cholera

Vibrio cholerae expresses two primary virulence factors, cholera toxin (CT) and the toxin-coregulated pilus (TCP). CT causes profuse watery diarrhea, and TCP (composed of repeating copies of the major pilin TcpA) is required for intestinal colonization by V. cholerae. Antibodies to CT or TcpA can protect against cholera in animal models. We developed a TcpA holotoxin-like chimera (TcpA-A2-CTB) to elicit both anti-TcpA and anti-CTB antibodies and evaluated its immunogenicity and protective efficacy in the infant mouse model of cholera. Adult female CD-1 mice were immunized intraperitoneally three times with the TcpA-A2-CTB chimera and compared with similar groups immunized with a TcpA+CTB mixture, TcpA alone, TcpA with Salmonella typhimurium flagellin subunit FliC as adjuvant, or CTB alone. Blood and fecal samples were analyzed for antigen-specific IgG or IgA, respectively, using quantitative ELISA. Immunized females were mated; their reared offspring were challenged orogastrically with 10 or 20 LD50 of V. cholerae El Tor N16961; and vaccine efficacy was assessed by survival of the challenged pups at 48 hrs. All pups from dams immunized with the TcpA-A2-CTB chimera or the TcpA+CTB mixture survived at both challenge doses. In contrast, no pups from dams immunized with TcpA+FliC or CTB alone survived at the 20 LD50 challenge dose, although the anti-TcpA or anti-CTB antibody level elicited by these immunizations was comparable to the corresponding antibody level achieved by immunization with TcpA-A2-CTB or TcpA+CTB. Taken together, these findings comprise strong preliminary evidence for synergistic action between anti-TcpA and anti-CTB antibodies in protecting mice against cholera. Weight loss analysis showed that only immunization of dams with TcpA-A2-CTB chimera or TcpA+CTB mixture protected their pups against excess weight loss from severe diarrhea. These data support the concept of including both TcpA and CTB as immunogens in development of an effective multivalent subunit vaccine against V. cholerae.

Preparation and immunogenicity of conjugate based on hydrazine-treated lipopolysaccharide antigen of Vibrio cholerae O139

A glycoconjugate construct was based on attachment of V. cholerae O139 hydrazine-treated lipopolysaccharide (LPS) to carboxylated bovine serum albumin (CBSA) via its amino group. The immunological properties of the glycoconjugate were tested using BALB/c mice, injected subcutaneously without any adjuvant three times at 2 weeks interval. The immunogenicity of the conjugate was estimated by enzyme-linked immunosorbent assay, testing of anti-LPS IgG, IgM, and IgA antibodies. The conjugate elicited a statistically significant increase of LPS-specific IgG levels in mice (p < 0.001). The specific anti-LPS IgG and IgA response after the second booster dose was significantly higher compared with reference and unconjugated detoxified LPS response. Antibodies elicited by the dLPS–CBSA conjugate were vibriocidal.

Vaccines against human diarrheal pathogens: current status and perspectives

Worldwide, nearly 1.7 billion people per year contract diarrheal infectious diseases (DID) and almost 760 000 of infections are fatal. DID are a major problem in developing countries where poor sanitation prevails and food and water may become contaminated by fecal shedding. Diarrhea is caused by pathogens such as bacteria, protozoans and viruses. Important diarrheal pathogens are Vibrio cholerae, Shigella spp. and rotavirus, which can be prevented with vaccines for several years. The focus of this review is on currently available vaccines against these three pathogens, and on development of new vaccines. Currently, various types of vaccines based on traditional (killed, live attenuated, toxoid or conjugate vaccines) and reverse vaccinology (DNA/mRNA, vector, recombinant subunit, plant vaccines) are in development or already available. Development of new vaccines demands high levels of knowledge, experience, budget, and time, yet promising new vaccines often fail in preclinical and clinical studies. Efficacy of vaccination also depends on the route of delivery, and mucosal immunization in particular is of special interest for preventing DID. Furthermore, adjuvants, delivery systems and other vaccine components are essential for an adequate immune response. These aspects will be discussed in relation to the improvement of existing and development of new vaccines against DID.

The state-of-the-art of approved and under-development cholera vaccines

Cholera remains a huge public health problem. Although in 1894, the first cholera vaccination was reported, an ideal vaccine that meets all the requirements of the WHO has not yet been produced. Among the different approaches used for cholera vaccination, attenuated vaccines represent a major category; these vaccines are beneficial in being able to induce a strong protective response after a single administration. However, they have possible negative effects on immunocompromised patient populations. Both the licensed CVD103-HgR and other vaccine approaches under development are detailed in this article, such as the Vibrio cholerae 638 vaccine candidate, Peru-15 or CholeraGarde(®) and the VA1.3, VA1.4, IEM 108 VCUSM2 and CVD 112 vaccine candidates. In another strategy, killed V. cholerae vaccines have been developed, including Dukoral(®), mORCAX(®) and Sanchol™. The killed vaccines are already sold, and they have successfully demonstrated their potential to protect populations in endemic areas or after natural disasters. However, these vaccines do not fulfill all the requirements of the WHO because they fail to confer long-term protection, are not suitable for children under two years, require more than a single dose and require a distribution chain with cold storage. Lastly, other vaccine strategies under development are summarized in this review. Among these strategies, vaccine candidates based on alternative drug delivery systems that have been reported lately in the literature are discussed, such as microparticles, proteoliposomes, LPS subunits, DNA vaccines and rice seeds containing toxin subunits. Preliminary results reported by many groups working on alternative delivery systems for cholera vaccines demonstrate the importance of new technologies in addressing old problems such as cholera. Although a fully ideal vaccine has not yet been designed, promising steps have been reported in the literature resulting in hope for the fight against cholera.

Lipopolysaccharide modifications of a cholera vaccine candidate based on outer membrane vesicles reduce endotoxicity and reveal the major protective antigen

The causative agent of the life-threatening gastrointestinal infectious disease cholera is the Gram-negative, facultative human pathogen Vibrio cholerae. We recently started to investigate the potential of outer membrane vesicles (OMVs) derived from V. cholerae as an alternative approach for a vaccine candidate against cholera and successfully demonstrated the induction of a long-lasting, high-titer, protective immune response upon immunization with OMVs using the mouse model. In this study, we present immunization data using lipopolysaccharide (LPS)-modified OMVs derived from V. cholerae, which allowed us to improve and identify the major protective antigen of the vaccine candidate. Our results indicate that reduction of endotoxicity can be achieved without diminishing the immunogenic potential of the vaccine candidate by genetic modification of lipid A. Although the protective potential of anti-LPS antibodies has been suggested many times, this is the first comprehensive study that uses defined LPS mutants to characterize the LPS-directed immune response of a cholera vaccine candidate in more detail. Our results pinpoint the O antigen to be the essential immunogenic structure and provide a protective mechanism based on inhibition of motility, which prevents a successful colonization. In a detailed analysis using defined antisera, we can demonstrate that only anti-O antigen antibodies, but not antibodies directed against the major flagellar subunit FlaA or the most abundant outer membrane protein, OmpU, are capable of effectively blocking the motility by binding to the sheathed flagellum and provide protection in a passive immunization assay.

New-generation vaccines against cholera

Cholera is a major global health problem, causing approximately 100,000 deaths annually, about half of which occur in sub-Saharan Africa. Although early-generation parenteral cholera vaccines were abandoned as public health tools owing to their limited efficacy, newer-generation oral cholera vaccines have attractive safety and protection profiles. Both killed and live oral vaccines have been licensed, although only killed oral vaccines are currently manufactured and available. These killed oral vaccines not only provide direct protection to vaccinated individuals, but also confer herd immunity. The combination of direct vaccine protection and vaccine herd immunity effects makes these vaccines highly cost-effective and, therefore, attractive for use in developing countries. Administration of these oral vaccines does not require qualified medical personnel, which makes their use practical--even in developing countries. Although new-generation oral cholera vaccines should not be considered in isolation from other preventive approaches, especially improved water quality and sanitation, they represent important tools in the public health armamentarium to control both endemic and epidemic cholera.

The utility of human challenge studies in vaccine development: lessons learned from cholera

Experiments in which virulent infectious organisms are administered to healthy adult volunteers with the intent to deliberately induce infection have been practiced for centuries. Many useful applications have developed from these experiments such as the provision of evidence of microbial pathogenicity and the identification of key virulence factors. Challenge studies have also played an important role in the evaluation of preliminary efficacy of potential vaccine candidates. Over the past 40 years, these experimental human challenge studies have found particular utility with regards to the development of both living and nonliving attenuated cholera vaccines. This review highlights some of the important contributions made by these challenge studies to cholera vaccine research.

Natural cholera infection-derived immunity in an endemic setting

BACKGROUND

Live oral cholera vaccines may protect against cholera in a manner similar to natural cholera infections. However, information on which to base these vaccines is limited.

METHODS

The study was conducted in a cholera-endemic population in Bangladesh. Patients with cholera (index patients) detected between 1991 and 2000 were age-matched to 4 cholera-free controls and then followed up during the subsequent 3 years.

RESULTS

El Tor cholera was associated with a 65% (95% confidence interval [CI], 37%-81%; P < .001) lower risk of a subsequent El Tor episode. Reduction of the risk of subsequent El Tor cholera was similar for children < 5 years and for older persons and was sustained during all 3 years of follow-up. Having El Tor Inaba cholera was associated with lower risks of both El Tor Inaba and El Tor Ogawa cholera, but having El Tor Ogawa cholera was associated only with a reduced risk of El Tor Ogawa cholera. O139 cholera was associated with a 63% (95% CI, -61% to 92%; P = .18) lower risk of subsequent O139 cholera, but there was no evidence of cross-protection between the O1 and O139 serogroups.

CONCLUSIONS

Live oral cholera vaccines designed to protect against the O1 and O139 serogroups should contain at least the Inaba serotype and strains of both serogroups.

Oral vaccines against cholera

The current seventh pandemic of cholera, caused by serogroup O1, El Tor biotype, has now involved almost the entire developing world. The ongoing dynamic epidemiology of cholera, involving evolution of new strains, prolonged and more frequent epidemics, increased antimicrobial resistance, and awareness of the role of climate change upon the global burden has returned cholera to the forefront of global public health discussions. Improved water and sanitation should continue to be the mainstays of cholera-prevention efforts, but major improvements are a far-off goal for much of the cholera-affected developing world. The advent of safe and effective, new-generation oral vaccines against cholera has created renewed interest in the use of vaccines as a tool to control cholera.

Vibrio cholerae: lessons for mucosal vaccine design

The ability of Vibrio cholerae to persist in bodies of water will continue to confound our ability to eradicate cholera through improvements to infrastructure, and thus cholera vaccines are needed. We aim for an inexpensive vaccine that can provide long-lasting protection from all epidemic cholera infections, currently caused by O1 or O139 serogroups. Recent insights into correlates of protection, epidemiology and pathogenesis may help us design improved vaccines. This notwithstanding, we have come to appreciate that even marginally protective vaccines, such as oral whole-cell killed vaccines, if widely distributed, can provide significant protection, owing to herd immunity. Further efforts are still required to provide more effective protection of young children.

Oral vaccines for preventing cholera

EDITORIAL NOTE

This review is superseded by the published Cochrane Review, Saif‐Ur‐Rahman 2024 [https://doi.org/10.1002/14651858.CD014573], which considers only the oral killed vaccines because the live oral vaccines do not have World Health Organization (WHO) prequalification. Saif‐Ur‐Rahman 2024 also considered only currently available WHO pre‐qualified oral killed cholera vaccines (Dukoral, Shanchol, and Euvichol/Euvichol‐Plus).

BACKGROUND

Cholera is a cause of acute watery diarrhoea which can cause dehydration and death if not adequately treated. It usually occurs in epidemics, and is associated with poverty and poor sanitation. Effective, cheap, and easy to administer vaccines could help prevent epidemics.

OBJECTIVES

To assess the effectiveness and safety of oral cholera vaccines in preventing cases of cholera and deaths from cholera.

SEARCH STRATEGY

In October 2010, we searched the Cochrane Infectious Disease Group Specialized Register; Cochrane Central Register of Controlled Trials (CENTRAL); MEDLINE; EMBASE; LILACS; the metaRegister of Controlled Trials (mRCT), and the WHO International Clinical Trials Registry Platform (ICTRP) for relevant published and ongoing trials.

SELECTION CRITERIA

Randomized or quasi-randomized controlled trials of oral cholera vaccines in healthy adults and children.

DATA COLLECTION AND ANALYSIS

Each trial was assessed for eligibility and risk of bias by two authors working independently. Data was extracted by two independent reviewers and analysed using the Review Manager 5 software. Outcomes are reported as vaccine protective efficacy (VE) with 95% confidence intervals (CIs).

MAIN RESULTS

Seven large efficacy trials, four small artificial challenge studies, and twenty-nine safety trials contributed data to this review.Five variations of a killed whole cell vaccine have been evaluated in large scale efficacy trials (four trials, 249935 participants). The overall vaccine efficacy during the first year was 52% (95% CI 35% to 65%), and during the second year was 62% (95% CI 51% to 62%). Protective efficacy was lower in children aged less than 5 years; 38% (95% CI 20% to 53%) compared to older children and adults; 66% (95% CI 57% to 73%).One trial of a killed whole cell vaccine amongst military recruits demonstrated 86% protective efficacy (95% CI 37% to 97%) in a small epidemic occurring within 4 weeks of the 2-dose schedule (one trial, 1426 participants). Efficacy data is not available beyond two years for the currently available vaccine formulations, but based on data from older trials is unlikely to last beyond three years.The safety data available on killed whole cell vaccines have not demonstrated any clinically significant increase in adverse events compared to placebo.Only one live attenuated vaccine has reached Phase III clinical evaluation and was not effective (one trial, 67508 participants). Two new candidate live attenuated vaccines have demonstrated clinical effectiveness in small artificial challenge studies, but are still in development.

AUTHORS' CONCLUSIONS

The currently available oral killed whole cell vaccines can prevent 50 to 60% of cholera episodes during the first two years after the primary vaccination schedule. The impact and cost-effectiveness of adopting oral cholera vaccines into the routine vaccination schedule of endemic countries will depend on the prevalence of cholera, the frequency of epidemics, and access to basic services providing rapid rehydration therapy.

Vibrio cholerae hemolysin is required for lethality, developmental delay, and intestinal vacuolation in Caenorhabditis elegans

BACKGROUND

Cholera toxin (CT) and toxin-co-regulated pili (TCP) are the major virulence factors of Vibrio cholerae O1 and O139 strains that contribute to the pathogenesis of disease during devastating cholera pandemics. However, CT and TCP negative V. cholerae strains are still able to cause severe diarrheal disease in humans through mechanisms that are not well understood.

METHODOLOGY/PRINCIPAL FINDINGS

To determine the role of other virulence factors in V. cholerae pathogenesis, we used a CT and TCP independent infection model in the nematode Caenorhabditis elegans and identified the hemolysin A (hlyA) gene as a factor responsible for animal death and developmental delay. We demonstrated a correlation between the severity of infection in the nematode and the level of hemolytic activity in the V. cholerae biotypes. At the cellular level, V. cholerae infection induces formation of vacuoles in the intestinal cells in a hlyA dependent manner, consistent with the previous in vitro observations.

CONCLUSIONS/SIGNIFICANCE

Our data strongly suggest that HlyA is a virulence factor in C. elegans infection leading to lethality and developmental delay presumably through intestinal cytopathic changes.

Vibrio cholerae cytolysin causes an inflammatory response in human intestinal epithelial cells that is modulated by the PrtV protease

Background

Vibrio cholerae is the causal intestinal pathogen of the diarrheal disease cholera. It secretes the protease PrtV, which protects the bacterium from invertebrate predators but reduces the ability of Vibrio-secreted factor(s) to induce interleukin-8 (IL-8) production by human intestinal epithelial cells. The aim was to identify the secreted component(s) of V. cholerae that induces an epithelial inflammatory response and to define whether it is a substrate for PrtV.

Methodology/Principal Findings

Culture supernatants of wild type V. cholerae O1 strain C6706, its derivatives and pure V. cholerae cytolysin (VCC) were analyzed for the capacity to induce changes in cytokine mRNA expression levels, IL-8 and tumor necrosis factor-α (TNF-α) secretion, permeability and cell viability when added to the apical side of polarized tight monolayer T84 cells used as an in vitro model for human intestinal epithelium. Culture supernatants were also analyzed for hemolytic activity and for the presence of PrtV and VCC by immunoblot analysis.

Conclusions/Significance

We suggest that VCC is capable of causing an inflammatory response characterized by increased permeability and production of IL-8 and TNF-α in tight monolayers. Pure VCC at a concentration of 160 ng/ml caused an inflammatory response that reached the magnitude of that caused by Vibrio-secreted factors, while higher concentrations caused epithelial cell death. The inflammatory response was totally abolished by treatment with PrtV. The findings suggest that low doses of VCC initiate a local immune defense reaction while high doses lead to intestinal epithelial lesions. Furthermore, VCC is indeed a substrate for PrtV and PrtV seems to execute an environment-dependent modulation of the activity of VCC that may be the cause of V. cholerae reactogenicity.

Characterization of Vibrio cholerae outer membrane vesicles as a candidate vaccine for cholera

Outer membrane vesicles (OMVs) offer a new approach for an effective cholera vaccine. We recently demonstrated that immunization of female mice with OMVs induces a long-lasting immune response and results in protection of their neonatal offspring from Vibrio cholerae intestinal colonization. This study investigates the induced protective immunity observed after immunization with OMVs in more detail. Analysis of the stomach contents and sera of the neonates revealed significant amounts of anti-OMV immunoglobulins (Igs). Swapping of litters born to immunized and nonvaccinated control mice allowed us to distinguish between prenatal and neonatal uptakes of Igs. Transfer of Igs to neonates via milk was sufficient for complete protection of the neonates from colonization with V. cholerae, while prenatal transfer alone reduced colonization only. Detection of IgA and IgG1 in the fecal pellets of intranasally immunized adult mice indicates an induced immune response at the mucosal surface in the gastrointestinal tract, which is the site of colonization by V. cholerae. When a protocol with three intranasal immunizations 14 days apart was used, the OMVs proved to be efficacious at doses as low as 0.025 microg per immunization. This is almost equivalent to OMV concentrations found naturally in the supernatants of LB-grown cultures of V. cholerae. Heterologous expression of the periplasmic alkaline phosphatase (PhoA) of Escherichia coli resulted in the incorporation of PhoA into OMVs derived from V. cholerae. Intranasal immunization with OMVs loaded with PhoA induced a specific immune response against this heterologous antigen in mice. The detection of an immune response against this heterologously expressed protein is a promising step toward the potential use of OMVs as antigen delivery vehicles in vaccine design.

Immunization with Vibrio cholerae outer membrane vesicles induces protective immunity in mice

The gram-negative bacterium Vibrio cholerae releases outer membrane vesicles (OMVs) during growth. In this study, we immunized female mice by the intranasal, intragastric, or intraperitoneal route with purified OMVs derived from V. cholerae. Independent of the route of immunization, mice induced specific, high-titer immune responses of similar levels against a variety of antigens present in the OMVs. After the last immunization, the half-maximum total immunoglobulin titer was stable over a 3-month period, indicating that the immune response was long lasting. The induction of specific isotypes, however, was dependent on the immunization route. Immunoglobulin A, for example, was induced to a significant level only by mucosal immunization, with the intranasal route generating the highest titers. We challenged the offspring of immunized female mice with V. cholerae via the oral route in two consecutive periods, approximately 30 and 95 days after the last immunization. Regardless of the route of immunization, the offspring was protected against colonization with V. cholerae in both challenge periods. Our results show that mucosal immunizations via both routes with OMVs derived from V. cholerae induce long-term protective immune responses against this gastrointestinal pathogen. These findings may contribute to the development of "nonliving," OMV-based vaccines against V. cholerae and other enteric pathogens, using the oral or intranasal route of immunization.

Prolonged colonization of mice by Vibrio cholerae El Tor O1 depends on accessory toxins

Cholera epidemics caused by Vibrio cholerae El Tor O1 strains are typified by a large number of asymptomatic carriers who excrete vibrios but do not develop diarrhea. This carriage state was important for the spread of the seventh cholera pandemic as the bacterium was mobilized geographically, allowing the global dispersion of this less virulent strain. Virulence factors associated with the development of the carriage state have not been previously identified. We have developed an animal model of cholera in adult C57BL/6 mice wherein V. cholerae colonizes the mucus layer and forms microcolonies in the crypts of the distal small bowel. Colonization occurred 1 to 3 h after oral inoculation and peaked at 10 to 12 h, when bacterial loads exceeded the inoculum by 10- to 200-fold, indicating bacterial growth within the small intestine. After a clearance phase, the number of bacteria within the small intestine, but not those in the cecum or colon, stabilized and persisted for at least 72 h. The ability of V. cholerae to prevent clearance and establish this prolonged colonization was associated with the accessory toxins hemolysin, the multifunctional autoprocessing RTX toxin, and hemagglutinin/protease and did not require cholera toxin or toxin-coregulated pili. The defect in colonization attributed to the loss of the accessory toxins may be extracellularly complemented by inoculation of the defective strain with an isogenic colonization-proficient V. cholerae strain. This work thus demonstrates that secreted accessory toxins modify the host environment to enable prolonged colonization of the small intestine in the absence of overt disease symptoms and thereby contribute to disease dissemination via asymptomatic carriers.

Hemolysin and the multifunctional autoprocessing RTX toxin are virulence factors during intestinal infection of mice with Vibrio cholerae El Tor O1 strains

The seventh cholera pandemic that started in 1961 was caused by Vibrio cholerae O1 strains of the El Tor biotype. These strains produce the pore-forming toxin hemolysin, a characteristic used clinically to distinguish classical and El Tor biotypes. Even though extensive in vitro data on the cytolytic activities of hemolysin exist, the connection of hemolysin to virulence in vivo is not well characterized. To study the contribution of hemolysin and other accessory toxins to pathogenesis, we utilized the model of intestinal infection in adult mice sensitive to the actions of accessory toxins. In this study, we showed that 4- to 6-week-old streptomycin-fed C57BL/6 mice were susceptible to intestinal infection with El Tor strains, which caused rapid death at high doses. Hemolysin had the predominant role in lethality, with a secondary contribution by the multifunctional autoprocessing RTX (MARTX) toxin. Cholera toxin and hemagglutinin/protease did not contribute to lethality in this model. Rapid death was not caused by increased dissemination due to a damaged epithelium since the numbers of CFU recovered from spleens and livers 6 h after infection did not differ between mice inoculated with hemolysin-expressing strains and those infected with non-hemolysin-expressing strains. Although accessory toxins were linked to virulence, a strain defective in the production of accessory toxins was still immunogenic since mice immunized with a multitoxin-deficient strain were protected from a subsequent lethal challenge with the wild type. These data suggest that hemolysin and MARTX toxin contribute to vaccine reactogenicity but that the genes for these toxins can be deleted from vaccine strains without affecting vaccine efficacy.

Live attenuated oral cholera vaccines

Live, orally administered, attenuated vaccine strains of Vibrio cholerae have many theoretical advantages over killed vaccines. A single oral inoculation could result in intestinal colonization and rapid immune responses, obviating the need for repetitive dosing. Live V. cholerae organisms can also respond to the intestinal environment and immunological exposure to in vivo expressed bacterial products, which could result in improved immunological protection against wild-type V. cholerae infection. The concern remains that live oral cholera vaccines may be less effective among partially immune individuals in cholera endemic areas as pre-existing antibodies can inhibit live organisms and decrease colonization of the gut. A number of live oral cholera vaccines have been developed to protect against cholera caused by the classical and El Tor serotypes of V. cholerae O1, including CVD 103-HgR, Peru-15 and V. cholerae 638. A number of live oral cholera vaccines have also been similarly developed to protect against cholera caused by V. cholerae O139, including CVD 112 and Bengal-15. Live, orally administered, attenuated cholera vaccines are in various stages of development and evaluation.

A Vibrio cholerae protease needed for killing of Caenorhabditis elegans has a role in protection from natural predator grazing

Vibrio cholerae is the causal bacterium of the diarrheal disease cholera, and its growth and survival are thought to be curtailed by bacteriovorous predators, e.g., ciliates and flagellates. We explored Caenorhabditis elegans as a test organism after finding that V. cholerae can cause lethal infection of this nematode. By reverse genetics we identified an extracellular protease, the previously uncharacterized PrtV protein, as being necessary for killing. The killing effect is associated with the colonization of bacteria within the Caenorhabditis elegans intestine. We also show that PrtV is essential for V. cholerae in the bacterial survival from grazing by the flagellate Cafeteria roenbergensis and the ciliate Tetrahymena pyriformis. The PrtV protein appears to have an indirect role in the interaction of V. cholerae with mammalian host cells as judged from tests with tight monolayers of human intestinal epithelial cells. Our results demonstrate a key role for PrtV in V. cholerae interaction with grazing predators, and we establish Caenorhabditis elegans as a convenient organism for identification of V. cholerae factors involved in host interactions and environmental persistence.

Construction and evaluation of a O139 Vibrio cholerae vaccine candidate based on a hemA gene mutation

In this paper, we describe the development of VCUSM2, a live metabolic auxotroph of Vibrio cholerae O139. Auxotrophy was achieved by mutating a house keeping gene, hemA, that encodes for glutamyl-tRNA reductase, an important enzyme in the C5 pathway for delta-aminolevulenic acid (ALA) biosynthesis, which renders this strain dependent on exogenous ALA for survival. Experiments using the infant mouse and adult rabbit models show that VCUSM2 is a good colonizer of the small intestine and elicits greater than a four-fold rise in vibriocidal antibodies in vaccinated rabbits. Rabbits vaccinated with VCUSM2 were fully protected against subsequent challenge with 1 x 10(11) CFU of the virulent wild type (WT) strain. Experiments using ligated ileal loops of rabbits show that VCUSM2 is 2.5-fold less toxic at the dose of 1 x 10(6) CFU compared to the WT strain. Shedding of VCUSM2 in rabbits were found to occur for no longer than 4 days and its maximum survival rate in environmental waters is 8 days compared to the greater than 20 days for the WT strain. VCUSM2 is thus a potential vaccine candidate against infection by V. cholerae O139.

Peptides mimicking Vibrio cholerae O139 capsular polysaccharide elicit protective antibody response

Vibrio cholerae is the etiological agent of cholera. V. cholerae serogroup O1 had been, until 1992, the only serogroup responsible for large epidemics and pandemics of cholera. In 1992, a new serotype of V. cholerae emerged in South-East Asia that caused a massive outbreak of cholera in India and neighboring countries. The new serotype was named V. cholerae O139. The main differences between V. cholerae O139 and O1 are that the former possesses a capsular polysaccharide and different lipopolysaccharide. Capsular polysaccharides are, in general, T-independent antigens giving rise to poor immune responses lacking immunological memory. In order to overcome this, monoclonal antibodies against the capsular polysaccharide of V. cholerae O139 were used to screen different phage-displayed random peptide libraries. Eight different phage clones were selected and characterized using enzyme immunoassay with the monoclonal antibodies, and then tested for specificity by competition with V. cholerae O139 capsular polysaccharide. Selected peptides were sequenced, synthesized and conjugated to bovine serum albumin (BSA) and keyhole limpet hemocyanin (KLH). The conjugated peptides were used to immunize mice. It is evident that the anti-peptide mouse antibodies bind to the V. cholerae O139 capsular polysaccharide. In addition, the anti-peptide antibodies are protective in a suckling mouse model. The protective efficacy is both specific and dose-dependent. A PCT (PCT/IT2003/000489) with the publication number WO 2004/056851 has been filed for the sequences of the eight peptides.

Reduction in capsular content and enhanced bacterial susceptibility to serum killing of Vibrio cholerae O139 associated with the 2002 cholera epidemic in Bangladesh

Vibrio cholerae O139 emerged in 1992 as a major cause of epidemic cholera. However, the incidence of disease due to this new serogroup subsequently decreased for almost a decade. In April 2002, there was a dramatic resurgence of V. cholerae O139 in Bangladesh. We compared the phenotypic properties of the bacterial isolates and the immunological responses in patients with disease due to V. cholerae O139 during the 2002 epidemic with those dating to the emergence of this disease in 1993 to 1995. Strains isolated from patients in the two time periods were compared with respect to capsular polysaccharide, their resistance to the bactericidal effect of serum, and their capacity to be used as target strains in complement-mediated vibriocidal assays. Phase-contrast microscopy showed that strains isolated in 2002 had less capsular material than those isolated from 1993 to 1995 (P = <0.001), a finding confirmed by electron microscopic studies. Strains isolated in 2002 were more susceptible to the bactericidal activity of serum compared to strains from 1993 to 1995 (P = 0.013). Compared to results using a standard O139 strain, a modified vibriocidal assay utilizing a 2002 strain, CIRS 134, as the target organism detected higher vibriocidal responses in both O139-infected cholera patients as well as O139 vaccine recipients. The vibriocidal assay utilizing the less encapsulated 2002 strain, CIRS 134, is a more sensitive indicator of adaptive immune responses to recent infection with V. cholerae O139. Consequently, this assay may be useful in studies of both O139-infected patients and recipients of O139 vaccines.

TcpF is a soluble colonization factor and protective antigen secreted by El Tor and classical O1 and O139 Vibrio cholerae serogroups

Vibrio cholerae causes diarrhea by colonizing the human small bowel and intoxicating epithelial cells. Colonization is a required step in pathogenesis, and strains defective for colonization are significantly attenuated. The best-characterized V. cholerae colonization factor is the toxin-coregulated pilus (TCP). It has been demonstrated that TCP is required for V. cholerae colonization in both humans and mice. TCP enhances bacterial interactions that allow microcolony formation and thereby promotes survival in the intestine. We have recently discovered that the TCP biogenesis apparatus also serves as a secretion system, mediating the terminal step in the extracellular secretion pathway of TcpF. TcpF was identified in classical isolates of V. cholerae O1 as a soluble factor essential for colonization in the infant mouse cholera model. In the present study, we expanded our analysis of TcpF to include the O1 El Tor and O139 serogroups and investigated how TCP and TcpF act together to mediate colonization. Additionally, we demonstrated that antibodies generated against TcpF are protective against experimental V. cholerae infection in the infant mouse cholera model. This observation, coupled with the fact that TcpF is a potent mediator of colonization, suggests that TcpF should be considered as a component of a polyvalent cholera vaccine formulation.

Transcriptional responses of intestinal epithelial cells to infection with Vibrio cholerae

Vibrio cholerae is a noninvasive enteric bacterium that causes the severe diarrheal disease cholera. Candidate cholera vaccines have been engineered by deleting genes encoding known virulence factors in V. cholerae; however, many of these attenuated strains were still reactogenic in human volunteers. In this study, DNA arrays were utilized to monitor the transcriptional responses of human intestinal epithelial cells (T84) to eight strains of V. cholerae, including attenuated, toxigenic, and environmental isolates. cDNA probes generated from host RNA samples were hybridized against low- and high-density gene arrays. V. cholerae induced the transcription of a variety of host genes and repressed the expression of a lower number of genes. Expression patterns were confirmed for certain genes by reverse transcriptase PCR and enzyme-linked immunosorbent assays. A core subset of genes was found to be differentially regulated in all experiments. These genes included genes involved in innate mucosal immunity, intracellular signaling, and cellular proliferation. Reactogenic vaccine strains induced greater expression of genes for certain proinflammatory cytokines than nonreactogenic strains. Wild-type and attenuated derivatives induced and repressed many genes in common, although there were differences in the transcription profiles. These results indicate that the types of host genes modulated by attenuated V. cholerae, and the extent of their induction, may mediate the symptoms seen with reactogenic cholera vaccine strains.

Induction of interleukin-8 in T84 cells by Vibrio cholerae

The induction of interleukin-8 (IL-8) in vitro has been suggested to correlate with the reactogenicity of Vibrio cholerae vaccine candidates. V. cholerae vaccine candidate 638, a hemagglutinin protease/hap-defective strain, was recently reported to be well tolerated in human volunteers, suggesting a role for Hap in reactogenicity. We examined the role of hap in the induction of IL-8 in intestinal epithelial T84 cells. Wild-type V. cholerae strains 3038 and C7258 and a vaccine candidate strain, JBK70, induced levels of IL-8 similar to those of their isogenic hap mutants. Supernatant containing Hap did not stimulate IL-8 production at a variety of concentrations tested, suggesting that Hap itself does not induce IL-8 production. Furthermore, supernatant from CVD115, which had deletions of hap and rtxA (encoding repeats in toxin) and was derived from a reactogenic strain, CVD110, induced IL-8 production in T84 cells in a dose-dependent manner. The IL-8-stimulating activity of CVD115 culture supernatants was growth phase dependent and was strongest in stationary phase cultures. This IL-8 stimulator(s) was resistant to heat treatment but sensitive to proteinase. Protease activity in vitro did not correlate with the reactogenicity of V. cholerae vaccine candidates. Our data suggest that Hap is not an IL-8 inducer in T84 cells and that the IL-8 stimulator in the supernatant of V. cholerae culture may play a role in reactogenicity.

Cholera

Intestinal infection with Vibrio cholerae results in the loss of large volumes of watery stool, leading to severe and rapidly progressing dehydration and shock. Without adequate and appropriate rehydration therapy, severe cholera kills about half of affected individuals. Cholera toxin, a potent stimulator of adenylate cyclase, causes the intestine to secrete watery fluid rich in sodium, bicarbonate, and potassium, in volumes far exceeding the intestinal absorptive capacity. Cholera has spread from the Indian subcontinent where it is endemic to involve nearly the whole world seven times during the past 185 years. V cholerae serogroup O1, biotype El Tor, has moved from Asia to cause pandemic disease in Africa and South America during the past 35 years. A new serogroup, O139, appeared in south Asia in 1992, has become endemic there, and threatens to start the next pandemic. Research on case management of cholera led to the development of rehydration therapy for dehydrating diarrhoea in general, including the proper use of intravenous and oral rehydration solutions. Appropriate case management has reduced deaths from diarrhoeal disease by an estimated 3 million per year compared with 20 years ago. Vaccination was thought to have no role for cholera, but new oral vaccines are showing great promise.

A constitutively active variant of the quorum-sensing regulator LuxO affects protease production and biofilm formation in Vibrio cholerae

Vibrio cholerae normally inhabits aquatic habitats but can cause a severe diarrheal illness in humans. Its arsenal of virulence factors includes a secreted hemagglutinin (HA) protease. An HA protease-deficient mutant of V. cholerae was isolated and designated E7946 mpc. E7946 mpc was found to contain a point mutation in the luxO quorum-sensing regulator. In accordance with this finding, E7946 mpc exhibits a defect in quorum sensing. The mutant luxO allele [luxO(Con)] produces a protein with a leucine-to-glutamine substitution at amino acid 104. Transfer of the luxO(Con) allele to an otherwise wild-type background was sufficient to eliminate HA protease expression; conversely, deletion of luxO(Con) from E7946 mpc restored protease activity. We demonstrate that LuxO(Con) constitutively represses the transcription of hapR, an essential positive regulator of HA protease. Interestingly, strains harboring luxO(Con) form enhanced biofilms, and enhanced biofilm formation does not appear to be dependent on reduced HA protease expression. Taken together, the results confirm the role of LuxO as a central "switch" that coordinately regulates virulence-related phenotypes such as protease production and biofilm formation.

Vibrio cholerae O139 Bengal: odyssey of a fortuitous variant

Vibrio cholerae O139, the new serogroup associated with epidemic cholera, came into being in the second half of the year 1992 in an explosive fashion and was responsible for several outbreaks in India and other neighbouring countries. This was an unprecedented event in the history of cholera and the genesis of the O139 serogroup was, at that time, thought to be the beginning of the next or the eighth pandemic of cholera. However, with the passage of time, the O1 serogroup of the El Tor biotype again reappeared and displaced the O139 serogroup on the Indian subcontinent, and there was a feeling among cholera workers that the appearance of this new serogroup may have been a one-time event. The resurgence of the O139 serogroup in September 1996 in Calcutta and the coexistence of both the O1 and O139 serogroups in much of the cholera endemic areas in India and elsewhere, suggested that the O139 serogroup has come to stay and is a permanent entity to contend with in the coming years. During the past 10 years, intensive work on all aspects of the O139 serogroup was carried out by cholera researchers around the world. The salient findings on this serogroup over the past 10 years pertinent to its prevalence, clinico-epidemiological features, virulence-associated genes, rapid screening and identification, molecular epidemiology, and vaccine developments have been highlighted.

Comparison of mucosal and systemic humoral immune responses after transcutaneous and oral immunization strategies

In order to compare the ability of transcutaneous and oral immunization strategies to induce mucosal and systemic immune responses, we inoculated mice transcutaneously with cholera toxin (CT) or the non-toxic B subunit of cholera toxin (CtxB), or orally with Peru2(pETR1), an attenuated vaccine strain of Vibrio cholerae expressing CtxB. In addition, we also evaluated dual immunization regimens (oral inoculation with transcutaneous boosting, and transcutaneous immunization with oral boosting) in an attempt to optimize induction of both mucosal and systemic immune responses. We found that transcutaneous immunization with purified CtxB or CT induces much more prominent systemic IgG anti-CtxB responses than does oral inoculation with a vaccine vector strain of V. cholerae expressing CtxB. In comparison, anti-CtxB IgA in serum, stool and bile were comparable in mice either transcutaneously or orally immunized. Overall, the most prominent systemic and mucosal anti-CtxB responses occurred in mice that were orally primed with Peru2(pETR1) and transcutaneously boosted with CT. Our results suggest that combination oral and transcutaneous immunization strategies may most prominently induce both mucosal and systemic humoral responses.

Sensitive microplate assay for detection of bactericidal antibodies to Vibrio cholerae O139

A microplate assay for the detection of bactericidal antibodies to Vibrio cholerae O139 is described. The assay is sensitive, highly reproducible, specific, and convenient to perform. It has been used to demonstrate the induction of serum bactericidal antibodies in Vietnamese recipients of an oral, inactivated, bivalent O1/O139 vaccine, as well as in Bangladeshi patients with O139 disease. In both study groups there was a significant inverse correlation between the preexposure level of antibodies in serum and the magnitude of the subsequent bactericidal response. Although infection generated stronger responses than vaccination, the proportion of responders was similar among individuals with low background titers.

A human volunteer challenge model using frozen bacteria of the new epidemic serotype, V. cholerae O139 in Thai volunteers

A total of 35 volunteers were recruited for an IRB-approved inpatient dose-escalation challenge. The goal was to identify a dose that produced an observed cholera attack rate > or =80% and an illness of sufficient severity during the defined study period such that the model would be useful for determining vaccine protection. Volunteers were challenged in groups of 5 with V. cholerae O139 that had been reconstituted immediately before use. Only 2 out of 5 volunteers who received the lowest dose (4.3 x 10(4) cfu) had diarrhea. As the inoculum size increased, the attack rate of diarrhea increased to 3-4 of 5 volunteers. At the highest dose tested, approximately 5 x 10(7) cfu, the attack rate was 73%. We recommend the use of frozen V. Cholera O139 in a human experimental challenge model to assess cholera vaccine efficacy (VE) in a cholera endemic area but with 4 days observation period before initiation of tetracycline to allow assessment of severity.

Active immunization in the United States: developments over the past decade

The Centers for Disease Control and Prevention has identified immunization as the most important public health advance of the 20th century. The purpose of this article is to review the changes that have taken place in active immunization in the United States over the past decade. Since 1990, new vaccines have become available to prevent five infectious diseases: varicella, rotavirus, hepatitis A, Lyme disease, and Japanese encephalitis virus infection. Improved vaccines have been developed to prevent Haemophilus influenzae type b, pneumococcus, pertussis, rabies, and typhoid infections. Immunization strategies for the prevention of hepatitis B, measles, meningococcal infections, and poliomyelitis have changed as a result of the changing epidemiology of these diseases. Combination vaccines are being developed to facilitate the delivery of multiple antigens, and improved vaccines are under development for cholera, influenza, and meningococcal disease. Major advances in molecular biology have enabled scientists to devise new approaches to the development of vaccines against diseases ranging from respiratory viral to enteric bacterial infections that continue to plague the world's population.