Safety, immunogenicity, and efficacy against cholera challenge in humans of a typhoid-cholera hybrid vaccine derived from Salmonella typhi Ty21a

The JMU-SalVac-System: A Novel, Versatile Approach to Oral Live Vaccine Development

Salmonella enterica Serovar Typhi Ty21a (Ty21a) is the only licensed oral vaccine against typhoid fever. Due to its excellent safety profile, it has been used as a promising vector strain for the expression of heterologous antigens for mucosal immunization. As the efficacy of any bacterial live vector vaccine correlates with its ability to express and present sufficient antigen, the genes for antigen expression are traditionally located on plasmids with antibiotic resistance genes for stabilization. However, for use in humans, antibiotic selection of plasmids is not applicable, leading to segregational loss of the antigen-producing plasmid. Therefore, we developed an oral Ty21a-based vaccine platform technology, the JMU-SalVac-system (Julius-Maximilians-Universität Würzburg) in which the antigen delivery plasmids (pSalVac-plasmid-series) are stabilized by a ΔtyrS/tyrS+-based balanced-lethal system (BLS). The system is made up of the chromosomal knockout of the essential tyrosyl-tRNA-synthetase gene (tyrS) and the in trans complementation of tyrS on the pSalVac-plasmid. Further novel functional features of the pSalVac-plasmids are the presence of two different expression cassettes for the expression of protein antigens. In this study, we present the construction of vaccine strains with BLS plasmids for antigen expression. The expression of cytosolic and secreted mRFP and cholera toxin subunit B (CTB) proteins as model antigens is used to demonstrate the versatility of the approach. As proof of concept, we show the induction of previously described in vivo inducible promoters cloned into pSalVac-plasmids during infection of primary macrophages and demonstrate the expression of model vaccine antigens in these relevant human target cells. Therefore, antigen delivery strains developed with the JMU-SalVac technology are promising, safe and stable vaccine strains to be used against mucosal infections in humans.

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.

Strategies for Enhancement of Live-Attenuated Salmonella-Based Carrier Vaccine Immunogenicity

The use of live-attenuated bacterial vaccines as carriers for the mucosal delivery of foreign antigens to stimulate the mucosal immune system was first proposed over three decades ago. This novel strategy aimed to induce immunity against at least two distinct pathogens using a single bivalent carrier vaccine. It was first tested using a live-attenuated Salmonella enterica serovar Typhi strain in clinical trials in 1984, with excellent humoral immune responses against the carrier strain but only modest responses elicited against the foreign antigen. Since then, clinical trials with additional Salmonella-based carrier vaccines have been conducted. As with the original trial, only modest foreign antigen-specific immunity was achieved in most cases, despite the incorporation of incremental improvements in antigen expression technologies and carrier design over the years. In this review, we will attempt to deconstruct carrier vaccine immunogenicity in humans by examining the basis of bacterial immunity in the human gastrointestinal tract and how the gut detects and responds to pathogens versus benign commensal organisms. Carrier vaccine design will then be explored to determine the feasibility of retaining as many characteristics of a pathogen as possible to elicit robust carrier and foreign antigen-specific immunity, while avoiding over-stimulation of unacceptably reactogenic inflammatory responses.

Clinical Testing of Tuberculosis Vaccine Candidates

It is almost 100 years since the development of bacille Calmette-Guérin (BCG), the only licensed vaccine against tuberculosis (TB). While BCG does confer consistent protection against disseminated disease, there is an urgent need for a more effective vaccine against pulmonary disease. There are several indications for such an improved vaccine, including prevention of infection, prevention of disease, and a therapeutic vaccine to prevent recurrent disease. The two main approaches to TB vaccine development are developing an improved whole mycobacterial priming agent to replace BCG and/or developing a subunit booster vaccine to be administered after a BCG or BCG replacement priming vaccination. In this article we review the status of the current candidate vaccines being evaluated in clinical trials. The critical challenges to successful TB vaccine development are the uncertain predictive value of the preclinical animal models and the lack of a validated immune correlate of protection. While it is relatively simple to evaluate safety and immunogenicity in phase 1/2 studies, the evaluation of efficacy requires complex studies with large numbers of subjects and long periods of follow-up. This article reviews the potential role for human Experimental Medicine studies, in parallel with product development, to help improve the predictive value of the early-stage trials.

Live-attenuatedSalmonellaas a prototype vaccine vector for passenger immunogens in humans: are we there yet?

It has been nearly 20 years since the first Phase I clinical trial of a live-attenuated bacterial vaccine was created by recombinant DNA methods, opening the door to the use of these organisms as mucosal delivery vehicles for passenger antigens. Over this time, a number of animal studies have indicated the feasibility of this approach. These include studies showing that bacteria can deliver antigens expressed by the bacterium itself and that bacteria can deliver DNA vaccines to be expressed in target eukaryotic cells. Concomitant studies have identified a number of attenuating mutations that render the bacterial vectors both safe and immunogenic in humans. Both avenues of research indicate the significant promise of this approach to mucosal vaccine development; however, this promise remains largely unrealized at the level of human clinical trials. This review sketches the history of this problem and points toward possible solutions using Salmonella vaccine vectors as the prototypes.

The ABCs (Antibody, B Cells, and Carbohydrate Epitopes) of Cholera Immunity: Considerations for an Improved Vaccine

Cholera, a diarrheal disease, is known for explosive epidemics that can quickly kill thousands. Endemic cholera is a seasonal torment that also has a significant mortality. Not all nations with extensive rural communities can achieve the required infrastructure or behavioral changes to prevent epidemic or endemic cholera. For some communities, a single-dose cholera vaccine that protects those at risk is the most efficacious means to reduce morbidity and mortality. It is clear that our understanding of what a protective cholera immune response is has not progressed at the rate our understanding of the pathogenesis and molecular biology of cholera infection has. This review addresses V. cholerae lipopolysaccharide (LPS)-based immunogens because LPS is the only immunogen proven to induce protective antibody in humans. We discuss the role of anti-LPS antibodies in protection from cholera, the importance and the potential role of B cell subsets in protection that is based on their anatomical location and the intrinsic antigen-receptor specificity of various subsets is introduced.

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.

Variable gene family usage of protective and non-protective anti-Vibrio cholerae O1 LPS antibody heavy chains

Vibrio cholerae causes cholera, an enteric disease of humans that is a worldwide problem. The O1 serogroup of Vibrio cholerae contains two predominant serotypes (Inaba and Ogawa) of LPS, a proven protective antigen for humans and experimental animals. We generated B-cell hybridomas from mice immunized with either: (i) two doses of purified Inaba LPS; (ii) two doses of an Inaba hexasaccharide conjugate (terminal six perosamine bound to a protein carrier), (iii) four doses of purified Inaba LPS; or (iv) a low dose of purified Inaba LPS followed by a booster with the Inaba conjugate. We showed previously that the first and third immunization protocols induce vibriocidal antibodies, as does the fourth; the second protocol induces antibodies that bind Inaba and Ogawa LPS but are not vibriocidal. Anti-LPS mAbs derived from hybridomas resulting from each immunization protocol were characterized for binding to Inaba and Ogawa LPS, their vibriocidal or protective capacity, and the variable heavy chain family they expressed. LPS immunogens selected different LPS-specific B cells expressing six different Vh chain families. Protective and non-protective mAbs could express variable regions from the same family. One mAb was specific for Inaba LPS, the other mAbs were cross-reactive with both LPS serotypes. Sequence comparison suggests that the pairing of a specific light chain, somatic mutation, or the specific VDJ recombination can modulate the protective capacity of mAbs that express a common variable heavy chain family member.

B-cell responses to lipopolysaccharide epitopes: Who sees what first - does it matter?

PROBLEM

Cholera is the paradigm for waterborne bacterial diseases. For over a 100 years, scientists have tried to develop a universally effective vaccine for cholera. We are hampered in our efforts because we do not know the details of the basic immune response to Vibrio cholerae antigens. What are the most proactive antigens? What special needs for immunization are engendered by previous exposure to cholera or the age of the individual? How long does immunity last, and is this immunity a classic immunologic memory or re-exposure and continual boosting?

METHOD OF STUDY

Immunization with synthetic derivatives of the carbohydrate moieties of V. cholerae lipopolysaccharide (LPS) coupled to different carrier proteins (neoglycoconjugates, NGC) has allowed dissection of the response to the disaccharide array of perosamine that represent either the Inaba or the Ogawa serotype. Studying serum anti-LPS endpoint titers and the serum vibriocidal response to NGC provides insight into the importance of LPS serotype-specific B-cell epitopes and how antibody response are influenced by the form of the LPS immunogen.

RESULTS

We found that murine serum antibody responses to V. cholerae LPS are dynamic. The magnitude of serum anti-LPS antibody titers and the capacity to induced vibriocidal antibodies (immunoglobulin M) are influenced by the initial immunizing serotype of LPS, the structure of the LPS immunogen (native LPS versus NGC), and the order of serotype immunization in a prime boost immunization strategy. The dynamic of the immune response to LPS immunogens is typified by the fact that the host species can affect the immunization response. We found mice do not make vibriocidal antibody to Inaba NGC but rabbits do. This is in contrast to the Ogawa NGC that induced vibriocidal antibody in mice.

CONCLUSION

The results suggest that the host's B-cell repertoire can influence the immunization efficacy; therefore, the development of the new generation of NGC V. cholerae vaccines should focus on human volunteers and their ability to mount protective responses.

Lipopolysaccharides of Vibrio cholerae: III. Biological functions

This review presents the salient features of the biological functions including the (i) endotoxic activities, (ii) antigenic properties, (iii) immunological responses to and (iv) phage receptor activities of the Vibrio cholerae lipopolysaccharides (LPS). The biological functions of the capsular polysaccharide (CPS) of V. cholerae have also been discussed briefly as a relevant topic. The roles of LPS and other extracellular polysaccharides in the (i) intestinal adherence and virulence of the vibrios and (ii) the biofilm formation by the organisms have been analysed on the basis of the available data. Every effort has been made to bring out, wherever applicable, the lacunae in our knowledge. The need for the continuous serogroup surveillance and monitoring of the environmental waters and the role of LPS in the designing of newer cholera vaccines has been discussed briefly in conclusion.

Salmonella typhi and S typhimurium derivatives harbouring deletions in aromatic biosynthesis and Salmonella Pathogenicity Island-2 (SPI-2) genes as vaccines and vectors

The S. typhimurium strain (TML deltaaroC deltassaV) WT05, harbouring defined deletions in genes involved in both the aromatic biosynthesis pathway (aroC) and the Salmonella Pathogenicity Island-2 (SPI-2) (ssaV) was shown to be significantly attenuated in C57 BL/6 interferon gamma knockout mice following oral inoculation. Similarly, the S. typhi strain (Ty2 deltaaroC deltassaV) ZH9 harbouring the aroC and ssaV mutations propagated less efficiently than wild type in human macrophages. These studies demonstrated the attractive safety profile of the aroC ssaV mutant combination. Strains S. typhimurium (TML deltaaroC deltassaV ) WT05 and S. typhi (Ty2 deltaaroC deltassaV) ZH9 were subsequently tested as vaccine vectors to deliver E. coli heat-labile toxin (LT-B) mucosally to mice. Mice inoculated orally with S. typhimurium (TML deltaaroC deltassaV) WT05 expressing LT-B (WT05/LT-B) elicited high titres of both LT-specific serum IgG and intestinal IgA, although no specific IgA was detected in the vagina. Similarly, intranasal inoculation of mice with S. typhi (Ty2 deltaaroC deltassaV) ZH9 expressing LT-B (ZH9/LT-B) elicited even higher titres of LT-specific serum antibody as well as LT-specific Ig in the vagina. We conclude that deltaaroC deltassaV strains of Salmonella are highly attenuated and are promising candidates both as human typhoid vaccines and as vaccine vectors for the delivery of heterologous antigens.

Animal models paving the way for clinical trials of attenuated Salmonella enterica serovar Typhi live oral vaccines and live vectors

Attenuated Salmonella enterica serovar Typhi (S. Typhi) strains can serve as safe and effective oral vaccines to prevent typhoid fever and as live vectors to deliver foreign antigens to the immune system, either by the bacteria expressing antigens through prokaryotic expression plasmids or by delivering foreign genes carried on eukaryotic expression systems (DNA vaccines). The practical utility of such live vector vaccines relies on achieving a proper balance between minimizing the vaccine's reactogenicity and maximizing its immunogenicity. To advance to clinical trials, vaccine candidates need to be pre-clinically evaluated in relevant animal models that attempt to predict what their safety and immunogenicity profile will be when administered to humans. Since S. Typhi is a human-restricted pathogen, a major obstacle that has impeded the progress of vaccine development has been the shortcomings of the animal models available to assess vaccine candidates. In this review, we summarize the usefulness of animal models in the assessment of the degree of attenuation and immunogenicity of novel attenuated S. Typhi strains as vaccine candidates for the prevention of typhoid fever and as live vectors in humans.

Salmonella vaccines for use in humans: present and future perspectives

In recent years there has been significant progress in the development of attenuated Salmonella enterica serovar Typhi strains as candidate typhoid fever vaccines. In clinical trials these vaccines have been shown to be well tolerated and immunogenic. For example, the attenuated S. enterica var. Typhi strains CVD 908-htrA (aroC aroD htrA), Ty800 (phoP phoQ) and chi4073 (cya crp cdt) are all promising candidate typhoid vaccines. In addition, clinical trials have demonstrated that S. enterica var. Typhi vaccines expressing heterologous antigens, such as the tetanus toxin fragment C, can induce immunity to the expressed antigens in human volunteers. In many cases, the problems associated with expression of antigens in Salmonella have been successfully addressed and the future of Salmonella vaccine development is very promising.

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.

Extending the CD4(+) T-cell epitope specificity of the Th1 immune response to an antigen using a Salmonella enterica serovar typhimurium delivery vehicle

We analyzed the CD4 T-cell immunodominance of the response to a model antigen (Ag), MalE, when delivered by an attenuated strain of Salmonella enterica serovar Typhimurium (SL3261*pMalE). Compared to purified MalE Ag administered with adjuvant, the mapping of the peptide-specific proliferative responses showed qualitative differences when we used the Salmonella vehicle. We observed the disappearance of one out of eight MalE peptides' T-cell reactivity upon SL3261*pMalE immunization, but this phenomenon was probably due to a low level of T-cell priming, since it could be overcome by further immunization. The most striking effect of SL3261*pMalE administration was the activation and stimulation of new MalE peptide-specific T-cell responses that were silent after administration of purified Ag with adjuvant. Ag presentation assays performed with MalE-specific T-cell hybridomas showed that infection of Ag-presenting cells by this intracellular attenuated bacterium did not affect the processing and presentation of the different MalE peptides by major histocompatibility complex (MHC) class II molecules and therefore did not account for immunodominance modulation. Thus, immunodominance of the T-cell response to microorganisms is governed not only by the frequency of the available T-cell repertoire or the processing steps in Ag-presenting cells that lead to MHC presentation but also by other parameters probably related to the infectious process and to the bacterial products. Our results indicate that, upon infection by a microorganism, the specificity of the T-cell response induced against its Ags can be much more effective than with purified Ags and that it cannot completely be mimicked by purified Ags administered with adjuvant.

Recombinant live Salmonella spp. for human vaccination against heterologous pathogens

Live attenuated Salmonella spp. are promising candidates as oral vaccine delivery systems for heterologous antigens. Clinical trials have demonstrated that this approach is feasible for human vaccinations but further optimisation is necessary to obtain a better efficacy. Here, we discuss how existing clinical and pre-clinical data can be used to guide such optimisation efforts.

Phase 2 clinical trial of attenuated Salmonella enterica serovar typhi oral live vector vaccine CVD 908-htrA in U.S. volunteers

Salmonella enterica serovar Typhi strain CVD 908-htrA is a live attenuated strain which may be useful as an improved oral typhoid vaccine and as a vector for cloned genes of other pathogens. We conducted a phase 2 trial in which 80 healthy adults received one of two dosage levels of CVD 908-htrA in a double-blind, placebo-controlled, crossover study. There were no differences in the rates of side effects among volunteers who received high-dose vaccine (4.5 x 10(8) CFU), lower-dose vaccine (5 x 10(7) CFU), or placebo in the 21 days after vaccination, although recipients of high-dose vaccine (8%) had more frequent diarrhea than placebo recipients (0%) in the first 7 days. Seventy-seven percent and 46% of recipients of high- and lower-dose vaccines, respectively, briefly excreted vaccine organisms in their stools. All blood cultures were negative. Antibody-secreting cells producing antilipopolysaccharide (LPS) immunoglobulin A (IgA) were detected in 100 and 92% of recipients of high- and lower-dose vaccines, respectively. Almost half the volunteers developed serum anti-LPS IgG. Lymphocyte proliferation and gamma interferon production against serovar Typhi antigens occurred in a significant proportion of vaccinees. This phase 2 study supports the further development of CVD 908-htrA as a single-dose vaccine against typhoid fever and as a possible live vector for oral delivery of other vaccine antigens.

A review of oral vaccination with transgenic vegetables

Mucosal immunization of the gastrointestinal tract is an effective way to stimulate local and systemic immune responses. Oral vaccines must be formulated in such a way that antigens are protected as they pass through the adverse environment of the stomach and are delivered to the mucosal inductive sites. Vaccine antigens cloned into edible transgenic plants are a promising new delivery system for oral vaccines. Such vaccines could be safe, inexpensive, and multicomponent.

Validation of a volunteer model of cholera with frozen bacteria as the challenge

To evaluate a standardized inoculum of Vibrio cholerae for volunteer challenge studies, 40 healthy adult volunteers were challenged at three different institutions with a standard inoculum prepared directly from vials of frozen, virulent, El Tor Inaba V. cholerae N16961, with no further incubation. Groups of 5 volunteers, with each group including 2 volunteers with blood group O, were given a dose of 10(5) CFU, and 34 of the 40 volunteers developed diarrhea (mean incubation time, 28 h). Transient fevers occurred in 15 (37.5%) of the volunteers. V. cholerae was excreted by 36 of 40 volunteers. Five additional volunteers received 10(4) CFU, and four developed diarrhea but with a lower average purging rate than required for the model. Of the 40 volunteers, 37 developed rises in their vibriocidal and antitoxin titers similar to those in previous groups challenged with freshly harvested bacteria. We conclude that challenge with frozen bacteria results in a reproducible illness similar to that induced by freshly harvested bacteria. Use of this model should minimize differences in attack rates or severity when groups are challenged at different times and in different institutions.

Immunogenicity of liposome-associated and refined antigen oral cholera vaccines in Thai volunteers

A mixture of Vibrio cholerae antigens made up of crude fimbrial extract, lipopolysaccharide and procholeragenoid was administered orally to Thai volunteers either as free antigen or associated with liposomes. All vaccines and controls were administered in three doses given at 14 day intervals. Nine volunteers received liposome-associated vaccine and seven received free vaccine. Liposomes without antigens were given to eight volunteers and seven volunteers received 5% NaHCO3 solution alone. Both vaccines had 100% immunogenicity as determined by serum vibriocidal antibody responses. Liposomes were shown by indirect ELISA to localize the immune response against lipopolysaccharide and fimbriae to the intestinal mucosa. Vaccines given liposome-associated antigens had a higher rate of antigen-specific antibody response than did individuals who had received free antigens. The vaccines induced intestinal antibodies of IgM and/or IgA isotypes, but not IgG antibody.

A murine model of intranasal immunization to assess the immunogenicity of attenuated Salmonella typhi live vector vaccines in stimulating serum antibody responses to expressed foreign antigens

The lack of a practical small animal model to study the immunogenicity of Salmonella typhi-based live vector vaccines expressing foreign antigens has seriously impeded the vaccine development process. For some foreign antigens, stimulation of serum IgG antibody is the desired, protective immune response. We administered to mice, by orogastric or intranasal (i.n.) routes, attenuated delta aroC delta aroD S. typhi CVD 908 carrying a plasmid encoding fragment C (fragC) of tetanus toxin fused to the eukaryotic cell receptor binding domain of diphtheria toxin (fragC-bDt), and monitored serum antibody. While orogastric inoculation of three doses was not immunogenic, i.n. immunization elicited high titers of serum IgG tetanus antitoxin, generating peak ELISA geometric mean titers (GMT) of 27024 and 35658 with 10(8) and 10(9) c.f.u. dosages, respectively; 10(9) c.f.u. i.n. of an delta aroA S. typhimurium live vector stimulated a peak antitoxin GMT of 376 405. Mice immunized with the S. typhi live vector were 100% protected against challenge with 100 50% lethal doses of tetanus toxin that rapidly killed all control mice. Intranasal immunization with two doses of S. typhi expressing unfused fragment C under control of an anaerobically-activated promoter derived from nirB stimulated significantly higher titers of serum neutralizing antitoxin than fused fragC-bDt controlled by the same promoter (GMT 0.10 AU ml-1 vs 0.01 AU ml-1, P = 0.0095). Two i.n. doses of S typhi encoding fragC under control of powerful constitutive promoter 1pp led to significantly higher peak serum neutralizing antitoxin titers than the otherwise identical construct utilizing the nirB promoter (peak GMT 0.72 AU ml-1 vs 0.10 AU ml-1, P = 0.022). The i.n. route of inoculation of mice may constitute a practical breakthrough that could expedite the development of some S. typhi-based live vector vaccines by allowing, for the first time, quantitative measurement of serum antibody responses to candidate constructs following i.n. mucosal immunization.

Safety of live oral Salmonella typhi vaccine strains with deletions in htrA and aroC aroD and immune response in humans

A single-dose, oral Salmonella typhi vaccine strain has been sought as a carrier or vector of cloned genes encoding protective antigens of other pathogens. Such a hybrid vaccine, administered orally, would stimulate immune responses both at the mucosal surface and in the systemic compartment and would potentially provide protection against multiple pathogens. S. typhi CVD 908 and CVD 906, which harbor deletions in aroC and aroD, were further engineered by deletion in htrA to produce strains CVD 908-htrA and CVD 906-htrA, which are unable to sustain growth and are severely impaired in their ability to survive in host tissues. These strains were fed to humans at doses of 5 x 10(7) to 5 x 10(9) CFU with buffer, and safety and immune responses were assessed. CVD 908-htrA and CVD 906-htrA were well tolerated in volunteers; mild diarrhea in 3 of 36 volunteers and mild fever in 1 volunteer were the only notable adverse responses. The vaccine strains were not detected in blood cultures and only transiently detected in stool. Serum immune responses to S. typhi lipopolysaccharide and H antigens were observed in 75 to 100% of volunteers who received 5 x 10(8) to 5 x 10(9) CFU, and cells secreting S. typhi-specific antibodies were found in all volunteers after ingestion of either strain. Sixty-three percent to 83% of volunteers developed lymphoproliferative responses to S. typhi flagellar and particulate antigens after the higher doses. These studies demonstrate the potential of CVD 908-htrA as a live vector for the delivery of heterologous genes, and a clinical trial of such a construct is planned.

Oral and rectal immunization of adult female volunteers with a recombinant attenuated Salmonella typhi vaccine strain

An attenuated strain of Salmonella typhi delta(cya) delta(crp-cdt) delta(asd) expressing a gene encoding a hepatitis B virus core-pre-S protein was tested in female adult volunteers for its ability to elicit a systemic and a mucosal immune response. Specifically, our purpose was to evaluate the potential of such a vaccine strain to induce specific secretory immunoglobulin A (sIgA) at genital and rectal surfaces. Oral and rectal routes of immunization were compared: oral immunization induced seroconversion against the bacterial lipopolysaccharide (LPS) in six out of seven volunteers, while after rectal immunization only one out of six volunteers seroconverted against LPS. To our disappointment, the latter volunteer was also the only one who seroconverted against the carried antigen (pre-S1), demonstrating the poor ability of this live vaccine to induce an immune response against the carried antigen. Anti-LPS sIgA was found in both the vaginal and cervical secretions of a volunteer who presented a strong seroconversion after oral immunization (16-fold increase in anti-LPS IgG). Smaller amounts of anti-LPS sIgA were found in the rectal secretions of one orally and one rectally immunized volunteer and in the saliva of three orally and one rectally immunized woman. Our data show for the first time that it is possible to induce specific sIgA in the genital and rectal tracts of women by using an S. typhi vaccine strain.

Control by H-2 genes of the Th1 response induced against a foreign antigen expressed by attenuated Salmonella typhimurium

Attenuated salmonellae represent an attractive vehicle for the delivery of heterologous protective antigens to the immune system. Here, we have investigated the influence of the genetic background of the host which regulates the growth and elimination of Salmonella cells on the cellular response induced against a foreign antigen delivered by an aroA Salmonella strain. We have tested CD4+ T-cell responses (cell proliferation and cytokine production) in various mouse strains following immunization with Salmonella typhimurium SL3261 expressing a high level of the recombinant Escherichia coli MalE protein. We were able to detect a CD4+ T-cell response against the recombinant MalE protein only in a restricted number of mouse strains, whereas all mice produced good levels of anti-MalE immunoglobulin G antibodies. The Ity gene did not play a major role in these differences in T-cell responses, since both Ity-resistant and -susceptible strains of mice were found to be unresponsive to MalE delivered by recombinant salmonellae. In contrast, when B10 congenic mice were used, a correlation was established between MalE-specific T-cell unresponsiveness and H-2 genes. The discrepancies described in this paper in the ability of various strains of mice to develop an efficient Th1 response against a recombinant antigen displayed by a live Salmonella vaccine underscore the difficulties that can be encountered in the vaccination of human populations by such a strategy.

Migration of Salmonella typhi through intestinal epithelial monolayers: an in vitro study

This study characterizes the transmigration of enteroinvasive Salmonella typhi in vitro, using a human intestinal epithelial cell line as a model of small intestinal epithelium. C2BBe cells, a subclone of CACO-2 with a highly differentiated enterocytic phenotype, were grown to maturity on Transwell filters. S. typhi Ty2 and the vaccine strain, Ty21a, the S. typhi mutant X7344 and parent strain SB130, and S. typhimurium 5771 in logarithmic phase were introduced to the upper chamber of the filter units. Numbers of bacteria in the lower chamber, TER and permeability of the monolayer to mannitol were measured over time. Monolayers were examined by light, electron and confocal microscopy to determine the pathway of bacterial transmigration, and intracellular bacteria were estimated by gentamicin assay. Epithelial cell injury was quantified by light microscopy. S. typhi transmigrated earlier and in larger numbers than S. typhimurium, inducing marked changes in electrical resistance and permeability. Unlike S. typhimurium, S. typhi selected epithelial cells in small number and caused their death and extrusion from the monolayers leaving holes through which S. typhi transmigrated. Ty2 consistently transmigrated in larger numbers and with more injury to monolayers than Ty21a. S. typhi crosses the monolayers of C2BBe cells by a paracellular route in contrast to the transcellular pathway described for other Salmonellae. This may be related to the unique pathophysiology of S. typhi infection and the restricted host specificity of this pathogen. In these assays the vaccine strain, Ty21a, is slightly less invasive than its parent, though more invasive than S. typhimurium.

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.

Immunogenicity of Vibrio cholerae ghosts following intraperitoneal immunization of mice

The immunogenic potential of Vibrio cholerae ghosts (VCG) in comparison with heat-killed whole-cell vibrios (WCV) was evaluated after intraperitoneal immunization of adult mice. Swiss white mice received four doses of VCG or WCV intraperitoneally, consisting of 500 micrograms of lyophilized material in 200 microliters of phosphate-buffered saline (PBS), pH 7.4. The control group received 200 microliters of PBS. Serum samples were collected from all mice on the day of immunization and on days 14, 24, 35 and 62 postimmunization. Sera were examined for vibriocidal antibodies by the microtitre and tube-dilution methods and Vibrio-specific serum IgG antibodies were assessed by ELISA. IgG antibodies to intact WCV were detected in sera from all animals immunized with VCG or WCV. The response was specific and of high magnitude. Significantly higher antibody responses were obtained when sera from both VCG- and WCV-immunized mice were titrated against VCG. The immunogenicity of VCG in evoking serum IgG responses was higher than that of WCV. However, the immunogenicity of the two antigen preparations was comparable in terms of seroconversion for vibriocidal antibodies. These results demonstrate that VCG administered intraperitoneally evoke Vibrio-specific serum IgG responses as well as vibriocidal antibody activity in mice.

Production of Vibrio cholerae ghosts (VCG) by expression of a cloned phage lysis gene: potential for vaccine development

The protein E-specific lysis mechanism of the Escherichia coli-specific bacteriophage PhiX174 was employed to produce Vibrio cholerae ghosts (VCG). VCG consist of both rounded and collapsed cells that have lost their cytoplasmic contents through an E-specific hole in the cell envelope. These ghosts are proposed as non-living material for immunization against cholera. A specific membrane anchor sequence was used to insert the human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) fusion protein into the cell envelope of V. cholerae. The identity of the expression products was confirmed by Western blot analysis employing an RT-specific monoclonal antibody. HIV-1 RT was chosen as a model for the purpose of evaluating heterologous gene expression in V. cholerae and the carrier potential of VCG. Intraperitoneal immunization of mice was used to evaluate the immunogenic potential of VCG. Preliminary results showed significant seroconversions to intact whole-cell vibrio antigens in mice immunized with VCG or a heat-killed whole-cell vibrio preparation.

Genetic control of antibody responses induced against an antigen delivered by recombinant attenuated Salmonella typhimurium

Recombinant derivatives of nonpathogenic bacteria such as attenuated Salmonella typhi have the potential to be used for delivery of heterologous antigens to the immune system. Genetic factors may modulate the immune responses to these live attenuated organisms and could therefore modify the immunogenicity of future human vaccines. In the present study, we compared the antibody responses of Ity or H-2 congenic strains of mice to a foreign antigen expressed by the murine attenuated aroA S. typhimurium strain. Our results demonstrate that the Ity gene may modulate the antibody responses to the foreign antigen but that the major genetic influence is exerted by H-2 genes, which control the capacity of mice to respond to the antigen expressed by recombinant attenuated Salmonella cells. This genetic control is related to differences in responsiveness of different strains of mice to low doses of antigen. Increasing the amount of foreign antigen expressed by recombinant Salmonella cells overcame the genetic restriction of these responses. These findings are potentially of great importance for the design of live vaccines for humans and show that care must be taken to optimize the amount of foreign antigen delivered to the immune system.

Immunization of BALB/c mice against Helicobacter felis infection with Helicobacter pylori urease

BACKGROUND/AIMS

Because Helicobacter pylori is a potentially dangerous human pathogen, the protective potential of oral immunization with H. pylori urease and its subunits was evaluated in an animal model.

METHODS

Mice were orally immunized with H. pylori sonicate, urease, or recombinant enzymatically inactive urease subunits and then challenged with Helicobacter felis. Control mice were sham-immunized.

RESULTS

H. felis colonization was present 5 days after challenge in 9 of 10 sham-immunized, 6 of 9 sonicate-immunized, and 3 of 10 urease-immunized animals (P = 0.031 vs. sham-immunized). Twelve days after challenge, urease B-immunized mice had a weaker colonization than sham-immunized controls, whereas urease A had no effect. After 70 days, most urease A- and urease B-immunized mice had cleared the colonization (10/17: P = 0.0019; 16/20: P = 0.00002 vs. sham-immunized). In urease B-immunized animals, protection was often associated with corpus gastritis.

CONCLUSIONS

Oral immunization with H. pylori urease protects mice against H. felis infection. Enzymatically inactive urease A and B subunits contain protective epitopes. It is unclear whether protection depends on the development of a mononuclear inflammatory response in the gastric corpus. Our observations should encourage the development of a human vaccine.

Use of the Vibrio cholerae irgA gene as a locus for insertion and expression of heterologous antigens in cholera vaccine strains

Vibrio cholerae may be a particularly effective organism for use in delivering heterologous antigens to stimulate a common mucosal immune response. A live attenuated vaccine strain of V. cholerae was constructed from the ctxA deletion mutant 0395-N1, containing the B subunit of Shiga-like toxin I under the transcriptional control of the iron-regulated irgA promoter. The B subunit of Shiga-like toxin I is identical to the B subunit of Shiga toxin (StxB). irgA encodes the major iron-regulated outer membrane protein of V. cholerae, which is a known virulence factor for this organism. Clones of the structural gene irgA from the classical V. cholerae strain 0395, with the gene for the Shiga-like toxin I B subunit inserted under the control of the irgA promoter, were used to introduce an internal deletion of irgA into the chromosome of 0395-N1 by in vivo marker exchange, using the suicide vector plasmid pCVD442. This plasmid contains the sacB gene from Bacillus subtilis, which allowed positive selection for loss of plasmid sequences on exposure to sucrose. The construction of vaccine strains was confirmed by Southern hybridization studies and outer membrane protein analysis. The expression of StxB in the vaccine strain VAC2 following growth in high- or low-iron conditions was shown to be tightly iron-regulated by Western blot analysis and by quantification of StxB using a sandwich enzyme-linked immunosorbent assay. The production of StxB by VAC2 under low-iron conditions was greater than that of the reference strain Shigella dysenteriae 60R.(ABSTRACT TRUNCATED AT 250 WORDS)

Vaccines to prevent enteric infections

Considerable progress has been made in the last decade in developing vaccines against the enteric infections of greatest public health importance. A quadrivalent rotavirus vaccine consisting of rhesus rotavirus vaccine (which contains serotype 3 neutralization antigen) and three reassortant viruses (rhesus virus expressing neutralization antigens of serotypes 1, 2 or 4) is undergoing placebo-controlled field trials of efficacy in the USA and in two developing countries. Two new vaccines against typhoid fever (oral Ty21a and parenteral Vi polysaccharide) have been licensed in many countries. Even newer generations of typhoid vaccines are undergoing clinical testing, including new attenuated S. typhi strains and Vi polysaccharide-carrier protein conjugate vaccines. Two inactivated oral cholera vaccines, consisting of inactivated V. cholerae O1 bacteria alone or in combination with purified B subunit of cholera toxin, each conferred 50-53% protection over 3 years in a field trial in Bangladesh where subjects were immunized with a three-dose regimen. In extensive clinical trials in adults and children in less-developed countries, an engineered live oral cholera vaccine, strain CVD 103-HgR, has been shown to be well tolerated and highly immunogenic following administration of just a single oral dose; a large-scale field trial in 70,000 subjects is underway to investigate the efficacy of this vaccine. Several candidate vaccines against Shigella and enterotoxigenic Escherichia coli are in clinical trials. Accumulating knowledge on pathogenesis of enteric infections and advances in mucosal and cellular immunology, coupled with the application of modern biotechnology, have resulted in a plethora of vaccine candidates. It is expected that in future years efforts will be directed to construct vaccines against other enteric pathogens.

Molecular cloning and characterization of the genetic determinants that express the complete Shigella serotype D (Shigella sonnei) lipopolysaccharide in heterologous live attenuated vaccine strains

The genetic determinants for the complete Shigella sonnei lipopolysaccharide (LPS) have been cloned, characterized by restriction mapping, and expressed in heterologous genetic backgrounds, including Salmonella typhi and Vibrio cholerae live attenuated vaccine strains. The rfb/rfc locus encoding the polymerized serotype-specific O polysaccharide was mapped within 23 kb of DNA isolated from S. sonnei virulence plasmid pWR105. A highly similar chromosomal DNA sequence was identified by Southern hybridization analysis in Plesiomonas shigelloides known to have the same O serotype specificity as S. sonnei. Expression studies of the rfb/rfc locus have shown that S. sonnei O polysaccharide is covalently bound to LPS cores of both the K-12 and R1 types, but neither to Salmonella (Ra-type) nor to V. cholerae O1 cores. In order to express a compatible core structure in the latter organisms, chromosomal rfa loci encoding R1-type LPS were isolated from both an Escherichia coli R1 strain (rfaR1) and from S. sonnei (rfasonnei). Restriction mapping and functional analysis of cloned DNA allowed us to localize the rfaR1 locus and to orient it with respect to the neighbouring cysE chromosomal marker. A high degree of sequence similarity was found at the DNA level between rfa loci of enterobacterial species characterized by R1-type LPS. Co-expression studies involving S. sonnei rfb/rfc and rfa loci propagated on compatible plasmids have shown that, at most, 13 to 14 kb of rfaR1 DNA are required for the expression of complete phase-I-like S. sonnei LPS in E. coli K-12 and S. typhi, whereas an adjacent region of about 3.5 kb is needed in the more stringent host, V. cholerae. S. sonnei O antigen expressed in a V. cholerae recombinant vaccine strain is present on the cell surface in a form suitable for the induction of a specific antibody response in vaccinated rabbits.

Live oral vaccines against cholera: an update

One hundred years elapsed between the first (live, parenteral) cholera vaccine that entered clinical trials in 1885 and the field trials of two oral inactivated cholera vaccines undertaken in Bangladesh in the mid-1980s. The oral inactivated vaccines advanced the art by establishing, convincingly, that oral vaccines could protect (although multiple doses were required) and that (at least in adults) protection could last 3 years. Attenuated Vibrio cholerae O1 strain CVD 103-HgR (deleted of the cholera toxin A subunit gene and harbouring a gene encoding resistance to Hg++) constitutes another significant advance. This live oral vaccine is well tolerated and highly immunogenic in adults and children and highly protective (in adult volunteer challenge studies) following ingestion of of a single dose.

Evaluation in volunteers of a candidate live oral attenuated Salmonella typhi vector vaccine

Candidate vector vaccine strain CVD 906 (aroC- and aroD- derivative of virulent Salmonella typhi strain ISP1820) was evaluated in phase 1 clinical trials. The first nine volunteers ingested a single dose of 5 x 10(7) CVD 906 bacilli. At this dose CVD 906 stimulates remarkable systemic and mucosal immune responses, inasmuch as 89% of volunteers developed marked serum antibody levels to S. typhi antigens and high numbers of antigen-specific gut-derived antibody-secreting cells. Four (44%) volunteers developed asymptomatic vaccinemia 4-10 d after immunization and all volunteers excreted CVD 906 on at least one occasion. However, two volunteers developed febrile adverse reactions, one on the day of vaccination and the other on day 4. Of 11 volunteers who ingested a single dose of 5 x 10(3) CVD 906 bacilli, none displayed side effects but 27% developed significant serum responses to S. typhi LPS. In vitro, CVD 906 replicates for only nine generations in pooled human serum, indicating that CVD 906 growth is limited in this physiologically relevant medium. In phorbol myristate acetate-induced U937 human macrophage-like cells, CVD 906 replicates intracellularly to a lesser extent than parent strain ISP1820. Although, strain CVD 906 is attenuated and highly immunogenic, the occasional febrile reactions at high doses indicate that further attenuation of this strain is necessary.

Risks connected with the use of conventional and genetically engineered vaccines

A review is given of real and potential risks connected with the use of conventional and genetically engineered live and dead vaccines. Special attention is given to live carrier vaccines expressing one or more heterologous genes of other microorganisms. Because most carrier vaccines are still in an experimental phase, there is only limited experience with the risks of carrier vaccines. There are three potential risks of live carrier vaccines which will be discussed: 1. Changes in cell, tissue, of host tropism, and virulence of the carrier through the incorporation of foreign genes. 2. Exchange of genetic information with other vaccine or wild-type strains of the carrier organism. 3. Spread in the environment. Only limited experimental data are available on changes in biological behaviour of microorganisms through the incorporation of foreign genes. For example, there are indications that vaccinia virus carrying the attachment protein G of respiratory syncytial virus (RSV) replicates better in lungs of mice than vaccinia virus carrying other genes of RSV. Poxviruses carry genes that probably determine their replication in different hosts. Exchange of such host tropism genes might alter their host spectrum. Recombination between herpesvirus vaccine or wild-type strains may lead to the appearance of virulent strains with of without heterologous genes. Before carrier vaccines are applied, these risks must be thoroughly evaluated case-by-case. Potential methods for the design of safe carrier vaccines are discussed.

Cholera as a model for research on mucosal immunity and development of oral vaccines

During the past year, the extensive investigational use of a recently developed oral vaccine against cholera has led to significant advances in our understanding of both immunity to cholera and related diarrhoeal diseases, and the mucosal immune response in general after oral immunization. The oral cholera vaccine has been shown to protect, through its cholera toxin B subunit component, against travellers' diarrhoea caused by enterotoxigenic Escherichia coli. The elaboration of sensitive new techniques has allowed detailed clonal analyses of the activation of specific B and T cells and immunologic memory in intestinal mucosa in humans after oral cholera vaccination. These techniques have also been used to demonstrate a transient appearance after immunization of specific gut-derived IgA antibody-producing cells in the circulation and also, a few days later, in a distant mucosal tissue such as the salivary glands.

Comparison of lipopolysaccharide biosynthesis genes rfaK, rfaL, rfaY, and rfaZ of Escherichia coli K-12 and Salmonella typhimurium

Analysis of the sequence of a 4.3-kb region downstream of rfaJ revealed four genes. The first two of these, which encode proteins of 27,441 and 32,890 Da, were identified as rfaY and rfaZ by homology of the derived protein sequences of their products to the products of similar genes of Salmonella typhimurium. The amino acid sequences of proteins RfaY and RfaZ showed, respectively, 70 and 72% identity. Genes 3 and 4 were identified as rfaK and rfaL on the basis of size and position, but the derived amino acid sequences of the products of these genes showed very little similarity (about 12% identity) between Escherichia coli K-12 and S. typhimurium. The next gene in the cluster, rfaC, encodes a product which also shows strong protein sequence homology between E. coli K-12 and S. typhimurium, as do the rfaF and rfaD genes which lie beyond it. Thus, the rfa gene cluster appears to consist of two blocks of genes which are conserved flanking a central region of two genes which are not conserved between these species. Although the RfaL protein sequence is not conserved, hydropathy plots of the two RfaL species are nearly identical and indicate that this is a typical integral membrane protein with 10 or more potential transmembrane domains. We noted the similarity of the structure of the rfa gene cluster to that of the rfb gene cluster, which has now been sequenced in several Salmonella serovars. The rfb cluster also contains a gene which lies within a central nonconserved region and encodes an integral membrane protein similar to protein RfaL. We speculate that protein RfaL may interact in a strain- or species-specific way with one or more Rfb proteins in the expression of surface O antigen.

Oral immunization using live attenuated Salmonella spp. as carriers of foreign antigens

A variety of techniques, including the use of live oral vaccines, have been used to deliver antigens to the gut-associated lymphoid tissues in an attempt to initiate production of specific secretory immunoglobulin A for protection against pathogens that colonize or cross mucosal surfaces to initiate infection. A number of attenuated Salmonella mutants are able to interact with the lymphoid tissues in the Peyer's patches but are not able to cause systemic disease. Some of these mutants are effective as live vaccines (i.e., able to protect against infection with the virulent Salmonella parent) and are candidates for use as carriers for virulence determinants of other mucosal pathogens. This has been shown to be an effective means of stimulating significant levels of specific mucosal secretory immunoglobulin A directed against the carrier strains and against a variety of heterologous antigens and has been shown to stimulate production of serum antibodies and cell-mediated responses as well. This review examines the history of this mechanism of vaccine delivery and summarizes the most recent applications of this evolving technology. This is a technique for vaccine delivery with significant potential for influencing the management of infectious diseases on a large scale. It can be used not only for vaccines against enteric bacterial pathogens but also for vaccines against a variety of other bacteria, viruses, and parasites. The results obtained to date are encouraging, and there is great potential for development of safe, effective, affordable vaccines.

Live attenuated vaccines for human use

Live attenuated vaccines have been successfully used for the prevention of a number of viral and bacterial diseases. Several vaccine strains have been utilized recently as expression vectors for cloned heterologous antigens. Through the use of recombinant DNA technology, candidate vaccine strains and vector systems have been developed and are undergoing clinical evaluation.

Indirect measurement of intestinal immune responses to an orally administered attenuated bacterial vaccine

Intestinal fluid, saliva, circulating peripheral blood lymphocytes (PBL), and serum samples obtained from 81 human adult subjects who had been orally vaccinated with either Salmonella typhi Ty21a or one of its recombinant derivatives were examined to determine the value of indirect measurements of an antigen-specific intestinal-immunoglobulin A (IgA) response. Salivary IgA failed to provide consistent or correlative responses, and no evidence of a significant relationship was apparent with the intestinal-IgA responses. No significant correlation between the specific increase in responses in serum IgA and intestinal IgA was evident. While the magnitude of the serum IgG response significantly correlated with the intestinal-IgA response (P = 0.00064), it failed to detect 14.8% of the intestinal-IgA responses. The observation that 16.6% of the subjects had delayed serum IgA responses, with a peak occurring after day 23 compared with days 12 to 14, may have contributed to the inadequacy of the serum IgA response as a correlative indicator. The serum IgG responses in these subjects were also of a diminished magnitude. Specific IgA production by circulating PBL was found to be the most sensitive (92.6% response rate) and correlative (P = 0.00071) indicator of a specific intestinal-IgA immune response. However, its value in predicting protective efficacy is untried. These studies confirm that for the assessment of an enteric bacterial vaccine, determination of in vitro specific IgA production by circulating PBL may offer a single measurement of specific immunity which is as useful as serum and intestinal measurements combined.

Comparison of the safety and immunogenicity of delta aroC delta aroD and delta cya delta crp Salmonella typhi strains in adult volunteers

Three attenuated Salmonella typhi strains have been constructed by introducing deletions in aroC and aroD or deletions in cya and crp into one of two wild-type parent strains, Ty2 or ISP1820. These mutant strains were designated CVD 906 (ISP1820 delta aroC delta aroD), CVD 908 (Ty2 delta aroC delta aroD), and chi 3927 (Ty2 delta cya delta crp). Two studies were conducted with 36 healthy adult inpatient volunteers to determine in a double-blind fashion the safety and immunogenicity of approximately 5 x 10(4) and 5 x 10(5) CFU of each of these three vaccine candidates given as a single dose. No statistically significant difference in the incidence of reactions among vaccinees was observed. Fever (oral temperature greater than or equal to 38.2 degrees C) occurred in 2 of 12 volunteers who received CVD 906, in 0 of 12 who received CVD 908, and in 1 of 12 who received chi 3927. Vaccine bacteremia without symptoms occurred in 1 of 12 vaccinees who received CVD 906, in 0 of 12 who received CVD 908, and in 2 of 12 who received chi 3927. Overall, 19 (53%) of 36 vaccinees developed immunoglobulin G antibody to S. typhi lipopolysaccharide after vaccination, with no statistically significant differences in the rate of seroconversion among volunteers in the three groups. We conclude that defined mutations in the aromatic biosynthetic pathway and in the cyclic AMP global regulatory system attenuate S. typhi. Mutant strains CVD 906, CVD 908, and chi 3927 are highly (and approximately equally) immunogenic but possibly differ in their propensity to induce fever. Further studies are needed to document the apparent relative safety of CVD 908 as a typhoid vaccine and as a vaccine carrier of foreign antigens.

Molecular design of cholera vaccines

Cholera is still a serious public health problem in developing countries, particularly those in tropical regions. This has stimulated considerable research into the molecular analysis of pathogenesis resulting in the identification of a number of critical components required for both colonization of the gut mucosa and the disease symptoms. These components are the targets for rational molecular approaches to vaccine development.