Genotypic and phenotypic characterization of an aroD deletion-attenuated Escherichia coli K12-Shigella flexneri hybrid vaccine expressing S. flexneri 2a somatic antigen

From Kiyoshi Shiga to Present-Day Shigella Vaccines: A Historical Narrative Review

We are at an exciting moment in time with the advancement of many vaccines, including a shigella vaccine for the world. It is instructive to look at the long road that some vaccines have traveled to recognize the remarkable accomplishments of those who were pioneers, appreciate the evolution of scientific and applied technology, and inform the future history of a vaccine that would have great potential for global health. To achieve this valuable retrospective, a narrative historical literature review was undertaken utilizing PubMed and Embase databases with relevant search terms. Retrieved articles were reviewed and information was organized into historical themes, landmark discoveries, and important vaccine development parallels. The literature reviewed was synthesized into major eras of shigella vaccine development from pathogen discovery and first attempts to empirical approaches of killed whole-cell and live-attenuated approaches, and a modern era that applied recombinant DNA engineering and structural vaccinology. The history of shigella vaccine development has largely followed the evolutionary path of vaccine development over the last 120 years, but with important lessons learned that should be considered as we embark on the future chapters of bringing to the world a safe and effective vaccine for global health.

Mucosal-associated invariant T-cell activation and accumulation after in vivo infection depends on microbial riboflavin synthesis and co-stimulatory signals

Despite recent breakthroughs in identifying mucosal-associated invariant T (MAIT) cell antigens (Ags), the precise requirements for in vivo MAIT cell responses to infection remain unclear. Using major histocompatibility complex-related protein 1 (MR1) tetramers, the MAIT cell response was investigated in a model of bacterial lung infection employing riboflavin gene-competent and -deficient bacteria. MAIT cells were rapidly enriched in the lungs of C57BL/6 mice infected with Salmonella Typhimurium, comprising up to 50% of αβ-T cells after 1 week. MAIT cell accumulation was MR1-dependent, required Ag derived from the microbial riboflavin synthesis pathway, and did not occur in response to synthetic Ag, unless accompanied by a Toll-like receptor agonist or by co-infection with riboflavin pathway-deficient S. Typhimurium. The MAIT cell response was associated with their long-term accumulation in the lungs, draining lymph nodes and spleen. Lung MAIT cells from infected mice displayed an activated/memory phenotype, and most expressed the transcription factor retinoic acid-related orphan receptor γt. T-bet expression increased following infection. The majority produced interleukin-17 while smaller subsets produced interferon-γ or tumor necrosis factor, detected directly ex vivo. Thus the activation and expansion of MAIT cells coupled with their pro-inflammatory cytokine production occurred in response to Ags derived from microbial riboflavin synthesis and was augmented by co-stimulatory signals.

Live-attenuatedShigellavaccines

Several live-attenuated Shigella vaccines, with well-defined mutations in specific genes, have shown great promise in eliciting significant immune responses when given orally to volunteers. These responses have been measured by evaluating antibody-secreting cells, serum antibody levels and fecal immunoglobulin A to bacterial lipopolysaccharide and to individual bacterial invasion plasmid antigens. In this review, data collected from volunteer trials with live Shigella vaccines from three different research groups are described. The attenuating features of the bacterial strains, as well as the immune response following the use of different dosing regimens, are also described. The responses obtained with each vaccine strain are compared with data obtained from challenge trials using wild-type Shigella strains. Although the exact correlates of protection have not been found, some consensus may be derived as to what may constitute a protective immune response. Future directions in the field of live Shigella vaccines are also discussed.

Shigella antigen-specific B memory cells are associated with decreased disease severity in subjects challenged with wild-type Shigella flexneri 2a

The role of Shigella-specific B memory (BM) in protection has not been evaluated in human challenge studies. We utilized cryopreserved pre- and post-challenge peripheral blood mononuclear cells and sera from wild-type Shigella flexneri 2a (wt-2457T) challenges. Challenged volunteers were either naïve or subjects who had previously ingested wt-2457T or been immunized with hybrid Escherichia coli-Shigella live oral candidate vaccine (EcSf2a-2). BM and antibody titers were measured against lipopolysaccharide (LPS) and recombinant invasion plasmid antigen B (IpaB); results were correlated with disease severity following challenge. Pre-challenge IgA IpaB-BM and post-challenge IgA LPS-BM in the previously exposed subjects negatively correlated with disease severity upon challenge. Similar results were observed with pre-challenge IgG anti-LPS and anti-IpaB titers in vaccinated volunteers. Inverse correlations between magnitude of pre-challenge IgG antibodies to LPS and IpaB, as well as IgA IpaB-BM and post-challenge IgA LPS-BM with disease severity suggest a role for antigen-specific BM in protection.

Bacterial conjugation in the cytoplasm of mouse cells

Intracellular pathogenic organisms such as salmonellae and shigellae are able to evade the effects of many antibiotics because the drugs are not able to penetrate the plasma membrane. In addition, these bacteria may be able to transfer genes within cells while protected from the action of drugs. The primary mode by which virulence and antibiotic resistance genes are spread is bacterial conjugation. Salmonellae have been shown to be competent for conjugation in the vacuoles of cultured mammalian cells. We now show that the conjugation machinery is also functional in the mammalian cytosol. Specially constructed Escherichia coli strains expressing Shigella flexneri plasmid and chromosomal virulence factors for escape from vacuoles and synthesizing the invasin protein from Yersinia pseudotuberculosis to enhance cellular entry were able to enter 3T3 cells and escape from the phagocytic vacuole. One bacterial strain (the donor) of each pair to be introduced sequentially into mammalian cells had a conjugative plasmid. We found that this plasmid could be transferred at high frequency. Conjugation in the cytoplasm of cells may well be a general phenomenon.

Clinical trials of Shigella vaccines: two steps forward and one step back on a long, hard road

More than 50 years of research has yielded numerous Shigella vaccine candidates that have exemplified both the promise of vaccine-induced prevention of shigellosis and the impediments to developing a safe and effective vaccine for widespread use, a goal that has yet to be attained. This Review discusses the most advanced strategies for Shigella vaccine development, the immune responses that are elicited following disease or vaccination, the factors that have accelerated or impeded Shigella vaccine development and our ideas for the way forward.

Prevention of Enteric Diseases

Enteric diseases remain a high public health priority for much of the world's population. Improvement of sanitation and hygiene would have a favorable impact on this problem, but resources are not available to effect these interventions worldwide. Thus, vaccines against some diarrheal diseases are needed urgently. There has been much success in this arena, but much more needs to be done. Solutions will depend on new and old technologies and on continued dedication of human and financial resources to address problems of global significance.

Complete DNA sequence and analysis of the large virulence plasmid of Shigella flexneri

The complete sequence analysis of the 210-kb Shigella flexneri 5a virulence plasmid was determined. Shigella spp. cause dysentery and diarrhea by invasion and spread through the colonic mucosa. Most of the known Shigella virulence determinants are encoded on a large plasmid that is unique to virulent strains of Shigella and enteroinvasive Escherichia coli; these known genes account for approximately 30 to 35% of the virulence plasmid. In the complete sequence of the virulence plasmid, 286 open reading frames (ORFs) were identified. An astonishing 153 (53%) of these were related to known and putative insertion sequence (IS) elements; no known bacterial plasmid has previously been described with such a high proportion of IS elements. Four new IS elements were identified. Fifty putative proteins show no significant homology to proteins of known function; of these, 18 have a G+C content of less than 40%, typical of known virulence genes on the plasmid. These 18 constitute potentially unknown virulence genes. Two alleles of shet2 and five alleles of ipaH were also identified on the plasmid. Thus, the plasmid sequence suggests a remarkable history of IS-mediated acquisition of DNA across bacterial species. The complete sequence will permit targeted characterization of potential new Shigella virulence determinants.

Native and mutant forms of cholera toxin and heat-labile enterotoxin effectively enhance protective efficacy of live attenuated and heat-killed Shigella vaccines

Both native and mutant forms of cholera toxin (CT) and heat-labile enterotoxin (LT) are effective adjuvants for antigens and killed whole-cell preparations. To determine whether these toxin molecules could also boost the immunogenicity and efficacy of live attenuated vaccines directed against shigellosis, the guinea pig keratoconjunctivitis model was used to evaluate the adjuvant effect of these toxin molecules on EcSf2a-3, a DeltavirG DeltaaroD Escherichia coli-Shigella flexneri 2a hybrid vaccine strain that was previously found to be less protective than its parent strain in the guinea pig model. Experiments using native and mutant toxin molecules showed that both CT and LT and mutant derivatives were effective as an adjuvant for EcSf2a-3 and that the mutant toxin molecules, which were developed to retain adjuvanticity without the toxicity associated with the native molecules, were as effective as the native toxin molecules as adjuvants. Protective efficacy was enhanced for both the oral and intranasal routes of immunization. Serum antibody response to the S. flexneri 2a O antigen, the primary antigen for protective immunity, was not dependent on the addition of an adjuvant. However, enumeration of the O-antigen-specific immunoglobulin G (IgG) and IgA antibody-secreting cells in the spleen and draining lymph nodes following intranasal immunization suggested that enhancement of the local immune response by the toxin molecules may contribute to the observed increase in protective efficacy. The efficacy of heat-killed S. flexneri 2a was enhanced only by mutant LT molecules. These results suggest that the best candidates for enhancing the efficacy of both live attenuated and heat-killed Shigella vaccines with minimal reactogenicity are the mutant toxin molecules.

Quantitative analysis of IgG class and subclass and IgA serum response to Shigella sonnei and Shigella flexneri 2a polysaccharides following vaccination with Shigella conjugate vaccines

It has been recently reported that a conjugate vaccine composed of the O-specific polysaccharide of S. sonnei bound to Pseudomonas aeruginosa recombinant exoprotein A (rEPA) conferred 74% protection against S. sonnei shigellosis. In the present study affinity purified Shigella antibodies were used as standards to quantify and characterize the serum antibody response to vaccination with Shigella sonnei or Shigella flexneri 2a polysaccharide conjugated to rEPA. The geometric mean concentrations of antibodies at the pre-vaccination stage were 3.8 microg/ml for IgG anti-S. sonnei LPS and 11.26 microg/ml for IgG anti-S. flexneri 2a LPS. Vaccination with S. sonnei-rEPA and S. flexneri 2a-rEPA induced the production of specific IgG antibodies to levels of 115.8 microg/ml and 126.5 microg/ml, respectively. The levels of specific antibodies above the pre-vaccination values persisted for at least 2 years. The IgG response to S. flexneri 2a-rEPA conjugate was almost entirely represented by the IgG2 subclass. The concentration of IgG1 anti-S. sonnei LPS was significantly higher than that of IgG2 14 days after vaccination with the homologous conjugate, but decreased to similar levels to those of IgG2 6, 12 and 24 months after immunization. Since the only difference between the S. sonnei and S. flexneri 2a conjugates lies in the different polysaccharides of the two Shigella serogroups (the protein rEPA, is identical in both cases), it follows that the different pattern of IgG subclass response is a result of the different structures of the two O-polysaccharides of S. sonnei and S. flexneri 2a.

Vaccination against shigellosis with attenuated Shigella flexneri 2a strain SC602

The Shigella flexneri 2a SC602 vaccine candidate carries deletions of the plasmid-borne virulence gene icsA (mediating intra- and intercellular spread) and the chromosomal locus iuc (encoding aerobactin) (S. Barzu, A. Fontaine, P. J. Sansonetti, and A. Phalipon, Infect. Immun. 64:1190-1196, 1996). Dose selection studies showed that SC602 causes shigellosis in a majority of volunteers when 3 x 10(8) or 2 x 10(6) CFU are ingested. In contrast, a dose of 10(4) CFU was associated with transient fever or mild diarrhea in 2 of 15 volunteers. All volunteers receiving single doses of >/=10(4) CFU excreted S. flexneri 2a, and this colonization induced significant antibody-secreting cell and enzyme-linked immunosorbent assay responses against S. flexneri 2a lipopolysaccharide in two-thirds of the vaccinees. Seven volunteers who had been vaccinated 8 weeks earlier with a single dose of 10(4) CFU and 7 control subjects were challenged with 2 x 10(3) CFU of virulent S. flexneri 2a organisms. Six of the control volunteers developed shigellosis with fever and severe diarrhea or dysentery, while none of the vaccinees had fever, dysentery, or severe symptoms (P = 0. 005). Three vaccinees experienced mild diarrhea, and these subjects had lower antibody titers than did the fully protected volunteers. Although the apparent window of safety is narrow, SC602 is the first example of an attenuated S. flexneri 2a candidate vaccine that provides protection against shigellosis in a stringent, human challenge model.

Construction and characterization of a live attenuated vaccine candidate against Shigella dysenteriae type 1

Vaccine candidates against Shigella dysenteriae type 1, which is associated with the most severe cases of bacillary dysentery, were constructed. The rfp and rfb gene clusters, which code for S. dysenteriae 1 O antigen biosynthesis, were randomly integrated into either the chromosome or the virulence plasmid of the rough attenuated Shigella flexneri aroD strain SFL124-27 with a minitransposon carrying an arsenite resistance selection marker. The recombinant clones efficiently expressed the recombinant O antigen, exhibited a normal growth pattern, were able to invade and survive within eukaryotic cells to the same extent as the parental strain, and expressed the recombinant antigen within invaded cells. A clone was selected as the vaccine candidate, which was demonstrated to be immunogenic and safe in animal models, leading to 47% full protection and 53% partial protection against challenge with the wild-type strain.

Specificity of monoclonal antibodies elicited by mucosal infection of BALB/c mice with virulent Shigella flexneri 2a

Protective immunity against shigellosis is thought to be determined by the O-antigen side chains of the lipopolysaccharide (LPS) molecule. To study possible common protective epitopes, monoclonal antibodies reacting with Shigella flexneri 2a LPS were generated from BALB/c mice infected ocularly with the virulent serotype 2a strain S. flexneri 2457T and tested against a panel of S. flexneri LPSs by enzyme-linked immunosorbent and immunoblot assays. Four monoclonal antibodies were identified, all of which showed restricted specificity patterns. Three different patterns of reactivity to LPS possessing the 3,4 group antigen were seen: (i) 2a only, (ii) 2a and 5a, and (iii) 2a, 4a, 5a, and Y. These results have implications for designing a Shigella vaccine that will be protective against related serotypes. Electron microscopy studies showed that the monoclonal antibodies bind to the bacterial surface in a patchy pattern, suggesting their potential use for examining the LPS distribution on the surface of the bacteria.

Safety and immunogenicity of the oral E. coli K12-S. flexneri 2a vaccine (EcSf2a-2) among Israeli soldiers

A double-blind placebo-controlled study was carried out on the safety and immunogenicity of the oral Shigella flexneri (EcSf2a-2) vaccine among Israeli soldiers. Sixty volunteers received the vaccine and 59 received placebo. Fifty-three were given the full vaccine regimen (four doses). Doses ranged between 4.1 x 10(8) and 1.1 x 10(9) c.f.u. Visits to the unit clinic for mild gastrointestinal symptoms were common after the first dose in vaccinees (13%) as compared with placebo recipients (5%), but the difference was not significant, p = 0.12. Similarly, there was no difference between the groups for either gastrointestinal or non-gastrointestinal complaints reported by questionnaire. The vaccine strain was excreted by 69% and 67% of the vaccinees one day after receiving the second and the fourth doses, respectively. As judged by antibiotic susceptibility, phage typing and restriction fragment length polymorphism (RFLP), the vaccine strain emerged as genetically stable after replication in human gut and shedding. There was neither bacteriological nor serological evidence of transmission of the vaccine from vaccinees to placebo recipients. Eighteen of 26 (69.2%) and 11 of 30 (36.7%) vaccinees had significant IgA secreting cell responses 7 and 21 days after the first dose, respectively. Significant IgA or IgG serum antibody response to S. flexneri 2a LPS was detected in 30% of the vaccinees. These results support further evaluation of EcSf2a-2 vaccine protective efficacy in field studies.

Local immune response and protection in the guinea pig keratoconjunctivitis model following immunization with Shigella vaccines

This study used the guinea pig keratoconjunctivitis model to examine the importance of route of administration (mucosal versus parenteral), frequency and timing of immunization (primary versus boosting immunization), and form of antigen given (live attenuated vaccine strain versus O-antigen-protein conjugate) on the production of protective immunity against Shigella infection. Since local immune response to the lipopolysaccharide (LPS) O-antigen of Shigella spp. is thought to be important for protection against disease, O-antigen-specific antibody-secreting cells (ASC) in the spleen and regional lymph nodes of immunized animals were measured by using an ELISPOT assay. Results indicated that protective efficacy was associated with a strong O-antigen-specific ASC response, particularly in the superficial ventral cervical lymph nodes draining the conjunctivae. In naive animals, a strong ASC response in the cervical lymph nodes and protection against challenge were detected only in animals that received a mucosal immunization. Protection in these animals was increased by a boosting mucosal immunization. While parenteral immunization alone with an O-antigen-protein conjugate vaccine did not protect naive animals against challenge, a combined parenteral-mucosal regimen elicited enhanced protection without the addition of a boosting immunization. Although O-antigen-specific serum immunoglobulin A titers were significantly higher in animals receiving a mucosal immunization, there was no apparent correlation between levels of serum antibody and protection against disease.

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.

Evaluation of Shigella vaccine safety and efficacy in an intranasally challenged mouse model

Five Shigella vaccine candidates (EcSf2a-1, EcSf2a-2, Sfl124, T32-Istrati and SMD) were tested for safety and efficacy in Balb/cJ mice using an intranasal challenge model. Experiments in this model suggest that (i) the relative attenuation of vaccines can be determined in mice by intranasal inoculation, (ii) all vaccines tested elicited antibacterial mucosal immunity protecting against pulmonary infection with Shigella flexneri 2a, (iii) protection was associated with serum IgA and/or IgG antibody recognizing the 2a somatic antigen.