Immunogenicity and characterization of WRSF2G11: a second generation live attenuated Shigella flexneri 2a vaccine strain

Shigella Vaccines: The Continuing Unmet Challenge

Shigellosis is a severe gastrointestinal disease that annually affects approximately 270 million individuals globally. It has particularly high morbidity and mortality in low-income regions; however, it is not confined to these regions and occurs in high-income nations when conditions allow. The ill effects of shigellosis are at their highest in children ages 2 to 5, with survivors often exhibiting impaired growth due to infection-induced malnutrition. The escalating threat of antibiotic resistance further amplifies shigellosis as a serious public health concern. This review explores Shigella pathology, with a primary focus on the status of Shigella vaccine candidates. These candidates include killed whole-cells, live attenuated organisms, LPS-based, and subunit vaccines. The strengths and weaknesses of each vaccination strategy are considered. The discussion includes potential Shigella immunogens, such as LPS, conserved T3SS proteins, outer membrane proteins, diverse animal models used in Shigella vaccine research, and innovative vaccine development approaches. Additionally, this review addresses ongoing challenges that necessitate action toward advancing effective Shigella prevention and control measures.

Oral immunization of Escherichia albertii strain DM104 induces protective immunity against Shigella dysenteriae type 4 in mouse model

The recent rise of antibiotic resistance and lack of an effective vaccine make the scenario of shigellosis alarming in developing countries like Bangladesh. In recent years, our group reported the vaccine efficacy of a non-pathogenic Escherichia albertii strain DM104 in different animal models, where an ocularly administered vaccine in the guinea pig eye model against Shigella dysenteriae type 4 challenge showed high protective efficacy and also induced a high titer of serum IgG against S. dysenteriae type 4 whole cell lysate (WCL) and LPS. In this study, we report further evaluation of the non-invasive and non-toxic environmental strain DM104 as a vaccine candidate against S. dysenteriae type 4 in mice model. Oral immunization of live DM104 bacterial strain demonstrated better protective immunity in mice model by showing 90% protection in mice against live S. dysenteriae type 4 lethal dose challenge and by inducing effective humoral and mucosal immune responses.

The Galleria mellonella larvae as an in vivo model for evaluation of Shigella virulence

Shigella spp. causing bacterial diarrhea and dysentery are human enteroinvasive bacterial pathogens that are orally transmitted through contaminated food and water and cause bacillary dysentery. Although natural Shigella infections are restricted to humans and primates, several smaller animal models are used to analyze individual steps in pathogenesis. No animal model fully duplicates the human response and sustaining the models requires expensive animals, costly maintenance of animal facilities, veterinary services and approved animal protocols. This study proposes the development of the caterpillar larvae of Galleria mellonella as a simple, inexpensive, informative, and rapid in-vivo model for evaluating virulence and the interaction of Shigella with cells of the insect innate immunity. Virulent Shigella injected through the forelegs causes larvae death. The mortality rates were dependent on the Shigella strain, the infectious dose, and the presence of the virulence plasmid. Wild-type S. flexneri 2a, persisted and replicated within the larvae, resulting in haemocyte cell death, whereas plasmid-cured mutants were rapidly cleared. Histology of the infected larvae in conjunction with fluorescence, immunofluorescence, and transmission electron microscopy indicate that S. flexneri reside within a vacuole of the insect haemocytes that ultrastructurally resembles vacuoles described in studies with mouse and human macrophage cell lines. Some of these bacteria-laden vacuoles had double-membranes characteristic of autophagosomes. These results suggest that G. mellonella larvae can be used as an easy-to-use animal model to understand Shigella pathogenesis that requires none of the time and labor-consuming procedures typical of other systems.

λ Recombination and Recombineering

The bacteriophage λ Red homologous recombination system has been studied over the past 50 years as a model system to define the mechanistic details of how organisms exchange DNA segments that share extended regions of homology. The λ Red system proved useful as a system to study because recombinants could be easily generated by co-infection of genetically marked phages. What emerged from these studies was the recognition that replication of phage DNA was required for substantial Red-promoted recombination in vivo, and the critical role that double-stranded DNA ends play in allowing the Red proteins access to the phage DNA chromosomes. In the past 16 years, however, the λ Red recombination system has gained a new notoriety. When expressed independently of other λ functions, the Red system is able to promote recombination of linear DNA containing limited regions of homology (∼50 bp) with the Escherichia coli chromosome, a process known as recombineering. This review explains how the Red system works during a phage infection, and how it is utilized to make chromosomal modifications of E. coli with such efficiency that it changed the nature and number of genetic manipulations possible, leading to advances in bacterial genomics, metabolic engineering, and eukaryotic genetics.

Alteration in apyrase enzyme attenuated virulence of Shigella flexneri

Virulence of Shigella is attributed to the genes presence in chromosome or in the megaplasmid. The apy gene which is located in the megaplasmid of Shigella species encodes for apyrase enzyme, a pathogenesis-associated enzyme causing mitochondrial damage and host cell death. In this study we constructed an apy mutant of Shigella flexneri by insertional activation using a kanamycin resistant gene cassette. The wild type apy gene of S. flexneri 2a was PCR amplified, cloned and mutated with insertion of kanamycin resistant gene cassette (aphA). The mutated construct (apy: aphA) was subcloned into a conjugative suicidal vector (pWM91) at the unique Sma1 and Sac1 sites. The mutation of the wild apy gene in the construct was confirmed by DNA sequencing. The mutated construct was introduced into wild type S. flexneri 2a by conjugation with Escherichia coli. After undergoing homologous recombination, the wild apy gene was deleted from the construct using the sucrose selection method. Non-functional activity of the apyrase enzyme in the constructed strain by colorimetric test indicated the successful mutation of the apyrase enzyme. This strain with mutated apy gene was evaluated for its protective efficacy using the guinea pig keratoconjunctivitis model. The strain was Sereny negative and it elicited a significant protection following challenge with wild S. flexneri strain. This apy mutant strain will form a base for the development of a vaccine target for shigellosis.

Vaccines for viral and bacterial pathogens causing acute gastroenteritis: Part II: Vaccines for Shigella, Salmonella, enterotoxigenic E. coli (ETEC) enterohemorragic E. coli (EHEC) and Campylobacter jejuni

In Part II we discuss the following bacterial pathogens: Shigella, Salmonella (non-typhoidal), diarrheogenic E. coli (enterotoxigenic and enterohemorragic) and Campylobacter jejuni. In contrast to the enteric viruses and Vibrio cholerae discussed in Part I of this series, for the bacterial pathogens described here there is only one licensed vaccine, developed primarily for Vibrio cholerae and which provides moderate protection against enterotoxigenic E. coli (ETEC) (Dukoral(®)), as well as a few additional candidates in advanced stages of development for ETEC and one candidate for Shigella spp. Numerous vaccine candidates in earlier stages of development are discussed.

An environmental Escherichia albertii strain, DM104, induces protective immunity to Shigella dysenteriae in guinea pig eye model

The environmental Escherichia albertii strain DM104, which cross-reacts serologically with Shigella dysenteriae was assessed for pathogenic properties, immunogenicity, and protective efficacy in different animal models to evaluate it as a vaccine candidate against S. dysenteriae, which causes the severe disease, shigellosis. The DM104 isolate was found to be non-invasive and did not produce any entero- or cyto-toxins. The strain also showed negative results in the mouse lethal activity assay. The non-pathogenic DM104 strain gave, however, a high protective efficacy as an ocularly administered vaccine in the guinea pig eye model against S. dysenteriae type 4 challenge. It also induced a high titer of serum IgG against S. dysenteriae type 4 whole cell lysate and lipopolysaccharide. Taken together, all these results indicate a good potential for the use of the DM104 as a live vaccine candidate against shigellosis.

Self-adjuvanting bacterial vectors expressing pre-erythrocytic antigens induce sterile protection against malaria

Genetically inactivated, Gram-negative bacteria that express malaria vaccine candidates represent a promising novel self-adjuvanting vaccine approach. Antigens expressed on particulate bacterial carriers not only target directly to antigen-presenting cells but also provide a strong danger signal thus circumventing the requirement for potent extraneous adjuvants. E. coli expressing malarial antigens resulted in the induction of either Th1 or Th2 biased responses that were dependent on both antigen and sub-cellular localization. Some of these constructs induced higher quality humoral responses compared to recombinant protein and most importantly they were able to induce sterile protection against sporozoite challenge in a murine model of malaria. In light of these encouraging results, two major Plasmodium falciparum pre-erythrocytic malaria vaccine targets, the Cell-Traversal protein for Ookinetes and Sporozoites (CelTOS) fused to the Maltose-binding protein in the periplasmic space and the Circumsporozoite Protein (CSP) fused to the Outer membrane (OM) protein A in the OM were expressed in a clinically relevant, attenuated Shigella strain (Shigella flexneri 2a). This type of live-attenuated vector has previously undergone clinical investigations as a vaccine against shigellosis. Using this novel delivery platform for malaria, we find that vaccination with the whole-organism represents an effective vaccination alternative that induces protective efficacy against sporozoite challenge. Shigella GeMI-Vax expressing malaria targets warrant further evaluation to determine their full potential as a dual disease, multivalent, self-adjuvanting vaccine system, against both shigellosis, and malaria.

Recent progress towards development of a Shigella vaccine

The burden of dysentery due to shigellosis among children in the developing world is still a major concern. A safe and efficacious vaccine against this disease is a priority, since no licensed vaccine is available. This review provides an update of vaccine achievements focusing on subunit vaccine strategies and the forthcoming strategies surrounding this approach. In particular, this review explores several aspects of the pathogenesis of shigellosis and the elicited immune response as being the basis of vaccine requirements. The use of appropriate Shigella antigens, together with the right adjuvants, may offer safety, efficacy and more convenient delivery methods for massive worldwide vaccination campaigns.

Progress and pitfalls in Shigella vaccine research

Renewed awareness of the substantial morbidity and mortality that Shigella infection causes among young children in developing countries, combined with technological innovations in vaccinology, has led to the development of novel vaccine strategies in the past 5 years. Along with advancement of classic vaccines in clinical trials and new sophisticated measurements of immunological responses, much new data has been produced, lending promise to the potential for production of safe and effective Shigella vaccines. Herein, we review the latest progress in Shigella vaccine development within the framework of persistent obstacles.

Two live attenuated Shigella flexneri 2a strains WRSf2G12 and WRSf2G15: a new combination of gene deletions for 2nd generation live attenuated vaccine candidates

Shigella infections are a major cause of inflammatory diarrhea and dysentery worldwide. First-generation virG-based live attenuated Shigella strains have been successfully tested in phase I and II clinical trials and are a leading approach for Shigella vaccine development. Additional gene deletions in senA, senB and msbB2 have been engineered into second-generation virG-based Shigella flexneri 2a strains producing WRSf2G12 and WRSf2G15. Both strains harbor a unique combination of gene deletions designed to increase the safety of live Shigella vaccines. WRSf2G12 and WRSf2G15 are genetically stable and highly attenuated in both cell culture and animal models of infection. Ocular immunization of guinea pigs with either strain induces robust systemic and mucosal immune responses that protect against homologous challenge with wild-type Shigella. The data support further evaluation of the second-generation strains in a phase I clinical trial.

Shigella flexneri infection in a newly acquired rhesus macaque (Macaca mulatta)

A 3.4 year-old rhesus macaque weighing 4.5 kg, was suffering from anorexia, acute mucous and bloody diarrhea. On physical examination, the monkey showed a loss of activity, hunched posture, abdominal pain, dehydration, mild gingivitis and unclean anus with discharge. Whole blood was collected for the examination of electrolytes, hematology and serum chemistry; fresh stool was also collected for bacterial culture. Blood profiles showed leukocytosis (14.5 K/µL) and neutrophilia (11.0 K/µL) on complete blood cell count and imbalanced electrolytes associated with diarrhea. As a result of bacterial culture, Shigella flexneri was identified through Mac/SS, IMVIC test, TCBS and VITEK II. Based on these results, this monkey was diagnosed as having acute enteritis caused by Shigella flexneri. Treatment was performed with enrofloxacin prior to the isolation of Shigella flexneri to prevent the transmission of disease. Fortunately, mucus and bloody diarrhea did not persist and general conditions fully recovered. Our results show that the use of enrofloxacin is effective in controlling Shigella flexneri infection in newly acquired rhesus monkeys.

Characterization of WRSs2 and WRSs3, new second-generation virG(icsA)-based Shigella sonnei vaccine candidates with the potential for reduced reactogenicity

Live, attenuated Shigella vaccine candidates, such as Shigella sonnei strain WRSS1, Shigella flexneri 2a strain SC602, and Shigella dysenteriae 1 strain WRSd1, are attenuated principally by the loss of the VirG(IcsA) protein. These candidates have proven to be safe and immunogenic in volunteer trials and in one study, efficacious against shigellosis. One drawback of these candidate vaccines has been the reactogenic symptoms of fever and diarrhea experienced by the volunteers, that increased in a dose-dependent manner. New, second-generation virG(icsA)-based S. sonnei vaccine candidates, WRSs2 and WRSs3, are expected to be less reactogenic while retaining the ability to generate protective levels of immunogenicity seen with WRSS1. Besides the loss of VirG(IcsA), WRSs2 and WRSs3 also lack plasmid-encoded enterotoxin ShET2-1 and its paralog ShET2-2. WRSs3 further lacks MsbB2 that reduces the endotoxicity of the lipid A portion of the bacterial LPS. Studies in cell cultures and in gnotobiotic piglets demonstrate that WRSs2 and WRSs3 have the potential to cause less diarrhea due to loss of ShET2-1 and ShET2-2 as well as alleviate febrile symptoms by loss of MsbB2. In guinea pigs, WRSs2 and WRSs3 were as safe, immunogenic and efficacious as WRSS1.

Virulence, inflammatory potential, and adaptive immunity induced by Shigella flexneri msbB mutants

The ability of genetically detoxified lipopolysaccharide (LPS) to stimulate adaptive immune responses is an ongoing area of investigation with significant consequences for the development of safe and effective bacterial vaccines and adjuvants. One approach to genetic detoxification is the deletion of genes whose products modify LPS. The msbB1 and msbB2 genes, which encode late acyltransferases, were deleted in the Shigella flexneri 2a human challenge strain 2457T to evaluate the virulence, inflammatory potential, and acquired immunity induced by strains producing underacylated lipid A. Consistent with a reduced endotoxic potential, S. flexneri 2a msbB mutants were attenuated in an acute mouse pulmonary challenge model. Attenuation correlated with decreases in the production of proinflammatory cytokines and in chemokine release without significant changes in lung histopathology. The levels of specific proinflammatory cytokines (interleukin-1beta [IL-1beta], macrophage inflammatory protein 1alpha [MIP-1alpha], and tumor necrosis factor alpha [TNF-alpha]) were also significantly reduced after infection of mouse macrophages with either single or double msbB mutants. Surprisingly, the msbB double mutant displayed defects in the ability to invade, replicate, and spread within epithelial cells. Complementation restored these phenotypes, but the exact nature of the defects was not determined. Acquired immunity and protective efficacy were also assayed in the mouse lung model, using a vaccination-challenge study. Both humoral and cellular responses were generally robust in msbB-immunized mice and afforded significant protection from lethal challenge. These data suggest that the loss of either msbB gene reduces the endotoxicity of Shigella LPS but does not coincide with a reduction in protective immune responses.

Shigellosis: the current status of vaccine development

PURPOSE OF REVIEW

Shigellosis, a major form of bacillary dysentery, is caused by infection with Shigella organisms. In poor countries, Shigella-caused dysentery is endemic and causes an estimated 163 million illness episodes annually and more than one million deaths. Although several strategies have been used to develop vaccines targeting shigellosis, none has been licensed for use outside China. Owing to the wide range of Shigella serotypes and subtypes, there is a need for a multivalent vaccine representing prevalent species and serotypes.

RECENT FINDINGS

Vaccine development has been limited by the lack of a suitable animal model for vaccine testing. This review discusses the most advanced strategies for Shigella vaccine development including live attenuated, conjugate, broad spectrum, and proteosome-based vaccines and describes current animal models under study.

SUMMARY

The greatest barrier to the use of vaccine against shigellosis in developing areas is poor immune responses to oral vaccines in children who have minimal maternal antibodies. Clinical studies of promising shigellosis vaccine candidates are urgently needed after confirmation of safety, immunogenicity, and protection in volunteer challenge models.

Bacterial genetics: past achievements, present state of the field, and future challenges

Genetic tools are required to take full advantage of the wealth of information generated by genome sequencing efforts and ensuing global gene and protein expression analyses. Bacterial genetics was originally developed and refined in Escherichia coli. As a consequence, elegant plasmid, cloning, expression, and mutagenesis systems were developed over the years and a good number of them are commercially available. This is not true for other bacteria. Although the development of genetic tools has generally not kept up with the sequencing pace, substantial progress has been made in this arena with many bacterial species. This short review highlights selected topics and achievements in the field over the past 25 years and presents some strategies that may help address future challenges. BioTechniques has played an integral part in the publication of important technological advances in the field over the first 25 years of its existence and it can be anticipated that it will continue to do so in the future.