Core-linked LPS expression of Shigella dysenteriae serotype 1 O-antigen in live Salmonella Typhi vaccine vector Ty21a: Preclinical evidence of immunogenicity and protection

Bactofection, Bacterial-Mediated Vaccination, and Cancer Therapy: Current Applications and Future Perspectives

From the first report in 1891 by Dr. Coley of the effective treatment of tumors in 1000 patients with Streptococcus and the first successful use of bacterial vectors for transferring therapeutic genes in 1980 by Dr. Schnaffer, bactofection has been shown to be a promising strategy in the fields of vaccination, gene therapy, and cancer therapy. This review describes the general theory of bactofection and its advantages, disadvantages, challenges, and expectations, compiling the most notable advances in 14 vaccination studies, 27 cancer therapy studies, and 13 clinical trials. It also describes the current scope of bactofection and promising results. The extensive knowledge of Salmonella biology, as well as the multiple adequacies of the Ty21a vaccination platform, has allowed notable developments worldwide that have mainly been reflected in therapeutic efforts against cancer. In this regard, we strongly recommend the creation of a recombinant Ty21a model that constitutively expresses the GtgE protease from S. typhimurium, allowing this vector to be used in animal trials, thus enhancing the likelihood of favorable results that could quickly transition to clinical trials. From the current perspective, it is necessary to explore a greater diversity of bacterial vectors and find the best combination of implemented attenuations, generating personalized models that guarantee the maximum effectiveness in cancer therapy and vaccination.

A biologically conjugated polysaccharide vaccine delivered by attenuated Salmonella Typhimurium provides protection against challenge of avian pathogenic Escherichia coli O1 infection

Avian pathogenic Escherichia coli (APEC) causes avian airsacculitis and colibacillosis, resulting in significant economic loss to the poultry industry. O1, O2 and O78 are the three predominant serotypes. O-antigen of lipopolysaccharide is serotype determinant and highly immunogenic, and O-antigen polysaccharide-based vaccines have great potential for preventing bacterial infections. In this study, we utilized a novel yeast/bacterial shuttle vector pSS26 to clone the 10.8 kb operon synthesizing APEC O1 O-antigen polysaccharide. The resulting plasmid was introduced into attenuated Salmonella vaccines to deliver this O-antigen polysaccharide. O1 O-antigen was stably synthesized in attenuated Salmonella Typhimurium, demonstrated by slide agglutination, silver staining and western blot. Our results also showed that APEC O1 O-antigen produced in the Salmonella vaccines was attached to bacterial cell surfaces, and the presence of heterologous O-antigen did not alter the resistance to surface-acting agents. Furthermore, birds immunized orally or intramuscularly provided protection against the virulent O1 APEC challenge. Salmonella vaccines carrying APEC O1 O-antigen gene cluster also induced high IgG and IgA immune responses against lipopolysaccharide from the APEC O1 strain. The use of our novel shuttle vector facilitates cloning of large DNA fragments, and this strategy could pave the way for production of Salmonella-vectored vaccines against prevalent APEC serotypes.

A bivalent vaccine derived from attenuated Salmonella expressing O-antigen polysaccharide provides protection against avian pathogenic Escherichia coli O1 and O2 infection

Avian pathogenic Escherichia coli (APEC), a leading cause of avian airsacculitis and colibacillosis, is responsible for significant economic loss in the poultry industry. APEC serogroups O1, O2, and O78 are predominantly associated with disease. Lipopolysaccharide (LPS) O-antigen has been shown to be a potent antigen for inducing specific protective immune responses. Therefore, we sought to develop a multivalent polysaccharide vaccine to prevent most APEC infections. We previously reported the stable expression of plasmid pSS27 encoding the APEC O1 O-antigen gene cluster (10.8 kb) in attenuated Salmonella enterica serovar Typhimurium S740 provided excellent protection against APEC O1 challenge. In this study, the plasmid pSS28 harboring the APEC O2 O-antigen polysaccharide gene cluster (15.5 kb) was constructed. Biosynthesis of pSS28-encoded APEC O2 O-antigen in Salmonella vaccine strain S740 was validated by Western blot. The recombinant Salmonella vaccine strain S740 (pSS28) elicited homologous protection against virulent wild-type APEC O2 challenge in a chicken model. Furthermore, through equal-volume mixing the two monovalent vaccine strains S740 (pSS27) and S740 (pSS28), a bivalent vaccine candidate against both APEC O1 and O2 was developed. Immunization of chickens with the bivalent vaccine elicited production of serum IgG and mucosal sIgA antibodies against the LPS of both APEC O1 and O2. Moreover, antibodies induced by the bivalent vaccine promoted opsonization, provoked complement-mediated bactericidal activity, and elicited protection against lethal challenge with both virulent APEC O1 and O2 strains. These results demonstrate that the bivalent vaccine comprised of S740 (pSS27) and S740 (pSS28) is a promising vaccine candidate against APEC O1 and O2 infection.

Bacterial Glycoengineering as a Biosynthetic Route to Customized Glycomolecules

Bacteria have garnered increased interest in recent years as a platform for the biosynthesis of a variety of glycomolecules such as soluble oligosaccharides, surface-exposed carbohydrates, and glycoproteins. The ability to engineer commonly used laboratory species such as Escherichia coli to efficiently synthesize non-native sugar structures by recombinant expression of enzymes from various carbohydrate biosynthesis pathways has allowed for the facile generation of important products such as conjugate vaccines, glycosylated outer membrane vesicles, and a variety of other research reagents for studying and understanding the role of glycans in living systems. This chapter highlights some of the key discoveries and technologies for equipping bacteria with the requisite biosynthetic machinery to generate such products. As the bacterial glyco-toolbox continues to grow, these technologies are expected to expand the range of glycomolecules produced recombinantly in bacterial systems, thereby opening up this platform to an even larger number of applications.

Stable Chromosomal Expression of Shigella flexneri 2a and 3a O-Antigens in the Live Salmonella Oral Vaccine Vector Ty21a

We have been exploring the use of the live attenuated Salmonella enterica serovar Typhi Ty21a vaccine strain as a versatile oral vaccine vector for the expression and delivery of multiple foreign antigens, including Shigella O-antigens. In this study, we separately cloned genes necessary for the biosynthesis of the Shigella flexneri serotype 2a and 3a O-antigens, which have been shown to provide broad cross-protection to multiple disease-predominant S. flexneri serotypes. The cloned S. flexneri 2a rfb operon, along with bgt and gtrII, contained on the SfII bacteriophage, was sufficient in Ty21a to express the heterologous S. flexneri 2a O-antigen containing the 3,4 antigenic determinants. Further, this rfb operon, along with gtrA, gtrB, and gtrX contained on the Sfx bacteriophage and oac contained on the Sf6 bacteriophage, was sufficient to express S. flexneri 3a O-antigen containing the 6, 7, and 8 antigenic determinants. Ty21a, with these plasmid-carried or chromosomally inserted genes, demonstrated simultaneous and stable expression of homologous S Typhi O-antigen plus the heterologous S. flexneri O-antigen. Candidate Ty21a vaccine strains expressing heterologous S. flexneri 2a or 3a lipopolysaccharide (LPS) elicited significant serum antibody responses against both homologous S Typhi and heterologous Shigella LPS and protected mice against virulent S. flexneri 2a or 3a challenges. These new S. flexneri 2a and 3a O-antigen-expressing Ty21a vaccine strains, together with our previously constructed Ty21a strains expressing Shigella sonnei or Shigella dysenteriae 1 O-antigens, have the potential to be used together for simultaneous protection against the predominant causes of shigellosis worldwide as well as against typhoid fever.

Development of an Acid-Resistant Salmonella Typhi Ty21a Attenuated Vector For Improved Oral Vaccine Delivery

The licensed oral, live-attenuated bacterial vaccine for typhoid fever, Salmonella enterica serovar Typhi strain Ty21a, has also been utilized as a vaccine delivery platform for expression of diverse foreign antigens that stimulate protection against shigellosis, anthrax, plague, or human papilloma virus. However, Ty21a is acid-labile and, for effective oral immunization, stomach acidity has to be either neutralized with buffer or by-passed with Ty21a in an enteric-coated capsule (ECC). Several studies have shown that efficacy is reduced when Ty21a is administered in an ECC versus as a buffered liquid formulation, the former limiting exposure to GI tract lymphoid tissues. However, the ECC was selected as a more practical delivery format for both packaging/shipping and vaccine administration ease. We have sought to increase Ty21a acid-resistance to allow for removal from the ECC and immune enhancement. To improve Ty21a acid-resistance, glutamate-dependent acid resistance genes (GAD; responsible for Shigella spp. survival at very low pH) were cloned on a multi-copy plasmid (pGad) under a controllable arabinose-inducible promoter. pGad enhanced acid survival of Ty21a by 5 logs after 3 hours at pH 2.5, when cells were pre-grown in arabinose and under conditions that promote an acid-tolerance response (ATR). For genetically 100% stable expression, we inserted the gad genes into the Ty21a chromosome, using a method that allowed for subsequent removal of a selectable antibiotic resistance marker. Further, both bacterial growth curves and survival assays in cultured human monocytes/macrophages suggest that neither the genetic methods employed nor the resulting acid-resistance conferred by expression of the Gad proteins in Ty21a had any effect on the existing attenuation of this vaccine strain.

Plague Vaccines: Status and Future

Three major plague pandemics caused by the gram-negative bacterium Yersinia pestis have killed nearly 200 million people in human history. Due to its extreme virulence and the ease of its transmission, Y. pestis has been used purposefully for biowarfare in the past. Currently, plague epidemics are still breaking out sporadically in most of parts of the world, including the United States. Approximately 2000 cases of plague are reported each year to the World Health Organization. However, the potential use of the bacteria in modern times as an agent of bioterrorism and the emergence of a Y. pestis strain resistant to eight antibiotics bring out severe public health concerns. Therefore, prophylactic vaccination against this disease holds the brightest prospect for its long-term prevention. Here, we summarize the progress of the current vaccine development for counteracting plague.

Live-Attenuated Bacterial Vectors: Tools for Vaccine and Therapeutic Agent Delivery

Genetically attenuated microorganisms, including pathogenic and commensal bacteria, can be engineered to carry and deliver heterologous antigens to elicit host immunity against both the vector as well as the pathogen from which the donor gene is derived. These live attenuated bacterial vectors have been given much attention due to their capacity to induce a broad range of immune responses including localized mucosal, as well as systemic humoral and/or cell-mediated immunity. In addition, the unique tumor-homing characteristics of these bacterial vectors has also been exploited for alternative anti-tumor vaccines and therapies. In such approach, tumor-associated antigen, immunostimulatory molecules, anti-tumor drugs, or nucleotides (DNA or RNA) are delivered. Different potential vectors are appropriate for specific applications, depending on their pathogenic routes. In this review, we survey and summarize the main features of the different types of live bacterial vectors and discussed the clinical applications in the field of vaccinology. In addition, different approaches for using live attenuated bacterial vectors for anti-cancer therapy is discussed, and some promising pre-clinical and clinical studies in this field are outlined.

Live bacterial vaccine vectors: an overview

Genetically attenuated microorganisms, pathogens, and some commensal bacteria can be engineered to deliver recombinant heterologous antigens to stimulate the host immune system, while still offering good levels of safety. A key feature of these live vectors is their capacity to stimulate mucosal as well as humoral and/or cellular systemic immunity. This enables the use of different forms of vaccination to prevent pathogen colonization of mucosal tissues, the front door for many infectious agents. Furthermore, delivery of DNA vaccines and immune system stimulatory molecules, such as cytokines, can be achieved using these special carriers, whose adjuvant properties and, sometimes, invasive capacities enhance the immune response. More recently, the unique features and versatility of these vectors have also been exploited to develop anti-cancer vaccines, where tumor-associated antigens, cytokines, and DNA or RNA molecules are delivered. Different strategies and genetic tools are constantly being developed, increasing the antigenic potential of agents delivered by these systems, opening fresh perspectives for the deployment of vehicles for new purposes. Here we summarize the main characteristics of the different types of live bacterial vectors and discuss new applications of these delivery systems in the field of vaccinology.

Vaccines against human diarrheal pathogens: current status and perspectives

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

Pushing the Bacterial Envelope

Over the past three decades, a powerful array of techniques has been developed for expressing heterologous proteins and saccharides on the surface of bacteria. Surface-engineered bacteria, in turn, have proven useful in a variety of settings, including high-throughput screening, biofuel production, and vaccinology. In this chapter, we provide a comprehensive review of methods for displaying polypeptides and sugars on the bacterial cell surface, and discuss the many innovative applications these methods have found to date. While already an important biotechnological tool, we believe bacterial surface display may be further improved through integration with emerging methodology in other fields, such as protein engineering and synthetic chemistry. Ultimately, we envision bacterial display becoming a multidisciplinary platform with the potential to transform basic and applied research in bacteriology, biotechnology, and biomedicine.

Genome Sequence of Salmonella enterica Serovar Typhi Oral Vaccine Strain Ty21a

Attenuated Salmonellaenterica serovar Typhi strain Ty21a is an important vaccine for controlling typhoid fever and serves as an oral vector for delivering heterologous antigens. The key attenuating features of this randomly mutated strain remain in question. Genome sequencing has revealed 679 single nucleotide polymorphisms (SNPs), and will help define alterations contributing to Ty21a safety and immunogenicity.

Serological cross-reaction between O-antigens of Shigella dysenteriae type 4 and an environmental Escherichia albertii isolate

An environmental freshwater bacterial isolate, DM104, appearing as Shigella-like colonies on selective agar plates was found to show strong and specific serological cross-reactivity with Shigella dysenteriae type 4. Biochemical identification according to the analytical profile index, molecular serotyping by restriction of the amplified O-antigen gene cluster (rfb-RFLP), together with phylogenetic analysis of the 16S rRNA gene and multi-locus sequence analysis, identified the isolate as Escherichia albertii. rfb-RFLP of DM104, revealed a profile different from that of S. dysenteriae type 4. However, western blot analysis of extracted lipopolysaccharides demonstrated strong cross-reactivity with S. dysenteriae type 4 using specific monovalent antisera and a lipopolysaccharide gel banding profile similar to that of S. dysenteriae type 4. The observed O-antigen cross-reaction between an E. albertii isolate and S. dysenteriae extends our knowledge of the extent of O-antigen cross-reaction within the Escherichia/Shigella group of organisms, and offers the possibility of using DM104 and similar cross-reacting strains as shigellosis vaccine candidates.

Survival of Salmonella Typhi and Shigella dysenteriae in dehydrated infant formula

Powdered infant formula has previously been linked to the transmission of various bacterial pathogens in infants resulting in life-threatening disease and death. Survival studies of 2 common foodborne pathogens, Salmonella enterica serovar Typhi and Shigella dysenteriae, in powdered infant formula have not been previously studied despite the potentially devastating consequences from ingestion of these organisms, particularly by newborns, in case of a natural or deliberate contamination event. Therefore, to better predict the risk of S. Typhi and S. dysenteriae infection from consumption of infant formula, the present study was undertaken to determine survival of these microorganisms in dry infant formula under varying atmospheric conditions. A 2-strain cocktail of S. Typhi and a 3-strain cocktail of S. dysenteriae were stored for up to 12 wk in dehydrated infant formula in an ambient air or nitrogen atmosphere. Viable counts of S. Typhi at 12 wk in infant formula revealed a 2.9- and 1.69-log decrease in ambient air and nitrogen atmosphere, respectively. Viable counts of S. dysenteriae at 12 wk in infant formula revealed a 0.81- and 0.42-log decrease in ambient air and nitrogen atmosphere, respectively. These results show that S. Typhi and S. dysenteriae can remain viable for prolonged periods of time in powdered infant formula, and the presence of nitrogen enhances survival.

PRACTICAL APPLICATION

Our goal in this work was to study the survival of S. Typhi and S. dysenteriae in dehydrated storage conditions in infant formula. This interest is partially generated by the possibility of using these 2 microorganisms to deliberately contaminate the food supply. The outcome of this study will help us to have a better idea how to respond and react to the risk of deliberate food contamination.

Enteric infections, diarrhea, and their impact on function and development

Enteric infections, with or without overt diarrhea, have profound effects on intestinal absorption, nutrition, and childhood development as well as on global mortality. Oral rehydration therapy has reduced the number of deaths from dehydration caused by infection with an enteric pathogen, but it has not changed the morbidity caused by such infections. This Review focuses on the interactions between enteric pathogens and human genetic determinants that alter intestinal function and inflammation and profoundly impair human health and development. We also discuss specific implications for novel approaches to interventions that are now opened by our rapidly growing molecular understanding.