Comparison of abilities of Salmonella enterica serovar typhimurium aroA aroD and aroA htrA mutants to act as live vectors

The Immunomodulatory Effects and Applications of Probiotic Lactiplantibacillus plantarum in Vaccine Development

Lactiplantibacillus plantarum (previously known as Lactobacillus plantarum) is a lactic acid bacterium that exists in various niches. L. plantarum is a food-grade microorganism that is commonly considered a safe and beneficial microorganism. It is widely used in food fermentation, agricultural enhancement, and environmental protection. L. plantarum is also part of the normal flora that can regulate the intestinal microflora and promote intestinal health. Some strains of L. plantarum are powerful probiotics that induce and modulate the innate and adaptive immune responses. Due to its outstanding immunoregulatory capacities, an increasing number of studies have examined the use of probiotic L. plantarum strains as natural immune adjuvants or alternative live vaccine carriers. The present review summarizes the main immunomodulatory characteristics of L. plantarum and discusses the preliminary immunological effects of L. plantarum as a vaccine adjuvant and delivery carrier. Different methods for improving the immune capacities of recombinant vector vaccines are also discussed.

Lacticaseibacillus rhamnosus: A Suitable Candidate for the Construction of Novel Bioengineered Probiotic Strains for Targeted Pathogen Control

Probiotics, with their associated beneficial effects, have gained popularity for the control of foodborne pathogens. Various sources are explored with the intent to isolate novel robust probiotic strains with a broad range of health benefits due to, among other mechanisms, the production of an array of antimicrobial compounds. One of the shortcomings of these wild-type probiotics is their non-specificity. A pursuit to circumvent this limitation led to the advent of the field of pathobiotechnology. In this discipline, specific pathogen gene(s) are cloned and expressed into a given probiotic to yield a novel pathogen-specific strain. The resultant recombinant probiotic strain will exhibit enhanced species-specific inhibition of the pathogen and its associated infection. Such probiotics are also used as vehicles to deliver therapeutic agents. As fascinating as this approach is, coupled with the availability of numerous probiotics, it brings a challenge with regard to deciding which of the probiotics to use. Nonetheless, it is indisputable that an ideal candidate must fulfil the probiotic selection criteria. This review aims to show how Lacticaseibacillus rhamnosus, a clinically best-studied probiotic, presents as such a candidate. The objective is to spark researchers’ interest to conduct further probiotic-engineering studies using L. rhamnosus, with prospects for the successful development of novel probiotic strains with enhanced beneficial attributes.

Insight of oral vaccines as an alternative approach to health and disease management: An innovative intuition and challenges

Vaccination is the most suitable and persuasive healthcare program for the prohibition of various deadly diseases. However, the higher production cost and purification strategies are out of reach for the developing nations. In this scenario, development of edible vaccine turns out to be the most promising alternative for remodeling the pharmaceutical industry with reduced production and purification costs. Generally, oral route of vaccination is mostly preferred due to its safety, compliance, low manufacturing cost and most importantly the ability to induce immunity in both systemic and mucosal sites. Genetically modified microorganisms and plants could efficiently be used as vehicles for edible vaccines. Edible vaccines are supposed to reduce the risk associated with traditional vaccines. Currently, oral vaccines are available in the market for several viral and bacterial diseases like cholera, hepatitis B, malaria, rabies etc. Herein, the review focuses on the breakthrough events in the area of edible vaccines associated with dietary microbes and plants for better control over diseases.

Contribution of the serine protease HtrA in Escherichia coli to infection in foxes

Escherichia coli can cause severe, acute hemorrhagic pneumonia and systemic infection in farmed foxes, raccoon dogs and minks, leading to considerable economic losses to the farmers. It is well established that the htrA-encoded serine protease HtrA is critical for bacterial growth and survival under stress, and HtrA has been determined to be a potential vaccine target. However, the roles of HtrA in E. coli pathogenesis remain unknown. In this study, we generated an htrA-deletion mutant of the E. coli protype strain HBCLE-12 that causes pneumonia in silver foxes and then evaluated the changes in bacterial physiological characteristics in the absence of HtrA. The data show that knockout of the htrA gene did not affect growth and biochemical characteristics but led to impaired virulence of the strain. Increased susceptibility to environmental stresses, impaired survival in serum, and reduced biofilm formation may contribute to the virulence attenuation of the mutant. Furthermore, the HtrA-deficient mutant was subjected to RNA-seq analysis, and 16 differentially expressed genes were determined. This study provided insight that HtrA plays a definitive role in E. coli-induced infection.

Live-Attenuated Bacterial Vectors for Delivery of Mucosal Vaccines, DNA Vaccines, and Cancer Immunotherapy

Vaccines save millions of lives each year from various life-threatening infectious diseases, and there are more than 20 vaccines currently licensed for human use worldwide. Moreover, in recent decades immunotherapy has become the mainstream therapy, which highlights the tremendous potential of immune response mediators, including vaccines for prevention and treatment of various forms of cancer. However, despite the tremendous advances in microbiology and immunology, there are several vaccine preventable diseases which still lack effective vaccines. Classically, weakened forms (attenuated) of pathogenic microbes were used as vaccines. Although the attenuated microbes induce effective immune response, a significant risk of reversion to pathogenic forms remains. While in the twenty-first century, with the advent of genetic engineering, microbes can be tailored with desired properties.

In this review, I have focused on the use of genetically modified bacteria for the delivery of vaccine antigens. More specifically, the live-attenuated bacteria, derived from pathogenic bacteria, possess many features that make them highly suitable vectors for the delivery of vaccine antigens. Bacteria can theoretically express any heterologous gene or can deliver mammalian expression vectors harboring vaccine antigens (DNA vaccines). These properties of live-attenuated microbes are being harnessed to make vaccines against several infectious and noninfectious diseases. In this regard, I have described the desired features of live-attenuated bacterial vectors and the mechanisms of immune responses manifested by live-attenuated bacterial vectors. Interestingly anaerobic bacteria are naturally attracted to tumors, which make them suitable vehicles to deliver tumor-associated antigens thus I have discussed important studies investigating the role of bacterial vectors in immunotherapy. Finally, I have provided important discussion on novel approaches for improvement and tailoring of live-attenuated bacterial vectors for the generation of desired immune responses.

Colonic immune stimulation by targeted oral vaccine

Background

Currently, sufficient data exist to support the use of lactobacilli as candidates for the development of new oral targeted vaccines. To this end, we have previously shown that Lactobacillus gasseri expressing the protective antigen (PA) component of anthrax toxin genetically fused to a dendritic cell (DC)-binding peptide (DCpep) induced efficacious humoral and T cell-mediated immune responses against Bacillus anthracis Sterne challenge.

Methodology/Principal Finding

In the present study, we investigated the effects of a dose dependent treatment of mice with L. gasseri expressing the PA-DCpep fusion protein on intestinal and systemic immune responses and confirmed its safety. Treatment of mice with different doses of L. gasseri expressing PA-DCpep stimulated colonic immune responses, resulting in the activation of innate immune cells, including dendritic cells, which induced robust Th1, Th17, CD4⁺Foxp3⁺ and CD8⁺Foxp3⁺ T cell immune responses. Notably, high doses of L. gasseri expressing PA-DCpep (10¹² CFU) were not toxic to the mice. Treatment of mice with L. gasseri expressing PA-DCpep triggered phenotypic maturation and the release of proinflammatory cytokines by dendritic cells and macrophages. Moreover, treatment of mice with L. gasseri expressing PA-DCpep enhanced antibody immune responses, including IgA, IgG₁, IgG_2b, IgG_2c and IgG₃. L. gasseri expressing PA-DCpep also increased the gene expression of numerous pattern recognition receptors, including Toll-like receptors, C-type lectin receptors and NOD-like receptors.

Conclusion/Significance

These findings suggest that L. gasseri expressing PA-DCpep has substantial immunopotentiating properties, as it can induce humoral and T cell-mediated immune responses upon oral administration and may be used as a safe oral vaccine against anthrax challenge.

Invited review: Application of omics tools to understanding probiotic functionality

The human gut microbiota comprises autochthonous species that colonize and reside at high levels permanently and allochthonous species that originate from another source and are transient residents of the human gut. The interactions between bacteria and the human host can be classified as a continuum from symbiosis and commensalism (mutualism) to pathogenesis. Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. Recent advances in omics tools and sequencing techniques have furthered our understanding of probiotic functionality and the specific interactions between probiotics and their human hosts. Although it is known that not all probiotics use the same mechanisms to confer benefits on hosts, some specific mechanisms of action have been revealed through omic investigations. These include competitive exclusion, bacteriocin-mediated protection against intestinal pathogens, intimate interactions with mucin and the intestinal epithelium, and modulation of the immune system. The ability to examine fully sequenced and annotated genomes has greatly accelerated the application of genetic approaches to elucidate many important functional roles of probiotic microbes.

Microbes, intestinal inflammation and probiotics

Inflammatory bowel disease (IBD) is known for causing disturbed homeostatic balance among the intestinal immune compartment, epithelium and microbiota. Owing to the emergence of IBD as a major cause of morbidity and mortality, great efforts have been put into understanding the sequence of intestinal inflammatory events. Intestinal macrophages and dendritic cells act in a synergistic fashion with intestinal epithelial cells and microbiota to initiate the triad that governs the intestinal immune responses (whether inflammatory or regulatory). In this review, we will discuss the interplay of intestinal epithelial cells, bacteria and the innate immune component. Moreover, whether or not genetic intervention of probiotic bacteria is a valid approach for attenuating/mitigating exaggerated inflammation and IBD will also be discussed.

Surface display of domain III of Japanese encephalitis virus E protein on Salmonella typhimurium by using an ice nucleation protein

A bacterial cell surface display technique based on an ice nucleation protein has been employed for the development of live vaccine against viral infection. Due to its ubiquitous ability to invade host cells, Salmonella typhimurium might be a good candidate for displaying viral antigens. We demonstrated the surface display of domain III of Japanese encephalitis virus E protein and the enhanced green fluorescent protein on S. typhimurium BRD509 using the ice nucleation protein. The effects of the motif in the ice nucleation protein on the effective display of integral protein were also investigated. The results showed that display motifs in the protein can target integral foreign protein on the surface of S. typhimurium BRD509. Moreover, recombinant strains with surface displayed viral proteins retained their invasiveness, suggesting that the recombinant S. typhimurium can be used as live vaccine vector for eliciting complete immunogenicity. The data may yield better understanding of the mechanism by which ice nucleation protein displays foreign proteins in the Salmonella strain.

Dendritic cell targeting of Bacillus anthracis protective antigen expressed by Lactobacillus acidophilus protects mice from lethal challenge

Efficient vaccines potentiate antibody avidity and increase T cell longevity, which confer protection against microbial lethal challenge. A vaccine strategy was established by using Lactobacillus acidophilus to deliver Bacillus anthracis protective antigen (PA) via specific dendritic cell-targeting peptides to dendritic cells (DCs), which reside in the periphery and mucosal surfaces, thus directing and regulating acquired immunity. The efficiency of oral delivery of L. acidophilus expressing a PA-DCpep fusion was evaluated in mice challenged with lethal B. anthracis Sterne. Vaccination with L. acidophilus expressing PA-DCpep induced robust protective immunity against B. anthracis Sterne compared with mice vaccinated with L. acidophilus expressing PA-control peptide or an empty vector. Additionally, serum anti-PA titers, neutralizing PA antibodies, and the levels of IgA-expressing cells were all comparable with the historical recombinant PA plus aluminum hydroxide vaccine administered s.c. Collectively, development of this strategy for oral delivery of DC-targeted antigens provides a safe and protective vaccine via a bacterial adjuvant that may potentiate mucosal immune responses against deadly pathogens.

A high-throughput comparison of recombinant gene expression parameters for E. coli-mediated gene transfer to P388D1 macrophage cells

Escherichia coli strain BL21(DE3) was tested as a delivery vector for gene transfer to a murine P388D1 macrophage cell line using a 96-well high-throughput assay. Five recombinant strains of E. coli were compared to identify the effect recombinant listeriolysin O (LLO) and associated gene expression parameters had on final delivery of a luciferase reporter gene. Listeriolysin O, native to Listeria monocytogenes and used here in an effort to improve final gene delivery, was expressed from plasmid and chromosomal locations under the control of constitutive Tet or inducible T7 promoters. The E. coli vectors delivered the luciferase reporter gene to the P388D1 line with success assessed by recording luciferase luminescence activity within the macrophage cells. The assay allowed rapid analysis and evaluation of each E. coli strain tested with strain BL21(DE3) harboring a chromosomal copy of the T7-driven LLO gene showing the greatest relative measure of gene delivery. Strains were separately assayed for LLO activity and exhibited a trend of maximum gene delivery between the lowest and highest recorded LLO activities.

Targeting mucosal dendritic cells with microbial antigens from probiotic lactic acid bacteria

The use of vaccines against infectious microbes has been critical to the advancement of medicine. Vaccine strategies combined with, or without, adjuvants have been established to eradicate various bacterial and viral pathogens. A new generation of vaccines is being developed using specific strains of Gram-positive, lactic acid bacteria and, notably, some probiotic lactobacilli. These bacteria have been safely consumed by humans for centuries in fermented foods. Thus, they can be orally administered, are well tolerated by recipients and could be easily and economically provided to large populations. In this overview, we focus on mucosal immunity and how its cellular component(s), particularly dendritic cells, can be specifically targeted to deliver immunogenic subunits, such as the protective antigen from Bacillus anthracis (the causative agent of anthrax). An antigen-specific immune response can be elicited using specific strains of Lactobacillus acidophilus expressing the protective antigen. A mucosal, dendritic cell-targeted approach increases the bioavailability of an immunogen of interest when delivered orally by L. acidophilus. This provides an efficiently elegant natural strategy and serves a dual function as an immune-stimulating adjuvant in vivo.

Genetic detoxification of an aroA Salmonella enterica serovar Typhimurium vaccine strain does not compromise protection against virulent Salmonella and enhances the immune responses towards a protective malarial antigen

Live Salmonella vaccines are limited in use by the inherent toxicity of the lipopolysaccharide. The waaN gene encodes a myristyl transferase required for the secondary acylation of lipid A in lipopolysaccharide. A waaN mutant exhibits reduced induction of the inflammatory cytokines associated with lipopolysaccharide toxicity. Here the characteristics of a Salmonella enterica serovar Typhimurium aroA waaN mutant (SK100) in vitro and in vivo compared with its parent aroA strain (SL3261) were described. Phenotypic analysis of purified lipopolysaccharide obtained from SK100 confirmed that the physical and biological activities of the lipopolysaccharide had been altered. Nevertheless both strains had similar patterns of colonization and persistence in mice and significantly the aroA waaN mutant was equally as effective as the parent at protecting against challenge with wild-type S. Typhimurium. Furthermore, a SK100 strain was constructed expressing both tetanus toxin fragment C and the circumsporozoite protein of a malaria parasite. In marked contrast to its isogenic parent, the new attenuated strain induces significantly enhanced immune responses against the circumsporozoite protein. The waaN mutation enhances the ability of this strain to elicit immune responses towards guest antigens. This study provides important insights into the development of safe and effective multivalent Salmonella vaccines.

Engineering bacterial vectors for delivery of genes and proteins to antigen-presenting cells

Bacterial vectors offer a biological route to gene and protein delivery with this article featuring delivery to antigen-presenting cells (APCs). Primarily in the context of immune stimulation against infectious disease or cancer, the goal of bacterially mediated delivery is to overcome the hurdles to effective macromolecule delivery. This review will present several bacterial vectors as macromolecule (protein or gene) delivery devices with both innate and acquirable (or engineered) biological features to facilitate delivery to APCs. The review will also present topics related to large-scale manufacture, storage, and distribution that must be considered if the bacterial delivery devices are ever to be used in a global market.

Live bacterial vaccines--a review and identification of potential hazards

The use of live bacteria to induce an immune response to itself or to a carried vaccine component is an attractive vaccine strategy. Advantages of live bacterial vaccines include their mimicry of a natural infection, intrinsic adjuvant properties and their possibility to be administered orally. Derivatives of pathogenic and non-pathogenic food related bacteria are currently being evaluated as live vaccines. However, pathogenic bacteria demands for attenuation to weaken its virulence. The use of bacteria as vaccine delivery vehicles implies construction of recombinant strains that contain the gene cassette encoding the antigen. With the increased knowledge of mucosal immunity and the availability of genetic tools for heterologous gene expression the concept of live vaccine vehicles gains renewed interest. However, administration of live bacterial vaccines poses some risks. In addition, vaccination using recombinant bacteria results in the release of live recombinant organisms into nature. This places these vaccines in the debate on application of genetically modified organisms. In this review we give an overview of live bacterial vaccines on the market and describe the development of new live vaccines with a focus on attenuated bacteria and food-related lactic acid bacteria. Furthermore, we outline the safety concerns and identify the hazards associated with live bacterial vaccines and try to give some suggestions of what to consider during their development.

Investigation of ansB and sspA derived promoters for multi- and single-copy antigen expression in attenuated Salmonella enterica var. typhimurium

Five candidate promoters were examined to determine their utility in directing immunogenic levels of expression of the C fragment from tetanus toxin in attenuated S. enterica used as an oral vaccine in mice. Promoters derived from the genes encoding the stringent starvation protein (sspA) from E. coli and S. enterica, but not ansB derived promoters, expressed immunogenic levels of C fragment from multi-copy plasmids in attenuated S. enterica in vivo and, following oral immunization, induced high titre specific anti-tetanus toxoid serum antibodies. We also demonstrate that not only the choice of promoter, replicon and growth conditions but also how expression constructs are assembled in the chosen plasmid is critical for the successful development of plasmid-based antigen delivery systems using attenuated S. enterica. In addition, the S. enterica sspA promoter is able to elicit anti-tetanus toxoid antibodies in mice when the psspA-tetC expression cassette is integrated in single copy on the S. enterica chromosome.

Mucosally delivered Salmonella live vector vaccines elicit potent immune responses against a foreign antigen in neonatal mice born to naive and immune mothers

The development of effective vaccines for neonates and very young infants has been impaired by their weak, short-lived, and Th-2 biased responses and by maternal antibodies that interfere with vaccine take. We investigated the ability of Salmonella enterica serovars Typhi and Typhimurium to mucosally deliver tetanus toxin fragment C (Frag C) as a model antigen in neonatal mice. We hypothesize that Salmonella, by stimulating innate immunity (contributing to adjuvant effects) and inducing Th-1 cytokines, can enhance neonatal dendritic cell maturation and T-cell activation and thereby prime humoral and cell-mediated immunity. We demonstrate for the first time that intranasal immunization of newborn mice with 10(9) CFU of S. enterica serovar Typhi CVD 908-htrA and S. enterica serovar Typhimurium SL3261 carrying plasmid pTETlpp on days 7 and 22 after birth elicits high titers of Frag C antibodies, previously found to protect against tetanus toxin challenge and similar to those observed in adult mice. Salmonella live vectors colonized and persisted primarily in nasal tissue. Mice vaccinated as neonates induced Frag C-specific mucosal and systemic immunoglobulin A (IgA)- and IgG-secreting cells, T-cell proliferative responses, and gamma interferon secretion. A mixed Th1- and Th2-type response to Frag C was established 1 week after the boost and was maintained thereafter. S. enterica serovar Typhi carrying pTETlpp induced Frag C-specific antibodies and cell-mediated immunity in the presence of high levels of maternal antibodies. This is the first report that demonstrates the effectiveness of Salmonella live vector vaccines in early life.

Delivery of the immunosuppressive antigen Salp15 to antigen-presenting cells by Salmonella enterica serovar Typhimurium aroA mutants

A Salmonella enterica serovar Typhimurium aroA-deficient delivery system was used to target the immunosuppressive protein Salp15 to antigen-presenting cells. In vitro and in vivo infections with Salp15-containing Salmonella resulted in an impaired CD4(+)-T-cell activation, suggesting that the protein was produced by antigen-presenting cells in a physiologically active form.

Use of Bordetella bronchiseptica and Bordetella pertussis as live vaccines and vectors for heterologous antigens

Bordetella pertussis and Bordetella bronchiseptica are respiratory pathogens of humans and animals respectively. Unlike many bacteria, they are able to efficiently colonise healthy ciliated respiratory mucosa. This characteristic of Bordetella spp. can potentially be exploited to develop efficient live vaccines and vectors for delivery of heterologous antigens to the respiratory tract. Here we review the progress in this area.

Escherichia coli DegP protease cleaves between paired hydrophobic residues in a natural substrate: the PapA pilin

The DegP protein, a multifunctional chaperone and protease, is essential for clearance of denatured or aggregated proteins from the inner-membrane and periplasmic space in Escherichia coli. To date, four natural targets for DegP have been described: colicin A lysis protein, pilin subunits and MalS from E. coli, and high-molecular-weight adherence proteins from Haemophilus influenzae. In vitro, DegP has shown weak protease activity with casein and several other nonnative substrates. We report here the identification of the major pilin subunit of the Pap pilus, PapA, as a natural DegP substrate and demonstrate binding and proteolysis of this substrate in vitro. Using overlapping peptide arrays, we identified three regions in PapA that are preferentially cleaved by DegP. A 7-mer peptide was found to be a suitable substrate for cleavage by DegP in vitro. In vitro proteolysis of model peptide substrates revealed that cleavage is dependent upon the presence of paired hydrophobic amino acids; moreover, cleavage was found to occur between the hydrophobic residues. Finally, we demonstrate that the conserved carboxyl-terminal sequence in pilin subunits, although not a cleavage substrate for DegP, activates the protease and we propose that the activating peptide is recognized by DegP's PDZ domains.

HtrA protease and processing of extracellular proteins of Streptococcus mutans

A homologue of the HtrA family of stress-response proteases was detected by analysis of the Streptococcus mutans genome sequence. Disabling of the S. mutans htrA gene by insertional inactivation resulted in bacterial clumping in liquid medium, altered colony morphology and a reduced ability to withstand high temperature, extremes of pH or oxidative stress. Seven different extracellular or wall-associated proteins that are known to be subject to post-translational proteolysis were examined in cultures of wild-type S. mutans and an htrA mutant. Inactivation of the htrA protease had no effect on degradation of the proteins.