Salmonella enterica serovar Typhi live vector vaccines finally come of age

Recent development of oral vaccines (Review)

Oral immunization can elicit an effective immune response and immune tolerance to specific antigens. When compared with the traditional injection route, delivering antigens via the gastrointestinal mucosa offers superior immune effects and compliance, as well as simplicity and convenience, making it a more optimal route for immunization. At present, various oral vaccine delivery systems exist. Certain modified bacteria, such as Salmonella, Escherichia coli and particularly Lactobacillus, are considered promising carriers for oral vaccines. These carriers can significantly enhance immunization efficiency by actively replicating in the intestinal tract following oral administration. The present review provided a discussion of the main mechanisms of oral immunity and the research progress made in the field of oral vaccines. Additionally, it introduced the advantages and disadvantages of the currently more commonly administered injectable COVID-19 vaccines, alongside the latest advancements in this area. Furthermore, recent developments in oral vaccines are summarized, and their potential benefits and side effects are discussed.

Immunogenicity and protective efficacy of nanoparticle formulations of L-SseB against Salmonella infection

Salmonella enterica, a Gram-negative pathogen, has over 2500 serovars that infect a wide range of hosts. In humans, S. enterica causes typhoid or gastroenteritis and is a major public health concern. In this study, SseB (the tip protein of the Salmonella pathogenicity island 2 type III secretion system) was fused with the LTA1 subunit of labile-toxin from enterotoxigenic E. coli to make the self-adjuvanting antigen L-SseB. Two unique nanoparticle formulations were developed to allow multimeric presentation of L-SseB. Mice were vaccinated with these formulations and protective efficacy determined via challenging the mice with S. enterica serovars. The polysaccharide (chitosan) formulation was found to elicit better protection when compared to the squalene nanoemulsion. When the polysaccharide formulation was used to vaccinate rabbits, protection from S. enterica challenge was elicited. In summary, L-SseB in a particulate polysaccharide formulation appears to be an attractive candidate vaccine capable of broad protection against S. enterica.

Coccidiosis: Recent Progress in Host Immunity and Alternatives to Antibiotic Strategies

Coccidiosis is an avian intestinal disease caused by several distinct species of Eimeria parasites that damage the host’s intestinal system, resulting in poor nutrition absorption, reduced growth, and often death. Increasing evidence from recent studies indicates that immune-based strategies such as the use of recombinant vaccines and various dietary immunomodulating feed additives can improve host defense against intracellular parasitism and reduce intestinal damage due to inflammatory responses induced by parasites. Therefore, a comprehensive understanding of the complex interactions between the host immune system, gut microbiota, enteroendocrine system, and parasites that contribute to the outcome of coccidiosis is necessary to develop logical strategies to control coccidiosis in the post-antibiotic era. Most important for vaccine development is the need to understand the protective role of the local intestinal immune response and the identification of various effector molecules which mediate anti-coccidial activity against intracellular parasites. This review summarizes the current understanding of the host immune response to coccidiosis in poultry and discusses various non-antibiotic strategies which are being developed for coccidiosis control. A better understanding of the basic immunobiology of pertinent host–parasite interactions in avian coccidiosis will facilitate the development of effective anti-Eimeria strategies to mitigate the negative effects of coccidiosis.

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.

Immunogenicity and protection efficacy of enhanced fitness recombinant Salmonella Typhi monovalent and bivalent vaccine strains against acute toxoplasmosis

The proficiency of Salmonella Typhi to induce cell-mediated immunity has allowed its exploitation as a live vector against the obligate intracellular protozoan Toxoplasma gondii. T. gondii vaccine research is of great medical value due to the lack of a suitable toxoplasmosis vaccine. In the present work, we integrated T. gondii antigen into a growth-dependent chromosome locus guaBA of S. Typhi CVD910 strain to form recombinant S. Typhi monovalent CVD910-SAG1 expressed T. gondii SAG1 antigen and monovalent CVD910-GRA2 expressed T. gondii GRA2 antigen. Furthermore, a low-copy stabilized recombinant plasmid encoding SAG1 antigen was transformed into CVD910-GRA2 to form bivalent CVD910-GS strain. An osmolarity-regulated promoter was also incorporated to control the gene transcription, whereas clyA export protein was included to translocate the antigen out of the cytoplasm. Both CVD910-GRA2 and CVD910-GS displayed healthy growth fitness and readily expressed the encoded T. gondii antigens. When administered in vivo, CVD910-GS successfully induced both humoral and cellular immunity in the immunized BALB/c mice, and extended mice survival against virulent T. gondii. In particular, the mice immunized with bivalent CVD910-GS presented the highest titers of IgG, percentages of CD4⁺ T, CD8⁺ T, B cells and memory T cells, and total IgG⁺ memory B cells as compared to the CVD910-GRA2 and control strains. The CVD910-GS group also generated mixed Th1/Th2 cytokine profile with secretions of IFN-ɣ, IL-2 and IL-10. This study demonstrated the importance of enhancing live vector fitness to sustain heterologous antigen expression for eliciting robust immune responses and providing effective protection against pathogen.

Salmonella Vaccine Vector System for Foot-and-Mouth Disease Virus and Evaluation of Its Efficacy with Virus-Like Particles

Foot-and-mouth disease virus (FMDV) causes a highly contagious and devastating disease in livestock animals and has a great potential to cause severe economic loss worldwide. The major antigen of FMDV capsid protein, VP1, contains the major B-cell epitope responsible for effectively eliciting protective humoral immunity. In this study, irradiated Salmonella Typhimurium (KST0666) were used as transgenic vectors containing stress-inducible plasmid pRECN-VP1 to deliver the VP1 protein from FMDV-type A/WH/CHA/09. Mice were orally inoculated with ATOMASal-L3 harboring pRECN-VP1, and FMDV virus-like particles, where (VLP_FMDV)-specific humoral, mucosal, and cellular immune responses were evaluated. Mice vaccinated with attenuated Salmonella (KST0666) expressing VP1 (named KST0669) showed high levels of VLP-specific IgA in feces and IgG in serum, with high FMDV neutralization titer. Moreover, KST0669-vaccinated mice showed increased population of IFN-γ (type 1 T helper cells; Th1 cells)-, IL-5 (Th2 cells)-, and IL-17A (Th17 cells)-expressing CD4⁺ as well as activated CD8⁺ T cells (IFN-γ⁺CD8⁺ cells), detected by stimulating VLP_FMDV. All data indicate that our Salmonella vector system successfully delivered FMDV VP1 to immune cells and that the humoral and cellular efficacy of the vaccine can be easily evaluated using VLP_FMDV in a Biosafety Level I (BSL1) laboratory.

Towards Innovative Design and Application of Recombinant Eimeria as a Vaccine Vector

Efficient delivery of antigenic cargo to trigger protective immune responses is critical to the success of vaccination. Genetically engineered microorganisms, including virus, bacteria, and protozoa, can be modified to carry and deliver heterologous antigens to the host immune system. The biological vectors can induce a broad range of immune responses and enhance heterologous antigen-specific immunological outcomes. The protozoan genus Eimeria is widespread in domestic animals, causing serious coccidiosis. Eimeria parasites with strong immunogenicity are potent coccidiosis vaccine candidates and offer a valuable model of live vaccines against infectious diseases in animals. Eimeria parasites can also function as a vaccine vector. Herein, we review recent advances in design and application of recombinant Eimeria as a vaccine vector, which has been a topic of ongoing research in our laboratory. By recapitulating the establishment of an Eimeria transfection platform and its application, it will help lay the foundation for the future development of effective parasite-based vaccine delivery vectors and beyond.

Prospects for Malaria Vaccines: Pre-Erythrocytic Stages, Blood Stages, and Transmission-Blocking Stages

Malaria is a disease of public health importance in many parts of the world. Currently, there is no effective way to eradicate malaria, so developing safe, efficient, and cost-effective vaccines against this disease remains an important goal. Current research on malaria vaccines is focused on developing vaccines against pre-erythrocytic stage parasites and blood-stage parasites or on developing a transmission-blocking vaccine. Here, we briefly describe the progress made towards a vaccine against Plasmodium falciparum, the most pathogenic of the malaria parasite species to infect humans.

Formulation technologies for oral vaccines

Many options now exist for constructing oral vaccines which, in experimental systems, have shown themselves to be able to generate highly effective immunity against infectious diseases. Their suitability for implementation in clinical practice, however, for prevention of outbreaks, particularly in low- and middle-income countries (LMIC), is not always guaranteed, because of factors such as cost, logistics and cultural and environmental conditions. This brief overview provides a summary of the various approaches which can be adopted, and evaluates them from a pharmaceutical point, taking into account potential regulatory issues, expense, manufacturing complexity, etc., all of which can determine whether a vaccine approach will be successful in the late stages of development. Attention is also drawn to problems arising from inadequate diet, which impacts upon success in stimulating effective immunity, and identifies the use of lipid-based carriers as a way to counteract the problem of nutritional deficiencies in vaccination campaigns.

A Live Salmonella Vaccine Delivering PcrV through the Type III Secretion System Protects against Pseudomonas aeruginosa

The Gram-negative bacterium Pseudomonas aeruginosa is an important opportunistic pathogen that causes infections in cystic fibrosis and hospitalized patients. Therapeutic treatments are limited due to the emergence and spread of new antibiotic-resistant strains. In this context, the development of a vaccine is a priority. Here, we used an attenuated strain of Salmonella enterica serovar Typhimurium as a vehicle to express and deliver the Pseudomonas antigen PcrV. This vaccine induced the generation of specific antibodies in mice and protected them from lethal infections with P. aeruginosa. This is an important step toward the development of an effective vaccine for the prevention of infections caused by P. aeruginosa in humans.

Pseudomonas aeruginosa is a common Gram-negative opportunistic pathogen that is intrinsically resistant to a wide range of antibiotics. The development of a broadly protective vaccine against P. aeruginosa remains a major challenge. Here, we used an attenuated strain of Salmonella enterica serovar Typhimurium as a vehicle to express P. aeruginosa antigens. A fusion between the S. enterica type III secretion effector protein SseJ and the P. aeruginosa antigen PcrV expressed under the control of the sseA promoter was translocated by Salmonella into host cells in vitro and elicited the generation of specific antibodies in mice. Mice immunized with attenuated Salmonella expressing this fusion had reduced bacterial loads in the spleens and lungs and lower serum levels of proinflammatory cytokines than control mice after P. aeruginosa infection. Importantly, immunized mice also showed significantly enhanced survival in this model. These results suggest that type III secretion effectors of S. enterica are appropriate carriers in the design of a live vaccine to prevent infections caused by P. aeruginosa.

IMPORTANCE The Gram-negative bacterium Pseudomonas aeruginosa is an important opportunistic pathogen that causes infections in cystic fibrosis and hospitalized patients. Therapeutic treatments are limited due to the emergence and spread of new antibiotic-resistant strains. In this context, the development of a vaccine is a priority. Here, we used an attenuated strain of Salmonella enterica serovar Typhimurium as a vehicle to express and deliver the Pseudomonas antigen PcrV. This vaccine induced the generation of specific antibodies in mice and protected them from lethal infections with P. aeruginosa. This is an important step toward the development of an effective vaccine for the prevention of infections caused by P. aeruginosa in humans.

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.

M cell targeting engineered biomaterials for effective vaccination

Vaccines are one of the greatest medical interventions of all time and have been successful in controlling and eliminating a myriad of diseases over the past two centuries. Among several vaccination strategies, mucosal vaccines have wide clinical applications and attract considerable interest in research, showing potential as innovative and novel therapeutics. In mucosal vaccination, targeting (microfold) M cells is a frontline prerequisite for inducing effective antigen-specific immunostimulatory effects. In this review, we primarily focus on materials engineered for use as vaccine delivery platforms to target M cells. We also describe potential M cell targeting areas, methods to overcome current challenges and limitations of the field. Furthermore, we present the potential of biomaterials engineering as well as various natural and synthetic delivery technologies to overcome the challenges of M cell targeting, all of which are absent in current literature. Finally, we briefly discuss manufacturing and regulatory processes to bring a robust perspective on the feasibility and potential of this next-generation vaccine technology.

Vaccines as alternatives to antibiotics for food producing animals. Part 2: new approaches and potential solutions

Vaccines and other alternative products are central to the future success of animal agriculture because they can help minimize the need for antibiotics by preventing and controlling infectious diseases in animal populations. To assess scientific advancements related to alternatives to antibiotics and provide actionable strategies to support their development, the United States Department of Agriculture, with support from the World Organisation for Animal Health, organized the second International Symposium on Alternatives to Antibiotics. It focused on six key areas: vaccines; microbial-derived products; non-nutritive phytochemicals; immune-related products; chemicals, enzymes, and innovative drugs; and regulatory pathways to enable the development and licensure of alternatives to antibiotics. This article, the second part in a two-part series, highlights new approaches and potential solutions for the development of vaccines as alternatives to antibiotics in food producing animals; opportunities, challenges and needs for the development of such vaccines are discussed in the first part of this series. As discussed in part 1 of this manuscript, many current vaccines fall short of ideal vaccines in one or more respects. Promising breakthroughs to overcome these limitations include new biotechnology techniques, new oral vaccine approaches, novel adjuvants, new delivery strategies based on bacterial spores, and live recombinant vectors; they also include new vaccination strategies in-ovo, and strategies that simultaneously protect against multiple pathogens. However, translating this research into commercial vaccines that effectively reduce the need for antibiotics will require close collaboration among stakeholders, for instance through public–private partnerships. Targeted research and development investments and concerted efforts by all affected are needed to realize the potential of vaccines to improve animal health, safeguard agricultural productivity, and reduce antibiotic consumption and resulting resistance risks.

A single vaccination with non-replicating MVA at birth induces both immediate and long-term protective immune responses

Newborns are considered difficult to protect against infections shortly after birth, due to their ineffective immune system that shows quantitative and qualitative differences compared to adults. However, here we show that a single vaccination of mice at birth with a replication-deficient live vaccine Modified Vaccinia Ankara [MVA] efficiently induces antigen-specific B- and T-cells that fully protect against a lethal Ectromelia virus challenge. Protection was induced within 2 weeks and using genetically modified mice we show that this protection was mainly T-cell dependent. Persisting immunological T-cell memory and neutralizing antibodies were obtained with the single vaccination. Thus, MVA administered as early as at birth induced immediate and long-term protection against an otherwise fatal disease and appears attractive as a new generation smallpox vaccine that is effective also in children. Moreover, it may have the potential to serve as platform for childhood vaccines as indicated by measles specific T- and B-cell responses induced in newborn mice vaccinated with recombinant MVA expressing measles antigens.

Salmonella Vaccines: Conduits for Protective Antigens

Vaccines afford a better and more cost-effective approach to combatting infectious diseases than continued reliance on antibiotics or antiviral or antiparasite drugs in the current era of increasing incidences of diseases caused by drug-resistant pathogens. Recombinant attenuated Salmonella vaccines (RASVs) have been significantly improved to exhibit the same or better attributes than wild-type parental strains to colonize internal lymphoid tissues and persist there to serve as factories to continuously synthesize and deliver rAgs. Encoded by codon-optimized pathogen genes, Ags are selected to induce protective immunity to infection by that pathogen. After immunization through a mucosal surface, the RASV attributes maximize their abilities to elicit mucosal and systemic Ab responses and cell-mediated immune responses. This article summarizes many of the numerous innovative technologies and discoveries that have resulted in RASV platforms that will enable development of safe efficacious RASVs to protect animals and humans against a diversity of infectious disease agents.

Lessons Learned from Protective Immune Responses to Optimize Vaccines against Cryptosporidiosis

In developing countries, cryptosporidiosis causes moderate-to-severe diarrhea and kills thousands of infants and toddlers annually. Drinking and recreational water contaminated with Cryptosporidium spp. oocysts has led to waterborne outbreaks in developed countries. A competent immune system is necessary to clear this parasitic infection. A better understanding of the immune responses required to prevent or limit infection by this protozoan parasite is the cornerstone of development of an effective vaccine. In this light, lessons learned from previously developed vaccines against Cryptosporidium spp. are at the foundation for development of better next-generation vaccines. In this review, we summarize the immune responses elicited by naturally and experimentally-induced Cryptosporidium spp. infection and by several experimental vaccines in various animal models. Our aim is to increase awareness about the immune responses that underlie protection against cryptosporidiosis and to encourage promotion of these immune responses as a key strategy for vaccine development. Innate and mucosal immunity will be addressed as well as adaptive immunity, with an emphasis on the balance between T_H1/T_H2 immune responses. Development of more effective vaccines against cryptosporidiosis is needed to prevent Cryptosporidium spp.-related deaths in infants and toddlers in developing countries.

Antigen Localization Influences the Magnitude and Kinetics of Endogenous Adaptive Immune Response to Recombinant Salmonella Vaccines

The use of recombinant attenuated Salmonella vaccine (RASV) strains is a promising strategy for presenting heterologous antigens to the mammalian immune system to induce both cellular and humoral immune responses. However, studies on RASV development differ on where heterologous antigens are expressed and localized within the bacterium, and it is unclear how antigen localization modulates the immune response. Previously, we exploited the plasmid-encoded toxin (Pet) autotransporter system for accumulation of heterologous antigens in cell culture supernatant. In the present study, this Pet system was used to express early secretory antigen 6 (ESAT-6), an immunodominant and diagnostic antigen from Mycobacterium tuberculosis, in Salmonella enterica serovar Typhimurium strain SL3261. Three strains were generated, whereby ESAT-6 was expressed as a cytoplasmic (SL3261/cyto), surface-bound (SL3261/surf), or secreted (SL3261/sec) antigen. Using these RASVs, the relationship between antigen localization and immunogenicity in infected C57BL/6 mice was systematically examined. Using purified antigen and specific tetramers, we showed that mice infected with the SL3261/surf or SL3261/sec strain generated large numbers of Th1 CD4⁺ ESAT-6⁺ splenic T cells compared to those of mice infected with SL3261/cyto. While all mice showed ESAT-6-specific antibody responses when infected with SL3261/surf or SL3261/sec, peak total serum IgG antibody titers were reached more rapidly in mice that received SL3261/sec. Thus, how antigen is localized after production within bacteria has a more marked effect on the antibody response than on the CD4⁺ T cell response, which might influence the chosen strategy to localize recombinant antigen in RASVs.

New Insights into the Roles of Long Polar Fimbriae and Stg Fimbriae in Salmonella Interactions with Enterocytes and M Cells

Salmonella enterica serovar Typhi causes the systemic disease typhoid fever. After ingestion, it adheres to and invades the host epithelium while evading the host innate immune response, causing little if any inflammation. Conversely, Salmonella enterica serovar Typhimurium causes gastroenteritis in humans and thrives in the inflamed gut. Upon entering the host, S Typhimurium preferentially colonizes Peyer's patches, a lymphoid organ in which microfold cells (M cells) overlay an arrangement of B cells, T cells, and antigen-presenting cells. Both serovars can adhere to and invade M cells and enterocytes, and it has been assumed that S Typhi also preferentially targets M cells. In this study, we present data supporting the alternative hypothesis that S Typhi preferentially targets enterocytes. Using a tissue culture M cell model, we examined S Typhi strains with a deletion in the stg fimbriae. The stg deletion resulted in increased adherence to M cells and, as expected, decreased adherence to Caco-2 cells. Adherence to M cells could be further enhanced by introduction of the long polar fimbriae (Lpf), which facilitate adherence of S Typhimurium to M cells. Deletion of stg and/or introduction of lpf enhanced M cell invasion as well, leading to significant increases in secretion of interleukin 8. These results suggest that S Typhi may preferentially target enterocytes in vivo.

Transfected Babesia bovis Expressing a Tick GST as a Live Vector Vaccine

The Rhipicephalus microplus tick is a notorious blood-feeding ectoparasite of livestock, especially cattle, responsible for massive losses in animal production. It is the main vector for transmission of pathogenic bacteria and parasites, including Babesia bovis, an intraerythrocytic apicomplexan protozoan parasite responsible for bovine Babesiosis. This study describes the development and testing of a live B. bovis vaccine expressing the protective tick antigen glutathione-S-transferase from Haemaphysalis longicornis (HlGST). The B. bovis S74-T3B parasites were electroporated with a plasmid containing the bidirectional Ef-1α (elongation factor 1 alpha) promoter of B. bovis controlling expression of two independent genes, the selectable marker GFP-BSD (green fluorescent protein–blasticidin deaminase), and HlGST fused to the MSA-1 (merozoite surface antigen 1) signal peptide from B. bovis. Electroporation followed by blasticidin selection resulted in the emergence of a mixed B. bovis transfected line (termed HlGST) in in vitro cultures, containing parasites with distinct patterns of insertion of both exogenous genes, either in or outside the Ef-1α locus. A B. bovis clonal line termed HlGST-Cln expressing intracellular GFP and HlGST in the surface of merozoites was then derived from the mixed parasite line HlGST using a fluorescent activated cell sorter. Two independent calf immunization trials were performed via intravenous inoculation of the HlGST-Cln and a previously described control consisting of an irrelevant transfected clonal line of B. bovis designated GFP-Cln. The control GFP-Cln line contains a copy of the GFP-BSD gene inserted into the Ef-1α locus of B. bovis in an identical fashion as the HIGST-Cln parasites. All animals inoculated with the HlGST-Cln and GFP-Cln transfected parasites developed mild babesiosis. Tick egg fertility and fully engorged female tick weight was reduced significantly in R. microplus feeding on HlGST-Cln-immunized calves. Collectively, these data show the efficacy of a transfected HlGST-Cln B. bovis parasite to induce detectable anti-glutathione-S-transferase antibodies and a reduction in tick size and fecundity of R. microplus feeding in experimentally inoculated animals.

The cattle tick Rhipicephalus microplus is a hematophagous ectoparasite, responsible for the transmission of lethal parasites such as Babesia sp, limiting cattle production in tropical and subtropical regions of the world. There is an urgent emerging need for improved methods of control for these currently neglected tick and tick borne diseases. It is hypothesized that a dual attenuated-live vector vaccine containing a stably transfected tick antigen elicits protective immune responses against the parasite and the tick vector in vaccinated cattle. Live Babesia vaccines based on attenuated parasites are the only effective method available for preventing acute babesiosis. On the other hand, glutathione-S-transferase from Haemaphysalis longicornis (HlGST) is a known effective antigen against Rhipicephalus microplus, the most common vector for B. bovis. This study describes the development and testing of a transfected, B. bovis vaccine expressing HlGST against the tick R. microplus. A B. bovis clonal line designated HlGST-Cln expressing HlGST and GFP/BSD, and separately a control transfected B. bovis clonal line expressing only GFP/BSD was used to vaccinate calves in two independent experiments. All immunized calves developed mild babesiosis, and only calves immunized with the HlGST-Cln parasite line generated anti-HlGST antibodies. Tick egg fertility and fully engorged female tick weight were reduced significantly in R. microplus feeding on HlGST-Cln-vaccinated calves. Taken together, these data demonstrates the ability of transfected B. bovis to elicit antibodies against a heterologous tick antigen in cattle and to induce partial protection in the vaccinated animals against the cattle tick for the first time, representing a step toward the goal to produce a live vector anti-tick vaccine.

Transgenic Eimeria tenella as a vaccine vehicle: expressing TgSAG1 elicits protective immunity against Toxoplasma gondii infections in chickens and mice

The surface antigen 1 of Toxoplasma gondii (TgSAG1) is a major immunodominant antigen and is widely considered an ideal candidate for the development of an effective recombinant vaccine against toxoplasmosis. Eimeria tenella, an affinis apicomplexan parasite with T. gondii, is a potential vaccine vector carrying exogenous antigens that stimulates specific immune responses. Here, we engineered TgSAG1 into E. tenella and obtained a stably transfected E. tenella line (Et-TgSAG1). We found TgSAG1 localized on the cell surface of Et-TgSAG1, which is similar to its native distribution in T. gondii tachyzoites. We immunized the chickens with Et-TgSAG1 orally and detected TgSAG1-specific immune responses, which partly reduced T. gondii infection. In the mouse model, we immunized the mice with Et-TgSAG1 sporozoites intraperitoneally and challenged them with T. gondii tachyzoites RH strain. We found that the mice immunized with Et-TgSAG1 showed a TgSAG1 specific Th 1-dominant immune response and a prolonged survival time compared with wild-type E. tenella and non-immunized mice. Collectively, our results demonstrated that Et-TgSAG1, utilized as a recombinant vaccine against toxoplasmosis, could be applied in both chickens and mice. Our findings also provide a promising persuasion for the development of transgenic Eimeria as vaccine vectors for use in birds and mammals.

An Oral Salmonella-Based Vaccine Inhibits Liver Metastases by Promoting Tumor-Specific T-Cell-Mediated Immunity in Celiac and Portal Lymph Nodes: A Preclinical Study

Primary tumor excision is one of the most widely used therapies of cancer. However, the risk of metastases development still exists following tumor resection. The liver is a common site of metastatic disease for numerous cancers. Breast cancer is one of the most frequent sources of metastases to the liver. The aim of this work was to evaluate the efficacy of the orally administered Salmonella Typhi vaccine strain CVD 915 on the development of liver metastases in a mouse model of breast cancer. To this end, one group of BALB/c mice was orogastrically immunized with CVD 915, while another received PBS as a control. After 24 h, mice were injected with LM3 mammary adenocarcinoma cells into the spleen and subjected to splenectomy. This oral Salmonella-based vaccine produced an antitumor effect, leading to a decrease in the number and volume of liver metastases. Immunization with Salmonella induced an early cellular immune response in mice. This innate stimulation rendered a large production of IFN-γ by intrahepatic immune cells (IHIC) detected within 24 h. An antitumor adaptive immunity was found in the liver and celiac and portal lymph nodes (LDLN) 21 days after oral bacterial inoculation. The antitumor immune response inside the liver was associated with increased CD4⁺ and dendritic cell populations as well as with an inflammatory infiltrate located around liver metastatic nodules. Enlarged levels of inflammatory cytokines (IFN-γ and TNF) were also detected in IHIC. Furthermore, a tumor-specific production of IFN-γ and TNF as well as tumor-specific IFN-γ-producing CD8 T cells (CD8⁺IFN-γ⁺) were found in the celiac and portal lymph nodes of Salmonella-treated mice. This study provides first evidence for the involvement of LDLN in the development of an efficient cellular immune response against hepatic tumors, which resulted in the elimination of liver metastases after oral Salmonella-based vaccination.

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 attenuated Salmonella enterica serovar Choleraesuis vaccine vector displaying regulated delayed attenuation and regulated delayed antigen synthesis to confer protection against Streptococcus suis in mice

Salmonella enterica serotype Choleraesuis (S. Choleraesuis) and Streptococcus suis (S. suis) are important swine pathogens. Development of a safe and effective attenuated S. Choleraesuis vaccine vector would open a new window to prevent and control pig diseases. To achieve this goal, the mannose and arabinose regulated delayed attenuated systems (RDAS), Δpmi and ΔPcrp::TT araC PBADcrp, were introduced into the wild type S. Choleraesuis strain C78-3. We also introduced ΔrelA::araC PBADlacI TT to achieve regulated delayed antigen synthesis and ΔasdA to constitute a balanced-lethal plasmid system. The safety and immunogenicity of the resulted RDAS S. Choleraesuis strain rSC0011 carrying 6-phosphogluconate dehydrogenase (6-PGD) of S. suis serotype 2 (SS2) were evaluated in vitro and in vivo. Compared with the wild type parent strain C78-3 and vaccine strain C500, a live attenuated S. Choleraesuis vaccine licensed for piglet in China, the results showed that the survival curves of the vaccine strain rSC0011 were similar to those of strains C78-3 and C500 at the early stage of infection, but lower than those of C78-3 and higher than those of C500 at the later stage in both porcine alveolar macrophages and peripheral porcine monocytes. The LD50 of the RDAS strains rSC0011 by oral route in mice was close to that of C500 and 10,000-fold higher than that of C78-3. Similar results were achieved by intraperitoneal (i.p.) route, suggesting that the RDAS strains rSC0011 achieved similar attenuation as C500. However, the RDAS strain rSC0011 was superior to C500 in colonization of Peyer's patches. Adult mice orally immunized with strain rSC0011 carrying a plasmid expression 6-phosphogluconate dehydrogenase (6-PGD) gene from SS2 developed strong immune responses against 6-PGD and Salmonella antigens, and conferred high protection against i.p. challenge with SS2.

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.

Antigen Delivery System II

This chapter reviews papers mostly written since 2005 that report results using live attenuated bacterial vectors to deliver after administration through mucosal surfaces, protective antigens, and DNA vaccines, encoding protective antigens to induce immune responses and/or protective immunity to pathogens that colonize on or invade through mucosal surfaces. Papers that report use of such vaccine vector systems for parenteral vaccination or to deal with nonmucosal pathogens or do not address induction of mucosal antibody and/or cellular immune responses are not reviewed.

Multicopy integration of mini-Tn7 transposons into selected chromosomal sites of a Salmonella vaccine strain

Chromosomal integration of expression modules for transgenes is an important aspect for the development of novel Salmonella vectors. Mini-Tn7 transposons have been used for the insertion of one such module into the chromosomal site attTn7, present only once in most Gram-negative bacteria. However, integration of multiple mini-Tn7 copies might be suitable for expression of appropriate amounts of antigen or combination of different modules. Here we demonstrate that integration of a 9.6 kb mini-Tn7 harbouring the luciferase luxCDABE (lux) occurs at the natural attTn7 site and simultaneously other locations of the Salmonella chromosome, which were engineered using λ-Red recombinase to contain one or two additional artificial attTn7 sites (a-attTn7). Multicopy integration even at closely spaced attTn7 sites was unexpected in light of the previously reported distance-dependent Tn7 target immunity. Integration of multiple copies of a mini-Tn7 containing a gfp cassette resulted in increasing green fluorescence of bacteria. Stable consecutive integration of two mini-Tn7 encoding lacZ and lux was achieved by initial transposition of lacZ-mini-Tn7, subsequent chromosomal insertion of a-attTn7 and a second round of transposition with lux-mini-Tn7. Mini-Tn7 thus constitutes a versatile method for multicopy integration of expression cassettes into the chromosome of Salmonella and possibly other bacteria.

Complex adaptive immunity to enteric fevers in humans: lessons learned and the path forward

Salmonella enterica serovar Typhi (S. Typhi), the causative agent of typhoid fever, and S. Paratyphi A and B, causative agents of paratyphoid fever, are major public health threats throughout the world. Although two licensed typhoid vaccines are currently available, they are only moderately protective and immunogenic necessitating the development of novel vaccines. A major obstacle in the development of improved typhoid, as well as paratyphoid vaccines is the lack of known immunological correlates of protection in humans. Considerable progress has been made in recent years in understanding the complex adaptive host responses against S. Typhi. Although the induction of S. Typhi-specific antibodies (including their functional properties) and memory B cells, as well as their cross-reactivity with S. Paratyphi A and S. Paratyphi B has been shown, the role of humoral immunity in protection remains undefined. Cell mediated immunity (CMI) is likely to play a dominant role in protection against enteric fever pathogens. Detailed measurements of CMI performed in volunteers immunized with attenuated strains of S. Typhi have shown, among others, the induction of lymphoproliferation, multifunctional type 1 cytokine production, and CD8(+) cytotoxic T-cell responses. In addition to systemic responses, the local microenvironment of the gut is likely to be of paramount importance in protection from these infections. In this review, we will critically assess current knowledge regarding the role of CMI and humoral immunity following natural S. Typhi and S. Paratyphi infections, experimental challenge, and immunization in humans. We will also address recent advances regarding cross-talk between the host's gut microbiota and immunization with attenuated S. Typhi, mechanisms of systemic immune responses, and the homing potential of S. Typhi-specific B- and T-cells to the gut and other tissues.

A bivalent typhoid live vector vaccine expressing both chromosome- and plasmid-encoded Yersinia pestis antigens fully protects against murine lethal pulmonary plague infection

Live attenuated bacteria hold great promise as multivalent mucosal vaccines against a variety of pathogens. A major challenge of this approach has been the successful delivery of sufficient amounts of vaccine antigens to adequately prime the immune system without overattenuating the live vaccine. Here we used a live attenuated Salmonella enterica serovar Typhi strain to create a bivalent mucosal plague vaccine that produces both the protective F1 capsular antigen of Yersinia pestis and the LcrV protein required for secretion of virulence effector proteins. To reduce the metabolic burden associated with the coexpression of F1 and LcrV within the live vector, we balanced expression of both antigens by combining plasmid-based expression of F1 with chromosomal expression of LcrV from three independent loci. The immunogenicity and protective efficacy of this novel vaccine were assessed in mice by using a heterologous prime-boost immunization strategy and compared to those of a conventional strain in which F1 and LcrV were expressed from a single low-copy-number plasmid. The serum antibody responses to lipopolysaccharide (LPS) induced by the optimized bivalent vaccine were indistinguishable from those elicited by the parent strain, suggesting an adequate immunogenic capacity maintained through preservation of bacterial fitness; in contrast, LPS titers were 10-fold lower in mice immunized with the conventional vaccine strain. Importantly, mice receiving the optimized bivalent vaccine were fully protected against lethal pulmonary challenge. These results demonstrate the feasibility of distributing foreign antigen expression across both chromosomal and plasmid locations within a single vaccine organism for induction of protective immunity.

The delicate balance in genetically engineering live vaccines

Contemporary vaccine development relies less on empirical methods of vaccine construction, and now employs a powerful array of precise engineering strategies to construct immunogenic live vaccines. In this review, we will survey various engineering techniques used to create attenuated vaccines, with an emphasis on recent advances and insights. We will further explore the adaptation of attenuated strains to create multivalent vaccine platforms for immunization against multiple unrelated pathogens. These carrier vaccines are engineered to deliver sufficient levels of protective antigens to appropriate lymphoid inductive sites to elicit both carrier-specific and foreign antigen-specific immunity. Although many of these technologies were originally developed for use in Salmonella vaccines, application of the essential logic of these approaches will be extended to development of other enteric vaccines where possible. A central theme driving our discussion will stress that the ultimate success of an engineered vaccine rests on achieving the proper balance between attenuation and immunogenicity. Achieving this balance will avoid over-activation of inflammatory responses, which results in unacceptable reactogenicity, but will retain sufficient metabolic fitness to enable the live vaccine to reach deep tissue inductive sites and trigger protective immunity. The breadth of examples presented herein will clearly demonstrate that genetic engineering offers the potential for rapidly propelling vaccine development forward into novel applications and therapies which will significantly expand the role of vaccines in public health.

Live-attenuated bacteria as a cancer vaccine vector

In the emerging field of active and specific cancer immunotherapy, strategies using live-attenuated bacterial vectors have matured in terms of academic and industrial development. Different bacterial species can be genetically engineered to deliver antigen to APCs with strong adjuvant effects due to their microbial origin. Proteic or DNA-encoding antigen delivery routes and natural bacterial tropisms might differ among species, permitting different applications. After many academic efforts to resolve safety and efficacy issues, some firms have recently engaged clinical trials with live Listeria or Salmonella spp. We describe here the main technological advances that allowed bacteria to become one of the most promising vectors in cancer immunotherapy.

IFN-γ-producing CD4+ T cells promote generation of protective germinal center-derived IgM+ B cell memory against Salmonella Typhi

Abs play a significant role in protection against the intracellular bacterium Salmonella Typhi. In this article, we investigated how long-term protective IgM responses can be elicited by a S. Typhi outer-membrane protein C- and F-based subunit vaccine (porins). We found that repeated Ag exposure promoted a CD4(+) T cell-dependent germinal center reaction that generated mutated IgM-producing B cells and was accompanied by a strong expansion of IFN-γ-secreting T follicular helper cells. Genetic ablation of individual cytokine receptors revealed that both IFN-γ and IL-17 are required for optimal germinal center reactions and production of porin-specific memory IgM(+) B cells. However, more profound reduction of porin-specific IgM B cell responses in the absence of IFN-γR signaling indicated that this cytokine plays a dominant role. Importantly, mutated IgM mAbs against porins exhibited bactericidal capacity and efficiently augmented S. Typhi clearance. In conclusion, repeated vaccination with S. Typhi porins programs type I T follicular helper cell responses that contribute to the diversification of B cell memory and promote the generation of protective IgM Abs.

Salmonella as a vaccine delivery vehicle

Attenuated Salmonella vaccines can be administered orally to deliver recombinant antigens to mucosal surfaces inducing a protective immune response against a variety of targeted pathogens. A number of exciting new approaches and technologies for attenuated Salmonella vaccines have been developed recently. However, a disconnect remains between results obtained with mice in preclinical studies and results obtained in human clinical trials. This is due to an incomplete understanding of Salmonella Typhi interactions with human hosts and inadequate animal models available for study. In this review, the authors describe recent progress in identifying important differences underlying S. Typhi-host interactions, the development of novel approaches to vaccine design and six recent clinical trials evaluating Salmonella-vectored vaccines.

Regulatory T-cell vaccination independent of auto-antigen

To date, efforts to treat autoimmune diseases have primarily focused on the disease symptoms rather than on the cause of the disease. In large part, this is attributed to not knowing the responsible auto-antigens (auto-Ags) for driving the self-reactivity coupled with the poor success of treating autoimmune diseases using oral tolerance methods. Nonetheless, if tolerogenic approaches or methods that stimulate regulatory T (T_reg) cells can be devised, these could subdue autoimmune diseases. To forward such efforts, our approach with colonization factor antigen I (CFA/I) fimbriae is to establish bystander immunity to ultimately drive the development of auto-Ag-specific T_reg cells. Using an attenuated Salmonella vaccine expressing CFA/I fimbriae, fimbriae-specific T_reg cells were induced without compromising the vaccine's capacity to protect against travelers' diarrhea or salmonellosis. By adapting the vaccine's anti-inflammatory properties, it was found that it could also dampen experimental inflammatory diseases resembling multiple sclerosis (MS) and rheumatoid arthritis. Because of this bystander effect, disease-specific T_reg cells are eventually induced to resolve disease. Interestingly, this same vaccine could elicit the required T_reg cell subset for each disease. For MS-like disease, conventional CD25⁺ T_reg cells are stimulated, but for arthritis CD39⁺ T_reg cells are induced instead. This review article will examine the potential of treating autoimmune diseases without having previous knowledge of the auto-Ag using an innocuous antigen to stimulate T_reg cells via the production of transforming growth factor-β and interleukin-10.

Serum bactericidal assays to evaluate typhoidal and nontyphoidal Salmonella vaccines

Invasive Salmonella infections for which improved or new vaccines are being developed include enteric fever caused by Salmonella enterica serovars Typhi, Paratyphi A, and Paratyphi B and sepsis and meningitis in young children in sub-Saharan Africa caused by nontyphoidal Salmonella (NTS) serovars, particularly S. enterica serovars Typhimurium and Enteritidis. Assays are needed to measure functional antibodies elicited by the new vaccines to assess their immunogenicities and potential protective capacities. We developed in vitro assays to quantify serum bactericidal antibody (SBA) activity induced by S. Typhi, S. Paratyphi A, S. Typhimurium, and S. Enteritidis vaccines in preclinical studies. Complement from various sources was tested in assays designed to measure antibody-dependent complement-mediated killing. Serum from rabbits 3 to 4 weeks of age provided the best complement source compared to serum from pigs, goats, horses, bovine calves, or rabbits 8 to 12 weeks of age. For S. Enteritidis, S. Typhimurium, and S. Typhi SBA assays to be effective, bacteria had to be harvested at log phase. In contrast, S. Paratyphi A was equally susceptible to killing whether it was grown to the stationary or log phase. The typhoidal serovars were more susceptible to complement-mediated killing than were the nontyphoidal serovars. Lastly, the SBA endpoint titers correlated with serum IgG anti-lipopolysaccharide (LPS) titers in mice immunized with mucosally administered S. Typhimurium, S. Enteritidis, and S. Paratyphi A but not S. Typhi live attenuated vaccines. The SBA assay described here is a useful tool for measuring functional antibodies elicited by Salmonella vaccine candidates.

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.

Recombinant Salmonella Bacteria Vectoring HIV/AIDS Vaccines

HIV/AIDS is an important public health problem globally. An affordable, easy-to-deliver and protective HIV vaccine is therefore required to curb the pandemic from spreading further. Recombinant Salmonella bacteria can be harnessed to vector HIV antigens or DNA vaccines to the immune system for induction of specific protective immunity. These are capable of activating the innate, humoral and cellular immune responses at both mucosal and systemic compartments. Several studies have already demonstrated the utility of live recombinant Salmonella in delivering expressed foreign antigens as well as DNA vaccines to the host immune system. This review gives an overview of the studies in which recombinant Salmonella bacteria were used to vector HIV/AIDS antigens and DNA vaccines. Most of the recombinant Salmonella-based HIV/AIDS vaccines developed so far have only been tested in animals (mainly mice) and are yet to reach human trials.

The Attenuated Brucella abortus Strain 19 Invades, Persists in, and Activates Human Dendritic Cells, and Induces the Secretion of IL-12p70 but Not IL-23

BACKGROUND

Bacterial vectors have been proposed as novel vaccine strategies to induce strong cellular immunity. Attenuated strains of Brucella abortus comprise promising vector candidates since they have the potential to induce strong CD4(+) and CD8(+) T-cell mediated immune responses in the absence of excessive inflammation as observed with other Gram-negative bacteria. However, some Brucella strains interfere with the maturation of dendritic cells (DCs), which is essential for antigen-specific T-cell priming. In the present study, we investigated the interaction of human monocyte-derived DCs with the smooth attenuated B. abortus strain (S) 19, which has previously been employed successfully to vaccinate cattle.

METHODOLOGY/PRINCIPAL FINDINGS

We first looked into the potential of S19 to hamper the cytokine-induced maturation of DCs; however, infected cells expressed CD25, CD40, CD80, and CD86 to a comparable extent as uninfected, cytokine-matured DCs. Furthermore, S19 activated DCs in the absence of exogeneous stimuli, enhanced the expression of HLA-ABC and HLA-DR, and was able to persist intracellularly without causing cytotoxicity. Thus, DCs provide a cellular niche for persisting brucellae in vivo as a permanent source of antigen. S19-infected DCs produced IL-12/23p40, IL-12p70, and IL-10, but not IL-23. While heat-killed bacteria also activated DCs, soluble mediators were not involved in S19-induced activation of human DCs. HEK 293 transfectants revealed cellular activation by S19 primarily through engagement of Toll-like receptor (TLR)2.

CONCLUSIONS/SIGNIFICANCE

Thus, as an immunological prerequisite for vaccine efficacy, B. abortus S19 potently infects and potently activates (most likely via TLR2) human DCs to produce Th1-promoting cytokines.

Host-Brucella interactions and the Brucella genome as tools for subunit antigen discovery and immunization against brucellosis

Vaccination is the most important approach to counteract infectious diseases. Thus, the development of new and improved vaccines for existing, emerging, and re-emerging diseases is an area of great interest to the scientific community and general public. Traditional approaches to subunit antigen discovery and vaccine development lack consideration for the critical aspects of public safety and activation of relevant protective host immunity. The availability of genomic sequences for pathogenic Brucella spp. and their hosts have led to development of systems-wide analytical tools that have provided a better understanding of host and pathogen physiology while also beginning to unravel the intricacies at the host-pathogen interface. Advances in pathogen biology, host immunology, and host-agent interactions have the potential to serve as a platform for the design and implementation of better-targeted antigen discovery approaches. With emphasis on Brucella spp., we probe the biological aspects of host and pathogen that merit consideration in the targeted design of subunit antigen discovery and vaccine development.

Evaluation of regulated delayed attenuation strategies for Salmonella enterica serovar Typhi vaccine vectors in neonatal and infant mice

We developed regulated delayed attenuation strategies for Salmonella vaccine vectors. In this study, we evaluated the combination of these strategies in recombinant attenuated Salmonella enterica serovar Typhi and Salmonella enterica serovar Typhimurium vaccine vectors with similar genetic backgrounds in vitro and in vivo. Our goal is to develop a vaccine to prevent Streptococcus pneumoniae infection in newborns; thus, all strains delivered a pneumococcal antigen PspA and the impact of maternal antibodies was evaluated. The results showed that all strains with the regulated delayed attenuated phenotype (RDAP) displayed an invasive ability stronger than that of the S. Typhi vaccine strain, Ty21a, but weaker than that of their corresponding wild-type parental strains. The survival curves of different RDAP vaccine vectors in vitro and in vivo exhibited diverse regulated delayed attenuation kinetics, which was different from S. Typhi Ty21a and the wild-type parental strains. Under the influence of maternal antibody, the persistence of the S. Typhimurium RDAP strain displayed a regulated delayed attenuation trend in nasal lymphoid tissue (NALT), lung, and Peyer's patches, while the persistence of S. Typhi RDAP strains followed the curve only in NALT. The bacterial loads of S. Typhi RDAP strains were lower in NALT, lung, and Peyer's patches in mice born to immune mothers than in those born to naive mothers. In accordance with these results, RDAP vaccine strains induced high titers of IgG antibodies against PspA and against Salmonella lipopolysaccharides. Immunization of mothers with S. Typhi RDAP strains enhanced the level of vaginal mucosal IgA, gamma interferon (IFN-γ), and interleukin 4 (IL-4) and resulted in a higher level of protection against S. pneumoniae challenge.

Identification of salmonella pathogenicity island-2 type III secretion system effectors involved in intramacrophage replication of S. enterica serovar typhimurium: implications for rational vaccine design

Salmonella enterica serovars cause severe diseases in humans, such as gastroenteritis and typhoid fever. The development of systemic disease is dependent on a type III secretion system (T3SS) encoded by Salmonella pathogenicity island-2 (SPI-2). Translocation of effector proteins across the Salmonella-containing vacuole, via the SPI-2 T3SS, enables bacterial replication within host cells, including macrophages. Here, we investigated the contribution of these effectors to intramacrophage replication of Salmonella enterica serovar Typhimurium using Fluorescence Dilution, a dual-fluorescence tool which allows direct measurement of bacterial replication. Of 32 strains, each carrying single mutations in genes encoding effectors, 10 (lacking sifA, sseJ, sopD2, sseG, sseF, srfH, sseL, spvD, cigR, or steD) were attenuated in replication in mouse bone marrow-derived macrophages. The replication profiles of strains combining deletions in effector genes were also investigated: a strain lacking the genes sseG, sopD2, and srfH showed an increased replication defect compared to single-mutation strains and was very similar to SPI-2 T3SS-deficient bacteria with respect to its replication defect. This strain was substantially attenuated in virulence in vivo and yet retained intracellular vacuole integrity and a functional SPI-2 T3SS. Moreover, this strain was capable of SPI-2 T3SS-mediated delivery of a model antigen for major histocompatibility complex (MHC) class I-dependent T-cell activation. This work establishes a basis for the use of a poly-effector mutant strain as an attenuated vaccine carrier for delivery of heterologous antigens directly into the cytoplasm of host cells.

Live attenuated strains of Salmonella enterica serotype Typhi have generated much interest in the search for improved vaccines against typhoid fever and as vaccine vectors for the delivery of heterologous antigens. A promising vaccine candidate is the ΔaroC ΔssaV S. Typhi strain, which owes its attenuation mainly to lack of a type III secretion system (SPI-2 T3SS). The SPI-2 T3SS is important for bacterial proliferation inside macrophages, but not all of the effectors involved in this process have been identified. Here, we show that 10 effectors of the related strain S. Typhimurium contribute to intracellular replication in macrophages. Moreover, we establish that a poly-effector mutant strain of S. Typhimurium can have a severe replication defect and maintain a functional SPI-2 T3SS, which can be exploited for delivery of heterologous antigens.

Novel methods for expression of foreign antigens in live vector vaccines

Bacterial live vector vaccines represent a vaccine development strategy that offers exceptional flexibility. In this approach, genes encoding protective antigens of unrelated bacterial, viral or parasitic pathogens are expressed in an attenuated bacterial vaccine strain that delivers these foreign antigens to the immune system, thereby eliciting relevant immune responses. Rather than expressing these antigens using low copy expression plasmids, here we pursue expression of foreign proteins from the live vector chromosome. Our strategy is designed to compensate for the inherent disadvantage of loss of gene dosage (vs. plasmid-based expression) by integrating antigen-encoding gene cassettes into multiple chromosomal sites already inactivated in an attenuated Salmonella enterica serovar Typhi vaccine candidate. We tested expression of a cassette encoding the green fluorescent protein (GFPuv) integrated separately into native guaBA, htrA or clyA chromosomal loci. Using single integrations, we show that expression levels of GFPuv are significantly affected by the site of integration, regardless of the inclusion of additional strong promoters within the incoming cassette. Using cassettes integrated into both guaBA and htrA, we observe cumulative synthesis levels from two integration sites superior to single integrations. Most importantly, we observe that GFPuv expression increases in a growth phase-dependent manner, suggesting that foreign antigen synthesis may be “tuned” to the physiology of the live vaccine. We expect this novel platform expression technology to prove invaluable in the development of a wide variety of multivalent live vector vaccines, capable of expressing multiple antigens from both chromosomal and plasmid-based expression systems within a single strain.

A prime-boost vaccination of mice with attenuated Salmonella expressing a 30-mer peptide from the Trichinella spiralis gp43 antigen

Protection against Trichinella infections has been achieved using various parasite antigens and adjuvants. Recently, we reported that immunization of mice with an attenuated Salmonella strain displaying a 30-mer peptide (residues 210-239) from the Trichinella spiralis gp43 antigen using the ShdA autotransporter induced partial protection against T. spiralis infection. To improve the efficacy of vaccination, we used the MisL autotransporter system to display the Ts30mer peptide on the surface of Salmonella enterica ser. Typhimurium in combination with a prime-boost vaccination strategy. This vector and immunization regimen induced superior protection against T. spiralis when compared to our previously reported approach. Data presented herein showed a significant reduction in adult worm and muscle larvae burdens, high IgG titers, and increased production of intestinal mucus with entrapped adult worms. This prime-boost vaccination scheme is a suitable strategy to elicit enhanced protective immunity against T. spiralis.

Expression and delivery of tetanus toxin fragment C fused to the N-terminal domain of SipB enhances specific immune responses in mice

Live attenuated bacteria can be used as a carrier for the delivery of foreign antigens to a host's immune system. The N-terminal domain of SipB, a translocon protein of the type III secretion system of Salmonella enterica serovar Typhimurium, is required for secretion and outer membrane localization. In the present study, vaccine plasmids for antigen delivery in which the non-toxic tetanus toxin fragment C (TTFC), which contains a T cell epitope, is fused to the N-terminal 160 amino acids of SipB were developed. It was found that the recombinant proteins are secreted into the culture media and localized to the bacterial surface. TTFC-specific antibody responses are significantly increased in mice orally immunized with attenuated S. Typhimurium BRD509 strains carrying TTFC delivery plasmids. When the TTFC delivery cassettes were introduced into a low copy vector, the plasmid was stably maintained in the BRD509 strain and induced an immune response to the TTFC antigen in mice. These results suggest that expression and delivery of heterologous antigens fused to the N-terminus of SipB enhance the induction of antigen-specific immune responses, and that the N-terminal domain of SipB can be used as a versatile delivery system for foreign antigens.

Intranasal vaccination in mice with an attenuated Salmonella enterica Serovar 908htr A expressing Cp15 of Cryptosporidium: impact of malnutrition with preservation of cytokine secretion

Cryptosporidium is a protozoan parasite associated with acute and persistent diarrhea that, even in asymptomatic persons, can impair normal growth and potentially cognitive and physical development in young children. The recent availability of the complete gene sequence for Cryptosporidium hominis antigen Cp15 allows examination of innovative vaccine regimens involving intra-nasal antigen priming with live bacterial vectors applicable to human populations. We used a recently described weaned mouse model of cryptosporidiosis, where nourished and malnourished vaccinated mice receive the Cp15 antigen recombinant with cytolysinA on a Salmonella serovar Typhi CVD 908-htr A vector, followed by parenteral exposure to antigen with adjuvant. After challenge with Cryptosporidium oocysts via gavage, parameters of infection and disease (stool shedding of parasites, growth rates) were quantified, and serum/lymphoid tissue harvested to elucidate the Cp15-specific adaptive immune response. In vaccinated nourished mice, the regimen was highly immunogenic, with strong antigen-specific IL-6 and IFN-γ secretion and robust Cp15-specific immunoglobulin titers. In vaccinated malnourished mice, secretion of cytokines, particularly IFN-γ, and antigen-specific humoral immunity were generally undiminished despite protein deprivation and stunted growth. In contrast, after natural (oral) challenge with an identical inoculum of Cryptosporidium oocysts, cytokine and humoral responses to Cp15 were less than one-fourth those in vaccinated mice. Nevertheless, vaccination resulted in only transient reduction in stool shedding of parasites and was not otherwise protective against disease. Overall, immunogenicity for a C. hominis antigen was documented in mice, even in the setting of prolonged malnutrition, using an innovative vaccine regimen involving intra-nasal antigen priming with a live enteric bacterial vector, that has potential applicability to vulnerable human populations irrespective of nutritional status.

New technologies in developing recombinant attenuated Salmonella vaccine vectors

Recombinant attenuated Salmonella vaccine (RASV) vectors producing recombinant gene-encoded protective antigens should have special traits. These features ensure that the vaccines survive stresses encountered in the gastrointestinal tract following oral vaccination to colonize lymphoid tissues without causing disease symptoms and to result in induction of long-lasting protective immune responses. We recently described ways to achieve these goals by using regulated delayed in vivo attenuation and regulated delayed in vivo antigen synthesis, enabling RASVs to efficiently colonize effector lymphoid tissues and to serve as factories to synthesize protective antigens that induce higher protective immune responses. We also developed some additional new strategies to increase vaccine safety and efficiency. Modification of lipid A can reduce the inflammatory responses without compromising the vaccine efficiency. Outer membrane vesicles (OMVs) from Salmonella-containing heterologous protective antigens can be used to increase vaccine efficiency. A dual-plasmid system, possessing Asd+ and DadB+ selection markers, each specifying a different protective antigen, can be used to develop multivalent live vaccines. These new technologies have been adopted to develop a novel, low-cost RASV synthesizing multiple protective pneumococcal protein antigens that could be safe for newborns/infants and induce protective immunity to diverse Streptococcus pneumoniae serotypes after oral immunization.

Recombinant vaccines and the development of new vaccine strategies

Vaccines were initially developed on an empirical basis, relying mostly on attenuation or inactivation of pathogens. Advances in immunology, molecular biology, biochemistry, genomics, and proteomics have added new perspectives to the vaccinology field. The use of recombinant proteins allows the targeting of immune responses focused against few protective antigens. There are a variety of expression systems with different advantages, allowing the production of large quantities of proteins depending on the required characteristics. Live recombinant bacteria or viral vectors effectively stimulate the immune system as in natural infections and have intrinsic adjuvant properties. DNA vaccines, which consist of non-replicating plasmids, can induce strong long-term cellular immune responses. Prime-boost strategies combine different antigen delivery systems to broaden the immune response. In general, all of these strategies have shown advantages and disadvantages, and their use will depend on the knowledge of the mechanisms of infection of the target pathogen and of the immune response required for protection. In this review, we discuss some of the major breakthroughs that have been achieved using recombinant vaccine technologies, as well as new approaches and strategies for vaccine development, including potential shortcomings and risks.

The Asd(+)-DadB(+) dual-plasmid system offers a novel means to deliver multiple protective antigens by a recombinant attenuated Salmonella vaccine

We developed means to deliver multiple heterologous antigens on dual plasmids with non-antibiotic-resistance markers in a single recombinant attenuated vaccine strain of Salmonella enterica serotype Typhimurium. The first component of this delivery system is a strain of S. Typhimurium carrying genomic deletions in alr, dadB, and asd, resulting in obligate requirements for diaminopimelic acid (DAP) and d-alanine for growth. The second component is the Asd(+)-DadB(+) plasmid pair carrying wild-type copies of asdA and dadB, respectively, to complement the mutations. To evaluate the protection efficacy of the dual-plasmid vaccine, S. Typhimurium strain χ9760 (a strain with multiple attenuating mutations: Δasd Δalr ΔdadB ΔrecF) was transformed with Asd(+) and DadB(+) plasmids specifying pneumococcal antigens PspA and PspC, respectively. Both plasmids were stable in χ9760 for 50 generations when grown in nonselective medium. This was significantly (P < 0.05) greater than the stability seen in its recF(+) counterpart χ9590 and could be attributed to reduced interplasmid recombination in χ9760. Oral immunization of BALB/c mice with 1 × 10(9) CFU of χ9760 (carrying Asd(+)-PspA and DadB(+)-PspC plasmids) elicited a dominant Th1-type serum IgG response against both antigens and protected mice against intraperitoneal challenge with 200 50% lethal doses (LD(50)s) of virulent Streptococcus pneumoniae strain WU2 or intravenous challenge with 100 LD(50)s of virulent S. pneumoniae strain L81905 or intranasal challenge with a lethal dose of S. pneumoniae A66.1 in a pneumonia model. Protection offered by χ9760 was superior to that offered by the mixture of two strains, χ9828 (Asd(+)-PspA) and χ11026 (DadB(+)-PspC). This novel dual-plasmid system marks a remarkable improvement in the development of live bacterial vaccines.