Safety and immune responses to attenuated Salmonella enterica serovar typhi oral live vector vaccines expressing tetanus toxin fragment C

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.

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.

Protection against Staphylococcus aureus and tetanus infections by a combined vaccine containing SasA and TeNT‑Hc in mice

In developing countries, trauma patients and neonates are vulnerable to Staphylococcus aureus (S. aureus) and Clostridium tetani infections. It has been suggested that a combined vaccine against the two infections may be a reliable and cost‑effective strategy. Previous studies have indicated that the S. aureus surface protein A (SasA) and the C fragment of tetanus neurotoxin (TeNT‑Hc) may be suitable candidates for a vaccine against S. aureus and tetanus infections, respectively. In the present study, mice were immunized with a combined vaccine containing SasA and TeNT‑Hc, which induced a robust immune response to both antigens, and mutual interference between SasA and TeNT‑Hc was not observed. In the S.aureus challenge model, the combined vaccine fully protected BALB/c mice against lethal intraperitoneal challenges with 3x109 colony‑forming units of a methicillin‑resistant S. aureus USA300 strain. In the TeNT challenge model, the combined vaccine conferred complete protection against a lethal dose of (2x103) xLD50 tetanus toxin. These results implied that SasA and TeNT‑Hc promising components for a combined vaccine against S. aureus and tetanus infections.

Cross-reactive multifunctional CD4+ T cell responses against Salmonella enterica serovars Typhi, Paratyphi A and Paratyphi B in humans following immunization with live oral typhoid vaccine Ty21a

The live oral typhoid vaccine Ty21a elicits predominantly CD8+, as well as CD4+ T cells mediated immune responses. Clinical field studies showed that Ty21a is moderately effective against S. Typhi and S. Paratyphi B, but not S. Paratyphi A infections. In this study we describe the in depth characterization of S. Typhi, S. Paratyphi A and S. Paratyphi B cross-reactive CD4+ T cell responses elicited following immunization with Ty21a. PBMC samples were collected from 16 healthy volunteers before and 42/84days after Ty21a immunization and stimulated ex-vivo with Salmonella-infected targets. Multiparametric flow cytometry was used to detect the vaccine elicited Salmonella-specific responses in T effector/memory (T_EM) and CD45RA+ T effector/memory (T_EMRA) CD4+ cell subsets, by measuring CD4+ multifunctional (MF) cells that concomitantly produced IFN-γ, TNF-α, IL-2, MIP-1β, IL-17A and/or expressed CD107a. Post-vaccination increases in S. Typhi-specific MF cells were observed in CD4+ T_EM and T_EMRA subsets which predominantly produced IFN-γ and/or TNF-α, while IL-2 was produced by a smaller cell subset. A small proportion of those MF cells also produced MIP-1β, IL-17A and expressed CD107a (a marker associated with cytotoxicity). Approximately one third of these specific MF cells have the potential to migrate to the gut mucosa, as evidenced by co-expression of the gut-homing molecule integrin α₄β₇. In contrast to our previous observations with CD8+ T cells, MF CD4+ T cell responses to the different Salmonella serovars evaluated were similar in magnitude and characteristics. We conclude that although induction of cross-reactive CD4+ MF effector T cells suggest a possible role in Salmonella-immunity, these responses are unlikely to provide an immunological basis for the observed efficacy of Ty21a against S. Typhi and S. Paratyphi B, but not to S. Paratyphi A.

Plague Vaccines: Status and Future

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

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.

Live-attenuatedSalmonellaas a prototype vaccine vector for passenger immunogens in humans: are we there yet?

It has been nearly 20 years since the first Phase I clinical trial of a live-attenuated bacterial vaccine was created by recombinant DNA methods, opening the door to the use of these organisms as mucosal delivery vehicles for passenger antigens. Over this time, a number of animal studies have indicated the feasibility of this approach. These include studies showing that bacteria can deliver antigens expressed by the bacterium itself and that bacteria can deliver DNA vaccines to be expressed in target eukaryotic cells. Concomitant studies have identified a number of attenuating mutations that render the bacterial vectors both safe and immunogenic in humans. Both avenues of research indicate the significant promise of this approach to mucosal vaccine development; however, this promise remains largely unrealized at the level of human clinical trials. This review sketches the history of this problem and points toward possible solutions using Salmonella vaccine vectors as the prototypes.

In vitro Intestinal Mucosal Epithelial Responses to Wild-Type Salmonella Typhi and Attenuated Typhoid Vaccines

Typhoid fever, caused by S. Typhi, is responsible for approximately 200,000 deaths per year worldwide. Little information is available regarding epithelium-bacterial interactions in S. Typhi infection. We have evaluated in vitro the effects of wild-type S. Typhi, the licensed Ty21a typhoid vaccine and the leading strains CVD 908-htrA and CVD 909 vaccine candidates on intestinal barrier function and immune response. Caco2 monolayers infected with wild-type S. Typhi exhibited alterations in the organization of tight junctions, increased paracellular permeability, and a rapid decrease in Trans-Epithelial Electrical Resistance as early as 4 h post-exposure. S. Typhi triggered the secretion of interleukin (IL)-8 and IL-6. Caco2 cells infected with the attenuated strains exhibited a milder pro-inflammatory response with minimal disruption of the barrier integrity. We conclude that wild-type S. Typhi causes marked transient alterations of the intestinal mucosa that are more pronounced than those observed with Ty21a or new generation attenuated typhoid vaccine candidates.

Salmonella expressing detoxified lipopolysaccharide is immunogenic and protective both as an attenuated vaccine and for delivery of foreign antigens

The construction of safe and protective vaccines, derived from human pathogens that have been genetically modified to remove pathogenicity, is often easier to accomplish on paper than it is in the laboratory. Kong and colleagues have pursued a clever strategy to reduce the reactogenicity of attenuated Salmonella oral vaccines by genetically modifying the surface lipopolysaccharide to lower endotoxic activity. The resulting candidate vaccine strains were highly reduced in virulence yet were able to confer protection in a mouse model against challenge with virulent Salmonella. Remarkably, these strains could also be further modified to present foreign antigens from unrelated human pathogens and again confer protection against heterologous challenge. This work brings important new tools to bear on solving the problem of creating efficacious attenuated bacterial vaccines that maximize both safety and immunogenicity in clinical trials.

Salmonella enterica as a vaccine carrier

Salmonella enterica is an invasive, facultative intracellular gastrointestinal pathogen causing human diseases such as gastroenteritis and typhoid fever. Virulence-attenuated strains of this pathogen have interesting capacities for the generation of live vaccines. Attenuated live typhoidal and nontyphoidal Salmonella strains can be used for vaccination against Salmonella infections and to target tumor tissue. Such strains may also serve as live carriers for the development of vaccination strategies against other bacterial, viral or parasitic pathogens. Various strategies have been developed to deploy regulatory circuits and protein secretion systems for efficient expression and delivery of foreign antigens by Salmonella carrier strains. One prominent example is the use of type III secretion systems to translocate recombinant antigens into antigen presenting cells. In this review, we will describe the recent developments in strategies that utilize live attenuated Salmonella as vaccine carriers for prophylactic vaccination against infectious diseases and therapeutic vaccination against tumors. Considerations for generating safe, attenuated carrier strains, designing stable expression systems and the use of adjuvants for live carrier strategies are discussed.

Comparison of a regulated delayed antigen synthesis system with in vivo-inducible promoters for antigen delivery by live attenuated Salmonella vaccines

Induction of strong immune responses against a vectored antigen in hosts immunized with live attenuated Salmonella vaccines is related in part to the amount of antigen delivered and the overall fitness of the Salmonella vector in relation to its ability to stimulate the host immune system. Constitutive high-level antigen synthesis causes a metabolic burden to the vaccine vector strain that can reduce the vaccine strain's ability to interact with host lymphoid tissues, resulting in a compromised immune response. A solution to this problem is the use of systems that regulate antigen gene expression, permitting high levels of antigen synthesis only after the vaccine strain has reached its target tissues. In vivo-inducible promoters (IVIPs) are often used to accomplish this. We recently developed an alternative strategy, a regulated delayed antigen synthesis (RDAS) system, in which the LacI-repressible P(trc) promoter controls antigen gene expression by adding arabinose. In this paper, we compared the RDAS system with two commonly used IVIPs, P(ssaG) and P(pagC). Three nearly identical plasmids, differing only in the promoter used to direct transcription of the pneumococcal pspA gene, P(trc), P(ssaG), or P(pagC), were constructed and introduced into isogenic Salmonella vaccine strains with or without arabinose-inducible LacI synthesis. Mice immunized with the RDAS strain developed slightly higher titers of mucosal and serum anti-PspA antibodies than P(pagC)-immunized mice, while titers in mice immunized with the P(ssaG) strain were 100-fold lower. Both the RDAS and P(pagC) strains conferred similar levels of protection against Streptococcus pneumoniae challenge, significantly greater than those for the P(ssaG) strain or controls. Thus, RDAS provides another choice for inclusion in the live vaccine design to increase immunogenicity.

Priming of Salmonella enterica serovar typhi-specific CD8(+) T cells by suicide dendritic cell cross-presentation in humans

BACKGROUND

The emergence of antibiotic-resistant strains of Salmonella enterica serovar Typhi (S. Typhi), the etiologic agent of typhoid fever, has aggravated an already important public health problem and added new urgency to the development of more effective typhoid vaccines. To this end it is critical to better understand the induction of immunity to S. Typhi. CD8(+) T cells are likely to play an important role in host defense against S. Typhi by several effector mechanisms, including killing of infected cells and IFN-gamma secretion. However, how S. Typhi regulates the development of specific CD8(+) responses in humans remains unclear. Recent studies in mice have shown that dendritic cells (DC) can either directly (upon uptake and processing of Salmonella) or indirectly (by bystander mechanisms) elicit Salmonella-specific CD8(+) T cells.

METHODOLOGY/PRINCIPAL FINDINGS

We report here that upon infection with live S. Typhi, human DC produced high levels of pro-inflammatory cytokines IL-6, IL-8 and TNF-alpha, but low levels of IL-12 p70 and IFN-gamma. In contrast, DC co-cultured with S. Typhi-infected cells, through suicide cross-presentation, uptake S. Typhi-infected human cells and release high levels of IFN-gamma and IL-12p70, leading to the subsequent presentation of bacterial antigens and triggering the induction of memory T cells, mostly CD3(+)CD8(+)CD45RA(-)CD62L(-) effector/memory T cells.

CONCLUSIONS/SIGNIFICANCE

This study is the first to demonstrate the effect of S. Typhi on human DC maturation and on their ability to prime CD8(+) cells and highlights the significance of these phenomena in eliciting adaptive immunity to S. Typhi.

Salmonella enterica serovar Typhi live vector vaccines finally come of age

Attenuated Salmonella Typhi vaccine strains hold great promise as live vectors for presentation of foreign antigens from unrelated bacterial, viral and parasitic pathogens to the immune system. Although this approach has proved quite successful in experimental animal models for eliciting antigen-specific mucosal, humoral and cellular responses, results have been disappointing for clinical trials carried out thus far. We hypothesize that the paucity of human responses to foreign antigens delivered by live vectors suggests that the strains and genetic approaches used to date have resulted in overattenuated vaccine strains with severely reduced immunogenicity. However, remarkable advances have now been made in the genetics of foreign antigen expression, understanding mechanisms of live vector immunity and refining immunization strategies. The time has now come for development of multivalent live vectors in which stable antigen expression and export is balanced with metabolic fitness to create highly immunogenic vaccines.

Mucosal immunization with attenuated Salmonella enterica serovar Typhi expressing protective antigen of anthrax toxin (PA83) primes monkeys for accelerated serum antibody responses to parenteral PA83 vaccine

Salmonella enterica serovar Typhi vaccine strain CVD 908-htrA was genetically engineered for stable plasmid-based expression of protective antigen of anthrax toxin (PA83) fused with the export protein ClyA (ClyA-PA83). The priming potential of CVD 908-htrA expressing ClyA-PA83 was assessed in 12 rhesus and 20 cynomolgus macaques that were immunized mucosally (i.e., intranasally) on days 0 and 14. A parenteral booster with purified PA83 plus alum was given to rhesus macaques on days 42 and 225; cynomolgus monkeys received a booster with either PA or licensed anthrax vaccine (BioThrax; Emergent Biosolutions) only one time, 3 months after priming. Monkeys primed with S. Typhi expressing ClyA-PA83 developed high levels of serum toxin-neutralization activity (TNA) antibodies (50% effective dose [ED50], >1.3x10(3)), 7 days after receipt of the booster, whereas unprimed controls lacked serum TNA (ED50, 0). In nonhuman primates, the success of this anthrax vaccine strategy based on heterologous mucosal priming followed by a parenteral subunit vaccine booster paves the way for clinical trials.

Mucosally delivered Salmonella typhi expressing the Yersinia pestis F1 antigen elicits mucosal and systemic immunity early in life and primes the neonatal immune system for a vigorous anamnestic response to parenteral F1 boost

Neonates respond poorly to conventional vaccines. This has been attributed, in part, to the immaturity of neonatal dendritic cells that lack full capacity for Ag presentation and T cell stimulation. We engineered an attenuated Salmonella Typhi strain to express and export the F1 Ag of Y. pestis (S. Typhi(F1)) and investigated its immunogenicity early in life using a heterologous prime-boost regimen. Newborn mice primed intranasally with a single dose of S. Typhi(F1) elicited mucosal Ab- and IFN-gamma-secreting cells 1 wk after immunization. They also developed a potent and fast anamnestic response to a subsequent parenteral boost with F1-alum, which surpassed those of mice primed and boosted with S. Typhi(F1) or F1-alum. Neonatal priming with S. Typhi(F1), as opposed to priming with F1-alum, resulted in a more balanced IgG2a/IgG1 profile, enhanced avidity maturation and stimulation of B memory cells, and strong Th1-type cell-mediated immunity. S. Typhi(F1) enhanced the activation and maturation of neonatal CD11c+ dendritic cells, shown by increased expression of CD80, CD86, CD40, and MHC-II cell surface markers and production of proinflammatory cytokines IL-12, TNF-alpha, IL-6, and MCP-1. S. Typhi(F1)-stimulated neonatal DC had improved capacity for Ag presentation and T cell stimulation in vitro and induced F1-specific CD4+ and CD8+ T cell responses when adoptively transferred to newborn mice. Mucosal immunization with S. Typhi expressing a foreign Ag effectively primes the neonatal immune system for potent, fast, and broader responses to a parenteral Ag boost. Such a strategy can prevent infectious diseases, including those considered biowarfare threats, early in life.

Plant-based oral vaccines: results of human trials

Vaccines consisting of transgenic plant-derived antigens offer a new strategy for development of safe, inexpensive vaccines. The vaccine antigens can be eaten with the edible part of the plant or purified from plant material. In phase 1 clinical studies of prototype potato- and corn-based vaccines, these vaccines have been safe and immunogenic without the need for a buffer or vehicle other than the plant cell. Transgenic plant technology is attractive for vaccine development because these vaccines are needle-less, stable, and easy to administer. This chapter examines some early human studies of oral transgenic plant-derived vaccines against enterotoxigenic Escherichia coli infection, norovirus, and hepatitis B.

Generation of specific effector and memory T cells with gut- and secondary lymphoid tissue- homing potential by oral attenuated CVD 909 typhoid vaccine in humans

Induction of effective memory T cells is likely to be critical to the level and duration of protection elicited by novel live oral typhoid vaccines. Using cells from volunteers who ingested Salmonella Typhi vaccine strain CVD 909, we characterized the induction of interferon (IFN)-gamma-secreting central (T(CM), CD45RO(+)CD62L(+)) and effector (T(EM), CD45RO(+)CD62L(-)) memory T populations, and their gut-homing potential based on integrin alpha4/beta7 expression. Both CD4(+) T(EM) and T(CM) populations secreted IFN-gamma. However, although CD4(+) T(EM) expressed, or not, integrin alpha(4)/beta(7), CD4(+) T(CM) cells were predominantly integrin alpha(4)/beta(7)(+). In contrast, IFN-gamma-secreting CD8(+) cells were predominantly classical T(EM) and CD45RA(+) T(EM) (T(EMRA), CD45RO(-)CD62L(-)) subsets. However, although CD8(+) T(EM) expressed, or not, integrin alpha(4)/beta(7), CD8(+) T(EMRA) were predominantly integrin alpha(4)/beta(7)(+). This is the first demonstration that oral immunization of humans with S. Typhi elicits diverse IFN-gamma-secreting CD4(+) and CD8(+) T(CM) and T(EM) subsets able to migrate to the gut and other lymphoid tissues.

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.

Rational design of Salmonella recombinant vaccines

Salmonella enterica is an important pathogen of animals and humans causing a variety of infectious diseases. The large number of cases of typhoid fever due to S. enterica serovar Typhi infections gives rise to the continuous need for improved vaccines against this life-threatening infection. However, S. enterica is also an interesting organism to act as a live attenuated carrier for the presentation of recombinant heterologous antigens. Comprehensive experimental studies have been performed and a detailed knowledge of the molecular mechanisms of important virulence factors is available. This allows the rationale design of improved Salmonella carrier strains and the development of novel strategies for the expression and presentation of recombinant antigens. Here, we review recent advances in generation of live attenuated Salmonella vaccines and discuss criteria for expression strategies of heterologous antigens by Salmonella carrier strains.

Cell-mediated immunity and antibody responses elicited by attenuated Salmonella enterica Serovar Typhi strains used as live oral vaccines in humans

The development of improved typhoid vaccines is a high global public health priority. However, their development has been hampered by a lack of information regarding the specific determinants of protective immunity to Salmonella enterica serovar Typhi (S. Typhi) infection in humans. Although antibodies to S. Typhi O, H, and Vi appear to be involved in protection against S. Typhi infection, it is unknown whether such antibodies mediate protection, act in conjunction with other adaptive responses, or serve as a surrogate for the presence of other, more dominant protective immune responses (e.g., cell-mediated immunity [CMI]). CMI responses elicited by immunization of subjects with attenuated S. Typhi oral vaccines include lymphoproliferation; production of type 1 cytokines (e.g., interferon- gamma and tumor necrosis factor- alpha ); and classical major histocompatibility complex (MHC) class Ia-restricted and novel, nonclassical MHC class Ib (human leukocyte antigen [HLA]-E)-restricted CD8(+) cytotoxic T cell responses. In sum, human immunity to S. Typhi elicited by immunization is unexpectedly broad and complex. However, the immunologic correlates of protection remain largely undefined.

Salmonella enterica serovar Typhi Ty21a expressing human papillomavirus type 16 L1 as a potential live vaccine against cervical cancer and typhoid fever

Human papillomavirus (HPV) vaccines based on L1 virus-like particles (VLPs) can prevent HPV-induced genital neoplasias, the precursors of cervical cancer. However, most cervical cancers occur in developing countries, where the implementation of expensive vaccines requiring multiple injections will be difficult. A live Salmonella-based vaccine could be a lower-cost alternative. We previously demonstrated that high HPV type 16 (HPV16)-neutralizing titers are induced after a single oral immunization of mice with attenuated Salmonella enterica serovar Typhimurium strains expressing a codon-optimized version of HPV16 L1 (L1S). To allow the testing of this type of vaccine in women, we constructed a new L1-expressing plasmid, kanL1S, and tested kanL1S recombinants of three Salmonella enterica serovar Typhi vaccine strains shown to be safe in humans, i.e., Ty21a, the actual licensed typhoid vaccine, and two highly immunogenic typhoid vaccine candidates, Ty800 and CVD908-htrA. In an intranasal mouse model of Salmonella serovar Typhi infection, Ty21a kanL1S was unique in inducing HPV16-neutralizing antibodies in serum and genital secretions, while anti-Salmonella responses were similar to those against the parental Ty21a vaccine. Electron microscopy examination of Ty21a kanL1S lysates showed that L1 assembled in capsomers and capsomer aggregates but not well-ordered VLPs. Comparison to the neutralizing antibody response induced by purified HPV16 L1 VLP immunizations in mice suggests that Ty21a kanL1S may be an effective prophylactic HPV vaccine. Ty21a has been widely used against typhoid fever in humans with a remarkable safety record. These finds encourage clinical testing of Ty21a kanL1S as a combined typhoid fever/cervical cancer vaccine with the potential for worldwide application.

Optimization of the delivery of heterologous proteins by the Salmonella enterica serovar Typhimurium type III secretion system for vaccine development

Type III protein secretion systems, which are organelles with the capacity to deliver bacterial proteins into host cells, have been adapted to deliver heterologous antigens for vaccine development. A limitation of these antigen delivery systems is that some proteins are not amenable to secretion through this pathway. We show here that proteins from the simian and human immunodeficiency viruses that are not permissive for secretion through a Salmonella enterica serovar Typhimurium type III secretion system can be modified to travel this secretion pathway by introduction of discrete mutations. Proteins optimized for secretion were presented more efficiently via the major histocompatibility complex class I pathway and were able to induce a better immune response.

Cell-mediated immune responses in humans after immunization with one or two doses of oral live attenuated typhoid vaccine CVD 909

CVD 909 is a novel live attenuated S. Typhi oral vaccine candidate derived from strain CVD 908-htrA which constitutively expresses Vi. Herein we investigated whether the genetic manipulations involved in modifying CVD 908-htrA altered its ability to induce potent T-cell immune responses (CMI) after a single dose (five subjects) and, in a separate trial, whether a second dose (eight subjects) further enhanced its immunogenicity. In these clinical trials we observed that CVD 909 immunization elicits a wide array of CMI, including cytotoxic T cells (CTL), IFN-gamma, TNF-alpha and IL-10 (but not IL-2, IL-4 or IL-5) production, and proliferation to S. Typhi antigens. However, the administration of a second dose did not result in increases in CMI. These results suggest that the genetic manipulations to constitutively express Vi did not adversely affect the ability of CVD 909 to elicit a wide array of CMI responses. These observations add impetus for the continuing evaluation of CVD 909 as a typhoid vaccine candidate.

Reduction of the ganglioside binding activity of the tetanus toxin HC fragment destroys immunogenicity: implications for development of novel tetanus vaccines

In this study, the immunogenicities of the nontoxic H(C) fragment of tetanus toxin and derivatives lacking ganglioside binding activity were compared with that of tetanus toxoid after subcutaneous immunization of mice. Wild-type H(C) (H(C)WT) protein and tetanus toxoid both elicited strong antibody responses against toxoid and H(C) antigens and provided complete protection against toxin challenge. Mutants of H(C) containing deletions essential for ganglioside binding elicited lower responses than H(C)WT. H(C)M115, containing two amino acid substitutions within the ganglioside binding site, provided reduced protection against tetanus toxin challenge compared with H(C)WT, consistent with lower anti-H(C) and anti-toxoid antibody titers. Circular-dichroism spectroscopy and intrinsic fluorescence spectroscopy showed minimal structural perturbation in H(C)M115. We conclude that the presence of the ganglioside binding site within H(C) may be essential for induction of a fully protective anti-tetanus response comparable to that induced by tetanus toxoid by subcutaneous injection.

Enhanced immunisation and expression strategies using bacterial spores as heat-stable vaccine delivery vehicles

Spores of the gram positive bacterium Bacillus subtilis have been used as oral vaccine vehicles and have been shown to confer protection against tetanus in mice. In this work, we have used tetanus as a model with which to examine different antigen presentation strategies as well as dosing regimes using the TTFC antigen of Clostridium tetani. We show that display of the antigen on the spore surface produces a more pronounced, but less rapid, response than when the antigen is expressed in the germinating spore; though this response has a clear Th1 bias. When using the nasal dosing route ten times less spores were needed to produce the same level of protective antibodies using surface display of TTFC. Expression of the antigen within the germinating spore by contrast enables as little as three oral doses on consecutive days to generate protection with a noticeable Th2 bias. We have also shown that TTFC can be expressed using two routes, using display on the spore surface as well as in the germinating spore. This dual route produced the best responses in terms of magnitude and speed of neutralising response as well as a clear Th1 bias indicating the involvement of cellular as well as humoral responses.

Rational design of Salmonella-based vaccination strategies

A permanently growing body of information is becoming available about the quality of protective immune responses induced by mucosal immunization. Attenuated live bacterial vaccines can be administered orally and induce long-lasting protective immunity in humans without causing major side effects. An attenuated Salmonella enterica serovar Typhi strain is registered as live oral vaccine against typhoid fever and has been in use for more than two decades. Recombinant attenuated Salmonella strains are also an attractive means of delivering heterologous antigens to the immune system, thereby, stimulating strong mucosal and systemic immune responses and consequently provide an efficient platform technology to design novel vaccination strategies. This includes the choice of heterologous protective antigens and their expression under the control of appropriate promoters within the carrier strain. The availability of well-characterized attenuated mutants of Salmonella concomitantly supports fine tuning of immune response triggered against heterologous antigens. Exploring different mucosal sites as a potential route of immunization has to be taken into account as an additional important way to modulate immune responses according to clinical requirements. This article focuses on the rational design of strategies to modulate appropriate immunological effector functions on the basis of selection of (i) attenuating mutations of the Salmonella strains, (ii) specific expression systems for the heterologous antigens, and (iii) route of mucosal administration.

Evaluation of Salmonella enterica serovar Typhi (Ty2 aroC-ssaV-) M01ZH09, with a defined mutation in the Salmonella pathogenicity island 2, as a live, oral typhoid vaccine in human volunteers

Salmonella enterica serovar Typhi strains with mutations in the Salmonella pathogenicity island-2 (SPI-2) may represent an effective strategy for human vaccine development, and a vectoring system for heterologous antigens. S. Typhi (Ty2 aroC-ssaV-) M01ZH09 is an attenuated, live, oral typhoid vaccine harboring defined deletion mutations in ssaV, which encodes an integral component in the SPI-2 type III secretion system (TTSS), as well as a mutation in an aromatic biosynthetic pathway needed for bacterial growth in vivo (aroC). SPI-2 mutant vaccines have yet to be evaluated in a large, randomized human trial. A simplified or single-oral dose oral typhoid vaccine using the SPI-2 strategy would offer significant advantages over the currently licensed typhoid vaccines. We performed a double-blinded, placebo-controlled, dose-escalating clinical trial in 60 healthy adult volunteers to determine the tolerability and immunogenicity of a single dose of M01ZH09. Three groups of 20 healthy adult volunteers were enrolled; 16 in each group received a single oral dose of the freeze-dried vaccine at 5 x 10(7), 5 x 10(8) or 5 x 10(9)CFU in a bicarbonate buffer. Four volunteers in each cohort received placebo in the same buffer. Adverse events were infrequent and not statistically different between vaccine and placebo recipients, although two subjects in the mid-range dose and three subjects in the highest dose had temperature measurements >37.5 degrees C. No blood or urine cultures were positive for M01ZH09, and fecal shedding was brief. The immune response was dose-related; the highest vaccine dose (5 x 10(9)CFU) was the most immunogenic. All tested subjects receiving the highest dose had a significant ASC response (mean 118 spots/10(6) cells). A >or=4-fold increase in antibody titer for S. Typhi LPS or flagellin was detected in 75% of volunteers in the highest-dose cohort by day 28. The SPI-2 mutant vaccine, M01ZH09, is a promising typhoid vaccine candidate and deserves further study as a vectoring system for heterologous vaccine antigens.

Plant-derived vaccines against diarrhoeal diseases

Transgenic plant-derived vaccines offer a new strategy for the development of safe, inexpensive vaccines against diarrhoeal diseases. In animal and Phase I clinical studies, these vaccines have been safe and immunogenic without the need for a buffer or vehicle other than the plant cell. This review examines some early attempts to develop oral transgenic plant vaccines against enteric infections such as enterotoxigenic Escherichia coli infection, cholera and norovirus infection.

Orally delivered malaria vaccines: not too hard to swallow

Vaccines offer efficient and cost-effective protection against a wide range of infectious diseases. Unfortunately, no effective vaccine is yet available against malaria, and this infection remains one of the most important causes of human morbidity and mortality in the developing world. Over the past two decades a number of candidate proteins for inclusion in a subunit vaccine have been identified. Malariologists believe that an effective malaria vaccine will need to include multiple proteins that induce protective immune responses against different stages of the Plasmodium life cycle. The construction of such multivalent vaccines is beset by considerable logistical difficulties, not least of which is how to deliver them to a population living in endemic areas. Compared with other routes of vaccine administration, oral delivery has several advantages that make it an attractive strategy for vaccine development. This review summarises the progress towards an oral vaccine delivery system for malaria and discusses the feasibility of this approach.

A Salmonella enterica serovar Typhi vaccine expressing Yersinia pestis F1 antigen on its surface provides protection against plague in mice

A recombinant strain of attenuated Salmonella enterica serovar Typhi surface-expressing Yersinia pestis F1 antigen was generated by transforming strain BRD1116 (aroA aroC htrA) with plasmid pAH34L encoding the Y. pestis caf operon. BRD1116/pAH34L was stable in vitro and in vivo. An immunisation regimen of two intranasal doses of 1 x 10(8) cfu of BRD1116/pAH34L given intranasally to mice 7 days apart induced the strongest immune response compared to other regimens and protected 13 out of 20 mice from lethal challenge with Y. pestis. Intranasal immunisation of mice constitutes a model for oral immunisation with Salmonella vaccines in humans. Thus, the results demonstrate that attenuated strains of S. enterica serovar Typhi which express Y. pestis F1 antigen may be developed to provide an oral vaccine against plague suitable for use in humans.

Recombinant Salmonella enterica serovar Typhi in a prime-boost strategy

This study investigated the utility of attenuated Salmonella enterica serovar Typhi strain CVD 908-htrA (908 h) in a heterologous prime-boost strategy. Mice primed intranasally (i.n.) with 908 h expressing fragment C (Frag C) of tetanus toxin and boosted intramuscularly (i.m.) with tetanus toxoid (TT) mounted enhanced and accelerated serum IgG anti-Frag C responses in comparison to unprimed, vector-primed and homologously-primed and boosted mice. Serum antitoxin responses were also determined; mice that were vaccinated following a heterologous prime-boost regimen exhibited the highest levels of Frag C-specific toxin neutralizing antibodies 1 week after boosting. Mice primed and boosted i.m. with TT developed a significantly greater proportion of serum IgG1 antibodies and weaker IFN-gamma levels in contrast to those primed intranasally (i.n.) with rS. Typhi that were homologously or heterologously boosted. These encouraging pre-clinical data provide a rational basis for undertaking a pilot clinical trial to evaluate this strategy. An ability to stimulate enhanced, accelerated responses to parenteral vaccination following mucosal priming may be advantageous in the immunoprophylaxis of many infectious diseases, including those of biodefense importance.

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.

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.

Attenuated Salmonella enterica serovar Typhi and Shigella flexneri 2a strains mucosally deliver DNA vaccines encoding measles virus hemagglutinin, inducing specific immune responses and protection in cotton rats

Measles remains a leading cause of child mortality in developing countries. Residual maternal measles antibodies and immunologic immaturity dampen immunogenicity of the current vaccine in young infants. Because cotton rat respiratory tract is susceptible to measles virus (MV) replication after intranasal (i.n.) challenge, this model can be used to assess the efficacy of MV vaccines. Pursuing a new measles vaccine strategy that might be effective in young infants, we used attenuated Salmonella enterica serovar Typhi CVD 908-htrA and Shigella flexneri 2a CVD 1208 vaccines to deliver mucosally to cotton rats eukaryotic expression plasmid pGA3-mH and Sindbis virus-based DNA replicon pMSIN-H encoding MV hemagglutinin (H). The initial i.n. dose-response with bacterial vectors alone identified a well-tolerated dosage (1 x 10(9) to 7 x 10(9) CFU) and a volume (20 micro l) that elicited strong antivector immune responses. Animals immunized i.n. on days 0, 28, and 76 with bacterial vectors carrying DNA plasmids encoding MV H or immunized parenterally with these naked DNA vaccine plasmids developed MV plaque reduction neutralizing antibodies and proliferative responses against MV antigens. In a subsequent experiment of identical design, cotton rats were challenged with wild-type MV 1 month after the third dose of vaccine or placebo. MV titers were significantly reduced in lung tissue of animals immunized with MV DNA vaccines delivered either via bacterial live vectors or parenterally. Since attenuated serovar Typhi and S. flexneri can deliver measles DNA vaccines mucosally in cotton rats, inducing measles immune responses (including neutralizing antibodies) and protection, boosting strategies can now be evaluated in animals primed with MV DNA vaccines.

Vaccination against type F botulinum toxin using attenuated Salmonella enterica var Typhimurium strains expressing the BoNT/F H(C) fragment

The utility of the htrA, pagC and nirB promoters to direct the expression of the carboxy-terminal (H(C)) fragment of botulinum toxin F (FH(C)) in Salmonella enterica var Typhimurium has been evaluated. Only low levels of serum antibody were induced after immunisation, and some protection against botulinum toxin type F was demonstrated after oral immunisation of mice with two doses of any of these recombinant Salmonella. Immunisation with two doses of recombinant Salmonella expressing FH(C) from the htrA promoter gave the greatest protection, against up to 10,000 mouse lethal doses of botulinum toxin type F. These results demonstrate the feasibility of an orally delivered vaccine against botulinum toxin type F.

Animal models paving the way for clinical trials of attenuated Salmonella enterica serovar Typhi live oral vaccines and live vectors

Attenuated Salmonella enterica serovar Typhi (S. Typhi) strains can serve as safe and effective oral vaccines to prevent typhoid fever and as live vectors to deliver foreign antigens to the immune system, either by the bacteria expressing antigens through prokaryotic expression plasmids or by delivering foreign genes carried on eukaryotic expression systems (DNA vaccines). The practical utility of such live vector vaccines relies on achieving a proper balance between minimizing the vaccine's reactogenicity and maximizing its immunogenicity. To advance to clinical trials, vaccine candidates need to be pre-clinically evaluated in relevant animal models that attempt to predict what their safety and immunogenicity profile will be when administered to humans. Since S. Typhi is a human-restricted pathogen, a major obstacle that has impeded the progress of vaccine development has been the shortcomings of the animal models available to assess vaccine candidates. In this review, we summarize the usefulness of animal models in the assessment of the degree of attenuation and immunogenicity of novel attenuated S. Typhi strains as vaccine candidates for the prevention of typhoid fever and as live vectors in humans.

Salmonella vaccines for use in humans: present and future perspectives

In recent years there has been significant progress in the development of attenuated Salmonella enterica serovar Typhi strains as candidate typhoid fever vaccines. In clinical trials these vaccines have been shown to be well tolerated and immunogenic. For example, the attenuated S. enterica var. Typhi strains CVD 908-htrA (aroC aroD htrA), Ty800 (phoP phoQ) and chi4073 (cya crp cdt) are all promising candidate typhoid vaccines. In addition, clinical trials have demonstrated that S. enterica var. Typhi vaccines expressing heterologous antigens, such as the tetanus toxin fragment C, can induce immunity to the expressed antigens in human volunteers. In many cases, the problems associated with expression of antigens in Salmonella have been successfully addressed and the future of Salmonella vaccine development is very promising.

Attenuated Salmonella enterica serovar Typhi expressing urease effectively immunizes mice against Helicobacter pylori challenge as part of a heterologous mucosal priming-parenteral boosting vaccination regimen

Recombinant vaccine strains of Salmonella enterica serovar Typhi capable of expressing Helicobacter pylori urease were generated by transforming strains CVD908 and CVD908-htrA with a plasmid harboring the ureAB genes under the control of an in vivo-inducible promoter. The plasmid did not interfere with the ability of either strain to replicate and persist in human monocytic cells or with their transient colonization of mouse lungs. When administered to mice intranasally, both recombinant strains elicited antiurease immune responses skewed towards a Th1 phenotype. Vaccinated mice exhibited strong immunoglobulin G2a (IgG2a)-biased antiurease antibody responses as well as splenocyte populations capable of proliferation and gamma interferon (IFNgamma) secretion in response to urease stimulation. Boosting of mice with subcutaneous injection of urease plus alum enhanced immune responses and led them to a more balanced Th1/Th2 phenotype. Following parenteral boost, IgG1 and IgG2a antiurease antibody titers were raised significantly, and strong urease-specific splenocyte proliferative responses, accompanied by IFNgamma as well as interleukin-4 (IL-4), IL-5, and IL-10 secretion, were detected. Neither immunization with urease-expressing S. enterica serovar Typhi alone nor immunization with urease plus alum alone conferred protection against challenge with a mouse-adapted strain of H. pylori; however, a vaccination protocol combining both immunization regimens was protective. This is the first report of effective vaccination against H. pylori with a combined mucosal prime-parenteral boost regimen in which serovar Typhi vaccine strains are used as antigen carriers. The significance of these findings with regard to development of a human vaccine against H. pylori and modulation of immune responses by heterologous prime-boost immunization regimens is discussed.

Salmonella enterica serovar Typhi live vector vaccines delivered intranasally elicit regional and systemic specific CD8+ major histocompatibility class I-restricted cytotoxic T lymphocytes

We investigated the ability of live attenuated Salmonella enterica serovar Typhi strains delivered to mice intranasally to induce specific cytotoxic T-lymphocyte (CTL) responses at regional and systemic levels. Mice immunized with two doses (28 days apart) of Salmonella serovar Typhi strain Ty21a, the licensed oral typhoid vaccine, and genetically attenuated mutants CVD 908 (DeltaaroC DeltaaroD), CVD 915 (DeltaguaBA), and CVD 908-htrA (DeltaaroC DeltaaroD DeltahtrA) induced CTL specific for Salmonella serovar Typhi-infected cells in spleens and cervical lymph nodes. CTL were detected in effector T cells that had been expanded in vitro for 7 days in the presence of Salmonella-infected syngeneic splenocytes. A second round of stimulation further enhanced the levels of specific cytotoxicity. CTL activity was observed in sorted alphabeta+ CD8+ T cells, which were remarkably increased after expansion, but not in CD4+ T cells. CTL from both cervical lymph nodes and spleens failed to recognize Salmonella-infected major histocompatibility complex (MHC)-mismatched cells, indicating that the responses were MHC restricted. Studies in which MHC blocking antibodies were used showed that H-2L(d) was the restriction element. This is the first demonstration that Salmonella serovar Typhi vaccines delivered intranasally elicit CD8+ MHC class I-restricted CTL. The results further support the usefulness of the murine intranasal model for evaluating the immunogenicity of typhoid vaccine candidates at the preclinical level.