Effect of attenuated Salmonella enterica serovar Typhimurium expressing a Streptococcus mutans antigen on secondary responses to the cloned protein

Immunomodulation with Enterotoxins for the Generation of Secretory Immunity or Tolerance: Applications for Oral Infections

The heat-labile enterotoxins, such as cholera toxin (CT), and the labile toxins types I and II (LT-I and LT-II) of Escherichia coli have been extensively studied for their immunomodulatory properties, which result in the enhancement of immune responses. Despite superficial similarity in structure, in which a toxic A subunit is coupled to a pentameric binding B subunit, different toxins have different immunological properties. Administration of appropriate antigens admixed with or coupled to these toxins by oral, intranasal, or other routes in experimental animals induces mucosal IgA and circulating IgG antibodies that have protective potential against a variety of enteric, respiratory, or genital infections. These include the generation of salivary antibodies that may protect against colonization with mutans streptococci and the development of dental caries. However, exploitation of these adjuvants for human use requires an understanding of their mode of action and the separation of their desirable immunomodulatory properties from their toxicity. Recent findings have revealed that adjuvant action is not critically dependent upon the enzymic activity of the A subunits, and that the isolated B subunits may exert different effects on cells of the immune system than do the intact toxins. Interaction of the toxins with immunocompetent cells is not exclusively dependent upon their conventional ganglioside receptors. Immunomodulatory effects have been observed on dendritic cells, macrophages, CD4+ and CD8+ T-cells, and B-cells. Numerous factors—including the precise form of the toxin adjuvant, properties of the antigen, whether and how they are coupled, route of administration, and species of animal model—affect the outcome, whether this is enhanced humoral and cellular immunity, or specific induced tolerance toward the antigen.

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.

Pre-existing immunity against vaccine vectors--friend or foe?

Over the last century, the successful attenuation of multiple bacterial and viral pathogens has led to an effective, robust and safe form of vaccination. Recently, these vaccines have been evaluated as delivery vectors for heterologous antigens, as a means of simultaneous vaccination against two pathogens. The general consensus from published studies is that these vaccine vectors have the potential to be both safe and efficacious. However, some of the commonly employed vectors, for example Salmonella and adenovirus, often have pre-existing immune responses in the host and this has the potential to modify the subsequent immune response to a vectored antigen. This review examines the literature on this topic, and concludes that for bacterial vectors there can in fact, in some cases, be an enhancement in immunogenicity, typically humoral, while for viral vectors pre-existing immunity is a hindrance for subsequent induction of cell-mediated responses.

PspA family fusion proteins delivered by attenuated Salmonella enterica serovar Typhimurium extend and enhance protection against Streptococcus pneumoniae

Pneumococcal surface protein A (PspA) is highly immunogenic and can induce a protective immune response against pneumococcal infection. PspA is divided into two major families based on serological variability: family 1 and family 2. To provide broad protection, PspA proteins from pneumococcal strains Rx1 (family 1) and EF5668 (family 2) were combined to form two PspA fusion proteins, PspA/Rx1-EF5668 and PspA/EF5668-Rx1. Each protein was fused to a type II secretion signal and delivered by a recombinant attenuated Salmonella vaccine (RASV). Both PspA/Rx1-EF5668 and PspA/EF5668-Rx1 were synthesized in the RASV and secreted into the periplasm and supernatant. The fusion proteins reacted strongly with both anti-PspA/Rx1 and anti-PspA/EF5668 antisera. Oral immunization of BALB/c mice with RASV synthesizing either PspA fusion protein elicited serum immunoglobulin G (IgG) and mucosal IgA responses against both families of PspA. Analysis of IgG isotypes (IgG2a and IgG1) indicated a strong Th1 bias to the immune responses to both proteins. Sera from mice immunized with RASV synthesizing PspA/Rx1-EF5668 bound to the surface and directed C3 complement deposition on representative strains from all five PspA clades. Immunization with RASV synthesizing either protein protected mice against intraperitoneal challenge with Streptococcus pneumoniae WU2 strain (family 1), intravenous challenge with S. pneumoniae 3JYP2670 strain (family 2), and intranasal challenge with S. pneumoniae A66.1 (family 1). The PspA/Rx1-EF5668 protein elicited significantly greater protection than PspA/EF5668-Rx1, PspA/Rx1, or PspA/EF5668. These results indicate an RASV synthesizing a PspA fusion protein representing both PspA families constitutes an effective antipneumococcal vaccine, extending and enhancing protection against multiple strains of S. pneumoniae.

Oral immunization with attenuated Salmonella enterica serovar Typhimurium encoding Cryptosporidium parvum Cp23 and Cp40 antigens induces a specific immune response in mice

Attenuated Salmonella enterica serovar Typhimurium vaccine strain SL3261 was used as an antigen delivery system for the oral immunization of mice against two Cryptosporidium parvum antigens, Cp23 and Cp40. Each antigen was subcloned into the pTECH1 vector system, which allows them to be expressed as fusion proteins with highly immunogenic fragment C of tetanus toxin under the control of the anaerobically inducible nirB promoter. The recombinant vector was introduced into Salmonella Typhimurium vaccine strain SL3261, and the stable soluble expression of the chimeric protein was evaluated and confirmed by Western blotting with polyclonal C. parvum antisera. Mice were inoculated orally with a single dose of SL3261/pTECH-Cp23 or Cp40, respectively, and plasmid stability was demonstrated both in vitro and in vivo. Specific serum immunoglobulin G (IgG) antibodies against the Cp23 or Cp40 antigen were detected by enzyme-linked immunosorbent assay 35 days after immunization. Also, serum IgA and mucosal (feces) IgA antibodies were detected in 30% of the mice immunized with Cp23. In addition, prime-boosting with Cp23 and Cp40 DNA vaccine vectors followed by Salmonella immunization significantly increased antibody responses to both antigens. Our data show that a single oral inoculation with recombinant S. Typhimurium SL3261 can induce specific antibody responses to the Cp23 or Cp40 antigen from C. parvum in mice, suggesting that recombinant Salmonella is a feasible delivery system for a vaccine against C. parvum infection.

Construction of a New Fusion Anti-caries DNA Vaccine

Mutans streptococci (MS) are generally considered to be the principal etiological agent of dental caries. MS have two important virulence factors: cell- surface protein PAc and glucosyltransferases (GTFs). GTFs have two functional domains: an N-terminal catalytic sucrose-binding domain (CAT) and a C-terminal glucan-binding domain (GLU). A fusion anti-caries DNA vaccine, pGJA-P/VAX, encoding two important antigenic domains, PAc and GLU, of S. mutans, was successful in reducing the levels of dental caries caused by S. mutans in gnotobiotic animals. However, its protective effect against S. sobrinus infection proved to be weak. Does the DNA vaccine need an antigen of S. sobrinus to enhance its ability to inhibit infection? To answer this question, in this study, we cloned the catalytic ( cat) fragment of S. sobrinus gtf-I, which demonstrated its ability to inhibit water-insoluble glucan synthesis by S. sobrinus, into pGJA-P/VAX to produce a new anti-caries DNA vaccine.

Influence of promoter, gene copy number, and preexisting immunity on humoral and cellular responses to a vectored antigen delivered by a Salmonella enterica vaccine

Attenuated Salmonella strains are currently in production as vaccines for protection of animals against salmonellosis. Such commercial strains offer the potential to deliver heterologous antigen to protect animals against other diseases. One vaccine strain, attenuated Salmonella enterica serovar Typhimurium (STM-1), was tested for the ability to deliver ovalbumin and to induce immune responses in mice. Two vaccine trials were performed testing the influence of promoter choice, the location of the encoding DNA (plasmid or chromosome), and the effect of preexisting homologous or heterologous immunity. The results demonstrated that humoral and T-cell responses were induced from either of two promoters, from either the plasmid or the chromosome, and that preexposure to the empty homologous vector, STM-1, or the heterologous vector, S. enterica serovar Enteritidis, had no detrimental effect on subsequent antigen-specific responses. In the case of homologous preexposure, responses were generally greater, and this was correlated with an increased uptake of Salmonella by macrophages in vitro after opsonization with immune sera.

Immunogenicity of Salmonella vector vaccines expressing SBR of Streptococcus mutans under the control of a T7-nirB (dual) promoter system

The purpose of the present study was to determine if a Salmonella vector expressing the cloned saliva-binding region (SBR) of Streptococcus mutans or SBR linked to the A2 and B subunits of cholera toxin (CTA2/B) under the control of both the T7 and nirB promoters (T7-nirB dual promoter) was more effective in inducing mucosal and systemic anti-SBR antibody responses than Salmonella clones expressing the same antigens but under the control of either the nirB or T7 promoter. Mice were immunized by the intranasal route on days 0, 18 and 320 with Salmonella enterica serovar Typhimurium strain BRD 509 containing one of six plasmids encoding SBR or SBR-CTA2/B under the control of the T7-nirB, T7, or nirB promoter. Serum, saliva and vaginal wash samples were collected throughout the experiment and assessed for antibody activity by ELISA. Evidence is provided that Salmonella clones expressing SBR or SBR-CAT2/B under the control of either the T7 or T7-nirB promoter induced a high and persistent mucosal and systemic anti-SBR antibody response. All Salmonella clones induced good anti-SBR responses following the boost on day 320.

Protection of neonatal mice from lethal enterovirus 71 infection by maternal immunization with attenuated Salmonella enterica serovar Typhimurium expressing VP1 of enterovirus 71

This study describes the potential use of attenuated Salmonella enterica serovar Typhimurium strains to express and deliver VP1 of enterovirus 71 (EV71) as a vaccination strategy to prevent EV71 infection in mice. When orally administered to BALB/c mice, both attenuated carrier strains, CNP101 and SL7207, were able to efficiently invade livers and spleens, while only the virulence plasmid-carrying strain SL7207 persisted for more than 30 days in these organs. A recombinant in vivo-regulated promoter expression plasmid expressing VP1 antigen of EV71 was constructed. The expression of the VP1, directed by the pagC promoter, in attenuated Salmonella was confirmed by Western blot hybridization. Both humoral and cellular immune responses were elicited in mice by oral immunization with such Salmonella-based VP1 vaccines. We evaluated the protective efficacy of the vaccines in mice using a maternal immunization protocol. With a lethal challenge, ICR newborn mice born to dams immunized with Salmonella-based VP1 vaccine showed a 50-60% survival; in contrast, none of the mice in the control group survived the challenge. Our data indicated that Salmonella-based VP1 subunit vaccines are a promising vaccine strategy in the prevention of EV71 infection.

Enhanced immunogenicity in the murine airway mucosa with an attenuated Salmonella live vaccine expressing OprF-OprI from Pseudomonas aeruginosa

We constructed an oral live vaccine based on the attenuated aroA mutant Salmonella enterica serovar Typhimurium strain SL3261 expressing outer membrane proteins F and I (OprF-OprI) from Pseudomonas aeruginosa and investigated it in a mouse model. Strains with in vivo inducible protein expression with the PpacC promoter showed good infection rates and immunogenicity but failed to engender detectable antibodies in the lung. However, a systemic booster vaccination following an oral primary immunization yielded high immunoglobulin A (IgA) and IgG antibody levels in both upper and lower airways superior to conventional systemic or mucosal booster vaccination alone. In addition, the proportion of IgG1 and IgG2a antibodies suggested that the systemic booster does not alter the more TH1-like type of response induced by the oral Salmonella primary vaccination. We conclude that an oral primary systemic booster vaccination strategy with an appropriate mucosal vector may be advantageous in diseases with the risk of P. aeruginosa airway infection, such as cystic fibrosis.

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.

Role of B7 costimulatory molecules in immune responses and T-helper cell differentiation in response to recombinant HagB from Porphyromonas gingivalis

In addition to antigen-specific signals mediated through the T-cell receptor, T cells also require antigen nonspecific costimulation for activation. The B7 family of molecules on antigen-presenting cells, which include B7-1 (CD80) and B7-2 (CD86), play important roles in providing costimulatory signals required for development of antigen-specific immune responses. Hemagglutinin B (HagB) is a nonfimbrial adhesin of the periodontopathic microorganism Porphyromonas gingivalis and is thought to be involved in the attachment of the bacterium to host tissues. However, the immune mechanisms involved in responses to HagB and their roles in pathogenesis have yet to be elucidated. Therefore, the purpose of this study was to determine the role of B7 costimulatory molecules on T-helper-cell differentiation for the induction of immune responses to HagB. Mice deficient in either or both of the costimulatory molecules B7-1 and B7-2 were used to explore their role in immune responses to HagB after subcutaneous immunization. B7-1(-/-) mice had levels of immunoglobulin G (IgG) anti-HagB antibody activity in serum similar to those of wild-type mice, whereas lower serum IgG anti-HagB antibody responses were seen in B7-2(-/-) mice. Moreover, significantly lower numbers of IgG antibody-secreting cells and lower levels of CD4(+)-T-cell proliferation were observed in B7-2(-/-) mice compared to wild-type mice. No serum IgG response to HagB was detected in B7-1/B7-2(-/-) mice. Analysis of the subclass of the serum IgG responses and the cytokines induced in response to HagB revealed that B7-2(-/-) mice had significantly lower IgG1 and higher IgG2a anti-HagB antibody responses compared to wild-type mice. The B7-2(-/-) mice also had significantly reduced levels of interleukin-4 (IL-4) and IL-5 and enhanced level of gamma interferon. Furthermore, assessment of B7-1 and B7-2 expression on B cells and macrophages derived from wild-type BALB/c mice after in vitro stimulation with HagB revealed a predominant upregulation in the expression of the B7-2 costimulatory molecule on B cells and macrophages. Essentially no change was seen in the expression of B7-1. Taken together, these results suggest a critical role for B7, especially B7-2, for the preferential induction of a Th2-like response to HagB.

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.

Enhanced immunogenicity of a genetic chimeric protein consisting of two virulence antigens of Streptococcus mutans and protection against infection

The saliva-binding region (SBR) of the cell surface antigen I/II (AgI/II) and the glucan-binding region (GLU) of the glucosyltransferase enzyme of Streptococcus mutans have been implicated in the initial adherence of S. mutans to saliva-coated tooth surfaces and the subsequent sucrose-dependent accumulation of S. mutans, respectively. Here, we describe the construction and characterization of a genetic chimeric protein consisting of the two virulence determinants SBR and GLU (SBR-GLU). The effectiveness of this construct in inducing mucosal and systemic immune responses to each virulence determinant following intranasal immunization was compared to that of each antigen alone or an equal mixture of SBR and GLU (SBR+GLU) in a mouse model. Furthermore, the ability of antibodies induced to SBR-GLU to protect against S. mutans infection was also investigated. Immunization of mice with the chimeric protein SBR-GLU resulted in significantly enhanced (P < 0.001) levels of serum immunoglobulin G (IgG) anti-SBR antibody activity compared to those in the SBR and SBR+GLU groups. The SBR-GLU-immunized mice also demonstrated a significant (P < 0.05) increase in salivary and vaginal IgA antibody responses to SBR and GLU. Analysis of the serum IgG subclass responses to SBR in mice immunized with SBR alone indicated a mixed IgG1 and IgG2a response. A preferential IgG1 response compared to an IgG2a anti-GLU response was induced in mice immunized with GLU alone. Similarly, a preferential IgG1 response was also induced to SBR when GLU was present in either a mixed or conjugated form. Finally, a significant reduction (P < 0.05) in S. mutans colonization was observed only in mice immunized with the SBR-GLU chimeric protein. Taken together, our results indicate that the chimeric protein SBR-GLU significantly enhanced mucosal immune responses to SBR and GLU and systemic immune responses to SBR. The ability of SBR-GLU to induce responses effective in protection against colonization of S. mutans suggests its potential as a vaccine antigen for dental caries.