Effect of nasal immunization with protective antigen of Bacillus anthracis on protective immune response against anthrax toxin

Vaccines against anthrax based on recombinant protective antigen: problems and solutions

Introduction: Anthrax is a dangerous bio-terror agent because Bacillus anthracis spores are highly resilient and can be easily aerosolized and disseminated. There is a threat of deliberate use of anthrax spores aerosol that could lead to serious fatal diseases outbreaks. Existing control measures against inhalation form of the disease are limited. All of this has provided an impetus to the development of new generation vaccines. Areas сovered: This review is devoted to challenges and achievements in the design of vaccines based on the anthrax recombinant protective antigen (rPA). Scientific databases have been searched, focusing on causes of PA instability and solutions to this problem, including new approaches of rPA expression, novel rPA-based vaccines formulations as well as the simultaneous usage of PA with other anthrax antigens. Expert opinion: PA is a central anthrax toxin component, playing a key role in the defense against encapsulated and unencapsulated strains. Subunit rPA-based vaccines have a good safety and protective profile. However, there are problems of PA instability that are greatly enhanced when using aluminum adjuvants. New adjuvant compositions, dry formulations and resistant to proteolysis and deamidation mutant PA forms can help to handle this issue. Devising a modern anthrax vaccine requires huge efforts.

Acetalated Dextran Microparticulate Vaccine Formulated via Coaxial Electrospray Preserves Toxin Neutralization and Enhances Murine Survival Following Inhalational Bacillus Anthracis Exposure

Subunit formulations are regarded as the safest type of vaccine, but they often contain a protein-based antigen that can result in significant challenges, such as preserving antigenicity during formulation and administration. Many studies have demonstrated that encapsulation of protein antigens in polymeric microparticles (MPs) via emulsion techniques results in total IgG antibody titers comparable to alum formulations, however, the antibodies themselves are non-neutralizing. To address this issue, a coaxial electrohydrodynamic spraying (electrospray) technique is used to formulate a microparticulate-based subunit anthrax vaccine under conditions that minimize recombinant protective antigen (rPA) exposure to harsh solvents and high shear stress. rPA and the adjuvant resiquimod are encapsulated either in separate or the same acetalated dextran MPs. Using a murine model, the electrospray formulations lead to higher IgG2a subtype titers as well as comparable total IgG antibody titers and toxin neutralization relative to the FDA-approved vaccine (BioThrax). BioThrax provides no protection against a lethal inhalational challenge of the highly virulent Ames Bacillus anthracis anthrax strain, whereas 50% of the mice vaccinated with separately encapsulated electrospray MPs survive. Overall, this study demonstrates the potential use of electrospray for encapsulating protein antigens in polymeric MPs.

Animal Models for the Pathogenesis, Treatment, and Prevention of Infection by Bacillus anthracis

This article reviews the characteristics of the major animal models utilized for studies on Bacillus anthracis and highlights their contributions to understanding the pathogenesis and host responses to anthrax and its treatment and prevention. Advantages and drawbacks associated with each model, to include the major models (murine, guinea pig, rabbit, nonhuman primate, and rat), and other less frequently utilized models, are discussed. Although the three principal forms of anthrax are addressed, the main focus of this review is on models for inhalational anthrax. The selection of an animal model for study is often not straightforward and is dependent on the specific aims of the research or test. No single animal species provides complete equivalence to humans; however, each species, when used appropriately, can contribute to a more complete understanding of anthrax and its etiologic agent.

Potential biological targets ofBacillus anthracisin anti-infective approaches against the threat of bioterrorism

The terrorist attacks of 2001 involving anthrax underscore the imperative that safe and effective medical countermeasures should be readily available. Vaccination appears to be the most effective form of mass protection against a biological attack, but the current vaccines have drawbacks that justify the enormous amount of effort currently being put into developing more effective vaccines and other treatment modalities. After providing a comprehensive overview of the organism Bacillus anthracis as a biological weapon and its pathogenicity, this review briefly summarizes the current knowledge vital to the management of anthrax disease. This knowledge has been acquired since 2001 as a result of the progress on anthrax research and focuses on the possible development of improved human anti-infective strategies targeting B. anthracis spore components, as well as strategies based on host-pathogen interactions.

Stable dry powder formulation for nasal delivery of anthrax vaccine

There is a current biodefense interest in protection against anthrax. Here, we developed a new generation of stable and effective anthrax vaccine. We studied the immune response elicited by recombinant protective antigen (rPA) delivered intranasally with a novel mucosal adjuvant, a mast cell activator compound 48/80 (C48/80). The vaccine formulation was prepared in a powder form by spray-freeze-drying (SFD) under optimized conditions to produce particles with a target size of D(50) = 25 μm, suitable for delivery to the rabbit nasal cavity. Physicochemical properties of the powder vaccines were characterized to assess their delivery and storage potential. Structural stability of rPA was confirmed by circular dichroism and attenuated total reflectance-Fourier transform infrared spectroscopy, whereas functional stability of rPA and C48/80 was monitored by cell-based assays. Animal study was performed using a unit-dose powder device for direct nasal application. Results showed that C48/80 provided effective mucosal adjuvant activity in rabbits. Freshly prepared SFD powder vaccine formulations or powders stored for over 2 years at room temperature elicited significantly elevated serum PA-specific and lethal toxin neutralization antibody titers that were comparable to that induced by intramuscular immunization with rPA. Nasal delivery of this vaccine formulation may be a viable alternative to the currently licensed vaccine or an attractive vaccine platform for other mucosally transmitted diseases.

A plant based protective antigen [PA(dIV)] vaccine expressed in chloroplasts demonstrates protective immunity in mice against anthrax

The currently available anthrax vaccines are limited by being incompletely characterized, potentially reactogenic and have an expanded dosage schedule. Plant based vaccines offer safe alternative for vaccine production. In the present study, we expressed domain IV of Bacillus anthracis protective antigen gene [PA(dIV)] in planta (by nuclear agrobacterium and chloroplast transformation) and E. coli [rPA(dIV)]. The presence of transgene and the expression of PA(dIV) in planta was confirmed by molecular analysis. Expression levels up to 5.3% of total soluble protein (TSP) were obtained with AT rich (71.8% AT content) PA(dIV) gene in transplastomic plants while 0.8% of TSP was obtained in nuclear transformants. Further, we investigated the protective response of plant and E. coli derived PA(dIV) in mice by intraperitoneal (i.p.) and oral immunizations with or without adjuvant. Antibody titers of >10(4) were induced upon i.p. and oral immunizations with plant derived PA(dIV) and oral immunization with E. coli derived PA(dIV). Intraperitoneal injections with adjuvanted E. coli derived PA(dIV), generated highest antibody titers of >10(5). All the immunized groups demonstrated predominant IgG1 titers over IgG2a indicating a polarized Th2 type response. We also evaluated the mucosal antibody response in orally immunized groups. When fecal extracts were analyzed, low sIgA titer was demonstrated in adjuvanted plant and E. coli derived PA(dIV) groups. Further, PA(dIV) antisera enhanced B. anthracis spore uptake by macrophages in vitro and also demonstrated an anti-germinating effect suggesting a potent role at mucosal surfaces. The antibodies from various groups were efficient in neutralizing the lethal toxin in vitro. When mice were challenged with B. anthracis, mice immunized with adjuvanted plant PA(dIV) imparted 60% and 40% protection while E. coli derived PA(dIV) conferred 100% and 80% protection upon i.p. and oral immunizations. Thus, our study is the first attempt in highlighting the efficacy of plant expressed PA(dIV) by oral immunization in murine model.

Anthrax prevention and treatment: utility of therapy combining antibiotic plus vaccine

The intentional release of anthrax spores in 2001 confirmed this pathogen's ability to cause widespread panic, morbidity and mortality. While individuals exposed to anthrax can be successfully treated with antibiotics, pre-exposure vaccination can reduce susceptibility to infection-induced illness. Concern over the safety and immunogenicity of the licensed US vaccine (Anthrax Vaccine Adsorbed (AVA)) has fueled research into alternatives. Second-generation anthrax vaccines based on purified recombinant protective antigen (rPA) have entered clinical trials. These rPA vaccines induce neutralizing antibodies that prevent illness, but the magnitude and duration of the resultant protective response is modest. Efforts are underway to bolster the immunogenicity of rPA by combining it with adjuvants and other immunostimulatory agents. Third generation vaccines are under development that utilize a wide variety of immunization platforms, antigens, adjuvants, delivery methods and routes of delivery to optimize the induction of a protective immunity. For the foreseeable future, vaccination will rely on first and second generation vaccines co-administered with immune adjuvants. Optimal post-exposure treatment of immunologically naive individuals should include a combination of vaccine plus antibiotic therapy.

Anthrax vaccination strategies

The biological attack conducted through the US postal system in 2001 broadened the threat posed by anthrax from one pertinent mainly to soldiers on the battlefield to one understood to exist throughout our society. The expansion of the threatened population placed greater emphasis on the reexamination of how we vaccinate against Bacillus anthracis. The currently-licensed Anthrax Vaccine, Adsorbed (AVA) and Anthrax Vaccine, Precipitated (AVP) are capable of generating a protective immune response but are hampered by shortcomings that make their widespread use undesirable or infeasible. Efforts to gain US Food and Drug Administration (FDA) approval for licensure of a second generation recombinant protective antigen (rPA)-based anthrax vaccine are ongoing. However, this vaccine's reliance on the generation of a humoral immune response against a single virulence factor has led a number of scientists to conclude that the vaccine is likely not the final solution to optimal anthrax vaccine design. Other vaccine approaches, which seek a more comprehensive immune response targeted at multiple components of the B. anthracis organism, are under active investigation. This review seeks to summarize work that has been done to build on the current PA-based vaccine methodology and to evaluate the search for future anthrax prophylaxis strategies.

Investigation of new dominant-negative inhibitors of anthrax protective antigen mutants for use in therapy and vaccination

The lethal toxin (LeTx) of Bacillus anthracis plays a key role in the pathogenesis of anthrax. The protective antigen (PA) is a primary part of the anthrax toxin and forms LeTx by combination with lethal factor (LF). Phenylalanine-427 (F427) is crucial for PA function. This study was designed to discover potential novel therapeutic agents and vaccines for anthrax. This was done by screening PA mutants that were mutated at the F427 residue for a dominant-negative inhibitory (DNI) phenotype which was nontoxic but inhibited the toxicity of the wild-type LeTx. For this, PA residue F427 was first mutated to each of the other 19 naturally occurring amino acids. The cytotoxicity and DNI phenotypes of the mutated PA proteins were tested in the presence of 1 microg/ml LF in RAW264.7 cells and were shown to be dependent on the individual amino acid replacements. A total of 16 nontoxic mutants with various levels of DNI activity were identified in vitro. Among them, F427D and F427N mutants had the highest DNI activities in RAW264.7 cells. Both mutants inhibited LeTx intoxication in mice in a dose-dependent way. Furthermore, they induced a Th2-predominant immune response and protected mice against a challenge with five 50% lethal doses of LeTx. The protection was correlated mainly with a low level of interleukin-1 beta (IL-1 beta) and with high levels of PA-specific immunoglobulin G1, IL-6, and tumor necrosis factor alpha. Thus, PA DNI mutants, such as F427D and F427N mutants, may serve in the development of novel therapeutic agents and vaccines to fight B. anthracis infections.

Efficacy of a vaccine based on protective antigen and killed spores against experimental inhalational anthrax

Protective antigen (PA)-based anthrax vaccines acting on toxins are less effective than live attenuated vaccines, suggesting that additional antigens may contribute to protective immunity. Several reports indicate that capsule or spore-associated antigens may enhance the protection afforded by PA. Addition of formaldehyde-inactivated spores (FIS) to PA (PA-FIS) elicits total protection against cutaneous anthrax. Nevertheless, vaccines that are effective against cutaneous anthrax may not be so against inhalational anthrax. The aim of this work was to optimize immunization with PA-FIS and to assess vaccine efficacy against inhalational anthrax. We assessed the immune response to recombinant anthrax PA from Bacillus anthracis (rPA)-FIS administered by various immunization protocols and the protection provided to mice and guinea pigs infected through the respiratory route with spores of a virulent strain of B. anthracis. Combined subcutaneous plus intranasal immunization of mice yielded a mucosal immunoglobulin G response to rPA that was more than 20 times higher than that in lung mucosal secretions after subcutaneous vaccination. The titers of toxin-neutralizing antibody and antispore antibody were also significantly higher: nine and eight times higher, respectively. The optimized immunization elicited total protection of mice intranasally infected with the virulent B. anthracis strain 17JB. Guinea pigs were fully protected, both against an intranasal challenge with 100 50% lethal doses (LD(50)) and against an aerosol with 75 LD(50) of spores of the highly virulent strain 9602. Conversely, immunization with PA alone did not elicit protection. These results demonstrate that the association of PA and spores is very much more effective than PA alone against experimental inhalational anthrax.

Towards a human oral vaccine for anthrax: the utility of a Salmonella Typhi Ty21a-based prime-boost immunization strategy

We previously demonstrated the ability of an orally administered attenuated Salmonella enterica serovar Typhimurium strain expressing the protective antigen (PA) of Bacillus anthracis to confer protection against lethal anthrax aerosol spore challenge [Stokes MG, Titball RW, Neeson BN, et al. Oral administration of a Salmonella enterica-based vaccine expressing Bacillus anthracis protective antigen confers protection against aerosolized B. anthracis. Infect Immun 2007;75(April (4)):1827-34]. To extend the utility of this approach to humans we constructed variants of S. enterica serovar Typhi Ty21a, an attenuated typhoid vaccine strain licensed for human use, which expressed and exported PA via two distinct plasmid-based transport systems: the Escherichia coli HlyA haemolysin and the S. Typhi ClyA export apparatus. Murine immunogenicity studies confirmed the ability of these constructs, especially Ty21a expressing the ClyA-PA fusion protein, to stimulate strong PA-specific immune responses following intranasal immunization. These responses were further enhanced by a subsequent boost with either parenterally delivered recombinant PA or the licensed US human alum-adsorbed anthrax vaccine (AVA). Anthrax toxin neutralizing antibody responses using this prime-boost regimen were rapid, vigorous and broad in nature. The results of this study demonstrate the feasibility of employing a mucosal prime with a licensed Salmonella Typhi vaccine strain followed by a parenteral protein boost to stimulate rapid protective immunity against anthrax.

A novel immunogenic spore coat-associated protein in Bacillus anthracis: characterization via proteomics approaches and a vector-based vaccine system

New generation anthrax vaccines have been actively explored with the aim of enhancing efficacies and decreasing undesirable side effects that could be caused by licensed vaccines. Targeting novel antigens and/or eliminating the requirements for multiple needle injections and adjuvants are major objectives in the development of new anthrax vaccines. Using proteomics approaches, we identified a spore coat-associated protein (SCAP) in Bacillus anthracis. An Escherichia coli vector-based vaccine system was used to determine the immunogenicity of SCAP. Mice generated detectable SCAP antibodies three weeks after intranasal immunization with an intact particle of ultraviolet (UV)-irradiated E. coli vector overproducing SCAP. The production of SCAP antibodies was detected via western blotting and SCAP-spotted antigen-arrays. The adjuvant effect of a UV-irradiated E. coli vector eliminates the necessity of boosting and the use of other immunomodulators which will foster the screening and manufacturing of new generation anthrax vaccines. More importantly, the immunogenic SCAP may potentially be a new candidate for the development of anthrax vaccines.

Mucosal immunization with a novel nanoemulsion-based recombinant anthrax protective antigen vaccine protects against Bacillus anthracis spore challenge

The currently available commercial human anthrax vaccine requires multiple injections for efficacy and has side effects due to its alum adjuvant. These factors limit its utility when immunizing exposed populations in emergent situations. We evaluated a novel mucosal adjuvant that consists of a nontoxic, water-in-oil nanoemulsion (NE). This material does not contain a proinflammatory component but penetrates mucosal surfaces to load antigens into dendritic cells. Mice and guinea pigs were intranasally immunized with recombinant Bacillus anthracis protective antigen (rPA) mixed in NE as an adjuvant. rPA-NE immunization was effective in inducing both serum anti-PA immunoglobulin G (IgG) and bronchial anti-PA IgA and IgG antibodies after either one or two mucosal administrations. Serum anti-PA IgG2a and IgG2b antibodies and PA-specific cytokine induction after immunization indicate a Th1-polarized immune response. rPA-NE immunization also produced high titers of lethal-toxin-neutralizing serum antibodies in both mice and guinea pigs. Guinea pigs nasally immunized with rPA-NE vaccine were protected against an intradermal challenge with approximately 1,000 times the 50% lethal dose ( approximately 1,000x LD(50)) of B. anthracis Ames strain spores (1.38 x 10(3) spores), which killed control animals within 96 h. Nasal immunization also resulted in 70% and 40% survival rates against intranasal challenge with 10x LD(50) and 100x LD(50) (1.2 x 10(6) and 1.2 x 10(7)) Ames strain spores. Our results indicate that NE can effectively adjuvant rPA for intranasal immunization. This potentially could lead to a needle-free anthrax vaccine requiring fewer doses and having fewer side effects than the currently available human vaccine.

Monitoring of ELISA-reactive antibodies against anthrax protective antigen (PA), lethal factor (LF), and toxin-neutralising antibodies in serum of individuals vaccinated against anthrax with the PA-based UK anthrax vaccine

The human anthrax vaccines currently licensed contain the protective antigen (PA) of Bacillus anthracis as main antigen together with traces of some other bacillus components, e.g. lethal factor (LF). The present study aimed at monitoring the course of specific antibody titres against PA and LF by enzyme linked immunosorbent assays (ELISA), as well as the levels of toxin-neutralising antibodies, in 11 volunteers vaccinated with the human anthrax vaccine UK. After an initial seroconversion in all vaccinees, a significant reduction of both antibody titres against PA and LF, and of neutralising antibodies, was detected just prior to a vaccine boost 6 months after completion of the basic immunisation. Following the booster injection, titres increased again to levels comparable to those after the fourth immunisation. ELISA titres against PA correlated significantly with neutralising antibodies (r=0.816, p<0.001). Therefore, the less work- and time-consuming ELISA should be favoured to monitor the efficacy of an anthrax vaccination.

Evaluation of combinatorial vaccines against anthrax and plague in a murine model

In this study, we examine the potential of a combinatorial vaccine consisting of the lead-candidate antigens for the next generations of vaccines against anthrax (rPA) and plague (F1-V) with the specific objective of determining synergy or interference between the vaccine components when they are administered separately or together by both traditional parenteral immunization (SC) and mucosal immunization (IN) in the presence of appropriate adjuvants. The most significant findings of the study reported here are that (1) a combinatorial vaccine consisting of equal amounts of F1-V and rPA administered SC is effective at eliciting a robust serum and bronchoalveolar lavage (BAL) antigen-specific IgG and IgG1 response against both antigens in immunized animals, and when administered IN, a robust antigen-specific IgG2a response in the serum and BAL is also induced; (2) there were few instances where either synergy or interference was observed in the combined vaccine administered by either route and those differences occurred soon after the final immunization and were not sustained over time; (3) IN immunization was as effective as SC immunization for induction of antigen-specific serum and BAL antibody responses using the same amount of antigen; (4) the IgG1/IgG2a ratios suggest a strongly biased Type 2 response following SC immunization, while IN immunization produced a more balanced Type 1/Type 2 response; (5) the IgG1/IgG2a ratio was influenced by the route of immunization, the adjuvant employed, and the nature of the antigen. As with previously published studies, there were still detectable levels of circulating anti-F1-V and anti-rPA even 6 months post-primary immunization. These studies provide important insights into the development of new generation biodefense vaccines.

Recombinant protective antigen 102 (rPA102): profile of a second-generation anthrax vaccine

Recent terrorist attacks involving the use of Bacillus anthracis spores have stimulated interest in the development of new vaccines for anthrax prevention. Studies of the pathogenesis of anthrax and of the immune responses following infection and immunization underscore the pivotal role that antibodies to the protective antigen play in protection. The most promising vaccine candidates contain purified recombinant protective antigen. Clinical trials of one of these, recombinant protective antigen (rPA)102, are underway. Initial results suggest that rPA102 is well tolerated and immunogenic. Additional trials are necessary to identify optimal formulations and immunization regimens for pre- and postexposure prophylaxis. Future licensure of these and other candidate vaccines will depend on their safety and immunogenicity profiles in humans, and their ability to confer protection in animal models of inhalational anthrax.

Past, imminent and future human medical countermeasures for anthrax

AIM

Anthrax is caused by the bacterium Bacillus anthracis. Although primarily a disease of animals, it can also infect man, sometimes with fatal consequences. As a result of concerns over the illicit use of this organism, considerable effort is focussed on the development of therapies capable of conferring protection against anthrax. This brief review will describe the efforts being made to address these issues.

METHODS AND RESULTS

A review of the literature and the proceedings of the sixth international conference on anthrax, held in Santa Fe, USA in 2005 shows intense activity, but there has been as yet no real progress. While effective antibiotics, antitoxins and vaccines are available, concerns over their toxicity and the emergence of resistant strains have driven the development of second-generation products. The principal target for vaccine development is Protective Antigen (PA), the nontoxic cell-binding component of anthrax lethal toxin. While the recombinant products currently undergoing human clinical trials will offer considerable advantages in terms of reduced side effects and ease of production, they would still require multiple, needle-based dosing, and the inclusion of the adjuvant alum makes them expensive to administer and stockpile. To address these issues, researchers are developing vaccine formulations, which stimulate rapid protection following needle-free injection (nasal, oral or transcutaneous), and are stable at room temperature to facilitate stockpiling and mass vaccination programs.

CONCLUSIONS

An array of medical countermeasures targeting B. anthracis will become available over the next 5-10 years.

SIGNIFICANCE AND IMPACT OF THE STUDY

The huge investment of research dollars is expected to dramatically expand the knowledge base. A better understanding of basic issues, such as survival in nature and pathogenesis in humans, will facilitate the development of new modalities to eliminate the threat posed by this organism.

Evaluation of the immune response induced by a nasal anthrax vaccine based on the protective antigen protein in anaesthetized and non-anaesthetized mice

To better protect against inhalational anthrax infection, a nasal anthrax vaccine based on the protective antigen (PA) protein of Bacillus anthracis could be an attractive alternative to the current Anthrax-Vaccine-Adsorbed (AVA), which was licensed for cutaneous anthrax prevention. Previously, we have demonstrated that an anti-PA immune response comparable with that in mice subcutaneously immunized with PA protein adjuvanted with aluminium hydroxide was induced in both the systemic compartment and the mucosal secretions of the nose and lung of anaesthetized mice when they were nasally immunized with PA protein incorporated into previously reported LPD (Liposome-Protamine-DNA) particles. In this study, we evaluated the anti-PA immune response induced by the nasal PA/LPD particles in non-anaesthetized mice and compared it with that in anaesthetized mice. Our data showed that the anti-PA antibody response and the anthrax lethal toxin-neutralization activity induced by the nasal PA/LPD in non-anaesthetized mice was relatively weaker than that in anaesthetized mice. However, the splenocytes isolated from the nasally immunized mice, anaesthetized and non-anaesthetized, proliferated comparably after in-vitro re-stimulation. By evaluating the uptake of fluorescence-labelled LPD particles by phagocytes in the nasal and broncho-alveolar lavages of mice after the nasal administration, we concluded that the relatively weaker anti-PA immune response in the non-anaesthetized mice might be partially attributed to the reduced retention of the PA/LPD particles in the nasal cavity of the non-anaesthetized mice. Data collected in this study are expected to be useful for future anthrax nasal vaccine studies when mice are used as a model.

Nasal immunization with anthrax protective antigen protein adjuvanted with polyriboinosinic-polyribocytidylic acid induced strong mucosal and systemic immunities

PURPOSE

The current anthrax vaccine adsorbed (AVA) was originally licensed for the prevention of cutaneous anthrax infection. It has many drawbacks, including the requirement for multiple injections and subsequent annual boosters. Thus, an easily administrable and efficacious anthrax vaccine is needed to prevent the most lethal form of anthrax infection, inhalation anthrax. We propose to develop a nasal anthrax vaccine using anthrax protective antigen (PA) protein as the antigen and synthetic double-stranded RNA in the form of polyriboinosinic-polyribocytidylic acid (pI:C) as an adjuvant.

METHODS

Mice were nasally immunized with recombinant PA admixed with pI:C. The resulting PA-specific antibody responses and the lethal toxin neutralization activity were measured. Moreover, the effect of pI:C on dendritic cells (DCs) was evaluated both in vivo and in vitro.

RESULTS

Mice nasally immunized with rPA adjuvanted with pI:C developed strong systemic and mucosal anti-PA responses with lethal toxin neutralization activity. These immune responses compared favorably to that induced by nasal immunization with rPA adjuvanted with cholera toxin. Poly(I:C) enhanced the proportion of DCs in local draining lymph nodes and stimulated DC maturation.

CONCLUSIONS

This pI:C-adjuvanted rPA vaccine has the potential to be developed into an efficacious nasal anthrax vaccine.

An intranasal vaccine targeting both the Bacillus anthracis toxin and bacterium provides protection against aerosol spore challenge in rabbits

An intranasal vaccine targeting the Bacillus anthracis toxin and vegetative bacterium was tested for the ability to protect immunized rabbits against aerosol B. anthracis spore exposure. Rabbits were vaccinated intranasally with PA-based vaccines formulated as dry powders with or without chitosan (ChiSys, Archimedes Development Limited), a compound that exhibits muco-adhesive properties, or as a liquid. Formulations also contained MPL adjuvant and PA. Some vaccines contained PA conjugated to a 10-mer peptide of the poly-d-glutamic acid capsule of B. anthracis. Rabbits were immunized on days 0 and 28 and aerosol challenged with an average 250LD50 Ames spores on day 85. Serum antibody was measured before and after challenge. Significant anti-PA serum IgG levels were obtained, particularly with use of ChiSys based formulations. PA-Conj induced significant anti-capsule responses, although a formulation containing free capsule peptide did not. All immunized rabbits survived the challenge, but differences in morbidity, as evidenced by anorexia, between vaccine groups were observed. Only rabbits immunized with PA+PA-Conj appeared normal throughout the post-challenge observation period (14 days), while all that received PA with the free capsule peptide appeared ill at times as evidenced by a failure to eat normally. One negative control rabbit received a lower inhaled spore dose (183LD50) and survived the challenge, although it was anorexic post-challenge. It also had a high level of anti-LF antibodies in its convalescent serum (5400 U/ml), indicating an extensive infection. In contrast, 75% of the immunized rabbits had no LF-specific antibody in their post-challenge sera, and the rest had low levels (< or = 138 U/ml), indicating that infections resulting in toxin production were avoided or greatly reduced. Thus, intranasal immunization with a chitosan-based powder vaccine combining PA and capsule epitopes provided superior protection against B. anthracis infection compared to a single antigen (PA) vaccine, as evidenced by a reduction in morbidity and prevention of death.

Strong mucosal and systemic immunities induced by nasal immunization with anthrax protective antigen protein incorporated in liposome-protamine-DNA particles

PURPOSE

The very lengthy and complicated dosing schedule of the current anthrax vaccine adsorbed, which was licensed in the USA for the prevention of cutaneous anthrax infection, calls for the development of an efficacious and easily administrable vaccine to prevent against the most lethal form of anthrax infection, the inhalation anthrax. We propose to develop a nasal anthrax vaccine using anthrax protective antigen (PA) protein carried by liposome-protamine-DNA (LPD) particles.

METHODS

PA was incorporated in LPD particles and nasally dosed to mice. The resulting PA-specific immune response and lethal toxin neutralization activity were measured.

RESULTS

Mice nasally immunized with PA incorporated into LPD particles developed both systemic and mucosal anti-PA responses. The anti-PA immunities induced included the production of anti-PA antibodies (IgG and IgM in the serum and IgA in nasal and lung mucosal secretions) and the proliferation of splenocytes after in vitro stimulation. The anti-PA IgG subtype induced was mainly IgG1. Finally, anthrax lethal toxin neutralization activity was detected both in the serum and in the mucosal secretions.

CONCLUSIONS

The anti-PA immune response induced by nasal PA incorporated in LPD was comparable to that induced by nasal PA adjuvanted with cholera toxin or subcutaneously injected PA adjuvanted with aluminum hydroxide.

Plant-based vaccine: mice immunized with chloroplast-derived anthrax protective antigen survive anthrax lethal toxin challenge

The currently available human vaccine for anthrax, derived from the culture supernatant of Bacillus anthracis, contains the protective antigen (PA) and traces of the lethal and edema factors, which may contribute to adverse side effects associated with this vaccine. Therefore, an effective expression system that can provide a clean, safe, and efficacious vaccine is required. In an effort to produce anthrax vaccine in large quantities and free of extraneous bacterial contaminants, PA was expressed in transgenic tobacco chloroplasts by inserting the pagA gene into the chloroplast genome. Chloroplast integration of the pagA gene was confirmed by PCR and Southern analysis. Mature leaves grown under continuous illumination contained PA as up to 14.2% of the total soluble protein. Cytotoxicity measurements in macrophage lysis assays showed that chloroplast-derived PA was equal in potency to PA produced in B. anthracis. Subcutaneous immunization of mice with partially purified chloroplast-derived or B. anthracis-derived PA with adjuvant yielded immunoglobulin G titers up to 1:320,000, and both groups of mice survived (100%) challenge with lethal doses of toxin. An average yield of about 150 mg of PA per plant should produce 360 million doses of a purified vaccine free of bacterial toxins edema factor and lethal factor from 1 acre of land. Such high expression levels without using fermenters and the immunoprotection offered by the chloroplast-derived PA should facilitate development of a cleaner and safer anthrax vaccine at a lower production cost. These results demonstrate the immunogenic and immunoprotective properties of plant-derived anthrax vaccine antigen.

Oral spore vaccine based on live attenuated nontoxinogenic Bacillus anthracis expressing recombinant mutant protective antigen

An attenuated nontoxinogenic nonencapsulated Bacillus anthracis spore vaccine expressing high levels of recombinant mutant protective antigen (PA), which upon subcutaneous immunization provided protection against a lethal B. anthracis challenge, was found to have the potential to serve also as an oral vaccine. Guinea pigs immunized per os with the recombinant spore vaccine were primed to B. anthracis vegetative antigens as well as to PA, yet only a fraction of the animals (30% to 50%) mounted a humoral response to all of these antigens. Protective immunity provided by per os immunization correlated with a threshold level of PA neutralizing antibody titers and was long-lasting. Protection conferred by per os immunization was attained when the vaccine was administered in the sporogenic form, which, unlike the vegetative cells, survived passage through the gastrointestinal tract. A comparison of immunization of unirradiated spores with immunization of gamma-irradiated spores demonstrated that germination and de novo synthesis of PA were prerequisites for mounting an immune protective response. Oral immunization of guinea pigs with attenuated B. anthracis spores resulted in a characteristic anti-PA immunoglobulin isotype profile (immunoglobulin [G1 IgG1] versus IgG2), as well as induction of specific anti-PA secretory IgA, indicating development of mucosal immunity.

Proteomics reveals that proteins expressed during the early stage of Bacillus anthracis infection are potential targets for the development of vaccines and drugs

In this review, we advance a new concept in developing vaccines and/or drugs to target specific proteins expressed during the early stage of Bacillus anthracis (anthrax) infection and address existing challenges to this concept. Three proteins (immune inhibitor A, GPR-like spore protease, and alanine racemase) initially identified by proteomics in our laboratory were found to have differential expressions during anthrax spore germination and early outgrowth. Other studies of different bacillus strains indicate that these three proteins are involved in either germination or cytotoxicity of spores, suggesting that they may serve as potential targets for the design of anti-anthrax vaccines and drugs.

DNA vaccines for biodefence

The advantages associated with DNA vaccines include the speed with which they may be constructed and produced at large-scale, the ability to produce a broad spectrum of immune responses, and the ability for delivery using non-invasive means. In addition, DNA vaccines may be manipulated to express multiple antigens and may be tailored for the induction of appropriate immune responses. These advantages make DNA vaccination a promising approach for the development of vaccines for biodefence. In this review, the potential of DNA vaccines for biodefence is discussed.

Molecular basis for improved anthrax vaccines

The current vaccine for anthrax has been licensed since 1970 and was developed based on the outcome of human trials conducted in the 1950s. This vaccine, known as anthrax vaccine adsorbed (AVA), consists of a culture filtrate from an attenuated strain of Bacillus anthracis adsorbed to aluminum salts as an adjuvant. This vaccine is considered safe and effective, but is difficult to produce and is associated with complaints about reactogenicity among users of the vaccine. Much of the work in the past decade on generating a second generation vaccine is based on the observation that antibodies to protective antigen (PA) are crucial in the protection against exposure to virulent anthrax spores. Antibodies to PA are thought to prevent binding to its cellular receptor and subsequent binding of lethal factor (LF) and edema factor (EF), which are required events for the action of the two toxins: lethal toxin (LeTx) and edema toxin (EdTx). The bacterial capsule as well as the two toxins are virulence factors of B. anthracis. The levels of antibodies to PA must exceed a certain minimal threshold in order to induce and maintain protective immunity. Immunity can be generated by vaccination with purified PA, as well as spores and DNA plasmids that express PA. Although antibodies to PA address the toxemia component of anthrax disease, antibodies to additional virulence factors, including the capsule or somatic antigens in the spore, may be critical in development of complete, sterilizing immunity to anthrax exposure. The next generation anthrax vaccines will be derived from the thorough understanding of the interaction of virulence factors with human and animal hosts and the role the immune response plays in providing protective immunity.

Anthrax Vaccines

The current human anthrax vaccines licensed in the US and UK consist of aluminum hydroxide-adsorbed or alum-precipitated culture supernatant material from fermentor cultures of toxigenic noncapsulated strains of Bacillus anthracis. The threat of B. anthracis being used as a biowarfare agent has led to a wider usage of these vaccines, which has heightened concerns regarding the need for frequent boosters and the occasional local reactogenicity associated with vaccination. These concerns have provided the impetus for the development of better characterized vaccines. This review summarizes the work of numerous laboratories in the search for alternative vaccines against anthrax that are well tolerated, provide long-lasting immunity, and are efficacious.

Rabbit and nonhuman primate models of toxin-targeting human anthrax vaccines

The intentional use of Bacillus anthracis, the etiological agent of anthrax, as a bioterrorist weapon in late 2001 made our society acutely aware of the importance of developing, testing, and stockpiling adequate countermeasures against biological attacks. Biodefense vaccines are an important component of our arsenal to be used during a biological attack. However, most of the agents considered significant threats either have been eradicated or rarely infect humans alive today. As such, vaccine efficacy cannot be determined in human clinical trials but must be extrapolated from experimental animal models. This article reviews the efficacy and immunogenicity of human anthrax vaccines in well-defined animal models and the progress toward developing a rugged immunologic correlate of protection. The ongoing evaluation of human anthrax vaccines will be dependent on animal efficacy data in the absence of human efficacy data for licensure by the U.S. Food and Drug Administration.

Binary bacterial toxins: biochemistry, biology, and applications of common Clostridium and Bacillus proteins

Certain pathogenic species of Bacillus and Clostridium have developed unique methods for intoxicating cells that employ the classic enzymatic "A-B" paradigm for protein toxins. The binary toxins produced by B. anthracis, B. cereus, C. botulinum, C. difficile, C. perfringens, and C. spiroforme consist of components not physically associated in solution that are linked to various diseases in humans, animals, or insects. The "B" components are synthesized as precursors that are subsequently activated by serine-type proteases on the targeted cell surface and/or in solution. Following release of a 20-kDa N-terminal peptide, the activated "B" components form homoheptameric rings that subsequently dock with an "A" component(s) on the cell surface. By following an acidified endosomal route and translocation into the cytosol, "A" molecules disable a cell (and host organism) via disruption of the actin cytoskeleton, increasing intracellular levels of cyclic AMP, or inactivation of signaling pathways linked to mitogen-activated protein kinase kinases. Recently, B. anthracis has gleaned much notoriety as a biowarfare/bioterrorism agent, and of primary interest has been the edema and lethal toxins, their role in anthrax, as well as the development of efficacious vaccines and therapeutics targeting these virulence factors and ultimately B. anthracis. This review comprehensively surveys the literature and discusses the similarities, as well as distinct differences, between each Clostridium and Bacillus binary toxin in terms of their biochemistry, biology, genetics, structure, and applications in science and medicine. The information may foster future studies that aid novel vaccine and drug development, as well as a better understanding of a conserved intoxication process utilized by various gram-positive, spore-forming bacteria.

Effective mucosal immunity to anthrax: neutralizing antibodies and Th cell responses following nasal immunization with protective antigen

Mucosal, but not parenteral, immunization induces immune responses in both systemic and secretory immune compartments. Thus, despite the reports that Abs to the protective Ag of anthrax (PA) have both anti-toxin and anti-spore activities, a vaccine administered parenterally, such as the aluminum-adsorbed anthrax vaccine, will most likely not induce the needed mucosal immunity to efficiently protect the initial site of infection with inhaled anthrax spores. We therefore took a nasal anthrax vaccine approach to attempt to induce protective immunity both at mucosal surfaces and in the peripheral immune compartment. Mice nasally immunized with recombinant PA (rPA) and cholera toxin (CT) as mucosal adjuvant developed high plasma PA-specific IgG Ab responses. Plasma IgA Abs as well as secretory IgA anti-PA Abs in saliva, nasal washes, and fecal extracts were also induced when a higher dose of rPA was used. The anti-PA IgG subclass responses to nasal rPA plus CT consisted of IgG1 and IgG2b Abs. A more balanced profile of IgG subclasses with IgG1, IgG2a, and IgG2b Abs was seen when rPA was given with a CpG oligodeoxynucleotide as adjuvant, suggesting a role for the adjuvants in the nasal rPA-induced immunity. The PA-specific CD4(+) T cells from mice nasally immunized with rPA and CT as adjuvant secreted low levels of CD4(+) Th1-type cytokines in vitro, but exhibited elevated IL-4, IL-5, IL-6, and IL-10 responses. The functional significance of the anti-PA Ab responses was established in an in vitro macrophage toxicity assay in which both plasma and mucosal secretions neutralized the lethal effects of Bacillus anthracis toxin.