Efficacy of a human anthrax vaccine in guinea pigs, rabbits, and rhesus macaques against challenge by Bacillus anthracis isolates of diverse geographical origin

Doxycycline, levofloxacin, and moxifloxacin are superior to ciprofloxacin in treating anthrax meningitis in rabbits and NHP

Efficient treatment of anthrax-related meningitis in patients poses a significant therapeutic challenge. Previously, we demonstrated in our anthrax meningitis rabbit model that ciprofloxacin treatment is ineffective with most of the treated animals succumbing to the infection. Herein we tested the efficacy of doxycycline in our rabbit model and found it highly effective. Since all of our findings are based on a rabbit model, we test the efficacy of ciprofloxacin or doxycycline in a specific central nervous system (CNS) model developed in non-human primates (NHPs). Similar to rabbits, ciprofloxacin treatment was ineffective, while doxycycline protected the infected rhesus macaques (n = 2) from the lethal CNS Bacillus anthracis infection. To test whether the low efficacy of Ciprofloxacin is an example of low efficacy of all fluoroquinolones or only this substance, we treated rabbits that were inoculated intracisterna magna (ICM) with levofloxacin or moxifloxacin. We found that in contrast to ciprofloxacin, levofloxacin and moxifloxacin were highly efficacious in treating lethal anthrax-related meningitis in rabbits and NHP (levofloxacin). We demonstrated (in naïve rabbits) that this difference probably results from variances in blood-brain-barrier penetration of the different fluoroquinolones. The combined treatment of doxycycline and any one of the tested fluoroquinolones was highly effective in the rabbit CNS infection model. The combined treatment of doxycycline and levofloxacin was effective in an inhalation rabbit model, as good as the doxycycline mono-therapy. These findings imply that while ciprofloxacin is highly effective as a post-exposure prophylactic drug, using this drug to treat symptomatic patients should be reconsidered.

Immunogenicity and Protective Efficacy of Recombinant Protective Antigen Anthrax Vaccine (GC1109) in A/J Mice Model

A recombinant protective antigen anthrax vaccine (GC1109) is being developed as a new-generation vaccine by the Korea Disease Control and Prevention Agency. In accordance with the ongoing step 2 of phase II clinical trials, the immunogenicity and protective efficacy of the booster dose of GC1109 were evaluated in A/J mice after 3 serial vaccinations at 4-week intervals. The results indicated that the booster dose significantly increased the production of anti-protective antigen (PA) IgG and toxin-neutralizing antibody (TNA) compared with those of the group without booster. An enhanced protective effect of the booster dose was not observed because the TNA titers of the group without booster were high enough to confer protection against spore challenge. Additionally, the correlation between TNA titers and probability of survival was determined for calculating the threshold TNA titer levels associated with protection. The threshold 50 % neutralization factor (NF₅₀) of TNA showing 70 % probability of protection was 0.21 in A/J mice with 1,200 LD₅₀ Sterne spores challenge. These results indicate that GC1109 is a promising candidate as a new-generation anthrax vaccine and that a booster dose might provide enhanced protection by producing toxin-neutralizing antibodies.

Efficacy of ANTHRASIL (Anthrax Immune Globulin Intravenous (Human)) in rabbit and nonhuman primate models of inhalational anthrax: Data supporting approval under animal rule

To meet the requirements of the Animal Rule, the efficacy of monotherapy with ANTHRASIL® (Anthrax Immune Globulin Intravenous (Human)) for inhalational anthrax was evaluated in blinded studies using rabbit and nonhuman primate models. Animals in both studies were randomized to treatment groups exposed to ~ 200 LD50 Bacillus anthracis (Ames strain) spores by the aerosol route to induce inhalational anthrax. Rabbits (N = 50/group) were treated with either 15 U/kg ANTHRASIL or a volume-matching dose of IGIV after disease onset as determined by the detection of bacterial toxin in the blood. At the end of the study, survival rates were 2% (1 of 48) in the IGIV control group, and 26% (13 of 50) in the ANTHRASIL-treated group (p = 0.0009). Similarly, ANTHRASIL was effective in cynomolgus monkeys (N = 16/group) when administered therapeutically after the onset of toxemia, with 6% survival in the IGIV control and a dose-related increase in survival of 36%, 43%, and 70% with 7.5, 15 or 30 U/kg doses of ANTHRASIL, respectively. These studies formed the basis for approval of ANTHRASIL by FDA under the Animal Rule.

New formulation of a recombinant anthrax vaccine stabilised with structurally modified plant viruses

Anthrax is a disease caused by Bacillus anthracis. The most promising approach to the development of anthrax vaccine is use of the anthrax protective antigen (PA). At the same time, recombinant PA is a very unstable protein. Previously, the authors have designed a stable modified recombinant anthrax protective antigen with inactivated proteolytic sites and substituted deamidation sites (rPA83m). As a second approach to recombinant PA stabilisation, plant virus spherical particles (SPs) were used as a stabiliser. The combination of these two approaches was shown to be the most effective. Here, the authors report the results of a detailed study of the stability, immunogenicity and protectiveness of rPA83m + SPs compositions. These compositions were shown to be stable, provided high anti-rPA83m antibody titres in guinea pigs and were able to protect them from a fully virulent 81/1 Bacillus anthracis strain. Given these facts, the formulation of rPA83m + SPs compositions is considered to be a prospective anthrax vaccine candidate.

Aerosolized Intratracheal Inoculation of Recombinant Protective Antigen (rPA) Vaccine Provides Protection Against Inhalational Anthrax in B10.D2-Hc0 Mice

Anthrax caused by Bacillus anthracis is a fatal zoonotic disease with a high lethality and poor prognosis. Inhalational anthrax is the most severe of the three forms of anthrax. The currently licensed commercial human anthrax vaccines require a complex immunization procedure for efficacy and have side effects that limit its use in emergent situations. Thus, development of a better anthrax vaccine is necessary. In this study, we evaluate the potency and efficacy of aerosolized intratracheal (i.t.) inoculation with recombinant protective antigen (rPA) subunit vaccines against aerosolized B. anthracis Pasteur II spores (an attenuated strain) challenge in a B10.D2-Hc⁰ mouse (deficient in complement component C5) model. Immunization of rPA in liquid, powder or powder reconstituted formulations via i.t. route conferred 100% protection against a 20× LD₅₀ aerosolized Pasteur II spore challenge in mice, compared with only 50% of subcutaneous (s.c.) injection with liquid rPA. Consistently, i.t. inoculation of rPA vaccines induced a higher lethal toxin (LeTx) neutralizing antibody titer, a stronger lung mucosal immune response and a greater cellular immune response than s.c. injection. Our results demonstrate that immunization with rPA dry powder vaccine via i.t. route may provide a stable and effective strategy to improve currently available anthrax vaccines and B10.D2-Hc⁰ mice challenged with B. anthracis attenuated strains might be an alternative model for anthrax vaccine candidate screening.

Sequence Variability of pXO1-Located Pathogenicity Genes of Bacillus anthracis Natural Strains of Different Geographic Origin

The main pathogenic factor of Bacillus anthracis is a three-component toxin encoded by the pagA, lef, and cya genes, which are located on the pXO1 plasmid. The atxA gene, which encodes the primary regulator of pathogenicity factor expression, is located on the same plasmid. In this work, we evaluated the polymorphism of the pagA, lef, cya, and atxA genes for 85 B. anthracis strains from different evolutionary lineages and canSNP groups. We have found a strong correlation of 19 genotypes with the main evolutionary lineages, but the correlation with the canSNP group of the strain was not as strong. We have detected several genetic markers indicating the geographical origin of the strains, for example, their source from the steppe zone of the former USSR. We also found that strains of the B.Br.001/002 group caused an anthrax epidemic in Russia in 2016 and strains isolated during paleontological excavations in the Russian Arctic have the same genotype as the strains of the B.Br.CNEVA group circulating in Central Europe. This data could testify in favor of the genetic relationship of these two groups of strains and hypothesize the ways of distribution of their ancestral forms between Europe and the Arctic.

Convergent evolution of diverse Bacillus anthracis outbreak strains toward altered surface oligosaccharides that modulate anthrax pathogenesis

Bacillus anthracis, a spore-forming gram-positive bacterium, causes anthrax. The external surface of the exosporium is coated with glycosylated proteins. The sugar additions are capped with the unique monosaccharide anthrose. The West African Group (WAG) B. anthracis have mutations rendering them anthrose deficient. Through genome sequencing, we identified 2 different large chromosomal deletions within the anthrose biosynthetic operon of B. anthracis strains from Chile and Poland. In silico analysis identified an anthrose-deficient strain in the anthrax outbreak among European heroin users. Anthrose-deficient strains are no longer restricted to West Africa so the role of anthrose in physiology and pathogenesis was investigated in B. anthracis Sterne. Loss of anthrose delayed spore germination and enhanced sporulation. Spores without anthrose were phagocytized at higher rates than spores with anthrose, indicating that anthrose may serve an antiphagocytic function on the spore surface. The anthrose mutant had half the LD50 and decreased time to death (TTD) of wild type and complement B. anthracis Sterne in the A/J mouse model. Following infection, anthrose mutant bacteria were more abundant in the spleen, indicating enhanced dissemination of Sterne anthrose mutant. At low sample sizes in the A/J mouse model, the mortality of ΔantC-infected mice challenged by intranasal or subcutaneous routes was 20% greater than wild type. Competitive index (CI) studies indicated that spores without anthrose disseminated to organs more extensively than a complemented mutant. Death process modeling using mouse mortality dynamics suggested that larger sample sizes would lead to significantly higher deaths in anthrose-negative infected animals. The model was tested by infecting Galleria mellonella with spores and confirmed the anthrose mutant was significantly more lethal. Vaccination studies in the A/J mouse model showed that the human vaccine protected against high-dose challenges of the nonencapsulated Sterne-based anthrose mutant. This work begins to identify the physiologic and pathogenic consequences of convergent anthrose mutations in B. anthracis.

Immunogenicity and Protective Efficacy of a Non-Living Anthrax Vaccine versus a Live Spore Vaccine with Simultaneous Penicillin-G Treatment in Cattle

Sterne live spore vaccine (SLSV) is the current veterinary anthrax vaccine of choice. Unlike the non-living anthrax vaccine (NLAV) prototype, SLSV is incompatible with concurrent antibiotics use in an anthrax outbreak scenario. The NLAV candidates used in this study include a crude recombinant protective antigen (CrPA) and a purified recombinant protective antigen (PrPA) complemented by formalin-inactivated spores and Emulsigen-D^®/Alhydrogel^® adjuvants. Cattle were vaccinated twice (week 0 and 3) with NLAVs plus penicillin-G (Pen-G) treatment and compared to cattle vaccinated twice with SLSV alone and with Pen-G treatment. The immunogenicity was assessed using ELISA against rPA and FIS, toxin neutralisation assay (TNA) and opsonophagocytic assay. The protection was evaluated using an in vivo passive immunisation mouse model. The anti-rPA IgG titres for NLAVs plus Pen-G and SLSV without Pen-G treatment showed a significant increase, whereas the titres for SLSV plus Pen-G were insignificant compared to pre-vaccination values. A similar trend was measured for IgM, IgG1, and IgG2 and TNA titres (NT₅₀) showed similar trends to anti-rPA titres across all vaccine groups. The anti-FIS IgG and IgM titres increased significantly for all vaccination groups at week 3 and 5 when compared to week 0. The spore opsonising capacity increased significantly in the NLAV vaccinated groups including Pen-G treatment and the SLSV without Pen-G but much less in the SLSV group with Pen-G treatment. Passive immunization of A/J mice challenged with a lethal dose of 34F2 spores indicated significant protective capacity of antibodies raised in the SLSV and the PrPA + FIS + adjuvants vaccinated and Pen-G treated groups but not for the NLAV with the CrPA + FIS + adjuvants and the SLSV vaccinated and Pen-G treated group. Our findings indicate that the PrPA + FIS + Emulsigen-D^®/Alhydrogel^® vaccine candidate may provide the same level of antibody responses and protective capacity as the SLSV. Advantageously, it can be used concurrently with Penicillin-G in an outbreak situation and as prophylactic treatment in feedlots and valuable breeding stocks.

Progress towards the Development of a NEAT Vaccine for Anthrax II: Immunogen Specificity and Alum Effectiveness in an Inhalational Model

Bacillus anthracis is the causative agent of anthrax disease, presents with high mortality, and has been at the center of bioweapon efforts. The only currently U.S. FDA-approved vaccine to prevent anthrax in humans is anthrax vaccine adsorbed (AVA), which is protective in several animal models and induces neutralizing antibodies against protective antigen (PA), the cell-binding component of anthrax toxin. However, AVA requires a five-course regimen to induce immunity, along with an annual booster, and is composed of undefined culture supernatants from a PA-secreting strain. In addition, it appears to be ineffective against strains that lack anthrax toxin. Here, we investigated a vaccine formulation consisting of recombinant proteins from a surface-localized heme transport system containing near-iron transporter (NEAT) domains and its efficacy as a vaccine for anthrax disease. The cocktail of five NEAT domains was protective against a lethal challenge of inhaled bacillus spores at 3 and 28 weeks after vaccination. The reduction of the formulation to three NEATs (IsdX1, IsdX2, and Bslk) was as effective as a five-NEAT domain cocktail. The adjuvant alum, approved for use in humans, was as protective as Freund's Adjuvant, and protective vaccination correlated with increased anti-NEAT antibody reactivity and reduced bacterial levels in organs. Finally, the passive transfer of anti-NEAT antisera reduced mortality and disease severity, suggesting the protective component is comprised of antibodies. Collectively, these results provide evidence that a vaccine based upon recombinant NEAT proteins should be considered in the development of a next-generation anthrax vaccine.

The Bacillus cereus Group: Bacillus Species with Pathogenic Potential

The Bacillus cereus group includes several Bacillus species with closely related phylogeny. The most well-studied members of the group, B. anthracis, B. cereus, and B. thuringiensis, are known for their pathogenic potential. Here, we present the historical rationale for speciation and discuss shared and unique features of these bacteria. Aspects of cell morphology and physiology, and genome sequence similarity and gene synteny support close evolutionary relationships for these three species. For many strains, distinct differences in virulence factor synthesis provide facile means for species assignment. B. anthracis is the causative agent of anthrax. Some B. cereus strains are commonly recognized as food poisoning agents, but strains can also cause localized wound and eye infections as well as systemic disease. Certain B. thuringiensis strains are entomopathogens and have been commercialized for use as biopesticides, while some strains have been reported to cause infection in immunocompromised individuals. In this article we compare and contrast B. anthracis, B. cereus, and B. thuringiensis, including ecology, cell structure and development, virulence attributes, gene regulation and genetic exchange systems, and experimental models of disease.

Using old antibiotics to treat ancient bacterium-β-lactams for Bacillus anthracis meningitis

As Bacillus anthracis spores pose a proven bio-terror risk, the treatment focus has shifted from exposed populations to anthrax patients and the need for effective antibiotic treatment protocols increases. The CDC recommends carbapenems and Linezolid (oxazolidinone), for the treatment of anthrax, particularly for the late, meningeal stages of the disease. Previously we demonstrated that treatment with Meropenem or Linezolid, either as a single treatment or in combination with Ciprofloxacin, fails to protect rabbits from anthrax-meningitis. In addition, we showed that the failure of Meropenem was due to slow BBB penetration rather than low antibacterial activity. Herein, we tested the effect of increasing the dose of the antibiotic on treatment efficacy. We found that for full protection (88% cure rate) the dose should be increased four-fold from 40 mg/kg to 150 mg/kg. In addition, B. anthracis is a genetically stable bacterium and naturally occurring multidrug resistant B. anthracis strains have not been reported. In this manuscript, we report the efficacy of classical β-lactams as a single treatment or in combination with β-lactamase inhibitors in treating anthrax meningitis. We demonstrate that Ampicillin based treatment of anthrax meningitis is largely efficient (66%). The high efficacy (88-100%) of Augmentin (Amoxicillin and Clavulonic acid) and Unasyn (Ampicillin and Sulbactam) makes them a favorable choice due to reports of β-lactam resistant B. anthracis strains. Tazocin (Piperacillin and Tazobactam) proved inefficient compared to the highly efficient Augmentin and Unasyn.

Nonclinical data supporting orphan medicinal product designations in the area of rare infectious diseases

This review provides an overview of nonclinical in vivo models that can be used to support orphan designation in selected rare infectious diseases in Europe, with the aim to inform and stimulate the planning of nonclinical development in this area of often neglected diseases.

Treating Anthrax-Induced Meningitis in Rabbits

Treatment of anthrax is challenging, especially during the advanced stages of the disease. Recently, the Centers for Disease Control and Prevention (CDC) updated its recommendations for postexposure prophylaxis and treatment of exposed populations (before and after symptom onset). These recommendations distinguished, for the first time, between systemic disease with and without meningitis, a common and serious complication of anthrax. The CDC considers all systemic cases meningeal unless positively proven otherwise. The treatment of patients suffering from systemic anthrax with suspected or confirmed meningitis includes the combination of three antibiotics, i.e., a fluoroquinolone (levofloxacin or ciprofloxacin), a β-lactam (meropenem or imipenem), and a protein synthesis inhibitor (linezolid or clindamycin). In addition, treatment with an antitoxin (anti-protective antigen antibodies) and dexamethasone should be applied. Since the efficacy of most of these treatments has not been demonstrated, especially in animal meningitis models, we developed an anthrax meningitis model in rabbits and tested several of these recommendations. We demonstrated that, in this model, ciprofloxacin, linezolid, and meropenem were ineffective as single treatments, while clindamycin was highly effective. Furthermore, combined treatments of ciprofloxacin and linezolid or ciprofloxacin and dexamethasone failed in treating rabbits with meningitis. We demonstrated that dexamethasone actually hindered blood-brain barrier penetration by antibiotics, reducing the effectiveness of antibiotic treatment of anthrax meningitis in this rabbit model.

Protection of farm goats by combinations of recombinant peptides and formalin inactivated spores from a lethal Bacillus anthracis challenge under field conditions

Background

Bacillus (B.) anthracis, the causal agent of anthrax, is effectively controlled by the Sterne live spore vaccine (34F2) in animals. However, live spore vaccines are not suitable for simultaneous vaccination and antibiotic treatment of animals being at risk of infection in an outbreak situation. Non-living vaccines could close this gap.

Results

In this study a combination of recombinant protective antigen and recombinant Bacillus collagen-like antigen (rBclA) with or without formalin inactivated spores (FIS), targeted at raising an immune response against both the toxins and the spore of B. anthracis, was tested for immunogenicity and protectiveness in goats. Two groups of goats received from local farmers of the Kars region of Turkey were immunized thrice in three weeks intervals and challenged together with non-vaccinated controls with virulent B. anthracis, four weeks after last immunization. In spite of low or none measurable toxin neutralizing antibodies and a surprisingly low immune response to the rBclA, 80% of the goats receiving the complete vaccine were protected against a lethal challenge. Moreover, the course of antibody responses indicates that a two-step vaccination schedule could be sufficient for protection.

Conclusion

The combination of recombinant protein antigens and FIS induces a protective immune response in goats. The non-living nature of this vaccine would allow for a concomitant antibiotic treatment and vaccination procedure. Further studies should clarify how this vaccine candidate performs in a post infection scenario controlled by antibiotics.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-017-1140-2) contains supplementary material, which is available to authorized users.

Protection of rhesus macaques against inhalational anthrax with a Bacillus anthracis capsule conjugate vaccine

The efficacy of currently licensed anthrax vaccines is largely attributable to a single Bacillus anthracis immunogen, protective antigen. To broaden protection against possible strains resistant to protective antigen-based vaccines, we previously developed a vaccine in which the anthrax polyglutamic acid capsule was covalently conjugated to the outer membrane protein complex of Neisseria meningitidis serotype B and demonstrated that two doses of 2.5μg of this vaccine conferred partial protection of rhesus macaques against inhalational anthrax . Here, we demonstrate complete protection of rhesus macaques against inhalational anthrax with a higher 50μg dose of the same capsule conjugate vaccine. These results indicate that B. anthracis capsule is a highly effective vaccine component that should be considered for incorporation in future generation anthrax vaccines.

Mechanistic Analysis of the Effect of Deamidation on the Immunogenicity of Anthrax Protective Antigen

The spontaneous modification of proteins, such as deamidation of asparagine residues, can significantly affect the immunogenicity of protein-based vaccines. Using a "genetically deamidated" form of recombinant protective antigen (rPA), we have previously shown that deamidation can decrease the immunogenicity of rPA, the primary component of new-generation anthrax vaccines. In this study, we investigated the biochemical and immunological mechanisms by which deamidation of rPA might decrease the immunogenicity of the protein. We found that loss of the immunogenicity of rPA vaccine was independent of the presence of adjuvant. We assessed the effect of deamidation on the immunodominant neutralizing B-cell epitopes of rPA and found that these epitopes were not significantly affected by deamidation. In order to assess the effect of deamidation on T-cell help for antibody production elicited by rPA vaccine, we examined the ability of the wild-type and genetically deamidated forms of rPA to serve as hapten carriers. We found that when wild-type and genetically deamidated rPA were modified to similar extents with 2,4-dinitrophenyl hapten (DNP) and then used to immunize mice, higher levels of anti-DNP antibodies were elicited by wild-type DNP-rPA than those elicited by the genetically deamidated DNP-rPA, indicating that wild-type rPA elicits more T-cell help than the genetically deamidated form of the protein. These results suggest that a decrease in the ability of deamidated rPA to elicit T-cell help for antibody production is a possible contributor to its lower immunogenicity.

Production of Functionally Active and Immunogenic Non-Glycosylated Protective Antigen from Bacillus anthracis in Nicotiana benthamiana by Co-Expression with Peptide-N-Glycosidase F (PNGase F) of Flavobacterium meningosepticum

Bacillus anthracis has long been considered a potential biological warfare agent, and therefore, there is a need for a safe, low-cost and highly efficient anthrax vaccine with demonstrated long-term stability for mass vaccination in case of an emergency. Many efforts have been made towards developing an anthrax vaccine based on recombinant protective antigen (rPA) of B. anthracis, a key component of the anthrax toxin, produced using different expression systems. Plants represent a promising recombinant protein production platform due to their relatively low cost, rapid scalability and favorable safety profile. Previous studies have shown that full-length rPA produced in Nicotiana benthamiana (pp-PA83) is immunogenic and can provide full protection against lethal spore challenge; however, further improvement in the potency and stability of the vaccine candidate is necessary. PA of B. anthracis is not a glycoprotein in its native host; however, this protein contains potential N-linked glycosylation sites, which can be aberrantly glycosylated during expression in eukaryotic systems including plants. This glycosylation could affect the availability of certain key epitopes either due to masking or misfolding of the protein. Therefore, a non-glycosylated form of pp-PA83 was engineered and produced in N. benthamiana using an in vivo deglycosylation approach based on co-expression of peptide-N-glycosidase F (PNGase F) from Flavobacterium meningosepticum. For comparison, versions of pp-PA83 containing point mutations in six potential N-glycosylation sites were also engineered and expressed in N. benthamiana. The in vivo deglycosylated pp-PA83 (pp-dPA83) was shown to have in vitro activity, in contrast to glycosylated pp-PA83, and to induce significantly higher levels of toxin-neutralizing antibody responses in mice compared with glycosylated pp-PA83, in vitro deglycosylated pp-PA83 or the mutated versions of pp-PA83. These results suggest that pp-dPA83 may offer advantages in terms of dose sparing and enhanced immunogenicity as a promising candidate for a safe, effective and low-cost subunit vaccine against anthrax.

Efficient generation of monoclonal antibodies from single rhesus macaque antibody secreting cells

Nonhuman primates (NHPs) are used as a preclinical model for vaccine development, and the antibody profiles to experimental vaccines in NHPs can provide critical information for both vaccine design and translation to clinical efficacy. However, an efficient protocol for generating monoclonal antibodies from single antibody secreting cells of NHPs is currently lacking. In this study we established a robust protocol for cloning immunoglobulin (IG) variable domain genes from single rhesus macaque (Macaca mulatta) antibody secreting cells. A sorting strategy was developed using a panel of molecular markers (CD3, CD19, CD20, surface IgG, intracellular IgG, CD27, Ki67 and CD38) to identify the kinetics of B cell response after vaccination. Specific primers for the rhesus macaque IG genes were designed and validated using cDNA isolated from macaque peripheral blood mononuclear cells. Cloning efficiency was averaged at 90% for variable heavy (VH) and light (VL) domains, and 78.5% of the clones (n = 335) were matched VH and VL pairs. Sequence analysis revealed that diverse IGHV subgroups (for VH) and IGKV and IGLV subgroups (for VL) were represented in the cloned antibodies. The protocol was tested in a study using an experimental dengue vaccine candidate. About 26.6% of the monoclonal antibodies cloned from the vaccinated rhesus macaques react with the dengue vaccine antigens. These results validate the protocol for cloning monoclonal antibodies in response to vaccination from single macaque antibody secreting cells, which have general applicability for determining monoclonal antibody profiles in response to other immunogens or vaccine studies of interest in NHPs.

Recent developments in the understanding and use of anthrax vaccine adsorbed: achieving more with less

Anthrax Vaccine Adsorbed (AVA, BioThrax™) is the only Food and Drug Administration (FDA) approved vaccine for the prevention of anthrax in humans. Recent improvements in pre-exposure prophylaxis (PrEP) use of AVA include intramuscular (IM) administration and simplification of the priming series to three doses over 6 months. Administration IM markedly reduced the frequency, severity and duration of injection site reactions. Refinement of animal models for inhalation anthrax, identification of immune correlates of protection and cross-species modeling have created opportunities for reductions in the PrEP booster schedule and were pivotal in FDA approval of a post-exposure prophylaxis (PEP) indication. Clinical and nonclinical studies of accelerated PEP schedules and divided doses may provide prospects for shortening the PEP antimicrobial treatment period. These data may assist in determining feasibility of expanded coverage in a large-scale emergency when vaccine demand may exceed availability. Enhancements to the AVA formulation may broaden the vaccine's PEP application.

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.

Quantitative Determination of Lethal Toxin Proteins in Culture Supernatant of Human Live Anthrax Vaccine Bacillus anthracis A16R

Bacillus anthracis (B. anthracis) is the etiological agent of anthrax affecting both humans and animals. Anthrax toxin (AT) plays a major role in pathogenesis. It includes lethal toxin (LT) and edema toxin (ET), which are formed by the combination of protective antigen (PA) and lethal factor (LF) or edema factor (EF), respectively. The currently used human anthrax vaccine in China utilizes live-attenuated B. anthracis spores (A16R; pXO1+, pXO2−) that produce anthrax toxin but cannot produce the capsule. Anthrax toxins, especially LT, have key effects on both the immunogenicity and toxicity of human anthrax vaccines. Thus, determining quantities and biological activities of LT proteins expressed by the A16R strain is meaningful. Here, we explored LT expression patterns of the A16R strain in culture conditions using another vaccine strain Sterne as a control. We developed a sandwich ELISA and cytotoxicity-based method for quantitative detection of PA and LF. Expression and degradation of LT proteins were observed in culture supernatants over time. Additionally, LT proteins expressed by the A16R and Sterne strains were found to be monomeric and showed cytotoxic activity, which may be the main reason for side effects of live anthrax vaccines. Our work facilitates the characterization of anthrax vaccines components and establishment of a quality control standard for vaccine production which may ultimately help to ensure the efficacy and safety of the human anthrax vaccine A16R.

Production and cell surface display of recombinant anthrax protective antigen on the surface layer of attenuated Bacillus anthracis

To investigate the surface display of the anthrax protective antigen (PA) on attenuated Bacillus anthracis, a recombinant B. anthracis strain, named AP429 was constructed by integrating into the chromosome a translational fusion harboring the DNA fragments encoding the cell wall-targeting domain of the S-layer protein EA1 and the anthrax PA. Crerecombinase action at the loxP sites excised the antibiotic marker. Western blot analysis, fluorescence-activated cell sorting and immunofluorescence analysis confirmed that PA was successfully expressed on the S-layer of the recombinant antibiotic marker-free strain. Notwithstanding extensive proteolytic degradation of the hybrid protein SLHs-PA, quantitative ELISA revealed that approximately 8.1 × 10(6) molecules of SLHs-PA were gained from each Bacillus cell. Moreover, electron microscopy assay indicated that the typical S-layer structures could be clearly observed from the recombinant strain micrographs.

Possible use of bacteriophages active against Bacillus anthracis and other B. cereus group members in the face of a bioterrorism threat

Anthrax is an infectious fatal disease with epidemic potential. Nowadays, bioterrorism using Bacillus anthracis is a real possibility, and thus society needs an effective weapon to neutralize this threat. The pathogen may be easily transmitted to human populations. It is easy to store, transport, and disseminate and may survive for many decades. Recent data strongly support the effectiveness of bacteriophage in treating bacterial diseases. Moreover, it is clear that bacteriophages should be considered a potential incapacitative agent against bioterrorism using bacteria belonging to B. cereus group, especially B. anthracis. Therefore, we have reviewed the possibility of using bacteriophages active against Bacillus anthracis and other species of the B. cereus group in the face of a bioterrorism threat.

Anthrax vaccines: present status and future prospects

The management of anthrax remains a top priority among the biowarfare/bioterror agents. It was the Bacillus anthracis spore attack through the US mail system after the September 11, 2001, terrorist attacks in the USA that highlighted the potential of B. anthracis as a bioterrorism agent and the threat posed by its deliberate dissemination. These attacks invigorated the efforts toward understanding the anthrax pathogenesis and development of more comprehensive medical intervention strategies for its containment in case of both natural disease and manmade, accidental or deliberate infection of a non-suspecting population. Currently, efforts are directed toward the development of safe and efficacious vaccines as well as intervention tools for controlling the disease in the advanced fulminant stage when toxemia has already developed. This work presents an overview of the current understanding of anthrax pathogenesis and recent advances made, particularly after 2001, for the successful management of anthrax and outlines future perspectives.

Deterministic models of inhalational anthrax in New Zealand white rabbits

Computational models describing bacterial kinetics were developed for inhalational anthrax in New Zealand white (NZW) rabbits following inhalation of Ames strain B. anthracis. The data used to parameterize the models included bacterial numbers in the airways, lung tissue, draining lymph nodes, and blood. Initial bacterial numbers were deposited spore dose. The first model was a single exponential ordinary differential equation (ODE) with 3 rate parameters that described mucociliated (physical) clearance, immune clearance (bacterial killing), and bacterial growth. At 36 hours postexposure, the ODE model predicted 1.7×10⁷ bacteria in the rabbit, which agreed well with data from actual experiments (4.0×10⁷ bacteria at 36 hours). Next, building on the single ODE model, a physiological-based biokinetic (PBBK) compartmentalized model was developed in which 1 physiological compartment was the lumen of the airways and the other was the rabbit body (lung tissue, lymph nodes, blood). The 2 compartments were connected with a parameter describing transport of bacteria from the airways into the body. The PBBK model predicted 4.9×10⁷ bacteria in the body at 36 hours, and by 45 hours the model showed all clearance mechanisms were saturated, suggesting the rabbit would quickly succumb to the infection. As with the ODE model, the PBBK model results agreed well with laboratory observations. These data are discussed along with the need for and potential application of the models in risk assessment, drug development, and as a general aid to the experimentalist studying inhalational anthrax.

A plant-produced protective antigen vaccine confers protection in rabbits against a lethal aerosolized challenge with Bacillus anthracis Ames spores

The potential use of Bacillus anthracis as a bioterrorism weapon threatens the security of populations globally, requiring the immediate availability of safe, efficient and easily delivered anthrax vaccine for mass vaccination. Extensive research efforts have been directed toward the development of recombinant subunit vaccines based on protective antigen (PA), the principal virulence factor of B. anthracis. Among the emerging technologies for the production of these vaccine antigens is our launch vector-based plant transient expression system. Using this system, we have successfully engineered, expressed, purified and characterized full-length PA (pp-PA83) in Nicotiana benthamiana plants using agroinfiltration. This plant-produced antigen elicited high toxin neutralizing antibody titers in mice and rabbits after two vaccine administrations with Alhydrogel. In addition, immunization with this vaccine candidate protected 100% of rabbits from a lethal aerosolized B. anthracis challenge. The vaccine effects were dose-dependent and required the presence of Alhydrogel adjuvant. In addition, the vaccine antigen formulated with Alhydrogel was stable and retained immunogenicity after two-week storage at 4°C, the conditions intended for clinical use. These results support the testing of this vaccine candidate in human volunteers and the utility of our plant expression system for the production of a recombinant anthrax vaccine.

Evaluation of mucoadhesive carrier adjuvant: toward an oral anthrax vaccine

The aim of present study was to evaluate the potential of mucoadhesive alginate-coated chitosan microparticles (A-CHMp) for oral vaccine against anthrax. The zeta potential of A-CHMp was -29.7 mV, and alginate coating could prevent the burst release of antigen in simulated gastric fluid. The results indicated that A-CHMp was mucoadhesive in nature and transported it to the peyer's patch upon oral delivery. The immunization studies indicated that A-CHMp resulted in the induction of potent systemic and mucosal immune responses, whereas alum-adjuvanted rPA could induce only systemic immune response. Thus, A-CHMp represents a promising acid carrier adjuvant for oral immunization against anthrax.

Pathology and pathophysiology of inhalational anthrax in a guinea pig model

Nonhuman primates (NHPs) and rabbits are the animal models most commonly used to evaluate the efficacy of medical countermeasures against anthrax in support of licensure under the FDA's "Animal Rule." However, a need for an alternative animal model may arise in certain cases. The development of such an alternative model requires a thorough understanding of the course and manifestation of experimental anthrax disease induced under controlled conditions in the proposed animal species. The guinea pig, which has been used extensively for anthrax pathogenesis studies and anthrax vaccine potency testing, is a good candidate for such an alternative model. This study was aimed at determining the median lethal dose (LD50) of the Bacillus anthracis Ames strain in guinea pigs and investigating the natural history, pathophysiology, and pathology of inhalational anthrax in this animal model following nose-only aerosol exposure. The inhaled LD50 of aerosolized Ames strain spores in guinea pigs was determined to be 5.0 × 10(4) spores. Aerosol challenge of guinea pigs resulted in inhalational anthrax with death occurring between 46 and 71 h postchallenge. The first clinical signs appeared as early as 36 h postchallenge. Cardiovascular function declined starting at 20 h postexposure. Hematogenous dissemination of bacteria was observed microscopically in multiple organs and tissues as early as 24 h postchallenge. Other histopathologic findings typical of disseminated anthrax included suppurative (heterophilic) inflammation, edema, fibrin, necrosis, and/or hemorrhage in the spleen, lungs, and regional lymph nodes and lymphocyte depletion and/or lymphocytolysis in the spleen and lymph nodes. This study demonstrated that the course of inhalational anthrax disease and the resulting pathology in guinea pigs are similar to those seen in rabbits and NHPs, as well as in humans.

Recombinant vaccine displaying the loop-neutralizing determinant from protective antigen completely protects rabbits from experimental inhalation anthrax

We previously showed that a multiple antigenic peptide (MAP) vaccine displaying amino acids (aa) 304 to 319 from the 2β2-2β3 loop of protective antigen was capable of protecting rabbits from an aerosolized spore challenge with Bacillus anthracis Ames strain. Antibodies to this sequence, referred to as the loop-neutralizing determinant (LND), are highly potent at neutralizing lethal toxin yet are virtually absent in rabbit and human protective antigen (PA) antiserum. While the MAP vaccine was protective against anthrax, it contains a single heterologous helper T cell epitope which may be suboptimal for stimulating an outbred human population. We therefore engineered a recombinant vaccine (Rec-LND) containing two tandemly repeated copies of the LND fused to maltose binding protein, with enhanced immunogenicity resulting from the p38/P4 helper T cell epitope from Schistosoma mansoni. Rec-LND was found to be highly immunogenic in four major histocompatibility complex (MHC)-diverse strains of mice. All (7/7) rabbits immunized with Rec-LND developed high-titer antibody, 6 out of 7 developed neutralizing antibody, and all rabbits were protected from an aerosolized spore challenge of 193 50% lethal doses (LD(50)) of the B. anthracis Ames strain. Survivor serum from Rec-LND-immunized rabbits revealed significantly increased neutralization titers and specific activity compared to prechallenge levels yet lacked PA or lethal factor (LF) antigenemia. Control rabbits immunized with PA, which were also completely protected, appeared sterilely immune, exhibiting significant declines in neutralization titer and specific activity compared to prechallenge levels. We conclude that Rec-LND may represent a prototype anthrax vaccine for use alone or potentially combined with PA-containing vaccines.

Use of site-directed mutagenesis to model the effects of spontaneous deamidation on the immunogenicity of Bacillus anthracis protective antigen

Long-term stability is a desired characteristic of vaccines, especially anthrax vaccines, which must be stockpiled for large-scale use in an emergency situation; however, spontaneous deamidation of purified vaccine antigens has the potential to adversely affect vaccine immunogenicity over time. In order to explore whether spontaneous deamidation of recombinant protective antigen (rPA)--the major component of new-generation anthrax vaccines--affects vaccine immunogenicity, we created a "genetically deamidated" form of rPA using site-directed mutagenesis to replace six deamidation-prone asparagine residues, at positions 408, 466, 537, 601, 713, and 719, with either aspartate, glutamine, or alanine residues. We found that the structure of the six-Asp mutant rPA was not significantly altered relative to that of the wild-type protein as assessed by circular dichroism (CD) spectroscopy and biological activity. In contrast, immunogenicity of aluminum-adjuvanted six-Asp mutant rPA, as measured by induction of toxin-neutralizing antibodies, was significantly lower than that of the corresponding wild-type rPA vaccine formulation. The six-Gln and six-Ala mutants also exhibited lower immunogenicity than the wild type. While the wild-type rPA vaccine formulation exhibited a high level of immunogenicity initially, its immunogenicity declined significantly upon storage at 25°C for 4 weeks. In contrast, the immunogenicity of the six-Asp mutant rPA vaccine formulation was low initially but did not change significantly upon storage. Taken together, results from this study suggest that spontaneous deamidation of asparagine residues predicted to occur during storage of rPA vaccines would adversely affect vaccine immunogenicity and therefore the storage life of vaccines.

Recombinant GroEL enhances protective antigen-mediated protection against Bacillus anthracis spore challenge

The fatal inhalation infection caused by Bacillus anthracis results from a complex pathogenic cycle involving release of toxins by bacteria that germinate from spores. Currently available vaccines against anthrax consist of protective antigen (PA), one of the anthrax toxin components. However, these PA-based vaccines are only partially protective against spore challenge in mice. This shows that exclusive elicitation of high anti-PA titer does not directly correlate with protection. Here, we demonstrate that inclusion of GroEL of B. anthracis with PA elicits enhanced protection against anthrax spore challenge in mice. GroEL was included as it has been reported to be present both on the exosporium and in the secretome in addition to the cell surface of B. anthracis. It has also been found protective against other pathogens. In the present study, immunization with GroEL alone was also potent enough to induce high humoral and cell-mediated response and significantly prolonged the mean time to death in spore-challenged mice. As a surface antigen, opsonization of spores with anti-GroEL IgG showed increased uptake of treated spores and therefore accelerated rate of spore destruction by phagocytic cells leading to the protection of mice. We found that GroEL was able to enhance nitric oxide release from lymphocytes and also reduce bacterial load from the organs, probably through the activation of macrophages and over-expression of certain innate immunity receptors. Therefore, the present study emphasizes that GroEL is an effective immunomodulator against B. anthracis infection.

Immunization of mice with formalin-inactivated spores from avirulent Bacillus cereus strains provides significant protection from challenge with Bacillus anthracis Ames

Bacillus anthracis spores are the infectious form of the organism for humans and animals. However, the approved human vaccine in the United States is derived from a vegetative culture filtrate of a toxigenic, nonencapsulated B. anthracis strain that primarily contains protective antigen (PA). Immunization of mice with purified spore proteins and formalin-inactivated spores (FIS) from a nonencapsulated, nontoxigenic B. anthracis strain confers protection against B. anthracis challenge when PA is also administered. To investigate the capacity of the spore particle to act as a vaccine without PA, we immunized mice subcutaneously with FIS from nontoxigenic, nonencapsulated B. cereus strain G9241 pBCXO1(-)/pBC210(-) (dcG9241), dcG9241 ΔbclA, or 569-UM20 or with exosporium isolated from dcG9241. FIS vaccination provided significant protection of mice from intraperitoneal or intranasal challenge with spores of the virulent B. anthracis Ames or Ames ΔbclA strain. Immunization with dcG9241 ΔbclA FIS, which are devoid of the immunodominant spore protein BclA, provided greater protection from challenge with either Ames strain than did immunization with FIS from BclA-producing strains. In addition, we used prechallenge immune antisera to probe a panel of recombinant B. anthracis Sterne spore proteins to identify novel immunogenic vaccine candidates. The antisera were variably reactive with BclA and with 10 other proteins, four of which were previously tested as vaccine candidates. Overall our data show that immunization with FIS from nontoxigenic, nonencapsulated B. cereus strains provides moderate to high levels of protection of mice from B. anthracis Ames challenge and that neither PA nor BclA is required for this protection.

An adenovirus-vectored nasal vaccine confers rapid and sustained protection against anthrax in a single-dose regimen

Bacillus anthracis is the causative agent of anthrax, and its spores have been developed into lethal bioweapons. To mitigate an onslaught from airborne anthrax spores that are maliciously disseminated, it is of paramount importance to develop a rapid-response anthrax vaccine that can be mass administered by nonmedical personnel during a crisis. We report here that intranasal instillation of a nonreplicating adenovirus vector encoding B. anthracis protective antigen could confer rapid and sustained protection against inhalation anthrax in mice in a single-dose regimen in the presence of preexisting adenovirus immunity. The potency of the vaccine was greatly enhanced when codons of the antigen gene were optimized to match the tRNA pool found in human cells. In addition, an adenovirus vector encoding lethal factor can confer partial protection against inhalation anthrax and might be coadministered with a protective antigen-based vaccine.

Anthrax lethal toxin and the induction of CD4 T cell immunity

Bacillus anthracis secretes exotoxins which act through several mechanisms including those that can subvert adaptive immunity with respect both to antigen presenting cell and T cell function. The combination of Protective Antigen (PA) and Lethal Factor (LF) forming Lethal Toxin (LT), acts within host cells to down-regulate the mitogen activated protein kinase (MAPK) signaling cascade. Until recently the MAPK kinases were the only known substrate for LT; over the past few years it has become evident that LT also cleaves Nlrp1, leading to inflammasome activation and macrophage death. The predicted downstream consequences of subverting these important cellular pathways are impaired antigen presentation and adaptive immunity. In contrast to this, recent work has indicated that robust memory T cell responses to B. anthracis antigens can be identified following natural anthrax infection. We discuss how LT affects the adaptive immune response and specifically the identification of B. anthracis epitopes that are both immunogenic and protective with the potential for inclusion in protein sub-unit based vaccines.

Disrupting the luxS quorum sensing gene does not significantly affect Bacillus anthracis virulence in mice or guinea pigs

Many bacterial species use secreted quorum-sensing autoinducer molecules to regulate cell density- and growth phase-dependent gene expression, including virulence factor production, as sufficient environmental autoinducer concentrations are achieved. Bacillus anthracis, the causative agent of anthrax, contains a functional autoinducer (AI-2) system, which appears to regulate virulence gene expression. To determine if the AI-2 system is necessary for disease, we constructed a LuxS AI-2 synthase-deficient mutant in the virulent Ames strain of B. anthracis. We found that growth of the LuxS-deficient mutant was inhibited and sporulation was delayed when compared with the parental strain. However, spores of the Ames luxS mutant remained fully virulent in both mice and guinea pigs.

A three-dose intramuscular injection schedule of anthrax vaccine adsorbed generates sustained humoral and cellular immune responses to protective antigen and provides long-term protection against inhalation anthrax in rhesus macaques

A 3-dose (0, 1, and 6 months) intramuscular (3-IM) priming series of a human dose (HuAVA) and dilutions of up to 1:10 of anthrax vaccine adsorbed (AVA) provided statistically significant levels of protection (60 to 100%) against inhalation anthrax for up to 4 years in rhesus macaques. Serum anti-protective antigen (anti-PA) IgG and lethal toxin neutralization activity (TNA) were detectable following a single injection of HuAVA or 1:5 AVA or following two injections of diluted vaccine (1:10, 1:20, or 1:40 AVA). Anti-PA and TNA were highly correlated (overall r(2) = 0.89 for log(10)-transformed data). Peak responses were seen at 6.5 months. In general, with the exception of animals receiving 1:40 AVA, serum anti-PA and TNA responses remained significantly above control levels at 28.5 months (the last time point measured for 1:20 AVA), and through 50.5 months for the HuAVA and 1:5 and 1:10 AVA groups (P < 0.05). PA-specific gamma interferon (IFN-γ) and interleukin-4 (IL-4) CD4(+) cell frequencies and T cell stimulation indices were sustained through 50.5 months (the last time point measured). PA-specific memory B cell frequencies were highly variable but, in general, were detectable in peripheral blood mononuclear cells (PBMC) by 2 months, were significantly above control levels by 7 months, and remained detectable in the HuAVA and 1:5 and 1:20 AVA groups through 42 months (the last time point measured). HuAVA and diluted AVA elicited a combined Th1/Th2 response and robust immunological priming, with sustained production of high-avidity PA-specific functional antibody, long-term immune cell competence, and immunological memory (30 months for 1:20 AVA and 52 months for 1:10 AVA). Vaccinated animals surviving inhalation anthrax developed high-magnitude anamnestic anti-PA IgG and TNA responses.

Structural and immunological analysis of anthrax recombinant protective antigen adsorbed to aluminum hydroxide adjuvant

New anthrax vaccines currently under development are based on recombinant protective antigen (rPA) and formulated with aluminum adjuvant. Because long-term stability is a desired characteristic of these vaccines, an understanding of the effects of adsorption to aluminum adjuvants on the structure of rPA is important. Using both biophysical and immunological techniques, we compared the structure and immunogenicity of freshly prepared rPA-Alhydrogel formulations to that of formulations stored for 3 weeks at either room temperature or 37°C in order to assess the changes in rPA structure that might occur upon long-term storage on aluminum adjuvant. Intrinsic fluorescence emission spectra of tryptophan residues indicated that some tertiary structure alterations of rPA occurred during storage on Alhydrogel. Using anti-PA monoclonal antibodies to probe specific regions of the adsorbed rPA molecule, we found that two monoclonal antibodies that recognize epitopes located in domain 1 of PA exhibited greater reactivity to the stored formulations than to freshly prepared formulations. Immunogenicity of rPA-Alhydrogel formulations in mice was assessed by measuring the induction of toxin-neutralizing antibodies, as well as antibodies reactive to 12-mer peptides spanning the length of PA. Mice immunized with freshly prepared formulations developed significantly higher toxin-neutralizing antibody titers than mice immunized with the stored preparations. In contrast, sera from mice immunized with stored preparations exhibited increased reactivity to nine 12-mer peptides corresponding to sequences located throughout the rPA molecule. These results demonstrate that storage of rPA-Alhydrogel formulations can lead to structural alteration of the protein and loss of the ability to elicit toxin-neutralizing antibodies.

Inhalational anthrax (Ames aerosol) in naïve and vaccinated New Zealand rabbits: characterizing the spread of bacteria from lung deposition to bacteremia

There is a need to better understand inhalational anthrax in relevant animal models. This understanding could aid risk assessment, help define therapeutic windows, and provide a better understanding of disease. The aim here was to characterize and quantify bacterial deposition and dissemination in rabbits following exposure to single high aerosol dose (> 100 LD₅₀) of Bacillus anthracis (Ames) spores immediately following exposure through 36 h. The primary goal of collecting the data was to support investigators in developing computational models of inhalational anthrax disease. Rabbits were vaccinated prior to exposure with the human vaccine (Anthrax Vaccine Adsorbed, AVA) or were sham-vaccinated, and were then exposed in pairs (one sham and one AVA) so disease kinetics could be characterized in equally-dosed hosts where one group is fully protected and is able to clear the infection (AVA-vaccinated), while the other is susceptible to disease, in which case the bacteria are able to escape containment and replicate uncontrolled (sham-vaccinated rabbits). Between 4–5% of the presented aerosol dose was retained in the lung of sham- and AVA-vaccinated rabbits as measured by dilution plate analysis of homogenized lung tissue or bronchoalveolar lavage (BAL) fluid. After 6 and 36 h, >80% and >96%, respectively, of the deposited spores were no longer detected in BAL, with no detectable difference between sham- or AVA-vaccinated rabbits. Thereafter, differences between the two groups became noticeable. In sham-vaccinated rabbits the bacteria were detected in the tracheobronchial lymph nodes (TBLN) 12 h post-exposure and in the circulation at 24 h, a time point which was also associated with dramatic increases in vegetative CFU in the lung tissue of some animals. In all sham-vaccinated rabbits, bacteria increased in both TBLN and blood through 36 h at which point in time some rabbits succumbed to disease. In contrast, AVA-vaccinated rabbits showed small numbers of CFU in TBLN between 24 and 36 h post-exposure with small numbers of bacteria in the circulation only at 24 h post-exposure. These results characterize and quantify disease progression in naïve rabbits following aerosol administration of Ames spores which may be useful in a number of different research applications, including developing quantitative models of infection for use in human inhalational anthrax risk assessment.

Achieving consistent multiple daily low-dose Bacillus anthracis spore inhalation exposures in the rabbit model

Repeated low-level exposures to biological agents could occur before or after the remediation of an environmental release. This is especially true for persistent agents such as B. anthracis spores, the causative agent of anthrax. Studies were conducted to examine aerosol methods needed for consistent daily low aerosol concentrations to deliver a low-dose (less than 10(6) colony forming units (CFU) of B. anthracis spores) and included a pilot feasibility characterization study, acute exposure study, and a multiple 15 day exposure study. This manuscript focuses on the state-of-the-science aerosol methodologies used to generate and aerosolize consistent daily low aerosol concentrations and resultant low inhalation doses to rabbits. The pilot feasibility characterization study determined that the aerosol system was consistent and capable of producing very low aerosol concentrations. In the acute, single day exposure experiment, targeted inhaled doses of 1 × 10(2), 1 × 10(3), 1 × 10(4), and 1 × 10(5) CFU were used. In the multiple daily exposure experiment, rabbits were exposed multiple days to targeted inhaled doses of 1 × 10(2), 1 × 10(3), and 1 × 10(4) CFU. In all studies, targeted inhaled doses remained consistent from rabbit-to-rabbit and day-to-day. The aerosol system produced aerosolized spores within the optimal mass median aerodynamic diameter particle size range to reach deep lung alveoli. Consistency of the inhaled dose was aided by monitoring and recording respiratory parameters during the exposure with real-time plethysmography. Overall, the presented results show that the animal aerosol system was stable and highly reproducible between different studies and over multiple exposure days.

Differential contribution of Bacillus anthracis toxins to pathogenicity in two animal models

The virulence of Bacillus anthracis, the causative agent of anthrax, stems from its antiphagocytic capsule, encoded by pXO2, and the tripartite toxins encoded by pXO1. The accepted paradigm states that anthrax is both an invasive and toxinogenic disease and that the toxins play major roles in pathogenicity. We tested this assumption by a systematic study of mutants with combined deletions of the pag, lef, and cya genes, encoding protective antigen (PA), lethal factor (LF), and edema factor (EF), respectively. The resulting seven mutants (single, double, and triple) were evaluated following subcutaneous (s.c.) and intranasal (i.n.) inoculation in rabbits and guinea pigs. In the rabbit model, virulence is completely dependent on the presence of PA. Any mutant bearing a pag deletion behaved like a pXO1-cured mutant, exhibiting complete loss of virulence with attenuation indices of over 2,500,000 or 1,250 in the s.c. or i.n. route of infection, respectively. In marked contrast, in guinea pigs, deletion of pag or even of all three toxin components resulted in relatively moderate attenuation, whereas the pXO1-cured bacteria showed complete attenuation. The results indicate that a pXO1-encoded factor(s), other than the toxins, has a major contribution to the virulence mechanism of B. anthracis in the guinea pig model. These unexpected toxin-dependent and toxin-independent manifestations of pathogenicity in different animal models emphasize the importance and need for a comprehensive evaluation of B. anthracis virulence in general and in particular for the design of relevant next-generation anthrax vaccines.

Expression of either lethal toxin or edema toxin by Bacillus anthracis is sufficient for virulence in a rabbit model of inhalational anthrax

The development of therapeutics against biothreats requires that we understand the pathogenesis of the disease in relevant animal models. The rabbit model of inhalational anthrax is an important tool in the assessment of potential therapeutics against Bacillus anthracis. We investigated the roles of B. anthracis capsule and toxins in the pathogenesis of inhalational anthrax in rabbits by comparing infection with the Ames strain versus isogenic mutants with deletions of the genes for the capsule operon (capBCADE), lethal factor (lef), edema factor (cya), or protective antigen (pagA). The absence of capsule or protective antigen (PA) resulted in complete avirulence, while the presence of either edema toxin or lethal toxin plus capsule resulted in lethality. The absence of toxin did not influence the ability of B. anthracis to traffic to draining lymph nodes, but systemic dissemination required the presence of at least one of the toxins. Histopathology studies demonstrated minimal differences among lethal wild-type and single toxin mutant strains. When rabbits were coinfected with the Ames strain and the PA- mutant strain, the toxin produced by the Ames strain was not able to promote dissemination of the PA- mutant, suggesting that toxigenic action occurs in close proximity to secreting bacteria. Taken together, these findings suggest that a major role for toxins in the pathogenesis of anthrax is to enable the organism to overcome innate host effector mechanisms locally and that much of the damage during the later stages of infection is due to the interactions of the host with the massive bacterial burden.