A single immunization with a dry powder anthrax vaccine protects rabbits against lethal aerosol challenge

Here we confirm that intranasal (IN) dry powder anthrax vaccine formulations are able to protect rabbits against aerosol challenge 9 weeks after a single immunization. The optimum dose of rPA in our dry powder anthrax vaccine formulation in rabbits was experimentally determined to be 150microg and therefore was chosen as the target dose for all subsequent experiments. Rabbits received a single dose of either 150microg rPA, 150microg rPA+150microg of a conjugated 10-mer peptide representing the Bacillus anthracis capsule (conj), or 150microg of conj alone. All dry powder formulations contained MPL and chitosan (ChiSys). Significant anti-rPA titers and anthrax lethal toxin neutralizing antibody (TNA) levels were seen with both rPA containing vaccines, although rPA-specific IgG and TNA levels were reduced in rabbits immunized with rPA plus conj. Nine weeks after immunization, rabbits were exposed to a mean aerosol challenge dose of 278 LD50 of Ames spores. Groups immunized with rPA or with rPA+conj had significant increases in survivor proportions compared to the negative control group by Logrank test (p=0.0001 and 0.003, respectively), and survival was not statistically different for the rPA and rPA+conj immunized groups (p=0.63). These data demonstrate that a single immunization with our dry powder anthrax vaccine can protect against a lethal aerosol spore challenge 9 weeks later.     

Development of an in vitro-based potency assay for anthrax vaccine

The potency assay currently used to evaluate consistency of manufacture for the anthrax vaccine is contingent upon meeting specified parameters after statistical analysis of the percent survival and time to death of vaccinated guinea pigs after challenge with spores of a virulent strain of Bacillus anthracis. During the development of a new anthrax vaccine based upon recombinant protective antigen (rPA) adsorbed to aluminum hydroxide gel (Alhydrogel), we found that the serological response of female A/J mice, as measured by a quantitative anti-rPA IgG ELISA, may be an effective method to monitor a manufacturer's consistency for rPA-based vaccines. An advantage of the proposed in vitro-based potency assay is that it will not need stringent biosafety containment measures as required by the current guinea pig potency assay.     

Temperature-mediated recombinant anthrax protective antigen aggregate development: Implications for toxin formation and immunogenicity

Anthrax vaccines containing recombinant PA (rPA) as the only antigen face a stability issue: rPA forms aggregates in solution after exposure to temperatures ⩾40 °C, thus losing its ability to form lethal toxin (LeTx) with Lethal Factor. To study rPA aggregation’s impact on immune response, we subjected rPA to several time and temperature combinations. rPA treated at 50 °C for 30 min formed high mass aggregates when analyzed by gel electrophoresis and failed to form LeTx as measured by a macrophage lysis assay (MLA). Aggregated rPA-formed LeTx was about 30 times less active than LeTx containing native rPA. Mice immunized with heat-treated rPA combined with Al(OH)₃ developed antibody titers about 49 times lower than mice immunized with native rPA, as measured by a Toxicity Neutralization Assay (TNA). Enzyme Linked Immunosorbent Assay (ELISA) of the same immune sera showed anti-rPA titers only 2–7 times lower than titers elicited by native rPA. Thus, rPA’s ability to form LeTx correlates with its production of neutralizing antibodies, and aggregation significantly impairs the protein’s antibody response. However, while these findings suggest MLA has some value as an in-process quality test for rPA in new anthrax vaccines, they also confirm the superiority of TNA for use in vaccine potency.     

Comparative performance of a licensed anthrax vaccine versus electroporation based delivery of a PA encoding DNA vaccine in rhesus macaques

DNA vaccination is a promising immunization strategy that could be applied in the development of vaccines for a variety of prophylactic and therapeutic indications. Utilizing anthrax protective antigen as a model antigen, we demonstrate that electroporation mediated delivery enhanced the immunogenicity of DNA vaccines in nonhuman primates over 100-fold as compared to conventional intramuscular injection. Two administrations of a DNA vaccine with electroporation elicited anthrax toxin neutralizing antibody responses in 100% of rhesus macaques. Toxin neutralizing antibodies were sustained for the nearly 1-year study duration and were correlated with protection against subsequent lethal Bacillus anthracis spore challenge. Collectively, electroporation mediated DNA vaccination conferred protection comparable to that observed following vaccination with an FDA approved anthrax vaccine.     

Efficacious, nontoxigenic Bacillus anthracis spore vaccines based on strains expressing mutant variants of lethal toxin components

We reported previously on the development of a Bacillus anthracis vaccine strain expressing high levels of recombinant protective antigen (rPA) [Cohen et al., Infec Immun 2000;68(8):4549-58]. To further explore the potential of the B. anthracis platform, we generated several attenuated strains expressing lethal toxin components PA and LF, which are biologically inactive, yet retain their antigenic properties. A single injection of 5 x 10(7) spores of one of these strains, carrying PA mutation at a site involved in effector translocation (residues 313-314) was shown to resemble wild type PA in inducing production of high levels of anti-PA neutralizing antibodies and producing effective protective immunity for 12 months. Long-term protection and persistence of functional antibody titers was observed after the gradual elimination of spores from guinea pig tissues 3 months after injection and in the measurable absence of bacteria in tissues. The mutant toxin components could, thus be an effective alternatives to their native counterparts when presented to the immune system in context of a live B. anthracis strain. These live vaccine prototypes may serve as a platform for future multi-component vaccines.     

Chimeric hepatitis B virus core particles carrying an epitope of anthrax protective antigen induce protective immunity against Bacillus anthracis

The major aim of present study is to develop and evaluate chimeric virus-like particles (VLPs) displaying a neutralizing epitope of anthrax protective antigen (PA) as a potential vaccine against anthrax. The truncated hepatitis B virus core (HBc) protein (aa 1-144) was used as a carrier, and the 2beta2-2beta3 loop of the PA domain 2 (aa 302-325) which has been shown contains a dominant neutralizing epitope was inserted into the major immunodominant region (MIR) of the HBc. The recombinant protein HBc-N144-PA-loop2 was expressed in Escherichia coli, and was able to form HBc-like particles confirmed by electron microscopy. The immunogenicity of these chimeric particles was evaluated in mice and guinea pigs. In mice the HBc-N144-PA-loop2 was able to induce PA-epitope specific antibodies; in guinea pigs it was able to induce PA-epitope specific antibodies and anthrax toxin-neutralizing antibodies regardless of whether alum adjuvant was used or not, and was able to partially protect the immunized guinea pigs against virulent anthrax spores challenge. This study suggests chimeric HBc particles carrying a neutralizing epitope of PA can induce protective immunity against Bacillus anthracis.     

Efficacy,heat stability and safety of intranasally administered Bacillus subtilis spore or vegetative cell vaccines expressing tetanus toxin fragment C

Bacillus subtilis strains expressing tetanus toxin fragment C (TTFC) were tested as vaccine candidates against tetanus in adult mice. Mice received three intranasal (IN) exposures to 10⁹ spores or 10⁸ vegetative cells of B. subtilis expressing recombinant TTFC. Immunized mice generated protective systemic and mucosal antibodies and survived challenge with 2× LD₁₀₀ of tetanus toxin. Isotype analysis of serum antibody indicated a balanced Th1/Th2 response. Lyophilized vaccines stored at 45 °C for ≥12 months, remained effective. Immunized conventional and SCID mice remained well, and no histological changes in brain or respiratory tract were detected. Lyophilized/reconstituted B. subtilis tetanus vaccines administered IN to mice appear safe, heat-stable, and protective against lethal tetanus challenge.     

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 >10⁴ 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 >10⁵. 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.     

An anthrax subunit vaccine candidate based on protective regions of Bacillus anthracis protective antigen and lethal factor

Studies have confirmed the key role of Bacillus anthracis protective antigen (PA) in the US and UK human anthrax vaccines. However, given the tripartite nature of the toxin, other components, including lethal factor (LF), are also likely to contribute to protection. We examined the antibody and T cell responses to PA and LF in human volunteers immunized with the UK anthrax vaccine (AVP). Individual LF domains were assessed for immunogenicity in mice when given alone or with PA. Based on the results obtained, a novel fusion protein comprising D1 of LF and the host cell-binding domain of PA (D4) was assessed for protective efficacy. Murine protection studies demonstrated that both full-length LF and D1 of LF conferred complete protection against a lethal intraperitoneal challenge with B. anthracis STI spores. Subsequent studies with the LFD1-PAD4 fusion protein showed a similar level of protection. LF is immunogenic in humans and is likely to contribute to the protection stimulated by AVP. A single vaccine comprising protective regions from LF and PA would simplify production and confer a broader spectrum of protection than that seen with PA alone.     

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.     

Protection against anthrax and plague by a combined vaccine in mice and rabbits

The protective antigen (PA) of Bacillus anthracis and the Fraction 1 Capsular Antigen (F1 antigen), V antigen of Yersinia pestis have been demonstrated to be potential immunogens and candidate vaccine sub-units against anthrax and plague respectively. In this study, the authors have investigated the antibody responses and the protective efficacy when the antigens were administered separately or in combination intramuscularly formulation adsorbed to an aluminum hydroxide adjuvant. Results show that immunized rF1 + rV and rPA antigen together was as effective as separately for induction of serological antibody response, and these titers were maintained for over 1 year in mice. An isotype analysis of the serum indicates that the co-administration of these antigens did not influence the antigen-specific IgG1/IgG2a ratio which was consistent with a Th2 bias. Furthermore, the combined vaccine comprising the protein antigens rF1 + rV + rPA has been demonstrated to protect mice from subcutaneous challenge with 10⁷ colony-forming units (CFU) virulent Y. pestis strain, and to fully protect rabbit against subcutaneous challenge with 1.2 × 10⁵ colony-forming units (CFU) virulent B. anthracis spores. These data show that the protective efficacy was unaffected when the antigens were administered in combination.     

Cell-wall preparation containing poly-γ-d-glutamate covalently linked to peptidoglycan,a straightforward extractable molecule,protects mice against experimental anthrax infection

Bacillus anthracis is the causative agent of anthrax that is characterized by septicemia and toxemia. Many vaccine strategies were described to counteract anthrax infection. In contrast with veterinary live vaccines, currently human vaccines are acellular with the protective antigen, a toxin component, as the main constituent. However, in animal models this vaccine is less efficient than the live vaccine. In this study, we analyzed the protection afforded by a single extractable surface element. The poly-γ-d-glutamate capsule is covalently linked to the peptidoglycan. A preparation of peptidoglycan-linked poly-γ-d-glutamate (GluPG) was tested for its immunogenicity and its protective effect. GluPG injection, in mice, elicited the production of specific antibodies directed against poly-glutamate and partially protected the animals against lethal challenges with a non-toxinogenic strain. When combined to protective antigen, GluPG immunization conferred full protection against cutaneous anthrax induced with a fully virulent strain.     

Defining a serological correlate of protection in rabbits for a recombinant anthrax vaccine

In these studies, a serological correlate of protection against anthrax was identified in New Zealand white (NZW) rabbits that had been given one or two injections of various amounts of recombinant protective antigen (rPA) combined with aluminum hydroxide adjuvant (Alhydrogel). Rabbits were subsequently challenged by the aerosol route with spores of the Ames isolate of Bacillus anthracis. Results suggested that the antibody response, as determined by the quantitative anti-rPA IgG ELISA and toxin neutralizing antibody (TNA) assay, were significant predictors (P<0.0015) of protection against a B. anthracis aerosol spore challenge in rabbits.     

BclA and toxin antigens augment each other to protect NMRI mice from lethal Bacillus anthracis challenge

While proving highly effective in controlling Anthrax in farm animals all over the world currently attenuated live anthrax vaccines employed in a veterinary context suffer from drawbacks such as residual virulence, short term protection, variation in quality and, most importantly, lack of efficacy if administered simultaneously with antibiotics. These limitations have stimulated the development of non-living component vaccines which induce a broad spectrum immune response capable of targeting both toxaemia (as in the case of PA based vaccines) and bacteraemia. To contribute to this several new approaches were tested in outbred NMRI mice for antibody titres and protectiveness. Plasmids encoding a recombinant toxin derived fusion peptide and a spore surface derived peptide were tested as DNA-vaccines in comparison to their protein counterparts utilising two adjuvant approaches and two DNA-vector backbones. The combination of two plasmids encoding LFD1PAD4-mIPS1 and TPA-BclAD1D3-LAMP1, when delivered by GeneGun, protected 90% of the animals against a lethal challenge with 25LD₅₀ spores of the Ames strain of Bacillus anthracis. Single applications of either antigen component showed significantly lower protection rates, indicating the beneficial interaction between anti-spore and anti-toxin components for an acellular vaccine formulation.     

Immunogenicity and safety of a novel recombinant protective antigen anthrax vaccine (GC1109), a randomized,single-blind,placebo controlled phase II clinical study

BackgroundThe demand on effective and safe anthrax vaccine is increasing as a part of the preparedness for possible bioterrorism in the future. We performed a randomized, single-blind, placebo controlled phase II clinical study to evaluate the immunogenicity and safety of a novel recombinant protective antigen (rPA) anthrax vaccine, GC1109, in healthy adult volunteers.MethodsParticipants were randomized to experiment groups (0.3 mL, 0.5 mL, and 1.0 mL of GC1109) or placebo group (normal saline 0.5 mL) in 2:2:2:1 ratio. They received respective vaccines intramuscularly at 0, 4 and 8 weeks. Immunogenicity was evaluated by seroconversion rate and geometric mean titer (GMT) of lethal toxin neutralizing assay (TNA) and anti-PA IgG by ELISA. Safety was assessed by laboratory tests, and solicited and unsolicited adverse events on diary cards.Results30, 29, 30 participants were randomized to 0.3, 0.5, and 1.0 mL of GC1109 groups, respectively, while 15 to placebo group. 92 participants received all three doses. In per-protocol analysis, TNA GMTs at week 12 were 296.5, 285.2, and 433.2 in the three groups, respectively. Seroconversion rates measured by ELISA were 100% at week 12 in the three groups. Local and systemic vaccine-related adverse events were frequent; however, most of them were mild, and no serious events were observed.ConclusionsA new rPA anthrax vaccine GC1109 was immunogenic after three doses of intramuscular administration, and was well-tolerated.     

Evaluation of the protective efficacy of recombinant protective antigen vaccine (GC1109)-immunized human sera using passive immunization in a mouse model

The protective efficacy of human sera from vaccinated individuals with a new recombinant protective antigen anthrax vaccine (GC1109) against lethal spore challenge was evaluated in a mouse model. Eighteen human sera were selected from the vaccinated individuals based on their toxin neutralizing assay (TNA) titer (ED₅₀ of 55 to 668). The selected sera were diluted and passively transferred to A/J mice and the mice were subsequently challenged with 100 × LD₅₀ of Bacillus anthracis Sterne spores. The correlation between the survival rate of passively immunized mice and the TNA ED₅₀ of transferred sera was presented (r = 0.873, P-value < 0.001). The estimated TNA titer for 50% survival rate against lethal challenge was 197 (95% confidence interval of 149 and 260). The result suggest that GC1109 is protective against exposure to B. anthracis and the TNA titer of vaccinated serum can be an indicator for protective efficacy.     

Efficacy of a novel Pasteurella multocida vaccine against progressive atrophic rhinitis of swine

The efficacy of a novel vaccine composed of three short recombinant subunit Pasteurella multocida toxin (PMT) proteins in combination with a bi-valent P. multocida whole-cell bacterin (rsPMT–PM) was evaluated in field studies for prevention and control of progressive atrophic rhinitis (PAR) of swine at 15 conventional farrow-to-finish farms. Experimental piglets that were immunized twice with the rsPMT–PM vaccine developed detectable titers of neutralizing antibodies (greater than 1:8) that prevented the growth retardation and pathological lesions typically observed following challenge with authentic PMT. A total of 542 sows were vaccinated once or twice prior to parturition and serum neutralizing antibody titers were evaluated. Both single and double vaccination protocols induced neutralizing antibody titers of 1:16 or higher in 62% and 74% of sows, respectively. Notably, neither sows nor piglets at a farm experiencing a severe outbreak of PAR at the time of the vaccination trial had detectable antibody titers, but antibody titers increased significantly to 1:16 or higher in 40% of sows following double vaccination. During the year after vaccination, clinical signs of PAR decreased in fattening pigs and growth performance improved sufficiently to reduce the rearing period until marketing by 2 weeks. Collectively, these results indicate that the rsPMT–PM vaccine could be used to provide protective immunity for controlling the prevalence and severity of PAR among farm-raised swine.     

Correlation between anthrax lethal toxin neutralizing antibody levels and survival in guinea pigs and nonhuman primates vaccinated with the AV7909 anthrax vaccine candidate

The anthrax vaccine candidate AV7909 is being developed as a next generation vaccine for a post-exposure prophylaxis (PEP) indication against anthrax. AV7909 consists of the Anthrax Vaccine Adsorbed (AVA, BioThrax®) bulk drug substance adjuvanted with the immunostimulatory oligodeoxynucleotide (ODN) compound, CPG 7909. The addition of CPG 7909 to AVA enhances both the magnitude and the kinetics of antibody responses in animals and human subjects, making AV7909 a suitable next-generation vaccine for use in a PEP setting. The studies described here provide initial information on AV7909-induced toxin-neutralizing antibody (TNA) levels associated with the protection of animals from lethal Bacillus anthracis challenge. Guinea pigs or nonhuman primates (NHPs) were immunized on Days 0 and 28 with various dilutions of AV7909, AVA or a saline or Alhydrogel + CPG 7909 control. Animals were challenged via the inhalational route with a lethal dose of aerosolized B. anthracis (Ames strain) spores and observed for clinical signs of disease and mortality. The relationship between pre-challenge serum TNA levels and survival following challenge was determined in order to calculate a threshold TNA level associated with protection. Immunisation with AV7909 induced a rapid, highly protective TNA response in guinea pigs and NHPs. Surprisingly, the TNA threshold associated with a 70% probability of survival for AV7909 immunized animals was substantially lower than the threshold which has been established for the licensed AVA vaccine. The results of this study suggest that the TNA threshold of protection against anthrax could be modified by the addition of an immune stimulant such as CPG 7909 and that the TNA levels associated with protection may be vaccine-specific.     

Anthrax vaccine efficacy in golden Syrian hamsters

The efficacy of a licensed human anthrax vaccine (anthrax vaccine adsorbed, AVA) was tested in golden Syrian hamsters against a virulent Bacillus anthracis spore challenge. Groups of golden Syrian hamsters were vaccinated at either 0 and 4 weeks or 0, 4 and 8 weeks, then challenged subcutaneously (s.c.) at 10 weeks with spores of various B. anthracis isolates. Although ELISA and toxin neutralization assays demonstrated high titers, none of the AVA-vaccinated hamsters were protected from challenge or demonstrated a significantly extended time to death compared to that of control animals. The results of the study demonstrate that the golden Syrian hamster is not an appropriate model for investigating human anthrax vaccine efficacy.     

Positive role for rApxIVN in the immune protection of pigs against infection by Actinobacillus pleuropneumoniae

The role of in vivo-induced ApxIV toxin of Actinobacillus pleuropneumoniae in protective immunity was evaluated in pigs by administering it alone or added to a multicomponent recombinant subunit vaccine composed of recombinant ApxI, ApxII, ApxIII toxin, and 42-kDa outer membrane protein (OMP). The pigs were immunized with vaccine I (rApxIVN), vaccine II (rApxI + rApxII + rApxIII + rApxIVN + rOMP), vaccine III (rApxI + rApxII + rApxIII + rOMP), or placebo (phosphate-buffered saline + adjuvant). A. pleuropneumoniae serovar 1 field isolate JMS 06 and serovar 2 field strain FX 01 were used as the challenge strains. Pigs that were immunized with vaccine I or vaccine II all developed high antibody titers against rApxIVN. The antibody titers against rApxI, rApxII, rApxIII, and rOMP in pigs immunized with vaccine II were higher than those in pigs vaccinated with vaccine III. Following the challenge, the pigs immunized with rApxIVN alone showed similar results to the pigs in the control group, such as severe respiratory symptoms and severe lung lesions. Pigs that had been immunized with vaccine II or vaccine III were protected against challenge with A. pleuropneumoniae serovar 1 and serovar 2. The pigs immunized with vaccine II had slighter lung lesions and fewer bacterial recovery than those of pigs immunized with vaccine III. These results indicate that rApxIVN contributes to the production of high level of antibodies directed against the vaccination antigens, and thus confers strong protection against challenges with different serovars of A. pleuropneumoniae.