Expression and secretion of the protective antigen of Bacillus anthracis in Bacillus brevis

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.

Construction of a bivalent vaccine against anthrax and smallpox using the attenuated vaccinia virus KVAC103

Background

Anthrax and smallpox are high-risk infectious diseases, and considered as potential agents for bioterrorism. To develop an effective countermeasure for these diseases, we constructed a bivalent vaccine against both anthrax and smallpox by integrating a gene encoding protective antigen (PA) of Bacillus anthracis to the genome of the attenuated vaccinia virus strain, KVAC103.

Results

Immunization with this bivalent vaccine induced antibodies against both PA and vaccinia virus in a mouse model. We also observed that the efficacy of this vaccine can be enhanced by combined immunization with immunoadjuvant-expressing KVAC103. Mouse groups co-immunized with PA-expressing KVAC103 and either interleukin-15 (IL-15) or cholera toxin subunit A (CTA1)-expressing KVAC103 showed increased anti-PA IgG titer and survival rate against B. anthracis spore challenge compared to the group immunized with PA-expressing KVAC103 alone.

Conclusions

We demonstrated that the attenuated smallpox vaccine KVAC103 is an available platform for a multivalent vaccine and co-immunization of immunoadjuvants can improve vaccine performance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-021-02121-5.

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.

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.

A therapeutic human antibody against the domain 4 of the Bacillus anthracis protective antigen shows protective efficacy in a mouse model

Since Bacillus anthracis is a high-risk pathogen and a potential tool for bioterrorism, numerous therapeutic methods including passive immunization have been actively developed. Using a human monoclonal antibody phage display library, we screened new therapeutic antibodies for anthrax infection against protective antigen (PA) of B. anthracis. Among 5 selected clones of antibodies based on enzyme-linked immunosorbent assay (ELISA) results, 7B1 showed neutralizing activity to anthrax lethal toxin (LT) by inhibiting binding of the domain 4 of PA (PD4) to its cellular receptors. Through light chain shuffling process, we improved the productivity of 7B1 up to 25 folds. The light chain shuffled 7B1 antibody showed protective activity against LT both in vitro and in vivo. Furthermore, the antibody also conferred protection of mice from 3 × LD₅₀ challenges of fully virulent anthrax spores. Our result expands the possibility of developing a new therapeutic antibody for anthrax cure.

Soluble Expression and Characterization of Biologically Active Bacillus anthracis Protective Antigen in Escherichia coli

Bacillus anthracis secretory protein protective antigen (PA) is primary candidate for subunit vaccine against anthrax. Attempts to obtain large quantity of PA from Escherichia coli expression system often result in the formation of insoluble inclusion bodies. Therefore, it is always better to produce recombinant proteins in a soluble form. In the present study, we have obtained biologically active recombinant PA in small scale E. coli shake culture system using three different expression constructs. The PA gene was cloned in expression vectors bearing trc, T5, and T7 promoters and transformed into their respective E. coli hosts. The growth conditions were optimized to obtain maximum expression of PA in soluble form. The expression construct PA-pET32c in DE3-pLysS E. coli host resulted in a maximum production of soluble PA (15 mg L(-1)) compared to other combinations. Purified PA was subjected to trypsin digestion and binding assay with lethal factor to confirm the protein's functionality. Biological activity was confirmed by cytotoxicity assay on J774.1 cells. Balb/c mice were immunized with PA and the immunogenicity was tested by ELISA and toxin neutralization assay. This study highlights the expression of soluble and biologically active recombinant PA in larger quantity using simpler E. coli production platform.

Immunoproteomically identified GBAA_0345, alkyl hydroperoxide reductase subunit C is a potential target for multivalent anthrax vaccine

Anthrax is caused by the spore-forming bacterium Bacillus anthracis, which has been used as a weapon for bioterrorism. Although current vaccines are effective, they involve prolonged dose regimens and often cause adverse reactions. High rates of mortality associated with anthrax have made the development of an improved vaccine a top priority. To identify novel vaccine candidates, we applied an immunoproteomics approach. Using sera from convalescent guinea pigs or from human patients with anthrax, we identified 34 immunogenic proteins from the virulent B. anthracis H9401. To evaluate vaccine candidates, six were expressed as recombinant proteins and tested in vivo. Two proteins, rGBAA_0345 (alkyl hydroperoxide reductase subunit C) and rGBAA_3990 (malonyl CoA-acyl carrier protein transacylase), have afforded guinea pigs partial protection from a subsequent virulent-spore challenge. Moreover, combined vaccination with rGBAA_0345 and rPA (protective antigen) exhibited an enhanced ability to protect against anthrax mortality. Finally, we demonstrated that GBAA_0345 localizes to anthrax spores and bacilli. Our results indicate that rGBAA_0345 may be a potential component of a multivalent anthrax vaccine, as it enhances the efficacy of rPA vaccination. This is the first time that sera from patients with anthrax have been used to interrogate the proteome of virulent B. anthracis vegetative cells.

Cloning, expression and purification of binding domains of lethal factor and protective antigen of Bacillus anthracis in Escherichia coli and evaluation of their related murine antibody

Anthrax is common disease between human and animals caused by Bacillus anthracis. The cell binding domain of protective antigen (PAD4) and the binding domain of lethal factor (LFD1) have high immunogenicity potential and always were considered as a vaccine candidate against anthrax. The aims of this study are cloning and expressing of PAD4 and LFD1 in Escherichia coli, purification of the recombinant proteins and determination of their immunogenicity through evaluating of the relative produced polyclonal antibodies in mice. PAD4 and LFD1 genes were cloned in pET28a(+) vector and expressed in E. coli Bl21(DE3)PlysS. Expression and purification of the two recombinant proteins were confirmed by SDS-PAGE and Western blotting techniques. The PAD4 and LFD1 were purified using Ni(+)-NTA affinity chromatography (95-98 %), yielding 37.5 and 45 mg/l of culture, respectively. The antigens were injected three times into mice and production of relative antibodies was evaluated by ELISA test. The results showed that both PAD4 and LFD1 are immunogenic, but LFD1 has higher potential to stimulate Murine immune system. With regard to the high level of LFD1 and PAD4 expression and also significant increment in produced polyclonal antibodies, these recombinant proteins can be considered as a recombinant vaccine candidate against anthrax.

Serological Correlate of Protection in Guinea Pigs for a Recombinant Protective Antigen Anthrax Vaccine Produced from Bacillus brevis

Objective

Recombinant protective antigen (rPA) is the active pharmaceutical ingredient of a second generation anthrax vaccine undergoing clinical trials both in Korea and the USA. By using the rPA produced from Bacillus brevis pNU212 expression system, correlations of serological immune response to anthrax protection efficacy were analyzed in a guinea pig model.

Methods

Serological responses of rPA anthrax vaccine were investigated in guinea pigs that were given single or two injections (interval of 4 weeks) of various amounts of rPA combined with aluminumhydroxide adjuvant. Guinea pigs were subsequently challenged by the intramuscular injection with 30 half-lethal doses (30LD₅₀) of virulent Bacillus anthracis spores. Serumantibody titerswere determined by anti-PA IgGELISA and the ability of antibodies to neutralize the cytotoxicity of lethal toxin on J774A.1 cell was measured through the toxin neutralizing antibody (TNA) assay.

Results

To examine correlations between survival rate and antibody titers, correlation between neutralizing antibody titers and the extent of protection was determined. Toxin neutralization titers of at least 1176 were sufficient to confer protection against a dose of 30LD₅₀ of virulent anthrax spores of the H9401 strain. Such consistency in the correlation was not observed from those antibody titers determined by ELISA.

Conclusion

Neutralizing-antibody titers can be used as a surrogate marker.

Monoclonal antibody against the poly-gamma-D-glutamic acid capsule of Bacillus anthracis protects mice from enhanced lethal toxin activity due to capsule and anthrax spore challenge

BACKGROUND

The poly-gamma-D-glutamic acid (PGA) capsule, a major virulence factor of Bacillus anthracis, protects bacilli from immune surveillance and allows its unimpeded growth in the host. Recently, the importance of the PGA in the pathogenesis of anthrax infection has been reported. The PGA capsule is associated with lethal toxin (LT) in the blood of experimentally infected animals and enhances the cytotoxicity of LT.

METHODS

To investigate the role of anti-PGA Abs on progression of anthrax infection, two mouse anti-PGA mAbs with K(d) values of 0.8 microM and 2.6 microM respectively were produced and in silico three dimensional (3D) models of mAbs with their cognitive PGA antigen complex were analyzed.

RESULTS

Anti-PGA mAbs specifically bound encapsulated B. anthracis H9401 and showed opsonophagocytosis activity against the bacteria with complement. The enhancement effect of PGA on LT-mediated cytotoxicity was confirmed ex vivo using mouse bone marrow-derived macrophages and was effectively inhibited by anti-PGA mAb. Passive immunization of mAb completely protected mice from PGA-enhanced LT toxicity and partially rescued mice from anthrax spore challenges. 3D structure models of these mAbs and PGA complex support specific interactions between CDR and cognitive PGA. These results indicate that mouse mAb against PGA capsule prevents the progress of anthrax disease not only by eliminating the vegetative form of encapsulated B. anthracis but also by inhibiting the enhanced cytotoxic activity of LT by PGA through specific binding with PGA capsule antigen.

GENERAL SIGNIFICANCE

Our results suggest a potential role for PGA antibodies in preventing and treating anthrax infection.

Increased long-term immunity to Bacillus anthracis protective antigen in mice immunized with a CIA06B-adjuvanted anthrax vaccine

Anthrax is an acute infectious disease caused by Bacillus anthracis. We previously reported that the adjuvant CIA06B, which consists of TLR4 agonist CIA05 and aluminum hydroxide (alum), enhanced the immune response to anthrax protective antigen (PA) in mice. This study was carried out to determine whether CIA06B can enhance long-term immune responses to PA in mice. BALB/c mice were immunized intramuscularly three times at 2-week intervals with recombinant PA alone or PA combined with alum or CIA06B. At 8 and 24 weeks post-immunization, the immunological responses including serum anti-PA IgG antibody titer, toxin-neutralizing antibody titer, splenic cytokine secretion and the frequency of PA-specific memory B cells were assessed. Compared with mice injected with PA alone or PA plus alum, mice injected with PA plus CIA06B had higher titers of serum anti-PA IgG antibodies, and higher frequencies of PA-specific memory B cells and interferon-γ secreting cells. Furthermore, anti-PA antibodies induced by CIA06B were more effective in neutralizing anthrax toxin. These results demonstrated that CIA06B is capable of providing long-term immunity when used as an adjuvant in a PA-based anthrax vaccine.

Complete genome sequence of Bacillus anthracis H9401, an isolate from a Korean patient with anthrax

Bacillus anthracis H9401 (NCCP 12889) is an isolate from a Korean patient with gastrointestinal anthrax. The whole genome of H9401 was sequenced. It is a circular chromosome containing 5,480 open reading frames (ORFs) and two plasmids, pXO1 containing 202 ORFs and pXO2 containing 110 ORFs. H9401 shows high pathogenicity and genome sequence similarity to Ames Ancestor.

The poly-γ-D-glutamic acid capsule of Bacillus anthracis enhances lethal toxin activity

The poly-γ-D-glutamic acid (PGA) capsule is one of the major virulence factors of Bacillus anthracis, which causes a highly lethal infectious disease. The PGA capsule disguises B. anthracis from immune surveillance and allows its unimpeded growth in the host. The PGA capsule recently was reported to be associated with lethal toxin (LT) in the blood of experimentally infected animals (M. H. Cho, et al., Infect. Immun. 78:387-392, 2010). The effect of PGA, either alone or in combination with LT, on macrophages, which play an important role in the progression of anthrax disease, has not been thoroughly investigated. In this study, we investigated the effect of PGA on LT cytotoxicity using the mouse macrophage cell line J774A.1. PGA produced a concentration-dependent enhancement of the cytotoxicity of LT on J774A.1 cells through an enhancement in the binding and accumulation of protective antigen to its receptors. The increase of LT activity was confirmed using Western blot analysis, which showed that the combination of PGA and LT produced a greater degree of degradation of mitogen-activated protein kinase kinases and an increased level of the activation of the proform of caspase-1 to its processed form compared to the effects of LT alone. In addition, mice that received a tail vein injection of both PGA and LT had a significantly increased rate of death compared to that of mice injected with LT alone. PGA had no effect when added to cultures or administered to mice in the absence of LT. These results emphasize the importance of PGA in the pathogenesis of anthrax infection.

A combination of the TLR4 agonist CIA05 and alum promotes the immune responses to Bacillus anthracis protective antigen in mice

Anthrax is an infectious disease caused by Bacillus anthracis. The currently licensed human anthrax vaccines contain protective antigen (PA) as a major protective component and alum as an adjuvant. In this study, we investigated whether CIA05, a TLR4 agonist, is able to promote the immune response to an anthrax vaccine adjuvanted with alum. BALB/c mice were immunized intraperitoneally three times at 2-week intervals with a recombinant B. anthracis PA alone or in combination with CIA05 in the absence or presence of alum, and immune responses were determined 2 or 3 weeks after the third immunization. The results showed that the combination of CIA05 and alum significantly increased both serum anti-PA IgG antibody and toxin-neutralizing antibody titers, and the adjuvant effects were greater when lower antigen doses were used for immunization. Both CIA05 and alum stimulated PA-specific splenocyte secretion of interleukin (IL)-4, IL-5, and IL-6. A combination of the two yielded synergistic effects on IL-4 secretion, but CIA05 tended to repress IL-5 and IL-6 secretions induced by alum. Co-administration of CIA05 and alum also increased GL7 expression in B220(+)CD24(+) splenic cells, indicating the ability to activate B cells. These data suggest that CIA05, combined with alum, could be used to achieve higher immune responses to PA, leading to the development of an effective anthrax vaccine.

Significant passive protective effect against anthrax by antibody to Bacillus anthracis inactivated spores that lack two virulence plasmids

The protective-antigen (PA)-based cell-free vaccine is the only vaccine licensed for use against Bacillus anthracis infection in humans. Although the PA shows strong immunogenicity, the capsule or spore-associated somatic antigens may be important as additional vaccine targets for full protection against anthrax. In this study, the protective effect of spore-associated antigens against B. anthracis infection was determined. Rabbits were immunized with formalin-fixed spores of a non-toxigenic unencapsulated B. anthracis strain that lacked the two virulence plasmids pXO1 and pXO2, and the protective effects of the immune antibody were evaluated. Immunostaining and Western blot analysis revealed that the anti-B. anthracis (anti-BA)-spore IgG specifically bound to the surface of spores or endospores of B. anthracis, but not to vegetative cells, or closely related Bacillus species, such as Bacillus cereus, Bacillus subtilis and Bacillus thuringiensis. Passively transferred anti-BA-spore IgG protected mice from intraperitoneal challenge with a lethal dose of fully virulent B. anthracis spores, and increased the survival rate in a dose-dependent manner. Pre-incubation of spores with antibody also reduced their infectivity in a dose-dependent manner. The number of bacteria (c.f.u.) in spleens and livers of infected mice was significantly lower in antibody-treated mice than in untreated mice. Treatment with anti-BA-spore IgG also inhibited the germination of spores in J774.1 macrophages, suggesting that opsonization of spores promotes phagocytosis and subsequent killing by macrophages. These results indicate the usefulness of spore surface antigens as vaccine targets. In combination with major virulence factors such as the PA, spore-associated antigens may offer a safer and more effective multicomponent vaccine for B. anthracis infection.

Efficacy of non-toxic deletion mutants of protective antigen from Bacillus anthracis

Current human anthrax vaccines available in the United States and Europe consist of alum-precipitated supernatant material from cultures of a toxigenic, nonencapsulated strain of Bacillus anthracis. The major component of human anthrax vaccine that confers protection is protective antigen (PA). A second-generation human vaccine using the recombinant PA (rPA) is being developed. In this study, to prevent the toxicity and the degradation of the native rPA by proteases, we constructed two PA variants, delPA (163-168) and delPA (313-314), that lack trypsin (S(163)-R(164)-K(165)-K(166)-R(167)-S(168)) or chymotrypsin cleavage sequence (F(313)-F(314)), respectively. These proteins were expressed in Bacillus brevis 47-5Q. The delPAs were fractionated from the culture supernatant of B. brevis by ammonium sulfate at 70% saturation, followed by anion exchange chromatography on a Hitrap Q, Hiload 16/60 superdex 200 gel filtration column and phenyl sepharose hydrophobic interaction column. In accordance with previous reports, both delPA proteins combined with lethal factor protein did not show any cytotoxicity on J774A.1 cells. The delPA (163-168) and delPA (313-314) formulated either in Rehydragel HPA or MPL-TDM-CWS (Ribi-Trimix), elicited a comparable amount of anti-PA and neutralizing antibodies to those of native rPA in guinea pigs, and confers full protection of guinea pigs from 50xLD50 of fully virulent B. anthracis spore challenges. Ribi-Trimix was significantly more effective in inducing anti-PA and neutralizing antibodies than Rehydragel HPA. These results indicate the possibility of delPA (163-168) and delPA (313-314) proteins being developed into nontoxic, effective and stable recombinant vaccine candidates.