N-fragment of edema factor as a candidate antigen for immunization against anthrax

Efficacy assessment of a triple anthrax chimeric antigen as a vaccine candidate in guinea pigs: challenge test with Bacillus anthracis 17 JB strain spores

CONTEXT

Bacillus anthracis secretes a tripartite toxin comprising protective antigen (PA), edema factor (EF), and lethal factor (LF). The human anthrax vaccine is mainly composed of the anthrax protective antigen (PA). Considerable efforts are being directed towards improving the efficacy of vaccines because the use of commercial anthrax vaccines (human/veterinary) is associated with several limitations.

OBJECTIVE

In this study, a triple chimeric antigen referred to as ELP (gene accession no: MT590758) comprising highly immunogenic domains of PA, LF, and EF was designed, constructed, and assessed for the immunization capacity against anthrax in a guinea pig model.

MATERIALS AND METHODS

Immunization was carried out considering antigen titration and immunization protocol. The immunoprotective efficacy of the ELP was evaluated in guinea pigs and compared with the potency of veterinary anthrax vaccine using a challenge test with B. anthracis 17JB strain spores.

RESULTS

The results demonstrated that the ELP antigen induced strong humoral responses. The T-cell response of the ELP was found to be similar to PA, and showed that the ELP could protect 100%, 100%, 100%, 80% and 60% of the animals from 50, 70, 90, 100 and 120 times the minimum lethal dose (MLD, equal 5 × 10⁵ spore/ml), respectively, which killed control animals within 48 h.

DISCUSSION AND CONCLUSIONS

It is concluded that the ELP antigen has the necessary requirement for proper immunization against anthrax and it can be used to develop an effective recombinant vaccine candidate against anthrax.

Anthrax Vaccine Precipitated Induces Edema Toxin-Neutralizing, Edema Factor-Specific Antibodies in Human Recipients

Edema toxin (ET), composed of edema factor (EF) and protective antigen (PA), is a virulence factor of Bacillus anthracis that alters host immune cell function and contributes to anthrax disease. Anthrax vaccine precipitated (AVP) contains low but detectable levels of EF and can elicit EF-specific antibodies in human recipients of AVP. Active and passive vaccination of mice with EF can contribute to protection from challenge with Bacillus anthracis spores or ET. This study compared humoral responses to ET in recipients of AVP (n = 33) versus anthrax vaccine adsorbed (AVA; n = 66), matched for number of vaccinations and time postvaccination, and further determined whether EF antibodies elicited by AVP contribute to ET neutralization. AVP induced higher incidence (77.8%) and titer (229.8 ± 58.6) of EF antibodies than AVA (4.2% and 7.8 ± 8.3, respectively), reflecting the reported low but detectable presence of EF in AVP. In contrast, PA IgG levels and ET neutralization measured using a luciferase-based cyclic AMP reporter assay were robust and did not differ between the two vaccine groups. Multiple regression analysis failed to detect an independent contribution of EF antibodies to ET neutralization in AVP recipients; however, EF antibodies purified from AVP sera neutralized ET. Serum samples from at least half of EF IgG-positive AVP recipients bound to nine decapeptides located in EF domains II and III. Although PA antibodies are primarily responsible for ET neutralization in recipients of AVP, increased amounts of an EF component should be investigated for the capacity to enhance next-generation, PA-based vaccines.

A dual purpose universal influenza vaccine candidate confers protective immunity against anthrax

Preventive influenza vaccines must be reformulated annually because of antigen shift and drift of circulating influenza viral strains. However, seasonal vaccines do not always match the circulating strains, and there is the ever-present threat that avian influenza viruses may adapt to humans. Hence, a universal influenza vaccine is needed to provide protective immunity against a broad range of influenza viruses. We designed an influenza antigen consisting of three tandem M2e repeats plus HA2, in combination with a detoxified anthrax oedema toxin delivery system (EFn plus PA) to enhance immune responses. The EFn-3×M2e-HA2 plus PA vaccine formulation elicited robust, antigen-specific, IgG responses; and was protective against heterologous influenza viral challenge when intranasally delivered to mice three times. Moreover, use of the detoxified anthrax toxin system as an adjuvant had the additional benefit of generating protective immunity against anthrax. Hence, this novel vaccine strategy could potentially address two major emerging public health and biodefence threats.

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.

Immunization of Mice with Anthrax Protective Antigen Limits Cardiotoxicity but Not Hepatotoxicity Following Lethal Toxin Challenge

Protective immunity against anthrax is inferred from measurement of vaccine antigen-specific neutralizing antibody titers in serum samples. In animal models, in vivo challenges with toxin and/or spores can also be performed. However, neither of these approaches considers toxin-induced damage to specific organ systems. It is therefore important to determine to what extent anthrax vaccines and existing or candidate adjuvants can provide organ-specific protection against intoxication. We therefore compared the ability of Alum, CpG DNA and the CD1d ligand α-galactosylceramide (αGC) to enhance protective antigen-specific antibody titers, to protect mice against challenge with lethal toxin, and to block cardiotoxicity and hepatotoxicity. By measurement of serum cardiac Troponin I (cTnI), and hepatic alanine aminotransferase (ALT), and aspartate aminotransferase (AST), it was apparent that neither vaccine modality prevented hepatic intoxication, despite high Ab titers and ultimate survival of the subject. In contrast, cardiotoxicity was greatly diminished by prior immunization. This shows that a vaccine that confers survival following toxin exposure may still have an associated morbidity. We propose that organ-specific intoxication should be monitored routinely during research into new vaccine modalities.

Targeted silencing of anthrax toxin receptors protects against anthrax toxins

Anthrax spores can be aerosolized and dispersed as a bioweapon. Current postexposure treatments are inadequate at later stages of infection, when high levels of anthrax toxins are present. Anthrax toxins enter cells via two identified anthrax toxin receptors: tumor endothelial marker 8 (TEM8) and capillary morphogenesis protein 2 (CMG2). We hypothesized that host cells would be protected from anthrax toxins if anthrax toxin receptor expression was effectively silenced using RNA interference (RNAi) technology. Thus, anthrax toxin receptors in mouse and human macrophages were silenced using targeted siRNAs or blocked with specific antibody prior to challenge with anthrax lethal toxin. Viability assays were used to assess protection in macrophages treated with specific siRNA or antibody as compared with untreated cells. Silencing CMG2 using targeted siRNAs provided almost complete protection against anthrax lethal toxin-induced cytotoxicity and death in murine and human macrophages. The same results were obtained by prebinding cells with specific antibody prior to treatment with anthrax lethal toxin. In addition, TEM8-targeted siRNAs also offered significant protection against lethal toxin in human macrophage-like cells. Furthermore, silencing CMG2, TEM8, or both receptors in combination was also protective against MEK2 cleavage by lethal toxin or adenylyl cyclase activity by edema toxin in human kidney cells. Thus, anthrax toxin receptor-targeted RNAi has the potential to be developed as a life-saving, postexposure therapy against anthrax.

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

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

Oral vaccination with an adenovirus-vectored vaccine protects against botulism

We have previously shown that an adenovirus vectored vaccine delivered intramuscularly or intranasally was effective in protection against botulism in a mouse model. The adenoviral vector encodes a human codon-optimized heavy chain C-fragment (H(C)50) of botulinum neurotoxin type C (BoNT/C). Here, we evaluate the same vaccine candidate as an oral vaccine against BoNT/C in a mouse model. To elicit protective immunity, the mice were orally vaccinated with a single dose of 1×10(4) to 1×10(7)plaque forming units (pfu) of the adenoviral vector. The immune sera, collected six weeks after oral vaccination with 2×10(7)pfu adenovirus, have shown an ability to neutralize the biological activity of BoNT/C in vitro. Additionally, animals receiving a single dose of 2×10(6)pfu adenovirus or greater were completely protected against challenge with 100×MLD(50) of BoNT/C. The data demonstrated the feasibility to develop an adenovirus-based oral vaccine against botulism.

Protective immunity against tularemia provided by an adenovirus-vectored vaccine expressing Tul4 of Francisella tularensis

Francisella tularensis, a category A bioterrorism agent, is a highly infectious organism that is passed on via skin contact and inhalation routes. A live attenuated vaccine strain (LVS) has been developed, but it has not been licensed for public use by the FDA due to safety concerns. Thus, there exists a need for a safer and improved vaccine. In this study, we have constructed a replication-incompetent adenovirus, Ad/opt-Tul4, carrying a codon-optimized gene for expression of a membrane protein, Tul4, of F. tularensis LVS. Its ability to protect against lethal challenge and its immunogenicity were evaluated in a murine model. An intramuscular injection of a single dose (1 × 10(7) PFU) of Ad/opt-Tul4 elicited a robust Tul4-specific antibody response. Assays suggest a Th1-driven response. A single dose elicited 20% protection against challenge with 100 × 50% lethal dose (LD(50)) F. tularensis LVS; two additional booster shots resulted in 60% protection. In comparison, three doses of 5 μg recombinant Tul4 protein did not elicit significant protection against challenge. Therefore, the Ad/opt-Tul4 vaccine was more effective than the protein vaccine, and protection was dose dependent. Compared to LVS, the protection rate is lower, but an adenovirus-vectored vaccine may be more attractive due to its enhanced safety profile and mucosal route of delivery. Furthermore, simple genetic modification of the vaccine may potentially produce antibodies protective against a fully virulent strain of F. tularensis. Our data support the development and further research of an adenovirus-vectored vaccine against Tul4 of F. tularensis LVS.

Inhibition of anthrax toxins with a bispecific monoclonal antibody that cross reacts with edema factor as well as lethal factor of Bacillus anthracis

Bacillus anthracis overwhelms its victims by way of two toxins, namely edema toxin and lethal toxin. Lethal toxin is formed by the combination of protective antigen with lethal factor while edema toxin is formed by the combination of Protective Antigen with edema factor. Overlapping regions between edema factor and lethal factor have been reported in past. For the first time, this study reports characterization of a bispecific monoclonal antibody (mAb), H10, which showed high affinity interaction with both edema factor and lethal factor of B. anthracis. H10 mAb not only neutralized the adenylate cyclase activity of edema toxin but it could also neutralize the cytotoxic activity of lethal toxin. Passive immunization with this antibody gave 100% protection to mice from in vivo challenge with lethal toxin and edema toxin. The results of this study suggest future application of this bispecific monoclonal antibody as passive immunization prophylactics in cases of B. anthracis exposure and infection.

A chimeric protein that functions as both an anthrax dual-target antitoxin and a trivalent vaccine

Effective measures for the prophylaxis and treatment of anthrax are still required for counteracting the threat posed by inhalation anthrax. In this study, we first demonstrated that the chimeric protein LFn-PA, created by fusing the protective antigen (PA)-binding domain of lethal factor (LFn) to PA, retained the functions of the respective molecules. On the basis of this observation, we attempted to develop an antitoxin that targets the binding of lethal factor (LF) and/or edema factor (EF) to PA and the transportation of LF/EF. Therefore, we replaced PA in LFn-PA with a dominant-negative inhibitory PA (DPA), i.e., PA(F427D). In in vitro models of anthrax intoxication, the LFn-DPA chimera showed 3-fold and 2-fold higher potencies than DPA in protecting sensitive cells against anthrax lethal toxin (LeTx) and edema toxin (EdTx), respectively. In animal models, LFn-DPA exhibited strong potency in rescuing mice from lethal challenge with LeTx. We also evaluated the immunogenicity and immunoprotective efficacy of LFn-DPA as an anthrax vaccine candidate. In comparison with recombinant PA, LFn-DPA induced significantly higher levels of the anti-PA immune response. Moreover, LFn-DPA elicited an anti-LF antibody response that could cross-react with EF. Mice immunized with LFn-DPA tolerated a LeTx challenge that was 5 times its 50% lethal dose. Thus, LFn-DPA represents a highly effective trivalent vaccine candidate for both preexposure and postexposure vaccination. Overall, we have developed a novel and dually functional reagent for the prophylaxis and treatment of anthrax.

The major neutralizing antibody responses to recombinant anthrax lethal and edema factors are directed to non-cross-reactive epitopes

Anthrax lethal and edema toxins (LeTx and EdTx, respectively) form by binding of lethal factor (LF) or edema factor (EF) to the pore-forming moiety protective antigen (PA). Immunity to LF and EF protects animals from anthrax spore challenge and neutralizes anthrax toxins. The goal of the present study is to identify linear B-cell epitopes of EF and to determine the relative contributions of cross-reactive antibodies of EF and LF to LeTx and EdTx neutralization. A/J mice were immunized with recombinant LF (rLF) or rEF. Pools of LF or EF immune sera were tested for reactivity to rLF or rEF by enzyme-linked immunosorbent assays, in vitro neutralization of LeTx and EdTx, and binding to solid-phase LF and EF decapeptides. Cross-reactive antibodies were isolated by column absorption of EF-binding antibodies from LF immune sera and by column absorption of LF-binding antibodies from EF immune sera. The resulting fractions were subjected to the same assays. Major cross-reactive epitopes were identified as EF amino acids (aa) 257 to 268 and LF aa 265 to 274. Whole LF and EF immune sera neutralized LeTx and EdTx, respectively. However, LF sera did not neutralize EdTx, nor did EF sera neutralize LeTx. Purified cross-reactive immunoglobulin G also failed to cross-neutralize. Cross-reactive B-cell epitopes in the PA-binding domains of whole rLF and rEF occur and have been identified; however, the major anthrax toxin-neutralizing humoral responses to these antigens are constituted by non-cross-reactive epitopes. This work increases understanding of the immunogenicity of EF and LF and offers perspective for the development of new strategies for vaccination against anthrax.

Potent neutralization of anthrax edema toxin by a humanized monoclonal antibody that competes with calmodulin for edema factor binding

This study describes the isolation and characterization of a neutralizing monoclonal antibody (mAb) against anthrax edema factor, EF13D. EF13D neutralized edema toxin (ET)-mediated cyclic AMP (cAMP) responses in cells and protected mice from both ET-induced footpad edema and systemic ET-mediated lethality. The antibody epitope was mapped to domain IV of EF. The mAb was able to compete with calmodulin (CaM) for EF binding and displaced CaM from EF-CaM complexes. EF-mAb binding affinity (0.05-0.12 nM) was 50- to 130-fold higher than that reported for EF-CaM. This anti-EF neutralizing mAb could potentially be used alone or with an anti-PA mAb in the emergency prophylaxis and treatment of anthrax infection.

Novel chimpanzee/human monoclonal antibodies that neutralize anthrax lethal factor, and evidence for possible synergy with anti-protective antigen antibody

Three chimpanzee Fabs reactive with lethal factor (LF) of anthrax toxin were isolated and converted into complete monoclonal antibodies (MAbs) with human gamma1 heavy-chain constant regions. In a macrophage toxicity assay, two of the MAbs, LF10E and LF11H, neutralized lethal toxin (LT), a complex of LF and anthrax protective antigen (PA). LF10E has the highest reported affinity for a neutralizing MAb against LF (dissociation constant of 0.69 nM). This antibody also efficiently neutralized LT in vitro, with a 50% effective concentration (EC(50)) of 0.1 nM, and provided 100% protection of rats against toxin challenge with a 0.5 submolar ratio relative to LT. LF11H, on the other hand, had a slightly lower binding affinity to LF (dissociation constant of 7.4 nM) and poor neutralization of LT in vitro (EC(50) of 400 nM) and offered complete protection in vivo only at an equimolar or higher ratio to toxin. Despite this, LF11H, but not LF10E, provided robust synergistic protection when combined with MAb W1, which neutralizes PA. Epitope mapping and binding assays indicated that both LF10E and LF11H recognize domain I of LF (amino acids 1 to 254). Although domain I is responsible for binding to PA, neither MAb prevented LF from binding to activated PA. Although two unique MAbs could protect against anthrax when used alone, even more efficient and broader protection should be gained by combining them with anti-PA MAbs.

Mucosal vaccination with a multicomponent adenovirus-vectored vaccine protects against Streptococcus pneumoniae infection in the lung

Streptococcus pneumoniae is a major bacterial respiratory pathogen. Current licensed pneumococcal polysaccharide and polysaccharide-protein conjugate vaccines are administered by an intramuscular injection. In order to develop a new-generation vaccine that can be administered in a needle-free mucosal manner, we have constructed early 1 and 3 gene regions (E1/E3) deleted, replication-defective adenoviral vectors encoding pneumococcal surface antigen A (PsaA), the N-fragment of pneumococcal surface protein A (N-PspA), and the detoxified mutant pneumolysin (PdB) from S. pneumoniae strain D39. Intranasal vaccination with the three adenoviral vectors (Ad/PsaA, Ad/N-PspA, and Ad/PdB) in mice resulted in robust antigen-specific serum immunoglobulin G responses, as demonstrated by an enzyme-linked immunosorbent assay. In addition, nasal mucosal vaccination with the combination of the three adenoviral vectors conferred protection against S. pneumoniae strain D39 colonization in mouse lungs. Taken together, these data demonstrate the feasibility of developing a mucosal vaccine against S. pneumoniae using recombinant adenoviruses for antigen delivery.

An adenoviral vector-based mucosal vaccine is effective in protection against botulism

A replication-incompetent adenoviral vector encoding the heavy chain C-fragment (H_C50) of botulinum neurotoxin type C (BoNT/C) was evaluated as a mucosal vaccine against botulism in a mouse model. Single intranasal inoculation of the adenoviral vector elicited a high level of H_C50-specific IgG, IgG1 and IgG2a in sera and IgA in mucosal secretions as early as 2 weeks after vaccination. The antigen-specific serum antibodies were maintained at a high level at least until the 27th week. Immune sera showed high potency in neutralizing BoNT/C as indicated by in vitro toxin neutralization assay. The mice receiving single dose of 2 × 10⁷ p.f.u. (plaque-forming unit) of adenoviral vector were completely protected against challenge with up to 10⁴ × MLD₅₀ of BoNT/C. The protective immunity showed vaccine dose dependence from 10⁵ to 2 × 10⁷ p.f.u. of adenoviral vector. In addition, animals receiving single intranasal dose of 2 × 10⁷ p.f.u. adenoviral vector could be protected against 100 × MLD₅₀ 27 weeks after vaccination. Animals with preexisting immunity to adenovirus could also be vaccinated intranasally and protected against lethal challenge with BoNT/C. These results suggest that the adenoviral vector is a highly effective gene-based mucosal vaccine against botulism.

Sequential B-cell epitopes of Bacillus anthracis lethal factor bind lethal toxin-neutralizing antibodies

The bipartite anthrax lethal toxin (LeTx) consisting of protective antigen (PA) and lethal factor (LF) is a major virulence factor contributing to death from systemic Bacillus anthracis infection. The current vaccine elicits antibodies directed primarily to PA; however, in experimental settings serologic responses to LF can neutralize LeTx and contribute to protection against infection. The goals of the present study were to identify sequential B-cell epitopes of LF and to determine the capacity of these determinants to bind neutralizing antibodies. Sera of recombinant LF-immunized A/J mice exhibited high titers of immunoglobulin G anti-LF reactivity that neutralized LeTx in vitro 78 days after the final booster immunization and protected the mice from in vivo challenge with 3 50% lethal doses of LeTx. These sera bound multiple discontinuous epitopes, and there were major clusters of reactivity on native LF. Strikingly, all three neutralizing, LF-specific monoclonal antibodies tested bound specific peptide sequences that coincided with sequential epitopes identified in polyclonal antisera from recombinant LF-immunized mice. This study confirms that LF induces high-titer protective antibodies in vitro and in vivo. Moreover, the binding of short LF peptides by LF-specific neutralizing monoclonal antibodies suggests that generation of protective antibodies by peptide vaccination may be feasible for this antigen. This study paves the way for a more effective anthrax vaccine by identifying discontinuous peptide epitopes of LF.

Analysis of epitope information related to Bacillus anthracis and Clostridium botulinum

We have reviewed the information about epitopes of immunological interest from Clostridium botulinum and Bacillus anthracis, by mining the Immune Epitope Database and Analysis Resource. For both pathogens, the vast majority of epitopes reported to date are derived from a single protein: the protective antigen of B. anthracis and the neurotoxin type A of C. botulinum. A detailed analysis of the data was performed to characterize the function, localization and conservancy of epitopes identified as neutralizing and/or protective. In order to broaden the scope of this analysis, we have also included data describing immune responses against defined fragments (over 50 amino acids long) of the relevant antigens. The scarce information on T-cell determinants and on epitopes from other antigens besides the toxins, highlights a gap in our knowledge and identifies areas for future research. Despite this, several distinct structures at the epitope and fragment level are described herein, which could be potential additions to future vaccines or targets of novel immunotherapeutics and diagnostic reagents.

Detoxified lethal toxin as a potential mucosal vaccine against anthrax

The nontoxic mutant lethal factor (mLF; which has the E687C substitution) and functional protective antigen (PA63) of Bacillus anthracis were evaluated for their use as mucosal vaccines against anthrax in A/J mice. Intranasal vaccination of three doses of 30 microg of mLF or 60 microg of PA63 elicited significant serum and mucosal antibody responses, with anthrax lethal toxin-neutralizing titers of 40 and 60 in immune sera, respectively. However, only 30% and 60% of the vaccinated animals in the two groups could survive a challenge with 100 times the 50% lethal dose of B. anthracis Sterne spores, respectively. In contrast, vaccination with three doses of the combination of 30 microg of mLF and 60 microg of PA63, the detoxified lethal toxin, elicited antibody responses against LF and PA significantly higher than those elicited after vaccination with mLF or PA63 individually by use of the same dose and schedule. Vaccination with the detoxified lethal toxin resulted in significantly higher lethal toxin-neutralizing antibody titers in sera (titer, 90). Animals vaccinated with three doses of the detoxified lethal toxin were completely protected against the spore challenge. The data suggest that mLF and PA63 have a mutual enhancement effect for evoking systemic and mucosal immune responses and that the detoxified lethal toxin can be used as an efficient mucosal vaccine against anthrax.

Catalytically inactive anthrax toxin(s) are potential prophylactic agents

The anthrax exotoxin, which is a key mediator of anthrax related pathogenesis, is composed of two separate toxins formed by pairwise combinations of three proteins that are encoded on the pXO1 plasmid of Bacillus anthracis. Lethal toxin is composed of protective antigen (PA) combined with lethal factor (LF) while edema toxin is composed of PA and edema factor (EF). The present study found that the catalytic mutants of LF (LFE687A) and EF (EFH351A) competitively inhibited lethal toxin and edema toxin-mediated activity in vitro and lethality in vivo and were non-toxic to sensitive cell lines when combined with PA. While PA combined with EFH351A was non-lethal in mice, PA combined with LFE687A was of reduced virulence. Full protection of mice against a lethal toxin challenge required injection of mice with PA combined with both LFE687A and EFH351A. The potential use of these full-length, biologically inactive mutant proteins combined with PA as prophylactics or therapeutics is discussed.

Transcriptional stimulation of anthrax toxin receptors by anthrax edema toxin and Bacillus anthracis Sterne spore

We used quantitative real-time RT-PCR to not only investigate the mRNA levels of anthrax toxin receptor 1 (ANTXR1) and 2 (ANTXR2) in the murine J774A.1 macrophage cells and different tissues of mice, but also evaluate the effect of anthrax edema toxin and Bacillus anthracis Sterne spores on the expression of mRNA of these receptors. The mRNA transcripts of both receptors were detected in J774A.1 cells and mouse tissues such as the lung, heart, kidney, spleen, stomach, jejunum, brain, skeleton muscle, and skin. The ANTXR2 mRNA level was significantly higher than that of ANTXR1 in J774A.1 cells and all tissues examined. The mRNA expression of both receptors in the lung was the highest among the tissues evaluated. Interestingly, the mRNA expression of both receptors in J774A.1 cells was upregulated by edema toxin. In addition, ANTXR mRNA expression in the lung was downregulated after subcutaneous inoculation of B. anthracis Sterne spores as well as after intranasal administration of anthrax toxin-based vaccine BioThrax. These results suggest that anthrax edema toxin and B. anthracis Sterne spore are involved in the ANTXR mRNA regulation in host cells.

Protection against anthrax by needle-free mucosal immunization with human anthrax vaccine

Human vaccination with BioThrax requires six injections followed by annual boosters. This makes it difficult for the compliance of the immunization program and underscores the need for development of a new and optimized vaccination protocol. Current research aims to demonstrate the proof of concept to develop a needle-free mucosal immunization protocol using a murine anthrax model. A/J mice were immunized with BioThrax via an intranasal route. Sera, saliva, vaginal, and nasal washes were evaluated for protective antigen (PA) specific antibody responses by ELISA. Antigen-specific, antibody-secreting lymphocytes were measured by ELISPOT. Sera neutralization antibody titers were determined by in vitro anthrax lethal toxin (Letx) neutralization assay. Immunized animals were challenged by a lethal dose of Bacillus anthracis Sterne spores to determine the efficacy of the vaccination. Nasal mucosal immunization with BioThrax elicited robust serum and mucosal antibody responses against PA. The antigen specific antibodies neutralized anthrax Letx, as demonstrated by in vitro neutralization assays. Two doses of intranasal BioThrax were sufficient to completely protect A/J mice against challenge with 100xLD50B. anthracis Sterne spores. The data suggests that intranasal administration may be an effective immunization modality for an improved immunization program against anthrax.

Adenovirus-based prime-boost immunization for rapid vaccination against anthrax

Prime-boost vaccination using plasmid DNA and replication-defective adenovirus vectors has emerged as a highly effective strategy for vaccinating against viral pathogens. However, its ability to provide protection against bacterial disease has never been assessed. Here we evaluate prime-boost vaccination approaches for immunizing against anthrax. We show that mice primed with DNA and boosted with an adenovirus vector, both expressing domain four of Bacillus anthracis protective antigen (PA), have higher antibody and toxin-neutralizing titers than mice immunized with either single modality alone. DNA-primed/adenovirus-boosted mice also had significantly higher antibody and toxin-neutralizing titers than mice immunized with Anthrax Vaccine Adsorbed. High levels of antigen-specific interferon-gamma-secreting cells were present in vaccinated mice indicating that a cell-mediated immune response had also been stimulated. Both DNA-primed/adenovirus-boosted and adenovirus-primed/adenovirus-boosted mice were fully protected from Sterne strain spore challenge. We also show that a single injection with an adenovirus vector-expressing domain four of PA can provide partial protection from spore challenge 2 weeks after immunization and full protection 3 weeks after immunization. These results demonstrate that adenovirus-based prime-boost vaccination can provide rapid protection from anthrax and that this approach may be an effective strategy for immunizing against bacterial as well as viral pathogens.

A new complementing cell line for replication-incompetent E1-deleted adenovirus propagation

Recombinant adenoviruses (Ad) are being explored as promising delivery systems for gene therapy and vaccination. However, there is a concern about the possibility of generating replication-competent adenoviruses (RCA) using the human embryonic kidney 293 cell line. We have constructed a new cell line named the UR cell line which can be used to produce Ad vectors free of RCA. This cell line is based on the human embryonic lung HEL 299 cell. We first constructed a shuttle plasmid which encodes the E1A/E1B sequence that is necessary for adenovirus replication. The shuttle plasmid was then transfected into HEL 299 cells. The presence of the E1A/E1B sequence and protein expression in the stably transformed UR cells was confirmed. Viruses produced in UR cells were still RCA-free after ten test passages, while adenovirus produced in 293 cells had generated RCA during the fourth passage. We conclude that the UR cell line is sufficiently stable, can effectively produce a virus yield comparable with 293 cells, and does not generate RCA formation during Ad propagation.

Multicomponent anthrax toxin display and delivery using bacteriophage T4

We describe a multicomponent antigen display and delivery system using bacteriophage T4. Two dispensable outer capsid proteins, Hoc (highly antigenic outer capsid protein, 155 copies) and Soc (small outer capsid protein, 810 copies), decorate phage T4 capsid. These proteins bind to the symmetrically localized capsid sites, which appear following prohead assembly and expansion. We hypothesized that multiple antigens fused to Hoc can be displayed on the same capsid and such particles can elicit broad immunological responses. Anthrax toxin proteins, protective antigen (PA), lethal factor (LF), and edema factor (EF), and their functional domains, were fused to Hoc with an N-terminal hexa-histidine tag and the recombinant proteins were over-expressed in E. coli and purified. Using a defined in vitro assembly system, the anthrax-Hoc fusion proteins were efficiently displayed on T4 capsid, either individually or in combinations. All of the 155 Hoc binding sites can be occupied by one antigen, or they can be split among two or more antigens by varying their molar ratio in the binding reaction. Immunization of mice with T4 phage carrying PA, LF, and EF elicited strong antigen-specific antibodies against all antigens as well as lethal toxin neutralization titers. The triple antigen T4 phage elicited stronger PA-specific immune responses than the phage displaying PA alone. These features offer novel avenues to develop customized multicomponent vaccines against anthrax and other pathogenic diseases.