Production and purification of anthrax toxin

Inhalable vaccine of bacterial culture supernatant extract mediates protection against fatal pulmonary anthrax

AbstractPulmonary anthrax is the most fatal clinical form of anthrax and currently available injectable vaccines do not provide adequate protection against it. Hence, next-generation vaccines that effectively induce immunity against pulmonary anthrax are urgently needed. In the present study, we prepared an attenuated and low protease activity Bacillus anthracis strain A16R-5.1 by deleting five of its extracellular protease activity-associated genes and its lef gene through the CRISPR-Cas9 genome editing system. This mutant strain was then used to formulate a lethal toxin (LeTx)-free culture supernatant extract (CSE) anthrax vaccine,of which half was protective antigen (PA). We generated liquid, powder, and powder reconstituted formulations that could be delivered by aerosolized intratracheal inoculation. All of them induced strong humoral, cellular, and mucosal immune responses. The vaccines also produced LeTx neutralizing antibodies and conferred full protection against the lethal aerosol challenges of B. anthracis Pasteur II spores in mice. Compared to the recombinant PA vaccine, the CSE anthrax vaccine with equal PA content provided superior immunoprotection against pulmonary anthrax. The preceding results suggest that the CSE anthrax vaccine developed herein is suitable and scalable for use in inhalational immunization against pulmonary anthrax.

The Transcription Activator AtxA from Bacillus Anthracis was Employed for Developing a Tight-Control, High-Level, Modulable, and Stationary-Phase Specific Transcription Activity in Escherichia Coli

The strong transcriptional activity of the virulent gene pagA in Bacillus anthracis has been proven to be anthrax toxin activator (AtxA)-regulated. However, the obscure pagA transcription mechanism hinders practical applications of this strong promoter. In this study, a 509-bp DNA fragment [termed 509sequence, (−508)-(+1) relative to the P2 transcription start site] was cloned upstream of rbs-GFPuv as pTOL02B to elucidate the AtxA-regulated transcription. The 509sequence was dissected into the −10 sequence, −35 sequence, AT_rich tract, SLI/SLII and upstream site. In conjunction with the heterologous co-expression of AtxA (under the control of the T7 promoter), the −10 sequence (TATACT) was sufficient for the AtxA-regulated transcription. Integration of pTOL02F + pTOLAtxA as pTOL03F showed that the AtxA-regulated transcription exhibited a strong specific fluorescence intensity/common analytical chemistry term (OD₆₀₀) of 40 597 ± 446 and an induction/repression ratio of 122. An improved induction/repression ratio of 276 was achieved by cultivating Escherichia coli/pTOL03F in M9 minimal medium. The newly developed promoter system termed P_AtxA consists of AtxA, the −10 sequence and Escherichia RNA polymerase. These three elements synergistically and cooperatively formed a previously undiscovered transcription system, which exhibited a tight-control, high-level, modulable and stationary-phase-specific transcription. The P_AtxA was used for phaCAB expression for the stationary-phase polyhydroxybutyrate production, and the results showed that a PHB yield, content and titer of 0.20 ± 0.27 g/g-glucose, 68 ± 11% and 1.5 ± 0.4 g/l can be obtained. The positive inducible P_AtxA, in contrast to negative inducible, should be a useful tool to diversify the gene information flow in synthetic biology.

Graphical Abstract

BrnQ-Type Branched-Chain Amino Acid Transporters Influence Bacillus anthracis Growth and Virulence

Bacillus anthracis, the anthrax agent, exhibits robust proliferation in diverse niches of mammalian hosts. The metabolic attributes of B. anthracis that permit rapid growth in multiple mammalian tissues have not been established. We posit that branched-chain amino acid (BCAA) (isoleucine, leucine, and valine) metabolism is key to B. anthracis pathogenesis. Increasing evidence indicates the relationships between B. anthracis virulence and the expression of BCAA-related genes. The expression of some BCAA-related genes is altered during culture in bovine blood in vitro, and the bacterium exhibits valine auxotrophy in a blood serum mimic medium. Transcriptome analyses have revealed that the virulence regulator AtxA, which positively affects the expression of the anthrax toxin and capsule genes, negatively regulates genes predicted to be associated with BCAA biosynthesis and transport. Here, we show that B. anthracis growth in defined medium is severely restricted in the absence of exogenous BCAAs, indicating that BCAA transport is required for optimal growth in vitro. We demonstrate functional redundancy among multiple BrnQ-type BCAA transporters. Three transporters are associated with isoleucine and valine transport, and the deletion of one, BrnQ3, attenuates virulence in a murine model for anthrax. Interestingly, an ilvD-null mutant lacking dihydroxy acid dehydratase, an enzyme essential for BCAA synthesis, exhibits unperturbed growth when cultured in medium containing BCAAs but is highly attenuated in the murine model. Finally, our data show that BCAAs enhance AtxA activity in a dose-dependent manner, suggesting a model in which BCAAs serve as a signal for virulence gene expression. IMPORTANCE Infection with B. anthracis can result in systemic disease with large numbers of the bacterium in multiple tissues. We found that branched-chain amino acid (BCAA) synthesis is insufficient for the robust growth of B. anthracis; access to BCAAs is necessary for the proliferation of the pathogen during culture and during infection in a murine model for anthrax. B. anthracis produces an unusually large repertoire of BCAA-related transporters. We identified three isoleucine/valine transporters with partial functional redundancy during culture. The deletion of one of these transporters, BrnQ3, resulted in attenuated virulence. Interestingly, a BCAA biosynthesis mutant grew well in medium containing BCAAs but, like BrnQ3, was attenuated for virulence. These results suggest that BCAAs are limiting in multiple niches during infection and further our understanding of the nutritional requirements of this important pathogen.

Direct Regulons of AtxA, the Master Virulence Regulator of Bacillus anthracis

AtxA, the master virulence regulator of Bacillus anthracis, regulates the expression of three toxins and genes for capsule formation that are required for the pathogenicity of B. anthracis. Recent transcriptome analyses showed that AtxA affects a large number of genes on the chromosome and plasmids, suggesting a role as a global regulator. However, information on genes directly regulated by AtxA is scarce. In this work, we conducted genome-wide analyses and cataloged the binding sites of AtxA in vivo and transcription start sites on the B. anthracis genome. By integrating these results, we detected eight genes as direct regulons of AtxA. These consisted of five protein-coding genes, including two of the three toxin genes, and three genes encoding the small RNAs XrrA and XrrB and a newly discovered 95-nucleotide small RNA, XrrC. Transcriptomes from single-knockout mutants of these small RNAs revealed changes in the transcription levels of genes related to the aerobic electron transport chain, heme biosynthesis, and amino acid metabolism, suggesting their function for the control of cell physiology. These results reveal the first layer of the gene regulatory network for the pathogenicity of B. anthracis and provide a data set for the further study of the genomics and genetics of B. anthracis.

IMPORTANCEBacillus anthracis is the Gram-positive bacterial species that causes anthrax. Anthrax is still prevalent in countries mainly in Asia and Africa, where it causes economic damage and remains a public health issue. The mechanism of pathogenicity is mainly explained by the three toxin proteins expressed from the pXO1 plasmid and by proteins involved in capsule formation expressed from the pXO2 plasmid. AtxA is a protein expressed from the pXO1 plasmid that is known to upregulate genes involved in toxin production and capsule formation and is thus considered the master virulence regulator of B. anthracis. Therefore, understanding the detailed mechanism of gene regulation is important for the control of anthrax. The significance of this work lies in the identification of genes that are directly regulated by AtxA via genome-wide analyses. The results reveal the first layer of the gene regulatory network for the pathogenicity of B. anthracis and provide useful resources for a further understanding of B. anthracis.

Effect of over expressing protective antigen on global gene transcription in Bacillus anthracis BH500

Protective antigen (PA) of Bacillus anthracis is being considered as a vaccine candidate against anthrax and its production has been explored in several heterologous host systems. Since the systems tested introduced adverse issues such as inclusion body formation and endotoxin contamination, the production from B. anthracis is considered as a preferred method. The present study examines the effect of PA expression on the metabolism of B. anthracis producing strain, BH500, by comparing it with a control strain carrying an empty plasmid. The strains were grown in a bioreactor and RNA-seq analysis of the producing and non-producing strain was conducted. Among the observed differences, the strain expressing rPA had increased transcription of sigL, the gene encoding RNA polymerase σ⁵⁴, sigB, the general stress transcription factor gene and its regulators rsbW and rsbV, as well as the global regulatory repressor ctsR. There were also decreased expression of intracellular heat stress related genes such as groL, groES, hslO, dnaJ, and dnaK and increased expression of extracellular chaperons csaA and prsA2. Also, major central metabolism genes belonging to TCA, glycolysis, PPP, and amino acids biosynthesis were up-regulated in the PA-producing strain during the lag phase and down-regulated in the log and late-log phases, which was associated with decreased specific growth rates. The information obtained from this study may guide genetic modification of B. anthracis to improve PA production.

The pag Gene of pXO1 Is Involved in Capsule Biosynthesis of Bacillus anthracis Pasteur II Strain

The poly-γ-D-glutamic acid capsule and anthrax toxins are major virulence factors of Bacillus anthracis. Genes responsible for capsule biosynthesis are located on pXO2, whereas genes encoding the toxins, which are composed of edema factors, lethal factors, and protective antigens (PA), are located on pXO1. In this study, we found that the pag null mutation not only eliminated the production of the protective antigen, it also eliminated the ability of the B. anthracis Pasteur II strain to form capsules. qPCR analysis revealed that the deletion of pag decreased the transcription levels of the capABCD operon and its regulatory genes acpA and acpB. The introduction of the acpA or acpB plasmid complemented the effect of the pag null mutation on capsule formation. Taken together, the above results suggest that PA probably affects capsule biosynthesis by altering the expression of acpA and acpB. In addition, we found that the deletion mutation of pag remarkably attenuated bacterial pathogenicity in a mouse model of infection. Our results indicate that besides encoding the protective antigen, the pag gene of pXO1 is also involved in the modulation of capsule biosynthesis. Our findings provide new insight into the regulation mechanisms of capsule formation in B. anthracis Pasteur II strain.

In vivo dynamics of active edema and lethal factors during anthrax

Lethal and edema toxins are critical virulence factors of Bacillus anthracis. However, little is known about their in vivo dynamics of production during anthrax. In this study, we unraveled for the first time the in vivo kinetics of production of the toxin components EF (edema factor) and LF (lethal factor) during cutaneous infection with a wild-type toxinogenic encapsulated strain in immuno-competent mice. We stratified the asynchronous infection process into defined stages through bioluminescence imaging (BLI), while exploiting sensitive quantitative methods by measuring the enzymatic activity of LF and EF. LF was produced in high amounts, while EF amounts steadily increased during the infectious process. This led to high LF/EF ratios throughout the infection, with variations between 50 to a few thousands. In the bloodstream, the early detection of active LF and EF despite the absence of bacteria suggests that they may exert long distance effects. Infection with a strain deficient in the protective antigen toxin component enabled to address its role in the diffusion of LF and EF within the host. Our data provide a picture of the in vivo complexity of the infectious process.

Ultrasensitive detection of protease activity of anthrax and botulinum toxins by a new PCR-based assay

Anthrax and botulism are dangerous infectious diseases that can be fatal unless detected and treated quickly. Fatalities from these diseases are primarily due to endopeptidase toxins secreted by the pathogens. Rapid and sensitive detection of the presence of active toxins is the key element for protection from natural outbreaks of anthrax and botulism, as well as from the threat of bioterrorism. We describe an ultrasensitive polymerase chain reaction (PCR)-based assay for detecting proteolytic activity of anthrax and botulinum toxins using composite probes consisting of covalent peptide-DNA conjugate for the detection of anthrax, and noncovalent protein-aptamer assembly to assay botulinum toxin activity. Probes immobilized on the solid-phase support are cleaved by toxins to release DNA, which is detected by real-time PCR. Both assays can detect subpicogram quantities of active toxins isolated from composite matrices. Special procedures were developed to isolate intact toxins from the matrices under mild conditions. The assay is rapid, uses proven technologies, and can be modified to detect other proteolytic and biopolymer-degrading enzymes.

Differential dependence on N-glycosylation of anthrax toxin receptors CMG2 and TEM8

ANTXR 1 and 2, also known as TEM8 and CMG2, are two type I membrane proteins, which have been extensively studied for their role as anthrax toxin receptors, but with a still elusive physiological function. Here we have analyzed the importance of N-glycosylation on folding, trafficking and ligand binding of these closely related proteins. We find that TEM8 has a stringent dependence on N-glycosylation. The presence of at least one glycan on each of its two extracellular domains, the vWA and Ig-like domains, is indeed necessary for efficient trafficking to the cell surface. In the absence of any N-linked glycans, TEM8 fails to fold correctly and is recognized by the ER quality control machinery. Expression of N-glycosylation mutants reveals that CMG2 is less vulnerable to sugar loss. The absence of N-linked glycans in one of the extracellular domains indeed has little impact on folding, trafficking or receptor function of the wild type protein expressed in tissue culture cells. N-glycans do, however, seem required in primary fibroblasts from human patients. Here, the presence of N-linked sugars increases the tolerance to mutations in cmg2 causing the rare genetic disease Hyaline Fibromatosis Syndrome. It thus appears that CMG2 glycosylation provides a buffer towards genetic variation by promoting folding of the protein in the ER lumen.

Bidirectional effect of Wnt signaling antagonist DKK1 on the modulation of anthrax toxin uptake

LRP6, a co-receptor for the morphogen Wnt, aids endocytosis of anthrax complexes. Here we report that Dickkopf1 (DKK1) protein, a secreted LRP6 ligand and antagonist, is also a modulator of anthrax toxin sensitivity. shRNA-mediated gene silencing or TALEN-mediated gene knockout of DKK1 reduced sensitivity of cells to PA-dependent hybrid toxins. However, unlike the solely inhibitory effect on Wnt signaling, the effects of DKK1 overexpression on anthrax toxicity were bidirectional, depending on its endogenous expression and cell context. Fluorescence microscopy and biochemical analyses showed that DKK1 facilitates internalization of anthrax toxins and their receptors, an event mediated by DKK1-LRP6-Kremen2 complex. Monoclonal antibodies against DKK1 provided dose-dependent protection to macrophages from killing by anthrax lethal toxin (LT). Our discovery that DKK1 forms ternary structure with LRP6 and Kremen2 in promoting PA-mediated toxin internalization provides a paradigm for bacterial exploitation of mechanisms that host cells use to internalize signaling proteins.

Vaccination of rhesus macaques with the anthrax vaccine adsorbed vaccine produces a serum antibody response that effectively neutralizes receptor-bound protective antigen in vitro

Anthrax toxin (ATx) is composed of the binary exotoxins lethal toxin (LTx) and edema toxin (ETx). They have separate effector proteins (edema factor and lethal factor) but have the same binding protein, protective antigen (PA). PA is the primary immunogen in the current licensed vaccine anthrax vaccine adsorbed (AVA [BioThrax]). AVA confers protective immunity by stimulating production of ATx-neutralizing antibodies, which could block the intoxication process at several steps (binding of PA to the target cell surface, furin cleavage, toxin complex formation, and binding/translocation of ATx into the cell). To evaluate ATx neutralization by anti-AVA antibodies, we developed two low-temperature LTx neutralization activity (TNA) assays that distinguish antibody blocking before and after binding of PA to target cells (noncomplexed [NC] and receptor-bound [RB] TNA assays). These assays were used to investigate anti-PA antibody responses in AVA-vaccinated rhesus macaques (Macaca mulatta) that survived an aerosol challenge with Bacillus anthracis Ames spores. Results showed that macaque anti-AVA sera neutralized LTx in vitro, even when PA was prebound to cells. Neutralization titers in surviving versus nonsurviving animals and between prechallenge and postchallenge activities were highly correlated. These data demonstrate that AVA stimulates a myriad of antibodies that recognize multiple neutralizing epitopes and confirm that change, loss, or occlusion of epitopes after PA is processed from PA83 to PA63 at the cell surface does not significantly affect in vitro neutralizing efficacy. Furthermore, these data support the idea that the full-length PA83 monomer is an appropriate immunogen for inclusion in next-generation anthrax vaccines.

Activation of the latent PlcR regulon in Bacillus anthracis

Many genes in Bacillus cereus and Bacillus thuringiensis are under the control of the transcriptional regulator PlcR and its regulatory peptide, PapR. In Bacillus anthracis, the causative agent of anthrax, PlcR is inactivated by truncation, and consequently genes having PlcR binding sites are expressed at very low levels when compared with B. cereus. We found that activation of the PlcR regulon in B. anthracis by expression of a PlcR–PapR fusion protein does not alter sporulation in strains containing the virulence plasmid pXO1 and thereby the global regulator AtxA. Using comparative 2D gel electrophoresis, we showed that activation of the PlcR regulon in B. anthracis leads to upregulation of many proteins found in the secretome of B. cereus, including phospholipases and proteases, such as the putative protease BA1995. Transcriptional analysis demonstrated expression of BA1995 to be dependent on PlcR–PapR, even though the putative PlcR recognition site of the BA1995 gene does not exactly match the PlcR consensus sequence, explaining why this protein had escaped recognition as belonging to the PlcR regulon. Additionally, while transcription of major PlcR-dependent haemolysins, sphingomyelinase and anthrolysin O is enhanced in response to PlcR activation in B. anthracis, only anthrolysin O contributes significantly to lysis of human erythrocytes. In contrast, the toxicity of bacterial culture supernatants from a PlcR-positive strain towards murine macrophages occurred independently of anthrolysin O expression in vitro and in vivo.

Targeted proteomics approach to species-level identification of Bacillus thuringiensis spores by AP-MALDI-MS

Anthrax infections progress at a rapid pace, making rapid detection methods of utmost importance. MALDI-MS proteomics methods focused on Bacillus anthracis detection have targeted chromosomally encoded proteins, which are highly conserved between closely related species, hindering species identification. Presented here is an AP-MALDI-MS method targeting plasmid-borne proteins from Bacillus spores for species-level identification. A bioinformatics analysis revealed that 60.3% and 75.4% of tryptic peptides from plasmid-borne proteins of B. anthracis and B. thuringiensis were species-specific, respectively. Reported here is a method in which plasmid-borne delta-endotoxins were extracted directly from B. thuringiensis spores in 100 mM KOH. The pH was then adjusted to 8 and a 5-min trypsin digestion was performed on the extracted proteins. The resulting tryptic peptides were analyzed by AP-MALDI-MS/MS, which produced a definitive identification the B. thuringiensis species-specific Cry1Ab protein with a MASCOT score of 278 and expect value of 7.5 x 10(-23). This method has demonstrated the detection and identification of B. thuringiensis spores at the species level following a 5-min trypsin digestion. The challenges in applying a similar approach to the detection of plasmid-borne protein toxins from B. anthracis are also discussed.

Endocytosis of the anthrax toxin is mediated by clathrin, actin and unconventional adaptors

The anthrax toxin is a tripartite toxin, where the two enzymatic subunits require the third subunit, the protective antigen (PA), to interact with cells and be escorted to their cytoplasmic targets. PA binds to cells via one of two receptors, TEM8 and CMG2. Interestingly, the toxin times and triggers its own endocytosis, in particular through the heptamerization of PA. Here we show that PA triggers the ubiquitination of its receptors in a β-arrestin-dependent manner and that this step is required for clathrin-mediated endocytosis. In addition, we find that endocytosis is dependent on the heterotetrameric adaptor AP-1 but not the more conventional AP-2. Finally, we show that endocytosis of PA is strongly dependent on actin. Unexpectedly, actin was also found to be essential for efficient heptamerization of PA, but only when bound to one of its 2 receptors, TEM8, due to the active organization of TEM8 into actin-dependent domains. Endocytic pathways are highly modular systems. Here we identify some of the key players that allow efficient heptamerization of PA and subsequent ubiquitin-dependent, clathrin-mediated endocytosis of the anthrax toxin.

Bacillus anthracis is the bacterium responsible for the anthrax disease. Its virulence is mainly due to 2 factors, the anthrax toxin and the anti-phagocytic capsule. This toxin is composed of three independent polypeptide chains. Two of these have enzymatic activity and are responsible for the effects of the toxin. The third has no activity but is absolutely required to bring the 2 enzymatic subunits into the cell where they act. If one blocks entry into the cells, one blocks the effects of these toxins, which is why it is important to understand how the toxin enters into the cell at the molecular level. Here we identified various molecules that are involved in efficiently bringing the toxin into the cell. First, we found that the actin cytoskeleton plays an important role in organizing one of the two anthrax toxin receptors at the cell surface. Second, we found a cytosolic protein, β-arrestin, that is required to modify the intracellular part of the toxin receptor, to allow uptake. Finally, we directly show, for the first time, that anthrax toxin uptake is mediated by the so-called clathrin-dependent pathway, a very modular entry pathway, but that the toxin utilizes this pathway in an unconventional way.

Bacillus anthracis physiology and genetics

Bacillus anthracis is a member of the Bacillus cereus group species (also known as the "group 1 bacilli"), a collection of Gram-positive spore-forming soil bacteria that are non-fastidious facultative anaerobes with very similar growth characteristics and natural genetic exchange systems. Despite their close physiology and genetics, the B. cereus group species exhibit certain species-specific phenotypes, some of which are related to pathogenicity. B. anthracis is the etiologic agent of anthrax. Vegetative cells of B. anthracis produce anthrax toxin proteins and a poly-d-glutamic acid capsule during infection of mammalian hosts and when cultured in conditions considered to mimic the host environment. The genes associated with toxin and capsule synthesis are located on the B. anthracis plasmids, pXO1 and pXO2, respectively. Although plasmid content is considered a defining feature of the species, pXO1- and pXO2-like plasmids have been identified in strains that more closely resemble other members of the B. cereus group. The developmental nature of B. anthracis and its pathogenic (mammalian host) and environmental (soil) lifestyles of make it an interesting model for study of niche-specific bacterial gene expression and physiology.

Anthrax LFn-PA Hybrid Antigens: Biochemistry, Immunogenicity, and Protection Against Lethal Ames Spore Challenge in Rabbits

We describe a novel hybrid anthrax toxin approach that incorporates multiple components into a single vaccine product. The key domains of protective antigen (PA) and lethal factor (LF) that may be critical for inducing protective immunity are combined into one recombinant molecule. Two LF N-terminal domain-PA hybrids, one with wild-type PA and another with furin cleavage-minus PA, were expressed in E. coli and purified in a native form. Both the hybrids bind to the extracellular domain of the host receptor, CMG2; the wild-type hybrid can be cleaved by furin exposing the LF interacting domain, allowing it to oligomerize into lethal toxin as well as translocation pore-like complexes. The hybrid antigens are immunogenic in Dutch-belted rabbits, eliciting strong PA-specific and LF-specific antibodies. However, the lethal toxin neutralizing antibody titers are 3-7 times lower than those elicited by PA-alum. The hybrid antigens conferred 100% (6/6) protection in rabbits challenged intranasally with a 100 LD(50) dose of Bacillus anthracis Ames strain spores.

Systemic hyalinosis mutations in the CMG2 ectodomain leading to loss of function through retention in the endoplasmic reticulum

Systemic hyalinosis is an autosomal recessive disease that encompasses two allelic syndromes, infantile systemic hyalinosis (ISH) and juvenile hyaline fibromatosis (JHF), which are caused by mutations in the CMG2 gene. Here we have analyzed the cellular consequences of five patient-derived point mutations in the extracellular von Willebrand domain or the transmembrane domain of the CMG2 protein. We found that four of the mutations led to retention of the protein in the endoplasmic reticulum (ER), albeit through different mechanisms. Analysis of recombinant CMG2 von Willebrand factor A (vWA) domains, to which three of the mutations map, indicated that the mutations did not prevent proper folding and ligand binding, suggesting that, in vivo, slow folding, rather than misfolding, is responsible for ER retention. Our work shows that systemic hyalinosis can be qualified as a conformational disease, at least for the mutations that have been mapped to the extracellular and transmembrane domains. The long ER half-life and the ligand binding ability of the mutated von Willebrand domains suggest that treatments based on chemical chaperones could be beneficial.

Two small c-type cytochromes affect virulence gene expression in Bacillus anthracis

Regulated expression of the genes for anthrax toxin proteins is essential for the virulence of the pathogenic bacterium Bacillus anthracis. Induction of toxin gene expression depends on several factors, including temperature, bicarbonate levels, and metabolic state of the cell. To identify factors that regulate toxin expression, transposon mutagenesis was performed under non-inducing conditions and mutants were isolated that untimely expressed high levels of toxin. A number of these mutations clustered in the haem biosynthetic and cytochrome c maturation pathways. Genetic analysis revealed that two haem-dependent, small c-type cytochromes, CccA and CccB, located on the extracellular surface of the cytoplasmic membrane, regulate toxin gene expression by affecting the expression of the master virulence regulator AtxA. Deregulated AtxA expression in early exponential phase resulted in increased expression of toxin genes in response to loss of the CccA-CccB signalling pathway. This is the first function identified for these two small c-type cytochromes of Bacillus species. Extension of the transposon screen identified a previously uncharacterized protein, BAS3568, highly conserved across many bacterial and archeal species, as involved in cytochrome c activity and virulence regulation. These findings are significant not only to virulence regulation in B. anthracis, but also to analysis of virulence regulation in many pathogenic bacteria and to the study of cytochrome c activity in Gram-positive bacteria.

Role of N-terminal amino acids in the potency of anthrax lethal factor

Anthrax lethal factor (LF) is a Zn(+2)-dependent metalloprotease that cleaves several MAPK kinases and is responsible for the lethality of anthrax lethal toxin (LT). We observed that a recombinant LF (LF-HMA) which differs from wild type LF (LF-A) by the addition of two residues (His-Met) to the native Ala (A) terminus as a result of cloning manipulations has 3-fold lower potency toward cultured cells and experimental animals. We hypothesized that the "N-end rule", which relates the half-life of proteins in cells to the identity of their N-terminal residue, might be operative in the case of LF, so that the N-terminal residue of LF would determine the cytosolic stability and thereby the potency of LF. Mutational studies that replaced the native N-terminal residue of LF with known N-end rule stabilizing or destabilizing residues confirmed that the N-terminal residue plays a significant role in determining the potency of LT for cultured cells and experimental animals. The fact that a commercially-available LF preparation (LF-HMA) that is widely used in basic research studies and for evaluation of vaccines and therapeutics is 3-fold less potent than native LF (LF-A) should be considered when comparing published studies and in the design of future experiments.

Dual promoters control expression of the Bacillus anthracis virulence factor AtxA

The AtxA virulence regulator of Bacillus anthracis is required for toxin and capsule gene expression. AtxA is a phosphotransferase system regulatory domain-containing protein whose activity is regulated by phosphorylation/dephosphorylation of conserved histidine residues. Here we report that transcription of the atxA gene occurs from two independent promoters, P1 (previously described by Dai et al. [Z. Dai, J. C. Sirard, M. Mock, and T. M. Koehler, Mol. Microbiol. 16:1171-1181, 1995]) and P2, whose transcription start sites are separated by 650 bp. Both promoters have -10 and -35 consensus sequences compatible with recognition by sigma(A)-containing RNA polymerase, and neither promoter depends on the sporulation sigma factor SigH. The dual promoter activity and the extended untranslated mRNA suggest that as-yet-unknown regulatory mechanisms may act on this region to influence the level of AtxA in the cell.

Virulence gene expression is independent of ResDE-regulated respiration control in Bacillus anthracis

The ResDE two-component system regulates the synthesis of several components of the aerobic and anaerobic respiratory pathways in bacilli. The ResD response regulator transcription factor has been implicated in the regulation of virulence factors in a number of gram-positive species, including Bacillus anthracis. The precise deletions of resD and resE in B. anthracis that retained the classical respiratory phenotypes did not affect the expression of the gene for the protective antigen of the anthrax toxin, pagA, or that of the toxin regulator, atxA. The results indicate that the loss of ResDE-controlled respiratory capacity does not affect the synthesis of anthrax toxin.

Maturation modulates caspase-1-independent responses of dendritic cells to Anthrax lethal toxin

Anthrax lethal toxin (LT) contributes to the immune evasion strategy of Bacillus anthracis by impairing the function of cells of the immune system, such as macrophages and dendritic cells (DCs). Macrophages from certain inbred mice strains undergo rapid death upon LT treatment mediated by caspase-1 activation dependent on Nalp1b, an inflammasome component. Rapid LT-induced death is however, not observed in macrophages from human and many mouse strains. Here, we focused on the responses of various murine DCs to LT. Using a variety of knockout mice, we found that depending on the mouse strain, death of bone marrow-derived DCs and macrophages was mediated either by a fast Nalp1b and caspase-1-dependent, or by a slow caspase-1-independent pathway that was triggered by the impairment of MEK1/2 pathways. Caspase-1-independent death was observed in cells of different genetic backgrounds and interestingly occurred only in immature DCs. Maturation, triggered by different types of stimuli, led to full protection of DCs. These studies illustrate that the cellular damage inflicted by LT depends not only on the innate responses but also on the maturation stage of the cell, which modulates the more general caspase-1-independent responses.

Efficient production and characterization of Bacillus anthracis lethal factor and a novel inactive mutant rLFm-Y236F

Lethal factor (LF) is a 90kDa zinc metalloprotease that plays an important role in the virulence of anthrax. Recombinant LF (rLF) is an effective tool to study anthrax pathogenesis and treatment. In this study, the LF gene was cloned into the Escherichia coli expression vector pGEX-6P-1 and expressed as a GST fusion protein (GST-rLF) in E. coli BL21-codonPlus (DE3)-RIL cells with 0.2mM IPTG induction at 28 degrees C. The GST-rLF protein was purified and the GST-tag was then cleaved in a single step by combining both GST-affinity column and treatment with 3C protease. This procedure yielded 5mg of rLF protein per liter of culture. The purified rLF was functional as confirmed by cytotoxicity assay in RAW264.7 cells and Western blot assay. Furthermore, the rLF could induce strong immune response in BALB/c mice and the presence of a specific antiserum could neutralize the cytotoxicity of rLF in vitro. In addition, a novel inactive mutant (rLFm-Y236F) was obtained. Compared to the wild-type rLF, an increase by 3700 folds of the purified rLFm-Y236F was needed to achieve a similar level of cytotoxicity of the wild-type rLF. This mutant might be of significance in the study of anthrax pathogenesis and treatment.

The seminal literature of anthrax research

A chronically weak area in research papers, reports, and reviews is the complete identification of seminal background documents that formed the building blocks for these papers. A method for systematically determining these seminal references is presented. Citation-Assisted Background (CAB) is based on the assumption that seminal documents tend to be highly cited. Application of CAB to the field of Anthrax research is presented. While CAB is a highly systematic approach for identifying seminal references, it is not a substitute for the judgment of the researchers, and serves as a supplement.

Multiple asparagine deamidation of Bacillus anthracis protective antigen causes charge isoforms whose complexity correlates with reduced biological activity

Protective antigen is essential for the pathology of Bacillus anthracis and is the proposed immunogen for an improved human anthrax vaccine. Known since discovery to comprise differentially charged isoforms, the cause of heterogeneity has eluded specific structural definition until now. Recombinant protective antigen (rPA) contains similar isoforms that appear early in fermentation and are mostly removed through purification. By liquid chromatography-tandem mass spectrometry sequencing of the entire protein and inspection of spectral data for amino acid modifications, pharmaceutical rPA contained measurable deamidation at seven of its 68 asparagine residues. A direct association between isoform complexity and percent deamidation was observed such that each decreased with purity and increased with protein aging. Position N537 consistently showed the highest level of modification, although its predicted rate of deamidation ranked 10th by theoretical calculation, and other asparagines of higher predicted rates were observed to be unmodified. rPA with more isoforms and greater deamidation displayed lower activities for furin cleavage, heptamerization, and holotoxin formation. Lethal factor-mediated macrophage toxicity correlated inversely with deamidation at residues N466 and N408. The described method measures deamidation without employing theoretical isotopic distributions, comparison between differentially treated samples or computational predictions of reactivity rates, and is broadly applicable to the characterization of other deamidated proteins.

Application of Paramagnetic Beads for Purifying Bacillus anthracis Protective Antigen

Paramagnetic beads coated with Protein G and Tosylactivated-280 dynabeads have been used to purify Bacillus anthracis protective antigen from a liquid culture. The obtained protein was used in the enzyme-linked immunosorbent assay test to detect B. anthracis protective antigen antibodies in human sera collected from immunized individuals. The purification method using paramagnetic beads is very effective. It is fast, easy and may be carried out practically in any laboratory.

Sepsis and pathophysiology of anthrax in a nonhuman primate model

Studies that define natural responses to bacterial sepsis assumed new relevance after the lethal bioterrorist attacks with Bacillus anthracis (anthrax), a spore-forming, toxigenic gram-positive bacillus. Considerable effort has focused on identifying adjunctive therapeutics and vaccines to prevent future deaths, but translation of promising compounds into the clinical setting necessitates an animal model that recapitulates responses observed in humans. Here we describe a nonhuman primate (Papio c. cynocephalus) model of B. anthracis infection using infusion of toxigenic B. anthracis Sterne 34F2 bacteria (5 x 10(5) to 6.5 x 10(9) CFU/kg). Similar to that seen in human patients, we observed changes in vascular permeability, disseminated intravascular coagulation, and systemic inflammation. The lung was a primary target organ with serosanguinous pleural effusions, intra-alveolar edema, and hemorrhagic lesions. This animal model reveals that a fatal outcome is dominated by the host septic response, thereby providing important insights into approaches for treatment and prevention of anthrax in humans.

The LDL receptor-related protein LRP6 mediates internalization and lethality of anthrax toxin

Toxins produced by Bacillus anthracis and other microbial pathogens require functions of host cell genes to yield toxic effects. Here we show that low density lipoprotein receptor-related protein 6 (LRP6), previously known to be a coreceptor for the Wnt signaling pathway, is required for anthrax toxin lethality in mammalian cells. Downregulation of LRP6 or coexpression of a truncated LRP6 dominant-negative peptide inhibited cellular uptake of complexes containing the protective antigen (PA) carrier of anthrax toxin moieties and protected targeted cells from death, as did antibodies against epitopes in the LRP6 extracellular domain. Fluorescence microscopy and biochemical analyses showed that LRP6 enables toxin internalization by interacting at the cell surface with PA receptors TEM8/ATR and/or CMG2 to form a multicomponent complex that enters cells upon PA binding. Our results, which reveal a previously unsuspected biological role for LRP6, identify LRP6 as a potential target for countermeasures against anthrax toxin lethality.

Receptor palmitoylation and ubiquitination regulate anthrax toxin endocytosis

The anthrax toxin is composed of three independent polypeptide chains. Successful intoxication only occurs when heptamerization of the receptor-binding polypeptide, the protective antigen (PA), allows binding of the two enzymatic subunits before endocytosis. We show that this tailored behavior is caused by two counteracting posttranslational modifications in the cytoplasmic tail of PA receptors. The receptor is palmitoylated, and this unexpectedly prevents its association with lipid rafts and, thus, its premature ubiquitination. This second modification, which is mediated by the E3 ubiquitin ligase Cbl, only occurs in rafts and is required for rapid endocytosis of the receptor. As a consequence, cells expressing palmitoylation-defective mutant receptors are less sensitive to anthrax toxin because of a lower number of surface receptors as well as premature internalization of PA without a requirement for heptamerization.

Molecular basis for improved anthrax vaccines

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

Development of high-throughput assay of lethal factor using native substrate

The design of inhibitors for anthrax lethal factor (LF) is currently of interest as an approach for the treatment of anthrax because LF plays a major role in the cytotoxicity of target cells. LF is a zinc-dependent metalloprotease that specifically cleaves the mitogen-activated protein kinase kinase (MKK) family. Current assay systems for the screening of LF inhibitor use the optimized synthetic peptide coupled with various kinds of fluorophores, enabling fast, sensitive, and robust assays suited to high-throughput screening. However, evidence suggests that the regions beside the cleavage site are also involved in specificity and proteolytic activity of LF. In the current study, we tried to develop a high-throughput assay for LF activity based on native substrate, mitogen-activated ERK kinase 1 (MEK1). The assay system relies on the enhanced chemiluminescence signal resulting from a specific antibody against the C-terminal region of native substrate. A glutathione-coated multiwell plate was used as a solid support to immobilize the native substrate by its N-terminal glutathione-S-transferase moiety. Immobilized substrate increases the specificity and sensitivity of LF-catalyzed substrate hydrolysis compared with the solution phase assay. This assay system might be used to discover a wide spectrum of anthrax inhibitors.

Glycerol monolaurate inhibits virulence factor production in Bacillus anthracis

Anthrax, caused by Bacillus anthracis, has been brought to the public's attention because of the 2001 bioterrorism attacks. However, anthrax is a disease that poses agricultural threats in the United States as well as human populations in Europe, China, Africa, and Australia. Glycerol monolaurate (GML) is a compound that has been shown to inhibit exotoxin production by Staphylococcus aureus and other gram-positive bacteria. Here, we study the effects of GML on growth and toxin production in B. anthracis. The Sterne strain of B. anthracis was grown to post-exponential phase with 0-, 10-, 15-, or 20-microg/ml concentrations of GML and then assayed quantitatively for protective antigen (PA) and lethal factor (LF). After 8 h, GML at concentrations greater than 20 microg/ml was bacteriostatic to growth of the organism. However, a 10-microg/ml concentration of GML was not growth inhibitory, but amounts of PA and LF made were greatly reduced. This effect was not global for all proteins when total secreted protein from culture fluids was examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Through quantitative reverse transcription-PCR assays, this toxin-inhibitory effect was shown to occur at the transcriptional level, since amounts of mRNA for pagA (PA), lef (LF), and cya (edema factor) were reduced. Surprisingly, mRNA levels of atxA, a regulator of exotoxin gene expression, rose in the presence of GML. These data will be useful in developing therapeutic tools to treat anthrax disease, whether in animals or humans. These results also suggest that mechanisms of virulence regulation exist independent of atxA.

Immunogenicity in mice of anthrax recombinant protective antigen in the presence of aluminum adjuvants

The only US-licensed anthrax vaccine for human use, as well as several experimental vaccines containing solely purified recombinant protective antigen (rPA), are formulated using aluminum hydroxide (Al(OH)3) as an adjuvant. It has been suggested that effective adjuvanticity of aluminum salts for protein antigens depends, at least partially, on the degree of adsorption of the antigen to the adjuvant. On the other hand, the ease of antigen desorption from the adjuvant in a quantitative fashion may facilitate the assessment of vaccine characteristics in the laboratory. In this regard, aluminum phosphate (AlPO4), although deemed a "weaker" adjuvant than Al(OH)3, appears superior to the latter. To investigate the possibility of formulating rPA vaccines with AlPO4, as well as the significance of the adsorption of this antigen to the aluminum salt for adjuvanticity, we studied the effect of AlPO4 and Al(OH)3 on the induction of anti-rPA antibodies in mice. In a first immunization experiment the adjuvanticity of AlPO4 combined with rPA was examined. Antibodies against rPA were measured using an ELISA. Results indicated that AlPO4 is able to significantly increase the antibody response to rPA, irrespective of its degree of adsorption to the adjuvant. Based on these results, in a second experiment mice were immunized twice, with different formulations of rPA containing either AlPO4 or Al(OH)3, and rPA-antibodies were measured using ELISA and an in vitro toxin neutralization assay. Comparable immune responses to rPA were obtained with both aluminum salts. Additionally, results with AlPO4 as adjuvant confirmed that, in this mouse model, binding of the protein to the adjuvant is not essential for adjuvanticity, whereas the amount of adjuvant has an influence on the antibody response induced.

The detection of protective antigen (PA) associated with spores of Bacillus anthracis and the effects of anti-PA antibodies on spore germination and macrophage interactions

The protective antigen (PA) component of the anthrax toxins is an essential virulence factor of Bacillus anthracis and is the major protective immunogen. The kinetics of PA production during growth of B. anthracis, and the roles of anti-PA antibody in host immunity are not clearly defined. Production of PA by the vegetative organisms peaks during the shift from exponential to stationary phase of growth. Recently, PA was also found to be associated with spores. In our study, PA-specific mRNA was detected in spores by RT-PCR within 15-min of exposure to germinant. PA protein was detected by immunomagnetic electrochemiluminescence (ECL) on spores within 1 h of exposure to a germination medium and was rapidly released into the supernatant. PA was not demonstrated on ungerminated spores by RNA analysis, ECL, or spore-based anti-PA ELISA; however, it was detected on ungerminated spores by immunoelectron microscopy (immunoem). In rabbits, PA induces polyclonal antibodies (Abs) that, in addition to their anti-toxin neutralizing activities, exhibit anti-spore activities. In this study, the anti-spore effects of a human monoclonal Ab specific for PA (AVP-hPA mAb, Avanir Pharmaceuticals) were characterized. AVP-hPA mAb retarded germination in vitro, and enhanced the phagocytic and sporicidal activities of macrophages. The activities were comparable to those of the polyclonal rabbit anti-rPA Ab. Assays to detect germination inhibitory activity (GIA) in serum from vaccinated mice and guinea pigs suggested a possible role for anti-PA Abs in protection. Thus, anti-PA Ab-mediated, anti-spore activities may play a role in protection during the early stages of an anthrax infection.

Enhancement of anthrax lethal toxin cytotoxicity: a subset of monoclonal antibodies against protective antigen increases lethal toxin-mediated killing of murine macrophages

We investigated the ability of using monoclonal antibodies (MAbs) against anthrax protective antigen (PA), an anthrax exotoxin component, to modulate exotoxin cytotoxic activity on target macrophage cell lines. Anthrax PA plays a critical role in the pathogenesis of Bacillus anthracis infection. PA is the cell-binding component of the two anthrax exotoxins: lethal toxin (LeTx) and edema toxin. Several MAbs that bind the PA component of LeTx are known to neutralize LeTx-mediated killing of target macrophages. Here we describe for the first time an overlooked population of anti-PA MAbs that, in contrast, function to increase the potency of LeTx against murine macrophage cell lines. The results support a possible mechanism of enhancement: binding of MAb to PA on the macrophage cell surface stabilizes the PA by interaction of MAb with macrophage Fcgamma receptors. This results in an increase in the amount of PA bound to the cell surface, which in turn leads to an enhancement in cell killing, most likely due to increased internalization of LF. Blocking of PA-receptor binding eliminates enhancement by MAb, demonstrating the importance of this step for the observed enhancement. The additional significance of these results is that, at least in mice, immunization with PA appears to elicit a poly-clonal response that has a significant prevalence of MAbs that enhance LeTx-mediated killing in macrophages.

Alanine-scanning mutations in domain 4 of anthrax toxin protective antigen reveal residues important for binding to the cellular receptor and to a neutralizing monoclonal antibody

A panel of variants with alanine substitutions in the small loop of anthrax toxin protective antigen domain 4 was created to determine individual amino acid residues critical for interactions with the cellular receptor and with a neutralizing monoclonal antibody, 14B7. Substituted protective antigen proteins were analyzed by cellular cytotoxicity assays, and their interactions with antibody were measured by plasmon surface resonance and analytical ultracentrifugation. Residue Asp683 was the most critical for cell binding and toxicity, causing an approximately 1000-fold reduction in toxicity, but was not a large factor for interactions with 14B7. Substitutions in residues Tyr681, Asn682, and Pro686 also reduced toxicity significantly, by 10-100-fold. Of these, only Asn682 and Pro686 were also critical for interactions with 14B7. However, residues Lys684, Leu685, Leu687, and Tyr688 were critical for 14B7 binding without greatly affecting toxicity. The K684A and L685A variants exhibited wild type levels of toxicity in cell culture assays; the L687A and Y688A variants were reduced only 1.5- and 5-fold, respectively.

Production and proteolytic assay of lethal factor from Bacillus anthracis

Bacillus anthracis is the causative agent of anthrax. The major virulence factors are a poly-D-glutamic acid capsule and three-protein component exotoxin, protective antigen (PA, 83 kDa), lethal factor (LF, 90 kDa), and edema factor (EF, 89 kDa), respectively. These three proteins individually have no known toxic activities, but in combination with PA form two toxins (lethal toxin or edema toxin), causing different pathogenic responses in animals and cultured cells. In this study, we constructed and produced rLF as a form of GST fusion protein in Escherichia coli. rLF was rapidly purified through a single affinity purification step to near homogeneity. Furthermore, we developed an in vitro immobilized proteolytic assay of LF under the condition containing full-length native substrate, MEK1, rather than short synthetic peptide. The availability of full-length substrate and of an immobilized LF assay could facilitate not only the in-depth investigation of structure-function relationship of the enzyme toward its substrate but also wide spectrum screening of inhibitor collections based on the 96-well plate system.

Cell surface tumor endothelium marker 8 cytoplasmic tail-independent anthrax toxin binding, proteolytic processing, oligomer formation, and internalization

The interaction of anthrax toxin protective antigen (PA) and target cells was assessed, and the importance of the cytosolic domain of tumor endothelium marker 8 (TEM8) in its function as a cellular receptor for PA was evaluated. PA binding and proteolytic processing on the Chinese hamster ovary cell surface occurred rapidly, with both processes nearly reaching steady state in 5 min. Remarkably, the resulting PA63 fragment was present on the cell surface only as an oligomer, and furthermore, the oligomer was the only PA species internalized, suggesting that oligomerization of PA63 triggers receptor-mediated endocytosis. Following internalization, the PA63 oligomer was rapidly and irreversibly transformed to an SDS/heat-resistant form, in a process requiring an acidic compartment. This conformational change was functionally correlated with membrane insertion, channel formation, and translocation of lethal factor into the cytosol. To explore the role of the TEM8 cytosolic tail, a series of truncated TEM8 mutants was transfected into a PA receptor-deficient Chinese hamster ovary cell line. Interestingly, all of the cytosolic tail truncated TEM8 mutants functioned as PA receptors, as determined by PA binding, processing, oligomer formation, and translocation of an lethal factor fusion toxin into the cytosol. Moreover, cells transfected with a TEM8 construct truncated before the predicted transmembrane domain failed to bind PA, demonstrating that residues 321-343 are needed for cell surface anchoring. Further evidence that the cytosolic domain plays no essential role in anthrax toxin action was obtained by showing that TEM8 anchored by a glycosylphosphatidylinositol tail also functioned as a PA receptor.

Anthrax toxin triggers endocytosis of its receptor via a lipid raft-mediated clathrin-dependent process

The protective antigen (PA) of the anthrax toxin binds to a cell surface receptor and thereby allows lethal factor (LF) to be taken up and exert its toxic effect in the cytoplasm. Here, we report that clustering of the anthrax toxin receptor (ATR) with heptameric PA or with an antibody sandwich causes its association to specialized cholesterol and glycosphingolipid-rich microdomains of the plasma membrane (lipid rafts). We find that although endocytosis of ATR is slow, clustering it into rafts either via PA heptamerization or using an antibody sandwich is necessary and sufficient to trigger efficient internalization and allow delivery of LF to the cytoplasm. Importantly, altering raft integrity using drugs prevented LF delivery and cleavage of cytosolic MAPK kinases, suggesting that lipid rafts could be therapeutic targets for drugs against anthrax. Moreover, we show that internalization of PA is dynamin and Eps15 dependent, indicating that the clathrin-dependent pathway is the major route of anthrax toxin entry into the cell. The present work illustrates that although the physiological role of the ATR is unknown, its trafficking properties, i.e., slow endocytosis as a monomer and rapid clathrin-mediated uptake on clustering, make it an ideal anthrax toxin receptor.

Anthrax toxins and the host: a story of intimacy

Although the dramatic events of the year 2001 have revitalized the interest in anthrax, research on Bacillus anthracis and its major virulence factors is one of the oldest theme in microbiology and started with the early works of Robert Koch and Louis Pasteur. The anthrax toxins are central to anthrax pathogenesis. They were discovered in the mid-1950s and since then there has been an enormous amount of work to elucidate both the molecular and physiopathological details of their mode of action. In this review, after a brief introduction of B. anthracis, we will focus on the latest findings that concern two aspects of anthrax toxin research: the environmental signals and the molecular mechanisms that regulate toxin synthesis, and the mechanisms of intoxication. We hope to convince the reader that the anthrax toxins are highly specialized determinants of B. anthracis pathogenicity: their synthesis is integrated within a global virulence programme and they target key eukaryotic cell proteins. We conclude with a consideration of the therapeutic perspectives arising from our current knowledge of how the toxins work.

Bacillus anthracis genetics and virulence gene regulation

The Bacillus anthracis genome consists of an approximately 5.3-Mb chromosome and two plasmids, pXO1 (182 kb) and pXO2 (96 kb). Genetic analysis has focused primarily on the structural genes for the anthrax toxin proteins, pagA, lef, and cya, the biosynthetic genes for capsule synthesis, capB, capC, and capA, and a gene associated with depolymerization of capsule, dep. The three toxin genes are located at distinct loci on pXO1, while the cap and dep genes are arranged in an apparent operon on pXO2. Additional genes that may play a role in B. anthracis virulence include the germination operon gerX and the general stress transcription factor sigB. Host-related signals affecting transcription of the toxin and capsule genes include temperature (37 degrees C) and bicarbonate/CO2. The B. anthracis plasmids carry two regulatory genes that share little sequence similarity with regulators in other bacteria. The pXO1-encoded gene atxA positively controls expression of the toxin and capsule genes, and has been implicated in control of other genes of unknown function. atxA mutants are avirulent in mice, and mice infected with atxA-null strains show a decreased immunological response to the toxin proteins. The pXO2-encoded regulator, acpA, shares sequence similarity with atxA. Yet acpA function appears to be restricted to positive control of capsule gene expression. The chromosomal gene abrB, a homologue of a well-characterized B. subtilis transition state regulator, controls growth phase-specific transcription of the toxin genes. Genetic manipulation of B. anthracis can be achieved by using natural means of DNA transfer and by electroporation of recombinant DNAs into B. anthracis. Genetic exchange can occur between B. anthracis strains and between B. anthracis and closely-related species. Although pXO1 and pXO2 are not self-transmissible, these plasmids and others can be transferred by conjugative plasmids originating in B. thuringiensis. Generalized transducing phage that permit inter-species transfer of chromosomal and plasmid DNA have also been described.

Delayed treatment with doxycycline has limited effect on anthrax infection in BLK57/B6 mice

Blk57/B6 mice were infected with LD90 dose of Sterne strain anthrax spores subcutaneously and then treated with doxycycline. Doxycycline at a dose of 1.5mg/kg, by intra-peritoneal injection, protected mice from death when given at the same time as spores. When doxycycline administration was delayed 4h survival is 90%. Delay of 24h increased survival time but had no impact on eventual mortality. When doxycycline was delayed 48h, mortality and time to death were comparable to sham injection. Peritoneal macrophages harvested from Blk57/B6 mice were examined for response to anthrax lethal toxin and are shown to be deficient in their ability to produce TNF-alpha and have increased expression of IL-6 compared to RAW 264.7 murine macrophage cell line. These findings suggest that antibiotic therapy has limited effects following lethal anthrax spore challenge, even when the host is of a phenotype that does not produce TNF-alpha in response to anthrax lethal toxin exposure.

Effect of Bacillus anthracis lethal toxin on human peripheral blood mononuclear cells

Lethal toxin (LeTx) plays a central role in anthrax pathogenesis, however a cytotoxicity of LeTx has been difficult to demonstrate in vitro. No cytolytic effect has been reported for human cells, in contrast to murine cell lines, indicating that cell lysis can not be considered as a marker of LeTx activity. We have recently shown that murine macrophage-like RAW 264.7 cells treated with LeTx or infected with anthrax spores underwent changes typical of apoptotic death. Here we demonstrate that cells from human peripheral blood display a proapoptotic behavior similar to murine cells. TUNEL assay detected a nucleosomal degradation typical of apoptosis in peripheral blood mononuclear cells (PBMC) treated with LeTx. Membrane staining with apoptotic dyes was detected in macrophages derived from monocytes in presence of LeTx. The toxin inhibited production of proinflammatory cytokines in PBMC stimulated with a preparation of Bacillus anthracis cell wall. Infection of PBMC with anthrax spores led to the appearance of a large population of cells stained positively for apoptosis, with a reduced capacity to eliminate spores and vegetative bacteria. The aminopeptidase inhibitor, bestatin, capable of protecting cells from LeTx, restored a bactericidal activity of infected cells. These findings may be explained by LeTx expression within phagocytes and support an important role of LeTx as an early intracellular virulence factor contributing to bacterial dissemination and disease progression.

Efficiency of protection of guinea pigs against infection with Bacillus anthracis spores by passive immunization

The efficacy of passive immunization as a postexposure prophylactic measure for treatment of guinea pigs intranasally infected with Bacillus anthracis spores was evaluated. Antisera directed either against the lethal toxin components (PA or LF) or against a toxinogenic strain (Sterne) were used for this evaluation. All antisera exhibited high enzyme-linked immunosorbent assay titers against the corresponding antigens, high titers of neutralization of cytotoxicity activity in an in vitro mouse macrophages cell line (J774A.1), as well as in vivo neutralization of toxicity when administered either directly to Fisher rats prior to challenge with the lethal toxin or after incubation with the lethal toxin. In these tests, anti-LF antiserum exhibited the highest neutralization efficiency, followed by anti-Sterne and anti-PA. The time dependence and antibody dose necessary for conferring postexposure protection by the various antibodies of guinea pigs infected with 25 50% lethal doses of Vollum spores was examined. Rabbit anti-PA serum was found to be the most effective. Intraperitoneal injections of anti-PA serum given 24 h postinfection protected 90% of the infected animals, whereas anti-Sterne and anti-LF were less effective. These results further emphasizes the importance of anti-PA antibodies in conferring protection against B. anthracis infection and demonstrated the ability of such antibodies to be effectively applied as an efficient postexposure treatment against anthrax disease.

Control of anthrax toxin gene expression by the transition state regulator abrB

Bacillus anthracis produces the anthrax toxin proteins protective antigen (PA), lethal factor (LF), and edema factor (EF) in a growth phase-dependent manner when cultured in liquid medium. Expression of the toxin genes pagA, lef, and cya peaks in late log phase, and steady-state levels of the toxin proteins are highest during the transition into stationary phase. Here we show that an apparent transition state regulator negatively regulates toxin gene expression. We identified two orthologues of the B. subtilis transition state regulator abrB in the B. anthracis genome: one on the chromosome and one on the 182-kb virulence plasmid pXO1. The orthologue located on the chromosome is predicted to encode a 94-amino-acid protein that is 85% identical to B. subtilis AbrB. The hypothetical protein encoded on pXO1 is 41% identical to B. subtilis AbrB but missing 27 amino acid residues from the amino terminus compared to the B. subtilis protein. Deletion of the pXO1-encoded abrB orthologue did not affect toxin gene expression under the conditions tested. However, a B. anthracis mutant in which the chromosomal abrB gene was deleted expressed pagA earlier and at a higher level than the parent strain. Expression of a transcriptional pagA-lacZ fusion in the abrB mutant was increased up to 20-fold during early exponential growth compared to the parent strain and peaked in mid-exponential rather than late exponential phase. In contrast to the strong effect of abrB on pagA expression, lef-lacZ and cya-lacZ expression during early-log-phase growth was increased only two- to threefold in the abrB null mutant. Western hybridization analysis showed increased PA, LF, and EF synthesis by the mutant. As is true in B. subtilis, the B. anthracis abrB gene is negatively regulated by spo0A. Our findings tie anthrax toxin gene expression to the complex network of postexponential phase adaptive responses that have been well studied in B. subtilis.

Rapid purification of recombinant anthrax-protective antigen under nondenaturing conditions

Anthrax-protective antigen is the central moiety of the anthrax toxin complex that mediates the entry of the other two toxin components, lethal factor and edema factor into the cells. It is also the main immunogen of the cell-free vaccine against anthrax. However, in addition to PA, the vaccine contains trace amounts of other culture-derived proteins that contribute to the side effects of the vaccine like pain, edema, erythrema, etc. Thus there is a need to develop high-resolution purification methods to purify PA to homogeneity. In this study we have presented a purification strategy for rapid purification of recombinant protective antigen under nondenaturing conditions, which ensures that not only biological activity but also the conformational integrity of immunological epitopes is well-preserved. The protein was purified to homogeneity in a two-step purification procedure that takes just 6 h for completion. Three milligrams of recombinant protective antigen obtained from 1-liter culture was comparable to B. anthracis protective antigen in terms of functional and biological activity. Moreover, the immunogenicity elicited by the purified protein in mice was also studied. The studies reported here are part of continuing research that aims to provide a safe and efficacious alternative to the current vaccine against anthrax.