Understanding Bacillus anthracis pathogenesis

Roles of macrophages and neutrophils in the early host response to Bacillus anthracis spores in a mouse model of infection

The development of new approaches to combat anthrax requires that the pathogenesis and host response to Bacillus anthracis spores be better understood. We investigated the roles that macrophages and neutrophils play in the progression of infection by B. anthracis in a mouse model. Mice were treated with a macrophage depletion agent (liposome-encapsulated clodronate) or with a neutrophil depletion agent (cyclophosphamide or the rat anti-mouse granulocyte monoclonal antibody RB6-8C5), and the animals were then infected intraperitoneally or by aerosol challenge with fully virulent, ungerminated B. anthracis strain Ames spores. The macrophage-depleted mice were significantly more susceptible to the ensuing infection than the saline-pretreated mice, whereas the differences observed between the neutropenic mice and the saline-pretreated controls were generally not significant. We also found that augmenting peritoneal neutrophil populations before spore challenge did not increase resistance of the mice to infection. In addition, the bacterial load in macrophage-depleted mice was significantly greater and appeared significantly sooner than that observed with the saline-pretreated mice. However, the bacterial load in the neutropenic mice was comparable to that of the saline-pretreated mice. These data suggest that, in our model, neutrophils play a relatively minor role in the early host response to spores, whereas macrophages play a more dominant role in early host defenses against infection by B. anthracis spores.

Strong mucosal and systemic immunities induced by nasal immunization with anthrax protective antigen protein incorporated in liposome-protamine-DNA particles

PURPOSE

The very lengthy and complicated dosing schedule of the current anthrax vaccine adsorbed, which was licensed in the USA for the prevention of cutaneous anthrax infection, calls for the development of an efficacious and easily administrable vaccine to prevent against the most lethal form of anthrax infection, the inhalation anthrax. We propose to develop a nasal anthrax vaccine using anthrax protective antigen (PA) protein carried by liposome-protamine-DNA (LPD) particles.

METHODS

PA was incorporated in LPD particles and nasally dosed to mice. The resulting PA-specific immune response and lethal toxin neutralization activity were measured.

RESULTS

Mice nasally immunized with PA incorporated into LPD particles developed both systemic and mucosal anti-PA responses. The anti-PA immunities induced included the production of anti-PA antibodies (IgG and IgM in the serum and IgA in nasal and lung mucosal secretions) and the proliferation of splenocytes after in vitro stimulation. The anti-PA IgG subtype induced was mainly IgG1. Finally, anthrax lethal toxin neutralization activity was detected both in the serum and in the mucosal secretions.

CONCLUSIONS

The anti-PA immune response induced by nasal PA incorporated in LPD was comparable to that induced by nasal PA adjuvanted with cholera toxin or subcutaneously injected PA adjuvanted with aluminum hydroxide.

Characterization of Bacillus anthracis germinant receptors in vitro

Bacillus anthracis begins its infectious cycle as a metabolically dormant cell type, the endospore. Upon entry into a host, endospores rapidly differentiate into vegetative bacilli through the process of germination, thus initiating anthrax. Elucidation of the signals that trigger germination and the receptors that recognize them is critical to understanding the pathogenesis of B. anthracis. Individual mutants deficient in each of the seven putative germinant receptor-encoding loci were constructed via temperature-dependent, plasmid insertion mutagenesis and used to correlate these receptors with known germinant molecules. These analyses showed that the GerK and GerL receptors are jointly required for the alanine germination pathway and also are individually required for recognition of either proline and methionine (GerK) or serine and valine (GerL) as cogerminants in combination with inosine. The germinant specificity of GerS was refined from a previous study in a nonisogenic background since it was required only for germination in response to aromatic amino acid cogerminants. The gerA and gerY loci were found to be dispensable for recognition of all known germinant molecules. In addition, we show that the promoter of each putative germinant receptor operon, except that of the gerA locus, is active during sporulation. A current model of B. anthracis endospore germination is presented.

Induction of protective immunity to anthrax lethal toxin with a nonhuman primate adenovirus-based vaccine in the presence of preexisting anti-human adenovirus immunity

Prevention or therapy for bioterrorism-associated anthrax infections requires rapidly acting effective vaccines. We recently demonstrated (Y. Tan, N. R. Hackett, J. L. Boyer, and R. G. Crystal, Hum. Gene Ther. 14:1673-1682, 2003) that a single administration of a recombinant serotype 5 adenovirus (Ad) vector expressing anthrax protective antigen (PA) provides rapid protection against anthrax lethal toxin challenge. However, approximately 35 to 50% of humans have preexisting neutralizing antibodies against Ad5. This study assesses the hypothesis that a recombinant adenovirus vaccine based on the nonhuman primate-derived serotype AdC7, against which humans do not have immunity, expressing PA (AdC7PA) will protect against anthrax lethal toxin even in the presence of preexisting anti-Ad5 immunity. Naive and Ad5-immunized BALB/c mice received (intramuscularly) 10(8) to 10(11) particle units (PU) of AdC7PA, Ad5PA (a human serotype Ad5-based vector expressing a secreted form of PA), or AdNull (an Ad5 vector with no transgene). Robust anti-PA immunoglobulin G and neutralizing antibodies were detected by 2 to 4 weeks following administration of AdC7PA to naive or Ad5 preimmunized mice, whereas low anti-PA titers were detected in Ad5-preimmunized mice following administration of Ad5PA. To assess protection in vivo, naive or mice previously immunized against Ad5 were immunized with AdC7PA or Ad5PA and then challenged with a lethal intravenous dose of Bacillus anthracis lethal toxin. Whereas Ad5PA protected naive mice against challenge with B. anthracis lethal toxin, Ad5PA was ineffective in mice that were previously immunized against Ad5. In contrast, AdC7PA functioned effectively not only to protect naive mice but also to protect Ad5-preimmunized mice, with 100% survival after lethal toxin challenge. These data suggest the nonhuman-based vector AdC7PA is an effective vaccine for the development of protective immunity against B. anthracis and importantly functions as a "sero-switch" base for an adenovirus vaccine to function in the context of preexisting anti-Ad immunity.

Adenovirus-based genetic vaccines for biodefense

The robust host responses elicited against transgenes encoded by (E1-)(E3-) adenovirus (Ad) gene transfer vectors can be used to develop Ad-based vectors as platform technologies for vaccines against potential bioterror pathogens. This review focuses on pathogens of major concern as bioterror agents and why Ad vectors are ideal as anti-bioterror vaccine platforms, providing examples from our laboratories of using Ad vectors as vaccines against potential bioterror pathogens and how Ad vectors can be developed to enhance vaccine efficacy in the bioterror war.

Anthrax lethal factor inhibition

The primary virulence factor of Bacillus anthracis is a secreted zinc-dependent metalloprotease toxin known as lethal factor (LF) that is lethal to the host through disruption of signaling pathways, cell destruction, and circulatory shock. Inhibition of this proteolytic-based LF toxemia could be expected to provide therapeutic value in combination with an antibiotic during and immediately after an active anthrax infection. Herein is shown the crystal structure of an intimate complex between a hydroxamate, (2R)-2-[(4-fluoro-3-methylphenyl)sulfonylamino]-N-hydroxy-2-(tetrahydro-2H-pyran-4-yl)acetamide, and LF at the LF-active site. Most importantly, this molecular interaction between the hydroxamate and the LF active site resulted in (i) inhibited LF protease activity in an enzyme assay and protected macrophages against recombinant LF and protective antigen in a cell-based assay, (ii) 100% protection in a lethal mouse toxemia model against recombinant LF and protective antigen, (iii) approximately 50% survival advantage to mice given a lethal challenge of B. anthracis Sterne vegetative cells and to rabbits given a lethal challenge of B. anthracis Ames spores and doubled the mean time to death in those that died in both species, and (iv) 100% protection against B. anthracis spore challenge when used in combination therapy with ciprofloxacin in a rabbit "point of no return" model for which ciprofloxacin alone provided 50% protection. These results indicate that a small molecule, hydroxamate LF inhibitor, as revealed herein, can ameliorate the toxemia characteristic of an active B. anthracis infection and could be a vital adjunct to our ability to combat anthrax.

Proteomic analysis of Bacillus anthracis Sterne vegetative cells

Mass spectrometry and proteomics have found increasing use as tools for the rapid detection of pathogenic bacteria, even when they are in a mixture of non-pathogenic relatives. The success of this technique is greatly augmented by the availability of publicly accessible proteomic databases for specific pathogenic bacteria. To aid proteomic detection analyses for the causative agent of anthrax, we have constructed a comprehensive proteomic catalogue of vegetative Bacillus anthracis Sterne cells using liquid chromatography tandem-mass spectrometry. Proteins were separated by molecular weight or isoelectric point prior to tryptic digestion. Alternatively, the whole protein extract was digested and tryptic peptides were separated by cation exchange chromatography prior to Reverse Phase-LC-MS/MS. The use of three complementary, pre-analytical separation techniques resulted in the identification of 1048 unique proteins, including 694 cytosolic, 153 membrane (including 27 cell wall), and 30 secreted proteins, accounting for 19% of the total predicted proteome. Each identified protein was functionally categorized using the gene attribute database from TIGR CMR. These results provide a large proteomic catalogue of vegetative B. anthracis cells and, coupled with the recent proteomic catalogue of B. anthracis spore proteins, form a thorough summary of proteins expressed in the active and dormant stages of this organism.

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.

Passive Immunotherapy for Anthrax Toxin Mediated by an Adenovirus Expressing an Anti-Protective Antigen Single-Chain Antibody

In the 2001 U.S. bioterror attacks, 33,000 individuals required postexposure prophylaxis, 18 subjects contracted anthrax (11 inhalation, 7 cutaneous), and despite optimal medical therapy, 5 deaths resulted. Rapid protection against anthrax is required in a bioterrorism scenario; this study describes an in vivo gene transfer-based therapy that uses a human adenovirus (Ad)-based vector (AdalphaPAscAb) encoding a single-chain antibody directed against protective antigen (PA), a critical component of Bacillus anthracis lethal toxin. Following AdalphaPAscAb administration to mice, anti-PA single-chain antibody and anti-PA neutralizing activity were detected in serum over a 2-week period. Substantial survival advantage from anthrax lethal toxin was conferred by AdalphaPAscAb following administration from 1 to 14 days prior to toxin challenge, compared to no survival associated with an Ad vector expressing a control single-chain antibody. Passive immunotherapy with an Ad-based vector may be a rapid, convenient approach for protecting a susceptible population against anthrax, including use as an adjunct to antibiotic therapy.

Differentiation of human monocytic cell lines confers susceptibility to Bacillus anthracis lethal toxin

Anthrax lethal toxin (LT) is comprised of protective antigen and lethal factor. Lethal factor enters mammalian cells in a protective antigen-dependent process and cleaves mitogen-activated protein kinase kinases. Although LT has no observable effect on many cell types, it causes necrosis in macrophages derived from certain mouse strains and apoptosis in activated mouse macrophages. In this study, we observed that LT treatment of three different human monocytic cell lines U-937, HL-60 and THP-1 did not induce cell death. Cells did become susceptible to the toxin, however, after differentiation into a macrophage-like state. Treatment with LT resulted in decreased phosphorylation of p38, ERK1/2 and JNK in both undifferentiated and differentiated HL-60 cells, suggesting that the change in susceptibility does not result from differences in toxin delivery or substrate cleavage. Death of differentiated HL-60 cells was accompanied by chromosome condensation and DNA fragmentation, but was not inhibited by the pan-caspase inhibitor Z-VAD-FMK. In addition, we observed that the macrophage differentiation process could be inhibited by LT. Our results indicate that LT-mediated death of mouse and human macrophages may occur through distinct processes and that the differentiation state of human cells can determine susceptibility or resistance to LT.

Rabbit and nonhuman primate models of toxin-targeting human anthrax vaccines

The intentional use of Bacillus anthracis, the etiological agent of anthrax, as a bioterrorist weapon in late 2001 made our society acutely aware of the importance of developing, testing, and stockpiling adequate countermeasures against biological attacks. Biodefense vaccines are an important component of our arsenal to be used during a biological attack. However, most of the agents considered significant threats either have been eradicated or rarely infect humans alive today. As such, vaccine efficacy cannot be determined in human clinical trials but must be extrapolated from experimental animal models. This article reviews the efficacy and immunogenicity of human anthrax vaccines in well-defined animal models and the progress toward developing a rugged immunologic correlate of protection. The ongoing evaluation of human anthrax vaccines will be dependent on animal efficacy data in the absence of human efficacy data for licensure by the U.S. Food and Drug Administration.

Identification and biochemical characterization of two novel collagen binding MSCRAMMs of Bacillus anthracis

Cell wall-anchored proteins play critical roles in the pathogenesis of infections caused by Gram-positive bacteria. Through the analysis of the genome of Bacillus anthracis Ames strain, we identified two novel putative cell wall-anchored proteins, BA0871 and BA5258, which have sequence homology to CNA, a cell wall-anchored collagen adhesin of Staphylococcus aureus. The two proteins have similar domain organization to that of CNA, with typical signal peptide sequences, a non-repetitive A region followed by repeats, and a characteristic cell wall-anchoring region. They are expressed on the surface of B. anthracis. The A regions of the two proteins were predicted to adopt similar structural folds as CNA. Circular dichroism analysis of the recombinant A regions of the two proteins (rBA0871A and rBA5258A) indicate that their secondary structure compositions are similar to those of the A regions of CNA and other cell wall-anchored adhesins. We demonstrate through solid phase binding assays and surface plasmon resonance analyses that rBA0871A and rBA5258A specifically bound type I collagen in a dose-dependent and saturable manner. Their dissociation constants (KD) for collagen are 1.6-3.2 microm for rBA0871A and 0.6-0.9 microm for rBA5258A, respectively. We further demonstrate that BA0871 and BA5258 can mediate cell attachment to collagen when expressed on the surface of a heterologous host bacterium. To our knowledge these are the first two adhesins of B. anthracis described, which may have important implications for our understanding of the pathogenic mechanisms explored by this organism.

Novel repression of the glucocorticoid receptor by anthrax lethal toxin

Death from anthrax has been reported to occur from systemic shock. The lethal toxin (LeTx) is the major effector of anthrax mortality. Although the mechanism of entry of this toxin into cells is well understood, its actions once inside the cell are not as well understood. LeTx is known to cleave and inactivate MAPKKs. We have recently shown that LeTx represses the glucocorticoid receptor (GR) both in vitro and in vivo. This repression is partial and specific, repressing the glucocorticoid, progesterone, and estrogen receptor alpha, but not the mineralocorticoid or estrogen receptor beta. This toxin does not affect GR ligand or DNA binding, and we have suggested that it may function by removing/inactivating one or more of the many cofactors involved in nuclear hormone receptor signaling. Although the precise involvement of this nuclear hormone receptor repression in LeTx toxicity is unknown, examples of blunted HPA axis and glucocorticoid signaling in numerous autoimmune/inflammatory diseases suggest that such repression of critically important receptors could have deleterious effects on health.

Anthrax lethal factor-cleavage products of MAPK (mitogen-activated protein kinase) kinases exhibit reduced binding to their cognate MAPKs

Anthrax lethal toxin is the major cause of death in systemic anthrax. Lethal toxin consists of two proteins: protective antigen and LF (lethal factor). Protective antigen binds to a cell-surface receptor and transports LF into the cytosol. LF is a metalloprotease that targets MKKs [MAPK (mitogen-activated protein kinase) kinases]/MEKs [MAPK/ERK (extracellular-signal-regulated kinase) kinases], cleaving them to remove a small N-terminal stretch but leaving the bulk of the protein, including the protein kinase domain, intact. LF-mediated cleavage of MEK1 and MKK6 has been shown to inhibit signalling through their cognate MAPK pathways. However, the precise mechanism by which this proteolytic cleavage inhibits signal transmission has been unclear. Here we show that the C-terminal LF-cleavage products of MEK1, MEK2, MKK3, MKK4, MKK6 and MKK7 are impaired in their ability to bind to their MAPK substrates, suggesting a common mechanism for the LF-induced inhibition of signalling.

Simulation modeling of anthrax spore dispersion in a bioterrorism incident

Recent events have increased awareness of the risk posed by terrorist attacks. Bacillus anthracis has resurfaced in the 21st century as a deadly agent of bioterrorism because of its potential for causing massive civilian casualties. This analysis presents the results of a computer simulation of the dispersion of anthrax spores in a typical 50-story, high-rise building after an intentional release during a bioterrorist incident. The model simulates aerosol dispersion in the case of intensive, small-scale convection, which equalizes the concentration of anthrax spores over the building volume. The model can be used to predict the time interval required for spore dispersion throughout a building after a terrorist attack in a high-rise building. The analysis reveals that an aerosol release of even a relatively small volume of anthrax spores during a terrorist incident has the potential to quickly distribute concentrations that are infectious throughout the building.

Protective Immunity Evoked Against Anthrax Lethal Toxin After a Single Intramuscular Administration of an Adenovirus-Based Vaccine Encoding Humanized Protective Antigen

Because of the need to develop a vaccine to rapidly protect the civilian population in response to a bioterrorism attack with Bacillus anthracis, we designed AdsechPA, a replication-deficient human serotype 5 adenovirus encoding B. anthracis protective antigen (PA) with codons optimized for expression in mammalian cells. With a single intramuscular administration to mice of 10(9) particle units of AdsechPA, a dose that can be scaled to human use, anti-PA antibodies were evoked more rapidly and at a higher level than with a single administration of the new U.S. military recombinant PA/Alhydrogel vaccine. Importantly, AdsechPA afforded approximately 2.7-fold more protection than the recombinant PA vaccine against B. anthracis lethal toxin challenge 4 weeks after a single vaccination. Even at 11 days postvaccination, AdsechPA provided some survival benefit, whereas the rPA/Alhydrogel vaccine provided none. In the context that equivalent human doses of Ad vectors have already been demonstrated to be safe in humans, a single administration of AdsechPA may provide the means to rapidly protect the civilian population against B. anthracis in response to a bioterrorism attack.

Impairment of dendritic cells and adaptive immunity by anthrax lethal toxin

Anthrax poses a clear and present danger as an agent of biological terrorism. Infection with Bacillus anthracis, the causative agent of anthrax, if untreated can result in rampant bacteraemia, multisystem dysfunction and death. Anthrax lethal toxin (LT) is a critical virulence factor of B. anthracis, which occurs as a complex of protective antigen and lethal factor. Here we demonstrate that LT severely impairs the function of dendritic cells--which are pivotal to the establishment of immunity against pathogens--and host immune responses by disrupting the mitogen-activated protein (MAP) kinase intracellular signalling network. Dendritic cells exposed to LT and then stimulated with lipopolysaccharide do not upregulate co-stimulatory molecules, secrete greatly diminished amounts of proinflammatory cytokines, and do not effectively stimulate antigen-specific T cells in vivo. Furthermore, injections of LT induce a profound impairment of antigen-specific T- and B-cell immunity. These data suggest a role for LT in suppressing host immunity during B. anthracis infections, and represent an immune evasion strategy, where a microbe targets MAP kinases in dendritic cells to disarm the immune response.

Macrophage-mediated germination of Bacillus anthracis endospores requires the gerH operon

The gerHABC operon of Bacillus anthracis, encoding a gerA-like family member of germinant sensors, was shown to be required for endospore germination in the presence of macrophages and in macrophage-conditioned media. The loss of the germination phenotype in macrophage cultures of B. anthracis gerH-null endospores was restored by complementation in trans with a wild-type copy of gerH expressed under the control of its own promoter. Using endospores from both the parental strain B. anthracis Sterne and an isogenic gerH-null strain, we partially characterized germinants secreted by macrophages into the extracellular medium.

Anthrax lethal factor represses glucocorticoid and progesterone receptor activity

We report here that a bacterial toxin, anthrax lethal toxin (LeTx), at very low concentrations represses glucocorticoid receptor (GR) transactivation in a transient transfection system and the activity of an endogenous GR-regulated gene in both a cellular system and an animal model. This repression is noncompetitive and does not affect ligand binding or DNA binding, suggesting that anthrax lethal toxin (LeTx) probably exerts its effects through a cofactor(s) involved in the interaction between GR and the basal transcription machinery. LeTx-nuclear receptor repression is selective, repressing GR, progesterone receptor B (PR-B), and estrogen receptor alpha (ERalpha), but not the mineralocorticoid receptor (MR) or ERbeta. GR repression was also caused by selected p38 mitogen-activated protein (MAP) kinase inhibitors, suggesting that the LeTx action may result in part from its known inactivation of MAP kinases. Simultaneous loss of GR and other nuclear receptor activities could render an animal more susceptible to lethal or toxic effects of anthrax infection by removing the normally protective antiinflammatory effects of these hormones, similar to the increased mortality seen in animals exposed to both GR antagonists and infectious agents or bacterial products. These finding have implications for development of new treatments and prevention of the toxic effects of anthrax.

Anaerobic induction of Bacillus anthracis hemolytic activity

A number of genes in Bacillus anthracis encode for proteins homologous to the membrane-damaging factors known as pathogenic determinants in different bacteria. B. anthracis, however, has been traditionally considered non-hemolytic, and the recently identified hemolytic genes have been suggested to be transcriptionally silent. We found that the hemolytic genes of B. anthracis, collectively designated as anthralysins (Anls), could be induced in strict anaerobic conditions. We also demonstrate that Anl genes are expressed at the early stages of infection within macrophages by vegetating bacilli after spore germination. Cooperative and synergistic enhancement of the pore-forming and phospholipase C (PLC) activities of the Anls was found in hemolytic tests on human, but not sheep, red blood cells (RBC). These findings imply Anls as B. anthracis pathogenic determinants and highlight oxygen limitation as environmental factor controlling their expression at both early and late stages of infection.

Bacillus anthracis: current knowledge in relation to contamination of food

In this article, information related to anthrax and its etiologic agent, Bacillus anthracis, in food is reviewed. The major topics discussed include the taxonomic relationship of B. anthracis to other Bacillus species, methods used for the recovery of the organism from surfaces and foods, routes of infection, the pathogenesis of the organism, the microbial ecology of the vegetative cell and spore in foods and the environment, chemical and physical treatments for spore inactivation, and the control of the disease in animals.

Search for potential vaccine candidate open reading frames in the Bacillus anthracis virulence plasmid pXO1: in silico and in vitro screening

A genomic analysis of the Bacillus anthracis virulence plasmid pXO1, aimed at identifying potential vaccine candidates and virulence-related genes, was carried out. The 143 previously defined open reading frames (ORFs) (R. T. Okinaka, K. Cloud, O. Hampton, A. R. Hoffmaster, K. K. Hill, P. Keim, T. M. Koehler, G. Lamke, S. Kumano, J. Mahillon, D. Manter, Y. Martinez, D. Ricke, R. Svensson, and P. J. Jackson, J. Bacteriol. 181:6509-6515, 1999) were subjected to extensive sequence similarity searches (with the nonredundant and unfinished microbial genome databases), as well as motif, cellular location, and domain analyses. A comparative genomics analysis was conducted with the related genomes of Bacillus subtilis, Bacillus halodurans, and Bacillus cereus and the pBtoxis plasmid of Bacillus thuringiensis var. israeliensis. As a result, the percentage of ORFs with clues about their functions increased from approximately 30% (as previously reported) to more than 60%. The bioinformatics analysis permitted identification of novel genes with putative relevance for pathogenesis and virulence. Based on our analyses, 11 putative proteins were chosen as targets for functional genomics studies. A rapid and efficient functional screening method was developed, in which PCR-amplified full-length linear DNA products of the selected ORFs were transcribed and directly translated in vitro and their immunogenicities were assessed on the basis of their reactivities with hyperimmune anti-B. anthracis antisera. Of the 11 ORFs selected for analysis, 9 were successfully expressed as full-length polypeptides, and 3 of these were found to be antigenic and to have immunogenic potential. The latter ORFs are currently being evaluated to determine their vaccine potential.

Macrophage-enhanced germination of Bacillus anthracis endospores requires gerS

Germination of Bacillus anthracis Sterne and plasmidless Delta-Sterne endospores was dramatically enhanced in RAW264.7 macrophage-like cells, while germination of nonpathogenic Bacillus endospores was not. Elimination of gerS, a germinant receptor locus, caused a complete loss of cell-enhanced germination, implicating gerS in the breaking of endospore dormancy in vivo.

In-vitro characterisation of the phagocytosis and fate of anthrax spores in macrophages and the effects of anti-PA antibody

Antibodies (Abs) to the protective antigen (PA) component of the anthrax toxins have anti-spore as well as anti-toxin activities. Anti-PA antisera and purified anti-PA Abs enhance the phagocytosis by murine-derived macrophages (MQs) of spores of the Ames and Sterne strains and retard the germination of extracellular spores in vitro. The fate after phagocytosis of untreated and anti-PA-treated spores was further studied in culture medium that supported phagocytosis without stimulating spore germination (Dulbecco's minimal essential medium with horse serum 10%). The spores germinated within cells of primary peritoneal murine MQs (C3H/HeN) and MQs of the RAW264.7 MQ-like cell line; germination was associated with a rapid decline in spore viability. Exposure of MQs to inhibitors of phago-endosomal acidification (bafilomycin A and chloroquine) reduced the efficiency of MQ killing and allowed outgrowth and replication of the organisms. Treatment of spores with anti-PA Abs stimulated their phagocytosis and was associated with enhanced MQ killing of the spores. The enhanced killing of spores correlated with the greater extent of germination of anti-PA-treated spores after phagocytosis. A PA null mutant of the Ames strain exhibited none of the effects associated with anti-PA Ab treatment ofthe parental strain. Thus, the anti-PA Ab-specific immunity induced by vaccines has anti-spore activities and its role in impeding the early stages of infection with Bacillusanthracis needs to be assessed.

Macrophage apoptosis by anthrax lethal factor through p38 MAP kinase inhibition

The bacterium Bacillus anthracis causes the death of macrophages, which may allow it to avoid detection by the innate immune system. We found that B. anthracis lethal factor (LF) selectively induces apoptosis of activated macrophages by cleaving the amino-terminal extension of mitogen-activated protein kinase (MAPK) kinases (MKKs) that activate p38 MAPKs. Because macrophages that are deficient in transcription factor nuclear factor kappaB (NF-kappaB) are also sensitive to activation-induced death and p38 is required for expression of certain NF-kappaB target genes, p38 is probably essential for synergistic induction of those NF-kappaB target genes that prevent apoptosis of activated macrophages. This dismantling of the p38 MAPK module represents a strategy used by B. anthracis to paralyze host innate immunity.

The alveolar macrophage: the Trojan horse of Bacillus anthracis

Bacillus anthracis, the causative agent of anthrax, has a particular strategy for invading the host and crossing the alveolar barrier. B. anthracis survives within alveolar macrophages, after germination within the phagolysosome, then enters the external medium where it proliferates. Recent data have shown that edema toxin and lethal toxin are the major genetic determinants mediating the survival of germinated spores within macrophages. Here, recent advances in the analysis of B. anthracis pathogenesis are summarized and future challenges discussed.

A two-component regulator induces the transmission phenotype of stationary-phase Legionella pneumophila

Pathogenic Legionella pneumophila evolved as a parasite of aquatic amoebae. To persist in the environment, the microbe must be proficient at both replication and transmission. In laboratory cultures, as nutrients become scarce a stringent response-like pathway coordinates exit from the exponential growth phase with induction of traits correlated with virulence, including motility. A screen for mutants that express the flagellin gene poorly identified five activators of virulence: LetA/LetS, a two-component regulator homologous to GacA/GacS of Pseudomonas and SirA/BarA of Salmonella; the stationary-phase sigma factor RpoS; the flagellar sigma factor FliA; and a new locus, letE. Unlike wild type, post-exponential-phase letA and letS mutants were not motile, cytotoxic, sodium sensitive or proficient at infecting macrophages. L. pneumophila also required fliA to become motile, cytotoxic and to infect macrophages efficiently and letE to express sodium sensitivity and maximal motility and cytotoxicity. When induced to express RelA, all of the strains exited the exponential phase, but only wild type converted to the fully virulent form. In contrast, intracellular replication was independent of letA, letS, letE or fliA. Together, the data indicate that, as the nutrient supply wanes, ppGpp triggers a regulatory cascade mediated by LetA/ LetS, RpoS, FliA and letE that coordinates differentiation of replicating L. pneumophila to a transmissible form.

TNF receptor 1, IL-1 receptor, and iNOS genetic knockout mice are not protected from anthrax infection

Anthrax produces at least two toxins that cause an intense systemic inflammatory response, edema, shock, and eventually death. The relative contributions of various elements of the immune response to mortality and course of disease progression are poorly understood. We hypothesized that knockout mice missing components of the immune system will have an altered response to infection. Parent strain mice and knockouts were challenged with LD95 of anthrax spores (5 x 10(6)) administered subcutaneously. Our results show that all genetic knockouts succumbed to anthrax infection at the same frequency as the parent. TNF antibody delayed death but TNF receptor 1 knockout had no effect. IL-1 receptor or iNOS knockouts died sooner. Anthrax was more abundant in the injection site of TNF-alpha and iNOS knockouts compared to parent suggesting that attenuated cellular response increases rate of disease progression. With the exception of edema and necrosis at the injection site pathological changes in internal organs were not observed.

Passive transfer of protection against Bacillus anthracis infection in a murine model

Passive transfer of lymphocytes and sera from mice immunised using two different formulations containing recombinant protective antigen (rPA) have been used to further elucidate the mechanism of protection against Bacillus anthracis infection. The results demonstrated that an antibody response maybe important in protection against B. anthracis infection, under the conditions tested. The results provide further data for the development of an improved anthrax vaccine.

The role of antibodies to Bacillus anthracis and anthrax toxin components in inhibiting the early stages of infection by anthrax spores

Vaccines which are efficacious against anthrax, such as the human vaccine, Anthrax Vaccine Absorbed (AVA), contain the protective antigen (PA) component of the anthrax toxins as the major protective immunogen. Although AVA protects against inhalational anthrax, the immune responses to and role in protection of PA and possibly other antigens have yet to be fully elucidated. Sera from animals immunized with a toxin-producing, unencapsulated live vaccine strain of Bacillus anthracis have been reported to have anti-spore activities associated with the antitoxin humoral response. The authors performed studies to determine whether anti-PA antibody (Ab)-containing preparations stimulated spore uptake by phagocytes and suppressed the germination of spores in vitro. AVA- and PA-immune sera from several species enhanced the phagocytosis by murine peritoneal macrophages of spores of the virulent Ames and the Sterne vaccine strains. Antitoxin Abs appeared to contribute significantly, although not solely, to the enhanced uptake. Rabbit antisera to PA purified from either Sterne or a PA-producing pX01-cured recombinant, affinity-purified anti-PA IgG, and monkey antisera to AVA were used to assess the role of anti-PA ABS: Rabbit anti-PA Abs promoted the uptake of spores of the PA-producing strains Sterne, Ames and RP42, a mutant of Sterne producing only PA, but not of the pX01-Sterne-1 strain, Ames strain, or RP4, a mutant of Sterne with deletions in the loci encoding PA and the oedema factor (EF) toxin component and producing only the lethal factor toxin component. Rabbit anti-PA and monkey anti-AVA Abs also significantly inhibited spore germination in vitro compared to preimmune serum or medium. Spore-associated proteins recognized by anti-PA Abs were detected by electron microscopy and confirmed by immunoblotting of spore coat extracts. Thus, the anti-PA Ab-specific immunity induced by AVA has anti-spore activity and might have a role in impeding the early stages of infection with B. anthracis spores.

Constitutive expression of protective antigen gene of Bacillus anthracis in Escherichia coli

The fatal bacterial infection caused by inhalation of the Bacillus anthracis spores results from the synthesis of protein toxins-protective antigen (PA), lethal factor (LF), and edema factor (EF)--by the bacterium. PA is the target-cell binding protein and is common to the two effector molecules, LF and EF, which exert their toxic effects once they are translocated to the cytosol by PA. PA is the major component of vaccines against anthrax since it confers protective immunity. The large-scale production of recombinant protein-based anthrax vaccines requires overexpression of the PA protein. We have constitutively expressed the protective antigen protein in E. coli DH5alpha strain. We have found no increase in degradation of PA when the protein is constitutively expressed and no plasmid instability was observed inside the expressing cells. We have also scaled up the expression by bioprocess optimization using batch culture technique in a fermentor. The protein was purified using metal-chelate affinity chromatography. Approximately 125 mg of recombinant protective antigen (rPA) protein was obtained per liter of batch culture. It was found to be biologically and functionally fully active in comparison to PA protein from Bacillus anthracis. This is the first report of constitutive overexpression of protective antigen gene in E. coli.

Search for correlates of protective immunity conferred by anthrax vaccine

Vaccination by anthrax protective antigen (PA)-based vaccines requires multiple immunization, underlying the need to develop more efficacious vaccines or alternative vaccination regimens. In spite of the vast use of PA-based vaccines, the definition of a marker for protective immunity is still lacking. Here we describe studies designed to help define such markers. To this end we have immunized guinea pigs by different methods and monitored the immune response and the corresponding extent of protection against a lethal challenge with anthrax spores. Active immunization was performed by a single injection using one of two methods: (i) vaccination with decreasing amounts of PA and (ii) vaccination with constant amounts of PA that had been thermally inactivated for increasing periods. In both studies a direct correlation between survival and neutralizing-antibody titer was found (r(2) = 0.92 and 0.95, respectively). Most significantly, in the two protocols a similar neutralizing-antibody titer range provided 50% protection. Furthermore, in a complementary study involving passive transfer of PA hyperimmune sera to naive animals, a similar correlation between neutralizing-antibody titers and protection was found. In all three immunization studies, neutralization titers of at least 300 were sufficient to confer protection against a dose of 40 50% lethal doses (LD(50)) of virulent anthrax spores of the Vollum strain. Such consistency in the correlation of protective immunity with anti-PA antibody titers was not observed for antibody titers determined by an enzyme-linked immunosorbent assay. Taken together, these results clearly demonstrate that neutralizing antibodies to PA constitute a major component of the protective immunity against anthrax and suggest that this parameter could be used as a surrogate marker for protection.

Bacillus anthracis, a bug with attitude!

The sequencing of the Bacillus anthracis genome and virulence plasmids represents the greatest advance in anthrax research in the past 100 years. The data will provide the foundation of all future work on this organism and will be invaluable to researchers in their battle to understand the basis of the host-microbe interaction.

Early Bacillus anthracis-macrophage interactions: intracellular survival survival and escape

This study describes early intracellular events occurring during the establishment phase of Bacillus anthracis infections. Anthrax infections are initiated by dormant endospores gaining access to the mammalian host and becoming engulfed by regional macrophages (Mphi). During systemic anthrax, late stage events include vegetative growth in the blood to very high titres and the synthesis of the anthrax exotoxin complex, which causes disease symptoms and death. Experiments focus on the early events occurring during the first few hours of the B. anthracis infectious cycle, from endospore germination up to and including release of the vegetative cell from phagocytes. We found that newly vegetative bacilli escape from the phagocytic vesicles of cultured Mphi and replicate within the cytoplasm of these cells. Release from the Mphi occurs 4-6 h after endospore phagocytosis, timing that correlates with anthrax infection of test animals. Genetic analysis from this study indicates that the toxin plasmid pXO1 is required for release from the Mphi, whereas the capsule plasmid pXO2 is not. The transactivator atxA, located on pXO1, is also found to be essential for release, but the toxin genes themselves are not required. This suggests that Mphi release of anthrax bacilli is atxA regulated. The putative 'escape' genes may be located on the chromosome and/or on pXO1.