Germination of Bacillus anthracis spores within alveolar macrophages

Importance of nitric oxide synthase in the control of infection by Bacillus anthracis

The spore-forming, gram-positive bacterium Bacillus anthracis, the causative agent of anthrax, has achieved notoriety due to its use as a bioterror agent. In the environment, B. anthracis exists as a dormant endospore. Upon infection, germination of endospores occurs during their internalization within the phagocyte, and the ability to survive exposure to antibacterial killing mechanisms, such as O2*-, NO*, and H2O2, is a key initial event in the infective process. Macrophages generate NO* from the oxidative metabolism of L-arginine, using an isoform of nitric oxide synthase (NOS 2). Exposure of murine macrophages (RAW264.7 cells) to B. anthracis endospores up-regulated the expression of NOS 2 12 h after exposure, and production of NO* was comparable to that achieved following other bacterial infections. Spore-killing assays demonstrated a NO*-dependent bactericidal response that was significantly decreased in the presence of the NOS 2 inhibitor L-N6-(1-iminoethyl)lysine and in L-arginine-depleted media. Interestingly, we also found that B. anthracis bacilli and endospores exhibited arginase activity, possibly competing with host NOS 2 for its substrate, L-arginine. As macrophage-generated NO* is an important pathway in microbial killing, the ability of endospores of B. anthracis to regulate production of this free radical has important implications in the control of B. anthracis-mediated infection.

The cytotoxic effect of anthrax lethal toxin on human lung cells in vitro and the protective action of bovine antibodies to PA and LF

The excretion of protein toxins by vegetative cells of Bacillus anthracis is critical to the development of the lethal consequences of anthrax, particularly inhalational anthrax. Whilst the lung macrophages and other phagocytic cells transfer the spores from the lung cavities into the lymphatic system, and provide an initial germination site for the proliferation of the vegetative cells, it appears that much of the tissue pathology at the time of the host's death could be due to the action of the toxins, especially lethal toxin-protective antigen (PA) plus lethal factor (LF). The widespread tissue oedema and hypoxia may in part reflect a direct attack by lethal toxin on vascular endothelial cells. Also the distribution of the receptor for PA on a variety of cell types including epithelial cells as well as endothelial cells, and the involvement of the lungs in the pathology raises the question of whether lung epithelial cells are also susceptible to lethal toxin. To investigate this possibility a series of in vitro cytotoxicity experiments were carried out with human lung epithelial cells and microvascular endothelial cells. In these experiments lethal toxin (PA 500 ng ml(-1) plus 10-100 ng ml(-1) LF) was shown to cause a progressive loss of cell viability that developed slowly over at least 3 days. Affinity purified bovine colostrum antibodies for both PA and LF were equally effective in providing a 100% protection for epithelial cells from this cytotoxic action of lethal toxin. This was achieved at a 10:1 molar ratio of the particular antibody to its respective target.

Inhibition of mitogen-activated protein kinase signalling by Bacillus anthracis lethal toxin causes destabilization of interleukin-8 mRNA

Bacillus anthracis must overcome host innate immune defences to establish a systemic anthrax infection. This is facilitated in part by lethal toxin (LT), a secreted virulence factor that consists of a cell-binding moiety, protective antigen (PA), and an enzymatic moiety, lethal factor (LF). PA binds cells through protein receptors and mediates the delivery of LF to the cytosol. LF is a protease that cleaves amino-terminal fragments from mitogen-activated protein kinase kinases (MAPKKs), preventing phosphorylation of their downstream targets. Here we report that LT reduces the amount of interleukin (IL)-8 produced and secreted by human endothelial cells. The reduction of IL-8 levels by LT was not attributable to reduced expression from the IL-8 promoter, but resulted from destabilization of IL-8 mRNA. Destabilization by LT was mediated through the 3' untranslated region of the IL-8 transcript and could be mimicked by pharmacological inhibitors of MAPK pathways. LT diminished the induction of IL-8 mRNA and protein by lipopolysaccharide, indicating that the toxin can impair the ability of these cells to initiate an immune response. Destabilization of a cytokine transcript represents a new interference strategy used by either a bacterial or viral pathogen to reduce cytokine expression and may help B. anthracis to evade host immune defences.

The InhA1 metalloprotease allows spores of the B. cereus group to escape macrophages

Bacteria of the Bacillus cereus group are resistant to the immune systems of various hosts and establish potent infections, implying that bacteria circumvent the bactericidal activity of host phagocytic cells. We investigated the fate of Bacillus spores after their internalization by macrophages. We found that these spores survive and escape from macrophages, and that the bacterial metalloprotease InhA1, the major component of the exosporium, is essential for efficient spore release from macrophages. InhA1 from Bacillus thuringiensis also enables Bacillus subtilis to escape from macrophages. Analysis of membrane permeability showed that the bacteria cause alterations in the macrophage membranes and that InhA1 is involved in these processes. Thus, InhA1 contributes to protect the bacteria against the host immune system. These findings provide further insight into the pathogenicity of B. cereus group members.

The dltABCD operon of Bacillus anthracis sterne is required for virulence and resistance to peptide, enzymatic, and cellular mediators of innate immunity

In the environment, the gram-positive bacterium Bacillus anthracis persists as a metabolically dormant endospore. Upon inoculation into the host the endospores germinate and outgrow into vegetative bacilli able to cause disease. The dramatic morphogenic changes to the bacterium during germination and outgrowth are numerous and include major rearrangement of and modifications to the bacterial surface. Such modifications occur during a time in the B. anthracis infectious cycle when the bacterium must guard against a multitude of innate immune mediators. The dltABCD locus of B. anthracis encodes a cell wall d-alanine esterification system that is initiated by transcriptional activation during endospore outgrowth. The level of transcription from the dltABCD operon determined B. anthracis resistance to cationic antibacterial peptides during vegetative growth and cationic peptide, enzymatic, and cellular mediators of innate immunity during outgrowth. Mutation of dltABCD was also attenuating in a mouse model of infection. We propose that the dltABCD locus is important for protection of endosporeforming bacteria from environmental assault during outgrowth and that such protection may be critical during the establishment phase of anthrax.

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.

Anthrax toxin receptor 2 mediates Bacillus anthracis killing of macrophages following spore challenge

Initiation of inhalation anthrax is believed to involve phagocytosis of Bacillus anthracis spores by alveolar macrophages, followed by spore germination within the phagolysosome. In order to establish a systemic infection, it is predicted that bacilli then escape from the macrophage and replicate extracellularly. Mechanisms utilized by B. anthracis to escape from the macrophage are not well characterized, but a role for anthrax toxin has been proposed. Here we report the isolation of an anthrax toxin-resistant cell line (R3D) following chemical mutagenesis of toxin-sensitive RAW 264.7 murine macrophage cells. Both R3D and RAW 264.7 cells phagocytize spores of a B. anthracis Sterne strain. However, RAW 264.7 cells are killed following spore challenge, whereas R3D cells survive. Resistance to toxin and spore challenge correlates with loss of expression of anthrax toxin receptor 2 (ANTXR2/CMG-2). When R3D cells are complemented with cDNA encoding either murine ANTXR2 or human anthrax toxin receptor 1 (ANTXR1/TEM-8), toxin and spore challenge susceptibility are restored, indicating that over-expression of either ANTXR can confer susceptibility to anthrax spore challenge. Taken together, these results indicate that anthrax toxin expression by the germinated spore enables B. anthracis killing of the macrophage from within.

DNA vaccines against anthrax

DNA vaccination is vaccination at its simplest. Due to renewed interest in vaccination against anthrax and other biothreat agents, a genetic immunisation approach offers attractive possibilities for rapid, responsive vaccine development. DNA vaccination against anthrax is an active area of research showing promising results at present, which in the short-term and in the future could form the basis for new advances in multi-agent vaccine development. The anthrax 'model' constitutes an important experimental system for genetic immunisation technology development.

Murine macrophages kill the vegetative form of Bacillus anthracis

Anti-protective antigen antibody was reported to enhance macrophage killing of ingested Bacillus anthracis spores, but it was unclear whether the antibody-mediated macrophage killing mechanism was directed against the spore itself or the vegetative form emerging from the ingested and germinating spore. To address this question, we compared the killing of germination-proficient (gp) and germination-deficient (DeltagerH) Sterne 34F2 strain spores by murine peritoneal macrophages. While macrophages similarly ingested both spores, only gp Sterne was killed at 5 h (0.37 log kill). Pretreatment of macrophages with gamma interferon (IFN-gamma) or opsonization with immunoglobulin G (IgG) isolated from a subject immunized with an anthrax vaccine enhanced the killing of Sterne to 0.49 and 0.73 log, respectively, but the combination of IFN-gamma and IgG was no better than either treatment alone. Under no condition was there killing of DeltagerH spores. To examine the ability of the exosporium to protect spores from macrophages, we compared the macrophage-mediated killing of nonsonicated (exosporium+) and sonicated (exosporium-) Sterne 34F2 spores. More sonicated spores than nonsonicated spores were killed at 5 h (0.98 versus 0.37 log kill, respectively). Pretreatment with IFN-gamma increased the sonicated spore killing to 1.39 log. However, the opsonization with IgG was no better than no treatment or pretreatment with IFN-gamma. We conclude that macrophages appear unable to kill the spore form of B. anthracis and that the exosporium may play a role in the protection of spores from macrophages.

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.

Human neutrophils kill Bacillus anthracis

Bacillus anthracis spores cause natural infections and are used as biological weapons. Inhalation infection with B. anthracis, the etiological agent of anthrax, is almost always lethal, yet cutaneous infections usually remain localized and resolve spontaneously. Neutrophils are typically recruited to cutaneous but seldom to other forms of anthrax infections, raising the possibility that neutrophils kill B. anthracis. In this study we infected human neutrophils with either spores or vegetative bacteria of a wild-type strain, or strains, expressing only one of the two major virulence factors. The human neutrophils engulfed B. anthracis spores, which germinated intracellularly and were then efficiently killed. Interestingly, neutrophil killing was independent of reactive oxygen species production. We fractionated a human neutrophil granule extract by high-performance liquid chromatography and identified α-defensins as the component responsible for B. anthracis killing. These data suggest that the timely recruitment of neutrophils can control cutaneous infections and possibly other forms of B. anthracis infections, and that α-defensins play an important role in the potent anti-B. anthracis activity of neutrophils.

Bacillus anthracis is the bacterium that causes anthrax, a disease that can occur through natural infections and also through intentional release. B. anthracis makes spores, which are in a dormant state, similar to seeds of a plant, and are extremely resistant to the environment. B. anthracis spores can infect through the skin or the lung. Lung infections disseminate through the body and are lethal. In contrast, skin infections often remain localized, and patients survive even without treatment. It is not well understood why these bacteria cause a localized infection through the skin and a lethal disease through the lung.

Little is known about how B. anthracis is controlled. Neutrophils are the first white blood cells recruited to a site of infection and are specialized in killing microbes. Previous studies show that neutrophils are abundant in the skin form, but not in the lung form of anthrax. The researchers report that human neutrophils can take up B. anthracis spores. Once inside, the spores germinate to form vegetative bacteria. The vegetative bacteria are extremely susceptible to neutrophil-killing mechanisms. The B. anthracis virulence factors (molecules that make bacteria cause diseases) manipulate other human cells but do not deter neutrophils. B. anthracis is indeed exquisitely sensitive to the neutrophil protein α-defensin. These data support a new model where B. anthracis skin, but not lung, infections are controlled by the antimicrobial activity of neutrophils.

The crystal structure of the Bacillus anthracis spore surface protein BclA shows remarkable similarity to mammalian proteins

The lethal disease anthrax is propagated by spores of Bacillus anthracis, which can penetrate into the mammalian host by inhalation, causing a rapid progression of the disease and a mostly fatal outcome. We have solved the three-dimensional structure of the major surface protein BclA on B. anthracis spores. Surprisingly, the structure resembles C1q, the first component of complement, despite there being no sequence homology. Although most assays for C1q-like activity, including binding to C1q receptors, suggest that BclA does not mimic C1q, we show that BclA, as well as C1q, interacts with components of the lung alveolar surfactant layer. Thus, to better recognize and invade its hosts, this pathogenic soil bacterium may have evolved a surface protein whose structure is strikingly close to a mammalian protein.

Imaging and analysis of Bacillus anthracis spore germination

External and internal changes occurring during the process of germination of Bacillus anthracis spores were observed through atomic force microscopy (AFM) and transmission electron microscopy (TEM), respectively. AFM studies showed that in response to L-alanine (4 mM), as a germinant, the spore germinates into a vegetative cell in 3 hours. The temporal size changes occurring during the germination were gradual but the major change in size was observed between the second and third hour. TEM of spores showed the presence of varied layers, which is in accordance with previous studies. However, the integrity of these layers was lost gradually during the process of germination. The inner spore membrane remains intact even until late stages of germination, whereas the coat, outer spore membrane, and the cortical layers are discarded at the second-hour stage. The results indicate that sequential changes during the germination of a B. anthracis spore are similar to other species of the Bacillus group.

The solute-binding component of a putative Mn(II) ABC transporter (MntA) is a novelBacillus anthracisvirulence determinant

Here we describe the characterization of a lipoprotein previously proposed as a potential Bacillus anthracis virulence determinant and vaccine candidate. This protein, designated MntA, is the solute-binding component of a manganese ion ATP-binding cassette transporter. Coupled proteomic-serological screen of a fully virulent wild-type B. anthracis Vollum strain, confirmed that MntA is expressed both in vitro and during infection. Expression of MntA is shown to be independent of the virulence plasmids pXO1 and pXO2. An mntA deletion, generated by allelic replacement, results in complete loss of MntA expression and its phenotypic analysis revealed: (i) impaired growth in rich media, alleviated by manganese supplementation; (ii) increased sensitivity to oxidative stress; and (iii) delayed release from cultured macrophages. The DeltamntA mutant expresses the anthrax-associated classical virulence factors, lethal toxin and capsule, in vitro as well as in vivo, and yet the mutation resulted in severe attenuation; a 10(4)-fold drop in LD(50) in a guinea pig model. MntA expressed in trans allowed to restore, almost completely, the virulence of the DeltamntA B. anthracis strain. We propose that MntA is a novel B. anthracis virulence determinant essential for the development of anthrax disease, and that B. anthracisDeltamntA strains have the potential to serve as platform for future live attenuated vaccines.

The use of bacterial spore formers as probiotics: Table 1

The field of probiosis has emerged as a new science with applications in farming and aqaculture as alternatives to antibiotics as well as prophylactics in humans. Probiotics are being developed commercially for both human use, primarily as novel foods or dietary supplements, and in animal feeds for the prevention of gastrointestinal infections, with extensive use in the poultry and aquaculture industries. The impending ban of antibiotics in animal feed, the current concern over the spread of antibiotic resistance genes, the failure to identify new antibiotics and the inherent problems with developing new vaccines make a compelling case for developing alternative prophylactics. Among the large number of probiotic products in use today are bacterial spore formers, mostly of the genus Bacillus. Used primarily in their spore form, these products have been shown to prevent gastrointestinal disorders and the diversity of species used and their applications are astonishing. Understanding the nature of this probiotic effect is complicated, not only because of the complexities of understanding the microbial interactions that occur within the gastrointestinal tract (GIT), but also because Bacillus species are considered allochthonous microorganisms. This review summarizes the commercial applications of Bacillus probiotics. A case will be made that many Bacillus species should not be considered allochthonous microorganisms but, instead, ones that have a bimodal life cycle of growth and sporulation in the environment as well as within the GIT. Specific mechanisms for how Bacillus species can inhibit gastrointestinal infections will be covered, including immunomodulation and the synthesis of antimicrobials. Finally, the safety and licensing issues that affect the use of Bacillus species for commercial development will be summarized, together with evidence showing the growing need to evaluate the safety of individual Bacillus strains as well as species on a case by case by basis.

Importance of srtA and srtB for growth of Bacillus anthracis in macrophages

We examined the effect of mutation of two sortase genes of Bacillus anthracis, srtA and srtB, on the ability of the bacterium to grow in J774A.1 cells, a mouse macrophage-like cell line. While disruption of either srtA or srtB had no effect on the ability of the bacteria to grow in rich culture media, mutations in each of these genes dramatically attenuated growth of the bacterium in J774A.1 cells. Complementation of the mutation restored the ability of bacteria to grow in the cells. Since the initial events in inhalation anthrax are believed to be uptake of B. anthracis spores by alveolar macrophages followed by germination of the spores and growth of the bacteria within the macrophages, these results suggest that two sortases of B. anthracis may be critical in the early stages of inhalation anthrax.

Identification of a second collagen-like glycoprotein produced by Bacillus anthracis and demonstration of associated spore-specific sugars

Certain carbohydrates (rhamnose, 3-O-methyl rhamnose, and galactosamine) have been demonstrated to be present in Bacillus anthracis spores but absent in vegetative cells. Others have demonstrated that these spore-specific sugars are constituents of the glycoprotein BclA. In the current work, spore extracts were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A second collagen-like glycoprotein, BclB, was identified in B. anthracis. The protein moiety of this glycoprotein was identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MS) and the carbohydrate components by gas chromatography-mass spectrometry and tandem mass spectrometry. Spore-specific sugars were also demonstrated to be components of BclB.

Analysis of epidermal entry in experimental cutaneous Bacillus anthracis infections in mice

Cutaneous infection is the most common form of human anthrax, but little is known of Bacillus anthracis-epidermal interactions. To study the latter, we used experimental inoculations of B. anthracis Sterne spores onto mouse flank skin. In DBA/2 mice (a sensitive strain) 10(7) spores injected intradermally or applied under occlusive dressings to abraded skin produced ipsilateral inguinal edema and rapid death. Epicutaneous application to shaved-only skin produced edema and death in most animals, but at longer times. Mortality after inoculation onto abraded skin was less in C57BL/6 mice (a relatively resistant strain). Inoculations onto shaved-only skin immunized C57BL/6 mice, and they survived later intradermal spore injections. Histology revealed massive organism proliferation in remaining epidermis and hair follicles of inoculated abraded skin, but less growth in the dermis itself. Conversely, no foci could be located by microscopic examination after inoculation onto shaved-only skin. High-dose nonocclusive dressing inoculations onto unshaved skin in DBA/2 mice revealed small numbers of infective foci, all in hair follicles. These results suggest that epidermal damage may increase infection susceptibility to B. anthracis of hair follicle contents and remaining epidermal remnants; the findings also indicate that access may occur through hair follicles and the denuded dermis.

Germination process of Bacillus anthracis endospores within macrophage RAW264.7

AIM: To confirm the germination process of Bacillus anthracis A16R endospores within murine macrophage RAW264.7

METHODS: Macrophage RAW264.7 cells were infected by Bacillus anthrax A16R (pXO2-) spores at a multiplicity of infection (MOI) of 20∶1. Then the cells were harvested at different time points (1, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5 and 8 h after infection). The growth of infected cells was observed under light microscope by fuchsin basic methylene blue staining.

RESULTS: The endospores began to germinate and develop into vegetative bodies 2 to 2.5 h after infection. The vegetative bodies entered the phase of binary fission 3.5 to 4 h after infection. At 5 to 5.5 h, the bacillus proliferated into exponential phase andthe macrophages began to lyse 7 to 8 h after infected.

CONCLUSION: For the fisrt time, fuchsin basic methylene blue staining is used to study the germination process of Bacillus anthracis endospores within macrophage RAW264.7.

Role of superoxide in the germination of Bacillus anthracis endospores

The spore forming Gram-positive bacterium Bacillus anthracis, the causative agent of anthrax, has achieved notoriety due to its use as a bioterror agent. In the environment, B. anthracis exists as a dormant endospore. Germination of endospores during their internalization within the myeloid phagocyte, and the ability of those endospores to survive exposure to antibacterial killing mechanisms such as superoxide (O(2)*-, is a key initial event in the infective process. We report herein that endospores exposed to fluxes of O(2)*- typically found in stimulated phagocytes had no effect on viability. Further endospores of the Sterne strain of B. anthracis were found to scavenge O(2)*-, which may enhance the ability of the bacterium to survive within the hostile environment of the phagolysosome. Most intriguing was the observation that endospore germination was stimulated by a flux of O(2)*- as low as 1 microM/min. Data presented herein suggest that B. anthracis may co-opt O(2)*- which is produced by stimulated myeloid phagocytes and is an essential element of host immunity, as a necessary step in productive infection of the host.

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.

Dendritic cells endocytose Bacillus anthracis spores: implications for anthrax pathogenesis

Phagocytosis of inhaled Bacillus anthracis spores and subsequent trafficking to lymph nodes are decisive events in the progression of inhalational anthrax because they initiate germination and dissemination of spores. Found in high frequency throughout the respiratory track, dendritic cells (DCs) routinely take up foreign particles and migrate to lymph nodes. However, the participation of DCs in phagocytosis and dissemination of spores has not been investigated previously. We found that human DCs readily engulfed fully pathogenic Ames and attenuated B. anthracis spores predominately by coiling phagocytosis. Spores provoked a loss of tissue-retaining chemokine receptors (CCR2, CCR5) with a concurrent increase in lymph node homing receptors (CCR7, CD11c) on the membrane of DCs. After spore infection, immature DCs displayed a mature phenotype (CD83(bright), HLA-DR(bright), CD80(bright), CD86(bright), CD40(bright)) and enhanced costimulatory activity. Surprisingly, spores activated the MAPK cascade (ERK, p38) within 30 min and stimulated expression of several inflammatory response genes by 2 h. MAPK signaling was extinguished by 6 h infection, and there was a dramatic reduction of secreted TNF-alpha, IL-6, and IL-8 in the absence of DC death. This corresponded temporally with enzymatic cleavage of proximal MAPK signaling proteins (MEK-1, MEK-3, and MAP kinase kinase-4) and may indicate activity of anthrax lethal toxin. Taken together, these results suggest that B. anthracis may exploit DCs to facilitate infection.

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.

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.

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.

TnXO1, a germination-associated class II transposon from Bacillus anthracis

Bacillus anthracis harbours two virulence plasmids, pXO1 (182 kb) and pXO2 (95 kb). Whereas pXO2 harbours the cap operon coding for the capsule, pXO1 contains the pag, lef, and cya genes coding for protective antigen, lethal, and oedema factors, respectively, as well as the atxA regulatory gene. These genes are located within a 44.8 kb long pathogenicity island flanked by insertion sequences. Here, we describe the presence in the same plasmid region of an 8679 bp genetic element displaying the structural features of a class II cointegrative transposon. This element, named TnXO1, bears a transposase and a site-specific recombinase and is delineated by 38 bp terminal inverted repeats sequences similar to those of other members of this group of transposons. A putative res site has been identified in the 200 bp region between these genes. Interestingly, TnXO1 also contains the gerX operon involved in the germination of B. anthracis spores within phagocytic cells. Such close association of a mobile DNA structure with known virulence determinants in a pathogen further prompted us to look for the presence of this transposable element in other members of the Bacillus cereus sensu lato group. No instance of TnXO1 was detected outside of B. anthracis in PCR experiments, although it was found to be present in the genome sequence draft of one strain of B. cereus which has recently been shown to harbour a plasmid almost identical to pXO1.

Differential susceptibility of macrophage cell lines to Bacillus anthracis–Vollum 1B

Bacillus anthracis (BA) is a spore forming bacterium and the causative agent of anthrax disease. Macrophages (Mphis) play a central role in anthrax disease. An important step in disease progression is the ability of BA to secrete lethal toxin (LeTx) that kills Mphis. LeTx is a heterodimer composed of protective antigen (PA) and lethal factor (LF). Researchers have shown that Mphi cell lines demonstrate differential susceptibility to purified LeTx; for example RAW264.7 and J774A.1 Mphis are sensitive to LeTx whereas IC-21 Mphis are resistant. Research has also suggested that exogenous factors, including other BA proteins, can influence the activity of LeTx. For this reason, the objective of the current work was to examine if RAW264.7, J774A.1, and IC-21 Mphis demonstrated differential susceptibility when cultured with a LeTx-producing strain of BA. Here, we co-cultured Mphis with LeTx+ Vollum 1B (V1B) spores for >15 h and assayed for Mphi cell death by morphology, trypan blue (TB) staining, neutral red (NR) activity, and lactate dehydrogenase (LDH) activity in the culture media. Following the addition of V1B spores, necrosis (approximately 50% mortality) was observed in RAW264.7 and J774A.1 Mphis at 7.5 and 10 h, respectively. By 15 h, both RAW264.7 and J774A.1 Mphis demonstrated 100% mortality. In contrast, IC-21 Mphis, under identical culture conditions, remained viable (98%) and activated throughout the course of the experiment (>24 h). The mechanism of RAW264.7 cell death appeared to involve LeTx because the V1B-induced cytotoxicity was dose-dependently reversed by the addition of anti-PA antibody to the culture media. These observations suggest there is differential susceptibility of Mphi cell lines to the LeTx+ V1B strain of BA. Further development of this in vitro model may be useful to further characterize the interactions between Mphis and BA spores.

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.

Murine macrophage transcriptional responses to Bacillus anthracis infection and intoxication

Interactions between Bacillus anthracis and host macrophages represent critical early events in anthrax pathogenesis, but their details are not clearly understood. Here we report the first genomewide characterization of the transcriptional changes within macrophages infected with B. anthracis and the identification of several hundred host genes that were differentially expressed during this intracellular stage of infection. These loci included both genes that are known to be regulated differentially in response to many other bacterial pathogens and those that appear to be differentially regulated in response to B. anthracis but not other bacterial species that have been tested. These data provide a transcriptional basis for a variety of physiological changes observed during infection, including the induction of apoptosis caused by the infecting bacteria. The expression patterns underlying B. anthracis-induced apoptosis led us to test further the importance of one very highly induced macrophage gene, that for ornithine decarboxylase. Our data show that this enzyme plays an important and previously unrecognized role in suppressing apoptosis in B. anthracis-infected cells. We have also characterized the transcriptional response to anthrax lethal toxin in activated macrophages and found that, following toxin treatment, many of the host inflammatory response pathways are dampened. These data provide insights into B. anthracis pathogenesis as well as potential leads for the development of new diagnostic and therapeutic options.

Cytokine response to infection with Bacillus anthracis spores

Bacillus anthracis, the etiological agent of anthrax, is a gram-positive, spore-forming bacterium. The inhalational form of anthrax is the most severe and is associated with rapid progression of the disease and the outcome is frequently fatal. Transfer from the respiratory epithelium to regional lymph nodes appears to be an essential early step in the establishment of infection. This transfer is believed to occur by means of carriage within alveolar macrophages following phagocytosis. Therefore, the ability of B. anthracis to transit through the host macrophage or dendritic cell appears to be an early and critical step in B. anthracis pathogenesis. In this work, we examined the cytokine responses to spore infection in mouse primary peritoneal macrophages, in primary human dendritic cells, and during a spore aerosol infection model utilizing the susceptible A/J mouse strain. We demonstrated that both mouse peritoneal macrophages and human dendritic cells exhibited significant intracellular bactericidal activity during the first hours following uptake, providing the necessary time to mount a cytokine response prior to cell lysis. Strong tumor necrosis factor (TNF-alpha) and interleukin-6 (IL-6) responses were seen in mouse peritoneal macrophages. In addition to TNF-alpha and IL-6, human dendritic cells produced the cytokines IL-1beta, IL-8, and IL-12. A mixture of Th1 and Th2 cytokines were detected in sera obtained from infected animals. In this study, we provide further evidence of an acute cytokine response when cells in culture and mice are infected with B. anthracis spores.

The adenylate cyclase toxins

Cyclic AMP is a ubiquitous messenger that integrates many processes of the cell. Diverse families of adenylate cyclases and phosphodiesterases stringently regulate the intracellular concentration of cAMP. Any alteration in the cytosolic concentration of cAMP has a profound effect on the various processes of the cell. Disruption of these cellular processes in vivo is often the most critical event in the pathogenesis of infectious diseases for animals and humans. Many pathogenic bacteria secrete toxins to alter the intracellular concentration of cAMP. These toxins either disrupt the normal regulation of the host cell's adenylate cyclases/phosphodiesterases or they themselves catalyze the synthesis of cAMP in the host cell. The latter are known as the adenylate cyclase toxins. Four such toxins have been identified: the invasive adenylate cyclase of Bordetella pertussis, the edema factor of Bacillus anthracis, ExoY of Pseudomonas aeruginosa, and the adenylate cyclase of Yersinia pestis. These adenylate cyclase toxins enter the eukaryotic host cells and get activated by eukaryotic cofactors, like calmodulin, to trigger the synthesis of cAMP in these cells. By accumulating cAMP in the target cells, these toxins either modulate the cellular function or completely deactivate the cell for further function. The immune effector cells appear to be the primary target of these adenylate cyclase toxins. By accumulating cAMP in the immune effector cells, these adenylate cyclase toxins poison the immune system and thus facilitate the survival of the bacteria in the host.

Binary bacterial toxins: biochemistry, biology, and applications of common Clostridium and Bacillus proteins

Certain pathogenic species of Bacillus and Clostridium have developed unique methods for intoxicating cells that employ the classic enzymatic "A-B" paradigm for protein toxins. The binary toxins produced by B. anthracis, B. cereus, C. botulinum, C. difficile, C. perfringens, and C. spiroforme consist of components not physically associated in solution that are linked to various diseases in humans, animals, or insects. The "B" components are synthesized as precursors that are subsequently activated by serine-type proteases on the targeted cell surface and/or in solution. Following release of a 20-kDa N-terminal peptide, the activated "B" components form homoheptameric rings that subsequently dock with an "A" component(s) on the cell surface. By following an acidified endosomal route and translocation into the cytosol, "A" molecules disable a cell (and host organism) via disruption of the actin cytoskeleton, increasing intracellular levels of cyclic AMP, or inactivation of signaling pathways linked to mitogen-activated protein kinase kinases. Recently, B. anthracis has gleaned much notoriety as a biowarfare/bioterrorism agent, and of primary interest has been the edema and lethal toxins, their role in anthrax, as well as the development of efficacious vaccines and therapeutics targeting these virulence factors and ultimately B. anthracis. This review comprehensively surveys the literature and discusses the similarities, as well as distinct differences, between each Clostridium and Bacillus binary toxin in terms of their biochemistry, biology, genetics, structure, and applications in science and medicine. The information may foster future studies that aid novel vaccine and drug development, as well as a better understanding of a conserved intoxication process utilized by various gram-positive, spore-forming bacteria.

Anthrax

Anthrax is an often fatal bacterial infection that occurs when Bacillus anthracis endospores enter the body through one of three major routes: inhalational, cutaneous, or gastrointestinal. Before the anthrax terrorist attacks in the United States in 2001, there was very little interest in anthrax as a serious human pathogen; anthrax was viewed mainly as a veterinarian problem of minor importance, with most cases attributed to occupational exposure. However, this cavalier attitude toward anthrax changed following the 2001 terrorist attacks. Although the number of cases was relatively small, the attacks have heightened concern about the feasibility of large-scale aerosol bioweapons attacks by terrorist groups. Many, if not most patients, would require some degree of critical care in the form of ventilator or hemodynamic support. It is for this reason that anesthesiologists and other critical care physicians have specific knowledge of the diagnosis, treatment, and prevention of anthrax.

The use of a model of in vivo macrophage depletion to study the role of macrophages during infection with Bacillus anthracis spores

The pathogenesis of infection by Bacillus anthracis has been the subject of many investigations, but remains incompletely understood. It has been shown that B. anthracis spores germinate in macrophages and perhaps require this intracellular niche to germinate in vivo before outgrowth of the vegetative organism. However, it has also been reported that macrophages are sporicidal in vitro. In our in vivo model, macrophages were depleted from mice by either silica treatment or treatment with liposome-encapsulated dichloromethylene disphosphonate (Cl(2)MDP), and the animals were infected parenterally with virulent ungerminated B. anthracis (Ames strain) spores. The mice in which macrophages had been depleted were killed more rapidly than untreated mice. In addition, augmenting peritoneal populations of macrophages with cultured RAW264.7 cells partially protected mice from disease, increasing the survival rate in a dose dependent relationship. Alveolar macrophages were depleted by intranasal instillation of liposome-encapsulated Cl(2)MDP. The animals with normal alveolar macrophage numbers had significantly greater survival rates after inhaling B. anthracis spores than the macrophage-depleted mice. These findings do not preclude the observations that macrophages provide a site permissive for spore germination, however, these data suggest that macrophages do play an important role in limiting and/or clearing a B. anthracis infection.

Targeting of Bacillus anthracis interaction factors for human macrophages using two-dimensional gel electrophoresis

Bacillus anthracis, a gram-positive, endospore-forming, aerobic rod-shaped bacterium, interacts with macrophages at various stages of the disease. Spore germination and the outgrowth of vegetative bacilli are crucial steps enabling the bacteria to proliferate actively and to synthesize the virulence factors leading to a massive septicemia. In this study, we performed a proteomic analysis and MALDI-TOF/MS were carried out to identify proteins using human macrophages infected with the spores of B. anthracis live-Sterne or inactivated-Sterne. We identified 21 proteins which are related to the infection of B. anthracis spores on human macrophages at the early stage events. These proteins function in processes such as cytoskeleton regulation, apoptosis, cell division, and protein degradation. Proteins such as PAK 2 revealed a relationship to apoptosis in human macrophages. These proteins play an important role in the macrophage survival and death on human macrophages with infected B. anthracis spores.

High bactericidal efficiency of type iia phospholipase A2 against Bacillus anthracis and inhibition of its secretion by the lethal toxin

There is a considerable body of evidence supporting the role of secretory type II-A phospholipase A(2) (sPLA(2)-IIA) as an effector of the innate immune response. This enzyme also exhibits bactericidal activity especially toward Gram-positive bacteria. In this study we examined the ability of sPLA(2)-IIA to kill Bacillus anthracis, the etiological agent of anthrax. Our results show that both germinated B. anthracis spores and encapsulated bacilli were sensitive to the bactericidal activity of recombinant sPLA(2)-IIA in vitro. In contrast, nongerminated spores were resistant. This bactericidal effect was correlated to the ability of sPLA(2)-IIA to hydrolyze bacterial membrane phospholipids. Guinea pig alveolar macrophages, the major source of sPLA(2)-IIA in an experimental model of acute lung injury, released enough sPLA(2)-IIA to kill extracellular B. anthracis. The production of sPLA(2)-IIA was significantly inhibited by B. anthracis lethal toxin. Human bronchoalveolar lavage fluids from acute respiratory distress syndrome patients are known to contain sPLA(2)-IIA; bactericidal activity against B. anthracis was detected in a high percentage of these samples. This anthracidal activity was correlated to the levels of sPLA(2)-IIA and was abolished by an sPLA(2)-IIA inhibitor. These results suggest that sPLA(2)-IIA may play a role in innate host defense against B. anthracis infection and that lethal toxin may help the bacteria to escape from the bactericidal action of sPLA(2)-IIA by inhibiting the production of this enzyme.

Murine model of pulmonary anthrax: kinetics of dissemination, histopathology, and mouse strain susceptibility

Bioweapons are most often designed for delivery to the lung, although this route is not the usual portal of entry for many of the pathogens in the natural environment. Vaccines and therapeutics that are efficacious for natural routes of infection may not be effective against the pulmonary route. Pulmonary models are needed to investigate the importance of specific bacterial genes in virulence, to identify components of the host immune system that are important in providing innate and acquired protection, and for testing diagnostic and therapeutic strategies. This report describes the characteristics of host and Bacillus anthracis interactions in a murine pulmonary-infection model. The infective dose varied depending on the route and method of inoculation. The germination process in the lung began within 1 h of inoculation into the lung, although growth within the lung was limited. B. anthracis was found in the lung-associated lymph nodes approximately 5 h after infection. Minimal pneumonitis was associated with the lung infection, but significant systemic pathology was noted after dissemination. Infected mice typically succumbed to infection approximately 3 to 4 days after inoculation. The 50% lethal doses differed among inbred strains of mice, but within a given mouse strain, neither the age nor the sex of the mice influenced susceptibility to B. anthracis.

Macrophages release tumor necrosis factor alpha and interleukin-12 in response to intracellular Bacillus anthracis spores

Herein we report that infection of a murine macrophage cell line with Bacillus anthracis results in the production of tumor necrosis factor alpha and interleukin-12 (IL-12). When infected with B. anthracis spores in combination with lipopolysaccharide, macrophages release increased amounts of IL-12. We found no evidence of inhibition of cytokine responses in macrophages infected with B. anthracis spores.

Intracellular fate and immunogenicity of B. subtilis spores

To support our work on the development of bacterial spores as oral vaccines we examined the immunogenicity and intracellular fate of Bacillus subtilis endospores in a murine model. Mice dosed orally with spores developed systemic IgG and mucosal sIgA responses. Analysis of IgG subclasses revealed a predominance of the IgG2a subclass during the early stages of immunisation. Analysis of cytokine mRNA in GALT and lymphoid organs showed early induction of IFN-gamma, a Th1 cytokine, as well as the pro-inflammatory cytokine TNF-alpha. Significant levels of IgG antibodies were produced against vegetative bacilli following dosing with spores. This showed that spores could germinate in the GI tract. In vitro studies detailing the intracellular fate and persistence of spores in a macrophage-like cell line (RAW264.7) demonstrated that spores could germinate efficiently in macrophages, initiate gene expression as well as inducing pro-inflammatory cytokines.

Bacillus anthracis requires siderophore biosynthesis for growth in macrophages and mouse virulence

Systemic anthrax infections can be characterized as proceeding in stages, beginning with an early intracellular establishment stage within phagocytes that is followed by extracelluar stages involving massive bacteraemia, sepsis and death. Because most bacteria require iron, and the host limits iron availability through homeostatic mechanisms, we hypothesized that B. anthracis requires a high-affinity mechanism of iron acquisition during its growth stages. Two putative types of siderophore synthesis operons, named Bacillus anthracis catechol, bac (anthrabactin), and anthrax siderophore biosynthesis, asb (anthrachelin), were identified. Directed gene deletions in both anthrabactin and anthrachelin pathways were generated in a B. anthracis (Sterne) 34F2 background resulting in mutations in asbA and bacCEBF. A decrease in siderophore production was observed during iron-depleted growth in both the DeltaasbA and DeltabacCEBF strains, but only the DeltaasbA strain was attenuated for growth under these conditions. In addition, the DeltaasbA strain was severely attenuated both for growth in macrophages (MPhi) and for virulence in mice. In contrast, the DeltabacCEBF strain did not differ phenotypically from the parental strain. These findings support a requirement for anthrachelin but not anthrabactin in iron assimilation during the intracellular stage of anthrax.

mAbs to Bacillus anthracis capsular antigen for immunoprotection in anthrax and detection of antigenemia

Bacillus anthracis is surrounded by an antiphagocytic polypeptide capsule composed of poly gamma-D-glutamic acid (gammaDPGA). gammaDPGA has been identified recently as a potential target for vaccine development. Studies of the role of gammaDPGA in disease have been hampered by the poor Ab response to this antigen and the lack of immunochemical reagents. As a consequence, neither the extent of gammaDPGA production during anthrax nor the protective activity of gammaDPGA Abs in inhalation anthrax are known. Here we report production of IgG Abs to gammaDPGA in mice following an immunization regimen using gammaDPGA in combination with agonist mAbs to CD40. mAbs were produced that are specific for gammaDPGA. Passive immunization with gammaDPGA mAbs protected >90% of mice in a pulmonary model of anthrax that was lethal in control mice (P < 0.0001). Use of gammaDPGA mAb in an antigen detection immunoassay found that the appearance of gammaDPGA in serum coincided with the emergence of bacteremia. These studies identify CD40 stimulation as a means for production of Ab and generation of mAbs against a weakly immunogenic antigen and demonstrate that the capsule is an effective target for immunoprotection and for antigen detection in the diagnosis of anthrax.

Spore germination

The germination of dormant spores of Bacillus species is the first crucial step in the return of spores to vegetative growth, and is induced by nutrients and a variety of non-nutrient agents. Nutrient germinants bind to receptors in the spore's inner membrane and this interaction triggers the release of the spore core's huge depot of dipicolinic acid and cations, and replacement of these components by water. These latter events trigger the hydrolysis of the spore's peptidoglycan cortex by either of two redundant enzymes in B. subtilis, and completion of cortex hydrolysis and subsequent germ cell wall expansion allows full spore core hydration and resumption of spore metabolism and macromolecular synthesis.

A microtiter fluorometric assay to detect the germination of Bacillus anthracis spores and the germination inhibitory effects of antibodies

Bacillus anthracis spore germination is usually detected in vitro by alterations in spore refractility, heat resistance, and stainability. We developed a more quantitative, sensitive, and semi-automated procedure for detecting germination by using a microtiter kinetic reader for fluorescence spectrophotometry. The procedure was based on the increase in fluorescence of spores with time during their incubation in germination medium containing a fluorescent nucleic acid-binding dye which stained germinated B. anthracis but not ungerminated (UG) spores. Spore germination in the presence of several germinants was characterized. Although L-alanine and inosine alone stimulated rapid germination in this assay, a medium containing optimal concentrations of L-alanine, adenosine, and casamino acids gave low background fluorescence, stimulated germination completely, and at a reasonable rate. Suspensions of heat-activated, UG spores of B. anthracis strain Ames were preincubated with antibodies (Abs) against whole spores to assess their effect on germination. Analyses of the germination data obtained revealed significant differences between spores pretreated with these Abs and those treated with non-immune sera or IgG. Germination inhibitory activity (GIA) was detected for several polyclonal rabbit anti-spore Ab preparations. These included anti-Ames strain spore antisera, IgG purified from the latter, and spore affinity-purified Abs from antisera elicited against four strains of B. anthracis. Abs elicited against UG as well as completely germinated Ames spores inhibited germination. Abs were ranked according to their GIA, and those specific for UG spores usually exhibited greater GIA. Direct binding to spores of these Abs was detected by an ELISA with whole un-germinated Ames spores. Although specific binding to spores by the anti-spore Abs was shown, their titers did not correlate with their GIA levels. Current efforts are focused on identifying the spore antigens recognized by the anti-spore Abs, characterizing the role of these targeted antigens in disease pathogenesis, and evaluating the ability of specific anti-spore Abs to protect against infection with B. anthracis.

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.

Innate immune activation as a broad-spectrum biodefense strategy: prospects and research challenges

Biodefense strategies require protection against a broad and largely unforeseen spectrum of pathogens—the forte of innate immune system defenses—that have evolved over millennia to function within moments of encountering either ancient or newly emerging pathogens. Although constitutive, the innate immune system is activated by the presence of microbes or their products, providing a rationale for a potential biodefense strategy. Both prophylactic and postexposure strategies involving innate immune stimulation have been shown to be plausible to prevent or ameliorate infections in animal models. Innate immune-activating compounds based on conserved microbial components recognized by toll-like molecules and other receptors could be synthesized and delivered like drugs by using an entirely different strategy from conventional vaccination. However, important theoretic and practical questions emerge about developing and deploying innate immune protective strategies for biodefense. This rostrum discusses prospects and problems in the overall approach itself. Important topics include microbe-specific issues about innate immune system effectiveness against highly virulent pathogens and general questions, such as whether innate immune responses will be safe and effective if used in a diverse human population of different age groups and with different genetic makeups.

Deletion mutants of protective antigen that inhibit anthrax toxin both in vitro and in vivo

The anthrax toxin complex is primarily responsible for most of the symptoms of anthrax. This complex is composed of three proteins, anthrax protective antigen, anthrax edema factor, and anthrax lethal factor. The three proteins act in binary combination of protective antigen plus edema factor (edema toxin) and protective antigen plus lethal factor (lethal toxin) that paralyze the host defenses and eventually kill the host. Both edema factor and lethal factor are intracellularly acting proteins that require protective antigen for their delivery into the host cell. In this study, we show that deletion of certain residues of protective antigen results in variants of protective antigen that inhibit the action of anthrax toxin both in vitro and in vivo. These mutants protected mice against both lethal toxin and edema toxin challenge, even when injected at a 1:8 ratio relative to the wild-type protein. Thus, these mutant proteins are promising candidates that may be used to neutralize the action of anthrax toxin.

In vitro-generated respiratory mucosa: a new tool to study inhalational anthrax

We generated a three-dimensional (3-D) model of human airway tissues in order to study initiation of inhalational form of anthrax infection. The system was designed to model the air-blood barrier of the respiratory tract represented by epithelial cells and macrophages. When grown on collagen/fibronectin gel support at an air-liquid interface, airway epithelial cells formed cell layers morphologically resembling those in vivo. These preformed epithelial cell cultures were further supplemented with monocytes/macrophages isolated from human blood. After 2-5 days of co-culture, monocytes differentiated into a phenotype of resident macrophages, which was evaluated by the expression of specific cell surface markers. This model allowed sorting out the role of each type of cell found at the air surface of the lung. The interdependence of macrophages and epithelial cells in the clearance of anthrax spores from airways and the capacity of the airway epithelial cells to protect from anthrax infection was demonstrated.