Anthrax protective antigen cleavage and clearance from the blood of mice and rats

Bacillus anthracis protective antigen (PA) is an 83-kDa (PA83) protein that is cleaved to the 63-kDa protein (PA63) as an essential step in binding and internalizing lethal factor (LF). To assess in vivo receptor saturating PA concentrations, we injected mice with PA variants and measured the PA remaining in the blood at various times using PA83- and PA63-specific enzyme-linked immunosorbent assays. We found that both wild-type PA (WT-PA) and a receptor-binding-defective mutant (Ub-PA) were cleaved to PA63 independent of their ability to bind cells. This suggested a PA-acting protease activity in the blood. The protease cleaved PA at the furin cleavage sequence because furin site-modified PA mutants were not cleaved. Cleavage measured in vitro was leupeptin sensitive and dependent on calcium. Cell surface cleavage was important for toxin clearance, however, as Ub-PA and uncleavable PA mutants were cleared at slower rates than WT-PA. The cell binding-independent cleavage of PA was also verified by using Ub-PA (which is still cleaved) to rescue mice from toxin challenge by competitively binding circulating LF. This mutant was able to rescue mice even when given 12 h before toxin challenge. Its therapeutic ability was comparable to that of dominant-negative PA, which binds cells but does not allow LF translocation, and to the protection afforded through receptor clearance by WT-PA and uncleavable receptor binding-competent mutants. The PA cleavage and clearance observed in mice did not appear to have a role in the differential mouse susceptibility as it occurred similarly in lethal toxin (LT)-resistant DBA/2J and LT-sensitive BALB/cJ mice. Interestingly, PA63 was not found in LT-resistant or -sensitive rats and PA83 clearance was slower in rats than in mice. Finally, to determine the minimum amount of PA required in circulation for LT toxicity in mice, we administered time-separated injections of PA and LF and showed that lethality of LF for mice after PA was no longer measurable in circulation, suggesting active PA sequestration at tissue surfaces.

Mycoplasma arthritidis-derived superantigen (MAM) displays DNase activity

Bacterial superantigens are potent stimulators of the immune system. In this study, we expressed recombinant superantigens, which were then affinity purified and used for growth curves and DNase activity assays. Overexpression of Mycoplasma arthritidis-derived superantigen in Escherichia coli reduced bacterial growth. This is unique, as staphylococcal enterotoxin A and toxic shock syndrome toxin-1, expressed in the same vector system, showed no growth impairment. The observed growth inhibition was caused by the DNase activity of recombinant M. arthritidis-derived superantigen, thus describing the first superantigen showing enzymatic activity, which may be a result of the separate evolution of this toxin.

In vitro and in vivo characterization of anthrax anti-protective antigen and anti-lethal factor monoclonal antibodies after passive transfer in a mouse lethal toxin challenge model to define correlates of immunity

Passive transfer of antibody may be useful for preexposure prophylaxis against biological agents used as weapons of terror, such as Bacillus anthracis. Studies were performed to evaluate the ability of anthrax antiprotective antigen (anti-PA) and antilethal factor (anti-LF) neutralizing monoclonal antibodies (mAbs) to protect against an anthrax lethal toxin (LeTx) challenge in a mouse model and to identify correlates of immunity to LeTx challenge. Despite having similar affinities for their respective antigens, anti-PA (3F11) and anti-LF (9A11), passive transfer of up to 1.5 mg of anti-PA 3F11 mAb did not provide significant protection when transferred to mice 24 h before LeTx challenge, while passive transfer of as low as 0.375 mg of anti-LF 9A11 did provide significant protection. Serum collected 24 h after passive transfer had LeTx-neutralizing activity when tested using a standard LeTx neutralization assay, but neutralization titers measured using this assay did not correlate with protection against LeTx challenge. However, measurement of LeTx-neutralizing serum responses with an LeTx neutralization assay in vitro employing the addition of LeTx to J774A.1 cells 15 min before the addition of the serum did result in neutralization titers that correlated with protection against LeTx challenge. Our results demonstrate that only the LeTx neutralization titers measured utilizing the addition of LeTx to J774A.1 cells 15 min before the addition of sample correlated with protection in vivo. Thus, this LeTx neutralization assay may be a more biologically relevant neutralization assay to predict the in vivo protective capacity of LeTx-neutralizing antibodies.

Accounting for ligand-bound metal ions in docking small molecules on adenylyl cyclase toxins

The adenylyl cyclase toxins produced by bacteria (such as the edema factor (EF) of Bacillus anthracis and CyaA of Bordetella pertussis) are important virulence factors in anthrax and whooping cough. Co-crystal structures of these proteins differ in the number and positioning of metal ions in the active site. Metal ions bound only to the ligands in the crystal structures are not included during the docking. To determine what effect these "missing" metals have on docking results, the AutoDock, LigandFit/Cerius2, and FlexX programs were compared for their ability to correctly place substrate analogues and inhibitors into the active sites of the crystal structures of EF, CyaA, and mammalian adenylate cyclase. Protonating the phosphates of substrate analogues improved the accuracy of docking into the active site of CyaA, where the grid did not account for one of the three Mg2+ ions in the crystal structure. The AutoDock ranking (based on docking energies) of a test group of compounds was relatively unaffected by protonation of carboxyl groups. However, the ranking by FlexX-ChemScore varied significantly, especially for docking to CyaA, suggesting that alternate protonation states should be tested when screening compound libraries with this program. When the charges on the bound metal were set correctly, AutoDock was the most reliable program of the three tested with respect to positioning substrate analogues and ranking compounds according to their experimentally determined ability to inhibit EF.

CD6 binds to pathogen-associated molecular patterns and protects from LPS-induced septic shock

CD6 is a lymphocyte receptor that belongs to the scavenger receptor cysteine-rich superfamily. Because some members of the scavenger receptor cysteine-rich superfamily act as pattern recognition receptors for microbial components, we studied whether CD6 shares this function. We produced a recombinant form of the ectodomain of CD6 (rsCD6), which was indistinguishable (in apparent molecular mass, antibody reactivity, and cell binding properties) from a circulating form of CD6 affinity-purified from human serum. rsCD6 bound to and aggregated several Gram-positive and -negative bacterial strains through the recognition of lipoteichoic acid and LPS, respectively. The Kd of the LPS-rsCD6 interaction was 2.69 +/- 0.32 x 10(-8) M, which is similar to that reported for the LPS-CD14 interaction. Further experiments showed that membrane CD6 also retains the LPS-binding ability, and it results in activation of the MAPK signaling cascade. In vivo experiments demonstrated that i.p. administration of rsCD6 before lethal LPS challenge significantly improved mice survival, and this was concomitant with reduced serum levels of the proinflammatory cytokines TNF-alpha, IL6, and IL-1beta. In conclusion, our results illustrate the unprecedented bacterial binding properties of rsCD6 and support its therapeutic potential for the intervention of septic shock syndrome or other inflammatory diseases of infectious origin.

Murine monoclonal antibodies against murine uPA receptor produced in gene-deficient mice: inhibitory effects on receptor-mediated uPA activity in vitro and in vivo

Binding of urokinase plasminogen activator (uPA) to its cellular receptor, uPAR, potentiates plasminogen activation and localizes it to the cell surface. Focal plasminogen activation is involved in both normal and pathological tissue remodeling processes including cancer invasion. The interaction between uPA and uPAR therefore represents a potential target for anti-invasive cancer therapy. Inhibitors of the human uPA-uPAR interaction have no effect in the murine system. To enable in-vivo studies in murine cancer models we have now generated murine monoclonal antibodies (mAbs) against murine uPAR (muPAR) by immunizing uPAR-deficient mice with recombinant muPAR and screened for antibodies, which inhibit the muPA-muPAR interaction. Two of the twelve mAbs obtained, mR1 and mR2, interfered with the interaction between muPAR and the amino-terminal fragment of muPA (mATF) when analyzed by surface plasmon resonance. The epitope for mR1 is located on domain I of muPAR, while that of mR2 is on domains (II-III). In cell binding experiments using radiolabelled mATF, the maximal inhibition obtained with mR1 was 85% while that obtained with mR2 was 50%. The IC(50) value for mR1 was 0.67 nM compared to 0.14 nM for mATF. In an assay based on modified anthrax toxins, requiring cell-bound muPA activity for its cytotoxity, an approximately 50% rescue of the cells could be obtained by addition of mR1. Importantly, in-vivo efficacy of mR1 was demonstrated by the ability of mR1 to rescue mice treated with a lethal dose of uPA-activatable anthrax toxins.

Anthrax toxins induce shock in rats by depressed cardiac ventricular function

Anthrax infections are frequently associated with severe and often irreversible hypotensive shock. The isolated toxic proteins of Bacillus anthracis produce a non-cytokine-mediated hypotension in rats by unknown mechanisms. These observations suggest the anthrax toxins have direct cardiovascular effects. Here, we characterize these effects. As a first step, we administered systemically anthrax lethal toxin (LeTx) and edema toxin (EdTx) to cohorts of three to twelve rats at different doses and determined the time of onset, degree of hypotension and mortality. We measured serum concentrations of the protective antigen (PA) toxin component at various time points after infusion. Peak serum levels of PA were in the µg/mL range with half-lives of 10–20 minutes. With doses that produced hypotension with delayed lethality, we then gave bolus intravenous infusions of toxins to groups of four to six instrumented rats and continuously monitored blood pressure by telemetry. Finally, the same doses used in the telemetry experiments were given to additional groups of four rats, and echocardiography was performed pretreatment and one, two, three and twenty-four hours post-treatment. LeTx and EdTx each produced hypotension. We observed a doubling of the velocity of propagation and 20% increases in left ventricular diastolic and systolic areas in LeTx-treated rats, but not in EdTx-treated rats. EdTx-but not LeTx-treated rats showed a significant increase in heart rate. These results indicate that LeTx reduced left ventricular systolic function and EdTx reduced preload. Uptake of toxins occurs readily into tissues with biological effects occurring within minutes to hours of serum toxin concentrations in the µg/mL range. LeTx and EdTx yield an irreversible shock with subsequent death. These findings should provide a basis for the rational design of drug interventions to reduce the dismal prognosis of systemic anthrax infections.

Synthesis of potent inhibitors of anthrax toxin based on poly-L-glutamic acid

We report the synthesis of biodegradable polyvalent inhibitors of anthrax toxin based on poly-L-glutamic acid (PLGA). These biocompatible polyvalent inhibitors are at least 4 orders of magnitude more potent than the corresponding monovalent peptides in vitro and are comparable in potency to polyacrylamide-based inhibitors of anthrax toxin assembly. We have elucidated the influence of peptide density on inhibitory potency and demonstrated that these inhibitory potencies are limited by kinetics, with even higher activities seen when the inhibitors are preincubated with the heptameric receptor-binding subunit of anthrax toxin prior to exposure to cells. These polyvalent inhibitors are also effective at neutralizing anthrax toxin in vivo and represent attractive leads for designing biocompatible anthrax therapeutics.

Anthrax lethal toxin paralyzes actin-based motility by blocking Hsp27 phosphorylation

Inhalation of anthrax causes fatal bacteremia, indicating a meager host immune response. We previously showed that anthrax lethal toxin (LT) paralyzes neutrophils, a major component of innate immunity. Here, we have found that LT also inhibits actin-based motility of the intracellular pathogen Listeria monocytogenes. LT inhibition of actin assembly is mediated by blockade of Hsp27 phosphorylation, and can be reproduced by treating cells with the p38 mitogen-activated protein (MAP) kinase inhibitor SB203580. Nonphosphorylated Hsp27 inhibits Listeria actin-based motility in cell extracts, and binds to and sequesters purified actin monomers. Phosphorylation of Hsp27 reverses these effects. RNA interference knockdown of Hsp27 blocks LT inhibition of Listeria actin-based motility. Rescue with wild-type Hsp27 accelerates Listeria speed in knockdown cells, whereas introduction of Hsp27 mutants incapable of phosphorylation or dephosphorylation causes slowing down. We propose that Hsp27 facilitates actin-based motility through a phosphorylation cycle that shuttles actin monomers to regions of new actin filament assembly. Our findings provide a previously unappreciated mechanism for LT virulence, and emphasize a central role for p38 MAP kinase-mediated phosphorylation of Hsp27 in actin-based motility and innate immunity.

Rat survival to anthrax lethal toxin is likely controlled by a single gene

We examined whether survival of different rat strains administered anthrax lethal toxin is genetically determined. A reproducible test population of first filial generation hybrid rats was bred based on the susceptibility of progenitors to anthrax lethal toxin and to maximize genetic diversity across the strains. These rats were then tested with varying doses of anthrax lethal toxin. We found that all 'sensitive' strains died within 2 h following systemic administration of 240 mug/kg lethal toxin, while one strain survived following a five times higher dose (1.4 mg/kg). The ability of lethal toxin to lyse macrophage cultures derived from the bone marrow of these strains corresponded with in vivo results. We conclude that a rat test population can detect strain differences in response to anthrax lethal toxin. Survival is influenced by the host genome background and is likely due to a single gene with a recessive mode of inheritance.

Systematic Urokinase-Activated Anthrax Toxin Therapy Produces Regressions of Subcutaneous Human Non–Small Cell Lung Tumor in Athymic Nude Mice

The novel recombinant anthrax toxin, PrAgU2/FP59, composed of the urokinase-activated protective antigen and a fusion protein of Pseudomonas exotoxin and lethal factor was tested for anti-lung cancer efficacy in an in vivo human tumor model. Male athymic nude mice (age 4-6 weeks) were inoculated s.c. with 10 million H1299 non-small cell lung cancer (NSCLC) cells in the left flank. When tumor volumes reached 200 mm(3) (6-8 days), i.p. injection of 100 muL saline or different ratios and doses of PrAgU2/FP59 in 100 muL saline were given every 3 days for four doses and an additional dose at day 29. Animals were monitored twice daily and tumor measurements were made by calipers. The maximum tolerated doses of PrAgU2/FP59 differed dependent on the ratios of PrAgU2 to FP59 over the range of 3:1 to 25:1, respectively. At tolerated doses, tumor regressions were seen in all animals. Complete histologic remission lasting 60 days occurred in 30% of animals. PrAgU2/FP59 showed dramatic anti-NSCLC efficacy and warrants further clinical development for therapy of patients with advanced NSCLC.

Anthrax edema toxin sensitizes DBA/2J mice to lethal toxin

Anthrax toxin is made up of three separate protein components: the receptor-binding protective antigen (PA), the adenylyl cyclase edema factor (EF), and the metalloproteinase lethal factor (LF). EF and PA constitute edema toxin (ET), which causes edema when injected subcutaneously. At higher doses, ET causes severe pathologies and death in BALB/cJ mice (A. M. Firoved et al., Am. J. Pathol. 167:1309-1320, 2005). A striking effect of ET at lethal doses is adrenal necrosis. Here we show that low doses of ET (10 microg) that produce no overt signs of illness in mice still cause substantial adrenal lesions. These lesions are not associated with reduced corticosterone production; instead, ET-treated mice have increased corticosterone production. Because the resistance of mice to the other component of anthrax toxin, lethal toxin (LT; LF plus PA), has been shown to be overcome by the perturbation of the endocrine system, we hypothesized that sublethal doses of ET might sensitize LT-resistant DBA/2J mice to LT-mediated lethality. We report that a low dose of ET (5 microg) is sufficient to sensitize DBA/2J mice when given concurrently with LT. Higher doses of ET (e.g., 15 microg) given to male and female DBA/2J mice 18 h prior to LT challenge also sensitize them to LT. This study using highly purified ET and LT demonstrates how the components of anthrax toxin can work together to increase lethality.

Chemical Genetic Screening Identifies Critical Pathways in Anthrax Lethal Toxin-Induced Pathogenesis

Anthrax lethal toxin (LT)-induced cell death via mitogen-activated protein kinase kinase (MAPKK) cleavage remains questionable. Here, a chemical genetics approach was used to investigate what pathways mediate LT-induced cell death. Several small molecules were found to protect macrophages from anthrax LT cytotoxicity and MAPKK from cleavage by lethal factor (LF), without inhibiting LF enzymatic activity or cellular proteasome activity. Interestingly, the compounds activated MAPK-signaling molecules, induced proinflammatory cytokine production, and inhibited LT-induced macrophage apoptosis in a concentration-dependent manner. We propose that induction of antiapoptotic responses by MAPK-dependent or -independent pathways and activation of host innate responses may protect macrophages from anthrax LT-induced cell death. Altering host responses through a chemical genetics approach can help identify critical cellular pathways involved in the pathogenesis of anthrax and can be exploited to further explore host-pathogen interactions.

Ultrastructural features of lymphocyte suppression induced by anthrax lethal toxin and treated with chloroquine

Antibacterial therapy does not fully protect against anthrax because of severe systemic intoxication. Lysosomal processing of anthrax lethal toxin (LTX) is a key event in the disease pathogenesis, and agents interfering with this process, like chloroquine (CQ), may have practical applications. Although LTX is known to induce T-cell suppression, precise mechanisms of this phenomenon are not completely characterized. In the present study, we investigated alterations of lymphocyte ultrastructure caused by LTX and associated with favorable effect of CQ on the LTX-related dysfunction. Peripheral blood lymphocytes were activated via CD3 crosslinking in the presence or absence of LTX and CQ, and examined by transmission electron microscopy, flow cytometry and immunoblotting. Crosslinking of CD3 induced ultrastructural signs of lymphocyte activation, mostly disappeared after LTX treatment. The cell ultrastructure was well preserved in LTX-treated cells, despite dose- and time-dependent inhibition of T-cell function associated with impaired activation of mitogen-activated protein kinase. Regardless of intracellular signaling abnormalities, LTX did not decrease T-cell viability. CQ restored expression of CD69 (P<0.001) and improved phosphorylation of p38 (P=0.022) in LTX-exposed T lymphocytes. The exposure of cells to CQ, with or without LTX, led to appearance of many phagolysosomes with heterogeneous content, possibly representing unprocessed internalized material. In conclusion, LTX suppressed T-cell functions, but did not affect the viability and caused no ultrastructural damage. Ultrastructural observations indicated that CQ reduced harmful effects of LTX, possibly by interfering with lysosomal activity.

Contributions of histamine, prostanoids, and neurokinins to edema elicited by edema toxin from Bacillus anthracis

Bacillus anthracis edema toxin (ET), composed of protective antigen and an adenylate cyclase edema factor (EF), elicits edema in host tissues, but the target cells and events leading from EF-mediated cyclic-AMP production to edema are unknown. We evaluated the direct effect of ET on several cell types in vitro and tested the possibility that mediators of vascular leakage, such as histamine, contribute to edema in rabbits given intradermal ET. ET increased the transendothelial electrical resistance of endothelial monolayers, a response that is mechanistically inconsistent with the in vivo vascular leakage induced by ET. Screening of several drugs by intradermal treatment prior to toxin injection demonstrated reduced ET-induced vascular leakage with a cyclo-oxygenase inhibitor (indomethacin), agents that interfere with histamine (pyrilamine or cromolyn), or a neurokinin antagonist (spantide). Systemic administration of indomethacin or celecoxib (cyclo-oxygenase inhibitors), pyrilamine, aprepitant (a neurokinin 1 receptor antagonist), or indomethacin with pyrilamine significantly reduced vascular leakage associated with ET. Although the effects of pyrilamine, cromolyn, or aprepitant on ET-induced vascular leakage suggest a possible role for mast cells (MC) and sensory neurons in ET-induced edema, ET did not elicit degranulation of human skin MC or substance P release from NT2N cells in vitro. Our results indicate that ET, acting indirectly or directly on a target yet to be identified, stimulates the production/release of multiple inflammatory mediators, specifically neurokinins, prostanoids, and histamine. These mediators, individually and through complex interactions, increase vascular permeability, and interventions directed at these mediators may benefit hosts infected with B. anthracis.

Bacillus anthracis edema and lethal toxin have different hemodynamic effects but function together to worsen shock and outcome in a rat model

INTRODUCTION

To better define the contribution of edema toxins (ETx) and lethal toxins (LeTx) to shock with Bacillus anthracis, recombinant preparations of each were investigated alone or together in rats.

METHODS AND RESULTS

Lethal dose ranges (0%-100% lethality) of ETx (200-800 microg/kg as a 24-h infusion) were higher than those of LeTx (12.5-200 microg/kg) (P<.0001). However, compared with LeTx, similarly lethal ETx doses produced earlier and greater reductions in mean blood pressure (MBP) and increased, rather than decreased, heart rate (HR) (P<.05 for all). Combining either similar weight or lethal doses of ETx and LeTx increased the hazard ratio for death (log +/- standard error) similar to the sum calculated with the toxin's effects alone (2.6+/-1.1 observed vs. 2.9+/-1.0 calculated for similar weight and 3.1+/-1.0 vs. 3.9+/-1.5 for similar lethal doses; P=.5 for both). Early (< or =10 h) and late during infusion, ETx and LeTx together also altered MBP and HR in patterns consistent with the sum of their individual effects.

CONCLUSIONS

ETx was approximately 10 times less lethal than LeTx but produced greater hypotension and added to the latter's harmful effects. These findings suggest that it may be appropriate for antitoxin therapies for B. anthracis to target both ETx and LeTx.

Anthrax toxin receptor 2-dependent lethal toxin killing in vivo

Anthrax toxin receptors 1 and 2 (ANTXR1 and ANTXR2) have a related integrin-like inserted (I) domain which interacts with a metal cation that is coordinated by residue D683 of the protective antigen (PA) subunit of anthrax toxin. The receptor-bound metal ion and PA residue D683 are critical for ANTXR1-PA binding. Since PA can bind to ANTXR2 with reduced affinity in the absence of metal ions, we reasoned that D683 mutant forms of PA might specifically interact with ANTXR2. We show here that this is the case. The differential ability of ANTXR1 and ANTXR2 to bind D683 mutant PA proteins was mapped to nonconserved receptor residues at the binding interface with PA domain 2. Moreover, a D683K mutant form of PA that bound specifically to human and rat ANTXR2 mediated killing of rats by anthrax lethal toxin, providing strong evidence for the physiological importance of ANTXR2 in anthrax disease pathogenesis.

The bacterium that causes anthrax produces a toxin which is largely responsible for the symptoms and death associated with this disease. The toxin acts by first docking onto specific proteins, called receptors, located on the host cell surface, and it is then taken up into cells where it can act on its cellular substrates. There are two known receptors for the toxin, anthrax toxin receptors 1 and 2 (ANTXR1 and ANTXR2). However, the physiological importance of each receptor in host organisms is not yet understood. To address this issue directly, the authors designed a form of the toxin which binds specifically to ANTXR2 but not to ANTXR1. They show that this ANTXR2-specific form of the toxin is capable of killing rats following intravenous injection. These studies provide direct evidence for the physiological importance of ANTXR2 in anthrax toxin action in a model host organism.

Lethal and edema toxins in the pathogenesis of Bacillus anthracis septic shock: implications for therapy

Recent research regarding the structure and function of Bacillus anthracis lethal (LeTx) and edema (ETx) toxins provides growing insights into the pathophysiology and treatment of shock with this lethal bacteria. These are both binary-type toxins composed of protective antigen necessary for their cellular uptake and either lethal or edema factors, the toxigenic moieties. The primary cellular receptors for protective antigen have been identified and constructed and key steps in the extracellular processing and internalization of the toxins clarified. Consistent with the lethal factor's primary action as an intracellular endopeptidase targeting mitogen-activated protein kinase kinases, growing evidence indicates that shock with this toxin does not result from an excessive inflammatory response. In fact, the potent immunosuppressive effects of LeTx may actually contribute to the establishment and persistence of infection. Instead, shock with LeTx may be related to the direct injurious effects of lethal factor on endothelial cell function. Despite the importance of LeTx, very recent studies show that edema factor, a potent adenyl cyclase, has the ability to make a substantial contribution to shock caused by B. anthracis and works additively with LeTx. Furthermore, ETx may contribute to the immunosuppressive effects of LeTx. Therapies under development that target several different steps in the cellular uptake and function of these two toxins have been effective in in vitro and in vivo systems. Understanding how best to apply these agents clinically and how they interact with conventional treatments should be goals for future research.

CagA protein of Helicobacter pylori: a hijacker of gastric epithelial cell signaling

Epidemiological study has shown strong correlation between the Helicobacter pylori (H. pylori) infection and gastric carcinogenesis. However, the mechanism by which H. pylori induces gastric carcinogenesis is not known. In this review, we focused on the product of cytotoxin-associated gene A (CagA), one of the important virulence factors of H. pylori. H. pylori injects CagA protein into the host gastric epithelial cells through its needle-like structure, type IV secretion system. Injected CagA hijacks physiological signal transduction and causes pathological cellular response such as increased cell proliferation, motility, apoptosis and morphological change through different mechanisms. H. pylori has been shown to produce reactive oxygen species (ROS) in infected gastric mucosa. Although the main source of ROS production is possibly host neutrophil, we propose novel source of ROS production in this review; CagA itself can induce ROS production in gastric epithelial cell. Excessive ROS production in gastric epithelial cells can cause DNA damage and thus might involve in gastric carcinogenesis. Understanding the molecular mechanism by which H. pylori-induced carcinogenesis is important for developing new strategies against gastric cancer.

Cisplatin inhibition of anthrax lethal toxin

Bacillus anthracis lethal toxin (LT) produces symptoms of anthrax in mice and induces rapid lysis of macrophages derived from certain inbred strains. LT is comprised of a receptor binding component, protective antigen (PA), which delivers the enzymatic component, lethal factor (LF), into cells. We found that mouse macrophages were protected from toxin by the antitumor drug cis-diammineplatinum (II) dichloride (cisplatin). Cisplatin was shown to inhibit LT-mediated cleavage of cellular mitogen-activated protein kinases (MEKs) without inhibiting LF's in vitro proteolytic activity. Cisplatin-treated PA lost 100% of its ability to function in toxicity assays when paired with untreated LF, despite maintaining the ability to bind to cells. Cisplatin-treated PA was unable to form heptameric oligomers required for LF binding and translocation. The drug was shown to modify PA in a reversible noncovalent manner. Not surprisingly, cisplatin also blocked the actions of anthrax edema toxin and of LF-Pseudomonas aeruginosa exotoxin A fusion peptide (FP59), both of which require PA for translocation. Treatment of BALB/cJ mice or Fischer F344 rats with cisplatin at biologically relevant concentrations completely protected the animals from a coadministered lethal dose of LT. However, treatment with cisplatin 2 hours before or after animals received a lethal bolus of toxin did not protect them.

Cytotoxic activity of Bacillus anthracis protective antigen observed in a macrophage cell line overexpressing ANTXR1

Anthrax toxin protective antigen (PA) binds cell surface receptors (e.g. ANTXR1,2), forms heptameric pores, and translocates lethal factor (LF) or oedema factor (OF) into the cytoplasm of mammalian cells. In the current study, we sought to determine how receptor levels influence these events, by examining PA heptamer stability and related processes in macrophages that overexpress ANTXR1 (RAW 264.7ANTXR1). In these experiments, PA-oligomers demonstrated an extended half-life in RAW 264.7ANTXR1 macrophages, with SDS-resistant heptamers detected up to 10 h following treatment, while levels of PA-oligomers declined within 3 h in control cells. RAW 264.7ANTXR1 macrophages were also more sensitive to lethal toxin, a combination of PA and LF. Surprisingly, we found that PA alone was cytotoxic to RAW 264.7ANTXR1 cells. Further analysis found that PA cytotoxicity required direct interaction with ANTXR1, oligomerization, channel formation, endosomal acidification, and was independent of the ANTXR1 cytoplasmic tail. PA intoxication of RAW 264.7ANTXR1 macrophages resulted in caspase-3 activation, with corresponding DNA fragmentation and proteolytic cleavage of poly-ADP-ribose polymerase, as well as activation of Bid, suggesting cell death occurred via apoptosis. Overall, results from the current study suggest that receptor levels dictate the extent of PA oligomer stability, and shifts in this normal process can lead to cell death via apoptosis in the absence of toxin catalytic subunits.

Oxidized ATP protection against anthrax lethal toxin

Bacillus anthracis lethal toxin (LT) induces rapid lysis (<90 min) of murine macrophages from certain inbred strains. The mechanism for LT-induced cytolysis is currently unknown. We hypothesized that the ATP-activated macrophage P2X7 receptors implicated in nucleotide-mediated macrophage lysis could play a role in LT-mediated cytolysis and discovered that a potent P2X7 antagonist, oxidized ATP (o-ATP), protects macrophages against LT. Other P2X7 receptor antagonists, however, had no effect on LT function, while oxidized nucleotides, o-ADP, o-GTP, and o-ITP, which did not act as receptor ligands, provided protection. Cleavage of the LT substrates, the mitogen-activated protein kinases, was inhibited by o-ATP in RAW274.6 macrophages and CHO cells. We investigated the various steps in the intoxication pathway and found that binding of the protective-antigen (PA) component of LT to cells and the enzymatic proteolytic ability of the lethal factor (LF) component of LT were unaffected by o-ATP. Instead, the drug inhibited formation of the sodium dodecyl sulfate-resistant PA oligomer, which occurs in acidified endosomes, but did not prevent cell surface PA oligomerization, as evidenced by binding and translocation of LF to a protease-resistant intracellular location. We found that o-ATP also protected cells from anthrax edema toxin and diphtheria toxin, which also require an acidic environment for escape from endosomes. Confocal microscopy using pH-sensitive fluorescent dyes showed that o-ATP increased endosomal pH. Finally, BALB/cJ mice injected with o-ATP and LT were completely protected against lethality.

Kinin receptor expression during Staphylococcus aureus infection

An inappropriate host response to invading bacteria is a critical parameter that often aggravates the outcome of an infection. Staphylococcus aureus is a major human Gram-positive pathogen that causes a wide array of community- and hospital-acquired diseases ranging from superficial skin infections to severe conditions such as staphylococcal toxic shock. Here we find that S aureus induces inflammatory reactions by modulating the expression and response of the B1 and B2 receptors, respectively. This process is initiated by a chain of events, involving staphylococcal-induced cytokine release from monocytes, bacteria-triggered contact activation, and conversion of bradykinin to its metabolite desArg(9)bradykinin. The data of the present study implicate an important and previously unknown role for kinin receptor regulation in S aureus infections.

Resident CD11c+ lung cells are impaired by anthrax toxins after spore infection

Bacillus anthracis secretes 2 toxins: lethal toxin (LT) and edema toxin (ET). We investigated their role in the physiopathologic mechanisms of inhalational anthrax by evaluating murine lung dendritic cell (LDC) functions after infection with B. anthracis strains secreting LT, ET, or both or with a nontoxinogenic strain. Three lung cell populations gated on CD11c/CD11b expression were obtained after lung digestion: (1) CD11c(high)/CD11b(low) (alveolar macrophages), (2) CD11c(intermediate (int))/CD11b(int) (LDCs), and (3) CD11c(low)/CD11b(high) (interstitial macrophages or monocytes). After infection with LT-secreting strains, a decrease in costimulatory molecule expression on LDCs was observed. All CD11c+ cells infected with a nontoxinogenic strain secreted tumor necrosis factor (TNF)- alpha , interleukin (IL)-10, and IL-6. LT-secreting strains inhibited overall cytokine secretion, whereas the ET-secreting strain inhibited only TNF- alpha secretion and increased IL-6 secretion. Similar results were obtained after preincubation with purified toxins. Our results suggest that anthrax toxins secreted during infection impair LDC function and suppress the innate immune response.

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

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

Anthrax Lethal Toxin Has Direct and Potent Inhibitory Effects on B Cell Proliferation and Immunoglobulin Production

Protective host immune responses to anthrax infection in humans and animal models are characterized by the development of neutralizing Abs against the receptor-binding anthrax protective Ag (PA), which, together with the lethal factor (LF) protease, composes anthrax lethal toxin (LT). We now report that B cells, in turn, are targets for LT. Anthrax PA directly binds primary B cells, resulting in the LF-dependent cleavage of the MAPK kinases (MAPKKs) and disrupted signaling to downstream MAPK targets. Although not directly lethal to B cells, anthrax LT treatment causes severe B cell dysfunction, greatly reducing proliferative responses to IL-4-, anti-IgM-, and/or anti-CD40 stimulation. Moreover, B cells treated with anthrax LT in vitro or isolated from mice treated with anthrax LT in vivo have a markedly diminished capacity to proliferate and produce IgM in response to TLR-2 and TLR-4 ligands. The suppressive effects of anthrax LT on B cell function occur at picomolar concentrations in vitro and at sublethal doses in vivo. These results indicate that anthrax LT directly inhibits the function of B cells in vitro and in vivo, revealing a potential mechanism through which the pathogen could bypass protective immune responses.

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.

Statistical pattern matching facilitates the design of polyvalent inhibitors of anthrax and cholera toxins

Numerous biological processes involve the recognition of a specific pattern of binding sites on a target protein or surface. Although ligands displayed by disordered scaffolds form stochastic rather than specific patterns, theoretical models predict that recognition will occur between patterns that are characterized by similar or "matched" statistics. Endowing synthetic biomimetic structures with statistical pattern matching capabilities may improve the specificity of sensors and resolution of separation processes. We demonstrate that statistical pattern matching enhances the potency of polyvalent therapeutics. We functionalized liposomes with an inhibitory peptide at different densities and observed a transition in potency at an interpeptide separation that matches the distance between ligand-binding sites on the heptameric component of anthrax toxin. Pattern-matched polyvalent liposomes inhibited anthrax toxin in vitro at concentrations four orders of magnitude lower than the corresponding monovalent peptide, and neutralized this toxin in vivo. Statistical pattern matching also enhanced the potency of polyvalent inhibitors of cholera toxin. This facile strategy should be broadly applicable to the detection and neutralization of toxins and pathogens.

New insights into the functions of anthrax toxin

Anthrax is the disease caused by the Gram-positive bacterium Bacillus anthracis. Two toxins secreted by B. anthracis - lethal toxin (LT) and oedema toxin (OT) - contribute significantly to virulence. Although these toxins have been studied for half a century, recent evidence indicates that LT and OT have several roles during infection not previously ascribed to them. Research on toxin-induced effects other than cytolysis of target cells has revealed that LT and OT influence cell types previously thought to be insensitive to toxin. Multiple host factors that confer sensitivity to anthrax toxin have been identified recently, and evidence indicates that the toxins probably contribute to colonisation and invasion of the host. Additionally, the toxins are now known to cause a wide spectrum of tissue and organ pathophysiologies associated with anthrax. Taken together, these new findings indicate that anthrax-toxin-associated pathogenesis is much more complex than has been traditionally recognised.

Anthrax lethal toxin induces ketotifen-sensitive intradermal vascular leakage in certain inbred mice

Bacillus anthracis lethal toxin (LT) is a bipartite toxin composed of protective antigen (PA) and lethal factor (LF). Injection of LT produces clinical signs characteristic of anthrax infection, including pleural edema and vascular collapse in various animal models. We utilized the classic Miles leakage assay to quantify vascular leakage in mice. LT injected intradermally induced leakage as early as 15 to 25 min in some inbred mouse strains, but not in others, whereas PA or LF individually did not induce leakage. A third component of anthrax toxin, edema factor, did not induce leakage alone or with PA. Leakage was quantified in eight mouse strains, and no correlation was found between sensitivity to intradermal leakage and sensitivity to the lethality of systemically administered LT. The leakage could be inhibited by ketotifen, an inhibitor of mast cell degranulation, but not by azelastine, a histamine receptor 1 antagonist, or by ketanserin, a serotonin 5-HT2A receptor antagonist. LT was cytotoxic to MC/9 mast cells (in vitro) by 7 h after toxin treatment but did not induce histamine release from these cells. Mast cell-deficient mice exhibited the leakage event and had no increased resistance to systemic LT. Human umbilical vein endothelial cells were resistant to LT over 12 h, with only 20% of cells succumbing by 24 h, suggesting that endothelial cell killing is not the cause of the rapid LT-mediated leakage event. We describe here a ketotifen-sensitive vascular leakage event induced by LT which is the most rapid in vivo or in vitro LT-mediated effect reported to date.

Efficient neutralization of anthrax toxin by chimpanzee monoclonal antibodies against protective antigen

Four single-chain variable fragments (scFvs) against protective antigen (PA) and 2 scFvs against lethal factor (LF) of anthrax were isolated from a phage display library generated from immunized chimpanzees. Only 2 scFvs recognizing PA (W1 and W2) neutralized the cytotoxicity of lethal toxin in a macrophage lysis assay. Full-length immunoglobulin G (IgG) of W1 and W2 efficiently protected rats from anthrax toxin challenge. The epitope recognized by W1 and W2 was conformational and was formed by C-terminal amino acids 614–735 of PA. W1 and W2 each bound to PA with an equilibrium dissociation constant of 4×10^-11 mol/L to 5 × 10⁻¹¹ mol/L, which is an affinity that is 20–100-fold higher than that for the interaction of the receptor and PA. W1 and W2 inhibited the binding of PA to the receptor, suggesting that this was the mechanism of protection. These data suggest that W1 and W2 chimpanzee monoclonal antibodies may serve as PA entry inhibitors for use in the emergency prophylaxis against and treatment of anthrax

Cytokine response and survival of mice immunized with an adenovirus expressing Bacillus anthracis protective antigen domain 4

Adenovirus vectors are promising for use in vaccinating against potential agents of bioterrorism and emerging infections because of their proven safety in humans and their ability to elicit rapid immune responses. Here, we describe the construction and evaluation of an adenovirus vaccine expressing domain 4 of Bacillus anthracis protective antigen, Ad.D4. Ad.D4 elicited antibodies to protective antigen 14 days after a single intramuscular injection, which were further increased upon boosting. Furthermore, two doses of Ad.D4 4 weeks apart were sufficient to protect 67% of mice from toxin challenge. Additionally, we have characterized the release of inflammatory cytokines from vaccinated mice after lethal-toxin challenge. We demonstrate that interleukin 1beta (IL-1beta) levels in mice that survive lethal toxin challenge are similar to levels in nonsurvivors and that IL-6 levels are higher in survivors than in nonsurvivors. These findings suggest that lethal-toxin-mediated death may not be a direct result of inflammatory-cytokine release.

Bacillus anthracis edema toxin causes extensive tissue lesions and rapid lethality in mice

Bacillus anthracis edema toxin (ET), an adenylyl cyclase, is an important virulence factor that contributes to anthrax disease. The role of ET in anthrax pathogenesis is, however, poorly understood. Previous studies using crude toxin preparations associated ET with subcutaneous edema, and ET-deficient strains of B. anthracis showed a reduction in virulence. We report the first comprehensive study of ET-induced pathology in an animal model. Highly purified ET caused death in BALB/cJ mice at lower doses and more rapidly than previously seen with the other major B. anthracis virulence factor, lethal toxin. Observations of gross pathology showed intestinal intralumenal fluid accumulation followed by focal hemorrhaging of the ileum and adrenal glands. Histopathological analyses of timed tissue harvests revealed lesions in several tissues including adrenal glands, lymphoid organs, bone, bone marrow, gastrointestinal mucosa, heart, and kidneys. Concomitant blood chemistry analyses supported the induction of tissue damage. Several cytokines increased after ET administration, including granulocyte colony-stimulating factor, eotaxin, keratinocyte-derived cytokine, MCP-1/JE, interleukin-6, interleukin-10, and interleukin-1beta. Physiological measurements also revealed a concurrent hypotension and bradycardia. These studies detail the extensive pathological lesions caused by ET and suggest that it causes death due to multiorgan failure.

Antitumor efficacy of a urokinase activation–dependent anthrax toxin

Previously, we have generated a potent prodrug consisting of modified anthrax toxins that is activated by urokinase plasminogen activator (uPA). The cytotoxicity of the drug, PrAg-U2 + FP59, is dependent on the presence of receptor-associated uPA activity. Local intradermal administration of PrAg-U2 + FP59 adjacent to the tumor nodules in mice with transplanted solid tumors had a potent antitumor effect. In succession of these experiments, we have now investigated the systemic antitumor efficacy of PrAg-U2 + FP59. C57Bl/6J mice bearing syngenic tumors derived from B16 melanoma, T241 fibrosarcoma, or Lewis lung carcinoma cells were treated with different mass ratios and doses of PrAg-U2 + FP59. Tumor volumes were recorded daily by caliper measurements. In some experiments, dexamethasone was coadministered. Our data show a significant antitumor effect of systemic administration of PrAg-U2 + FP59 in three syngenic tumor models. Optimal antitumor effect and low toxicity was obtained with a 25:1 mass ratio between the two components (PrAg-U2 and FP59). The experiments show that PrAg-U2 + FP59 displays a clear dose-response relationship with regard to both antitumor efficacy and systemic toxicity. Dose-limiting toxicity seemed to be due to activation of the prodrug by uPA and its receptor in the intestinal mucosa. Concurrent treatment with dexamethasone was found to prevent dose-limiting toxicity. Taken together, these data indicate that uPA-activated toxins may be promising candidates for targeted therapy of human cancers that overexpress uPA and its receptor.

Nalp1b controls mouse macrophage susceptibility to anthrax lethal toxin

The pathogenesis of Bacillus anthracis, the bacterium that causes anthrax, depends on secretion of three factors that combine to form two bipartite toxins. Edema toxin, consisting of protective antigen (PA) and edema factor (EF), causes the edema associated with cutaneous anthrax infections, whereas lethal toxin (LeTx), consisting of PA and lethal factor (LF), is believed to be responsible for causing death in systemic anthrax infections. EF and LF can be transported by PA into the cytosol of many cell types. In mouse macrophages, LF can cause rapid necrosis that may be related to the pathology of systemic infections. Inbred mouse strains display variable sensitivity to LeTx-induced macrophage necrosis. This trait difference has been mapped to a locus on chromosome 11 named Ltxs1 (refs. 7,8). Here we show that an extremely polymorphic gene in this locus, Nalp1b, is the primary mediator of mouse macrophage susceptibility to LeTx. We also show that LeTx-induced macrophage death requires caspase-1, which is activated in susceptible, but not resistant, macrophages after intoxication, suggesting that Nalp1b directly or indirectly activates caspase-1 in response to LeTx.

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.

Bacillus anthracis oedema toxin as a cause of tissue necrosis and cell type-specific cytotoxicity

Oedema factor (OF) and protective antigen (PA) are secreted by Bacillus anthracis, and their binary combination yields oedema toxin (OT). Following PA-mediated delivery to the cytosol, OF functions as an adenylate cyclase generating high levels of cAMP. To assess OT as a possible cause of tissue damage and cell death, a novel approach was developed, which utilized a developing zebrafish embryo model to study toxin activity. Zebrafish embryos incubated with OT exhibited marked necrosis of the liver, cranium and gastrointestinal tract, as well as reduced swim bladder inflation. The OT-treated embryos survived after all stages of development but succumbed to the toxin within 7 days. Additional analysis of specific cell lines, including macrophage and non-macrophage, showed OT-induced cell death is cell type-specific. There was no discernible correlation between levels of OF-generated cAMP and cell death. Depending on the type of cell analysed, cell death could be detected in low levels of cAMP, and, conversely, cell survival was observed in one cell line in which high levels of cAMP were found following treatment with OT. Collectively, these data suggest OT is cytotoxic in a cell-dependent manner and may contribute to disease through direct cell killing leading to tissue necrosis.

Pathophysiological manifestations in mice exposed to anthrax lethal toxin

Pathophysiological changes associated with anthrax lethal toxin included loss of plasma proteins, decreased platelet count, slower clotting times, fibrin deposits in tissue sections, and gross and histopathological evidence of hemorrhage. These findings suggest that blood vessel leakage and hemorrhage lead to disseminating intravascular coagulation and/or circulatory shock as an underlying pathophysiological mechanism.

Protection from anthrax toxin-mediated killing of macrophages by the combined effects of furin inhibitors and chloroquine

Cell surface proteolytic processing of anthrax protective antigen by furin or other furin-related proteases is required for its oligomerization, endocytosis, and function as a translocon for anthrax lethal and edema factors. Countering toxin lethality is essential to developing effective chemotherapies for anthrax infections that have proceeded beyond the stage at which antibiotics are effective. The primary target for toxin is the macrophage, which can be killed by lethal factor via both necrotic and apoptotic pathways. Here we show that three high-affinity inhibitors of furin efficiently blocked killing of murine J774A.1 macrophages by recombinant protective antigen plus lethal factor: RRD-eglin and RRDG-eglin, developed by engineering the protein protease inhibitor eglin c, and the peptide boronic acid inhibitor acetyl-Arg-Glu-Lys-boroArg pinanediol. Inhibition of killing was dose dependent and correlated with prevention of protective antigen processing. Previous studies have shown that weak bases, such as chloroquine, which neutralize acidic compartments, also interfere with toxin-dependent killing. Here we show that combining furin inhibitors and chloroquine strongly augments the inhibition of toxin-dependent killing, suggesting that combined use of antifurin drugs and chloroquine might provide enhanced therapeutic benefits. Reversible furin inhibitors protected against anthrax toxin killing for at least 5 h, but by 8 h, toxin-dependent killing resumed even though furin inhibitors were still active. An irreversible chloromethylketone inhibitor did not exhibit this loss of protection.

Bacillus anthracis: toxicology, epidemiology and current rapid-detection methods

B. anthracis, the causative agent for anthrax, has been well studied for over 150 years. Due to the genetic similarities among various Bacillus species, as well as its existence in both a spore form and a vegetative state, the detection and specific identification of B. anthracis have been proven to require complex techniques and/or laborious methods. With the heightened interest in the organism as a potential biological threat agent, a large number of interesting detection technologies have recently been developed, including methods involving immunological and nucleic acid-based assay formats. The technologies range from culture-based methods to portable Total Analysis Systems based on real-time PCR. This review with 170 references provides a brief background on the toxicology and epidemiology of B. anthracis, discusses challenges associated with its detection related to genetic similarities to other species, and reviews immunological and, with greater emphasis, nucleic acid-based detection systems.

Whole-cell Voltage Clamp Measurements of Anthrax Toxin Pore Current

Protective antigen (PA) of anthrax toxin binds cellular receptors and forms pores in target cell membranes, through which catalytic lethal factor (LF) and edema factor (EF) are believed to translocate to the cytoplasm. Using patch clamp electrophysiological techniques, we assayed pore formation by PA in real time on the surface of cultured cells. The membranes of CHO-K1 cells treated with activated PA had little to no electrical conductivity at neutral pH (7.3) but exhibited robust mixed ionic currents in response to voltage stimuli at pH 5.3. Pore formation depended on specific cellular receptors and exhibited voltage-dependent inactivation at large potentials (>60 mV). The pH requirement for pore formation was receptor-specific as membrane insertion occurs at significantly different pH values when measured in cells specifically expressing tumor endothelial marker 8 (TEM8) or capillary morphogenesis protein 2 (CMG2), the two known cellular receptors for anthrax toxin. Pores were inhibited by an N-terminal fragment of LF and by micromolar concentrations of tetrabutylammonium ions. These studies demonstrated basic biophysical properties of PA pores in cell membranes and served as a foundation for the study of LF and EF translocation in vivo.

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

We used the Bacillus brevis-pNU212 system to develop a mass production system for the protective antigen (PA) of Bacillus anthracis. A moderately efficient expression-secretion system for PA was constructed by fusing the PA gene from B. anthracis with the B. brevis cell-wall protein signal-peptide encoding region of pNU212, and by introducing the recombinant plasmid, pNU212-mPA, into B. brevis 47-5Q. The clone producing PA secreted about 300 microg of recombinant PA (rPA) per ml of 5PY-erythromycin medium after 4 days incubation at 30 degrees C. The rPA was fractionated from the culture supernatant of B. brevis 47-5Q carrying pNU212-mPA using ammonium sulfate at 70% saturation followed by anion exchange chromatography on a Hitrap Q, a Hiload 16/60 Superdex 200 gel filtration column and a phenyl sepharose hydrophobic interaction column, yielding 70 mg rPA per liter of culture. The N-terminal sequence of the purified rPA was identical to that of native PA from B. anthracis. The purified rPA exhibited cytotoxicity towards J774A.1 cells when combined with lethal factor. The rPA formulated in either Rehydragel HPA or MPL-TDM-CWS adjuvant (Ribi-Trimix) elicited the expression of a large amount of anti-PA and neutralizing antibodies in guinea pigs and completely protected them against a 100 LD50 challenge with fully virulent B. anthracis spores.

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

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

Antiplatelet Activities of Anthrax Lethal Toxin Are Associated with Suppressed p42/44 and p38 Mitogen‐Activated Protein Kinase Pathways in the Platelets

Anthrax lethal toxin (LT) is the major virulence factor produced by Bacillus anthracis, but the mechanism by which it induces high mortality remains unclear. We found that LT treatment could induce severe hemorrhage in mice and significantly suppress human whole-blood clotting and platelet aggregation in vitro. In addition, LT could inhibit agonist-induced platelet surface P-selectin expression, resulting in the inhibition of platelet-endothelial cell engagements. Data from Western blot analysis indicated that LT treatment resulted in the suppression of p42/44 and p38 mitogen-activated protein kinase pathways in platelets. Combined treatments with LT and antiplatelet agents such as aspirin and the RGD-containing disintegrin rhodostomin significantly increased mortality in mice. Our data suggest that platelets are a pathogenic target for anthrax LT.

Cerebral diffusional changes in the early phase of anthrax: is cutaneous anthrax only limited to skin?

OBJECTIVES

Characteristics of cerebral diffusion in the acute period of the anthrax infection were investigated to understand the pathophysiology of the disease.

METHODS

Six cutaneous anthrax patients (mean age: 33.3, SD: 18.1) and six healthy control subjects (mean age: 33.7, SS: 19.6) were examined at the acute phase of the infection with diffusion weighted imaging on 1.5 T scanner. ADC values were measured from five different cerebral locations. T-tests, logistic regression and ROC curves were used.

RESULTS

Anthrax patients were significantly different than controls regarding cortical ADC values (p<0.05). Logistic regression model accurately classified five out of the six anthrax cases (83.3%). A cut-off value of 574 mm2/s x 10(-3) was found by using ROC curve coordinates. A sensitivity of 100% and a specificity of 67% were attained by means of this value.

CONCLUSIONS

This study shows the existence of cerebral parenchymal changes at microstructural level in cutaneous anthrax without neurological findings. These changes are possibly related to the components of the toxin. Our results support the general but unproven opinion that anthrax treatment does not change the existence and the effects of the toxin. Pathophysiological mechanisms towards classification should therefore be reviewed.

BRAF status and mitogen-activated protein/extracellular signal-regulated kinase kinase 1/2 activity indicate sensitivity of melanoma cells to anthrax lethal toxin

Anthrax lethal toxin, composed of protective antigen and lethal factor, was tested for cytotoxicity to human melanoma cell lines and normal human cells. Eleven of 18 melanoma cell lines were sensitive to anthrax lethal toxin (IC(50) < 400 pmol/L) and 10 of these 11 sensitive cell lines carried the V599E BRAF mutation. Most normal cell types (10 of 15) were not sensitive to anthrax lethal toxin and only 5 of 15 normal human cell types were sensitive to anthrax lethal toxin (IC(50) < 400 pmol/L). These cells included monocytes and a subset of endothelial cells. In both melanoma cell lines and normal cells, anthrax toxin receptor expression levels did not correlate with anthrax lethal toxin cytotoxicity. Furthermore, an anthrax toxin receptor-deficient cell line (PR230) did not show any enhanced sensitivity to anthrax lethal toxin when transfected with anthrax toxin receptor. Anthrax lethal toxin toxicity correlated with elevated phosphorylation levels of mitogen-activated protein/extracellular signal-regulated kinase kinase (MEK) 1/2 in both melanoma cell lines and normal cells. Anthrax lethal toxin-sensitive melanoma cell lines and normal cells had higher phospho-MEK1/2 levels than anthrax lethal toxin-resistant melanoma cell lines and normal tissue types. U0126, a specific MEK1/2 inhibitor, was not toxic to anthrax lethal toxin-resistant melanoma cell lines but was toxic to 8 of 11 anthrax lethal toxin-sensitive cell lines. These results show that anthrax lethal toxin toxicity correlates with elevated levels of active MEK1/2 pathway but not with anthrax toxin receptor expression levels in both normal and malignant tissues. Anthrax lethal toxin may be a useful therapeutic for melanoma patients, especially those carrying the V599E BRAF mutation with constitutive activation of the mitogen-activated protein kinase pathway.

A Soluble Receptor Decoy Protects Rats against Anthrax Lethal Toxin Challenge

Successful postexposure treatment for inhalation anthrax is thought to include neutralization of anthrax toxin. The soluble anthrax toxin receptor/tumor endothelial marker 8 and capillary morphogenesis protein 2 (sATR/TEM8 and sCMG2, respectively) receptor decoys bind to anthrax toxin protective antigen (PA) and compete with cellular receptors for binding. Here, we show that, in a tissue-culture model of intoxication, sCMG2 is a 11.4-fold more potent antitoxin than sATR/TEM8 and that this increased activity corresponds to an approximately 1000-fold higher PA-binding affinity. Stoichiometric concentrations of sCMG2 protect rats against lethal toxin challenge, making sCMG2 one of the most effective anthrax antitoxins described to date.

[Use of infusoria Paramecium caudatum for the evaluation of the toxicity of antigens of different causative quarantine agents]

The results of the evaluation of the toxicity of bacterial antigens obtained from the causative agents of plaque, glanders, melioidosis, cholera on infusoria of the species P. caudatum, as well as on cell lines L-929, CHO K-1 and peritoneal macrophages of BALB/c mice, are presented. As revealed in this study, the method of toxicity determination on infusoria is similar in its sensitivity to the methods of testing on. CHO K-1 and L-929 cells, but the former is simpler, more available and permits the determination of toxic doses producing disturbances in the vital activity of the infusoria, but not leading to their death.