Systemic cytokine response in murine anthrax

Rational corticosteroids administration and antibiotic treatment is key to managing cutaneous anthrax

BACKGROUND

Anthrax is a global health concern, with cutaneous anthrax accounting for over 95% of cases and generally promising outcomes. Nonetheless, the absence of timely intervention can result in mortality rates of 10-40%. This research aims to explore the clinical presentations and phenotypic characteristics of cutaneous anthrax patients and evaluate the efficacy of various therapeutic approaches.

METHODS

A retrospective study was performed on 76 cutaneous anthrax patients identified at three hospitals from 2017 to 2022. Patients were categorized based on their hospital stay into two groups: those hospitalized for at least seven days and those for shorter durations. We assessed their clinical and phenotypic profiles, including symptoms, general health status, and laboratory findings, alongside treatment outcomes, focusing on corticosteroids therapy and antibiotic regimens.

RESULTS

The study encompassed 76 diagnosed individuals, predominantly young adult males (78.9%). A significant gender disparity was noted. Hormonal treatment markedly improved edema regression in patients (P < 0.002), highlighting its therapeutic value. The impact of various antibiotic treatments on disease progression differed significantly based on corticosteroids treatment status, with specific combinations showing more effectiveness in non-corticosteroids-treated patients.

CONCLUSIONS

The predominance of young male adults among cutaneous anthrax cases was observed, with corticosteroids treatment significantly reducing edema duration. In cases where corticosteroids therapy is not utilized, employing piperacillin-tazobactam alone or in combination with quinolones effectively shortens the illness duration, suggesting a tailored approach to treatment can enhance patient outcomes.

Animal Models for the Pathogenesis, Treatment, and Prevention of Infection by Bacillus anthracis

This article reviews the characteristics of the major animal models utilized for studies on Bacillus anthracis and highlights their contributions to understanding the pathogenesis and host responses to anthrax and its treatment and prevention. Advantages and drawbacks associated with each model, to include the major models (murine, guinea pig, rabbit, nonhuman primate, and rat), and other less frequently utilized models, are discussed. Although the three principal forms of anthrax are addressed, the main focus of this review is on models for inhalational anthrax. The selection of an animal model for study is often not straightforward and is dependent on the specific aims of the research or test. No single animal species provides complete equivalence to humans; however, each species, when used appropriately, can contribute to a more complete understanding of anthrax and its etiologic agent.

Rapid vaccination using an acetalated dextran microparticulate subunit vaccine confers protection against triplicate challenge by bacillus anthracis

PURPOSE

A rapid immune response is required to prevent death from Anthrax, caused by Bacillus anthracis.

METHOD

We formulated a vaccine carrier comprised of acetalated dextran microparticles encapsulating recombinant protective antigen (rPA) and resiquimod (a toll-like receptor 7/8 agonist).

RESULTS

We were able to protect against triplicate lethal challenge by vaccinating twice (Days 0, 7) and then aggressively challenging on Days 14, 21, 28. A significantly higher level of antibodies was generated by day 14 with the encapsulated group compared to the conventional rPA and alum group. Antibodies produced by the co-encapsulated group were only weakly-neutralizing in toxin neutralization; however, survival was not dependent on toxin neutralization, as all vaccine formulations survived all challenges except control groups. Post-mortem culture swabs taken from the hearts of vaccinated groups that did not produce significant neutralizing titers failed to grow B. anthracis.

CONCLUSIONS

Results indicate that protective antibodies are not required for rapid protection; indeed, cytokine results indicate that T cell protection may play a role in protection from anthrax. We report the first instance of use of a particulate carrier to generate a rapid protective immunity against anthrax.

Immunization of mice with formalin-inactivated spores from avirulent Bacillus cereus strains provides significant protection from challenge with Bacillus anthracis Ames

Bacillus anthracis spores are the infectious form of the organism for humans and animals. However, the approved human vaccine in the United States is derived from a vegetative culture filtrate of a toxigenic, nonencapsulated B. anthracis strain that primarily contains protective antigen (PA). Immunization of mice with purified spore proteins and formalin-inactivated spores (FIS) from a nonencapsulated, nontoxigenic B. anthracis strain confers protection against B. anthracis challenge when PA is also administered. To investigate the capacity of the spore particle to act as a vaccine without PA, we immunized mice subcutaneously with FIS from nontoxigenic, nonencapsulated B. cereus strain G9241 pBCXO1(-)/pBC210(-) (dcG9241), dcG9241 ΔbclA, or 569-UM20 or with exosporium isolated from dcG9241. FIS vaccination provided significant protection of mice from intraperitoneal or intranasal challenge with spores of the virulent B. anthracis Ames or Ames ΔbclA strain. Immunization with dcG9241 ΔbclA FIS, which are devoid of the immunodominant spore protein BclA, provided greater protection from challenge with either Ames strain than did immunization with FIS from BclA-producing strains. In addition, we used prechallenge immune antisera to probe a panel of recombinant B. anthracis Sterne spore proteins to identify novel immunogenic vaccine candidates. The antisera were variably reactive with BclA and with 10 other proteins, four of which were previously tested as vaccine candidates. Overall our data show that immunization with FIS from nontoxigenic, nonencapsulated B. cereus strains provides moderate to high levels of protection of mice from B. anthracis Ames challenge and that neither PA nor BclA is required for this protection.

Disrupting the luxS quorum sensing gene does not significantly affect Bacillus anthracis virulence in mice or guinea pigs

Many bacterial species use secreted quorum-sensing autoinducer molecules to regulate cell density- and growth phase-dependent gene expression, including virulence factor production, as sufficient environmental autoinducer concentrations are achieved. Bacillus anthracis, the causative agent of anthrax, contains a functional autoinducer (AI-2) system, which appears to regulate virulence gene expression. To determine if the AI-2 system is necessary for disease, we constructed a LuxS AI-2 synthase-deficient mutant in the virulent Ames strain of B. anthracis. We found that growth of the LuxS-deficient mutant was inhibited and sporulation was delayed when compared with the parental strain. However, spores of the Ames luxS mutant remained fully virulent in both mice and guinea pigs.

Characterization of a multi-component anthrax vaccine designed to target the initial stages of infection as well as toxaemia

Current vaccine approaches to combat anthrax are effective; however, they target only a single protein [the protective antigen (PA) toxin component] that is produced after spore germination. PA production is subsequently increased during later vegetative cell proliferation. Accordingly, several aspects of the vaccine strategy could be improved. The inclusion of spore-specific antigens with PA could potentially induce protection to initial stages of the disease. Moreover, adding other epitopes to the current vaccine strategy will decrease the likelihood of encountering a strain of Bacillus anthracis (emerging or engineered) that is refractory to the vaccine. Adding recombinant spore-surface antigens (e.g. BclA, ExsFA/BxpB and p5303) to PA has been shown to augment protection afforded by the latter using a challenge model employing immunosuppressed mice challenged with spores derived from the attenuated Sterne strain of B. anthracis. This report demonstrated similar augmentation utilizing guinea pigs or mice challenged with spores of the fully virulent Ames strain or a non-toxigenic but encapsulated ΔAmes strain of B. anthracis, respectively. Additionally, it was shown that immune interference did not occur if optimal amounts of antigen were administered. By administering the toxin and spore-based immunogens simultaneously, a significant adjuvant effect was also observed in some cases. Thus, these data further support the inclusion of recombinant spore antigens in next-generation anthrax vaccine strategies.

Electroporation of a multivalent DNA vaccine cocktail elicits a protective immune response against anthrax and plague

Electroporation of DNA vaccines represents a platform technology well positioned for the development of multivalent biodefense vaccines. To evaluate this hypothesis, three vaccine constructs were produced using codon-optimized genes encoding Bacillus anthracis Protective Antigen (PA), and the Yersinia pestis genes LcrV and F1, cloned into pVAX1. A/J mice were immunized on a prime-boost schedule with these constructs using the electroporation-based TriGrid Delivery System. Immunization with the individual pDNA vaccines elicited higher levels of antigen-specific IgG than when used in combination. DNA vaccine effectiveness was proven, the pVAX-PA titers were toxin neutralizing and fully protective against a lethal B. anthracis spore challenge when administered alone or co-formulated with the plague pDNA vaccines. LcrV and F1 pVAX vaccines against plague were synergistic, resulting in 100% survival, but less protective individually and when co-formulated with pVAX-PA. These DNA vaccine responses were Th1/Th2 balanced with high levels of IFN-γ and IL-4 in splenocyte recall assays, contrary to complimentary protein Alum vaccinations displaying a Th2 bias with increased IL-4 and low levels of IFN-γ. These results demonstrate the feasibility of electroporation to deliver and maintain the overall efficacy of an anthrax-plague DNA vaccine cocktail whose individual components have qualitative immunological differences when combined.

Platelets, inflammatory cells, von Willebrand factor, syndecan-1, fibrin, fibronectin, and bacteria co-localize in the liver thrombi of Bacillus anthracis-infected mice

Vascular dysfunction and thrombosis have been described in association with anthrax infection in humans and animals but the mechanisms of these dysfunctions, as well as the components involved in thrombi formation are poorly understood. Immunofluorescent microscopy was used to define the composition of thrombi in the liver of mice challenged with the Bacillus anthracis Sterne spores. Lethal infection with the toxigenic Sterne strain, in contrast to the non-lethal, non-toxigenic delta-Sterne strain, demonstrated time-dependent increase in the number of vegetative bacteria inside the liver sinusoids and central vein. Massive appearance of thrombi typically occluding the lumen of the vessels coincided with the sudden death of infected animals. Bacterial chains in the thrombi were stained positive for syndecan-1 (SDC-1), fibronectin, and were surrounded by fibrin polymers, GPIIb-positive platelets, von Willebrand Factor (vWF), CD45-positive leukocytes, and massive amount of shed SDC-1. Experiments with human umbilical vein endothelial cells (HUVECs) demonstrated the active role of the host response to the secreted pathogenic factors of bacteria during the onset of the pro-thrombotic condition. The bacterial culture supernatants, as well as the isolated proteins (the pore-forming toxin anthrolysin O and phospholipase C) induced release of vWF, while anthrolysin O, sphingomyelinase and edema toxin induced release of thrombin from HUVECs and polymerization of fibrin in the presence of human plasma.

CONCLUSION

Our findings suggest that activation of endothelium in response to infection can contribute to the formation of occlusive thrombi consisting of aggregated bacteria, vWF, shed SDC-1, fibrin, activated platelets, fibronectin and leukocytes.

Targeting the inflammasome and adenosine type-3 receptors improves outcome of antibiotic therapy in murine anthrax

AIM: To establish whether activation of adenosine type-3 receptors (A3Rs) and inhibition of interleukin-1β-induced inflammation is beneficial in combination with antibiotic therapy to increase survival of mice challenged with anthrax spores.

METHODS: DBA/2 mice were challenged with Bacillus anthracis spores of the toxigenic Sterne strain 43F2. Survival of animals was monitored for 15 d. Ciprofloxacin treatment (50 mg/kg, once daily, intraperitoneally) was initiated at day +1 simultaneously with the administration of inhibitors, and continued for 10 d. Two doses (2.5 mg/kg and 12.5 mg/kg) of acetyl-tyrosyl-valyl-alanyl-aspartyl-chloromethylketone (YVAD) and three doses (0.05, 0.15 and 0.3 mg/kg) of 1-[2-Chloro-6-[[(3-iodophenyl) methyl]amino]-9H-purin-9-yl]-1-deoxy-N-methyl-β-D- ribofuranuronamide (Cl-IB-MECA) were tested. Animals received YVAD on days 1-4, and Cl-IB-MECA on days 1-10 once daily, subcutaneously. Human lung epithelial cells in culture were challenged with spores or edema toxin and the effects of IB-MECA on phosphorylation of AKT and generation of cAMP were tested.

RESULTS: We showed that the outcome of antibiotic treatment in a murine anthrax model could be substantially improved by co-administration of the caspase-1/4 inhibitor YVAD and the A3R agonist Cl-IB-MECA. Combination treatment with these substances and ciprofloxacin resulted in up to 90% synergistic protection. All untreated mice died, and antibiotic alone protected only 30% of animals. We conclude that both substances target the aberrant host signaling that underpins anthrax mortality.

CONCLUSION: Our findings suggest new possibilities for combination therapy of anthrax with antibiotics, A3R agonists and caspase-1 inhibitors.

Anthrax lethal factor activates K(+) channels to induce IL-1β secretion in macrophages

Anthrax lethal toxin (LeTx) is a virulence factor of Bacilillus anthracis that is a bivalent toxin, containing lethal factor (LF) and protective Ag proteins, which causes cytotoxicity and altered macrophage function. LeTx exposure results in early K(+) efflux from macrophages associated with caspase-1 activation and increased IL-1β release. The mechanism of this toxin-induced K(+) efflux is unknown. The goals of the current study were to determine whether LeTx-induced K(+) efflux from macrophages is mediated by toxin effects on specific K(+) channels and whether altered K(+)-channel activity is involved in LeTx-induced IL-1β release. Exposure of macrophages to LeTx induced a significant increase in the activities of two types of K(+) channels that have been identified in mouse macrophages: Ba(2+)-sensitive inwardly rectifying K(+) (Kir) channels and 4-aminopyridine-sensitive outwardly rectifying voltage-gated K(+) (Kv) channels. LeTx enhancement of both Kir and Kv required the proteolytic activity of LF, because exposure of macrophages to a mutant LF-protein (LF(E687C)) combined with protective Ag protein had no effect on the currents. Furthermore, blocking Kir and Kv channels significantly decreased LeTx-induced release of IL-1β. In addition, retroviral transduction of macrophages with wild-type Kir enhanced LeTx-induced release of IL-1β, whereas transduction of dominant-negative Kir blocked LeTx-induced release of IL-1β. Activation of caspase-1 was not required for LeTx-induced activation of either of the K(+) channels. These data indicate that a major mechanism through which LeTx stimulates macrophages to release IL-1β involves an LF-protease effect that enhances Kir and Kv channel function during toxin stimulation.

Modeling the host response to inhalation anthrax

Inhalation anthrax, an often fatal infection, is initiated by endospores of the bacterium Bacillus anthracis, which are introduced into the lung. To better understand the pathogenesis of an inhalation anthrax infection, we propose a two-compartment mathematical model that takes into account the documented early events of such an infection. Anthrax spores, once inhaled, are readily taken up by alveolar phagocytes, which then migrate rather quickly out of the lung and into the thoracic/mediastinal lymph nodes. En route, these spores germinate to become vegetative bacteria. In the lymph nodes, the bacteria kill the host cells and are released into the extracellular environment where they can be disseminated into the blood stream and grow to a very high level, often resulting in the death of the infected person. Using this framework as the basis of our model, we explore the probability of survival of an infected individual. This is dependent on several factors, such as the rate of migration and germination events and treatment with antibiotics.

Targeting caspases in intracellular protozoan infections

Caspases are cysteine aspartases acting either as initiators (caspases 8, 9, and 10) or executioners (caspases 3, 6, and 7) to induce programmed cell death by apoptosis. Parasite infections by certain intracellular protozoans increase host cell life span by targeting caspase activation. Conversely, caspase activation, followed by apoptosis of lymphocytes and other cells, prevents effective immune responses to chronic parasite infection. Here we discuss how pharmacological inhibition of caspases might affect the immunity to protozoan infections, by either blocking or delaying apoptosis.

The human-bacterial pathogen protein interaction networks of Bacillus anthracis, Francisella tularensis, and Yersinia pestis

Background

Bacillus anthracis, Francisella tularensis, and Yersinia pestis are bacterial pathogens that can cause anthrax, lethal acute pneumonic disease, and bubonic plague, respectively, and are listed as NIAID Category A priority pathogens for possible use as biological weapons. However, the interactions between human proteins and proteins in these bacteria remain poorly characterized leading to an incomplete understanding of their pathogenesis and mechanisms of immune evasion.

Methodology

In this study, we used a high-throughput yeast two-hybrid assay to identify physical interactions between human proteins and proteins from each of these three pathogens. From more than 250,000 screens performed, we identified 3,073 human-B. anthracis, 1,383 human-F. tularensis, and 4,059 human-Y. pestis protein-protein interactions including interactions involving 304 B. anthracis, 52 F. tularensis, and 330 Y. pestis proteins that are uncharacterized. Computational analysis revealed that pathogen proteins preferentially interact with human proteins that are hubs and bottlenecks in the human PPI network. In addition, we computed modules of human-pathogen PPIs that are conserved amongst the three networks. Functionally, such conserved modules reveal commonalities between how the different pathogens interact with crucial host pathways involved in inflammation and immunity.

Significance

These data constitute the first extensive protein interaction networks constructed for bacterial pathogens and their human hosts. This study provides novel insights into host-pathogen interactions.

Bacillus anthracis endospores regulate ornithine decarboxylase and inducible nitric oxide synthase through ERK1/2 and p38 mitogen-activated protein kinases

Interactions between Bacillus anthracis (B. anthracis) and host cells are of particular interest given the implications of anthrax as a biological weapon. Inhaled B. anthracis endospores encounter alveolar macrophages as the first line of defense in the innate immune response. Yet, the consequences of this interaction remain unclear. We have demonstrated that B. anthracis uses arginase, inherent in the endospores, to reduce the ability of macrophages to produce nitric oxide ((•)NO) from inducible nitric oxide synthase (NOS2) by competing for L-arginine, producing L-ornithine at the expense of (•)NO. In the current study, we used genetically engineered B. anthracis endospores to evaluate the contribution of germination and the lethal toxin (LT) in mediating signaling pathways responsible for the induction of NOS2 and ornithine decarboxylase (ODC), which is the rate-limiting enzyme in the conversion of L-ornithine into polyamines. We found that induction of NOS2 and ODC expression in macrophages exposed to B. anthracis occurs through the activation of p38 and ERK1/2 MAP kinases, respectively. Optimal induction of NOS2 was observed following exposure to germination-competent endospores, whereas ODC induction occurred irrespective of the endospores' germination capabilities and was more prominent in macrophages exposed to endospores lacking LT. Our findings suggest that activation of kinase signaling cascades that determine macrophage defense responses against B. anthracis infection occurs through distinct mechanisms.

Bacillus anthracis capsule activates caspase-1 and induces interleukin-1beta release from differentiated THP-1 and human monocyte-derived dendritic cells

The poly-gamma-d-glutamic acid (PGA) capsule is one of the major virulence factors of Bacillus anthracis, which causes a highly lethal infection. The antiphagocytic PGA capsule disguises the bacilli from immune surveillance and allows unimpeded growth of bacilli in the host. Recently, efforts have been made to include PGA as a component of anthrax vaccine; however, the innate immune response of PGA itself has been poorly investigated. In this study, we characterized the innate immune response elicited by PGA in the human monocytic cell line THP-1, which was differentiated into macrophages with phorbol 12-myristate 13-acetate (PMA) and human monocyte-derived dendritic cells (hMoDCs). PGA capsules were isolated from the culture supernatant of either the pXO1-cured strain of B. anthracis H9401 or B. licheniformis ATCC 9945a. PGA treatment of differentiated THP-1 cells and hMoDCs led to the specific extracellular release of interleukin-1beta (IL-1beta) in a dose-dependent manner. Evaluation of IL-1beta processing by Western blotting revealed that cleaved IL-1beta increased in THP-1 cells and hMoDCs after PGA treatment. Enhanced processing of IL-1beta directly correlated with increased activation of its upstream regulator, caspase-1, also known as IL-1beta-converting enzyme (ICE). The extracellular release of IL-1beta in response to PGA was ICE dependent, since the administration of an ICE inhibitor prior to PGA treatment blocked induction of IL-1beta. These results demonstrate that B. anthracis PGA elicits IL-1beta production through activation of ICE in PMA-differentiated THP-1 cells and hMoDCs, suggesting the potential for PGA as a therapeutic target for anthrax.

Investigation of new dominant-negative inhibitors of anthrax protective antigen mutants for use in therapy and vaccination

The lethal toxin (LeTx) of Bacillus anthracis plays a key role in the pathogenesis of anthrax. The protective antigen (PA) is a primary part of the anthrax toxin and forms LeTx by combination with lethal factor (LF). Phenylalanine-427 (F427) is crucial for PA function. This study was designed to discover potential novel therapeutic agents and vaccines for anthrax. This was done by screening PA mutants that were mutated at the F427 residue for a dominant-negative inhibitory (DNI) phenotype which was nontoxic but inhibited the toxicity of the wild-type LeTx. For this, PA residue F427 was first mutated to each of the other 19 naturally occurring amino acids. The cytotoxicity and DNI phenotypes of the mutated PA proteins were tested in the presence of 1 microg/ml LF in RAW264.7 cells and were shown to be dependent on the individual amino acid replacements. A total of 16 nontoxic mutants with various levels of DNI activity were identified in vitro. Among them, F427D and F427N mutants had the highest DNI activities in RAW264.7 cells. Both mutants inhibited LeTx intoxication in mice in a dose-dependent way. Furthermore, they induced a Th2-predominant immune response and protected mice against a challenge with five 50% lethal doses of LeTx. The protection was correlated mainly with a low level of interleukin-1 beta (IL-1 beta) and with high levels of PA-specific immunoglobulin G1, IL-6, and tumor necrosis factor alpha. Thus, PA DNI mutants, such as F427D and F427N mutants, may serve in the development of novel therapeutic agents and vaccines to fight B. anthracis infections.

Activation of plasminogen activator inhibitor implicates protease InhA in the acute-phase response to Bacillus anthracis infection

Anthrax is a zoonotic disease caused by Bacillus anthracis. The infection is associated with inflammation and sepsis, but little is known about the acute-phase response during disease and the nature of the bacterial factors causing it. In this study, we examined the levels of the acute-phase proteins (APPs) in comparative experiments using mice challenged with spores and a purified B. anthracis protease InhA as a possible factor mediating the response. A strong increase in the plasma levels of APPs such as haptoglobin and serum amyloid A was observed during infection. Protein and mRNA levels of plasminogen activator inhibitor (PAI)-1 in the liver were also increased concurrently with bacterial dissemination at 72 h post-infection. Similar effects were observed at 6 h post injection with InhA. Induction of hepatic transforming growth factor-beta1, a PAI-1 inducer, was also found in the liver of InhA-injected mice. PAI-1 elevation by InhA resulted in an increased level of urokinase-type plasminogen activator complex with PAI-1 and a decreased level of D-dimers indicating inhibition of blood fibrinolysis. These results reveal an acute liver response to anthrax infection and provide a plausible pathophysiological link between the host inflammatory response and the pro-thrombotic haemostatic imbalance in the course of disease through PAI-1 induction in the liver.

Anthrax infection inhibits the AKT signaling involved in the E-cadherin-mediated adhesion of lung epithelial cells

The effect of anthrax infection on phosphoprotein signaling was studied in human small airway lung epithelial cells exposed to B. anthracis spores of the plasmidless dSterne strain in comparison with the Sterne strain containing the toxigenic plasmid (pXO1). The differential regulation of phosphorylation was found in the mitogen-activated protein kinase cascade (ERK, p38, and P90RSK), the PI3K cascade (AKT, GSK-3alpha/beta), and downstream in the case of the proapoptotic BAD and the transcription factor STAT3. Both strains stimulate phosphorylation of CREB and inhibit phosphorylation of 4E-BP1 required for activation of cap-dependent translation. Downregulation of the survival AKT phosphorylation by the Sterne strain inhibits the process of Ca(2+)-dependent homophilic interaction of E-cadherin (EC) upon formation or repair of cell-cell contacts. Both lethal and edema toxins produced by the Sterne strain inhibit the AKT phosphorylation induced during the EC-mediated signaling. Activity of ERK1/2 and p38 inhibitors indicates that inhibition of AKT phosphorylation takes place through the ERK1/2-PI3K crosstalk. In Sterne spore-challenged mice, a specific inhibitor of PI3K/AKT, wortmannin, accelerates the lethal outcome, and reduction of AKT phosphorylation in the circulating blood cells coincides with the death of animals. We conclude that the PI3K/AKT pathway controlling the integrity of epithelium plays an important survival role in anthrax infection.

Transcriptional and apoptotic responses of THP-1 cells to challenge with toxigenic, and non-toxigenic Bacillus anthracis

Background

Bacillus anthracis secretes several virulence factors targeting different host organs and cell types during inhalational anthrax infection. The bacterial expression of a key virulence factor, lethal toxin (LeTx) is closely tied to another factor, edema toxin (EdTx). Both are transcribed on the same virulence plasmid (pXO1) and both have been the subject of much individual study. Their combined effect during virulent anthrax likely modulates both the global transcriptional and the phenotypic response of macrophages and phagocytes. In fact, responses brought about by the toxins may be different than each of their individual effects.

Results

Here we report the transcriptional and apoptotic responses of the macrophage-like phagocytic cell line THP-1 exposed to B. anthracis Sterne (pXO1⁺) spores, and B. anthracis Δ Sterne (pXO1^-) spores. These cells are resistant to LeTx-induced cytolysis, a phenotype seen in macrophages from several mouse strains which are sensitive to toxigenic anthrax infection. Our results indicate that the pXO1-containing strain induces higher pro-inflammatory transcriptional responses during the first 4 hours of interaction with bacterium, evident in the upregulation of several genes relevant to Nf-κB, phosphatases, prostaglandins, and TNF-α, along with decreases in expression levels of genes for mitochondrial components. Both bacterial strains induce apoptosis, but in the toxigenic strain-challenged cells, apoptosis is delayed.

Conclusion

This delay in apoptosis occurs despite the much higher level of TNF-α secretion induced by the toxigenic-strain challenge. Interestingly, CFLAR, an important apoptotic inhibitor which blocks apoptosis induced by large amounts of extracellular TNF-α, is upregulated significantly during toxigenic-strain infection, but not at all during non-toxigenic-strain infection, indicating that it may play a role in blocking or delaying TNF-α-mediated apoptosis. The suppression of apoptosis by the toxigenic anthrax strain is consistent with the notion that apoptosis itself may represent a protective host cell response.

Discriminating virulence mechanisms among Bacillus anthracis strains by using a murine subcutaneous infection model

Bacillus anthracis strains harboring virulence plasmid pXO1 that encodes the toxin protein protective antigen (PA), lethal factor, and edema factor and virulence plasmid pXO2 that encodes capsule biosynthetic enzymes exhibit different levels of virulence in certain animal models. In the murine model of pulmonary infection, B. anthracis virulence was capsule dependent but toxin independent. We examined the role of toxins in subcutaneous (s.c.) infections using two different genetically complete (pXO1(+) pXO2(+)) strains of B. anthracis, strains Ames and UT500. Similar to findings for the pulmonary model, toxin was not required for infection by the Ames strain, because the 50% lethal dose (LD(50)) of a PA-deficient (PA(-)) Ames mutant was identical to that of the parent Ames strain. However, PA was required for efficient s.c. infection by the UT500 strain, because the s.c. LD(50) of a UT500 PA(-) mutant was 10,000-fold higher than the LD(50) of the parent UT500 strain. This difference between the Ames strain and the UT500 strain could not be attributed to differences in spore coat properties or the rate of germination, because s.c. inoculation with the capsulated bacillus forms also required toxin synthesis by the UT500 strain to cause lethal infection. The toxin-dependent phenotype of the UT500 strain was host phagocyte dependent, because eliminating Gr-1(+) phagocytes restored virulence to the UT500 PA(-) mutant. These experiments demonstrate that the dominant virulence factors used to establish infection by B. anthracis depend on the route of inoculation and the bacterial strain.

Role of anthrax toxins in dissemination, disease progression, and induction of protective adaptive immunity in the mouse aerosol challenge model

Anthrax toxins significantly contribute to anthrax disease pathogenesis, and mechanisms by which the toxins affect host cellular responses have been identified with purified toxins. However, the contribution of anthrax toxin proteins to dissemination, disease progression, and subsequent immunity after aerosol infection with spores has not been clearly elucidated. To better understand the role of anthrax toxins in pathogenesis in vivo and to investigate the contribution of antibody to toxin proteins in protection, we completed a series of in vivo experiments using a murine aerosol challenge model and a collection of in-frame deletion mutants lacking toxin components. Our data show that after aerosol exposure to Bacillus anthracis spores, anthrax lethal toxin was required for outgrowth of bacilli in the draining lymph nodes and subsequent progression of infection beyond the lymph nodes to establish disseminated disease. After pulmonary exposure to anthrax spores, toxin expression was required for the development of protective immunity to a subsequent lethal challenge. However, immunoglobulin (immunoglobulin G) titers to toxin proteins, prior to secondary challenge, did not correlate with the protection observed upon secondary challenge with wild-type spores. A correlation was observed between survival after secondary challenge and rapid anamnestic responses directed against toxin proteins. Taken together, these studies indicate that anthrax toxins are required for dissemination of bacteria beyond the draining lymphoid tissue, leading to full virulence in the mouse aerosol challenge model, and that primary and anamnestic immune responses to toxin proteins provide protection against subsequent lethal challenge. These results provide support for the utility of the mouse aerosol challenge model for the study of inhalational anthrax.

Anthrax toxins inhibit neutrophil signaling pathways in brain endothelium and contribute to the pathogenesis of meningitis

BACKGROUND

Anthrax meningitis is the main neurological complication of systemic infection with Bacillus anthracis approaching 100% mortality. The presence of bacilli in brain autopsies indicates that vegetative bacteria are able to breach the blood-brain barrier (BBB). The BBB represents not only a physical barrier but has been shown to play an active role in initiating a specific innate immune response that recruits neutrophils to the site of infection. Currently, the basic pathogenic mechanisms by which B. anthracis penetrates the BBB and causes anthrax meningitis are poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

Using an in vitro BBB model, we show for the first time that B. anthracis efficiently invades human brain microvascular endothelial cells (hBMEC), the single cell layer that comprises the BBB. Furthermore, transcriptional profiling of hBMEC during infection with B. anthracis revealed downregulation of 270 (87%) genes, specifically key neutrophil chemoattractants IL-8, CXCL1 (Gro alpha) and CXCL2 (Gro beta), thereby strongly contrasting hBMEC responses observed with other meningeal pathogens. Further studies using specific anthrax toxin-mutants, quantitative RT-PCR, ELISA and in vivo assays indicated that anthrax toxins actively suppress chemokine production and neutrophil recruitment during infection, allowing unrestricted proliferation and dissemination of the bacteria. Finally, mice challenged with B. anthracis Sterne, but not the toxin-deficient strain, developed meningitis.

CONCLUSIONS/SIGNIFICANCE

These results suggest a significant role for anthrax toxins in thwarting the BBB innate defense response promoting penetration of bacteria into the central nervous system. Furthermore, establishment of a mouse model for anthrax meningitis will aid in our understanding of disease pathogenesis and development of more effective treatment strategies.

Bacillus anthracis spores and lethal toxin induce IL-1beta via functionally distinct signaling pathways

Previous reports suggested that lethal toxin (LT)-induced caspase-1 activity and/or IL-1beta accounted for Bacillus anthracis (BA) infection lethality. In contrast, we now report that caspase-1-mediated IL-1beta expression in response to BA spores is required for anti-BA host defenses. Caspase-1(-/-) and IL-1beta(-/-) mice are more susceptible than wild-type (WT) mice to lethal BA infection, are less able to kill BA both in vivo and in vitro, and addition of rIL-1beta to macrophages from these mice restored killing in vitro. Non-germinating BA spores induced caspase-1 activity, IL-1beta and nitric oxide, by which BA are killed in WT but not in caspase-1(-/-) mice, suggesting that the spore itself stimulated inflammatory responses. While spores induced IL-1beta in LT-susceptible and -resistant macrophages, LT induced IL-1beta only in LT-susceptible macrophages. Cooperation between MyD88-dependent and -independent signaling pathways was required for spore-induced, but not LT-induced, IL-1beta. While both spores and LT induced caspase-1 activity and IL-1beta, LT did not induce IL-1beta mRNA, and spores did not induce cell death. Thus different components of the same bacterium each induce IL-1beta by distinct signaling pathways. Whereas the spore-induced IL-1beta limits BA infection, LT-induced IL-1beta enables BA to escape host defenses.

In vivo efficacy of a phosphodiester TLR-9 aptamer and its beneficial effect in a pulmonary anthrax infection model

Immunostimulatory oligonucleotide (ISS-ODN) used as adjuvants are commonly modified with phosphorothioate (PS). The PS backbone prevents nuclease degradation, but confers undesired side effects, including systemic cytokine release. Previously, R10-60, a phosphodiester (PO) ISS-ODN, was structurally optimized as an intracellular Toll-like receptor-9 agonist. Here intravenous, intradermal and intranasal administration of PO R10-60 elicit local or adaptive immune responses with minimal systemic effects compared to a prototypic PS ISS-ODN in mice. Furthermore, prophylactic intranasal administration of PO R10-60 significantly delayed death in mice exposed to respiratory anthrax comparable to the PS ISS-ODN. The pattern of cytokine release suggested that early IL-1beta production might contribute to this protective effect, which was replicated with recombinant IL-1beta injections during infection. Hence, the transient effects from a PO TLR-9 agonist may be beneficial for protection in a bacterial bioterrorism attack, by delaying the onset of systemic infection without the induction of a cytokine syndrome.

Anthrax lethal toxin and Salmonella elicit the common cell death pathway of caspase-1-dependent pyroptosis via distinct mechanisms

Caspase-1 cleaves the inactive IL-1beta and IL-18 precursors into active inflammatory cytokines. In Salmonella-infected macrophages, caspase-1 also mediates a pathway of proinflammatory programmed cell death termed "pyroptosis." We demonstrate active caspase-1 diffusely distributed in the cytoplasm and localized in discrete foci within macrophages responding to either Salmonella infection or intoxication by Bacillus anthracis lethal toxin (LT). Both stimuli triggered caspase-1-dependent lysis in macrophages and dendritic cells. Activation of caspase-1 by LT required binding, uptake, and endosome acidification to mediate translocation of lethal factor (LF) into the host cell cytosol. Catalytically active LF cleaved cytosolic substrates and activated caspase-1 by a mechanism involving proteasome activity and potassium efflux. LT activation of caspase-1 is known to require the inflammasome adapter Nalp1. In contrast, Salmonella infection activated caspase-1 through an independent pathway requiring the inflammasome adapter Ipaf. These distinct mechanisms of caspase-1 activation converged on a common pathway of caspase-1-dependent cell death featuring DNA cleavage, cytokine activation, and, ultimately, cell lysis resulting from the formation of membrane pores between 1.1 and 2.4 nm in diameter and pathological ion fluxes that can be blocked by glycine. These findings demonstrate that distinct activation pathways elicit the conserved cell death effector mechanism of caspase-1-mediated pyroptosis and support the notion that this pathway of proinflammatory programmed cell death is broadly relevant to cell death and inflammation invoked by diverse stimuli.

Early interactions between fully virulent Bacillus anthracis and macrophages that influence the balance between spore clearance and development of a lethal infection

The role of macrophages in the pathogenesis of anthrax is unresolved. Macrophages are believed to support the initiation of infection by Bacillus anthracis spores, yet are also sporicidal. Furthermore, it is believed that the anthrax toxins suppress normal macrophage function. However, the significance of toxin effects on macrophages has not been addressed in an in vivo infection model. We used mutant derivatives of murine macrophage RAW264.7 cells that are toxin receptor-negative (R3D) to test the role of toxin-targeting of macrophages during a challenge with spores of the Ames strain of B. anthracis in both in vivo and in vitro models. We found that the R3D cells were able to control challenge with Ames when mice were inoculated with the cells prior to spore challenge. These findings were confirmed in vitro by high dose spore infection of macrophages. Interestingly, whereas the R3D cells provided a significantly greater survival advantage against spores than did the wild type RAW264.7 cells or R3D-complemented cells, the protection afforded the mutant and wild type cells was equivalent against a bacillus challenge. The findings appear to be the first specific test of the role of toxin targeting of macrophages during infection with B. anthracis spores.

Degradation of circulating von Willebrand factor and its regulator ADAMTS13 implicates secreted Bacillus anthracis metalloproteases in anthrax consumptive coagulopathy

Pathology data from the anthrax animal models show evidence of significant increases in vascular permeability coincident with hemostatic imbalances manifested by thrombocytopenia, transient leucopenia, and aggressive disseminated intravascular coagulation. In this study we hypothesized that anthrax infection modulates the activity of von Willebrand factor (VWF) and its endogenous regulator ADAMTS13, which play important roles in hemostasis and thrombosis, including interaction of endothelial cells with platelets. We previously demonstrated that purified anthrax neutral metalloproteases Npr599 and InhA are capable of cleaving a variety of host structural and regulatory proteins. Incubation of human plasma with these proteases at 37 degrees C in the presence of urea as a mild denaturant results in proteolysis of VWF. Also in these conditions, InhA directly cleaves plasma ADAMTS13 protein. Npr599 and InhA digest synthetic VWF substrate FRETS-VWF73. Amino acid sequencing of VWF fragments produced by InhA suggests that one of the cleavage sites of VWF is located at domain A2, the target domain of ADAMTS13. Proteolysis of VWF by InhA impairs its collagen binding activity (VWF:CBA) and ristocetin-induced platelet aggregation activity. In plasma from anthrax spore-challenged DBA/2 mice, VWF antigen levels increase up to 2-fold at day 3 post-infection with toxigenic Sterne 34F(2) strain, whereas VWF:CBA levels drop in a time-dependent manner, suggesting dysfunction of VWF instead of its quantitative deficiency. This conclusion is further supported by significant reduction in the amount of VWF circulating in blood in the ultra-large forms. In addition, Western blot analysis shows proteolytic depletion of ADAMTS13 from plasma of spore-challenged mice despite its increased expression in the liver. Our results suggest a new mechanism of anthrax coagulopathy affecting the levels and functional activities of both VWF and its natural regulator ADAMTS13. This mechanism may contribute to hemorrhage and thrombosis typical in anthrax.

Bacillus anthracis: interactions with the host and establishment of inhalational anthrax

Due to its potential as a bioweapon, Bacillus anthracis has received a great deal of attention in recent years, and a significant effort has been devoted to understanding how this organism causes anthrax. There has been a particular focus on the inhalational form of the disease, and studies over the past several years have painted an increasingly complex picture of how B. anthracis enters the mammalian host, survives the host's defense mechanisms, disseminates throughout the body and causes death. This article reviews recent advances in these areas, with a focus on how the bacterium interacts with its host in establishing infection and causing anthrax.

Human monoclonal antibodies against anthrax lethal factor and protective antigen act independently to protect against Bacillus anthracis infection and enhance endogenous immunity to anthrax

The unpredictable nature of bioterrorism and the absence of real-time detection systems have highlighted the need for an efficient postexposure therapy for Bacillus anthracis infection. One approach is passive immunization through the administration of antibodies that mitigate the biological action of anthrax toxin. We isolated and characterized two protective fully human monoclonal antibodies with specificity for protective antigen (PA) and lethal factor (LF). These antibodies, designated IQNPA (anti-PA) and IQNLF (anti-LF), were developed as hybridomas from individuals immunized with licensed anthrax vaccine. The effective concentration of IQNPA that neutralized 50% of the toxin in anthrax toxin neutralization assays was 0.3 nM, while 0.1 nM IQNLF neutralized the same amount of toxin. When combined, the antibodies had additive neutralization efficacy. IQNPA binds to domain IV of PA containing the host cell receptor binding site, while IQNLF recognizes domain I containing the PA binding region in LF. A single 180-mug dose of either antibody given to A/J mice 2.5 h before challenge conferred 100% protection against a lethal intraperitoneal spore challenge with 24 50% lethal doses [LD50s] of B. anthracis Sterne and against rechallenge on day 20 with a more aggressive challenge dose of 41 LD50s. Mice treated with either antibody and infected with B. anthracis Sterne developed detectable murine anti-PA and anti-LF immunoglobulin G antibody responses by day 17 that were dependent on which antibody the mice had received. Based on these results, IQNPA and IQNLF act independently during prophylactic anthrax treatment and do not interfere with the establishment of endogenous immunity.

Role of Bacillus anthracis spore structures in macrophage cytokine responses

The innate immune response of macrophages (Mphi) to spores, the environmentally acquired form of Bacillus anthracis, is poorly characterized. We therefore examined the early Mphi cytokine response to B. anthracis spores, before germination. Mphi were exposed to bacilli and spores of Sterne strain 34F2 and its congenic nongerminating mutant (DeltagerH), and cytokine expression was measured by real-time PCR and an enzyme-linked immunosorbent assay. The exosporium spore layer was retained (exo+) or removed by sonication (exo-). Spores consistently induced a strong cytokine response, with the exo- spores eliciting a two- to threefold-higher response than exo+ spores. The threshold for interleukin-1beta (IL-1beta) production by wild-type Mphi was significantly lower than that required for tumor necrosis factor alpha expression. Cytokine production was largely dependent on MyD88, suggesting Toll-like receptor involvement; however, the expression of beta interferon in MyD88-/- Mphi suggests involvement of a MyD88-independent pathway. We conclude that (i) the B. anthracis spore is not immunologically inert, (ii) the exosporium masks epitopes recognized by the Mphi, (iii) the Mphi cytokine response to B. anthracis involves multiple pattern recognition receptors and signaling pathways, and (iv) compared to other cytokines, IL-1beta is expressed at a lower spore concentration.

Murine splenocytes produce inflammatory cytokines in a MyD88-dependent response to Bacillus anthracis spores

Bacillus anthracis is a sporulating Gram-positive bacterium that causes the disease anthrax. The highly stable spore is the infectious form of the bacterium that first interacts with the prospective host, and thus the interaction between the host and spore is vital to the development of disease. We focused our study on the response of murine splenocytes to the B. anthracis spore by using paraformaldehyde-inactivated spores (FIS), a treatment that prevents germination and production of products associated with vegetative bacilli. We found that murine splenocytes produce IL-12 and IFN-gamma in response to FIS. The IL-12 was secreted by CD11b cells, which functioned to induce the production of IFN-gamma by CD49b (DX5) NK cells. The production of these cytokines by splenocytes was not dependent on TLR2, TLR4, TLR9, Nod1, or Nod2; however, it was dependent on the signalling adapter protein MyD88. Unlike splenocytes, Nod1- and Nod2-transfected HEK cells were activated by FIS. Both IL-12 and IFN-gamma secretion were inhibited by treatment with B. anthracis lethal toxin. These observations suggest that the innate immune system recognizes spores with a MyD88-dependent receptor (or receptors) and responds by secreting inflammatory cytokines, which may ultimately aid in resisting infection.

Large scale genotype-phenotype correlation analysis based on phylogenetic trees

We provide two methods for identifying changes in genotype that are correlated with changes in a phenotype implied by phylogenetic trees. The first method, VENN, works when the number of branches over which the change occurred are modest. VENN looks for genetic changes that are completely penetrant with phenotype changes on a tree. The second method, CCTSWEEP, allows for a partial matching between changes in phenotypes and genotypes and provides a score for each change using Maddison's concentrated changes test. The mutations that are highly correlated with phenotypic change can be ranked by score. We use these methods to find SNPs correlated with resistance to Bacillus anthracis in inbred mouse strains. Our findings are consistent with the current biological literature, and also suggest potential novel candidate genes.

Modulation of cytokine production and enhancement of cell viability by TLR7 and TLR9 ligands during anthrax infection of macrophages

Inhalation of Bacillus anthracis, a bioterrorism agent, results in a high mortality rate despite appropriate antibiotic therapy. Macrophages appear to be a key factor in B. anthracis pathogenesis. The burst of pro-inflammatory cytokines from macrophages could be a major cause of death in anthrax. However, preactivation of Toll-like receptors (TLRs) could modify the host response. TLR ligands stimulate the release of activating cytokines but may also down-modulate the subsequent deleterious cytokine response to pathogens. We developed a cell culture model to measure macrophage responses to B. anthracis spores and bacilli. We found that germination from spores to bacilli produced a substantial stimulus for the secretion of the cytokines IL-6, TNF-alpha, IL-10, and IL-12 p40. Our studies showed that pretreatment of mouse macrophages with the TLR9 ligand ISS-1018, or the TLR7 ligands R-848 and IT-37, results in a substantial decrease in the subsequent secretion of IL-6 and TNF-alpha in response to B. anthracis infection of macrophages. Furthermore, the TLR7 and TLR9 ligands significantly decreased anthrax-induced cytotoxicity in the macrophages. These findings suggest that TLR ligands may contribute to the enhancement of innate immunity in B. anthracis infection by suppressing potentially deleterious pro-inflammatory cytokine responses and by improving macrophage viability.

Bacillus anthracis spores stimulate cytokine and chemokine innate immune responses in human alveolar macrophages through multiple mitogen-activated protein kinase pathways

Contact with the human alveolar macrophage plays a key role in the innate immune response to Bacillus anthracis spores. Because there is a significant delay between the initial contact of the spore with the host and clinical evidence of disease, there appears to be temporary containment of the pathogen by the innate immune system. Therefore, the early macrophage response to Bacillus anthracis exposure is important in understanding the pathogenesis of this disease. In this paper, we studied the initial events after exposure to spores, beginning with the rapid internalization of spores by the macrophages. Spore exposure rapidly activated the mitogen-activated protein kinase signaling pathways extracellular signal-regulated kinase, c-Jun-NH2-terminal kinase, and p38. This was followed by the transcriptional activation of cytokine and primarily monocyte chemokine genes as determined by RNase protection assays. Transcriptional induction is reflected at the translational level, as interleukin-1alpha (IL-1alpha), IL-1beta, IL-6, and tumor necrosis factor alpha (TNF-alpha) cytokine protein levels were markedly elevated as determined by enzyme-linked immunosorbent assay. Induction of IL-6 and TNF-alpha, and, to a lesser extent, IL-1alpha and IL-1beta, was partially inhibited by the blockade of individual mitogen-activated protein kinases, while the complete inhibition of cytokine induction was achieved when multiple signaling pathway inhibitors were used. Taken together, these data clearly show activation of the innate immune system in human alveolar macrophages by Bacillus anthracis spores. The data also show that multiple signaling pathways are involved in this cytokine response. This report is the first comprehensive examination of this process in primary human alveolar macrophages.

Secreted neutral metalloproteases of Bacillus anthracis as candidate pathogenic factors

To evaluate the pathogenic potential of Bacillus anthracis-secreted proteases distinct from lethal toxin, two neutral zinc metalloproteases were purified to apparent homogeneity from the culture supernatant of a non-virulent delta Ames strain (pXO1-, pXO2-). The first (designated Npr599) is a thermolysin-like enzyme highly homologous to bacillolysins from other Bacillus species. The second (designated InhA) is a homolog of the Bacillus thuringiensis immune inhibitor A. These proteases belong to the M4 and M6 families, respectively. Both enzymes digested various substrates, including extracellular matrix proteins, endogenous inhibitors, and coagulation proteins, with some differences in specificity. In addition, InhA accelerated urokinase-mediated plasminogen activation, suggesting that InhA acts as a modulator of plasmin in the host inflammatory system. Relevant to epithelial barrier function, Npr599 and InhA significantly enhanced syndecan-1 shedding from cultured normal murine mammary gland cells without affecting their viability through stimulation of the host cell ectodomain shedding mechanism. In addition, Npr599 and InhA directly cleaved recombinant syndecan-1 fused to glutathione S-transferase. Mass spectrometric analysis suggested that the cleavage sites of Npr599 and InhA are the Asp(39)-Asp(40) and Gly(48)-Thr(49) bonds, respectively. We propose that Npr599 and InhA from B. anthracis are multifunctional pathogenic factors that may contribute to anthrax pathology through direct degradation of host tissues, increases in barrier permeability, and/or modulation of host defenses.

Histopathology in a murine model of anthrax

Systemic anthrax infection is usually fatal even with optimal medical care. Further insights into anthrax pathogenesis are therefore urgently needed to develop more effective therapies. Animal models that reproduce human disease will facilitate this research. Here, we describe the detailed histopathology of systemic anthrax infection in A/J mice infected with Bacillus anthracis Sterne, a strain with reduced virulence for humans. Subcutaneous infection leads to systemic disease with multiple pathologies including oedema, haemorrhage, secondary pneumonia and lymphocytolysis. These pathologies bear marked similarity to primary pathologies observed during human disease. Therefore, this simple, small animal model will allow researchers to study the major pathologies observed in humans, while permitting experimentation in more widely available Biosafety Level 2 facilities.

Regulation of Apoptosis by Gram-Positive Bacteria: Mechanistic Diversity and Consequences for Immunity

Apoptosis, or programmed cell death (PCD), is an important physiological mechanism, through which the human immune system regulates homeostasis and responds to diverse forms of cellular damage. PCD may also be involved in immune counteraction to microbial infection. Over the past decade, the amount of research on bacteria-induced PCD has grown tremendously, and the implications of this mechanism on immunity are being elucidated. Some pathogenic bacteria actively trigger the suicide response in critical lineages of leukocytes that orchestrate both the innate and adaptive immune responses; other bacteria proactively prevent PCD to benefit their own survival and persistence. Currently, the microbial virulence factors, which represent the keys to unlocking the suicide response in host cells, are a primary focus of this field. In this review, we discuss these bacterial "apoptosis regulatory molecules" and the apoptotic events they either trigger or prevent, the host target cells of this regulatory activity, and the possible ramifications for immunity to infection. Gram-positive pathogens including Staphylococcus, Streptococcus, Bacillus, Listeria, and Clostridia species are discussed as important agents of human infection that modulate PCD pathways in eukaryotic cells.

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.

Acceleration of epithelial cell syndecan-1 shedding by anthrax hemolytic virulence factors

BACKGROUND

It has been recently reported that major pathogens Staphylococcus aureus and Pseudomonas aeruginosa accelerate a normal process of cell surface syndecan-1 (Synd1) ectodomain shedding as a mechanism of host damage due to the production of shedding-inducing virulence factors. We tested if acceleration of Synd1 shedding takes place in vitro upon treatment of epithelial cells with B. anthracis hemolysins, as well as in vivo during anthrax infection in mice.

RESULTS

The isolated anthrax hemolytic proteins AnlB (sphingomyelinase) and AnlO (cholesterol-binding pore-forming factor), as well as ClnA (B. cereus homolog of B. anthracis phosphatidyl choline-preferring phospholipase C) cause accelerated shedding of Synd1 and E-cadherin from epithelial cells and compromise epithelial barrier integrity within a few hours. In comparison with hemolysins in a similar range of concentrations, anthrax lethal toxin (LT) also accelerates shedding albeit at slower rate. Individual components of LT, lethal factor and protective antigen are inactive with regard to shedding. Inhibition experiments favor a hypothesis that activities of tested bacterial shedding inducers converge on the stimulation of cytoplasmic tyrosine kinases of the Syk family, ultimately leading to activation of cellular sheddase. Both LT and AnlO modulate ERK1/2 and p38 MAPK signaling pathways, while JNK pathway seems to be irrelevant to accelerated shedding. Accelerated shedding of Synd1 also takes place in DBA/2 mice challenged with Bacillus anthracis (Sterne) spores. Elevated levels of shed ectodomain are readily detectable in circulation after 24 h.

CONCLUSION

The concerted acceleration of shedding by several virulence factors could represent a new pathogenic mechanism contributing to disruption of epithelial or endothelial integrity, hemorrhage, edema and abnormal cell signaling during anthrax infection.

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.

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.

MyD88-dependent signaling contributes to protection following Bacillus anthracis spore challenge of mice: implications for Toll-like receptor signaling

Bacillus anthracis is a spore-forming, gram-positive organism that is the causative agent of the disease anthrax. Recognition of Bacillus anthracis by the host innate immune system likely plays a key protective role following infection. In the present study, we examined the role of TLR2, TLR4, and MyD88 in the response to B. anthracis. Heat-killed Bacillus anthracis stimulated TLR2, but not TLR4, signaling in HEK293 cells and stimulated tumor necrosis factor alpha (TNF-alpha) production in C3H/HeN, C3H/HeJ, and C57BL/6J bone marrow-derived macrophages. The ability of heat-killed B. anthracis to induce a TNF-alpha response was preserved in TLR2-/- but not in MyD88-/- macrophages. In vivo studies revealed that TLR2-/- mice and TLR4-deficient mice were resistant to challenge with aerosolized Sterne strain spores but MyD88-/- mice were as susceptible as A/J mice. We conclude that, although recognition of B. anthracis occurs via TLR2, additional MyD88-dependent pathways contribute to the host innate immune response to anthrax infection.

Metalloproteinase inhibitors, nonantimicrobial chemically modified tetracyclines, and ilomastat block Bacillus anthracis lethal factor activity in viable cells

Lethal toxin, produced by the bacterium Bacillus anthracis, is a major contributor to morbidity and mortality in animals and humans who have contracted anthrax. One component of this toxin, lethal factor (LF), proteolytically inactivates members of the mitogen-activated protein kinase kinase (MAPKK or MEK) family. In this study we show that CMT-300, CMT-308, and Ilomastat, agents initially characterized as matrix metalloproteinase inhibitors which are in early stages of development as pharmaceuticals, effectively inhibit the zinc metalloproteinase activity of LF. All three inhibitors, CMT-300, CMT-308, and Ilomastat, inhibit LF-mediated cleavage of a synthetic peptide substrate based on the N-terminal domain of MEKs. Inhibition of LF-mediated MEK proteolysis by all three agents was also achieved using lysates of the human monocytoid line MonoMac 6 as sources of MAPKKs and visualization of the extent of cleavage after separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by detection by Western blotting. Finally, we have demonstrated inhibition of intracellular MEKs in viable human monocytes and MonoMac 6 cells by these agents after incubation of the cells with a reconstituted preparation of recombinant lethal toxin. All three agents are effective inhibitors when incubated with LF prior to exposure to cells, while the CMTs, but not Ilomastat, are also effective when added after LF has already entered the viable cell targets. These results offer promise for strategies to combat effects of the lethal toxin of B. anthracis.

Anthrax lethal toxin enhances cytokine-induced VCAM-1 expression on human endothelial cells

Vascular endothelial dysfunction is thought to play a prominent role in systemic anthrax pathogenesis. We examined the effect of anthrax lethal toxin (LTx), a key virulence factor of Bacillus anthracis, on the expression of vascular cell adhesion molecule-1 (VCAM-1) on normal and cytokine-stimulated human lung microvascular endothelial cells. Confluent endothelial monolayers were treated with lethal factor (LF), protective antigen (PA), or both (LTx) in the presence or absence of tumor necrosis factor-alpha (TNFalpha). LTx enhanced cytokine-induced VCAM-1 expression and monocyte adhesion. LTx alone had no effect on VCAM-1 expression. LF, PA or the combination of a catalytically inactive mutant LF and PA failed to enhance cytokine-induced VCAM-1 expression. Treatment with inhibitors of mitogen-activated protein kinase kinases (MEKs) and mitogen-activated protein kinases did not reproduce the VCAM-1 enhancement effect of LTx, a known MEK metalloprotease, suggesting LTx-mediated MEK cleavage may not be a contributing factor.

Effective antiprotease-antibiotic treatment of experimental anthrax

Background

Inhalation anthrax is characterized by a systemic spread of the challenge agent, Bacillus anthracis. It causes severe damage, including multiple hemorrhagic lesions, to host tissues and organs. It is widely believed that anthrax lethal toxin secreted by proliferating bacteria is a major cause of death, however, the pathology of intoxication in experimental animals is drastically different from that found during the infectious process. In order to close a gap between our understanding of anthrax molecular pathology and the most prominent clinical features of the infectious process we undertook bioinformatic and experimental analyses of potential proteolytic virulence factors of B. anthracis distinct from lethal toxin.

Methods

Secreted proteins (other than lethal and edema toxins) produced by B. anthracis were tested for tissue-damaging activity and toxicity in mice. Chemical protease inhibitors and rabbit immune sera raised against B. anthracis proteases were used to treat mice challenged with B. anthracis (Sterne) spores.

Results

B. anthracis strain delta Ames (pXO1^-, pXO2^-) producing no lethal and edema toxins secrets a number of metalloprotease virulence factors upon cultivation under aerobic conditions, including those with hemorrhagic, caseinolytic and collagenolytic activities, belonging to M4 and M9 thermolysin and bacterial collagenase families, respectively.

These factors are directly toxic to DBA/2 mice upon intratracheal administration at 0.5 mg/kg and higher doses. Chemical protease inhibitors (phosphoramidon and 1, 10-phenanthroline), as well as immune sera against M4 and M9 proteases of B. anthracis, were used to treat mice challenged with B. anthracis (Sterne) spores. These substances demonstrate a substantial protective efficacy in combination with ciprofloxacin therapy initiated as late as 48 h post spore challenge, compared to the antibiotic alone.

Conclusion

Secreted proteolytic enzymes are important pathogenic factors of B. anthrasis, which can be considered as effective therapeutic targets in the development of anthrax treatment and prophylactic approaches complementing anti-lethal toxin therapy.

Anthrolysin O and other gram-positive cytolysins are toll-like receptor 4 agonists

Exposure of bone marrow–derived macrophages (BMDMs) to low concentrations of Bacillus anthracis lethal toxin (LT), whose catalytic subunit is lethal factor (LF), results in induction of a robust apoptotic response dependent on activation of Toll-like receptor (TLR)4. A similar TLR4-dependent apoptotic response is observed when BMDMs are infected with live B. anthracis (Sterne strain). However, TLR4 is considered to be a specific signaling receptor for lipopolysaccharide (LPS), a typical product of gram-negative bacteria, whereas B. anthracis is gram-positive. To understand how B. anthracis can activate TLR4, we analyzed its culture supernatants and found them to contain a potent TLR4-stimulating activity that can also induce apoptosis in macrophages in which the antiapoptotic p38 MAP kinase (whose activation is prevented by LF) was inhibited. Purification of this activity suggested it consists of anthrolysin O (ALO), a member of the cholesterol-dependent cytolysin (CDC) family. We show that recombinant ALO can activate TLR4 in a manner independent of LPS contamination and, together with LT, can induce macrophage apoptosis. We also provide genetic evidence that ALO is required for induction of macrophage apoptosis in response to infection with live B. anthracis and that other CDC family members share the ability to activate TLR4.

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.