Anthrax lethal toxin impairs innate immune functions of alveolar macrophages and facilitates Bacillus anthracis survival

Spores and soil from six sides: interdisciplinarity and the environmental biology of anthrax (Bacillus anthracis)

Environmentally transmitted diseases are comparatively poorly understood and managed, and their ecology is particularly understudied. Here we identify challenges of studying environmental transmission and persistence with a six-sided interdisciplinary review of the biology of anthrax (Bacillus anthracis). Anthrax is a zoonotic disease capable of maintaining infectious spore banks in soil for decades (or even potentially centuries), and the mechanisms of its environmental persistence have been the topic of significant research and controversy. Where anthrax is endemic, it plays an important ecological role, shaping the dynamics of entire herbivore communities. The complex eco-epidemiology of anthrax, and the mysterious biology of Bacillus anthracis during its environmental stage, have necessitated an interdisciplinary approach to pathogen research. Here, we illustrate different disciplinary perspectives through key advances made by researchers working in Etosha National Park, a long-term ecological research site in Namibia that has exemplified the complexities of the enzootic process of anthrax over decades of surveillance. In Etosha, the role of scavengers and alternative routes (waterborne transmission and flies) has proved unimportant relative to the long-term persistence of anthrax spores in soil and their infection of herbivore hosts. Carcass deposition facilitates green-ups of vegetation to attract herbivores, potentially facilitated by the role of anthrax spores in the rhizosphere. The underlying seasonal pattern of vegetation, and herbivores' immune and behavioural responses to anthrax risk, interact to produce regular 'anthrax seasons' that appear to be a stable feature of the Etosha ecosystem. Through the lens of microbiologists, geneticists, immunologists, ecologists, epidemiologists, and clinicians, we discuss how anthrax dynamics are shaped at the smallest scale by population genetics and interactions within the bacterial communities up to the broadest scales of ecosystem structure. We illustrate the benefits and challenges of this interdisciplinary approach to disease ecology, and suggest ways anthrax might offer insights into the biology of other important pathogens. Bacillus anthracis, and the more recently emerged Bacillus cereus biovar anthracis, share key features with other environmentally transmitted pathogens, including several zoonoses and panzootics of special interest for global health and conservation efforts. Understanding the dynamics of anthrax, and developing interdisciplinary research programs that explore environmental persistence, is a critical step forward for understanding these emerging threats.

Tetrazole-Based trans-Translation Inhibitors Kill Bacillus anthracis Spores To Protect Host Cells

Bacillus anthracis, the causative agent of anthrax, remains a significant threat to humans, including potential use in bioterrorism and biowarfare. The capacity to engineer strains with increased pathogenicity coupled with the ease of disseminating lethal doses of B. anthracis spores makes it necessary to identify chemical agents that target and kill spores. Here, we demonstrate that a tetrazole-based trans-translation inhibitor, KKL-55, is bactericidal against vegetative cells of B. anthracis in culture. Using a fluorescent analog, we show that this class of compounds colocalizes with developing endospores and bind purified spores in vitro KKL-55 was effective against spores at concentrations close to its MIC for vegetative cells. Spore germination was inhibited at 1.2× MIC, and spores were killed at 2× MIC. In contrast, ciprofloxacin killed germinants at concentrations close to its MIC but did not prevent germination even at 32× MIC. Because toxins are released by germinants, macrophages infected by B. anthracis spores are killed early in the germination process. At ≥2× MIC, KKL-55 protected macrophages from death after infection with B. anthracis spores. Ciprofloxacin required concentrations of ≥8× MIC to exhibit a similar effect. Taken together, these data indicate that KKL-55 and related tetrazoles are good lead candidates for therapeutics targeting B. anthracis spores and suggest that there is an early requirement for trans-translation in germinating spores.

Targeting Bacillus anthracis toxicity with a genetically selected inhibitor of the PA/CMG2 protein-protein interaction

The protein-protein interaction between the human CMG2 receptor and the Bacillus anthracis protective antigen (PA) is essential for the transport of anthrax lethal and edema toxins into human cells. We used a genetically encoded high throughput screening platform to screen a SICLOPPS library of 3.2 million cyclic hexapeptides for inhibitors of this protein-protein interaction. Unusually, the top 3 hits all contained stop codons in the randomized region of the library, resulting in linear rather than cyclic peptides. These peptides disrupted the targeted interaction in vitro; two act by binding to CMG2 while one binds PA. The efficacy of the most potent CMG2-binding inhibitor was improved through the incorporation of non-natural phenylalanine analogues. Cell based assays demonstrated that the optimized inhibitor protects macrophages from the toxicity of lethal factor.

A Biologically-Based Computational Approach to Drug Repurposing for Anthrax Infection

Developing drugs to treat the toxic effects of lethal toxin (LT) and edema toxin (ET) produced by B. anthracis is of global interest. We utilized a computational approach to score 474 drugs/compounds for their ability to reverse the toxic effects of anthrax toxins. For each toxin or drug/compound, we constructed an activity network by using its differentially expressed genes, molecular targets, and protein interactions. Gene expression profiles of drugs were obtained from the Connectivity Map and those of anthrax toxins in human alveolar macrophages were obtained from the Gene Expression Omnibus. Drug rankings were based on the ability of a drug/compound’s mode of action in the form of a signaling network to reverse the effects of anthrax toxins; literature reports were used to verify the top 10 and bottom 10 drugs/compounds identified. Simvastatin and bepridil with reported in vitro potency for protecting cells from LT and ET toxicities were computationally ranked fourth and eighth. The other top 10 drugs were fenofibrate, dihydroergotamine, cotinine, amantadine, mephenytoin, sotalol, ifosfamide, and mefloquine; literature mining revealed their potential protective effects from LT and ET toxicities. These drugs are worthy of investigation for their therapeutic benefits and might be used in combination with antibiotics for treating B. anthracis infection.

Inhibition of Interleukin 1β (IL-1β) Expression by Anthrax Lethal Toxin (LeTx) Is Reversed by Histone Deacetylase 8 (HDAC8) Inhibition in Murine Macrophages

Many pathogenic microbes often release toxins that subvert the host's immune responses to render the environment suitable for their survival and proliferation. LeTx is one of the toxins causing immune paralysis by cleaving and inactivating the mitogen-activated protein kinase (MAPK) kinases (MEKs). Here, we show that inhibition of the histone deacetylase 8 (HDAC8) by either the HDAC8-specific inhibitor PCI-34051 or small interference (si)RNAs rendered LeTx-exposed murine macrophages responsive to LPS in pro-IL-1β production. HDAC8 selectively targeted acetylated histone H3 lysine 27 (H3K27Ac), which is known to associate with active enhancers. LeTx induced HDAC8 expression, in part through inhibiting p38 MAPK, which resulted in a decrease of H3K27Ac levels. Inhibition of HDAC8 increased H3K27Ac levels and enhanced NF-κB-mediated pro-IL-1β enhancer and messenger RNA production in LeTx-exposed macrophages. Collectively, this study demonstrates a novel role of HDAC8 in LeTx immunotoxicity and regulation of pro-IL-1β production likely through eRNAs. Targeting HDAC8 could be a strategy for enhancing immune responses in macrophages exposed to LeTx or other toxins that inhibit MAPKs.

Bacterial Toxins as Pathogen Weapons Against Phagocytes

Bacterial toxins are virulence factors that manipulate host cell functions and take over the control of vital processes of living organisms to favor microbial infection. Some toxins directly target innate immune cells, thereby annihilating a major branch of the host immune response. In this review we will focus on bacterial toxins that act from the extracellular milieu and hinder the function of macrophages and neutrophils. In particular, we will concentrate on toxins from Gram-positive and Gram-negative bacteria that manipulate cell signaling or induce cell death by either imposing direct damage to the host cells cytoplasmic membrane or enzymatically modifying key eukaryotic targets. Outcomes regarding pathogen dissemination, host damage and disease progression will be discussed.

Crossing of the epithelial barriers by Bacillus anthracis: the Known and the Unknown

Anthrax, caused by Bacillus anthracis, a Gram-positive spore-forming bacterium, is initiated by the entry of spores into the host body. There are three types of human infection: cutaneous, inhalational, and gastrointestinal. For each form, B. anthracis spores need to cross the cutaneous, respiratory or digestive epithelial barriers, respectively, as a first obligate step to establish infection. Anthrax is a toxi-infection: an association of toxemia and rapidly spreading infection progressing to septicemia. The pathogenicity of Bacillus anthracis mainly depends on two toxins and a capsule. The capsule protects bacilli from the immune system, thus promoting systemic dissemination. The toxins alter host cell signaling, thereby paralyzing the immune response of the host and perturbing the endocrine and endothelial systems. In this review, we will mainly focus on the events and mechanisms leading to crossing of the respiratory epithelial barrier, as the majority of studies have addressed inhalational infection. We will discuss the critical gaps of knowledge that need to be addressed to gain a comprehensive view of the initial steps of inhalational anthrax. We will then discuss the few data available on B. anthracis crossing the cutaneous and digestive epithelia.

Global metabolomic analysis of a mammalian host infected with Bacillus anthracis

Whereas DNA provides the information to design life and proteins provide the materials to construct it, the metabolome can be viewed as the physiology that powers it. As such, metabolomics, the field charged with the study of the dynamic small-molecule fluctuations that occur in response to changing biology, is now being used to study the basis of disease. Here, we describe a comprehensive metabolomic analysis of a systemic bacterial infection using Bacillus anthracis, the etiological agent of anthrax disease, as the model pathogen. An organ and blood analysis identified approximately 400 metabolites, including several key classes of lipids involved in inflammation, as being suppressed by B. anthracis. Metabolite changes were detected as early as 1 day postinfection, well before the onset of disease or the spread of bacteria to organs, which testifies to the sensitivity of this methodology. Functional studies using pharmacologic inhibition of host phospholipases support the idea of a role of these key enzymes and lipid mediators in host survival during anthrax disease. Finally, the results are integrated to provide a comprehensive picture of how B. anthracis alters host physiology. Collectively, the results of this study provide a blueprint for using metabolomics as a platform to identify and study novel host-pathogen interactions that shape the outcome of an infection.

Anthrax prophylaxis: recent advances and future directions

Anthrax is a serious, potentially fatal disease that can present in four distinct clinical patterns depending on the route of infection (cutaneous, gastrointestinal, pneumonic, or injectional); effective strategies for prophylaxis and therapy are therefore required. This review addresses the complex mechanisms of pathogenesis employed by the bacterium and describes how, as understanding of these has developed over many years, so too have current strategies for vaccination and therapy. It covers the clinical and veterinary use of live attenuated strains of anthrax and the subsequent identification of protein sub-units for incorporation into vaccines, as well as combinations of protein sub-units with spore or other components. It also addresses the application of these vaccines for conventional prophylactic use, as well as post-exposure use in conjunction with antibiotics. It describes the licensed acellular vaccines AVA and AVP and discusses the prospects for a next generation of recombinant sub-unit vaccines for anthrax, balancing the regulatory requirement and current drive for highly defined vaccines, against the risk of losing the “danger” signals required to induce protective immunity in the vaccinee. It considers novel approaches to reduce time to immunity by means of combining, for example, dendritic cell vaccination with conventional approaches and considers current opportunities for the immunotherapy of anthrax.

Anthrax Pathogenesis

Anthrax is caused by the spore-forming, gram-positive bacterium Bacillus anthracis. The bacterium's major virulence factors are (a) the anthrax toxins and (b) an antiphagocytic polyglutamic capsule. These are encoded by two large plasmids, the former by pXO1 and the latter by pXO2. The expression of both is controlled by the bicarbonate-responsive transcriptional regulator, AtxA. The anthrax toxins are three polypeptides-protective antigen (PA), lethal factor (LF), and edema factor (EF)-that come together in binary combinations to form lethal toxin and edema toxin. PA binds to cellular receptors to translocate LF (a protease) and EF (an adenylate cyclase) into cells. The toxins alter cell signaling pathways in the host to interfere with innate immune responses in early stages of infection and to induce vascular collapse at late stages. This review focuses on the role of anthrax toxins in pathogenesis. Other virulence determinants, as well as vaccines and therapeutics, are briefly discussed.

Anthrax toxin-induced rupture of artificial lipid bilayer membranes

We demonstrate experimentally that anthrax toxin complexes rupture artificial lipid bilayer membranes when isolated from the blood of infected animals. When the solution pH is temporally acidified to mimic that process in endosomes, recombinant anthrax toxin forms an irreversibly bound complex, which also destabilizes membranes. The results suggest an alternative mechanism for the translocation of anthrax toxin into the cytoplasm.

Alveolar macrophages infected with Ames or Sterne strain of Bacillus anthracis elicit differential molecular expression patterns

Alveolar macrophages (AMs) phagocytose Bacillus anthracis following inhalation and induce the production of pro-inflammatory cytokines and chemokines to mediate the activation of innate immunity. Ames, the virulent strain of B. anthracis, contains two plasmids that encode the antiphagocytic poly-γ-d-glutamic acid capsule and the lethal toxin. The attenuated Sterne strain of B. anthracis, which lacks the plasmid encoding capsule, is widely adapted as a vaccine strain. Although differences in the outcome of infection with the two strains may have originated from the presence or absence of an anti-phagocytic capsule, the disease pathogenesis following infection will be manifested via the host responses, which is not well understood. To gain understanding of the host responses at cellular level, a microarray analysis was performed using primary rhesus macaque AMs infected with either Ames or Sterne spores. Notably, 528 human orthologs were identified to be differentially expressed in AMs infected with either strain of the B. anthracis. Meta-analyses revealed genes differentially expressed in response to B. anthracis infection were also induced upon infections with multiple pathogens such as Francisella Novicida or Staphylococcus aureus. This suggests the existence of a common molecular signature in response to pathogen infections. Importantly, the microarray and protein expression data for certain cytokines, chemokines and host factors provide further insights on how cellular processes such as innate immune sensing pathways, anti-apoptosis versus apoptosis may be differentially modulated in response to the virulent or vaccine strain of B. anthracis. The reported differences may account for the marked difference in pathogenicity between these two strains.

Cellular and physiological effects of anthrax exotoxin and its relevance to disease

Bacillus anthracis, the causative agent of anthrax, secretes a tri-partite exotoxin that exerts pleiotropic effects on the host. The purification of the exotoxin components, protective antigen, lethal factor, and edema factor allowed the rapid characterization of their physiologic effects on the host. As molecular biology matured, interest focused on the molecular mechanisms and cellular alterations induced by intoxication. Only recently have researchers begun to connect molecular and cellular knowledge back to the broader physiological effects of the exotoxin. This review focuses on the progress that has been made bridging molecular knowledge back to the exotoxin’s physiological effects on the host.

Expression of either lethal toxin or edema toxin by Bacillus anthracis is sufficient for virulence in a rabbit model of inhalational anthrax

The development of therapeutics against biothreats requires that we understand the pathogenesis of the disease in relevant animal models. The rabbit model of inhalational anthrax is an important tool in the assessment of potential therapeutics against Bacillus anthracis. We investigated the roles of B. anthracis capsule and toxins in the pathogenesis of inhalational anthrax in rabbits by comparing infection with the Ames strain versus isogenic mutants with deletions of the genes for the capsule operon (capBCADE), lethal factor (lef), edema factor (cya), or protective antigen (pagA). The absence of capsule or protective antigen (PA) resulted in complete avirulence, while the presence of either edema toxin or lethal toxin plus capsule resulted in lethality. The absence of toxin did not influence the ability of B. anthracis to traffic to draining lymph nodes, but systemic dissemination required the presence of at least one of the toxins. Histopathology studies demonstrated minimal differences among lethal wild-type and single toxin mutant strains. When rabbits were coinfected with the Ames strain and the PA- mutant strain, the toxin produced by the Ames strain was not able to promote dissemination of the PA- mutant, suggesting that toxigenic action occurs in close proximity to secreting bacteria. Taken together, these findings suggest that a major role for toxins in the pathogenesis of anthrax is to enable the organism to overcome innate host effector mechanisms locally and that much of the damage during the later stages of infection is due to the interactions of the host with the massive bacterial burden.

Debridement increases survival in a mouse model of subcutaneous anthrax

Anthrax is caused by infection with Bacillus anthracis, a spore-forming gram-positive bacterium. A major virulence factor for B. anthracis is an immunomodulatory tripartite exotoxin that has been reported to alter immune cell chemotaxis and activation. It has been proposed that B. anthracis infections initiate through entry of spores into the regional draining lymph nodes where they germinate, grow, and disseminate systemically via the efferent lymphatics. If this model holds true, it would be predicted that surgical removal of infected tissues, debridement, would have little effect on the systemic dissemination of bacteria. This model was tested through the development of a mouse debridement model. It was found that removal of the site of subcutaneous infection in the ear increased the likelihood of survival and reduced the quantity of spores in the draining cervical lymph nodes (cLN). At the time of debridement 12 hours post-injection measurable levels of exotoxins were present in the ear, cLN, and serum, yet leukocytes within the cLN were activated; countering the concept that exotoxins inhibit the early inflammatory response to promote bacterial growth. We conclude that the initial entry of spores into the draining lymph node of cutaneous infections alone is not sufficient to cause systemic disease and that debridement should be considered as an adjunct to antibiotic therapy.

Cytoskeleton as an emerging target of anthrax toxins

Bacillus anthracis, the agent of anthrax, has gained virulence through its exotoxins produced by vegetative bacilli and is composed of three components forming lethal toxin (LT) and edema toxin (ET). So far, little is known about the effects of these toxins on the eukaryotic cytoskeleton. Here, we provide an overview on the general effects of toxin upon the cytoskeleton architecture. Thus, we shall discuss how anthrax toxins interact with their receptors and may disrupt the interface between extracellular matrix and the cytoskeleton. We then analyze what toxin molecular effects on cytoskeleton have been described, before discussing how the cytoskeleton may help the pathogen to corrupt general cell processes such as phagocytosis or vascular integrity.

New insights into the biological effects of anthrax toxins: linking cellular to organismal responses

The anthrax toxins lethal toxin (LT) and edema toxin (ET) are essential virulence factors produced by Bacillus anthracis. These toxins act during two distinct phases of anthrax infection. During the first, prodromal phase, which is often asymptomatic, anthrax toxins act on cells of the immune system to help the pathogen establish infection. Then, during the rapidly progressing (or fulminant) stage of the disease bacteria disseminate via a hematological route to various target tissues and organs, which are typically highly vascularized. As bacteria proliferate in the bloodstream, LT and ET begin to accumulate rapidly reaching a critical threshold level that will cause death even when the bacterial proliferation is curtailed by antibiotics. During this final phase of infection the toxins cause an increase in vascular permeability and a decrease in function of target organs including the heart, spleen, kidney, adrenal gland, and brain. In this review, we examine the various biological effects of anthrax toxins, focusing on the fulminant stage of the disease and on mechanisms by which the two toxins may collaborate to cause cardiovascular collapse. We discuss normal mechanisms involved in maintaining vascular integrity and based on recent studies indicating that LT and ET cooperatively inhibit membrane trafficking to cell-cell junctions we explore several potential mechanisms by which the toxins may achieve their lethal effects. We also summarize the effects of other potential virulence factors secreted by B. anthracis and consider the role of toxic factors in the evolutionarily recent emergence of this devastating disease.

Parameters affecting spore recovery from wipes used in biological surface sampling

The need for the precise and reliable collection of potential biothreat contaminants has motivated research in developing a better understanding of the variability in biological surface sampling methods. In this context, the objective of this work was to determine parameters affecting the efficiency of extracting Bacillus anthracis Sterne spores from commonly used wipe sampling materials and to describe performance using the interfacial energy concept. In addition, surface thermodynamics was applied to understand and predict surface sampling performance. Wipe materials were directly inoculated with known concentrations of B. anthracis spores and placed into extraction solutions, followed by sonication or vortexing. Experimental factors investigated included wipe material (polyester, cotton, and polyester-rayon), extraction solution (sterile deionized water [H(2)O], deionized water with 0.04% Tween 80 [H(2)O-T], phosphate-buffered saline [PBS], and PBS with 0.04% Tween 80 [PBST]), and physical dissociation method (vortexing or sonication). The most efficient extraction from wipes was observed for solutions containing the nonionic surfactant Tween 80. The increase in extraction efficiency due to surfactant addition was attributed to an attractive interfacial energy between Tween 80 and the centrifuge tube wall, which prevented spore adhesion. Extraction solution significantly impacted the extraction efficiency, as determined by statistical analysis (P < 0.05). Moreover, the extraction solution was the most important factor in extraction performance, followed by the wipe material. Polyester-rayon was the most efficient wipe material for releasing spores into solution by rank; however, no statistically significant difference between polyester-rayon and cotton was observed (P > 0.05). Vortexing provided higher spore recovery in H(2)O and H(2)O-T than sonication, when all three wipe materials and the reference control were considered (P < 0.05).

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.

Comparison of TNFα to lipopolysaccharide as an inflammagen to characterize the idiosyncratic hepatotoxicity potential of drugs: Trovafloxacin as an example

Idiosyncratic drug reactions (IDRs) are poorly understood, unpredictable, and not detected in preclinical studies. Although the cause of these reactions is likely multi-factorial, one hypothesis is that an underlying inflammatory state lowers the tolerance to a xenobiotic. Previously used in an inflammation IDR model, bacterial lipopolysaccharide (LPS) is heterogeneous in nature, making development of standardized testing protocols difficult. Here, the use of rat tumor necrosis factor-α (TNFα) to replace LPS as an inflammatory stimulus was investigated. Sprague-Dawley rats were treated with separate preparations of LPS or TNFα, and hepatic transcriptomic effects were compared. TNFα showed enhanced consistency at the transcriptomic level compared to LPS. TNFα and LPS regulated similar biochemical pathways, although LPS was associated with more robust inflammatory signaling than TNFα. Rats were then codosed with TNFα and trovafloxacin (TVX), an IDR-associated drug, and evaluated by liver histopathology, clinical chemistry, and gene expression analysis. TNFα/TVX induced unique gene expression changes that clustered separately from TNFα/levofloxacin, a drug not associated with IDRs. TNFα/TVX cotreatment led to autoinduction of TNFα resulting in potentiation of underlying gene expression stress signals. Comparison of TNFα/TVX and LPS/TVX gene expression profiles revealed similarities in the regulation of biochemical pathways. In conclusion, TNFα could be used in lieu of LPS as an inflammatory stimulus in this model of IDRs.

Human transferrin confers serum resistance against Bacillus anthracis

The innate immune system in humans consists of both cellular and humoral components that collaborate to eradicate invading bacteria from the body. Here, we discover that the gram-positive bacterium Bacillus anthracis, the causative agent of anthrax, does not grow in human serum. Fractionation of serum by gel filtration chromatography led to the identification of human transferrin as the inhibiting factor. Purified transferrin blocks growth of both the fully virulent encapsulated B. anthracis Ames and the non-encapsulated Sterne strain. Growth inhibition was also observed in serum of wild-type mice but not of hypotransferrinemic mice that only have approximately 1% circulating transferrin levels. We were able to definitely assign the bacteriostatic activity of transferrin to its iron-binding function: neither iron-saturated transferrin nor a recombinant transferrin mutant unable to bind iron could inhibit growth of B. anthracis. Additional iron could restore bacterial growth in human serum. The observation that other important gram-positive pathogens are not inhibited by transferrin suggests they have evolved effective mechanisms to circumvent serum iron deprivation. These findings provide a better understanding of human host defense mechanisms against anthrax and provide a mechanistic basis for the antimicrobial activity of human transferrin.

Activation of NF-κB pathway and TNF-α are involved in the cytotoxicity of anthrax lethal toxin in bovine BoMac macrophages

Anthrax lethal toxin (LeTx) is an important virulence factor of Bacillus anthracis and causes illness and lethality for both animals and humans. Because species demonstrate varied sensitivity to anthrax intoxication, we investigated signaling pathways involved in anthrax LeTx cytotoxicity using a bovine macrophage cell line (BoMac). We found that bovine macrophages are sensitive to LeTx as displayed by a concentration-dependent increase in cell death. LeTx induced the degradation of I-κB and increased the nuclear translocation of NF-κB in BoMac cells. Blocking NF-κB activation with either chemical inhibitors or a dominant negative super-repressor I-κBαm eliminated LeTx-induced cell death. LeTx-induced production of TNF-α that contributed dramatically to cellular cytotoxicity. Inhibiting NF-κB activation eliminated TNF-α release and decreased cytotoxicity. The caspase pathway was also important for cytotoxicity as specific inhibitors abrogated LeTx-induced cell death. Taken together, our results show that activation of the NF-κB pathway and TNF-α production contribute to the cytotoxicity of anthrax LeTx in bovine macrophages.

Transport of Bacillus anthracis from the lungs to the draining lymph nodes is a rapid process facilitated by CD11c+ cells

Inhalational anthrax is established after inhaled Bacillus anthracis spores are transported to the lung associated lymph nodes. Dendritic cells (CD11c+ cells) located in the lungs are phagocytes that maintain many capabilities consistent with transport. This study investigates the role of dendritic cells as conduits of spores from the lung to the draining lymph nodes. The intratracheally spore-challenged mouse model of inhalational anthrax was utilized to investigate in vivo activities of CD11c+ cells. FITC labeled spores were delivered to the lungs of mice. Subsequently lung associated lymph nodes were isolated after infection and CD11c+ cells were found in association with the labeled spores. Further investigation of CD11c+ cells in early anthrax events was facilitated by use of the CD11c-diphtheria toxin (DT) receptor-green fluorescent protein transgenic mice in which CD11c+ cells can be transiently depleted by treatment with DT. We found that the presence of CD11c+ cells was necessary for efficient traffic of the spore to lung associated lymph nodes at early times after infection. Cultured dendritic cells were used to determine that these cells are capable of B. anthracis spore phagocytosis, and support germination and outgrowth. This data demonstrates that CD11c+ cells are likely carriers of B. anthracis spores from the point of inhalation in the lung to the lung associated lymph nodes. The cultured dendritic cell allows for spore germination and outgrowth supporting the concept that the CD11c+ cell responsible for this function can be a dendritic cell.

Resistance of human alveolar macrophages to Bacillus anthracis lethal toxin

The etiologic agent of inhalational anthrax, Bacillus anthracis, produces virulence toxins that are important in the disease pathogenesis. Current studies suggest that mouse and human macrophages are susceptible to immunosuppressive effects of one of the virulence toxins, lethal toxin (LT). Thus a paradigm has emerged that holds that the alveolar macrophage (AM) does not play a significant role in the innate immune response to B. anthracis or defend against the pathogen as it is disabled by LT. This is inconsistent with animal models and autopsy studies that show minimal disease at the alveolar surface. We examined whether AM are immunosuppressed by LT. We found that human AM were relatively resistant to LT-mediated innate immune cytokine suppression, MEK cleavage, and induction of apoptosis as compared with mouse RAW 264.7 macrophages. Mouse AM and murine bone marrow-derived macrophages were also relatively resistant to LT-mediated apoptosis despite intermediate sensitivity to MEK cleavage. The binding component of LT, protective Ag, does not attach to human AM, although it did bind to mouse AM, murine bone marrow-derived macrophages, and RAW 264.7 macrophages. Human AM do not produce significant amounts of the protective Ag receptor anthrax toxin receptor 1 (TEM8/ANTXR1) and anthrax toxin receptor 2 (CMG2/ANTXR2). Thus, mature and differentiated AM are relatively resistant to the effects of LT as compared with mouse RAW 264.7 macrophages. AM resistance to LT may enhance clearance of the pathogen from the alveolar surface and explain why this surface is relatively free of B. anthracis in animal models and autopsy studies.

Anthrax vaccination strategies

The biological attack conducted through the US postal system in 2001 broadened the threat posed by anthrax from one pertinent mainly to soldiers on the battlefield to one understood to exist throughout our society. The expansion of the threatened population placed greater emphasis on the reexamination of how we vaccinate against Bacillus anthracis. The currently-licensed Anthrax Vaccine, Adsorbed (AVA) and Anthrax Vaccine, Precipitated (AVP) are capable of generating a protective immune response but are hampered by shortcomings that make their widespread use undesirable or infeasible. Efforts to gain US Food and Drug Administration (FDA) approval for licensure of a second generation recombinant protective antigen (rPA)-based anthrax vaccine are ongoing. However, this vaccine's reliance on the generation of a humoral immune response against a single virulence factor has led a number of scientists to conclude that the vaccine is likely not the final solution to optimal anthrax vaccine design. Other vaccine approaches, which seek a more comprehensive immune response targeted at multiple components of the B. anthracis organism, are under active investigation. This review seeks to summarize work that has been done to build on the current PA-based vaccine methodology and to evaluate the search for future anthrax prophylaxis strategies.

Zoonoses of dermatologic interest

Zoonoses are infectious diseases that can be transmitted from animals to humans. Transmission occurs directly or through vectors such as ticks, mosquitoes, or flies. The causative agents include bacteria, parasites, viruses, and fungi. Domestic pets and livestock, as well as wild animals, can be the source of disease. In this summary, we will focus on a number of dermatologically relevant examples.

Anthrax toxins: a weapon to systematically dismantle the host immune defenses

Successful colonization of the host by bacterial pathogens relies on their capacity to evade the complex and powerful defenses opposed by the host immune system, at least in the initial phases of infection. The two toxins of Bacillus anthracis, lethal toxin and edema toxin, appear to have been shaped by evolution to assist the microorganism in this crucial function, in addition to act as general toxins acting on almost all cell types. Edema toxin causes a consistent elevation of cAMP, an important second messenger the production of which is normally strictly controlled in mammalian cells, whereas lethal toxin cleaves most isoforms of mitogen-activated protein kinase kinases. By disrupting or subverting central modules common to all the principal signaling networks which control immune cell activation, effector function and migration, the anthrax toxins effectively and systematically dismantle both the innate and the adaptive immune defenses of the host. Here, we review the specific effects of the lethal and edema toxins of B. anthracis on the activation and function of phagocytes, dendritic cells and lymphocytes. We also discuss some open issues which should be addressed to gain a comprehensive insight into the complex relationship that B. anthracis establishes with the host.

Reduced expression of CD45 protein-tyrosine phosphatase provides protection against anthrax pathogenesis

The modulation of cellular processes by small molecule inhibitors, gene inactivation, or targeted knockdown strategies combined with phenotypic screens are powerful approaches to delineate complex cellular pathways and to identify key players involved in disease pathogenesis. Using chemical genetic screening, we tested a library of known phosphatase inhibitors and identified several compounds that protected Bacillus anthracis infected macrophages from cell death. The most potent compound was assayed against a panel of sixteen different phosphatases of which CD45 was found to be most sensitive to inhibition. Testing of a known CD45 inhibitor and antisense phosphorodiamidate morpholino oligomers targeting CD45 also protected B. anthracis-infected macrophages from cell death. However, reduced CD45 expression did not protect anthrax lethal toxin (LT) treated macrophages, suggesting that the pathogen and independently added LT may signal through distinct pathways. Subsequent, in vivo studies with both gene-targeted knockdown of CD45 and genetically engineered mice expressing reduced levels of CD45 resulted in protection of mice after infection with the virulent Ames B. anthracis. Intermediate levels of CD45 expression were critical for the protection, as mice expressing normal levels of CD45 or disrupted CD45 phosphatase activity or no CD45 all succumbed to this pathogen. Mechanism-based studies suggest that the protection provided by reduced CD45 levels results from regulated immune cell homeostasis that may diminish the impact of apoptosis during the infection. To date, this is the first report demonstrating that reduced levels of host phosphatase CD45 modulate anthrax pathogenesis.

Pathophysiology of anthrax

Infection by Bacillus anthracis in animals and humans results from accidental or intentional exposure, by oral, cutaneous or pulmonary routes, to spores, which are normally present in the soil. Treatment includes administration of antibiotics, vaccination or treatment with antibody to the toxin. A better understanding of the molecular basis of the processes involved in the pathogenesis of anthrax namely, spore germination in macrophages and biological effects of the secreted toxins on heart and blood vessels will lead to improved management of infected animals and patients. Controlling germination will be feasible by inhibiting macrophage paralysis and cell death. On the other hand, the control of terminal hypotension might be achieved by inhibition of cardiomyocyte mitogen-activated protein kinase and stimulation of vessel cAMP.

Efficacy of a vaccine based on protective antigen and killed spores against experimental inhalational anthrax

Protective antigen (PA)-based anthrax vaccines acting on toxins are less effective than live attenuated vaccines, suggesting that additional antigens may contribute to protective immunity. Several reports indicate that capsule or spore-associated antigens may enhance the protection afforded by PA. Addition of formaldehyde-inactivated spores (FIS) to PA (PA-FIS) elicits total protection against cutaneous anthrax. Nevertheless, vaccines that are effective against cutaneous anthrax may not be so against inhalational anthrax. The aim of this work was to optimize immunization with PA-FIS and to assess vaccine efficacy against inhalational anthrax. We assessed the immune response to recombinant anthrax PA from Bacillus anthracis (rPA)-FIS administered by various immunization protocols and the protection provided to mice and guinea pigs infected through the respiratory route with spores of a virulent strain of B. anthracis. Combined subcutaneous plus intranasal immunization of mice yielded a mucosal immunoglobulin G response to rPA that was more than 20 times higher than that in lung mucosal secretions after subcutaneous vaccination. The titers of toxin-neutralizing antibody and antispore antibody were also significantly higher: nine and eight times higher, respectively. The optimized immunization elicited total protection of mice intranasally infected with the virulent B. anthracis strain 17JB. Guinea pigs were fully protected, both against an intranasal challenge with 100 50% lethal doses (LD(50)) and against an aerosol with 75 LD(50) of spores of the highly virulent strain 9602. Conversely, immunization with PA alone did not elicit protection. These results demonstrate that the association of PA and spores is very much more effective than PA alone against experimental inhalational anthrax.

Bacillus anthracis peptidoglycan stimulates an inflammatory response in monocytes through the p38 mitogen-activated protein kinase pathway

We hypothesized that the peptidoglycan component of B. anthracis may play a critical role in morbidity and mortality associated with inhalation anthrax. To explore this issue, we purified the peptidoglycan component of the bacterial cell wall and studied the response of human peripheral blood cells. The purified B. anthracis peptidoglycan was free of non-covalently bound protein but contained a complex set of amino acids probably arising from the stem peptide. The peptidoglycan contained a polysaccharide that was removed by mild acid treatment, and the biological activity remained with the peptidoglycan and not the polysaccharide. The biological activity of the peptidoglycan was sensitive to lysozyme but not other hydrolytic enzymes, showing that the activity resides in the peptidoglycan component and not bacterial DNA, RNA or protein. B. anthracis peptidoglycan stimulated monocytes to produce primarily TNFα; neutrophils and lymphocytes did not respond. Peptidoglycan stimulated monocyte p38 mitogen-activated protein kinase and p38 activity was required for TNFα production by the cells. We conclude that peptidoglycan in B. anthracis is biologically active, that it stimulates a proinflammatory response in monocytes, and uses the p38 kinase signal transduction pathway to do so. Given the high bacterial burden in pulmonary anthrax, these findings suggest that the inflammatory events associated with peptidoglycan may play an important role in anthrax pathogenesis.

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.

Inactivation of rho GTPases by statins attenuates anthrax lethal toxin activity

Anthrax lethal factor (LF), secreted by Bacillus anthracis, interacts with protective antigen to form a bipartite toxin (lethal toxin [LT]) that exerts pleiotropic biological effects resulting in subversion of the innate immune response. Although the mitogen-activated protein kinase kinases (MKKs) are the major intracellular protein targets of LF, the pathology induced by LT is not well understood. The statin family of HMG-coenzyme A reductase inhibitors have potent anti-inflammatory effects independent of their cholesterol-lowering properties, which have been attributed to modulation of Rho family GTPase activity. The Rho GTPases regulate vesicular trafficking, cytoskeletal dynamics, and cell survival and proliferation. We hypothesized that disruption of Rho GTPase function by statins might alter LT action. We show here that statins delay LT-induced death and MKK cleavage in RAW macrophages and that statin-mediated effects on LT action are attributable to disruption of Rho GTPases. The Rho GTPase-inactivating toxin, toxin B, did not significantly affect LT binding or internalization, suggesting that the Rho GTPases regulate trafficking and/or localization of LT once internalized. The use of drugs capable of inhibiting Rho GTPase activity, such as statins, may provide a means to attenuate intoxication during B. anthracis infection.

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.

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.

C/EBPβ phosphorylation rescues macrophage dysfunction and apoptosis induced by anthrax lethal toxin

Bacillus anthracis lethal toxin (LT) impairs innate and adaptive immunity. Anthrax lethal factor stimulates cleavage of MAPK kinases, which prevents the activation of antiapoptotic MAPK targets. However, these MAPK targets have not been yet identified. Here, we found that LT induces macrophage apoptosis by enhancing caspase 8 activation and by preventing the activation of ribosomal S6 kinase-2 (RSK), a MAPK target, and the phosphorylation of CCAAT/enhancer binding protein-β (C/EBPβ) on T217, a RSK target. Expression of the dominant positive, phosphorylation mimic C/EBPβ-E217rescued macrophages from LT-induced apoptosis by blocking the activation of procaspase 8. LT inhibited macrophage phagocytosis and oxidative burst and induced apoptosis in normal mice but not in C/EBPβ-E217transgenic mice. These findings suggest that C/EBPβ may play a critical role in anthrax pathogenesis, at least in macrophages.

Human lung innate immune response to Bacillus anthracis spore infection

Bacillus anthracis, the causative agent of inhalational anthrax, enters a host through the pulmonary system before dissemination. We have previously shown that human alveolar macrophages participate in the initial innate immune response to B. anthracis spores through cell signal-mediated cytokine release. We proposed that the lung epithelia also participate in the innate immune response to this pathogen, and we have developed a human lung slice model to study this process. Exposure of our model to B. anthracis (Sterne) spores rapidly activated the mitogen-activated protein kinase signaling pathways ERK, p38, and JNK. In addition, an RNase protection assay showed induction of mRNA of several cytokines and chemokines. This finding was reflected at the translational level by protein peak increases of 3-, 25-, 9-, 34-, and 5-fold for interleukin-6 (IL-6), tumor necrosis factor alpha, IL-8, macrophage inflammatory protein 1alpha/beta, and monocyte chemoattractant protein 1, respectively, as determined by an enzyme-linked immunosorbent assay. Inhibition of individual pathways by UO126, SP600125, and SB0203580 decreased induction of chemokines and cytokines by spores, but this depended on the pathways inhibited and the cytokines and chemokines induced. Combining all three inhibitors reduced induction of all cytokines and chemokines tested to background levels. An immunohistochemistry analysis of IL-6 and IL-8 revealed that alveolar epithelial cells and macrophages and a few interstitial cells are the source of the cytokines and chemokines. Taken together, these data showed the activation of the pulmonary epithelium in response to B. anthracis spore exposure. Thus, the lung epithelia actively participate in the innate immune response to B. anthracis infection through cell signal-mediated elaboration of cytokines and chemokines.

Manipulation of host signalling pathways by anthrax toxins

Infectious microbes face an unwelcoming environment in their mammalian hosts, which have evolved elaborate multicelluar systems for recognition and elimination of invading pathogens. A common strategy used by pathogenic bacteria to establish infection is to secrete protein factors that block intracellular signalling pathways essential for host defence. Some of these proteins also act as toxins, directly causing pathology associated with disease. Bacillus anthracis, the bacterium that causes anthrax, secretes two plasmid-encoded enzymes, LF (lethal factor) and EF (oedema factor), that are delivered into host cells by a third bacterial protein, PA (protective antigen). The two toxins act on a variety of cell types, disabling the immune system and inevitably killing the host. LF is an extraordinarily selective metalloproteinase that site-specifically cleaves MKKs (mitogen-activated protein kinase kinases). Cleavage of MKKs by LF prevents them from activating their downstream MAPK (mitogen-activated protein kinase) substrates by disrupting a critical docking interaction. Blockade of MAPK signalling functionally impairs cells of both the innate and adaptive immune systems and induces cell death in macrophages. EF is an adenylate cyclase that is activated by calmodulin through a non-canonical mechanism. EF causes sustained and potent activation of host cAMP-dependent signalling pathways, which disables phagocytes. Here I review recent progress in elucidating the mechanisms by which LF and EF influence host signalling and thereby contribute to disease.

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.

Contribution of toxins to the pathogenesis of inhalational anthrax

Inhalational anthrax is a life-threatening infectious disease of considerable concern, especially as a potential bioterrorism agent. Progress is gradually being made towards understanding the mechanisms used by Bacillus anthracis to escape the immune system and to induce severe septicaemia associated with toxaemia and leading to death. Recent advances in fundamental research have revealed previously unsuspected roles for toxins in various cell types. We summarize here pathological data for animal models and macroscopic histological examination data from recent clinical records, which we link to the effects of toxins. We describe three major steps in infection: (i) an invasion phase in the lung, during which toxins have short-distance effects on lung phagocytes; (ii) a phase of bacillus proliferation in the mediastinal lymph nodes, with local effects of toxins; and (iii) a terminal, diffusion phase, characterized by a high blood bacterial load and by long-distance effects of toxins, leading to host death. The pathophysiology of inhalational anthrax thus involves interactions between toxins and various cell partners, throughout the course of infection.

Targeting proteins to the cell wall of sporulating Bacillus anthracis

Dormant spores of Bacillus anthracis germinate during host infection and their vegetative growth and dissemination precipitate anthrax disease. Upon host death, bacilli engage a developmental programme to generate infectious spores within carcasses. Hallmark of sporulation in Bacillus spp. is the formation of an asymmetric division septum between mother cell and forespore compartments. We show here that sortase C (SrtC) cleaves the LPNTA sorting signal of BasH and BasI, thereby targeting both polypeptides to the cell wall of sporulating bacilli. Sortase substrates are initially produced in different cell compartments and at different developmental stages but penultimately decorate the envelope of the maturing spore. srtC mutants appear to display no defect during the initial stages of infection and precipitate lethal anthrax disease in guinea pigs at a similar rate as wild-type B. anthracis strain Ames. Unlike wild-type bacilli, srtC mutants do not readily form spores in guinea pig tissue or sheep blood unless their vegetative forms are exposed to air.