Contribution of lethal toxin and edema toxin to the pathogenesis of anthrax meningitis

Comprehensive characterization of toxins during progression of inhalation anthrax in a non-human primate model

Inhalation anthrax has three clinical stages: early-prodromal, intermediate-progressive, and late-fulminant. We report the comprehensive characterization of anthrax toxins, including total protective antigen (PA), total lethal factor (LF), total edema factor (EF), and their toxin complexes, lethal toxin and edema toxin in plasma, during the course of inhalation anthrax in 23 cynomolgus macaques. The toxin kinetics were predominantly triphasic with an early rise (phase-1), a plateau/decline (phase-2), and a final rapid rise (phase-3). Eleven animals had shorter survival times, mean±standard deviation of 58.7±7.6 hours (fast progression), 11 animals had longer survival times, 113±34.4 hours (slow progression), and one animal survived. Median (lower-upper quartile) LF levels at the end-of-phase-1 were significantly higher in animals with fast progression [138 (54.9-326) ng/mL], than in those with slow progression [23.8 (15.6-26.3) ng/mL] (p = 0.0002), and the survivor (11.1 ng/mL). The differences were also observed for other toxins and bacteremia. Animals with slow progression had an extended phase-2 plateau, with low variability of LF levels across all time points and animals. Characterization of phase-2 toxin levels defined upper thresholds; critical levels for exiting phase-2 and entering the critical phase-3, 342 ng/mL (PA), 35.8 ng/mL (LF), and 1.10 ng/mL (EF). The thresholds were exceeded earlier in animals with fast progression (38.5±7.4 hours) and later in animals with slow progression (78.7±15.2 hours). Once the threshold was passed, toxin levels rose rapidly in both groups to the terminal stage. The time from threshold to terminal was rapid and similar; 20.8±7.4 hours for fast and 19.9±7.5 hours for slow progression. The three toxemic phases were aligned with the three clinical stages of anthrax for fast and slow progression which showed that anthrax progression is toxin- rather than time-dependent. This first comprehensive evaluation of anthrax toxins provides new insights into disease progression.

Risk Factors for Death or Meningitis in Adults Hospitalized for Cutaneous Anthrax, 1950-2018: A Systematic Review

BACKGROUND

Cutaneous anthrax accounts for approximately 95% of anthrax cases worldwide. About 24% of untreated patients die, and many cases are complicated by meningitis. Here, we explore clinical features of cutaneous disease associated with poor outcomes.

METHODS

A systematic review identified 303 full-text articles published from 1950 through 2018 that met predefined inclusion criteria. Cases were abstracted, and descriptive analyses and univariate logistic regression were conducted to identify prognostic indicators for cutaneous anthrax.

RESULTS

Of 182 included patients, 47 (25.8%) died. Previously reported independent predictors for death or meningitis that we confirmed included fever or chills; nausea or vomiting; headache; severe headache; nonheadache, nonmeningeal signs; leukocytosis; and bacteremia. Newly identified predictors included anxiety, abdominal pain, diastolic hypotension, skin trauma, thoracic edema, malignant pustule edema, lymphadenopathy, and evidence of coagulopathy (all with P < .05).

CONCLUSIONS

We identified patient presentations not previously associated with poor outcomes.

Anthrax toxins regulate pain signaling and can deliver molecular cargoes into ANTXR2+ DRG sensory neurons

Bacterial products can act on neurons to alter signaling and function. In the present study, we found that dorsal root ganglion (DRG) sensory neurons are enriched for ANTXR2, the high-affinity receptor for anthrax toxins. Anthrax toxins are composed of protective antigen (PA), which binds to ANTXR2, and the protein cargoes edema factor (EF) and lethal factor (LF). Intrathecal administration of edema toxin (ET (PA + EF)) targeted DRG neurons and induced analgesia in mice. ET inhibited mechanical and thermal sensation, and pain caused by formalin, carrageenan or nerve injury. Analgesia depended on ANTXR2 expressed by Na_v1.8⁺ or Advillin⁺ neurons. ET modulated protein kinase A signaling in mouse sensory and human induced pluripotent stem cell-derived sensory neurons, and attenuated spinal cord neurotransmission. We further engineered anthrax toxins to introduce exogenous protein cargoes, including botulinum toxin, into DRG neurons to silence pain. Our study highlights interactions between a bacterial toxin and nociceptors, which may lead to the development of new pain therapeutics.

Clindamycin Protects Nonhuman Primates Against Inhalational Anthrax But Does Not Enhance Reduction of Circulating Toxin Levels When Combined With Ciprofloxacin

BACKGROUND

Inhalational anthrax is rare and clinical experience limited. Expert guidelines recommend treatment with combination antibiotics including protein synthesis-inhibitors to decrease toxin production and increase survival, although evidence is lacking.

METHODS

Rhesus macaques exposed to an aerosol of Bacillus anthracis spores were treated with ciprofloxacin, clindamycin, or ciprofloxacin + clindamycin after becoming bacteremic. Circulating anthrax lethal factor and protective antigen were quantitated pretreatment and 1.5 and 12 hours after beginning antibiotics.

RESULTS

In the clindamycin group, 8 of 11 (73%) survived demonstrating its efficacy for the first time in inhalational anthrax, compared to 9 of 9 (100%) with ciprofloxacin, and 8 of 11 (73%) with ciprofloxacin + clindamycin. These differences were not statistically significant. There were no significant differences between groups in lethal factor or protective antigen levels from pretreatment to 12 hours after starting antibiotics. Animals that died after clindamycin had a greater incidence of meningitis compared to those given ciprofloxacin or ciprofloxacin + clindamycin, but numbers of animals were very low and no definitive conclusion could be reached.

CONCLUSION

Treatment of inhalational anthrax with clindamycin was as effective as ciprofloxacin in the nonhuman primate. Addition of clindamycin to ciprofloxacin did not enhance reduction of circulating toxin levels.

Structural Integrity of the Alveolar-Capillary Barrier in Cynomolgus Monkeys Challenged with Fully Virulent and Toxin-Deficient Strains of Bacillus anthracis

Inhalational anthrax, a disease caused by inhaling Bacillus anthracis spores, leads to respiratory distress, vascular leakage, high-level bacteremia, and often death within days. Anthrax lethal toxin and edema toxin, which are composed of protective antigen (PA) plus either lethal factor (LF) or edema factor (EF), respectively, play an important yet incompletely defined role in the pulmonary pathophysiology. To better understand their contribution, we examined the structural integrity of the alveolar-capillary barrier in archival formalin-fixed lungs of cynomolgus monkeys challenged with the fully virulent B. anthracis Ames wild-type strain or the isogenic toxin-deficient mutants ΔEF, ΔLF, and ΔPA. Pulmonary spore challenge with the wild-type strain caused high mortality, intra-alveolar hemorrhages, extensive alveolar septal sequestration of bacteria and neutrophils, diffuse destabilization of epithelial and endothelial junctions, increased markers of coagulation and complement activation (including tissue factor and C5a), and multifocal intra-alveolar fibrin deposition. ΔEF challenge was lethal and showed similar alveolar-capillary alterations; however, intra-alveolar hemorrhages, bacterial deposition, and markers of coagulation or complement were absent or markedly lower. In contrast, ΔLF or ΔPA challenges were nonlethal and showed no signs of alveolar bacterial deposition or alveolar-capillary changes. These findings provide evidence that lethal toxin plays a determinative role in bacterial dissemination and alveolar-capillary barrier dysfunction, and edema toxin may significantly exacerbate pulmonary pathologies in a systemic infection.

Disruption of the Blood-Brain Barrier During Neuroinflammatory and Neuroinfectious Diseases

As the organ of highest metabolic demand, utilizing over 25% of total body glucose utilization via an enormous vasculature with one capillary every 73 μm, the brain evolves a barrier at the capillary and postcapillary venules to prevent toxicity during serum fluctuations in metabolites and hormones, to limit brain swelling during inflammation, and to prevent pathogen invasion. Understanding of neuroprotective barriers has since evolved to incorporate the neurovascular unit (NVU), the blood-cerebrospinal fluid (CSF) barrier, and the presence of CNS lymphatics that allow leukocyte egress. Identification of the cellular and molecular participants in BBB function at the NVU has allowed detailed analyses of mechanisms that contribute to BBB dysfunction in various disease states, which include both autoimmune and infectious etiologies. This chapter will introduce some of the cellular and molecular components that promote barrier function but may be manipulated by inflammatory mediators or pathogens during neuroinflammation or neuroinfectious diseases.

Mechanisms of Blood Brain Barrier Disruption by Different Types of Bacteria, and Bacterial-Host Interactions Facilitate the Bacterial Pathogen Invading the Brain

This review aims to elucidate the different mechanisms of blood brain barrier (BBB) disruption that may occur due to invasion by different types of bacteria, as well as to show the bacteria-host interactions that assist the bacterial pathogen in invading the brain. For example, platelet-activating factor receptor (PAFR) is responsible for brain invasion during the adhesion of pneumococci to brain endothelial cells, which might lead to brain invasion. Additionally, the major adhesin of the pneumococcal pilus-1, RrgA is able to bind the BBB endothelial receptors: polymeric immunoglobulin receptor (pIgR) and platelet endothelial cell adhesion molecule (PECAM-1), thus leading to invasion of the brain. Moreover, Streptococcus pneumoniae choline binding protein A (CbpA) targets the common carboxy-terminal domain of the laminin receptor (LR) establishing initial contact with brain endothelium that might result in BBB invasion. Furthermore, BBB disruption may occur by S. pneumoniae penetration through increasing in pro-inflammatory markers and endothelial permeability. In contrast, adhesion, invasion, and translocation through or between endothelial cells can be done by S. pneumoniae without any disruption to the vascular endothelium, upon BBB penetration. Internalins (InlA and InlB) of Listeria monocytogenes interact with its cellular receptors E-cadherin and mesenchymal-epithelial transition (MET) to facilitate invading the brain. L. monocytogenes species activate NF-κB in endothelial cells, encouraging the expression of P- and E-selectin, intercellular adhesion molecule 1 (ICAM-1), and Vascular cell adhesion protein 1 (VCAM-1), as well as IL-6 and IL-8 and monocyte chemoattractant protein-1 (MCP-1), all these markers assist in BBB disruption. Bacillus anthracis species interrupt both adherens junctions (AJs) and tight junctions (TJs), leading to BBB disruption. Brain microvascular endothelial cells (BMECs) permeability and BBB disruption are induced via interendothelial junction proteins reduction as well as up-regulation of IL-1α, IL-1β, IL-6, TNF-α, MCP-1, macrophage inflammatory proteins-1 alpha (MIP1α) markers in Staphylococcus aureus species. Streptococcus agalactiae or Group B Streptococcus toxins (GBS) enhance IL-8 and ICAM-1 as well as nitric oxide (NO) production from endothelial cells via the expression of inducible nitric oxide synthase (iNOS) enhancement, resulting in BBB disruption. While Gram-negative bacteria, Haemophilus influenza OmpP2 is able to target the common carboxy-terminal domain of LR to start initial interaction with brain endothelium, then invade the brain. H. influenza type b (HiB), can induce BBB permeability through TJ disruption. LR and PAFR binding sites have been recognized as common routes of CNS entrance by Neisseria meningitidis. N. meningitidis species also initiate binding to BMECs and induces AJs deformation, as well as inducing specific cleavage of the TJ component occludin through the release of host MMP-8. Escherichia coli bind to BMECs through LR, resulting in IL-6 and IL-8 release and iNOS production, as well as resulting in disassembly of TJs between endothelial cells, facilitating BBB disruption. Therefore, obtaining knowledge of BBB disruption by different types of bacterial species will provide a picture of how the bacteria enter the central nervous system (CNS) which might support the discovery of therapeutic strategies for each bacteria to control and manage infection.

Gastric pH and Toxin Factors Modulate Infectivity and Disease Progression After Gastrointestinal Exposure to Bacillus anthracis

Gastrointestinal (GI) anthrax is the most prevalent form of naturally acquired Bacillus anthracis infection, which is associated with exposure to vegetative bacteria in infected meat (carnivores) or to fermented rumen contents (herbivores). We assessed whether key host and pathogen factors modulate infectivity and progression of infection using a mouse model of GI infection. Gastric acid neutralization increases infectivity, but 30%-40% of mice succumb to infection without neutralization. Mice either fed or fasted before exposure showed similar infectivity rates. Finally, the pathogen's anthrax lethal factor is required to establish lethal infection, whereas its edema factor modulates progression and dissemination of infection.

Pathology of wild-type and toxin-independent Bacillus anthracis meningitis in rabbits

Hemorrhagic meningitis is considered a complication of anthrax and was reported in about 50% of deadly cases in humans and non-human primates (NHP). Recently we demonstrated in Guinea pigs and rabbits that 100% of the B. anthracis-infected animals presented histopathology of meningitis at the time of death, some without any sign of hemorrhage. A similar pathology was observed in animals that succumbed following infection with the toxin deficient mutant, thus indicating that anthrax meningitis is a toxin-independent phenomenon. In this manuscript we describe a histopathological study of the B. anthracis infection of the central nervous system (CNS). Though we could find sporadic growth of the bacteria around blood vessels in the cortex, we report that the main infiltration route is the choroid plexus. We found massive destruction of entire sections of the choroid plexus coupled with massive aggregation of bacilli in the ventricles, in close proximity to the parenchyma. The choroid plexus also contained significant amounts of intravascular bacterial aggregates, often enclosed in what appear to be fibrin-like clots. The high concentration of these aggregates in areas of significant tissue destruction combined with the fact that capsular B. anthracis bacteria have a low tendency to adhere to endothelial cells, might suggest that these clots are used as an adherence mechanism by the bacteria. The major histopathological finding is meningitis. We find massive bacterial growth in the meninges without evidence of encephalitis, even when the bacteria emerge from a parenchymal blood vessel. Erythrocytes were present within the meningeal space but no clear vasculitis could be detected. Histology of the brain stem indicates meningitis, edema and hemorrhages that might explain death from suffocation due to direct damage to the respiratory center. All of these processes are toxin-independent, since they were observed following infection with either the wild type strain or the toxin-deficient mutant. Herein, we propose that the first step of anthrax-meningitis is bacterial adhesion to the blood vessels by manipulating coagulation, mainly in the choroid plexus. The trapped bacteria then destroy sections of the choroid plexus, resulting in penetration into the CSF, leading to meningitis and hemorrhage. Death could be the result of increased intracranial pressure and/or damage to the brain stem.

Streptococcus pneumoniae induces pyroptosis through the regulation of autophagy in murine microglia

Streptococcus pneumoniae is responsible for significant mortality and morbidity worldwide and causes invasive pneumococcal diseases including pneumococcal meningitis. Pyroptosis is caspase-1-dependent inflammatory cell death and is known to be induced by various microbial infections. In the present study, we investigated the molecular mechanisms that regulate pyroptosis induced by S. pneumoniae in microglia. Our results revealed that S. pneumoniae induced pyroptosis through caspase-1 activation and IL-1β production. We also found that the activation of caspase-1 and the maturation of IL-1β and IL-18 in the S. pneumoniae-triggered pyroptotic cell death process were mediated by NLRP3 inflammasome. In addition, pneumococcal infection increased the expression of autophagy-related genes and induced autophagosome formation. We also showed that the inhibition of autophagy promoted pneumococcus-induced pyroptosis. Furthermore, ROS was generated by pneumococcal infection and inhibited caspase-1 activation within 4 h of infection. However, in the late phase of infection, IL-1β secretion and caspase-1-dependent cell death were induced by ROS. These results suggest that autophagy induction transiently delay pyroptosis induced by S. pneumoniae in microglia. Our study also revealed that the activation of caspase-1 and the production of IL-1β were induced by pneumolysin and that pneumolysin triggered pyroptosis in microglial cells. Similar to the in vitro results, S. pneumoniae induced caspase-1 activation and caspase-1-dependent cytokine maturation in the mouse meningitis model. Thus, the present data demonstrate that S. pneumoniae induces pyroptosis in murine microglia and that NLRP3 inflammasome is critical for caspase-1 activation during the process. Furthermore, the induction of autophagy could transiently protect microglia from pyroptosis.

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.

Diminished but Not Abolished Effect of Two His351 Mutants of Anthrax Edema Factor in a Murine Model

Edema toxin (ET), which is composed of a potent adenylate cyclase (AC), edema factor (EF), and protective antigen (PA), is one of the major toxicity factors of Bacillus anthracis. In this study, we introduced mutations in full-length EF to generate alanine EF(H351A) and arginine EF(H351R) variants. In vitro activity analysis displayed that the adenylyl cyclase activity of both the mutants was significantly diminished compared with the wild-type EF. When the native and mutant toxins were administered subcutaneously in a mouse footpad edema model, severe acute swelling was evoked by wild-type ET, while the symptoms induced by mutant toxins were very minor. Systemic administration of these EF variants caused non-lethal hepatotoxicity. In addition, EF(H351R) exhibited slightly higher activity in causing more severe edema than EF(H351A). Our findings demonstrate that the toxicity of ET is not abolished by substitution of EF residue His351 by alanine or arginine. These results also indicate the potential of the mouse footpad edema model as a sensitive method for evaluating both ET toxicity and the efficacy of candidate therapeutic agents.

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.

Raxibacumab augments hemodynamic support and improves outcomes during shock with B. anthracis edema toxin alone or together with lethal toxin in canines

BACKGROUND

Lethal and edema toxin contribute to shock and lethality with Bacillus anthracis. We showed previously in a 96-h sedated canine model that raxibacumab, a monoclonal antibody against protective antigen, augmented hemodynamic support (HS) and improved survival with lethal toxin challenge. Here we study raxibacumab further. Using this model, we have now studied raxibacumab with 24 h edema toxin challenges (Study 1), and lethal and edema toxin challenges together (Study 2).

METHODS

Using our canine model, we have now studied raxibacumab with 24h edema toxin challenges (Study-1), and lethal and edema toxin challenges together (Study-2).

RESULTS

In Study 1, compared to no treatment, HS (titrated fluid and norepinephrine) increased mean arterial blood pressure (MAP, p ≤ 0.05) but not survival [0 of 10 (0/10) animals survived in each group] or median survival time [43.8 h (range 16.8 to 80.3) vs. 45.2 h (21.0 to 57.1)]. Compared to HS, HS with raxibacumab treatment at or 6 h after the beginning of edema toxin increased MAP and survival rate (6/7 and 7/8, respectively) and time [96.0 h (39.5 to 96.0) and 96.0 h (89.5 to 96.0), respectively]; (p ≤ 0.05). HS with raxibacumab at 12 h increased MAP (p ≤ 0.05) but not survival [1/5; 55.3 h (12.6 to 96.0)]. In Study-2, survival rate and time increased with HS and raxibacumab at 0 h (4/4) or 6 h after (3/3) beginning lethal and edema toxin compared to HS [0/5; 71.5 h (65 to 93)] (p = 0.01 averaged over raxibacumab groups).

CONCLUSIONS

Raxibacumab augments HS and improves survival during shock with lethal and edema toxin.

The role of autophagy during group B Streptococcus infection of blood-brain barrier endothelium

Bacterial meningitis occurs when bloodborne pathogens invade and penetrate the blood-brain barrier (BBB), provoking inflammation and disease. Group B Streptococcus (GBS), the leading cause of neonatal meningitis, can enter human brain microvascular endothelial cells (hBMECs), but the host response to intracellular GBS has not been characterized. Here we sought to determine whether antibacterial autophagy, which involves selective recognition of intracellular organisms and their targeting to autophagosomes for degradation, is activated in BBB endothelium during bacterial infection. GBS infection resulted in increased punctate distribution of GFP-microtubule-associated protein 1 light chain 3 (LC3) and increased levels of endogenous LC3-II and p62 turnover, two hallmark indicators of active autophagic flux. Infection with GBS mutants revealed that bacterial invasion and the GBS pore-forming β-hemolysin/cytolysin (β-h/c) trigger autophagic activation. Cell-free bacterial extracts containing β-h/c activity induced LC3-II conversion, identifying this toxin as a principal provocative factor for autophagy activation. These results were confirmed in vivo using a mouse model of GBS meningitis as infection with WT GBS induced autophagy in brain tissue more frequently than a β-h/c-deficient mutant. Elimination of autophagy using Atg5-deficient fibroblasts or siRNA-mediated impairment of autophagy in hBMECs led to increased recovery of intracellular GBS. However, electron microscopy revealed that GBS was rarely found within double membrane autophagic structures even though we observed GBS-LC3 co-localization. These results suggest that although autophagy may act as a BBB cellular defense mechanism in response to invading and toxin-producing bacteria, GBS may actively thwart the autophagic pathway.

Lethal factor, but not edema factor, is required to cause fatal anthrax in cynomolgus macaques after pulmonary spore challenge

Inhalational anthrax is caused by inhalation of Bacillus anthracis spores. The ability of B. anthracis to cause anthrax is attributed to the plasmid-encoded A/B-type toxins, edema toxin (edema factor and protective antigen) and lethal toxin (lethal factor and protective antigen), and a poly-d-glutamic acid capsule. To better understand the contribution of these toxins to the disease pathophysiology in vivo, we used B. anthracis Ames strain and isogenic toxin deletion mutants derived from the Ames strain to examine the role of lethal toxin and edema toxin after pulmonary spore challenge of cynomolgus macaques. Lethal toxin, but not edema toxin, was required to induce sustained bacteremia and death after pulmonary challenge with spores delivered via bronchoscopy. After intravenous challenge with bacilli to model the systemic phase of infection, lethal toxin contributed to bacterial proliferation and subsequent host death to a greater extent than edema toxin. Deletion of protective antigen resulted in greater loss of virulence after intravenous challenge with bacilli than deletion of lethal toxin or edema toxin alone. These findings are consistent with the ability of anti-protective antigen antibodies to prevent anthrax and suggest that lethal factor is the dominant toxin that contributes to the escape of significant numbers of bacilli from the thoracic cavity to cause anthrax after inhalation challenge with spores.

Colonic immune suppression, barrier dysfunction, and dysbiosis by gastrointestinal bacillus anthracis Infection

Gastrointestinal (GI) anthrax results from the ingestion of Bacillus anthracis. Herein, we investigated the pathogenesis of GI anthrax in animals orally infected with toxigenic non-encapsulated B. anthracis Sterne strain (pXO1⁺ pXO2⁻) spores that resulted in rapid animal death. B. anthracis Sterne induced significant breakdown of intestinal barrier function and led to gut dysbiosis, resulting in systemic dissemination of not only B. anthracis, but also of commensals. Disease progression significantly correlated with the deterioration of innate and T cell functions. Our studies provide critical immunologic and physiologic insights into the pathogenesis of GI anthrax infection, whereupon cleavage of mitogen-activated protein kinases (MAPKs) in immune cells may play a central role in promoting dysfunctional immune responses against this deadly pathogen.

Impaired colonic B-cell responses by gastrointestinal Bacillus anthracis infection

Ingestion of Bacillus anthracis spores causes gastrointestinal (GI) anthrax. Humoral immune responses, particularly immunoglobulin A (IgA)-secreting B-1 cells, play a critical role in the clearance of GI pathogens. Here, we investigated whether B. anthracis impacts the function of colonic B-1 cells to establish active infection. GI anthrax led to significant inhibition of immunoglobulins (eg, IgA) and increased expression of program death 1 on B-1 cells. Furthermore, infection also diminished type 2 innate lymphoid cells (ILC2) and their ability to enhance differentiation and immunoglobulin production by secreting interleukin 5 (IL-5). Such B-1-cell and ILC2 dysfunction is potentially due to cleavage of p38 and Erk1/2 mitogen-activated protein kinases in these cells. Conversely, mice that survived infection generated neutralizing antibodies via the formation of robust germinal center B cells in Peyer's patches and had restored B-1-cell and ILC2 function. These data may provide additional insight for designing efficacious vaccines and therapeutics against this deadly pathogen.

Cholera toxin disrupts barrier function by inhibiting exocyst-mediated trafficking of host proteins to intestinal cell junctions

Cholera toxin (CT), a virulence factor elaborated by Vibrio cholerae, is sufficient to induce the severe diarrhea characteristic of cholera. The enzymatic moiety of CT (CtxA) increases cAMP synthesis in intestinal epithelial cells, leading to chloride ion (Cl(-)) efflux through the CFTR Cl(-) channel. To preserve electroneutrality and osmotic balance, sodium ions and water also flow into the intestinal lumen via a paracellular route. We find that CtxA-driven cAMP increase also inhibits Rab11/exocyst-mediated trafficking of host proteins including E-cadherin and Notch signaling components to cell-cell junctions in Drosophila, human intestinal epithelial cells, and ligated mouse ileal loops, thereby disrupting barrier function. Additionally, CtxA induces junctional damage, weight loss, and dye leakage in the Drosophila gut, contributing to lethality from live V. cholerae infection, all of which can be rescued by Rab11 overexpression. These barrier-disrupting effects of CtxA may act in parallel with Cl(-) secretion to drive the pathophysiology of cholera.

Inflammasomes in the CNS

Microglia and macrophages in the CNS contain multimolecular complexes termed inflammasomes. Inflammasomes function as intracellular sensors for infectious agents as well as for host-derived danger signals that are associated with neurological diseases, including meningitis, stroke and Alzheimer's disease. Assembly of an inflammasome activates caspase 1 and, subsequently, the proteolysis and release of the cytokines interleukin-1β and interleukin-18, as well as pyroptotic cell death. Since the discovery of inflammasomes in 2002, there has been burgeoning recognition of their complexities and functions. Here, we review the current understanding of the functions of different inflammasomes in the CNS and their roles in neurological diseases.

B. anthracis associated cardiovascular dysfunction and shock: the potential contribution of both non-toxin and toxin components

The development of cardiovascular dysfunction and shock in patients with invasive Bacillus anthracis infection has a particularly poor prognosis. Growing evidence indicates that several bacterial components likely play important pathogenic roles in this injury. As with other pathogenic Gram-positive bacteria, the B. anthracis cell wall and its peptidoglycan constituent produce a robust inflammatory response with its attendant tissue injury, disseminated intravascular coagulation and shock. However, B. anthracis also produces lethal and edema toxins that both contribute to shock. Growing evidence suggests that lethal toxin, a metalloprotease, can interfere with endothelial barrier function as well as produce myocardial dysfunction. Edema toxin has potent adenyl cyclase activity and may alter endothelial function, as well as produce direct arterial and venous relaxation. Furthermore, both toxins can weaken host defense and promote infection. Finally, B. anthracis produces non-toxin metalloproteases which new studies show can contribute to tissue injury, coagulopathy and shock. In the future, an understanding of the individual pathogenic effects of these different components and their interactions will be important for improving the management of B. anthracis infection and shock.

Anthrax lethal toxin downregulates claudin-5 expression in human endothelial tight junctions

Vascular leakage pathologies such as pleural effusion and hemorrhage are hallmarks of anthrax pathogenesis. We previously reported that anthrax lethal toxin (LT), the major virulence factor of anthrax, reduces barrier function in cultured primary human microvascular endothelial cells. Here, we show that LT-induced barrier dysfunction is accompanied by the reduced expression of the endothelial tight junction (TJ) protein claudin-5 but no change in the expression of other TJ components occludin, ZO-1, ZO-2, or the adherens junction (AJ) protein VE-cadherin. The downregulation of claudin-5 correlated temporally and dose-dependently with the reduction of transendothelial electrical resistance. LT-induced loss of claudin-5 was independent of cell death and preceded the appearance of actin stress fibers and altered AJ morphology. Pharmacological inhibition of MEK-1/2, two kinases that are proteolytically inactivated by LT, showed a similar reduction in claudin-5 expression. We found that LT reduced claudin-5 mRNA levels but did not accelerate the rate of claudin-5 degradation. Mice challenged with LT also showed significant reduction in claudin-5 expression. Together, these findings support a possible role for LT disruption of endothelial TJs in the vascular leakage pathologies of anthrax.

B. anthracis edema toxin increases cAMP levels and inhibits phenylephrine-stimulated contraction in a rat aortic ring model

B. anthracis edema toxin (ET) and lethal toxin (LT) are each composed of protective antigen (PA), necessary for toxin uptake by host cells, and their respective toxic moieties, edema factor (EF) and lethal factor (LF). Although both toxins likely contribute to shock during infection, their mechanisms are unclear. To test whether ET and LT produce arterial relaxation, their effects on phenylephrine (PE)-stimulated contraction in a Sprague-Dawley rat aortic ring model were measured. Rings were prepared and connected to pressure transducers. Their viability was confirmed, and peak contraction with 60 mM KCl was determined. Compared with PA pretreatment (control, 60 min), ET pretreatment at concentrations similar to those noted in vivo decreased the mean (±SE) maximum contractile force (MCF; percent peak contraction) in rings generated during stimulation with increasing PE concentrations (96.2 ± 7.0 vs. 57.3 ± 9.1) and increased the estimated PE concentration producing half the MCF (EC50; 10(-7) M, 1.1 ± 0.3 vs. 3.7 ± 0.8, P ≤ 0.002). ET inhibition with PA-directed monoclonal antibodies, selective EF inhibition with adefovir, or removal of the ring endothelium inhibited the effects of ET on MCF and EC50 (P ≤ 0.02). Consistent with its adenyl cyclase activity, ET increased tissue cAMP in endothelium-intact but not endothelium-denuded rings (P < 0.0001 and 0.25, respectively). LT pretreatment, even in high concentrations, did not significantly decrease MCF or increase EC50 (all P > 0.05). In rings precontracted with PE compared with posttreatment with PA (90 min), ET posttreatment produced progressive reductions in contractile force and increases in relaxation in endothelium-intact rings (P < 0.0001) but not endothelium-denuded rings (P = 0.51). Thus, ET may contribute to shock by producing arterial relaxation.

Vector-mediated selective expression of lethal factor, a toxic element of Bacillus anthracis, damages A549 cells via inhibition of MAPK and AKT pathways

Lethal factor (LF), a major toxic element of Bacillus anthracis combined with its protective antigen (PA), enters the cells through the cytomembrane receptors and causes damage to the host cells, thereby leading to septicemia, toxemia, and meningitis with high mortality. LF has been identified as a potential biotech-weapon, which can impede cancer growth in vascular endothelial cells because of its cytotoxicity. However, the feasibility of LF application and further investigations has been limited because LF is nonspecific. To solve this problem, we constructed a vector that contained the LF sequence, which was regulated by a tumor-specific human telomerase reverse transcriptase promoter (hTERTp). Results showed that LF was selectively expressed in lung cancer A549 cells but not in normal cells, thereby resulting in A549 cell apoptosis. The results also revealed that the inhibition of mitogen-activated protein kinase and AKT pathways was partially involved in the process. Thus, hTERTp-regulated LF increase could be a promising approach in lung cancer-targeted therapy.

Bacillus anthracis factors for phagosomal escape

The mechanism of phagosome escape by intracellular pathogens is an important step in the infectious cycle. During the establishment of anthrax, Bacillus anthracis undergoes a transient intracellular phase in which spores are engulfed by local phagocytes. Spores germinate inside phagosomes and grow to vegetative bacilli, which emerge from their resident intracellular compartments, replicate and eventually exit from the plasma membrane. During germination, B. anthracis secretes multiple factors that can help its resistance to the phagocytes. Here the possible role of B. anthracis toxins, phospholipases, antioxidant enzymes and capsules in the phagosomal escape and survival, is analyzed and compared with that of factors of other microbial pathogens involved in the same type of process.

Bacillus anthracis edema factor substrate specificity: evidence for new modes of action

Since the isolation of Bacillus anthracis exotoxins in the 1960s, the detrimental activity of edema factor (EF) was considered as adenylyl cyclase activity only. Yet the catalytic site of EF was recently shown to accomplish cyclization of cytidine 5'-triphosphate, uridine 5'-triphosphate and inosine 5'-triphosphate, in addition to adenosine 5'-triphosphate. This review discusses the broad EF substrate specificity and possible implications of intracellular accumulation of cyclic cytidine 3':5'-monophosphate, cyclic uridine 3':5'-monophosphate and cyclic inosine 3':5'-monophosphate on cellular functions vital for host defense. In particular, cAMP-independent mechanisms of action of EF on host cell signaling via protein kinase A, protein kinase G, phosphodiesterases and CNG channels are discussed.

Defense at the border: the blood-brain barrier versus bacterial foreigners

Bacterial meningitis is among the top ten causes of infectious disease-related deaths worldwide, with up to half of the survivors left with permanent neurological sequelae. The blood-brain barrier (BBB), composed mainly of specialized brain microvascular endothelial cells, maintains biochemical homeostasis in the CNS by regulating the passage of nutrients, molecules and cells from the blood to the brain. Despite its highly restrictive nature, certain bacterial pathogens are able to gain entry into the CNS resulting in serious disease. In recent years, important advances have been made in understanding the molecular and cellular events that are involved in the development of bacterial meningitis. In this review, we summarize the progress made in elucidating the molecular mechanisms of bacterial BBB-crossing, highlighting common themes of host-pathogen interaction, and the potential role of the BBB in innate defense during infection.

Differential contribution of Bacillus anthracis toxins to pathogenicity in two animal models

The virulence of Bacillus anthracis, the causative agent of anthrax, stems from its antiphagocytic capsule, encoded by pXO2, and the tripartite toxins encoded by pXO1. The accepted paradigm states that anthrax is both an invasive and toxinogenic disease and that the toxins play major roles in pathogenicity. We tested this assumption by a systematic study of mutants with combined deletions of the pag, lef, and cya genes, encoding protective antigen (PA), lethal factor (LF), and edema factor (EF), respectively. The resulting seven mutants (single, double, and triple) were evaluated following subcutaneous (s.c.) and intranasal (i.n.) inoculation in rabbits and guinea pigs. In the rabbit model, virulence is completely dependent on the presence of PA. Any mutant bearing a pag deletion behaved like a pXO1-cured mutant, exhibiting complete loss of virulence with attenuation indices of over 2,500,000 or 1,250 in the s.c. or i.n. route of infection, respectively. In marked contrast, in guinea pigs, deletion of pag or even of all three toxin components resulted in relatively moderate attenuation, whereas the pXO1-cured bacteria showed complete attenuation. The results indicate that a pXO1-encoded factor(s), other than the toxins, has a major contribution to the virulence mechanism of B. anthracis in the guinea pig model. These unexpected toxin-dependent and toxin-independent manifestations of pathogenicity in different animal models emphasize the importance and need for a comprehensive evaluation of B. anthracis virulence in general and in particular for the design of relevant next-generation anthrax vaccines.

Analysis of defined combinations of monoclonal antibodies in anthrax toxin neutralization assays and their synergistic action

Antibodies against the protective antigen (PA) component of anthrax toxin play an important role in protection against disease caused by Bacillus anthracis. In this study, we examined defined combinations of PA-specific monoclonal antibodies for their ability to neutralize anthrax toxin in cell culture assays. We observed additive, synergistic, and antagonistic effects of the antibodies depending on the specific antibody combination examined and the specific assay used. Synergistic toxin-neutralizing antibody interactions were examined in more detail. We found that one mechanism that can lead to antibody synergy is the bridging of PA monomers by one antibody, with resultant bivalent binding of the second antibody. These results may aid in optimal design of new vaccines and antibody therapies against anthrax.

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.

The effect of deletion of the edema factor on Bacillus anthracis pathogenicity in guinea pigs and rabbits

Bacillus anthracis secretes three major components, which assemble into two bipartite toxins: lethal toxin (LT), composed of lethal factor (LF) and protective antigen (PA) and edema toxin (ET), composed of edema factor (EF) and PA. EF is a potent calmodulin-dependent adenylate cyclase, which is internalized into the target cell following PA binding. Once inside the cell, EF elevates cAMP levels, interrupting intracellular signaling. Effects of ET were demonstrated on monocytes, neutrophils and T-cells. In an earlier work we demonstrated that a deletion of LF in a fully virulent strain had no effect in guinea pigs and a significant, but not major, effect in the rabbit model. These results suggested that EF might play an important role in the development of infection and mortality following exposure to B. anthracis spores. To evaluate the role of EF in B. anthracis pathogenicity we deleted the cya gene, which encodes the EF protein, in the fully virulent Vollum strain. The Δcya mutant was fully virulent in the guinea pig model as determined by LD(50) experiments. In the rabbit model, when infected subcutaneously, the absence of EF had no effect on the virulence of the mutant. However an increase of two orders of magnitude in the LD(50) was demonstrated when the rabbits were infected by intranasal instillation accompanied with partial mortality and increased mean time to death. These results argue that in the guinea pig model the presence of one of the toxins, ET or LT is sufficient for the development of the infection. In the rabbit model ET plays a role in respiratory infection, most probably mediating the early steps of host colonization.

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.