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

Bacillus anthracis edema toxin inhibits hypoxic pulmonary vasoconstriction via edema factor and cAMP-mediated mechanisms in isolated perfused rat lungs

Bacillus anthracis edema toxin (ET) inhibited lethal toxin-stimulated pulmonary artery pressure (Ppa) and increased lung cAMP levels in our previous study. We therefore examined whether ET inhibits hypoxic pulmonary vasoconstriction (HPV). Following baseline hypoxic measures in isolated perfused lungs from healthy rats, compared with diluent, ET perfusion reduced maximal Ppa increases (mean ± SE percentage of maximal Ppa increase with baseline hypoxia) during 6-min hypoxic periods (FIO2 = 0%) at 120 min (16 ± 6% vs. 51 ± 6%, P = 0.004) and 180 min (11.4% vs. 55 ± 6%, P = 0.01). Protective antigen-mAb (PA-mAb) and adefovir inhibit host cell edema factor uptake and cAMP production, respectively. In lungs perfused with ET following baseline measures, compared with placebo, PA-mAb treatment increased Ppa during hypoxia at 120 and 180 min (56 ± 6% vs. 10 ± 4% and 72 ± 12% vs. 12 ± 3%, respectively, P ≤ 0.01) as did adefovir (84 ± 10% vs. 16.8% and 123 ± 21% vs. 26 ± 11%, respectively, P ≤ 0.01). Compared with diluent, lung perfusion with ET for 180 min reduced the slope of the relationships between Ppa and increasing concentrations of endothelin-1 (ET-1) (21.12 ± 2.96 vs. 3.00 ± 0.76 × 108 cmH2O/M, P < 0.0001) and U46619, a thromboxane A2 analogue (7.15 ± 1.01 vs. 3.74 ± 0.31 × 107 cmH2O/M, P = 0.05) added to perfusate. In lungs isolated from rats after 15 h of in vivo infusions with either diluent, ET alone, or ET with PA-mAb, compared with diluent, the maximal Ppa during hypoxia and the slope of the relationship between change in Ppa and ET-1 concentration added to the perfusate were reduced in lungs from animals challenged with ET alone (P ≤ 0.004) but not with ET and PA-mAb together (P ≥ 0.73). Inhibition of HPV by ET could aggravate hypoxia during anthrax pulmonary infection.NEW & NOTEWORTHY The most important findings here are edema toxin's potent adenyl cyclase activity can interfere with hypoxic pulmonary vasoconstriction, an action that could worsen hypoxemia during invasive anthrax infection with lung involvement. These findings, coupled with other studies showing that lethal toxin can disrupt pulmonary vascular integrity, indicate that both toxins can contribute to pulmonary pathophysiology during infection. In combination, these investigations provide a further basis for the use of antitoxin therapies in patients with worsening invasive anthrax disease.

Hydrocortisone decreases lethality and inflammatory cytokine and nitric oxide production in rats challenged with B. anthracis cell wall peptidoglycan

Background

Lethal B. anthracis infection produces high proinflammatory peptidoglycan (PGN) burdens in hosts. We investigated whether the lethality and inflammation anthrax PGN can produce are related.

Methods

At 6 h before and the start of 24 h anthrax PGN infusions, rats (n = 198) were treated with diluent (controls) or one of three IV-doses of either hydrocortisone (125, 12.5 or 1.25 mg/kg) or TNF-soluble receptor (TNFsr; 2000, 1000 or 333 μg/kg), non-selective and selective anti-inflammatory agents, respectively.

Results

Compared to controls, hydrocortisone 125 and 12.5 mg/kg each decreased 7-day lethality (p ≤ 0.004). Hydrocortisone 125 mg/kg decreased IL-1β, IL-6, TNFα, MCP, MIP-1α, MIP-2, RANTES and nitric oxide (NO) blood levels at 4 and 24 h after starting PGN (except MCP at 24 h). Each decrease was significant at 4 h (except MIP-1α that was significant at 24 h) (p ≤ 0.05). Similarly, hydrocortisone 12.5 mg/kg decreased each measure at 4, 24 and 48 h (except TNFα at 24 h and MIP-1α at 24 and 48 h and NO at 48 h). Decreases were significant for IL-6 and NO at 4 h and RANTES at 48 h (p ≤ 0.05). Hydrocortisone 1.25 mg/kg had non-significant effects. Each TNFsr dose decreased lethality but non-significantly. However, when doses were analyzed together, TNFsr decreased lethality in a potential trend (p = 0.16) and IL-6 and NO significantly at 4 h (p = 0.05).

Conclusions

Peptidoglycan-stimulated host inflammation may contribute to B. anthracis lethality.

Bacillus anthracis lethal toxin, but not edema toxin, increases pulmonary artery pressure and permeability in isolated perfused rat lungs

Although lethal toxin (LT) and edema toxin (ET) contribute to lethality during Bacillus anthracis infection, whether they increase vascular permeability and the extravascular fluid accumulation characterizing this infection is unclear. We employed an isolated perfused Sprague-Dawley rat lung model to investigate LT and ET effects on pulmonary vascular permeability. Lungs (n ≥ 6 per experimental group) were isolated, ventilated, suspended from a force transducer, and perfused. Lung weight and pulmonary artery (P_pa) and left atrial pressures were measured over 4 h, after which pulmonary capillary filtration coefficients (Kf.c) and lung wet-to-dry weight ratios (W/D) were determined. When compared with controls, LT increased P_pa over 4 h and Kf.c and W/D at 4 h (P < 0.0001). ET decreased P_pa in a significant trend (P = 0.09) but did not significantly alter Kf.c or W/D (P ≥ 0.29). Edema toxin actually blocked LT increases in P_pa but not LT increases in Kf.c and W/D. When P_pa was maintained at control levels, LT still increased Kf.c and W/D (P ≤ 0.004). Increasing the dose of each toxin five times significantly increased and a toxin-directed monoclonal antibody decreased the effects of each toxin (P ≤ 0.05). Two rho-kinase inhibitors (GSK269962 and Y27632) decreased LT increases in P_pa (P ≤ 0.02) but actually increased Kf.c and W/D in LT and control lungs (P ≤ 0.05). A vascular endothelial growth factor receptor inhibitor (ZM323881) had no significant effect (P ≥ 0.63) with LT. Thus, LT but not ET can increase pulmonary vascular permeability independent of increased P_pa and could contribute to pulmonary fluid accumulation during anthrax infection. However, pulmonary vascular dilation with ET could disrupt protective hypoxic vasoconstriction. NEW & NOTEWORTHY The most important findings from the present study are that Bacillus anthracis lethal toxin increases pulmonary artery pressure and pulmonary permeability independently in the isolated rat lung, whereas edema toxin decreases the former and does not increase permeability. Each effect could be a basis for organ dysfunction in patients with this lethal infection. These findings further support the need for adjunctive therapies that limit the effects of both toxins during infection.

The Potential Pathogenic Contributions of Endothelial Barrier and Arterial Contractile Dysfunction to Shock Due to B. anthracis Lethal and Edema Toxins

Shock with B. anthracis infection is particularly resistant to conventional cardiovascular support and its mortality rate appears higher than with more common bacterial pathogens. As opposed to many bacteria that lack exotoxins directly depressing hemodynamic function, lethal and edema toxin (LT and ET respectively) both cause shock and likely contribute to the high lethality rate with B. anthracis. Selective inhibition of the toxins is protective in infection models, and administration of either toxin alone in animals produces hypotension with accompanying organ injury and lethality. Shock during infection is typically due to one of two mechanisms: (i) intravascular volume depletion related to disruption of endothelial barrier function; and (ii) extravasation of fluid and/or maladaptive dilation of peripheral resistance arteries. Although some data suggests that LT can produce myocardial dysfunction, growing evidence demonstrates that it may also interfere with endothelial integrity thereby contributing to the extravasation of fluid that helps characterize severe B. anthracis infection. Edema toxin, on the other hand, while known to produce localized tissue edema when injected subcutaneously, has potent vascular relaxant effects that could lead to pathologic arterial dilation. This review will examine recent data supporting a role for these two pathophysiologic mechanisms underlying the shock LT and ET produce. Further research and a better understanding of these mechanisms may lead to improved management of B. anthracis in patients.

Anthrax immune globulin improves hemodynamics and survival during B. anthracis toxin-induced shock in canines receiving titrated fluid and vasopressor support

Background

Although anthrax immune globulin (AIG) improved survival in antibiotic-treated Bacillus anthracis-challenged animal models, whether it adds to the benefit of conventional hemodynamic support for B. anthracis toxin-associated shock is unknown.

Methods

We therefore tested AIG in sedated, mechanically ventilated canines challenged with 24-h B. anthracis lethal and edema toxin infusions and supported for 96 h with a previously demonstrated protective regimen of titrated normal saline and norepinephrine.

Results

Compared to controls, proportional survival (%) was increased with AIG treatment started 4 h before (33 vs. 100%, n = 6 each) or 2 h (17 vs. 86%, n = 6 and 7 respectively) or 5 h (0 vs. 67%, n = 3 each) after the start of toxin (p ≤ 0.05) and overall [3 survivors of 15 controls (20%) vs. 14 of 16 AIG animals (88%); p = 0.006]. Averaged across treatment times, AIG increased blood pressure at 48 h and decreased norepinephrine requirements at 72 h (p ≤ 0.02), increased left ventricular ejection fraction at 48 and 72 h (p ≤ 0.02), and increased urine output and decreased net fluid balance at 72 and 96 h (p ≤ 0.04). AIG also reduced acidosis and renal and hepatic injury markers between 24 and 96 h.

Conclusions

These findings further support AIG’s potential benefit for patients with B. anthracis infection and developing toxin-associated shock.

Electronic supplementary material

The online version of this article (10.1186/s40635-017-0159-9) contains supplementary material, which is available to authorized users.

Shock and lethality with anthrax edema toxin in rats are associated with reduced arterial responsiveness to phenylephrine and are reversed with adefovir

Although edema toxin (ETx) and lethal toxin (LTx) contribute to Bacillus anthracis shock and lethality, the mechanisms underlying their cardiovascular effects are unclear. We have previously shown that ETx but not LTx inhibited phenylephrine-stimulated contraction of aortic rings prepared from healthy rats and that adefovir, a selective inhibitor of ETx cAMP production, blocked this effect. Here, we examined arterial function in rats that received 24-h ETx or LTx infusions. Compared with control rats, ETx reduced mean arterial pressure (MAP) and survival over 48 h (P ≤ 0.0003) and increased plasma cAMP at 4, 24, and 48 h (P < 0.0001) and nitric oxide (NO) at 24 and 48 h (P ≤ 0.01). Compared with control animals, at 24- and 48-h phenylephrine stimulation of aortic rings from ETx animals produced decreased maximal contractile force (MCF; P = 0.05 and 0.006) and in vivo phenylephrine infusion in ETx animals produced decreased proportional increases in MAP (P < 0.0001 and P = 0.05). In ETx-treated animals, compared with placebo-treated animals, adefovir treatment prevented all lethality (P = 0.01), increased MAP (P ≤ 0.0001), decreased plasma and aortic tissue cAMP at 24 and 48 h, respectively (P ≤ 0.03), and plasma NO at both times (P ≤ 0.004), and increased phenylephrine-stimulated increases in MCF in aortic rings and MAP in vivo at 48 h (P = 0.02). LTx decreased MAP and survival also, but it did not alter the response to phenylephrine of MCF in aortic rings prepared from LTx animals or of MAP in vivo. In conclusion, in rats, hypotension and lethality are associated with reduced arterial contractile function with ETx but not LTx and adefovir improves ETx-induced hypotension and lethality.NEW & NOTEWORTHY The most important aspects of the present study are the findings that 1) in vivo challenge with anthrax edema but not lethal toxin depresses arterial contractile function measured both ex vivo and in vivo and 2) adefovir inhibits the effects of edema toxin on arterial hypotension and improves survival with lethal dose of edema toxin challenge.

Bacillus anthracis Edema Toxin Increases Fractional Free Water and Sodium Reabsorption in an Isolated Perfused Rat Kidney Model

Bacillus anthracis edema toxin (ET) consists of protective antigen (PA), necessary for host cell toxin uptake, and edema factor (EF), the toxic moiety which increases host cell cyclic AMP (cAMP). Since vasopressin stimulates renal water and sodium reabsorption via increased tubular cell cAMP levels, we hypothesized the ET would also do so. To test this hypothesis, we employed an isolated perfused rat kidney model. Kidneys were isolated and perfused with modified Krebs-Henseleit buffer. Perfusate and urine samples were obtained at baseline and every 10 min over 150 min following the addition of challenges with or without treatments to the perfusate. In kidneys perfused under constant flow or constant pressure, compared to PA challenge (n = 14 or 15 kidneys, respectively), ET (13 or 15 kidneys, respectively) progressively increased urine cAMP levels, water and sodium reabsorption, and urine osmolality and decreased urine output (P ≤ 0.04, except for sodium reabsorption under constant pressure [P = 0.17]). In ET-challenged kidneys, compared to placebo treatment, adefovir, an EF inhibitor, decreased urine cAMP levels, water and sodium reabsorption, and urine osmolality and increased urine output, while raxibacumab, a PA-directed monoclonal antibody (MAb), decreased urine cAMP levels, free water reabsorption, and urine osmolality and increased urine output (P ≤ 0.03 except for urine output with raxibacumab [P = 0.17]). Upon immunohistochemistry, aquaporin 2 was concentrated along the apical membrane of tubular cells with ET but not PA, and urine aquaporin 2 levels were higher with ET (5.52 ± 1.06 ng/ml versus 1.51 ± 0.44 ng/ml [means ± standard errors of the means {SEM}; P = 0.0001). Edema toxin has renal effects that could contribute to extravascular fluid collection characterizing anthrax infection clinically.

Nitric oxide production contributes to Bacillus anthracis edema toxin-associated arterial hypotension and lethality: ex vivo and in vivo studies in the rat

We showed previously that Bacillus anthracis edema toxin (ET), comprised of protective antigen (PA) and edema factor (EF), inhibits phenylephrine (PE)-induced contraction in rat aortic rings and these effects are diminished in endothelial-denuded rings. Therefore, employing rat aortic ring and in vivo models, we tested the hypothesis that nitric oxide (NO) contributes to ET's arterial effects. Compared with rings challenged with PA alone, ET (PA + EF) reduced PE-stimulated maximal contractile force (MCF) and increased the PE concentration producing 50% MCF (EC50) (P < 0.0001). Compared with placebo, l-nitro-arginine methyl-ester (l-NAME), an NO synthase (NOS) inhibitor, reduced ET's effects on MCF and EC50 in patterns that approached or were significant (P = 0.06 and 0.03, respectively). In animals challenged with 24-h ET infusions, l-NAME (0.5 or 1.0 mg·kg(-1)·h(-1)) coadministration increased survival to 17 of 28 animals (60.7%) compared with 4 of 27 (14.8%) given placebo (P = 0.01). Animals receiving l-NAME but no ET all survived. Compared with PBS challenge, ET increased NO levels at 24 h and l-NAME decreased these increases (P < 0.0001). ET infusion decreased mean arterial blood pressure (MAP) in placebo and l-NAME-treated animals (P < 0.0001) but l-NAME reduced decreases in MAP with ET from 9 to 24 h (P = 0.03 for the time interaction). S-methyl-l-thiocitrulline, a selective neuronal NOS inhibitor, had effects in rings and, at a high dose in vivo models, comparable to l-NAME, whereas N'-[3-(aminomethyl)benzyl]-acetimidamide, a selective inducible NOS inhibitor, did not. NO production contributes to ET's arterial relaxant, hypotensive, and lethal effects in the rat.

Bacillus anthracis edema but not lethal toxin challenge in rats is associated with depressed myocardial function in hearts isolated and tested in a Langendorff system

Although direct myocardial depression has been implicated in the lethal effects of Bacillus anthracis lethal toxin (LT), in hearts isolated from healthy rats and perfused under constant pressure, neither LT or edema toxin (ET) in typically lethal concentrations depressed myocardial function. In the present study, we challenged rats with LT and ET and performed in vivo and ex vivo heart measures. Sprague-Dawley rats infused over 24 h with LT (n = 94), ET (n = 99), or diluent (controls; n = 50) were studied at 8, 24, or 48 h. Compared with control rats (all survived), survival rates with LT (56.1%) and ET (37.3%) were reduced (P < 0.0001) similarly (P = 0.66 for LT vs. ET). LT decreased mean arterial blood pressure from 12 to 20 h (P ≤ 0.05), whereas ET decreased it progressively throughout (P < 0.05). On echocardiography, LT decreased left ventricular (LV) ejection fraction at 8 and 48 h but increased it at 24 h and decreased cardiac output (P ≤ 0.05 for the time interaction or averaged over time). ET decreased systolic and diastolic volumes and increased LV ejection fraction at 24 h (P ≤ 0.05). In isolated hearts perfused for 120 min under constant pressure, LT did not significantly alter LV systolic or developed pressures at any time point, whereas ET decreased both of these at 24 h (P < 0.0001 initially). ET but not LT progressively increased plasma creatine phosphokinase and cardiac troponin levels (P < 0.05). In conclusion, despite echocardiographic changes, in vivo lethal LT challenge did not produce evidence of myocardial depression in isolated rat hearts. While lethal ET challenge did depress isolated heart function, this may have resulted from prior hypotension and ischemia.

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.

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.

Potential biological targets ofBacillus anthracisin anti-infective approaches against the threat of bioterrorism

The terrorist attacks of 2001 involving anthrax underscore the imperative that safe and effective medical countermeasures should be readily available. Vaccination appears to be the most effective form of mass protection against a biological attack, but the current vaccines have drawbacks that justify the enormous amount of effort currently being put into developing more effective vaccines and other treatment modalities. After providing a comprehensive overview of the organism Bacillus anthracis as a biological weapon and its pathogenicity, this review briefly summarizes the current knowledge vital to the management of anthrax disease. This knowledge has been acquired since 2001 as a result of the progress on anthrax research and focuses on the possible development of improved human anti-infective strategies targeting B. anthracis spore components, as well as strategies based on host-pathogen interactions.

Molecular determinants for a cardiovascular collapse in anthrax

Bacillus anthracis releases two bipartite proteins, lethal toxin and edema factor, that contribute significantly to the progression of anthrax-associated shock. As blocking the anthrax toxins prevents disease, the toxins are considered the main virulence factors of the bacterium. The anthrax bacterium and the anthrax toxins trigger multi-organ failure associated with enhanced vascular permeability, hemorrhage and cardiac dysfunction in animal challenge models. A recent study using mice that either lacked the anthrax toxin receptor in specific cells and corresponding mice expressing the receptor in specific cell types demonstrated that cardiovascular cells are critical for disease mediated by anthrax lethal toxin. These studies are consistent with involvement of the cardiovascular system, and with an increase of cardiac failure markers observed in human anthrax and in animal models using B. anthracis and anthrax toxins. This review discusses the current state of knowledge regarding the pathophysiology of anthrax and tries to provide a mechanistic model and molecular determinants for the circulatory shock in anthrax.

Anthrax lethal toxin induces acute diastolic dysfunction in rats through disruption of the phospholamban signaling network

BACKGROUND

Anthrax lethal toxin (LT), secreted by Bacillus anthracis, causes severe cardiac dysfunction by unknown mechanisms. LT specifically cleaves the docking domains of MAPKK (MEKs); thus, we hypothesized that LT directly impairs cardiac function through dysregulation of MAPK signaling mechanisms.

METHODS AND RESULTS

In a time-course study of LT toxicity, echocardiography revealed acute diastolic heart failure accompanied by pulmonary regurgitation and left atrial dilation in adult Sprague-Dawley rats at time points corresponding to dysregulated JNK, phospholamban (PLB) and protein phosphatase 2A (PP2A) myocardial signaling. Using isolated rat ventricular myocytes, we identified the MEK7-JNK1-PP2A-PLB signaling axis to be important for regulation of intracellular calcium (Ca(2+)(i)) handling, PP2A activation and targeting of PP2A-B56α to Ca(2+)(i) handling proteins, such as PLB. Through a combination of gain-of-function and loss-of-function studies, we demonstrated that over-expression of MEK7 protects against LT-induced PP2A activation and Ca(2+)(i) dysregulation through activation of JNK1. Moreover, targeted phosphorylation of PLB-Thr(17) by Akt improved sarcoplasmic reticulum Ca(2+)(i) release and reuptake during LT toxicity. Co-immunoprecipitation experiments further revealed the pivotal role of MEK7-JNK-Akt complex formation for phosphorylation of PLB-Thr(17) during acute LT toxicity.

CONCLUSIONS

Our findings support a cardiogenic mechanism of LT-induced diastolic dysfunction, by which LT disrupts JNK1 signaling and results in Ca(2+)(i) dysregulation through diminished phosphorylation of PLB by Akt and increased dephosphorylation of PLB by PP2A. Integration of the MEK7-JNK1 signaling module with Akt represents an important stress-activated signalosome that may confer protection to sustain cardiac contractility and maintain normal levels of Ca(2+)(i) through PLB-T(17) phosphorylation.

The sepsis model: an emerging hypothesis for the lethality of inhalation anthrax

Inhalation anthrax is often described as a toxin-mediated disease. However, the toxaemia model does not account for the high mortality of inhalation anthrax relative to other forms of the disease or for the pathology present in inhalation anthrax. Patients with inhalation anthrax consistently show extreme bacteraemia and, in contrast to animals challenged with toxin, signs of sepsis. Rather than toxaemia, we propose that death in inhalation anthrax results from an overwhelming bacteraemia that leads to severe sepsis. According to our model, the central role of anthrax toxin is to permit the vegetative bacteria to escape immune detection. Other forms of B. anthracis infection have lower mortality because their overt symptoms early in the course of disease cause patients to seek medical care at a time when the infection and its sequelae can still be reversed by antibiotics. Thus, the sepsis model explains key features of inhalation anthrax and may offer a more complete understanding of disease pathology for researchers as well as those involved in the care of patients.

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.

Neutralization of B. anthracis toxins during ex vivo phagocytosis

Glycoconjugates (GCs) are recognized as stimulation and signaling agents, affecting cell adhesion, activation, and growth of living organisms. Among GC targets, macrophages are considered ideal since they play a central role in inflammation and immune responses against foreign agents. In this context, we studied the effects of highly selective GCs in neutralizing toxin factors produced by B. anthracis during phagocytosis using murine macrophages. The effects of GCs were studied under three conditions: A) prior to, B) during, and C) following exposure of macrophages to B. anthracis individual toxin (protective antigen [PA], edema factor [EF], lethal factor [LF] or toxin complexes (PA-EF-LF, PA-EF, and PA-LF). We employed ex vivo phagocytosis and post-phagocytosis analysis including direct microscopic observation of macrophage viability, and macrophage activation. Our results demonstrated that macrophages are more prone to adhere to GC-altered PA-EF-LF, PA-EF, and PA-LF toxin complexes. This adhesion results in a higher phagocytosis rate and toxin complex neutralization during phagocytosis. In addition, GCs enhance macrophage viability, activate macrophages, and stimulate nitric oxide (NO) production. The present study may be helpful in identifying GC ligands with toxin-neutralizing and/or immunomodulating properties. In addition, our study could suggest GCs as new targets for existing vaccines and the prospective development of vaccines and immunomodulators used to combat the effects of B. anthracis.

Novel approaches to the treatment of systemic anthrax

Anthrax continues to generate concern as an agent of bioterrorism and as a natural cause of sporadic disease outbreaks. Despite the use of appropriate antimicrobial agents and advanced supportive care, the mortality associated with the systemic disease remains high. This is primarily due to the pathogenic exotoxins produced by Bacillus anthracis as well as other virulence factors of the organism. For this reason, new therapeutic strategies that target events in the pathogenesis of anthrax and may potentially augment antimicrobials are being investigated. These include anti-toxin approaches, such as passive immune-based therapies; non-antimicrobial drugs with activity against anthrax toxin components; and agents that inhibit binding, processing, or assembly of toxins. Adjunct therapies that target spore germination or downstream events in anthrax intoxication are also under investigation. In combination, these modalities may enhance the management of systemic anthrax.

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.

An overview of anthrax infection including the recently identified form of disease in injection drug users

PURPOSE

Bacillus anthracis infection (anthrax) can be highly lethal. Two recent outbreaks related to contaminated mail in the USA and heroin in the UK and Europe and its potential as a bioterrorist weapon have greatly increased concerns over anthrax in the developed world.

METHODS

This review summarizes the microbiology, pathogenesis, diagnosis, and management of anthrax.

RESULTS AND CONCLUSIONS

Anthrax, a gram-positive bacterium, has typically been associated with three forms of infection: cutaneous, gastrointestinal, and inhalational. However, the anthrax outbreak among injection drug users has emphasized the importance of what is now considered a fourth disease form (i.e., injectional anthrax) that is characterized by severe soft tissue infection. While cutaneous anthrax is most common, its early stages are distinct and prompt appropriate treatment commonly produces a good outcome. However, early symptoms with the other three disease forms can be nonspecific and mistaken for less lethal conditions. As a result, patients with gastrointestinal, inhalational, or injectional anthrax may have advanced infection at presentation that can be highly lethal. Once anthrax is suspected, the diagnosis can usually be made with gram stain and culture from blood or tissue followed by confirmatory testing (e.g., PCR). While antibiotics are the mainstay of anthrax treatment, use of adjunctive therapies such as anthrax toxin antagonists are a consideration. Prompt surgical therapy appears to be important for successful management of injectional anthrax.

Protective antigen antibody augments hemodynamic support in anthrax lethal toxin shock in canines

BACKGROUND

Anthrax-associated shock is closely linked to lethal toxin (LT) release and is highly lethal despite conventional hemodynamic support. We investigated whether protective antigen-directed monoclonal antibody (PA-mAb) treatment further augments titrated hemodynamic support.

METHODS AND RESULTS

Forty sedated, mechanically ventilated, instrumented canines challenged with anthrax LT were assigned to no treatment (controls), hemodynamic support alone (protocol-titrated fluids and norepinephrine), PA-mAb alone (administered at start of LT infusion [0 hours] or 9 or 12 hours later), or both, and observed for 96 hours. Although all 8 controls died, 2 of 8 animals receiving hemodynamic support alone survived (median survival times 65 vs 85 hours, respectively; P = .03). PA-mAb alone at 0 hour improved survival (5 of 5 animals survived), but efficacy decreased progressively with delayed treatment (9 hours, 2 of 3 survived; 12 hours, 0 of 4 survived) (P = .004 comparing survival across treatment times). However, combined treatment increased survival irrespective of PA-mAb administration time (0 hours, 4 of 5 animals; 9 hours, 3 of 3 animals; and 12 hours, 4 of 5 animals survived) (P = .95 comparing treatment times). Compared to hemodynamic support alone, when combined over PA-mAb treatment times (0, 9, and 12 hours), combination therapy produced higher survival (P = .008), central venous pressures, and left ventricular ejection fractions, and lower heart rates, norepinephrine requirements and fluid retention (P ≤ .03).

CONCLUSIONS

PA-mAb may augment conventional hemodynamic support during anthrax LT-associated shock.

Bacillus anthracis lethal toxin alters regulation of visceral sympathetic nerve discharge

Bacillus anthracis infection is a pathophysiological condition that is complicated by progressive decreases in mean arterial pressure (MAP). Lethal toxin (LeTx) is central to the pathogenesis of B. anthracis infection, and the sympathetic nervous system plays a critical role in physiological regulation of acute stressors. However, the effect of LeTx on sympathetic nerve discharge (SND), a critical link between central sympathetic neural circuits and MAP regulation, remains unknown. We determined visceral (renal, splenic, and adrenal) SND responses to continuous infusion of LeTx [lethal factor (100 μg/kg) + protective antigen (200 μg/kg) infused at 0.5 ml/h for ≤6 h] and vehicle (infused at 0.5 ml/h) in anesthetized, baroreceptor-intact and baroreceptor (sinoaortic)-denervated (SAD) Sprague-Dawley rats. LeTx infusions produced an initial state of cardiovascular and sympathetic nervous system activation in intact and SAD rats. Subsequent to peak LeTx-induced increases in arterial blood pressure, intact rats demonstrated a marked hypotension that was accompanied by significant reductions in SND (renal and splenic) and heart rate (HR) from peak levels. After peak LeTx-induced pressor and sympathoexcitatory responses in SAD rats, MAP, SND (renal, splenic, and adrenal), and HR were progressively and significantly reduced, supporting the hypothesis that LeTx alters the central regulation of sympathetic nerve outflow. These findings demonstrate that the regulation of visceral SND is altered in a complex manner during continuous anthrax LeTx infusions and suggest that sympathetic nervous system dysregulation may contribute to the marked hypotension accompanying B. anthracis infection.

Anthrax edema toxin impairs clearance in mice

The anthrax edema toxin (ET) of Bacillus anthracis is composed of the receptor-binding component protective antigen (PA) and of the adenylyl cyclase catalytic moiety, edema factor (EF). Uptake of ET into cells raises intracellular concentrations of the secondary messenger cyclic AMP, thereby impairing or activating host cell functions. We report here on a new consequence of ET action in vivo. We show that in mouse models of toxemia and infection, serum PA concentrations were significantly higher in the presence of enzymatically active EF. These higher concentrations were not caused by ET-induced inhibition of PA endocytosis; on the contrary, ET induced increased PA binding and uptake of the PA oligomer in vitro and in vivo through upregulation of the PA receptors TEM8 and CMG2 in both myeloid and nonmyeloid cells. ET effects on protein clearance from circulation appeared to be global and were not limited to PA. ET also impaired the clearance of ovalbumin, green fluorescent protein, and EF itself, as well as the small molecule biotin when these molecules were coinjected with the toxin. Effects on injected protein levels were not a result of general increase in protein concentrations due to fluid loss. Functional markers for liver and kidney were altered in response to ET. Concomitantly, ET caused phosphorylation and activation of the aquaporin-2 water channel present in the principal cells of the collecting ducts of the kidneys that are responsible for fluid homeostasis. Our data suggest that in vivo, ET alters circulatory protein and small molecule pharmacokinetics by an as-yet-undefined mechanism, thereby potentially allowing a prolonged circulation of anthrax virulence factors such as EF during infection.

The potential contributions of lethal and edema toxins to the pathogenesis of anthrax associated shock

Outbreaks of Bacillus anthracis in the US and Europe over the past 10 years have emphasized the health threat this lethal bacteria poses even for developed parts of the world. In contrast to cutaneous anthrax, inhalational disease in the US during the 2001 outbreaks and the newly identified injectional drug use form of disease in the UK and Germany have been associated with relatively high mortality rates. One notable aspect of these cases has been the difficulty in supporting patients once shock has developed. Anthrax bacilli produce several different components which likely contribute to this shock. Growing evidence indicates that both major anthrax toxins may produce substantial cardiovascular dysfunction. Lethal toxin (LT) can alter peripheral vascular function; it also has direct myocardial depressant effects. Edema toxin (ET) may have even more pronounced peripheral vascular effects than LT, including the ability to interfere with the actions of conventional vasopressors. Additionally, ET also appears capable of interfering with renal sodium and water retention. Importantly, the two toxins exert their actions via quite different mechanisms and therefore have the potential to worsen shock and outcome in an additive fashion. Finally, both toxins have the ability to inhibit host defense and microbial clearance, possibly contributing to the very high bacterial loads noted in patients dying with anthrax. This last point is clinically relevant since emerging data has begun to implicate other bacterial components such as anthrax cell wall in the shock and organ injury observed with infection. Taken together, accumulating evidence regarding the potential contribution of LT and ET to anthrax-associated shock supports efforts to develop adjunctive therapies that target both toxins in patients with progressive shock.

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.

Delayed treatment with W1-mAb, a chimpanzee-derived monoclonal antibody against protective antigen, reduces mortality from challenges with anthrax edema or lethal toxin in rats and with anthrax spores in mice

OBJECTIVE

W1-mAb is a chimpanzee-derived monoclonal antibody to protective antigen that improved survival when administered before anthrax lethal toxin challenge in rats. To better define W1-mAb's efficacy for anthrax, we administered it after initiation of 24-hr infusions of edema toxin and lethal toxin either alone or together in rats or following anthrax spore challenge in mice.

INTERVENTIONS

W1-mAb or placebo treatment.

METHODS AND MAIN RESULTS

In toxin-challenged rats treated with placebo, survival rates were lower with edema toxin (500 μg/kg) compared to lethal toxin either alone (175 μg/kg) or with edema toxin (175 μg/kg each) (8%, 33%, and 32%, respectively), but the median time to death was longer (36, 11, and 9 hrs, respectively) (p ≤ .01 for all comparisons). W1-mAb administered up to 12 hrs after edema toxin and 6 hrs after lethal toxin increased survival and reduced hypotension (p ≤ .01). However, only administration of W1-mAb at 0 hrs improved these variables with lethal toxin and edema toxin together (p ≤ .0002). In C57BL/6J mice challenged with anthrax spores subcutaneously, compared to placebo treatment (0 of 15 animals survived), W1-mAb administered beginning 24 hrs after challenge increased survival (13 of 15 survived) (p ≤ .0001).

CONCLUSION

While rapidity of lethality may influence the effectiveness of delayed W1-mAb treatment, these rat and mouse studies provide a basis for further exploring this agent's usefulness for anthrax.

Tumor endothelial marker 8 amplifies canonical Wnt signaling in blood vessels

Tumor Endothelial Marker 8/Anthrax Toxin Receptor 1 (TEM8/ANTXR1) expression is induced in the vascular compartment of multiple tumors and therefore, is a candidate molecule to target tumor therapies. This cell surface molecule mediates anthrax toxin internalization, however, its physiological function in blood vessels remains largely unknown. We identified the chicken chorioallantoic membrane (CAM) as a model system to study the endogenous function of TEM8 in blood vessels as we found that TEM8 expression was induced transiently between day 10 and 12 of embryonic development, when the vascular tree is undergoing final development and growth. We used the cell-binding component of anthrax toxin, Protective Antigen (PA), to engage endogenous TEM8 receptors and evaluate the effects of PA-TEM8 complexes on vascular development. PA applied at the time of highest TEM8 expression reduced vascular density and disrupted hierarchical branching as revealed by quantitative morphometric analysis of the vascular tree after 48h. PA-dependent reduced branching phenotype was partially mimicked by Wnt3a application and ameliorated by the Wnt antagonist, Dikkopf-1. These results implicate TEM8 expression in endothelial cells in regulating the canonical Wnt signaling pathway at this day of CAM development. Consistent with this model, PA increased beta catenin levels acutely in CAM blood vessels in vivo and in TEM8 transfected primary human endothelial cells in vitro. TEM8 expression in Hek293 cells, which neither express endogenous PA-binding receptors nor Wnt ligands, stabilized beta catenin levels and amplified beta catenin-dependent transcriptional activity induced by Wnt3a. This agonistic function is supported by findings in the CAM, where the increase in TEM8 expression from day 10 to day 12 and PA application correlated with Axin 2 induction, a universal reporter gene for canonical Wnt signaling. We postulate that the developmentally controlled expression of TEM8 modulates endothelial cell response to canonical Wnt signaling to regulate vessel patterning and density.

Hemodynamic effects of anthrax toxins in the rabbit model and the cardiac pathology induced by lethal toxin

Anthrax lethal toxin (LeTx) and edema toxin (EdTx) have been shown to alter hemodynamics in the rodent model, while LeTx primarily is reported to induce extensive tissue pathology. However, the rodent model has limitations when used for comparison to higher organisms such as humans. The rabbit model, on the other hand, has gained recognition as a useful model for studying anthrax infection and its pathophysiological effects. In this study, we assessed the hemodynamic effects of lethal toxin (LeTx) and edema toxin (EdTx) in the rabbit model using physiologically relevant amounts of the toxins. Moreover, we further examine the pathological effects of LeTx on cardiac tissue. We intravenously injected Dutch-belted rabbits with either low-dose and high-dose recombinant LeTx or a single dose of EdTx. The animals’ heart rate and mean arterial pressure were continuously monitored via telemetry until either 48 or 72 h post-challenge. Additional animals challenged with LeTx were used for cardiac troponin I (cTnI) quantitation, cardiac histopathology, and echocardiography. LeTx depressed heart rate at the lower dose and mean arterial pressure (MAP) at the higher dose. EdTx, on the other hand, temporarily intensified heart rate while lowering MAP. Both doses of LeTx caused cardiac pathology with the higher dose having a more profound effect. Lastly, left-ventricular dilation due to LeTx was not apparent at the given time-points. Our study demonstrates the hemodynamic effects of anthrax toxins, as well as the pathological effects of LeTx on the heart in the rabbit model, and it provides further evidence for the toxins’ direct impact on the heart.

Glycogen synthase kinase 3 activation is important for anthrax edema toxin-induced dendritic cell maturation and anthrax toxin receptor 2 expression in macrophages

Anthrax edema toxin (ET) is one of two binary toxins produced by Bacillus anthracis that contributes to the virulence of this pathogen. ET is an adenylate cyclase that generates high levels of cyclic AMP (cAMP), causing alterations in multiple host cell signaling pathways. We previously demonstrated that ET increases cell surface expression of the anthrax toxin receptors (ANTXR) in monocyte-derived cells and promotes dendritic cell (DC) migration toward the lymph node-homing chemokine MIP-3β. In this work, we sought to determine if glycogen synthase kinase 3 (GSK-3) is important for ET-induced modulation of macrophage and DC function. We demonstrate that inhibition of GSK-3 dampens ET-induced maturation and migration processes of monocyte-derived dendritic cells (MDDCs). Additional studies reveal that the ET-induced expression of ANTXR in macrophages was decreased when GSK-3 activity was disrupted with chemical inhibitors or with small interfering RNA (siRNA) targeting GSK-3. Further examination of the ET induction of ANTXR revealed that a dominant negative form of CREB could block the ET induction of ANTXR, suggesting that CREB or a related family member was involved in the upregulation of ANTXR. Because CREB and GSK-3 activity appeared to be important for ET-induced ANTXR expression, the impact of GSK-3 on ET-induced CREB activity was examined in RAW 264.7 cells possessing a CRE-luciferase reporter. As with ANTXR expression, the ET induction of the CRE reporter was decreased by reducing GSK-3 activity. These studies not only provide insight into host pathways targeted by ET but also shed light on interactions between GSK-3 and CREB pathways in host immune cells.

Anthrax edema toxin has cAMP-mediated stimulatory effects and high-dose lethal toxin has depressant effects in an isolated perfused rat heart model

While anthrax edema toxin produces pronounced tachycardia and lethal toxin depresses left ventricular (LV) ejection fraction in in vivo models, whether these changes reflect direct cardiac effects as opposed to indirect ones related to preload or afterload alterations is unclear. In the present study, the effects of edema toxin and lethal toxin were investigated in a constant pressure isolated perfused rat heart model. Compared with control hearts, edema toxin at doses comparable to or less than a dose that produced an 80% lethality rate (LD(80)) in vivo in rats (200, 100, and 50 ng/ml) produced rapid increases in heart rate (HR), coronary flow (CF), LV developed pressure (LVDP), dP/dt(max), and rate-pressure product (RPP) that were most pronounced and persisted with the lowest dose (P ≤ 0.003). Edema toxin (50 ng/ml) increased effluent and myocardial cAMP levels (P ≤ 0.002). Compared with dobutamine, edema toxin produced similar myocardial changes, but these occurred more slowly and persisted longer. Increases in HR, CF, and cAMP with edema toxin were inhibited by a monoclonal antibody blocking toxin uptake and by adefovir, which inhibits the toxin's intracellular adenyl cyclase activity (P ≤ 0.05). Lethal toxin at an LD(80) dose (50 ng/ml) had no significant effect on heart function but a much higher dose (500 ng/ml) reduced all parameters (P ≤ 0.05). In conclusion, edema toxin produced cAMP-mediated myocardial chronotropic, inotropic, and vasodilatory effects. Vasodilation systemically with edema toxin could contribute to shock during anthrax while masking potential inotropic effects. Although lethal toxin produced myocardial depression, this only occurred at high doses, and its relevance to in vivo findings is unclear.

Anthrax lethal and edema toxins produce different patterns of cardiovascular and renal dysfunction and synergistically decrease survival in canines

BACKGROUND

High mortality in the 2001 US and recent European anthrax outbreaks suggests that better understanding of the effects of the toxins produced by this bacterium is needed to improve treatment.

METHODS AND RESULTS

Here, 24-h edema (ETx) and lethal (LeTx) toxin infusions were investigated for 96 hin sedated canines receiving mechanical ventilation. The initial study compared similarly lethal doses of ETx (n=8) or LeTx (n=15) alone. ETx was 24 times less lethal than LeTx, and the median time to death in nonsurvivors (n=6 and n=9, respectively) was shorter with ETx (42 vs 67 h; P=.04). Compared with controls(n=9), both toxins decreased arterial and central venous pressures and systemic vascular resistance and increased heart rate, cardiac index, blood urea nitrogen (BUN) level, creatinine (Cr) concentration, BUN:Cr ratio, and hepatic transaminase levels (P ≤ .05 for toxin effect or time interaction). However, ETx stimulated early diuresis,reduced serum sodium levels, and had more pronounced vasodilatory effects, compared with LeTx, as reflected by greater or earlier central venous pressures, systemic vascular resistance, and changes in the BUN:Cr ratio(P ≤ .01). LeTx progressively decreased the left ventricular ejection fraction (P ≤ .002). In a subsequent study, a lethal dose of LeTx with an equimolar nonlethal ETx dose (n=8) increased mortality, compared with LeTx alone (n=8; P= .05).

CONCLUSION

Shock with ETx or LeTx may require differing supportive therapies, whereas toxin antagonists should likely target both toxins.

Anthrax lethal toxin suppresses murine cardiomyocyte contractile function and intracellular Ca2+ handling via a NADPH oxidase-dependent mechanism

Objectives

Anthrax infection is associated with devastating cardiovascular sequelae, suggesting unfavorable cardiovascular effects of toxins originated from Bacillus anthracis namely lethal and edema toxins. This study was designed to examine the direct effect of lethal toxins on cardiomyocyte contractile and intracellular Ca²⁺ properties.

Methods

Murine cardiomyocyte contractile function and intracellular Ca²⁺ handling were evaluated including peak shortening (PS), maximal velocity of shortening/ relengthening (± dL/dt), time-to-PS (TPS), time-to-90% relengthening (TR₉₀), intracellular Ca²⁺ rise measured as fura-2 fluorescent intensity (ΔFFI), and intracellular Ca²⁺ decay rate. Stress signaling and Ca²⁺ regulatory proteins were assessed using Western blot analysis.

Results

In vitro exposure to a lethal toxin (0.05 – 50 nM) elicited a concentration-dependent depression on cardiomyocyte contractile and intracellular Ca²⁺ properties (PS, ± dL/dt, ΔFFI), along with prolonged duration of contraction and intracellular Ca²⁺ decay, the effects of which were nullified by the NADPH oxidase inhibitor apocynin. The lethal toxin significantly enhanced superoxide production and cell death, which were reversed by apocynin. In vivo lethal toxin exposure exerted similar time-dependent cardiomyocyte mechanical and intracellular Ca²⁺ responses. Stress signaling cascades including MEK1/2, p38, ERK and JNK were unaffected by in vitro lethal toxins whereas they were significantly altered by in vivo lethal toxins. Ca²⁺ regulatory proteins SERCA2a and phospholamban were also differentially regulated by in vitro and in vivo lethal toxins. Autophagy was drastically triggered although ER stress was minimally affected following lethal toxin exposure.

Conclusions

Our findings indicate that lethal toxins directly compromised murine cardiomyocyte contractile function and intracellular Ca²⁺ through a NADPH oxidase-dependent mechanism.

Medical countermeasures to protect humans from anthrax bioterrorism

The deliberate dissemination of Bacillus anthracis spores via the US mail system in 2001 confirmed their potential use as a biological weapon for mass human casualties. This dramatically highlighted the need for specific medical countermeasures to enable the authorities to protect individuals from a future bioterrorism attack. Although vaccination appears to be the most effective and economical form of mass protection, current vaccines have significant drawbacks that justify the immense research effort to develop improved treatment modalities. After eight years and an expenditure of more than $50 billion, only marginal progress has been made in developing effective therapeutics. This article summarizes the most important medical countermeasures that have mostly been developed since the 2001 events, and highlights current problems and possible avenues for future research.

Bacillus anthracis cell wall produces injurious inflammation but paradoxically decreases the lethality of anthrax lethal toxin in a rat model

OBJECTIVES

The in vivo inflammatory effects of the Bacillus anthracis cell wall are unknown. We therefore investigated these effects in rats and, for comparison, those of known inflammatory stimulants, Staphylococcus aureus cell wall or lipopolysaccharide (LPS).

METHOD AND RESULTS

Sprague-Dawley rats (n = 103) were challenged with increasing B. anthracis cell wall doses (10, 20, 40, 80, or 160 mg/kg) or diluent (control) as a bolus or 24-h infusion. The three highest bolus doses were lethal (20-64% lethality rates) as were the two highest infused doses (13% with each). Comparisons among lethal or nonlethal doses on other measured parameters were not significantly different, and these were combined for analysis. Over the 24 h after challenge initiation with lethal bolus or infusion, compared to controls, ten inflammatory cytokines and NO levels were increased and circulating neutrophils and platelets decreased (P <or= 0.05). Changes with lethal doses were greater than changes with nonlethal doses (P <or= 0.01). Lethal bolus or infusion doses produced hypotension or hypoxemia, respectively (P <or= 0.05). The effects with B. anthracis cell wall were similar to those of S. aureus cell wall or LPS. However, paradoxically administration of B. anthracis cell wall or LPS decreased the lethality of concurrently administered B. anthracis lethal toxin (P < 0.0001 and 0.04, respectively).

CONCLUSION

B. anthracis cell wall has the potential to produce inflammatory injury during anthrax infection clinically. However, understanding why cell wall or LPS paradoxically reduced lethality with lethal toxin may help understand this toxin's pathogenic effects.

Cellular and systemic effects of anthrax lethal toxin and edema toxin

Anthrax lethal toxin (LT) and edema toxin (ET) are the major virulence factors of anthrax and can replicate the lethality and symptoms associated with the disease. This review provides an overview of our current understanding of anthrax toxin effects in animal models and the cytotoxicity (necrosis and apoptosis) induced by LT in different cells. A brief reexamination of early historic findings on toxin in vivo effects in the context of our current knowledge is also presented.

Membrane translocation by anthrax toxin

Much attention has been focused on anthrax toxin recently, both because of its central role in the pathogenesis of Bacillus anthracis and because it has proven to be one of the most tractable toxins for studying how enzymic moieties of intracellularly acting toxins traverse membranes. The Protective Antigen (PA) moiety of the toxin, after being proteolytically activated at the cell surface, self-associates to form a heptameric pore precursor (prepore). The prepore binds up to three molecules of Edema Factor (EF), Lethal Factor (LF), or both, forming a series of complexes that are then endocytosed. Under the influence of acidic pH within the endosome, the prepore undergoes a conformational transition to a mushroom-shaped pore, with a globular cap and 100A-long stem that spans the membrane. Electrophysiological studies in planar bilayers indicate that EF and LF translocate through the pore in unfolded form and in the N- to C-terminal direction. The pore serves as an active transporter, which translocates its proteinaceous cargo across the endosomal membrane in response to DeltapH and perhaps, to a degree, Deltapsi. A ring of seven Phe residues (Phe427) in the lumen of the pore forms a seal around the translocating polypeptide and blocks the passage of ions, presumably preserving the pH gradient. A charge state-dependent Brownian ratchet mechanism has been proposed to explain how the pore translocates EF and LF. This transport mechanism of the pore may function in concert with molecular chaperonins to effect delivery of effector proteins in catalytically active form to the cytosolic compartment of host cells.

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

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

Norepinephrine increases blood pressure but not survival with anthrax lethal toxin in rats

OBJECTIVES

The response of anthrax lethal toxin (LeTx) induced shock and lethality to conventional therapies has received little study. Previously, fluids worsened outcome in LeTx-challenged rats in contrast to its benefit with lipopolysaccharide (LPS) or Escherichia coli. The current study investigated norepinephrine treatment.

MEASUREMENTS AND MAIN RESULTS

Sprague-Dawley rats (n = 232) weighing between 230 and 250 g were challenged with similar lethal (80%) 24-hour infusions of either LPS or LeTx, or with diluent only. Toxin-challenged animals were also randomized to receive 24-hour infusions with one of three doses of norepinephrine (0.03, 0.3, or 3.0 microg/kg/min) or placebo started 1 hour after initiation of challenge. All toxin animals received similar volumes of fluid over the 24 hours (equivalent to 4.0-4.3 mL/kg/hr). Although the intermediate norepinephrine dose (0.3 microg/kg/min for 24 hours) improved survival with LPS (p = 0.04) and increased blood pressure before the onset of lethality with LeTx (p < 0.0001), it did not improve survival with the latter (p = ns). Furthermore, neither increasing nor decreasing norepinephrine doses improved survival with LeTx.

CONCLUSION

Hypotension with LeTx may not be a primary cause of lethality in this model. Rather, LeTx may cause direct cellular injury insensitive to vasopressors. These findings suggest that during anthrax infection and shock, along with hemodynamic support, toxin-directed treatments may be necessary as well.

Suppression of dendritic cell activation by anthrax lethal toxin and edema toxin depends on multiple factors including cell source, stimulus used, and function tested

Bacillus anthracis produces lethal toxin (LT) and edema toxin (ET), and they suppress the function of LPS-stimulated dendritic cells (DCs). Because DCs respond differently to various microbial stimuli, we compared toxin effects in bone marrow DCs stimulated with either LPS or Legionella pneumophila (Lp). LT, not ET, was more toxic for cells from BALB/c than from C57BL/6 (B6) as measured by 7-AAD uptake; however, ET suppressed CD11c expression. LT suppressed IL-12, IL-6, and TNF-alpha in cells from BALB/c and B6 mice but increased IL-1beta in LPS-stimulated cultures. ET also suppressed IL-12 and TNF-alpha, but increased IL-6 and IL-1beta in Lp-stimulated cells from B6. Regarding maturation marker expression, LT increased MHCII and CD86 while suppressing CD40 and CD80; ET generally decreased marker expression across all groups. We conclude that the suppression of cytokine production by anthrax toxins is dependent on variables, including the source of the DCs, the type of stimulus and cytokine measured, and the individual toxin tested. However, LT and ET enhancement or suppression of maturation marker expression is more related to the marker studied than the stimuli or cell source. Anthrax toxins are not uniformly suppressive of DC function but instead can increase function under defined conditions.

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.

Genetic vaccines for anthrax based on recombinant adeno-associated virus vectors

Bacillus anthracis represents a formidable bioterrorism and biowarfare threat for which new vaccines are needed with improved safety and efficacy over current options. Toward this end, we created recombinant adeno-associated virus type 1 (rAAV1) vectors containing synthetic genes derived from the protective antigen (PA) or lethal factor (LF) of anthrax lethal toxin (LeTx) and tested them for immunogenicity and induction of toxin-neutralizing antibodies in rabbits. Codon-optimized segments encoding activated PA (PA63), or LF, were synthesized and cloned into optimized rAAV1 vectors containing a human cytomegalovirus (hCMV) promoter and synthetic optimized leader. Serum from rabbits immunized intramuscularly with rAAV1/PA (monovalent), rAAV1/LF (monovalent), rAAV1/PA + rAAV1/LF (bivalent), or rAAV1/enhanced green fluorescent protein (control) exhibited substantial PA- and LF-specific antibody responses at 4 weeks by both western blot (> 1:10,000 dilution) and enzyme-linked immunosorbent assay (ELISA) (mean end-point titer: 32,000-260,000), and contained anthrax LeTx-neutralizing activity in vitro, with peak titers approximating those of a rabbit hyperimmune antisera raised against soluble PA and LF. Compared to the monovalent groups (rAAV1/PA or rAAV1/LF), the bivalent group (rAAV1/PA + rAAV1/LF) exhibited marginally higher ELISA and neutralization activity with dual specificity for both PA and LF. The finding of robust neutralizing antibody responses after a single injection of these rAAV1-based vectors supports their further development as candidate anthrax vaccines.

Anthrax lethal toxin induces cell death-independent permeability in zebrafish vasculature

Vascular dysfunction has been reported in human cases of anthrax, in mammalian models of Bacillus anthracis, and in animals injected with anthrax toxin proteins. To examine anthrax lethal toxin effects on intact blood vessels, we developed a zebrafish model that permits in vivo imaging and evaluation of vasculature and cardiovascular function. Vascular defects monitored in hundreds of embryos enabled us to define four stages of phenotypic progression leading to circulatory dysfunction. We demonstrated increased endothelial permeability as an early consequence of toxin action by tracking the extravasation of fluorescent microspheres in toxin-injected embryos. Lethal toxin did not induce a significant amount of cell death in embryonic tissues or blood vessels, as shown by staining with acridine orange, and endothelial cells in lethal toxin-injected embryos continued to divide at the normal rate. Vascular permeability is strongly affected by the VEGF/vascular permeability factor (VPF) signaling pathway, and we were able to attenuate anthrax lethal toxin effects with chemical inhibitors of VEGFR function. Our study demonstrates the importance of vascular permeability in anthrax lethal toxin action and the need for further investigation of the cardiovascular component of human anthrax disease.

Standardized, mathematical model-based and validated in vitro analysis of anthrax lethal toxin neutralization

Quantification of anthrax lethal toxin (LTx) neutralization activity (TNA) is pivotal in assessing protective antibody responses to anthrax vaccines and for evaluation of immunotherapies for anthrax. We have adapted and redesigned the TNA assay to establish a unifying, standardized, quantitative and validated technology platform for LTx neutralization in the J774A.1 murine cell line. Critical design features of this platform are 1) the application of a free-form or constrained 4 parameter logistic (4-PL) function to model neutralization responses within and between boundary limits of 100% cell survival and 95% cell lysis and 2) to exploit innovative assay curve recognition algorithms for interpretive endpoints. The assay was validated using human serum ED50 (dilution of serum effecting 50% neutralization) as the primary reportable value (RV). Intra-operator and intermediate precision, expressed as the coefficient of variation (%CV), were high at 10.5-15.5%CV and 13.5-14.5%CV respectively. TNA assay dilutional linearity was demonstrated for human sera using linear regression analysis of log(10) transformed data with slope=0.99, intercept=-0.03 and r(2)=0.985. Assay accuracy, inferred from the precision and linearity data and using a spike-recovery approach, was high with a percent error (%E) range of only 3.4-20.5%E. The lower limit of detection (LLOD) was ED50=12 and the lower limit of quantification (LLOQ) was ED50=36. The cell-based assay was robust, tolerating incubation temperatures from 35 to 39 degrees C, CO(2) concentrations from 3% to 7% and reporter substrate (MTT) concentrations of 2.5-7.5 mg/ml. Strict assay quality control parameters were met for up to 25 cell culture passages. The long term (50 month) assay stability, determined using human reference standards AVR414 and AVR801, indicated high precision, consistent accuracy and no detectable assay drift. A customized software program provided two additional assay metrics, Quantification Titer (QT) and Threshold Titer (TT), both of which demonstrate acceptable accuracy, precision and dilutional linearity. The TT was also used to establish the assay reactivity threshold (RT). The application of the assay to sera from humans, Rhesus macaques and rabbits was demonstrated separately and by aggregate dilutional linearity analysis of the ED50 (slope=0.98, intercept=0.003, r(2)=0.989). We propose this TNA assay format with a qualified standard reference serum and customized interpretive software as a unifying platform technology for determination of functional serologic responses to anthrax vaccines and for evaluation of anthrax immunotherapeutics.

Anthrax toxin-induced shock in rats is associated with pulmonary edema and hemorrhage

Bacillus anthracis infections are frequently associated with severe and often irreversible hypotensive shock despite appropriate antibiotics and aggressive hemodynamic and pulmonary support. Based on the observations that the anthrax secreted proteins-protective antigen (PA), lethal factor (LF), and edema factor (EF) also produce shock and mortality in animal models, we chose to characterize further the clinical chemistries and microscopic pathology of toxin treated rats. Groups of three male Sprague Dawley rats received bolus intravenous infusions of PA/LF, PA/EF, LF, or EF alone and blood samples and tissues were collected and assayed for chemistries and tissue pathology. In PA/LF and PA/EF treated animals but not other groups, chemistries showed transaminasemia and elevated lactate dehydrogenase. PA/LF treated animals alone showed elevated hemoglobin and hematocrits; PA/EF treated animals alone showed lymphopenia. Pathology was remarkable for pulmonary edema in PA/LF treated rat lungs and pulmonary hemorrhage in PA/EF treated rat lungs. These results are consistent with our and others' previous findings that the morbidity and mortality associated with anthrax are not cytokine-mediated but due to a direct effect of the toxins on the cardiovascular system along with toxin-specific alterations in blood counts. PA/LF pathology matches that seen with acute cardiac failure, and PA/EF pathology coincides with direct vascular endothelial injury. These observations provide a rational basis for drug interventions to reduce the effect of these toxins on the heart and blood vessels.

Risk of death does not alter the efficacy of hydrocortisone therapy in a mouse E. coli pneumonia model: risk and corticosteroids in sepsis

BACKGROUND

Risk of death may influence the efficacy of anti-inflammatory agents in sepsis. "Physiologic" dose corticosteroids, while improving survival in earlier trials with higher control mortality rates (>50%), were not beneficial in the recent CORTICUS trial with lower control mortality (31%). We investigated whether risk of death altered the effects of hydrocortisone in a mouse pneumonia model.

METHODS

Mice (n=637) challenged with high, medium or low intratracheal E. coli doses were randomized to receive one of three hydrocortisone doses (5, 25 or 125 mg/kg) or normal saline (NS) only (control) for 4 days. All animals were treated with similar volumes of ceftriaxone and NS support following E. coli and were observed for 168 h.

RESULTS

Decreasing E. coli doses reduced control mortality rates (from 94 to 12%). In similar patterns (not significant) each hydrocortisone dose increased the odds ratio (OR) of survival (95% confidence interval) with each E. coli dose (ORs ranging from 1.2 [0.4, 3.7] to 6.1 [0.6, 61.0]). The effect of hydrocortisone on the OR was not related to control mortality rate (r=-0.13, p=0.29) and overall was highly significant (2.04 [1.37, 3.03], p=0.0004). In randomly selected animals 48 h after the highest E. coli dose, compared with the control, hydrocortisone (125 mg/kg) significantly decreased IL-6, INFgamma, and nitric oxide levels.

CONCLUSIONS

In this mouse model the beneficial effects of hydrocortisone were independent of risk of death. These findings suggest that factors other than risk of death may underlie the differing effects of corticosteroids in recent sepsis trials.

Transcriptional profiling of murine organ genes in response to infection with Bacillus anthracis Ames spores

Bacillus anthracis is the Gram-positive, spore-forming etiological agent of anthrax, an affliction studied because of its importance as a potential bioweapon. Although in vitro transcriptional responses of macrophages to either spore or anthrax toxins have been previously reported, little is known regarding the impact of infection on gene expression in host tissues. We infected Swiss-Webster mice intranasally with 5 LD(50) of B. anthracis-virulent Ames spores and observed the global transcriptional profiles of various tissues over a 48 h time period. RNA was extracted from spleen, lung, and heart tissues of infected and control mice and examined by Affymetrix GeneChip analysis. Approximately 580 host genes were significantly over or under expressed among the lung, spleen, and heart tissues at 8 and 48 h time points. Expression of genes encoding for surfactant and major histocompatibility complex (MHC) presentation was diminished during the early phase of infection in lungs. By 48 h, a significant number of genes were modulated in the heart, including up-regulation of calcium-binding-related gene expression, and down-regulation of multiple genes related to cell adhesion, formation of the extracellular matrix, and the cell cytoskeleton. Interestingly, the spleen 8h post-infection showed striking increases in the expression of genes that encode hydrolytic enzymes, and these levels remained elevated throughout infection. Further, genes involving antigen presentation and interferon responses were down-regulated in the spleen at 8 h. In late stages of infection, splenic genes related to the inflammatory response were up-regulated. This study is the first to describe the in vivo global transcriptional response of multiple organs during inhalational anthrax. Although numerous genes related to the host immunological response and certain protection mechanisms were up-regulated in these organs, a vast list of genes important for fully developing and maintaining this response were decreased. Additionally, the lung, spleen, and heart showed differential responses to the infection, further validating the demand for a better understanding of anthrax pathogenesis in order to design therapies against novel targets.

New insights into the pathogenesis and treatment of anthrax toxin-induced shock

Inhalational Bacillus anthracis infection is a leading bioterrorist health threat in the US today. Lethal (LeTx) and edema toxin production are key to the virulent effects of this lethal bacteria. Recent insights into the structure and function of these toxins have increased the understanding of both the pathogenesis and treatment of anthrax. These are binary type toxins comprised of protective antigen necessary for their cellular uptake and either lethal or edema factors, the toxigenic moieties. Primary cellular receptors for protective antigen have been identified and the processing of the completed toxins clarified. Consistent with the ability of lethal factor to cleave mitogen activated protein kinase kinases, the evidence indicates that an excessive inflammatory response does not contribute to shock with LeTx. Rather, the immunosuppressive effects of LeTx could promote infection; however, direct endothelial dysfunction may have an important role in shock due to LeTx. Recent studies show that edema factor, a potent adenyl cyclase, may have a major role in shock during anthrax and that it may also be immunosuppresive. Therapies under development which target several steps in the cellular uptake and function of these two toxins have been effective in both in vitro and in vivo systems. Understanding how best to apply these agents in combination with conventional treatments should be a goal of future research.