Anthrax Edema Toxin Research Articles

Vaccine Delivery with a Detoxified Bacterial Toxin.

It is still a challenge to develop needle-free mucosal vaccines. Despite progress in the development of the influenza vaccine, it must be reformulated annually because of antigenic changes in circulating influenza viral strains. Due to seasonal drift and shift of circulating strains, the influenza vaccine does not always match the circulating strains, and included adjuvants are not sufficient to induce a protective effect with long-lived memory cells. The adjuvants play a major role in the immune responses to a vaccine. Interestingly, the Bacillus anthracis detoxified anthrax edema toxin, which composes of protective antigen PA and N-fragment of edema factor (EFn), has shown improved effects for humoral and cellular immune responses. Here we describe the design of a universal influenza vaccine construct that consists of three tandem M2e repeats of the influenza antigen plus HA2 and detoxified toxin EFn, which is associated with the PA component, as well as the techniques used to corroborate protection. We present two major parts of description to demonstrate the vaccine strategy, using detoxified anthrax toxin for intranasal delivery of influenza antigen: (1) vaccine candidate design, production, and purification; (2) influenza virus microneutralization assay and cellular responses and lethal challenge with influenza viruses and B. anthracis Sterne spores. In the methods detailed here, we used different versions of the M2e-HA2 proteins.

Anthrax Edema Toxin Co-Opts IGF1R and EGFR Signaling to Promote Cell-Cell Barrier Dysfunction

Cell-cell barrier dysfunction induced by the anthrax edema toxin (ET) is central to bacterial dissemination and lethal vascular collapse during disease progression. Available treatments including antimicrobial and early-stage antitoxin measures do not prevent lethal anthrax toxemia during fulminant stages of infection. This study identifies cell-based therapeutic measures that target the late-stage intracellular activities of ET and their potential to mitigate consequences of ET toxemia. We uncover a critical involvement of IGF1R and EGFR in promoting ET-dependent barrier disruption through effects on cellular F-actin network that are separate from, but have an impact upon, previously characterized cAMP-mediated inhibition of vesicular trafficking. ET rapidly trans-activates IGF1R and EGFR, resulting in the activation of downstream effectors PI3K and MEK, and subsequently a Rac1- and cofilin-dependent actin remodeling. Finally, we provide in vivo pre-clinical validation of the therapeutic potential of PI3K, MEK and Rac1 inhibitors in preventing ET-dependent edema in a mouse footpad model.

Rapid Discovery and Characterization of Synthetic Neutralizing Antibodies against Anthrax Edema Toxin.

Anthrax, a lethal, weaponizable disease caused by Bacillus anthracis, acts through exotoxins that are primary mediators of systemic toxicity and also targets for neutralization by passive immunotherapy. The ease of engineering B. anthracis strains resistant to established therapy and the historic use of the microbe in bioterrorism present a compelling test case for platforms that permit the rapid and modular development of neutralizing agents. In vitro antigen-binding fragment (Fab) selection offers the advantages of speed, sequence level molecular control, and engineering flexibility compared to traditional monoclonal antibody pipelines. By screening an unbiased, chemically synthetic phage Fab library and characterizing hits in cell-based assays, we identified two high-affinity neutralizing Fabs, A4 and B7, against anthrax edema factor (EF), a key mediator of anthrax pathogenesis. Engineered homodimers of these Fabs exhibited potency comparable to that of the best reported neutralizing monoclonal antibody against EF at preventing EF-induced cyclic AMP production. Using internalization assays in COS cells, B7 was found to block steps prior to EF internalization. This work demonstrates the efficacy of synthetic alternatives to traditional antibody therapeutics against anthrax while also demonstrating a broadly generalizable, rapid, and modular screening pipeline for neutralizing antibody generation.

New technology for development of host-targeted therapeutic vaccine against anthrax

Abstract Anthrax is a serious disease caused by Bacillus anthracis, a bacterium that can form spores for long-term survival and secrete anthrax toxins including protective antigen (PA), lethal factor (LF), and edema factor (EF). However, current anthrax postexposure treatments are inadequate at later stages of infection. Previously, our research have shown RNA inhibition (RNAi) on host cells silences anthrax toxin receptors (ATRs): tumor endothelial marker 8 (TEM8), capillary morphogenesis protein 2 (CMG2). This approach was protective against the cytotoxicity of anthrax toxins, indicating that ATR-targeted RNAi can be used as a therapy against anthrax. However, inefficient cytosolic delivery and toxicity of siRNA delivery vehicles limit the use siRNA as therapeutics. In this study, we have developed a detoxified anthrax edema toxin, which consists of PA and nontoxic N-terminal fragment of EF (EFn) conjugated with a peptide nona-D-arginine residues (EFn-9dR) to enable siRNA binding. The detoxified toxin was able to deliver specific siRNA to induce cmg2 gene silencing in different cell lines and C57BL/6 mice, and provide significant protection against anthrax lethal toxin challenge in vitro and in mice. Survived mice from toxin challenge were fully protected against lethal challenge with B. anthracis Sterne spores. The immune protective mechanism is mainly due to the significantly high serum neutralizing antibody response against anthrax toxins in these mice. These results suggest that detoxified anthrax toxin provides a tool for delivery of host-targeted siRNA into anthrax pathogenesis-associated host cells, and this new system could potentially be developed as a lifesaving postexposure therapeutic vaccine against anthrax.

Anthrax edema toxin disrupts distinct steps in Rab11-dependent junctional transport.

Various bacterial toxins circumvent host defenses through overproduction of cAMP. In a previous study, we showed that edema factor (EF), an adenylate cyclase from Bacillus anthracis, disrupts endocytic recycling mediated by the small GTPase Rab11. As a result, cargo proteins such as cadherins fail to reach inter-cellular junctions. In the present study, we provide further mechanistic dissection of Rab11 inhibition by EF using a combination of Drosophila and mammalian systems. EF blocks Rab11 trafficking after the GTP-loading step, preventing a constitutively active form of Rab11 from delivering cargo vesicles to the plasma membrane. Both of the primary cAMP effector pathways -PKA and Epac/Rap1- contribute to inhibition of Rab11-mediated trafficking, but act at distinct steps of the delivery process. PKA acts early, preventing Rab11 from associating with its effectors Rip11 and Sec15. In contrast, Epac functions subsequently via the small GTPase Rap1 to block fusion of recycling endosomes with the plasma membrane, and appears to be the primary effector of EF toxicity in this process. Similarly, experiments conducted in mammalian systems reveal that Epac, but not PKA, mediates the activity of EF both in cell culture and in vivo. The small GTPase Arf6, which initiates endocytic retrieval of cell adhesion components, also contributes to junctional homeostasis by counteracting Rab11-dependent delivery of cargo proteins at sites of cell-cell contact. These studies have potentially significant practical implications, since chemical inhibition of either Arf6 or Epac blocks the effect of EF in cell culture and in vivo, opening new potential therapeutic avenues for treating symptoms caused by cAMP-inducing toxins or related barrier-disrupting pathologies.

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.

Intranasal immunization with a chimeric influenza antigen delivered by detoxified anthrax toxin induces broad spectrum immunity against influenza viruses (TECH2P.756)

Abstract Influenza remains to be a significant public health problem worldwide, despite the heavy campaign for flu vaccination in many countries. The conventional influenza vaccines are based on stimulating immune responses against hemagglutinin by virus attenuation or inactivation. However, they must be reformulated annually because of antigenic shift and drift of circulating influenza viral strains. These seasonal vaccines do not always match the circulating strains, and there is the ever-present threat that avian influenza viruses may adapt to be transmitted in humans. Development of a broadly protective or universal influenza vaccine would have a significant impact on public health. We constructed a new chimeric influenza antigen, which is composed of the relatively conserved matrix protein 2 (M2) and stem region of hemagglutinin (HA2) of influenza A viruses in combination with a detoxified anthrax edema toxin delivery system. This vaccine candidate has been shown to elicit T cell and humoral responses against influenza antigens and cross-strain protective immunity against influenza viruses, when it is intranasally delivered to mice. In addition, the vaccination of CD4+ and CD8+ T cell knock out mice do not protect against influenza virus infection. Thus, this novel vaccine strategy induces the CD4+ and CD8+ memory T cell responses which play an important role in cross-strain protection against influenza viruses.

Toxin Inhibition of Antimicrobial Factors Induced by Bacillus anthracis Peptidoglycan in Human Blood

Here, we describe the capacity of Bacillus anthracis peptidoglycan (BaPGN) to trigger an antimicrobial response in human white blood cells (WBCs). Analysis of freshly isolated human blood cells found that monocytes and neutrophils, but not B and T cells, were highly responsive to BaPGN and produced a variety of cytokines and chemokines. This BaPGN-induced response was suppressed by anthrax lethal toxin (LT) and edema toxin (ET), with the most pronounced effect on human monocytes, and this corresponded with the higher levels of anthrax toxin receptor 1 (ANTXR1) in these cells than in neutrophils. The supernatant from BaPGN-treated cells altered the growth of B. anthracis Sterne, and this effect was blocked by LT, but not by ET. An FtsX mutant of B. anthracis known to be resistant to the antimicrobial effects of interferon-inducible Glu-Leu-Arg (ELR)-negative CXC chemokines was not affected by the BaPGN-induced antimicrobial effects. Collectively, these findings describe a system in which BaPGN triggers expression of antimicrobial factors in human WBCs and reveal a distinctive role, not shared with ET, in LT's capacity to suppress this response.

Human aortic endothelial cells compare favourably with macrophages for the study of anthrax toxins

Anthrax is an infectious disease caused by Bacillus anthracis. Although B. anthracis is rare in the natural environment, anthrax spores can be produced in vitro and potentially used as a biological weapon. Thus, understanding the molecular pathogenesis of anthrax is a critical concern for national security. The aim of this study was to compare the effects of anthrax toxins, lethal toxin (LeTx), and edema toxin (EdTx) on human aortic endothelial cells (HAECs) and the J774A.1 murine macrophage cell line. We analysed cell viability, adhesion, morphology, and lipopolysaccharide (LPS)-stimulated cytokine production after incubation of cells with varying concentrations of the toxins. Both LeTx and EdTx markedly inhibited LPS-induced transcription of tumour necrosis factor alpha (TNF-α), interleukin (IL)-1β, and IL-6 in J774A.1 cells. In contrast, EdTx synergised with LPS to increase the transcription of IL-6 and IL-8 in HAECs. We showed that HAECs are suitable for anthrax toxin research and express higher levels of the two anthrax toxin receptors – tumour endothelial marker 8 (TEM8/ANTXR1) and capillary morphogenesis protein 2 (CMG2/ANTXR2) – than do J774A.1 cells. Collectively, our results suggest that HAECs may be superior to macrophages for the study of anthrax pathogenesis.

Anthrax Lethal and Edema Toxins Fail to Directly Impair Human Platelet Function

Hemorrhage is a prominent clinical manifestation of systemic anthrax. Therefore, we have examined the effects of anthrax lethal and edema toxins on human platelets. We find that anthrax lethal toxin fails to cleave its target, mitogen-activated protein kinase 1, and anthrax edema toxin fails to increase intracellular cyclic adenosine monophosphate. Surface expression of toxin receptors tumor endothelial marker 8 and capillary morphogenesis gene 2, as well as coreceptor low density lipoprotein receptor-related protein 6 (LRP6), are markedly reduced, preventing toxin binding to platelets. Our studies suggest that the hemorrhagic clinical manifestations of systemic anthrax are unlikely to be caused by the direct binding and entry of anthrax toxins into human platelets.

In situgastrointestinal protection against anthrax edema toxin by single-chain antibody fragment producing lactobacilli

BackgroundAnthrax is caused by the bacterium Bacillus anthracis and is regarded as one of the most prominent bioterrorism threats. Anthrax toxicity is induced by the tripartite toxin complex, composed of the receptor-binding anthrax protective antigen and the two enzymatic subunits, lethal factor and edema factor. Recombinant lactobacilli have previously been used to deliver antibody fragments directed against surface epitopes of a variety of pathogens, including Streptococcus mutans, Porphyromonas gingivalis, and rotavirus. Here, we addressed whether or not anthrax toxins could be targeted and neutralised in the gastrointestinal tract by lactobacilli producing recombinant antibody fragments as a model system for toxin neutralisation in the gastrointestinal lumen.ResultsThe neutralising anti-PA scFv, 1H, was expressed in L. paracasei as a secreted protein, a cell wall-anchored protein or both secreted and wall-anchored protein. Cell wall display on lactobacilli and PA binding of the anchored constructs was confirmed by flow cytometry analysis. Binding of secreted or attached scFv produced by lactobacilli to PA were verified by ELISA. Both construct were able to protect macrophages in an in vitro cytotoxicity assay. Finally, lactobacilli producing the cell wall attached scFv were able to neutralise the activity of anthrax edema toxin in the GI tract of mice, in vivo.ConclusionWe have developed lactobacilli expressing a neutralising scFv fragment against the PA antigen of the anthrax toxin, which can provide protection against anthrax toxins both in vitro and in vivo. Utilising engineered lactobacilli therapeutically for neutralising toxins in the gastrointestinal tract can potential be expanded to provide protection against a range of additional gastrointestinal pathogens. The ability of lactobacilli to colonise the gastrointestinal tract may allow the system to be used both prophylactically and therapeutically.

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.

CAMP Signaling by Anthrax Edema Toxin Induces Transendothelial Cell Tunnels, which Are Resealed by MIM via Arp2/3-Driven Actin Polymerization

RhoA-inhibitory bacterial toxins, such as Staphylococcus aureus EDIN toxin, induce large transendothelial cell macroaperture (TEM) tunnels that rupture the host endothelium barrier and promote bacterial dissemination. Host cells repair these tunnels by extending actin-rich membrane waves from the TEM edges. We reveal that cyclic-AMP signaling produced by Bacillus anthracis edema toxin (ET) also induces TEM formation, which correlates with increased vascular permeability. We show that ET-induced TEM formation resembles liquid dewetting, a physical process of nucleation and growth of holes within a thin liquid film. We also identify the cellular mechanisms of tunnel closure and reveal that the I-BAR domain protein Missing in Metastasis (MIM) senses de novo membrane curvature generated by the TEM, accumulates at the TEM edge, and triggers Arp2/3-dependent actin polymerization, which induces actin-rich membrane waves that close the TEM. Thus, the balance between ET-induced TEM formation and resealing likely determines the integrity of the host endothelium barrier.

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.

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.

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

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. W1-mAb or placebo treatment. 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). 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.

Contribution of Lethal Toxin and Edema Toxin to the Pathogenesis of Anthrax Meningitis

Bacillus anthracis is a Gram-positive spore-forming bacterium that causes anthrax disease in humans and animals. Systemic infection is characterized by septicemia, toxemia, and meningitis, the main neurological complication associated with high mortality. We have shown previously that B. anthracis Sterne is capable of blood-brain barrier (BBB) penetration, establishing the classic signs of meningitis, and that infection is dependent on the expression of both major anthrax toxins, lethal toxin (LT) and edema toxin (ET). Here we further investigate the contribution of the individual toxins to BBB disruption using isogenic toxin mutants deficient in lethal factor, ΔLF, and edema factor, ΔEF. Acute infection with B. anthracis Sterne and the ΔLF mutant resulted in disruption of human brain microvascular endothelial cell (hBMEC) monolayer integrity and tight junction protein zona occludens-1, while the result for cells infected with the ΔEF mutant was similar to that for the noninfected control. A significant decrease in bacterial invasion of BBB endothelium in vitro was observed during infection with the ΔLF strain, suggesting a prominent role for LT in promoting BBB interaction. Further, treatment of hBMECs with purified LT or chemicals that mimic LT action on host signaling pathways rescued the hypoinvasive phenotype of the ΔLF mutant and resulted in increased bacterial uptake. We also observed that toxin expression reduced bacterial intracellular survival by inducing the bulk degradative autophagy pathway in host cells. Finally, in a murine model of anthrax meningitis, mice infected with the ΔLF mutant exhibited no mortality, brain bacterial load, or evidence of meningitis compared to mice infected with the parental or ΔEF strains.

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.

Inter-α Inhibitor Proteins

Human inter-α inhibitor proteins are endogenous human plasma proteins that function as serine protease inhibitors. Inter-α inhibitor proteins can block the systemic release of proteases in sepsis and block furin-mediated assembly of protective antigen, an essential stop in the intracellular delivery of the anthrax exotoxins, lethal toxin and edema toxin. Inter-α inhibitor proteins administered on hour or up to 24 h after spore challenge with Bacillus anthracis Sterne strain protected mice from lethality if administered with antimicrobial therapy (P < 0.001). These human plasma proteins possess combined actions against anthrax as general inhibitors of excess serine proteases in sepsis and specific inhibitors of anthrax toxin assembly. Inter-α inhibitor proteins could represent a novel adjuvant therapy for the treatment of established anthrax infection.

A Chimeric Protein That Functions as both an Anthrax Dual-Target Antitoxin and a Trivalent Vaccine

Effective measures for the prophylaxis and treatment of anthrax are still required for counteracting the threat posed by inhalation anthrax. In this study, we first demonstrated that the chimeric protein LFn-PA, created by fusing the protective antigen (PA)-binding domain of lethal factor (LFn) to PA, retained the functions of the respective molecules. On the basis of this observation, we attempted to develop an antitoxin that targets the binding of lethal factor (LF) and/or edema factor (EF) to PA and the transportation of LF/EF. Therefore, we replaced PA in LFn-PA with a dominant-negative inhibitory PA (DPA), i.e., PA(F427D). In in vitro models of anthrax intoxication, the LFn-DPA chimera showed 3-fold and 2-fold higher potencies than DPA in protecting sensitive cells against anthrax lethal toxin (LeTx) and edema toxin (EdTx), respectively. In animal models, LFn-DPA exhibited strong potency in rescuing mice from lethal challenge with LeTx. We also evaluated the immunogenicity and immunoprotective efficacy of LFn-DPA as an anthrax vaccine candidate. In comparison with recombinant PA, LFn-DPA induced significantly higher levels of the anti-PA immune response. Moreover, LFn-DPA elicited an anti-LF antibody response that could cross-react with EF. Mice immunized with LFn-DPA tolerated a LeTx challenge that was 5 times its 50% lethal dose. Thus, LFn-DPA represents a highly effective trivalent vaccine candidate for both preexposure and postexposure vaccination. Overall, we have developed a novel and dually functional reagent for the prophylaxis and treatment of anthrax.