Fluid support worsens outcome and negates the benefit of protective antigen-directed monoclonal antibody in a lethal toxin-infused rat Bacillus anthracis shock model

A Review of the Efficacy of FDA-Approved B. anthracis Anti-Toxin Agents When Combined with Antibiotic or Hemodynamic Support in Infection- or Toxin-Challenged Preclinical Models

Anti-toxin agents for severe B. anthracis infection will only be effective if they add to the benefit of the two mainstays of septic shock management, antibiotic therapy and titrated hemodynamic support. Both of these standard therapies could negate benefits related to anti-toxin treatment. At present, three anthrax anti-toxin antibody preparations have received US Food and Drug Administration (FDA) approval: Raxibacumab, Anthrax Immune Globulin Intravenous (AIGIV) and ETI-204. Each agent is directed at the protective antigen component of lethal and edema toxin. All three agents were compared to placebo in antibiotic-treated animal models of live B. anthracis infection, and Raxibacumab and AIGIV were compared to placebo when combined with standard hemodynamic support in a 96 h canine model of anthrax toxin-associated shock. However, only AIG has actually been administered to a group of infected patients, and this experience was not controlled and offers little insight into the efficacy of the agents. To provide a broader view of the potential effectiveness of these agents, this review examines the controlled preclinical experience either in antibiotic-treated B. anthracis models or in titrated hemodynamic-supported toxin-challenged canines. The strength and weaknesses of these preclinical experiences are discussed.

Anthrax Pathogenesis

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

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.

Two anthrax cases with soft tissue infection, severe oedema and sepsis in Danish heroin users

Background

Anthrax had become extremely rare in Europe, but in 2010 an outbreak of anthrax among heroin users in Scotland increased awareness of contaminated heroin as a source of anthrax. We present the first two Danish cases of injectional anthrax and discuss the clinical presentations, which included both typical and more unusual manifestations.

Case presentations

The first patient, a 55-year old man with HIV and hepatitis C virus co-infection, presented with severe pain in the right thigh and lower abdomen after injecting heroin into the right groin. Computed tomography and ultrasonographic examination of the abdomen and right thigh showed oedematous thickened peritoneum, distended oedematous mesentery and subcutaneous oedema of the right thigh. At admission the patient was afebrile but within 24 hours he progressed to severe septic shock and abdominal compartment syndrome. Cultures of blood and intraperitoneal fluid grew Bacillus anthracis. The patient was treated with meropenem, clindamycin, ciprofloxacin and metronidazole. Despite maximum supportive care including mechanical ventilation, vasopressor treatment and continuous veno-venous hemodiafiltration the patient died on day four.

The second patient, a 39-year old man with chronic hepatitis C virus infection, presented with fever and a swollen right arm after injecting heroin into his right arm. The arm was swollen from the axilla to the wrist with tense and discoloured skin. He was initially septic with low blood pressure but responded to crystalloids. During the first week, swelling progressed and the patient developed massive generalised oedema with a weight gain of 40 kg. When blood cultures grew Bacillus anthracis antibiotic treatment was changed to meropenem, moxifloxacin and metronidazole, and on day 7 hydroxycloroquin was added. The patient responded to treatment and was discharged after 29 days.

Conclusions

These two heroin-associated anthrax cases from Denmark corroborate that heroin contaminated with anthrax spores may be a continuous source of injectional anthrax across Europe. Clinicians and clinical microbiologists need to stay vigilant and suspect anthrax in patients with a history of heroin use who present with soft tissue or generalised infection. Marked swelling of affected soft tissue or unusual intra-abdominal oedema should strengthen clinical suspicion.

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.

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.

Anthrax and the inflammasome

Anthrax lethal toxin (LT), a major virulence determinant of anthrax disease, induces vascular collapse in mice and rats. LT activates the Nlrp1 inflammasome in macrophages and dendritic cells, resulting in caspase-1 activation, IL-1β and IL-18 maturation and a rapid cell death (pyroptosis). This review presents the current understanding of LT-induced activation of Nlrp1 in cells and its consequences for toxin-mediated effects in rodent toxin and spore challenge models.

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.

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.

The individual survival benefits of tumor necrosis factor soluble receptor and fluid administration are not additive in a rat sepsis model

BACKGROUND

Tumor necrosis factor (TNF) antagonists [e.g., TNF soluble receptor (TNFsr)] improved survival in preclinical but not clinical sepsis trials. However fluid support-itself beneficial-is standard clinically but rarely employed in preclinical sepsis models. We hypothesized that these therapies may not have additive benefit.

METHODS AND RESULTS

Antibiotic-treated rats (n = 156) were randomized to intratracheal or intravenous Escherichia coli challenges (>LD50) and either placebo or TNFsr and 24 h fluid treatments alone or together. The survival effects of these therapies did not differ significantly comparing challenge routes. When averaged across route, while TNFsr or fluid alone decreased the hazard ratio of death significantly [ln ± standard error (SE): -0.65 ± 0.30 and -0.62 ± 0.30, respectively, p ≤ 0.05], together they did not (p = 0.16). Furthermore, the observed effect of TNFsr and fluid together on reducing the hazard ratio was significantly less than estimated (-0.37 ± 0.29 versus -1.27 ± 0.43, respectively, p = 0.027) based on TNFsr and fluid alone. While each treatment increased central venous pressure at 6 and 24 h, the observed effects of the combination were also less than estimated ones (p ≤ 0.0005).

CONCLUSIONS

The individual survival benefits of TNFsr and fluids were not additive in this rat sepsis model. Investigating new sepsis therapies together with conventional ones during preclinical testing may be informative.

Proteasome inhibitors prevent caspase-1-mediated disease in rodents challenged with anthrax lethal toxin

NOD-like receptors (NLRs) and caspase-1 are critical components of innate immunity, yet their over-activation has been linked to a long list of microbial and inflammatory diseases, including anthrax. The Bacillus anthracis lethal toxin (LT) has been shown to activate the NLR Nalp1b and caspase-1 and to induce many symptoms of the anthrax disease in susceptible murine strains. In this study we tested whether it is possible to prevent LT-mediated disease by pharmacological inhibition of caspase-1. We found that caspase-1 and proteasome inhibitors blocked LT-mediated caspase-1 activation and cytolysis of LT-sensitive (Fischer and Brown-Norway) rat macrophages. The proteasome inhibitor NPI-0052 also prevented disease progression and death in susceptible Fischer rats and increased survival in BALB/c mice after LT challenge. In addition, NPI-0052 blocked rapid disease progression and death in susceptible Fischer rats and BALB/c mice challenged with LT. In contrast, Lewis rats, which harbor LT-resistant macrophages, showed no signs of caspase-1 activation after LT injection and did not exhibit rapid disease progression. Taken together, our findings indicate that caspase-1 activation is critical for rapid disease progression in rodents challenged with LT. Our studies indicate that pharmacological inhibition of NLR signaling and caspase-1 can be used to treat inflammatory diseases.

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.

The heart is an early target of anthrax lethal toxin in mice: a protective role for neuronal nitric oxide synthase (nNOS)

Anthrax lethal toxin (LT) induces vascular insufficiency in experimental animals through unknown mechanisms. In this study, we show that neuronal nitric oxide synthase (nNOS) deficiency in mice causes strikingly increased sensitivity to LT, while deficiencies in the two other NOS enzymes (iNOS and eNOS) have no effect on LT-mediated mortality. The increased sensitivity of nNOS-/- mice was independent of macrophage sensitivity to toxin, or cytokine responses, and could be replicated in nNOS-sufficient wild-type (WT) mice through pharmacological inhibition of the enzyme with 7-nitroindazole. Histopathological analyses showed that LT induced architectural changes in heart morphology of nNOS-/- mice, with rapid appearance of novel inter-fiber spaces but no associated apoptosis of cardiomyocytes. LT-treated WT mice had no histopathology observed at the light microscopy level. Electron microscopic analyses of LT-treated mice, however, revealed striking pathological changes in the hearts of both nNOS-/- and WT mice, varying only in severity and timing. Endothelial/capillary necrosis and degeneration, inter-myocyte edema, myofilament and mitochondrial degeneration, and altered sarcoplasmic reticulum cisternae were observed in both LT-treated WT and nNOS-/- mice. Furthermore, multiple biomarkers of cardiac injury (myoglobin, cardiac troponin-I, and heart fatty acid binding protein) were elevated in LT-treated mice very rapidly (by 6 h after LT injection) and reached concentrations rarely reported in mice. Cardiac protective nitrite therapy and allopurinol therapy did not have beneficial effects in LT-treated mice. Surprisingly, the potent nitric oxide scavenger, carboxy-PTIO, showed some protective effect against LT. Echocardiography on LT-treated mice indicated an average reduction in ejection fraction following LT treatment in both nNOS-/- and WT mice, indicative of decreased contractile function in the heart. We report the heart as an early target of LT in mice and discuss a protective role for nNOS against LT-mediated cardiac damage.

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.

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.

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.