Macrophage interactions

Anthrax Toxins in Context of Bacillus anthracis Spores and Spore Germination

The interaction of anthrax toxin or toxin components with B. anthracis spores has been demonstrated. Germinating spores can produce significant amounts of toxin components very soon after the initiation of germination. In this review, we will summarize the work performed that has led to our understanding of toxin and spore interactions and discuss the complexities associated with these interactions.

Cryo-EM analysis of the organization of BclA and BxpB in the Bacillus anthracis exosporium

Bacillus anthracis and other pathogenic Bacillus species form spores that are surrounded by an exosporium, a balloon-like layer that acts as the outer permeability barrier of the spore and contributes to spore survival and virulence. The exosporium consists of a hair-like nap and a paracrystalline basal layer. The filaments of the nap are comprised of trimers of the collagen-like glycoprotein BclA, while the basal layer contains approximately 20 different proteins. One of these proteins, BxpB, forms tight complexes with BclA and is required for attachment of essentially all BclA filaments to the basal layer. Another basal layer protein, ExsB, is required for the stable attachment of the exosporium to the spore. To determine the organization of BclA and BxpB within the exosporium, we used cryo-electron microscopy, cryo-sectioning and crystallographic analysis of negatively stained exosporium fragments to compare wildtype spores and mutant spores lacking BclA, BxpB or ExsB (ΔbclA, ΔbxpB and ΔexsB spores, respectively). The trimeric BclA filaments are attached to basal layer surface protrusions that appear to be trimers of BxpB. The protrusions interact with a crystalline layer of hexagonal subunits formed by other basal layer proteins. Although ΔbxpB spores retain the hexagonal subunits, the basal layer is not organized with crystalline order and lacks basal layer protrusions and most BclA filaments, indicating a central role for BxpB in exosporium organization.

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.

Cost-effectiveness comparison of response strategies to a large-scale anthrax attack on the chicago metropolitan area: impact of timing and surge capacity

Rapid public health response to a large-scale anthrax attack would reduce overall morbidity and mortality. However, there is uncertainty about the optimal cost-effective response strategy based on timing of intervention, public health resources, and critical care facilities. We conducted a decision analytic study to compare response strategies to a theoretical large-scale anthrax attack on the Chicago metropolitan area beginning either Day 2 or Day 5 after the attack. These strategies correspond to the policy options set forth by the Anthrax Modeling Working Group for population-wide responses to a large-scale anthrax attack: (1) postattack antibiotic prophylaxis, (2) postattack antibiotic prophylaxis and vaccination, (3) preattack vaccination with postattack antibiotic prophylaxis, and (4) preattack vaccination with postattack antibiotic prophylaxis and vaccination. Outcomes were measured in costs, lives saved, quality-adjusted life-years (QALYs), and incremental cost-effectiveness ratios (ICERs). We estimated that postattack antibiotic prophylaxis of all 1,390,000 anthrax-exposed people beginning on Day 2 after attack would result in 205,835 infected victims, 35,049 fulminant victims, and 28,612 deaths. Only 6,437 (18.5%) of the fulminant victims could be saved with the existing critical care facilities in the Chicago metropolitan area. Mortality would increase to 69,136 if the response strategy began on Day 5. Including postattack vaccination with antibiotic prophylaxis of all exposed people reduces mortality and is cost-effective for both Day 2 (ICER=$182/QALY) and Day 5 (ICER=$1,088/QALY) response strategies. Increasing ICU bed availability significantly reduces mortality for all response strategies. We conclude that postattack antibiotic prophylaxis and vaccination of all exposed people is the optimal cost-effective response strategy for a large-scale anthrax attack. Our findings support the US government's plan to provide antibiotic prophylaxis and vaccination for all exposed people within 48 hours of the recognition of a large-scale anthrax attack. Future policies should consider expanding critical care capacity to allow for the rescue of more victims.

Glycoconjugates prevent B. anthracis toxin-induced cell death through binding while activating macrophages

Bacillus anthracis toxins may be attenuated if macrophages could neutralize toxins upon contact or exposure. Glycoconjugate-bearing polymers, which have been shown to bind to Bacillus spores, were tested for recognition and binding of protective antigen (PA), lethal factor (LF), and edema factor (EF) toxins. We have demonstrated modulation of macrophage activity following exposure to these toxins. Without glycoconjugate (GC) activation, murine macrophages were killed by Bacillus toxins. GCs were shown to have a protective influence, sparing macrophages from toxin-induced cell death, as shown by increased macrophage cell viability based on trypan blue assay. Increased levels of inducible nitric oxide (NO) production by macrophages in presence of GCs suggest that GCs provide an activation signal for macrophages and stimulate their function. Results hint to GCs that promote neutralization of Bacillus toxins, block toxin-induced macrophage death, while increasing macrophage activation. Polymeric GCs may suggest novel approaches to improve existing or develop new vaccines as well as immunotherapeutics.

Bacillus anthracis spore interactions with mammalian cells: relationship between germination state and the outcome of in vitro

Background

During inhalational anthrax, internalization of Bacillus anthracis spores by host cells within the lung is believed to be a key step for initiating the transition from the localized to disseminated stages of infection. Despite compelling in vivo evidence that spores remain dormant within the bronchioalveolar spaces of the lungs, and germinate only after uptake into host cells, most in vitro studies of infection have been conducted under conditions that promote rapid germination of spores within the culture medium.

Results

Using an in vitro model of infection, we evaluated the influence of the germination state of B. anthracis spores, as controlled by defined culture conditions, on the outcome of infection. Spores prepared from B. anthracis Sterne 7702 germinated in a variety of common cell culture media supplemented with fetal bovine serum (FBS) while, in the absence of FBS, germination was strictly dependent on medium composition. RAW264.7 macrophage-like cells internalized spores to the same extent in either germinating or non-germinating media. However, significantly more viable, intracellular B. anthracis were recovered from cells infected under non-germinating conditions compared to germinating conditions. At the same time, RAW264.7 cells demonstrated a significant loss in viability when infected under non-germinating conditions.

Conclusions

These results suggest that the outcome of host cell infection is sensitive to the germination state of spores at the time of uptake. Moreover, this study demonstrates the efficacy of studying B. anthracis spore infection of host cells within a defined, non-germinating, in vitro environment.

Development of automated amperometric detection of antibodies against Bacillus anthracis protective antigen

Picogram levels of antibodies against the protective antigen (PA) of Bacillus anthracis were detected in an automated electrochemical sandwich-type enzyme-linked immunosorbant assay. The antibodies were captured and detected using an 8 x 3 array of 50-microm-diameter cavities. The reagent and sample volumes were as low as 200 nL in a less than 25-min assay from capture to signal generation. The electrochemical detection of the antibodies was demonstrated at 0.05-10 microg/mL containing only 10-5,000 pg antibodies. The limit of detection is 10 fg for a 200-nL sample. Detection of anti-PA immunoglobulin G performed in spiked normal human serum and fresh whole human blood did not show a significant difference from detection in a buffer. The initial automation of the assay involved the use of a digital syringe pump for the delivery of reagents to the capture surface.

Protective antigen as a correlative marker for anthrax in animal models

The most aggressive form of anthrax results from inhalation of airborne spores of Bacillus anthracis and usually progresses unnoticed in the early stages because of unspecific symptoms. The only reliable marker of anthrax is development of bacteremia, which increases with disease progress. Rapid diagnosis of anthrax is imperative for efficient treatment and cure. Herein we demonstrate that the presence and level of a bacterial antigen, the protective antigen (PA), a component of B. anthracis toxins, in host sera can serve as a reliable marker of infection. This was tested in two animal models of inhalation anthrax, rabbits and guinea pigs infected by intranasal instillation of Vollum spores. In both models, we demonstrated qualitative and quantitative correlations between levels of bacteremia and PA concentrations in the sera of sick animals. The average time to death in infected animals was about 16 h after the appearance of bacteremia, leaving a small therapeutic window. As the time required for immunodetection of PA can be very short, the use of this marker will be beneficial for faster diagnosis and treatment of inhalation anthrax.

Sensor domains encoded in Bacillus anthracis virulence plasmids prevent sporulation by hijacking a sporulation sensor histidine kinase

Anthrax toxin and capsule, determinants for successful infection by Bacillus anthracis, are encoded on the virulence plasmids pXO1 and pXO2, respectively. Each of these plasmids also encodes proteins that are highly homologous to the signal sensor domain of a chromosomally encoded major sporulation sensor histidine kinase (BA2291) in this organism. B. anthracis Sterne overexpressing the plasmid pXO2-61-encoded signal sensor domain exhibited a significant decrease in sporulation that was suppressed by the deletion of the BA2291 gene. Expression of the sensor domains from the pXO1-118 and pXO2-61 genes in Bacillus subtilis strains carrying the B. anthracis sporulation sensor kinase BA2291 gene resulted in BA2291-dependent inhibition of sporulation. These results indicate that sporulation sensor kinase BA2291 is converted from an activator to an inhibitor of sporulation in its native host by the virulence plasmid-encoded signal sensor domains. We speculate that activation of these signal sensor domains contributes to the initiation of B. anthracis sporulation in the bloodstream of its infected host, a salient characteristic in the virulence of this organism, and provides an additional role for the virulence plasmids in anthrax pathogenesis.

Rap phosphatase of virulence plasmid pXO1 inhibits Bacillus anthracis sporulation

This study shows that the Bacillus anthracis pXO1 virulence plasmid carries a Rap-Phr system, BXA0205, which regulates sporulation initiation in this organism. The BXA0205Rap protein was shown to dephosphorylate the Spo0F response regulator intermediate of the phosphorelay signal transduction system that regulates the initiation of the developmental pathway in response to environmental, metabolic, and cell cycle signals. The activity of the Rap protein was shown to be inhibited by the carboxy-terminal pentapeptide generated through an export-import processing pathway from the associated BXA0205Phr protein. Deregulation of the Rap activity by either overexpression or lack of the Phr pentapeptide resulted in severe inhibition of sporulation. Five additional Rap-Phr encoding systems were identified on the chromosome of B. anthracis, one of which, BA3790-3791, also affected sporulation initiation. The results suggest that the plasmid-borne Rap-Phr system may provide a selective advantage to the virulence of B. anthracis.

Human neutrophils kill Bacillus anthracis

Bacillus anthracis spores cause natural infections and are used as biological weapons. Inhalation infection with B. anthracis, the etiological agent of anthrax, is almost always lethal, yet cutaneous infections usually remain localized and resolve spontaneously. Neutrophils are typically recruited to cutaneous but seldom to other forms of anthrax infections, raising the possibility that neutrophils kill B. anthracis. In this study we infected human neutrophils with either spores or vegetative bacteria of a wild-type strain, or strains, expressing only one of the two major virulence factors. The human neutrophils engulfed B. anthracis spores, which germinated intracellularly and were then efficiently killed. Interestingly, neutrophil killing was independent of reactive oxygen species production. We fractionated a human neutrophil granule extract by high-performance liquid chromatography and identified α-defensins as the component responsible for B. anthracis killing. These data suggest that the timely recruitment of neutrophils can control cutaneous infections and possibly other forms of B. anthracis infections, and that α-defensins play an important role in the potent anti-B. anthracis activity of neutrophils.

Bacillus anthracis is the bacterium that causes anthrax, a disease that can occur through natural infections and also through intentional release. B. anthracis makes spores, which are in a dormant state, similar to seeds of a plant, and are extremely resistant to the environment. B. anthracis spores can infect through the skin or the lung. Lung infections disseminate through the body and are lethal. In contrast, skin infections often remain localized, and patients survive even without treatment. It is not well understood why these bacteria cause a localized infection through the skin and a lethal disease through the lung.

Little is known about how B. anthracis is controlled. Neutrophils are the first white blood cells recruited to a site of infection and are specialized in killing microbes. Previous studies show that neutrophils are abundant in the skin form, but not in the lung form of anthrax. The researchers report that human neutrophils can take up B. anthracis spores. Once inside, the spores germinate to form vegetative bacteria. The vegetative bacteria are extremely susceptible to neutrophil-killing mechanisms. The B. anthracis virulence factors (molecules that make bacteria cause diseases) manipulate other human cells but do not deter neutrophils. B. anthracis is indeed exquisitely sensitive to the neutrophil protein α-defensin. These data support a new model where B. anthracis skin, but not lung, infections are controlled by the antimicrobial activity of neutrophils.

Anthrax toxins and the host: a story of intimacy

Although the dramatic events of the year 2001 have revitalized the interest in anthrax, research on Bacillus anthracis and its major virulence factors is one of the oldest theme in microbiology and started with the early works of Robert Koch and Louis Pasteur. The anthrax toxins are central to anthrax pathogenesis. They were discovered in the mid-1950s and since then there has been an enormous amount of work to elucidate both the molecular and physiopathological details of their mode of action. In this review, after a brief introduction of B. anthracis, we will focus on the latest findings that concern two aspects of anthrax toxin research: the environmental signals and the molecular mechanisms that regulate toxin synthesis, and the mechanisms of intoxication. We hope to convince the reader that the anthrax toxins are highly specialized determinants of B. anthracis pathogenicity: their synthesis is integrated within a global virulence programme and they target key eukaryotic cell proteins. We conclude with a consideration of the therapeutic perspectives arising from our current knowledge of how the toxins work.