Research
The National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health, conducts and funds research to improve our ability to prevent, diagnose, and treat anthrax. Anthrax research was under way prior to the 2001 bioterror attack, but it has expanded significantly since then. New research findings are improving our understanding of how B. anthracis causes disease and how to better prevent and treat it.
Several biologic factors contribute to B. anthracis’ ability to cause disease. NIAID researchers and grantees are uncovering the molecular pathways that enable the bacterium to form spores, survive in people, and cause illness. Scientists envision this basic research to be the pathway to new vaccines, drugs, and diagnostic tools.
Natural history of anthrax
One goal of NIAID physician researchers is to look at the infectious disease process over time, from initial infection through the clinical course and beyond recovery. A small number of anthrax survivors from the 2001 attacks are enrolled in a long-term clinical study for this purpose. Many years of clinical observation are needed for this type of study to yield definitive results. Because the medical literature on anthrax does not include any findings regarding long-term complications in survivors, information gained in this study will be valuable to patients and healthcare workers.
Toxin biology
Scientists are studying anthrax toxins to learn how to block their production and action. Recently, NIAID-supported scientists have shown that protective antigen can bind edema factor and lethal factor at the same time, forming a greater variety of toxin complexes than were previously known. This finding could help researchers develop antitoxin therapies.
Previously, scientists discovered the three-dimensional molecular structure of the anthrax protective antigen protein bound to one of the receptors (CMG2) it uses to enter cells. Using a specific fragment of the CMG2 receptor protein, researchers have been able to block the attachment of protective antigen in test-tube experiments, thereby inhibiting all anthrax toxin activity.
NIAID-funded scientists also had synthesized a small cyclic molecule that blocks anthrax toxin in cell culture and in rodents. The molecule blocks the pore formed by anthrax protective antigen. Blocking the pore effectively prevents lethal factor and edema factor toxins from entering cells.
Scientists anticipate that these findings will lead to new and effective treatments.
Anthrax bacterium genome
Genes are the instructions for making proteins, which in turn build components of the cell or carry out its biochemical processes. The instructions that dictate how a microbe works are encoded within its genes. Bacteria keep most of their genes in a chromosome, a very long stretch of DNA. Smaller circular pieces of DNA called plasmids also carry genes that bacteria may exchange with each other. Because plasmids often contain genes for toxins and antibiotic resistance, knowing the DNA sequence of such plasmids is important.
Scientists have sequenced plasmids carrying the toxin genes of B. anthracis. In addition, researchers have sequenced the complete chromosomal DNA sequence of several B. anthracis strains, including one that killed a Florida man in the 2001 anthrax bioterror attack.
By comparing the DNA blueprints of different B. anthracis strains, researchers are learning why some strains are more virulent than others. Small variations among the DNA sequences of different strains may also help investigators pinpoint the origin of an anthrax outbreak.
Knowing the genetic fingerprint of B. anthracis might lead to gene-based detection mechanisms that can alert scientists to the bacteria in the environment or allow rapid diagnosis of anthrax in infected people. Variations between strains might also point to differences in antibiotic susceptibility, permitting doctors to immediately determine the appropriate treatment.
Scientists are now analyzing the B. anthracis genome sequence to determine the function of each of its genes and to learn how those genes interact with each other or with host-cell components to cause disease. Knowing the sequence of B. anthracis genes will help scientists discover key bacterial proteins that can then be targeted by new drugs or vaccines.
Spore biology
B. anthracis spores are essentially dormant and must “wake up,” or germinate, to become reproductive, disease-causing bacteria. Researchers are studying the germination process to learn more about the signals that cause spores to become active once inside an animal or person. Efforts are under way to develop models of spore germination in laboratory animals. Scientists hope those models will enable discoveries leading to drugs that block the germination process in B. anthracis spores.
Host immunity
People who contract anthrax produce antibodies to protective antigen protein. Similar antibodies appear to block infection in animals. Recent studies also suggest that some animals can produce antibodies to components of B. anthracis spores. Those antibodies, when studied in a test tube, prevent spores from germinating and increase their uptake by the immune system’s microbe-eating cells. These discoveries suggest that scientists might be able to develop a vaccine to fight both B. anthracis cells and spores.
Researchers also are studying how the immune system responds to B. anthracis infection. Part of the immune system response, known as adaptive immunity, consists of B and T cells that specifically recognize components of the anthrax bacterium. The other type of immune response—innate immunity—aims more generally to combat a wide range of microbial invaders and likely plays a key role in the body’s front-line defenses. Scientists are conducting studies of how those two arms of the immune system act to counter infection, including how B. anthracis spore germination affects individual immune responses.
In another study, NIAID-supported scientists have discovered a potential target for developing new measures to prevent and treat anthrax toxicity. Their study shows that a human gene called LRP6 plays a role in the delivery of anthrax toxins into cells. Antibodies directed against LRP6 protected cell cultures from anthrax lethal toxin. These results suggest that targeting LRP6 may prove useful in developing ways to protect against the effects of accumulated toxin.
Vaccine
NIAID is supporting research on next-generation anthrax vaccines designed to prevent infection using fewer doses than the currently licensed vaccine. The new vaccines, called recombinant protective antigen, or rPA vaccines, are based on the gene for just one anthrax toxin. These vaccines have been tested in rabbits and monkeys and have completed two phases of clinical trials in humans. The rPA vaccines appear to produce an effective immune response in people with intact immune systems. In general, the goal is to make rPA vaccines that are safer, more reliable, can be produced in large quantities, and may also be given to people with compromised immune systems.
Diagnostics
Research is under way to develop improved techniques for spotting B. anthracis in the environment and diagnosing it in infected individuals. As mentioned previously, a key part of that research is the functional genomic analysis of the bacterium, which should lead to new genetic markers for sensitive and rapid identification. Genomic analysis will also reveal differences in individual B. anthracis strains that may affect how those bacteria cause disease or respond to treatment.
Therapies
Following the discoveries of how the protective antigen and lethal factor proteins interact with cells, researchers are screening thousands of small molecules in hopes of finding an anti-anthrax drug. In addition, NIAID is working with FDA, CDC, and the Department of Defense to accelerate testing of collections of compounds for their effectiveness against inhalational anthrax. Many of those compounds already have been approved by FDA for other conditions and therefore could quickly be approved for use in treating anthrax, should they prove effective.
NIAID is also seeking new drugs that attack B. anthracis at different levels. These include agents that prevent the bacterium from attaching to cells, compounds that inhibit spore germination, and inhibitors that block the activity of key enzymes such as anthrax lethal factor. NIAID also will develop the capacity to synthesize promising anti-anthrax compounds in sufficient purity and quantity for preclinical testing.
NIAID-supported scientists have solved the structure of enzymes called sortases, which are known to anchor bacterial surface proteins to the cell walls. These enzymes may be essential to bacterial survival, and therefore could be an attractive potential target for therapies.
Scientists have designed a compound that blocks anthrax toxins from attaching to receptors on the surface of host cells in animal models. If the toxin cannot attach to and enter the cell, it is effectively neutralized. The new inhibitor is much more potent than current therapies and shows promise against some antibiotic-resistant strains as well. The general concept could also be applied to designing inhibitors for other pathogens.
Researchers have also found that human monoclonal antibodies protect against inhalation anthrax in three animal models. New anthrax therapies, such as monoclonal and polyclonal antibodies that can neutralize anthrax toxins, are being further developed.
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