Anthrax is a serious infectious disease caused by the bacterium Bacillus anthracis. The severe and often lethal effects of anthrax are primarily due to potent toxins it produces. These compounds interfere with normal biological processes within the host, leading to the characteristic symptoms of the disease. Understanding these chemical agents helps explain how anthrax causes harm and how its effects can be counteracted.
The Chemical Agents of Anthrax
Bacillus anthracis secretes a tripartite exotoxin, composed of three distinct protein components. These proteins are individually non-toxic, but when combined, they form the active anthrax toxins. The three components are Protective Antigen (PA), Lethal Factor (LF), and Edema Factor (EF).
Protective Antigen (PA) is an 83-kilodalton (kDa) protein that serves as the cell-binding and delivery component of the toxin. It acts as the “key” that allows the other two components to enter host cells. Lethal Factor (LF) is a 90-kDa protein with enzymatic activity, specifically a zinc-dependent metalloprotease. Edema Factor (EF) is an 89-kDa protein that functions as a calmodulin-dependent adenylate cyclase. These three proteins are secreted by the bacterium during infection and mediate disease progression.
How Anthrax Toxins Cause Harm
The individual toxin components, Protective Antigen (PA), Lethal Factor (LF), and Edema Factor (EF), must work in concert to inflict cellular damage. The process begins when PA binds to specific receptors on the surface of host cells, such as tumor endothelium marker-8 (TEM8) and capillary morphogenesis protein 2 (CMG2). Once bound, cellular proteases, like furin, cleave PA83, leaving behind a 63-kDa fragment called PA63.
The remaining PA63 fragments then assemble into a ring-shaped structure on the cell surface. This oligomerized PA63 forms a “pre-pore” structure, which then binds to LF and/or EF. The entire complex is subsequently internalized by the cell through endocytosis, enclosed within an endosome. The acidic environment within the endosome causes a conformational change in the PA63 pre-pore, transforming it into a functional pore that spans the endosomal membrane. This pore facilitates the translocation of LF and EF from the endosome into the cell’s cytoplasm.
Once inside the cytoplasm, LF and EF exert their damaging effects. Lethal Factor (LF) acts as an enzyme that cleaves specific proteins called mitogen-activated protein kinase kinases (MAPKKs), thereby disrupting important cellular signaling pathways. This disruption can lead to cell death, particularly in immune cells like macrophages. Edema Factor (EF) increases the levels of cyclic AMP (cAMP) inside the cell. This excessive cAMP disrupts cellular water balance and signaling pathways, leading to fluid accumulation and swelling characteristic of the edema seen in anthrax infections.
Detecting and Neutralizing Anthrax Toxins
Detecting anthrax toxins involves identifying these specific protein components in various samples. Immunoassays, such as ELISA (Enzyme-Linked Immunosorbent Assay), are commonly used to detect the presence of Protective Antigen (PA), Lethal Factor (LF), or Edema Factor (EF) proteins directly. These tests rely on antibodies that specifically bind to the toxin components, providing a direct indication of their presence. Methods that assess the enzymatic activity of LF and EF can also detect toxin presence, offering high sensitivity for early infection detection in plasma.
Molecular methods, such as Polymerase Chain Reaction (PCR), can identify the genes that encode these toxins, indicating the presence of toxin-producing Bacillus anthracis. Mass spectrometry is another specific and sensitive technique used to detect LF protein in clinical samples like serum or plasma. Plasma is often the preferred sample for LF toxin testing and can be collected up to 18 days after suspected exposure or symptom onset.
Neutralizing anthrax toxins involves strategies that target the toxins themselves or prevent their entry into cells. Antitoxins, which are antibodies, can bind to and neutralize the toxins, particularly Protective Antigen (PA), preventing them from interacting with host cells. These antitoxins are administered as a treatment, often in conjunction with antibiotics, especially in severe cases where toxins have already been released.
Anthrax vaccines primarily work by stimulating the body to produce antibodies against Protective Antigen (PA). By targeting PA, the vaccine helps the immune system generate a protective response that can neutralize the toxin before it can enter and damage host cells. The currently available human vaccine, Anthrax Vaccine Adsorbed (AVA), aims to induce these toxin-neutralizing antibodies, which are considered a reliable indicator of protection against lethal toxin challenge.