Anthrax Evasion of the Immune System
If there is one thing that B. anthracis does well, it is evading and confounding the immune system. The sections on innate immunity, humoral immunity, and cell mediated immunity all delve into that concept as well. Here, the ability of B. anthracis of avoiding destruction by the immune system will be looked at in (relatively) brief summary, with specific attention being devoted to the capsule, protective antigen, lethal factor, and edema factor.
The capsule of B. anthracis is composed of poly-D glutamic acid, and is part of what makes it such a successful pathogen. It is this capsule that allows B. anthracis to climb into a macrophage and ride it into the lymph node, where the bacteria can divide and multiply. The capsule is able to inhibit macrophage phagocytosis due to its negative charge. Enzymes that synthesize the capsule are encoded on a plasmid called pX02. The enzymes are membrane associated, and are synthesized in response to elevated carbon dioxide. The regulation depends on bicarbonate interaction with the transcripts of atxA, a gene on pX01, the toxin encoding plasmid (25).
Protective antigen (PA) is an 83 kDa protein that is cleaved into a 63 kDa fragment in vivo called PA63 (4). PA63 forms a heptamer which can bind to host cells by an unknown surface receptor (26). The heptamer is referred to as a prepore, because at low pH, it forms a pore in the cell surface (4). The prepore serves as a binding site for both edema factor and lethal factor. There seem to be different binding sites on PA for edema factor and lethal factor (27), and each monomer in the heptamer binds one molecule of edema factor or lethal factor. The complex is internalized by the targeted cell, at which point it converts from the prepore to the pore due to acidic conditions. The pore then allows the bound edema factor and lethal factor to leak into the cytoplasm, where they begin to do their work (4).
Action of protective antigen. Image found at http://pathport.vbi.vt.edu/pathinfo/pathogens/Bacillus-anthracis.html
Edema factor (EF) is an adenylate cyclase, which converts ATP into cAMP, with the goal of dramatically increasing intracellular cAMP concentrations. It has been shown to cause edema in the skin of rabbits and guinea pigs when introduced with PA (28). The goal of increasing cAMP is altering secretion of TNF-a and IL-6 by macrophages. Macrophages exposed to EF are unable to generate TNF-a in response to LPS stimulation, suggesting impairment of the innate immune response.
Crystal structure of anthrax edema factor. Image found at http://www.bnl.gov/bnlweb/pubaf/pr/2002/bnlpr040502.htm
Lethal factor (LF) is the most studied of the anthrax toxins. It has been characterized as a zinc-metalloprotease (4). Combined with PA, it forms lethal toxin (LeTx). LeTx is an elegant defense for B. anthracis against host immunity, as it targets the MAPK kinases that are ubiquitous to almost every immune system cell and are critical in immune cell signaling. For example, the toll like receptors (TLRs) on macrophages are a potent first line of defense against infection, with many pathogen interactions having been identified. The diverse set of receptors, however, almost all function through the NFkB and MAPK pathways. These signaling cascades can promote secretion of inflammatory cytokines, MHC I and II, co-stimulatory molecules, and antimicrobial peptides. In this way, LeTx disables many of the macrophage’s abilities to respond to the presence of pathogen (9).
Anthrax Lethal Factor. Image found at http://www.nature.com/nature/journal/v414/n6860/fig_tab/414229a0_F1.html
LeTx also attacks dendritic cells, cutting off the bridge from innate immunity to adaptive immunity. Dendritic cells maturing in the presence of LeTx are not able to activate T-cells. In vivo this translates to an absolute absence of adaptive immunity to anthrax, severely decreasing the ability to clear in the infection (9, 18). Also, LeTx causes the apoptosis of activated macrophages, also by cleaving MAPK kinases. By targeting activated macrophages for apoptosis, the bacteria kills any of the immune system’s guards that may have recognized it (17). Finally, LeTx can also directly target and disable cytoxic T lymphocytes (23) as well as B-cells (20), again by cleaving MAPK kinases and disrupting cell signaling.
Above is only a small sampling of the ability of anthrax to subvert the efforts of the immune system to destroy it. More specific examples are found in each of the other sections of the website. It is clear that a large part of what makes B. anthracis so dangerous and lethal is how well it can evade detection by the immune system.