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Lethal toxin of Bacillus anthracis causes apoptosis of macrophages
Serguei G Popov1, Rafael Villasmil, Jessica Bernardi
1Hadron Advanced Biosystems, Inc., 10900 University Boulevard, MSN 1A8, Manassas, VA 20110, USA. serguei.popov@hadron.com
Abstract:
Lethal toxin is a major anthrax virulence factor, causing the rapid death of experimental animals. Lethal toxin can enter most cell types, but only certain macrophages and cell lines are susceptible to toxin-mediated cytolysis. We have shown that in murine RAW 264.7 cells, sublytic amounts of lethal toxin trigger intracellular signaling events typical for apoptosis, including changes in membrane permeability, loss of mitochondrial membrane potential, and DNA fragmentation. The cells were protected from the toxin by specific inhibitors of caspase-1, -2, -3, -4, -6, and -8. Phagocytic activity of macrophages was inhibited by sublytic concentrations of lethal toxin. Infection of cells with anthrax (Sterne) spores impaired their bactericidal capacity, which could be reversed by a lethal toxin inhibitor, bestatin. We suggest that apoptosis rather than direct lysis is biologically relevant to lethal toxin intracellular activity.
Insights
Anthrax lethal toxin induces apoptosis in macrophages, not direct lysis. This finding reveals a key mechanism of anthrax pathogenesis and potential therapeutic targets.
Area of Science:
- Cell Biology
- Microbiology
- Immunology
Background:
- Bacillus anthracis lethal toxin is a primary virulence factor responsible for severe disease.
- While lethal toxin affects many cell types, only specific macrophages exhibit sensitivity to its cytolytic effects.
- Understanding the precise cellular mechanisms of lethal toxin action is crucial for developing effective countermeasures.
Purpose of the Study:
- To investigate the intracellular signaling pathways triggered by sublytic doses of anthrax lethal toxin in murine RAW 264.7 cells.
- To determine whether lethal toxin induces apoptosis or direct cell lysis.
- To assess the impact of lethal toxin on macrophage phagocytic activity and bactericidal capacity.
Main Methods:
- Treatment of RAW 264.7 cells with sublytic amounts of lethal toxin.
- Assessment of apoptosis markers: membrane permeability, mitochondrial membrane potential, and DNA fragmentation.
- Inhibition studies using specific caspase inhibitors.
- Evaluation of macrophage phagocytosis and bactericidal function following infection with anthrax Sterne spores.
Main Results:
- Sublytic lethal toxin exposure initiated apoptosis-like intracellular events, including membrane permeability changes, mitochondrial dysfunction, and DNA fragmentation.
- Specific caspase inhibitors (caspase-1, -2, -3, -4, -6, -8) protected cells from toxin-induced damage.
- Lethal toxin inhibited macrophage phagocytic activity.
- Infection with anthrax spores impaired macrophage bactericidal capacity, an effect reversed by the lethal toxin inhibitor bestatin.
Conclusions:
- Apoptosis, rather than direct cell lysis, is the biologically relevant intracellular activity of anthrax lethal toxin.
- Lethal toxin's interference with macrophage function contributes to anthrax pathogenesis.
- Targeting apoptosis pathways or inhibiting lethal toxin may represent viable therapeutic strategies against anthrax.