Macrophages are sensitive to anthrax lethal toxin through an acid-dependent process

Abstract

Anthrax lethal toxin, which consists of two proteins, protective antigen and lethal factor, is lethal for experimental animals. This study describes the first in vitro system demonstrating lethality of the toxin. Mouse peritoneal macrophages are killed within 1 h of exposure to the toxin. Neither protein component alone shows any toxic activity. The minimal effective concentration of protective antigen and lethal factor was approximately equal to 10(-2) and approximately equal to 10(-3) micrograms/ml, respectively. None of the several established cell lines examined was killed. Cells could be completely protected from the toxin by pretreatment with agents, such as amines or monensin, which dissipate intracellular proton gradients and raise the pH of intracellular vesicles. This protection was reversible and could be overcome by lowering the intravesicular pH. Antitoxin added after preincubation with amines was unable to protect cells subsequently exposed to low pH treatment. These results suggest that anthrax lethal toxin requires passage through an acidic endocytic vesicle in order to exert its toxic effect within the cytosol.