Anthrax Lethal Toxin Rapidly Activates Caspase-1/ICE and Induces Extracellular Release of Interleukin (IL)-1β and IL-18

Anthrax lethal toxin (LT), a critical virulence factor for Bacillus anthracis, has been demonstrated to cleave and to inactivate mitogen-activated protein kinase kinases (MAPKKs) that propagate prosurvival signals in macrophages (1-5). Whether this action of anthrax LT leads to the production of proinflammatory cytokines by macrophages has been more controversial (6, 7). We now report that anthrax LT treatment leads to the specific extracellular release of interleukin (IL)-1 beta and IL-18 by the murine macrophage cell lines, RAW264.7 and J774A.1. Studies of the processing of IL-1 beta reveal that the levels of activated/cleaved IL-1 beta in RAW264.7 and J774.A1 cells are increased following treatment with anthrax LT. Enhanced processing of IL-1 beta directly correlates with increased levels in the activation of its upstream regulator, IL-1 beta -converting enzyme/Caspase-1 (ICE). The extracellular release of IL-1 beta and IL-18 in response to anthrax LT is ICE-dependent, as an ICE-specific inhibitor blocks this process. These data indicate that ICE, IL- 1 beta , and IL-18 are downstream effectors of anthrax LT in macrophages, providing the basis for new bioassays for anthrax LT activity and representing potential therapeutic targets.