Intracellular type III secretion by cytoplasmic Shigella flexneri promotes caspase-1-dependent macrophage cell death.

Abstract

The Gram-negative bacterium Shigella flexneri triggers pro-inflammatory apoptotic cell death in macrophages, which is crucial for the onset of an acute inflammatory diarrhoea termed bacillary dysentery. The Mxi-Spa type III secretion system promotes bacterial uptake and escape into the cytoplasm, where, dependent on the translocator/effector protein IpaB, caspase-1 [interleukin (IL)-1beta-converting enzyme] and its substrate IL-1beta are activated. Here, we show that in the course of a macrophage infection, IpaB is secreted intracellularly for more than 1 h post-infection and progressively accumulates in aggregates on the bacterial surface. Concomitantly, the bacterial pool of IpaB is gradually depleted. The protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) dose-dependently inhibited the Mxi-Spa-dependent secretion of IpaB triggered by the dye Congo red in vitro and abolished translocation of IpaB into the host-cell cytoplasm of S. flexneri-infected macrophages. CCCP specifically inhibited S. flexneri-triggered macrophage death in a dose-dependent manner, even if added up to 60 min post-infection. Addition of CCCP 15 min after infection blocked macrophage cell death, the activation of caspase-1 and the maturation of IL-1beta, without affecting uptake or escape of S. flexneri from the phagosome. By contrast, CCCP used at the same concentration had no effect on ATP-induced caspase-1 activation or staurosporine-induced apoptosis. Our results indicate that under the conditions used, CCCP rapidly and specifically blocks bacterial type III secretion, and thus, intracellular type III secretion promotes cytotoxicity of S. flexneri.

• Authors: Gunnar N Schroeder, Naja J Jann, Hubert Hilbi
• Journal: Microbiology (Reading, England) (Microbiology (Reading)) Vol. 153 Issue Pt 9 Pg. 2862-2876 (Sep 2007) ISSN: 1350-0872 [Print] England
• PMID: 17768231 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)

Chemical References

• Bacterial Proteins
• Interleukin-1beta
• ipaB protein, Shigella
• Carbonyl Cyanide m-Chlorophenyl Hydrazone
• Caspase 1

Topics

• Animals
• Apoptosis
• Bacterial Proteins (metabolism)
• Carbonyl Cyanide m-Chlorophenyl Hydrazone (pharmacology)
• Caspase 1 (metabolism)
• Cell Line
• Cytoplasm (microbiology)
• Enzyme Activation
• Interleukin-1beta (metabolism)
• Macrophages (microbiology, physiology)
• Macrophages, Peritoneal (microbiology, physiology)
• Mice
• Mice, Inbred C57BL
• Shigella flexneri (drug effects, pathogenicity)