Cholesterol is required to trigger caspase-1 activation and macrophage apoptosis after phagosomal escape of Shigella

Pro-inflammatory macrophage apoptosis is pivotal in the aetiology of bacillary dysentery, an acute inflammatory diarrhoea caused by Shigella spp. S. flexneri triggers its uptake by macrophages, escapes the phagosome and kills the host cell by a cytotoxic pathway, which activates and requires caspase-1 [interleukin (IL)-1 beta-converting enzyme] and releases mature IL-1 beta. The bacterial type III-secreted translocator/effector protein IpaB triggers cell death and directly binds to caspase-1. Here, we demonstrate that in S. flexneri-infected macrophages, activated caspase-1 is present in the cytoplasm, the nucleus and on vesicular membranes. IpaB partitions with membrane and cytoplasmic fractions and colocalizes with activated caspase-1 on the surface of bacteria, in the macrophage cytoplasm and on vesicular membranes. Macrophages treated with the cholesterol-sequestering compound methyl-beta-cyclodextrin (MCD) were depleted from cholesterol within minutes and were impaired for phagocytosis of S. flexneri. Consequently, cytotoxicity as determined by lactate dehydrogenase release was blocked. Interestingly, if MCD was added 15-30 min post infection, cytotoxicity, activation of caspase-1, and apoptosis were inhibited, while phagocytosis of the bacteria, escape from the phagosome and type III secretion of IpaB was not affected. Inhibition of Shigella cytotoxicity by MCD coincided with a reduced association of IpaB to host cell membranes. Contrarily, the activation of caspase-1 and cytotoxicity triggered by the K+/H+ antiport ionophore nigericin or by ATP was not affected or even increased by MCD. These results indicate that cholesterol is specifically required for caspase-1 activation and apoptosis triggered by Shigella after the escape from phagosomes, and suggest that membrane association of IpaB contributes to the activation of caspase-1.