Autophagic Control of Listeria Determines the Infection-Induced Death of Macrophages

Listeria induces cell death in macrophages, hepatocytes and dendritic cells. It is accepted that the cell death is induced by listeriolysin O (LLO), which has a crucial role in the escape of Listeria from the vacuole. To re-evaluate the requirement of LLO for inducing cell death, we used an hlyW492A mutant that possesses a mutation in codon Trp-492 of LLO, with leads to a 1000-fold decrease in the hemolytic activity. Although the hlyW492A mutant possesses decreased hemolytic activity, it can still escape from the phagosome into the cytosol and induces cell death in macrophages. An aroA-deficient mutant that has normal LLO activity and a reduced replication capacity in the cytosol because of the deficiency of menaquinone failed to induce cell death in macrophages. The addition of menaquinone, which is a component of the electron-transfer pathway, accelerated the cytotoxicity and bacterial growth in aroA-deficient mutant-infected macrophages. These results indicate that the Listeria-infected cell death in macrophages depends on the cytosolic Listeria numberin addition to the LLO activity. Although the autophagy system fails to exclude wild-type Listeria from the infected macrophages, the aroA-deficient mutant was co-localized with autophagy marker LC3, suggesting the involvement of an autophagic mechanism to rescue the host cells when Listeria grows slowly in the cytosol. In cases with the growth of wild-type Listeria, the intermediate substances of oxidative metabolism or the rapid replication of Listeria might allow for an escape from the autophagy machinery, which induces death in the host cells. Therefore, our findings suggest that LLO is necessary for the bacteria to invade the cytosol, but is not sufficient for inducing Listeria-infected cell death in macrophages. An autophagy system apparently rescues host cells from infection-induced cell death even in the presence of LLO when the Listeria is growing slowly in the cytosol.