Human monocytes kill Shigella flexneri but then die by apoptosis associated with suppression of proinflammatory cytokine production.

Shigella flexneri infection of human macrophages is followed by rapid bacterial escape into the cytosol and secretion of IpaB, which activates caspase-1 to mediate cell death and release of mature interleukin (IL)-1 beta. Here we report a different outcome following infection of human peripheral blood monocytes. S. flexneri infects monocytes inefficiently in the absence of complement and, following complement-dependent uptake, cannot escape the endosomal compartment. Consequently, bacteria are killed within the first 60 min in the absence of monocyte cell death, as demonstrated by immunofluorescence and electron microscopy and enumeration of colonies in a gentamicin protection assay. Despite early bacterial death, wild-type S. flexneri influenced the subsequent monocyte proinflammatory cytokine response and cell fate. Infection with wild-type S. flexneri resulted in IpaB-dependent suppression of IL-1 beta, tumor necrosis factor alpha, and IL-6 compared with that of plasmid-cured avirulent S. flexneri-infected cells. Furthermore, over the following 6 to 8 h, virulent S. flexneri-infected monocytes died by apoptosis whereas avirulent infected monocytes died by necrosis. Together, these results imply that monocytes migrating into the inflammatory site during the early stages of shigellosis kill S. flexneri but that during bacterial uptake, they receive virulence signals from S. flexneri which induce delayed apoptosis associated with suppression of the proinflammatory cytokine response to bacterial phagocytosis. This delayed apoptosis may have important effects on the ordered initiation of the innate immune response, leading to the excessive inflammatory response characteristic of shigellosis.