Human Monocytes KillShigella flexneribut Then Die by Apoptosis Associated with Suppression of Proinflammatory Cytokine Production

Abstract

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␤. 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␤, 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.

Virulent Shigella flexneri Causes Damage to Mitochondria and Triggers Necrosis in Infected Human Monocyte-Derived Macrophages

“…The appearance of PI-positive monocytes by 20 h reflected the delayed change in the cell membrane permeability that is seen when cells die by apoptosis. This interpretation is consistent with previously published reports demonstrating that Shigella triggers apoptosis of infected human monocytes (7,12). In contrast, the rapid loss of cell membrane permeability in approximately 60% of the HMDM within 2 h after infection strongly argues that Shigella killed these cells by a more acute mechanism resembling oncosis or necrosis (7).…”

“…A contrary conclusion, that Shigella kills human macrophages by an apoptotic mechanism, has also been reported (11,16). In a recent report the workers concluded that human monocytes phagocytose and kill virulent Shigella and then die by apoptosis within 6 to 8 h after infection (12).…”

Two Distinct Mechanisms For Induction of Dendritic Cell Apoptosis in Response to IntactStreptococcus pneumoniae

“…P rogrammed cell death or apoptosis plays a major regulatory role in immune responses (1). TNF-␣ released early in response to bacterial infections (2) and, specifically, several bacterial pathogens can directly trigger apoptosis in cells of the monocyte/macrophage lineage (3)(4)(5)(6)(7)(8)(9)(10). However, the molecular mechanisms of apoptosis induced by bacteria are incompletely delineated, particularly regarding the involvement of death receptors.…”

An attenuated plasmid-cured strain of Aeromonas hydrophila elicits protective immunity in Clarias batrachus L.

“…Plasmid-cured and wild type A. hydrophila isolates were grown to mid log phase, harvested and infected into macrophages at a multiplicity of infection (MOI) of 5 bacteria per cell for 40 min [29]. Following incubation, the cells were washed in RPMI containing 25 mg mL À1 gatifloxacin to remove the extra-cellular bacteria.…”