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Institute for Medical Microbiology, Immunology, and Hygiene, Technische Universität München, Munich, Germany
A number of highly virulent, intracellular bacteria are known to
induce cell death by apoptosis in infected host cells. In this work we
demonstrate that phagocytosis of bacteria from the Escherichia
coli laboratory strain K12 DH5
is a potent cell death
stimulus for mouse macrophages. RAW264.7 mouse macrophages took up
bacteria and digested them within 2–4 h as investigated with green
fluorescent protein-expressing bacteria. No evidence of apoptosis was
seen at 8 h postexposure, but at 24 h 
70% of macrophages
displayed an apoptotic phenotype by a series of parameters. Apoptosis
was blocked by inhibition of caspases or by forced expression of the
apoptosis-inhibiting protein Bcl-2. Processing of caspase-3 and
caspase-9 but not caspase-8 was seen suggesting that the mitochondrial
branch of the apoptotic pathway was activated. Active effector caspases
could be detected in two different assays. Because the adapter molecule
myeloid differentiation factor 88 (MyD88) has been implicated in
apoptosis, involvement of the Toll-like receptor pathway was
investigated. In RAW264.7 cells, heat-treated bacteria were taken up
poorly and failed to induce significant apoptosis. However, cell
activation was almost identical between live and heat-inactivated
bacteria as measured by extracellular signal-regulated kinase
activation, generation of free radicals, and TNF secretion.
Furthermore, primary bone marrow-derived macrophages from wild-type as
well as from MyD88-deficient mice underwent apoptosis upon phagocytosis
of bacteria. These results show that uptake and digestion of bacteria
leads to MyD88-independent apoptosis in mouse macrophages. This form of
cell death might have implications for the generation of the immune
response.
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