Caspase-8 Activation Precedes Alterations of Mitochondrial Membrane Potential during Monocyte Apoptosis Induced by Phagocytosis and Killing of Staphylococcus aureus

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Abstract
Human peripheral blood monocytes become apoptotic following phagocytosis and killing of Staphylococcus aureus . Although this type of monocyte apoptosis is known to be initiated by Fas-Fas ligand (FasL) interactions, the downstream signaling pathway has not been determined. In this work the involvement of mitochondria and the kinetics of caspase-8 and caspase-3 activation after phagocytosis of S. aureus were studied. Caspase-8 activity was measured in cell lysates by using the fluorogenic substrate Ac-IETD-AFC. Active caspase-3 levels and mitochondrial membrane potential (Δψ m ) were measured in whole cells by flow cytometry using monoclonal antibodies reacting with activated caspase-3 and chloromethyl-X-rosamine, respectively. The results show that caspase-8 was activated shortly after phagocytosis of bacteria. Caspase-8 activation was followed by progressive disruption of Δψ m , which is associated with the production of reactive oxygen intermediates. The irreversible caspase-8 inhibitor zIETD-FMK prevented the disruption of Δψ m and the release of cytochrome c from S. aureus -exposed monocytes. Caspase-3 activation occurred following disruption of Δψ m . These results strongly suggest that apoptosis of monocytes that have phagocytosed and killed S. aureus is driven by the Fas-FasL-initiated pathway, which is typical for type II cells.

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