A functional NADPH oxidase prevents caspase involvement in the clearance of phagocytic neutrophils

Neutrophils play a prominent role in host defense. Phagocytosis of bacteria leads to the formation of an active NADPH oxidase complex that generates reactive oxygen species for bactericidal purposes. A critical step in the resolution of inflammation is the uptake of neutrophils by macrophages; however, there are conflicting reports on the mechanisms leading to the apoptosis of phagocytic neutrophils. The aim of this study was to clarify the role of effector caspases in these processes. Caspase activity was measured by DEVDase activity assays or immunofluorescence detection of active caspase-3. With normal human and wild-type murine neutrophils there was no caspase activation following phagocytosis of Staphylococcus aureus. However, caspase activity was observed in phagocytic neutrophils with a defective NADPH oxidase, including neutrophils isolated from X-linked gp91(phox) knockout chronic granulomatous disease mice. These results indicate that a functional NADPH oxidase and the generation of oxidants in the neutrophil phagosome prevent the activation of the cytoplasmic caspase cascade.