Involvement of caspases in neutrophil apoptosis: Regulation by reactive oxygen species

Human neutrophils have a short half-life and are believed to die by apoptos is or programmed cell death both in vivo and in vitro. We found that caspas es are activated in a time-dependent manner in neutrophils undergoing spont aneous apoptosis, concomitant with other characteristic features of apoptot ic cell death such as morphologic changes, phosphatidylserine (PS) exposure , and DNA fragmentation. The treatment of neutrophils with agonistic anti-f as monoclonal antibodies (MoAbs) significantly accelerated this process, Ho wever, in cells treated with the potent neutrophil activator phorbol 12-myr istate 13-acetate (PMA), caspase activity was only evident after pharmacolo gic inhibition of the nicotinamide adenine dinucleotide phosphate (NADPH) o xidase. Similarily, inhibition of the NADPH oxidase in constitutive and Fas /APO-1-triggered apoptosis resulted in increased rather than suppressed lev els of caspase activity, suggesting that reactive oxygen species may preven t caspases from functioning optimally in these cells. Moreover, oxidants ge nerated via the NADPH oxidase were essential for PS exposure during PMA-ind uced cell death, but not for neutrophils undergoing spontaneous apoptosis. We conclude that caspases are an important component of constitutive and Fa s/APO-1-triggered neutrophil apoptosis. However, these redox sensitive enzy mes are suppressed in activated neutrophils, and an alternate oxidant-depen dent pathway is used to mediate PS exposure and neutrophil clearance under these conditions. (C) 1998 by The American Society of Hematology.