Regulation of apoptosis by cytokines as measured by flow cytometry of propidium iodide–stained PMN. PMN were aged for 2, 6, 22, or 34 hours in culture without further stimulation (control), or treated with 300 U/mL TNF- or 300 U/mL GM-CSF, respectively. Data represent apoptotic PMN in percent of total cell number (top panel) or apoptotic PMN in percent of the values seen in the untreated samples at the same time points (middle and bottom panels). Mean ± SD of 12 independent experiments. *P < .05 versus unstimulated control. The effects of TNF- and GM-CSF were dose-dependent (data not shown).