Differential expression of Bcl-X_l and Bax- upon induction or prevention of apoptosis by cytokines. PMN were stimulated for indicated times with 300 U/mL TNF-, 300 U/mL GM-CSF, or left untreated (control), respectively. Whole-cell lysates were subjected to SDS-PAGE and Western blot was performed using the anti–Bcl-X (A) or anti–Bax- Abs and a peroxidase-conjugated secondary antibody (B). Numbers indicate mean OD of each lane obtained from three representative and independent experiments. Bak was present in BL41 cells but not detectable in PMN at any time point (data not shown).