PF4 prevents monocytes from undergoing spontaneous apoptosis.

Monocytes were cultured for 72 hours (A) in medium alone, (B) in the presence of 4 μmol/L PF4, or (C) in 1 ng/mL GM-CSF. After simultaneous staining with annexin-V–FITC and PI, cells were analyzed by flow cytometry, as described under “Materials and Methods.” The upper right quadrant represents necrotic cells; the lower right quadrant represents apoptotic cells; and the lower left quadrant represents viable, nonapoptotic cells. Data are derived from 1 representative experiment out of 13. Statistical differences between numbers of apoptotic cells of untreated and PF4-stimulated or GM-CSF–stimulated cells were observed with P < .002 (n = 13).