Phosphatidylserine-dependent phagocytosis of apoptotic glioma cells by normal human microglia, astrocytes, and glioma cells

Apoptotic cells display signals that trigger phagocytic removal by macropha ges or neighboring cells. To better understand the signals triggering phago cytosis:of apoptotic glioma cells, and to identify the cells that might be involved in the phagocytic process, U-251 MG glioma cells were made apoptot ic by etoposide (25 mug/ml) treatment and were incubated with normal human astrocytes (NHA), glioma cells, or microglia, Extent of phagocytosis was as sessed by an in vitro phagocytosis assay. After 3 h of incubation with apop totic cells, phagocytes tested were washed to remove nonengulfed cells, the n fixed, stained, and counted to determine phagocytosis index (PI). NHA, gl ioma cells, and microglia all phagocytosed apoptotic, but not nonapoptotic, glioma cells. Microglia, however, had a pi approximately 4-fold higher tha n did either NHA or glioma cells. Binding of phosphatidylserine (PS) on apo ptotic glioma cell membranes by annexin-V inhibited phagocytosis by 90% in both microglia and NHA, The activity of an enzyme (scramblase) that moves P S from the inner cell membrane to the outer cell membrane was also increase d in apoptotic glioma cells, These results suggest that a variety of cells present in and near gliomas in vivo can remove glioma cells in a PS-depende nt scramblase-mediated fashion, Manipulation of scramblase and/or PS exposu re in glioma cells may therefore be a means of triggering phagocytic remova l of glioma cells.