Hormone-induced cell death. 2. Surface changes in thymocytes undergoing apoptosis

In vivo, apoptotic cells are swiftly recognized by phagocytes, presumably because of changes on their surface. This article describes surface changes in rat cortical thymocytes undergoing apoptosis induced by glucocorticoid treatment in vitro. Homogeneous populations of thymocytes early in apoptosis were prepared by isopyknic centrifugation. These cells were compared with purified nonapoptotic cells in terms of several surface characteristics, including binding to macrophages, surface ultrastructure, microelectrophoretic mobility (a measure of surface charge density), and ability to bind four lectins and four monoclonal antibodies to thymocyte antigens. Apoptotic cells bound to macrophages more avidly than did nonapoptotic cells by a process not dependent upon serum factors. Their surfaces lost microvilli and became " blistered ," apparently through fusion of vesicles of endoplasmic reticulum with the plasma membrane. The surface charge density of apoptotic cells was less than that of nonapoptotic cells. Surface antigens and lectin-binding sites were less abundant on apoptotic than on normal cells, in proportion to the general reduction in cell size observed in apoptosis. Differences between apoptotic and normal cells were not detected, however, in the relative quantities of exposed galactose, N-acetyl galactosamine, N-acetyl glucosamine, N-acetyl neuraminic acid, or of several surface antigens, including the major sialoglycoproteins of the thymocyte membrane. It appears that although several changes occur in the surface of apoptotic cells, many cell membrane structures remain intact. The changes responsible for the recognition of apoptotic cells by phagocytes are more subtle than those detectable by the binding of lectin and antibody probes, but preliminary data suggest that a lectin-sugar interaction is involved.