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

Area of Science:

• Cell Biology
• Immunology

Background:

• Apoptotic cells undergo rapid recognition and clearance by phagocytes in vivo.
• Surface changes on apoptotic cells are hypothesized to mediate this recognition.

Purpose of the Study:

• To characterize surface alterations in rat cortical thymocytes during glucocorticoid-induced apoptosis.
• To investigate how these changes affect interactions with macrophages and cell surface properties.

Main Methods:

• Isopyknic centrifugation to isolate early apoptotic thymocytes.
• Assays for macrophage binding, surface ultrastructure, microelectrophoretic mobility, and lectin/antibody binding.

Main Results:

• Apoptotic thymocytes showed enhanced avid binding to macrophages, independent of serum factors.
• Surface ultrastructure changed, with loss of microvilli and "blistering" due to endoplasmic reticulum-plasma membrane fusion.
• Apoptotic cells had reduced surface charge density and fewer surface antigens/lectin-binding sites, correlating with decreased cell size.
• No significant differences were found in the relative amounts of specific sugars or major sialoglycoproteins.

Conclusions:

• Glucocorticoid-induced apoptosis in thymocytes triggers distinct surface modifications.
• While many membrane structures remain intact, specific changes facilitate phagocyte recognition.
• The precise mechanisms for phagocyte recognition are subtle, with preliminary evidence suggesting a lectin-sugar interaction.