Carbohydrate-mediated phagocytic recognition of early apoptotic cells undergoing transient capping of CD43 glycoprotein

A novel mechanism of phagocytic recognition of apoptotic cells was found and characterized. Jurkat cells incubated with appropriate concentrations of etoposide or anti-Fas antibody transiently became susceptible to binding and phagocytosis by THP-1 cell-derived macrophages at 2 h. The bound Jurkat cells showed no chromatin condensation, but the binding was prevented by a caspase inhibitor, indicating that they were recognized at an early stage of apoptosis. The ligands recognized on the apoptotic cells were sialylpolylactosaminyl sugar chains because 1) the binding was inhibited by an oligosaccharide preparation of erythrocyte membrane, and its inhibitory activity was destroyed by polylactosaminoglycan-specific endo-beta-galactosidase or neuraminidase; 2) Jurkat cells pretreated with endo-beta-galactosidase or neuraminidase failed to be recognized; and 3) treatment of the apoptotic cells with polylactosaminoglycan-binding Datura stramonium agglutinin prevented recognition. The sialylpolylactosaminyl chains involved were most likely those of a major sialoglycoprotein CD43 because anti-CD43 antibody inhibited recognition. CD43 on apoptotic Jurkat cells was found to form a cap at 2 h, and the cap disappeared at 4 h. This transient capping of CD43 coincided with the transient increase in the susceptibility of the cells to macrophage recognition, suggesting that CD43 capping is responsible for generation of the carbohydrate ligands for recognition. Furthermore, microscopic observation suggested that the apoptotic cells were recognized at the CD43 cap. Taken together, we conclude that apoptotic Jurkat cells transiently undergo CD43 capping at an early stage of apoptosis and are recognized by macrophages through the cluster of sialylpolylactosaminyl chains of the capped CD43.