Bcl‐2 delays macrophage engulfment of human B cells induced to undergo apoptosis

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Abstract
The capacity to be recognized and engulfed by phagocytes is an important characteristic of cells dying by apoptosis. Phagocytosis of apoptotic cells occurs rapidly in vivo, probably prior to plasma membrane breakdown. While the molecular mechanisms mediating phagocytosis of apoptotic cells are beginning to be defined, little is yet known of the relationship between the cell-death program itself and the surface changes on the dying cells that signal for engulfment. Here, we investigate to what extent the apoptosis repressor Bcl-2 can modulate the recognition and phagocytosis of human B cells exposed to triggers of apoptosis. Burkitt lymphoma (BL)-derived, Bcl-2− B cells were induced into apoptosis either by the Ca2+-ionophore ionomycin or by the inhibitor of protein synthesis cycloheximide. Apoptotic BL cells, but not viable BL cells, were recognized and phagocytosed by monocyte-derived macrophages. bcl-2-transfected BL populations showed a reduced capacity both to undergo apoptosis in response to these inducing agents and to interact with macrophages. Like their Bcl-2− counterparts, Bcl-2+ BL cells interacted with macrophages only after activation of their apoptotic program as assessed by changes in nuclear morphology. These results demonstrate not only that continued protein synthesis in B cells undergoing apoptosis is not essential for their recognition by macrophages, but also that macrophage recognition of apoptotic B cells cannot be uncoupled from the cell-death program that is controlled by Bcl-2. In this respect, the behavior of B cells contrasts markedly with that of neutrophils in which Bcl-2 has been reported to inhibit apoptosis without affecting phagocytic clearance (Lagasse and Weissman, J. Exp. Med. 1994. 179: 1047).