CD31 promotes beta1 integrin-dependent engulfment of apoptotic Jurkat T lymphocytes opsonized for phagocytosis by fibronectin.

Phagocyte integrins, by binding "bridging" molecules, mediate the ingestion of late apoptotic cells and apoptotic bodies by mechanisms that remain obscure. We recently reported that human monocyte-derived macrophages capture viable and apoptotic human leukocytes through homophilic interactions involving CD31 and that CD31 then promotes the engulfment of apoptotic cells or the detachment of viable cells. We now report that CD31 homophilic interactions between phagocyte and target cells lead to activation of phagocyte alpha5beta1 integrin and the engulfment of apoptotic Jurkat T lymphocytes via a fibronectin (Fn) "bridge." Although Fn and serum served as an opsonin for beta1 integrin-dependent phagocytosis of apoptotic leukemic T cells, they failed to do so for neutrophils. Given the complexities and inherent variability of working with primary cells, we have refined our model to show that ligation of CD31 on THP-1 macrophages also regulates beta1 integrin-dependent phagocytosis of Fn-coated Latex beads. Thus, selective "tethering" of apoptotic leukocytes by phagocyte CD31 not only discriminates dying from viable cells but also selectively activates phagocyte integrins for the engulfment of apoptotic cells.