Recognition ligands on apoptotic cells: a perspective.

The process of apoptosis includes critically important changes on the cell surface that lead to its recognition and removal. The recognition also generates a number of other local tissue responses including suppression of inflammation and immunity. It is surprising that the ligands generated on the apoptotic cell, which mediates these effects, have received relatively little attention. Some of these candidate molecules and possible mechanisms for their surface expression are addressed herein, with particular emphasis on phosphatidylserine and calreticulin. However, exposure of such ligands is exclusive to apoptosis and may, in fact, occur on viable cells. To partially explain the lack of response to such potential stimuli, the presence on viable cells of "don't eat me" signals, in this case, CD47 is suggested to prevent such unwarranted actions. Loss or inactivation of the don't eat me CD47 effects accompanies apoptosis and now allow the cells to be recognized and cleared.