Recognition and phagocytosis of cells undergoing apoptosis.

In vivo, the normal fate of cells undergoing apoptosis is recognition, uptake and degradation of the intact dying cell by phagocytes. Cell clearance by this mechanism is fast, efficient and injury-limiting, being mediated by macrophages and semi-professional phagocytes. Apoptotic cells are marked for disposal by mechanisms which remain poorly understood, although in some circumstances surface sugar changes and exposure of phosphatidylserine lead to recognition by uncharacterised phagocyte receptors. Furthermore, there is specific evidence in vitro for involvement of phagocyte receptors including the thrombospondin receptors alpha v beta 3 and CD36, scavenger receptors, the 61D3 antigen and the ABC 1 transporter. It is conceivable that recognition mechanisms may be ordered in a hierarchy of 'back ups', each recognising cells at different stages of the death program. Nevertheless, a full understanding of this complexity will require definition of recognition mechanisms which operate in vivo in higher organisms.