Recognizing death: liver phagocytosis of apoptotic cells

Apoptosis impacts on nearly all areas of cell biology and continues to draw increasing numbers of investigators to join in the multifaceted race to understand it. Within the study of cell death the area that has less benefited from the fast advances has been the study of the phagocytic process of apoptotic cells. But finally this field is now converging the attention and the studies of an increasing number of researchers that are highlighting its importance. This review deals with removal of apototic cells; in particular, the liver cell mediated removal of apoptotic blood cells will be considered. The involvement of carbohydrate-specific receptors of liver cells in the recognition and engulfment of apoptotic cells has been tested using three different experimental approaches: i- in vivo induction of apoptosis; ii- in vitro phagocytosis; iii- in situ adhesion experiments. All three main cell liver types are able to recognize and internalize apoptotic cells mainly by means of carbohydrate-specific receptors (galactose and mannose). By up-regulating the cell surface expression of mannose receptors of the endothelial cells, the recognition and the internalization of apoptotic lymphocytes can be increased. Of note is the discrimination in the recognition of apoptotic lymphocytes by the sinusoidal cells: only homologous cells are rapidly and efficiently deleted from the circulation.