In programmed cell death, a large number of cells undergo apoptosis, and are engulfed by macrophages to avoid the release of noxious materials from the dying cells 1,2 . In definitive erythropoiesis, nuclei are expelled from erythroid precursor cells and are engulfed by macrophages. Phosphatidylserine is exposed on the surface of apoptotic cells 3 and on the nuclei expelled from erythroid precursor cells 4 ; it works as an 'eat me' signal for phagocytes 5,6 . Phosphatidylserine is also expressed on the surface of exosomes involved in intercellular signalling 7 . Here we established a library of hamster monoclonal antibodies against mouse peritoneal macrophages, and found an antibody that strongly inhibited the phosphatidylserine-dependent engulfment of apoptotic cells. The antigen recognized by the antibody was identified by expression cloning as a type I transmembrane protein called Tim4 (T-cell immunoglobulin-and mucin-domain-containing molecule; also known as Timd4) 8 . Tim4 was expressed in Mac1 1 cells in various mouse tissues, including spleen, lymph nodes and fetal liver. Tim4 bound apoptotic cells by recognizing phosphatidylserine via its immunoglobulin domain. The expression of Tim4 in fibroblasts enhanced their ability to engulf apoptotic cells. When the anti-Tim4 monoclonal antibody was administered into mice, the engulfment of apoptotic cells by thymic macrophages was significantly blocked, and the mice developed autoantibodies. Among the other Tim family members, Tim1, but neither Tim2 nor Tim3, specifically bound phosphatidylserine. Tim1-or Tim4-expressing Ba/F3 B cells were bound by exosomes via phosphatidylserine, and exosomes stimulated the interaction between Tim1 and Tim4. These results indicate that Tim4 and Tim1 are phosphatidylserine receptors for the engulfment of apoptotic cells, and may also be involved in intercellular signalling in which exosomes are involved.Caspase-activated DNase (CAD)-deficient cells do not undergo apoptotic DNA fragmentation, but their DNA is degraded in phagocytes after they are engulfed 9 . We used this knowledge to assay the

Apoptotic cells are swiftly phagocytosed by macrophages and immature dendritic cells. In this study, we found that one mouse macrophage cell line (BAM3) engulfed apoptotic thymocytes, but not a lymphoma cell line (WR19L). mAbs that inhibited the phagocytosis of apoptotic thymocytes by BAM3 were identified. Purification of the Ag revealed that it was Src homology 2 domain-bearing protein tyrosine phosphatase substrate-1 (SHPS-1). CD47, the ligand for SHPS-1, was expressed in mouse thymocytes, but not in WR19L. When WR19L was transformed with CD47, the transformants, after induction of apoptosis, could be phagocytosed by BAM3. The WR19L transformants expressing CD47 were more efficiently engulfed in vivo by splenic dendritic cells than the parental WR19L. Masking of the phosphatidylserine exposed on apoptotic thymocytes inhibited the engulfment, whereas the anti-SHPS-1 mAb inhibited not only the engulfment, but also the binding of apoptotic cells to phagocytes. These results indicate that macrophages require CD47 and phosphatidylserine on apoptotic cells for engulfment, and suggest that the interaction between CD47 and SHPS-1 works as a tethering step in the phagocytosis.

Polarized localization of membrane proteins to axons or dendrites is important for a variety of neuronal functions, including neurite outgrowth and synaptogenesis during neural development. Src homology 2 domain-containing protein tyrosine phosphatase (SHP) substrate-1 (SHPS-1) and its ligand cluster of differentiation 47 (CD47), both of which are members of the Ig superfamily of proteins, are thought to constitute an intercellular communication system in the CNS, although the physiological functions of this CD47-SHPS-1 system remain unknown. To provide insight into these functions, we have now examined the localization of SHPS-1 and CD47 in cultured hippocampal neurons. Endogenous SHPS-1 was detected at the surface of both axons and dendrites, whereas endogenous CD47 was localized predominantly to the surface of dendrites. Forced expression of these two proteins confirmed their distinct localizations. The extracellular regions of SHPS-1 and CD47 were responsible, at least in part, for their axonal and dendritic localizations, respectively; however, the axonal localization of SHPS-1 was not mediated by any one of the three Ig domains in its extracellular region. Overexpression of SHPS-1 and CD47 in distinct neurons resulted in marked accumulation of these proteins at sites of contact between SHPS-1-expressing axons and CD47-expressing dendrites. Such contact sites exhibited an enlarged structure but did not contain the synaptic marker protein vesicle-associated membrane protein-2. These results suggest that differential localization of SHPS-1 and CD47 at axons and dendrites generates a directional intercellular communication system that potentially contributes to regulation of synaptogenesis and the formation of neural networks.