Molecular Mechanisms of Apoptotic Cell Phagocytosis by Macrophages

巨噬细胞和树突状细胞不仅能识别来自体外的微生物，还能识别垂死的自身细胞，通过吞噬来消灭它们。这种现象阻止了潜在有害或免疫原性的细胞内物质从死亡细胞中释放出来。我们之前已经证明乳脂球egf -因子8 （MFG-E8）参与吞噬细胞吞噬凋亡细胞。MFG-E8通过c端因子viii同源结构域特异性结合凋亡细胞上的PS。当MFG-E8被凋亡细胞参与时，通过n端egf样结构域与吞噬细胞中表达的α_vβ_3整合素结合，促进凋亡细胞的吞噬作用。这些结果表明，活化巨噬细胞分泌的MFG-E8与凋亡细胞结合，并将其带到吞噬细胞中吞噬。我们发现，一种小鼠巨噬细胞（BAM3）吞噬了凋亡的胸腺细胞，但不吞噬凋亡的t细胞WR19L细胞。发现了抑制胸腺细胞吞噬的单克隆抗体。经纯化，抗原为SHPS-1。SHPS-1的配体CD47在小鼠胸腺细胞中表达，而在WR19L中不表达。用CD47转染WR19L后，转化子诱导细胞凋亡后可被BAM3吞噬。与亲代WR19L相比，表达CD47的WR19L在体内更有效地被脾树突状细胞吞噬。暴露在凋亡胸腺细胞上的磷脂酰丝氨酸的掩蔽抑制了吞噬。而抗shps -1单抗不仅能抑制凋亡细胞与吞噬细胞的吞噬，还能抑制凋亡细胞与吞噬细胞的结合。这些结果表明，巨噬细胞需要CD47和磷脂酰丝氨酸来吞噬凋亡细胞，并表明CD47和SHPS-1之间的相互作用在吞噬过程中起着系住作用。我们还发现Del-1与MFG-E8高度同源，在促进凋亡细胞吞噬方面具有相似的功能。少

Macrophages and dendritic cells recognize not only microorgaisms from outside of the body, but also dying self-cells to eliminate them by engulfment. This phenomenon prevents the release of potentially harmful or immunogenic intracellular materials from dying cells. We have previously shown that milk fat globule-EGF-factor 8 (MFG-E8) is involved in apoptotic cell phagocytosis by phagocytes. MFG-E8 specifically binds to PS exposed on apoptotic cells via C-terminal factor VIII-homologous domains. When MFG-E8 is engaged by apoptotic cells, it binds to α_vβ_3 integrin expressed in phagocytes via a N-terminal EGF-like domain, and promotes the phagocytosis of apoptotic cells. These results indicated that MFG-E8 secreted from activated macrophages binds to apoptotic cells, and brings them to phagocytes for engulfment. We found that one mouse macrophage cell line (BAM3) engulfed apoptotic thymocytes, but not apoptotic WR19L cells, a T-cell line. Monoclonal antibodies that inhibited the phagocy … More tosis of apoptotic thymocytes by BAM3 were identified. Purification of the antigen revealed that it was 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. We also found that Del-1 is highly homologous to MFG-E8,and has a similar function in terms of promoting phagocytosis of apoptotic cells. Less