Molecular Mechanisms of Apoptotic Cell Phagocytosis by Macrophages

巨噬细胞和树突状细胞不仅识别来自体外的微生物，而且还识别死亡的自身细胞，以通过吞噬来消除它们。这种现象阻止死亡细胞释放潜在的有害或免疫原性的细胞内物质。我们之前已经证明，乳脂球-EGF-8(MFG-E8)参与了吞噬细胞吞噬细胞凋亡的过程。MFG-E8通过C-末端因子VIII-同源结构域与暴露在凋亡细胞上的PS特异性结合。当MFG-E8被凋亡细胞结合时，它通过N端的表皮生长因子样结构域与吞噬细胞表达的α_vβ_3整合素结合，促进凋亡细胞的吞噬。这些结果表明，活化的巨噬细胞分泌的MFG-E8与凋亡细胞结合，并将其带到吞噬细胞吞噬。我们发现一个小鼠巨噬细胞系(BAM3)吞噬了凋亡的胸腺细胞，但没有吞噬T细胞系WR19L细胞。抑制吞噬…的单抗BAM3诱导的胸腺细胞凋亡率增加。纯化后的抗原为SHPS-1。SHPS-1的配体CD47在小鼠胸腺细胞中表达，但在WR19L中不表达。用CD47转化WR19L后，转化子经诱导凋亡后可被BAM3吞噬。与亲本WR19L相比，表达CD47的WR19L能更有效地被脾树突状细胞吞噬。遮盖暴露在凋亡胸腺细胞上的磷脂酰丝氨酸可抑制吞噬作用。而抗SHPS-1单抗不仅能抑制吞噬细胞的吞噬，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，提示CD47和SHPS-1之间的相互作用在吞噬过程中起着拴系步骤的作用。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类似的功能。较少

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