Physiological roles of apoptotic cell clearance

乳脂球-EGF-8(MFG-E8)由活化的巨噬细胞和未成熟的树突状细胞分泌，连接凋亡细胞和吞噬细胞，促进凋亡细胞的吞噬。我们发现一个MFG-E8突变体，命名为D89E，在RGD基序中携带点突变，抑制了多种吞噬细胞对凋亡细胞的吞噬。当D89E蛋白静脉注射到小鼠体内时，诱导产生包括抗磷脂抗体和抗核抗体在内的自身抗体。通过共注射同基因的凋亡胸腺细胞，增强了自身抗体的产生。在D89E诱导产生自身抗体后，治疗组小鼠的自身抗体效价长期升高，并在腹股沟内形成了IgG沉积。这些结果表明，凋亡细胞吞噬功能的障碍导致了自身抗体的产生。我们产生了转基因小鼠，在这些转基因小鼠中，II型肺上皮细胞和巨噬细胞被瞬时消融。人白喉毒素受体在溶菌酶M基因启动子的控制下在小鼠体内表达。白喉毒素致小鼠急性肺损伤，4d内死亡。白喉毒素处理的小鼠肺泡巨噬细胞和II型细胞均被清除。经白喉毒素处理的转基因小鼠支气管肺泡灌洗液中表面活性蛋白A和B的含量显著减少。将野生型小鼠的骨髓移植到照射后的转基因小鼠体内，肺泡巨噬细胞对白喉毒素产生了抵抗力，但小鼠仍存在急性肺损伤。这些结果表明，消融II型细胞导致致死性急性呼吸窘迫，并与表面活性蛋白的减少有关。

Milk fat globule-EGF-factor 8 (MFG-E8), secreted by activated macrophages and immature dendritic cells, links apoptotic cells and phagocytes, and promotes phagocytosis of apoptotic cells. We found that an MFG-E8 mutant, designated as D89E, carrying a point mutation in an RGD motif, inhibited the phagocytosis of apoptotic cells by a wide variety of phagocytes. When intravenously injected into mice, the D89E protein induced the production of auto-antibodies including anti-phospholipids antibodies and anti-nuclear antibodies. The production of auto-antibodies was enhanced by the co-injection of syngeneic apoptotic thymocytes. Following the induction of auto-antibody production by D89E, the treated mice showed a long-term elevation of the titer for auto-antibodies, and developed IgG deposition in the gromeruli. These results indicated that the impairment of apoptotic cell phagocytosis led to auto-antibody production.We generated transgenic mice in which type II lung epithelial cells as well as macrophages were transiently ablated. Human diphtheria toxin receptor was expressed under the control of the lysozyme M gene promoter in the mice. When diphtheria toxin was administrated to the mice, they suffered from acute lung injury, and died within 4 days. Type II cells as well as alveolar macrophages were deleted in the mice treated with diphtheria toxin. The amount of surfactant proteins A and B in bronchoalveolar lavage fluid was greatly reduced in the diphtheria toxin-treated transgenic mice. When the bone marrow from wild-type mice was transplanted into the irradiated transgenic mice, the alveolar macrophages became resistant to diphtheria toxin, but the mice still suffered from acute lung injury. These results indicated that ablation of type II cells caused lethal acute respiratory distress in association with the reduced amount of surfactant proteins.