Theofilopoulos A N, Bokisch V A, Dixon F J
J Exp Med. 1974 Mar 1;139(3):696-711. doi: 10.1084/jem.139.3.696.
This study describes the presence of a receptor for fluid phase human C3 and C3b on Raji cell membranes. The binding of C3 and C3b was demonstrated indirectly by a fluoresceinated anti-C3 serum and directly by using radioiodinated proteins. No other complement proteins or serum factors were needed to mediate binding of C3 and C3b to the receptor. The possibility of enzymatic cleavage of C3 before or after its attachment on the cell membrane was ruled out by the demonstration of antigenically intact C3 on Raji cells. Inhibition and dissociation of Raji cell-EAC1423 rosettes by C3 and C3b indicated that both of these proteins bind to the same receptor site or closely associated receptor sites on Raji cells. C3b-bearing Raji cells were immune adherence negative, indicating that C3b binding to the receptor is brought about through the immune adherence region of the molecule and not the C3d portion. The C3 receptor on Raji cell membranes is uniformly distributed and can move on the membrane plane. Approximately 4 x 10(5) molecules of C3 or C3b bind per Raji cell. The receptor had a higher affinity for C3 than C3b, as was shown by uptake experiments and inhibition of Raji cell-EAC1423 rosette formation. Apart from the described receptor for C3 and C3b another specific receptor for C3b inactivator-cleaved C3b (C3d) bound to red cells was shown to be present on Raji cells. Raji cells cultured in medium containing fresh normal human serum and cobra venom factor were lysed. Similar results were obtained when C3b-bearing Raji cells were cultured in medium with fresh normal human serum. The lytic effect could be abolished by inactivating serum C3 proactivator (C3PA) and required C6. It was concluded that C3b bound to the Raji cell membrane activates the complement system through the alternate pathway and results in membrane damage and cytolysis. It is postulated that cell destruction by this mechanism may play an important role in vivo in controlling cell growth.
本研究描述了拉吉细胞膜上存在液相人C3和C3b的受体。C3和C3b的结合通过荧光素化抗C3血清间接证明，以及通过使用放射性碘化蛋白直接证明。不需要其他补体蛋白或血清因子来介导C3和C3b与受体的结合。通过在拉吉细胞上证明抗原完整的C3，排除了C3在附着于细胞膜之前或之后进行酶解的可能性。C3和C3b对拉吉细胞-EAC1423玫瑰花结的抑制和解离表明，这两种蛋白都结合到拉吉细胞上的相同受体位点或紧密相关的受体位点。携带C3b的拉吉细胞免疫黏附阴性，表明C3b与受体的结合是通过分子的免疫黏附区域而非C3d部分实现的。拉吉细胞膜上的C3受体均匀分布且可在膜平面上移动。每个拉吉细胞约结合4×10⁵个C3或C3b分子。摄取实验和拉吉细胞-EAC1423玫瑰花结形成的抑制表明，该受体对C3的亲和力高于C3b。除了所描述的C3和C3b受体外，拉吉细胞上还存在另一种与红细胞结合的C3b灭活剂裂解的C3b（C3d）特异性受体。在含有新鲜正常人血清和眼镜蛇毒因子的培养基中培养的拉吉细胞被裂解。当携带C3b的拉吉细胞在含有新鲜正常人血清的培养基中培养时，也获得了类似结果。通过灭活血清C3前活化剂（C3PA）可消除裂解作用，且需要C6。得出结论，结合到拉吉细胞膜上的C3b通过替代途径激活补体系统并导致膜损伤和细胞溶解。据推测，通过这种机制导致的细胞破坏可能在体内控制细胞生长中起重要作用。
