Devine D V, Rosse W F
Proc Natl Acad Sci U S A. 1987 Aug;84(16):5873-7. doi: 10.1073/pnas.84.16.5873.
The alternative pathway of complement is regulated on the surface of homologous blood cells at the C3 amplification step by the membrane protein decay-accelerating factor, as well as by the plasma protein factor H. We have reported elsewhere that platelets from patients with paroxysmal nocturnal hemoglobinuria regulate the activity of the C3 convertase C3bBb, even though they lack decay-accelerating factor. We now report that normal human platelets contain factor H, which was released from the platelet in response to complement deposition or thrombin stimulation. Factor H was localized to the platelet alpha granules by immunocytochemical techniques. As determined by a solid-phase radioimmunoassay, thrombin-stimulated platelets released approximately equal to 54 ng of factor H per 10(8) platelets. The release of factor H in response to complement or thrombin was inhibited by treating the platelets with metabolic inhibitors. Such inhibition resulted in a 3-fold increase in the activity of C3bBb. Platelets that released factor H bound only half as many molecules of radiolabeled factor B to platelet-bound C3b than platelets that could not release factor H. Treatment of platelets with anti-decay-accelerating factor antibody had no effect on the activity of C3bBb unless the release of factor H was blocked. Therefore, so far as we know, human platelets have a unique mechanism for the regulation of the alternative pathway of complement.
补体替代途径在同源血细胞表面的C3扩增步骤受到膜蛋白衰变加速因子以及血浆蛋白H因子的调节。我们在其他地方报道过，阵发性夜间血红蛋白尿患者的血小板尽管缺乏衰变加速因子，但仍能调节C3转化酶C3bBb的活性。我们现在报道，正常人血小板含有H因子，该因子在补体沉积或凝血酶刺激时从血小板中释放出来。通过免疫细胞化学技术将H因子定位到血小板α颗粒中。通过固相放射免疫测定法确定，凝血酶刺激的血小板每10⁸个血小板释放约54 ng的H因子。用代谢抑制剂处理血小板可抑制补体或凝血酶刺激引起的H因子释放。这种抑制导致C3bBb的活性增加3倍。释放H因子的血小板与结合在血小板上的C3b结合的放射性标记因子B分子数量仅为不能释放H因子的血小板的一半。用抗衰变加速因子抗体处理血小板对C3bBb的活性没有影响，除非H因子的释放被阻断。因此，据我们所知，人类血小板具有调节补体替代途径的独特机制。
