Structure and function of the complement receptor type 2(CR2) in fish : regulation of the adaptive immunity by the complement system.

在哺乳动物中，补体第三组分(C3)的激活片段(C3d)通过一种称为CR2的特殊受体刺激B细胞，在支持正常体液免疫反应中发挥关键作用。在目前的研究中，我们检查了适应性免疫调节效应是否在多骨鱼类中保守。首先，我们分析了酵母多糖激活的鲤鱼C3对血清中C3的生理抑制作用，发现鲤鱼C3产生了相当于人C3d的片段。还证明鲤鱼C3的C3-H1和C3-S亚型均被片段化为C3d。接下来，我们构建了谷胱甘肽-S-转移酶与C3d融合蛋白的表达载体，并成功制备了重组蛋白。以GST-C3d蛋白为探针，检测鲤鱼外周血淋巴细胞上识别C3d片段的受体。结果，我们观察到GST-C3d与淋巴细胞的显著和特异性结合。抗鲤鱼C3的兔免疫球蛋白可抑制这种结合，但不能抑制EDTA，提示鲤鱼淋巴细胞携带C3d特异性受体。二价非依赖性的性质与哺乳动物的CR2很好地吻合。

In mammals, an activation fragment(C3d) of the third complement component(C3) plays a crucial role in supporting nomal humoral immune responces, by stimulating B cells thrugh a specific receptor termed CR2. In the present study, we have examined if the adaptive immunity-regulatory effect is conserved in bony fish. First, we have analyzed physiolocial brakedown of carp C3 in serum activated by zymosan, and found that carp C3 generates a fragment equivalent to human C3d. It was also demonstrated that both C3-H1 and C3-S isoforms of carp C3 are fragmented into C3d. Next we have constructed expression vectors for fusion proteins of glutathione-S-transferase(GST) and C3d, and succeeded to prepare the recombinant proteins. Using the GST-C3d protein as a probe, we have attemted to detect receptors that recognize carp C3d fragment on carp peripheral lymphocytes. As a result, we observed significant and specific binding of GST-C3d to the lymphocytes. Anti-carp C3 rabbit IgG, but not EDTA, inhibited the binding, suggesting that carp lymphocytes carry receptors specific for C3d. The divalent-independent nature agrees well with that of mammalian CR2.