Functional analysis of the classical complement pathway in bony fish

哺乳动物的经典补体途径被抗原结合抗体激活，在清除入侵病原体中起着至关重要的作用。虽然在硬骨鱼中经典途径的存在是明显的，但其反应机制仍有待阐明。本研究通过分子克隆和功能分析，对鲤鱼补体经典激活途径中的相关组分进行了鉴定。从鲤鱼肝胰腺cDNA文库中克隆了编码甘露糖结合蛋白相关丝氨酸蛋白酶（MASP）的两个C1 r/s亚型、两个C4亚型和一个新的截短亚型的互补DNA克隆。这些在几个功能位点中显示出大量的序列多样性，表明它们的功能差异。此外，还从同一文库中克隆了三个B/C2-A同种型。它们在不同的组织中以不同的水平合成。特别是，命名为B/C2-A3的新同种型仅在淋巴器官如肾和脾中表达，并且其表达在β-葡聚糖注射后上调，表明B/C2-A3同种型是急性期反应物。补体成分同种型的功能差异暗示了这种多样性是硬骨鱼为扩展其先天免疫识别而采取的独特进化策略。

The classical complement pathway of mammals is activated by antigen-bound antibody and plays a crucial role in elimination of invading pathgens. Although the presence of the classical pathway is evident in bony fish, its reaction mechanism remains to be elucidated. In the present study, components involved in the classical pathway activation of the common carp complement was identified by molecular cloning and functional analysis. Complementary DNA clones encoding two C1r/s isotypes, two C4 isotypes and a novel truncated isoform of the mannose-binding protein-associated serine protease (MASP) have been isolated from carp hepatopancreas CDNA library. These show a substantial sequence diversity in several functional sites, suggesting their functional difference. Furthermore, three B/C2-A isotypes were also cloned from the same library. They are synthesized in different tissues at different levels. In particular, a novel isotype, designated B/C2-A3, is expressed exclusively in the lymphoid organs such as kidney and spleen, and its expression is up-regulated, on beta-glucan-injection, suggesting that B/C2-A3 isotype is an acute-phase reactant. The functional divergence of the complement component isotypes implicates that the diversity is a unique evolutioary strategy taken by bony fish to expand its innate immune-recognition.