Grass carp (Ctenopharyngodon idella) is the most widely produced freshwater aquaculture product worldwide. However, its susceptibility to microbial infections during its cultivation restricts its development. The complement system plays an important role in immune defence. To investigate the transcription of the complement factor C5 and production of its cleavage product C5a in grass carp infected with grass carp reovirus (GCRV), the full-length C5 gene sequence was cloned and sequenced using the rapid amplification of cDNA ends method, and C5 transcription and C5a production in different grass carp tissues at different GCRV infection stages were determined by RT-qPCR and western blotting, respectively. Our results showed that the full-length cDNA of the C5 gene was 5365-bp long, encoding 1687 amino acids. Signal peptide prediction showed that C5 had a secretory signal peptide. After artificial infection with GCRV, C5 transcription in grass carp liver increased significantly before the appearance of symptoms. C5a generation in the liver increased significantly from the onset of disease symptoms, and continued to increase until recovery. GCRV induced C5 cleavage in grass carp, with C5a production.

草鱼（Ctenopharyngodon idella）是世界上生产最广泛的淡水水产养殖产品。然而，它在培养过程中对微生​​物感染的易感性限制了它的发展。补体系统在免疫防御中起着重要作用。为了研究感染草鱼呼肠孤病毒 (GCRV) 的草鱼中补体因子 C5 的转录及其裂解产物 C5a 的产生，使用 cDNA 末端快速扩增法克隆和测序了全长C5基因序列，C5 RT-qPCR 和蛋白质印迹法分别测定了不同 GCRV 感染阶段不同草鱼组织中的转录和 C5a 产生。我们的结果表明，C5的全长 cDNA基因全长 5365 bp，编码 1687 个氨基酸。信号肽预测显示C5具有分泌信号肽。GCRV人工感染后，草鱼肝脏中C5转录在症状出现前显着增加。从疾病症状开始，肝脏中的 C5a 生成显着增加，并持续增加直到恢复。GCRV 在草鱼中诱导 C5 裂解，产生 C5a。