A novel complement factor I involving in the complement system immune response from Lampetra morii.

Complement factor I (CFI) is a serine protease which plays a key role in the modulation of complement system and the induced-fit factor responsible for controlling the complement-mediated processes. In this study, a CFI gene was cloned and characterized from Lampetra morii (designated as L-CFI) at molecular and cellular levels. The L-CFI protein included a factor I membrane attack complex domain (FIMAC), a scavenger receptor cysteine-rich domain (SRCR), a trypsin-like serine protease domain (Tryp_SPc) and 2 low-density lipoprotein receptor class A domains (LDLa) which would exhibit functional similarities to CFI superfamily proteins. Tissue expression profile analysis showed that L-CFI mRNA constitutively expressed in all tested tissues except erythrocytes, with the predominant expression in liver. The mRNA expression level of L-CFI increased significantly after Vibrio anguillarum and Staphylocccus aureus stimulation. It is demonstrated that L-CFI interacted with L-C3 protein and affected the deposition of L-C3 on the cell surface. Furthermore, lamprey serum after deplete L-CFI and L-C3 reduced the cytotoxic activity against HeLa cells. These findings suggest that L-CFI plays an important role in lamprey immunity and involved in the lamprey complement system.