Structural and functional identification of complement components of the bony fish, carp (Cyprinus carpio).

Complement is a humoral factor of innate immunity and plays important roles in immune surveillance and clearance of invading pathogens. Mammalian complement consists of the classical (antibody-dependent), the alternative (antibody-independent) and the lectin (triggered by mannose-binding lectin) pathways of activation, and of the lytic pathway. Phylogenetically, bony fish are one of the lowest groups of vertebrates with serum that shows hemolytic activity indistinguishable from that of mammalian complement. In our series of trials to address the question "How many components constitute the bony fish complement?" Functional analyses, protein isolation, and molecular cloning revealed the presence of major components constituting each pathway, the similarity between bony fish and mammalian complement being considerable. Recent investigations on carp (Cyprinus carpio) and other fish species revealed striking features unique to bony fish complement, including a remarkable diversity in structure and function of the third component, C3, and the presence of a newly identified lineage in evolution of the factor B and C2 family. For a significant insight into the evolution of complement systems and clinical applications to aquaculture industry, further extended studies are warranted.