Polymorphism of immune-related genes has the potential to be utilized as a genetic marker to survey the disease-resistant fish population. The complement is a major factor functioning both in innate and adaptive immune systems. In the present paper it was found that C3-H1, a major isoform of the complement component C3 of carp, exhibits four genetic variants defined as electrophoretic mobility, and that the four variants have different hemolytic activity. Carp serum was electrophoresed on an agarose gel and blotted onto a nitrocellulose membrane, and C3 bands were detected by immunoblotting using monoclonal antibody specific to intact α-chain of carp C3-H1 isoform. Four C3-H1 variants, designated I, II, III and IV, with different mobility were detected, and the phenotypes II and II/IV were dominant. Each of four C3-H1 variants free of cross-contamination was purified from carp sera and examined for the hemolytic activity. It was found that C3-H1-I had the highest specific activity, which was approximately fourfold higher than that of C3-H1-II, which had the lowest activity. The results suggest the possibility that carp individuals possessing a different type of C3-H1 variants have a different level of complement activity, and that the C3-H1 polymorphism can possibly be useful to screen fish with a higher level of innate immune defense.
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