Characteristics of the alternative complement pathway in fish serum

Divalent cation requirements for the hemolysis of rabbit red blood cells (RaRBC) by the alternative complement pathway (ACP) of fish (rainbow trout Salmo gairdneri, ayu Plecoglossus altivelis, carp Cyprinus carpio, tilapia Tilapia nilotica, yellowtail Seriola quinqueradiata, porgy Pagrus major and flounder Paralichthys olivaceus) were investigated using sera freed of divalent cations and natural antibody. Among divalent cations tested (Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Zn2+, Sr2+, Cd2+ and Ba2+), Mg2+, Co2+ and Ni2+ activated the ACP of all fishes, on the other hand, Ca2+, Mn2+, Zn2+, Sr2+ and Cd2+ activated the ACP of one to four fishes. It is of great interest that carp serum hemolyzed RaRBC in the presence of Ca2+ alone, while mammalian ACP is known to be activated by Mg2+, but not with Ca2+. The fish sera hemolyzed sheep red blood cells (SRBC), which are not hemolyzed by mammalian sera, in the presence of Mg2+ and EGTA (ethylene glycol-bis (2-aminoethyl ether) tetraacetic acid). In particular, the degree of hemolysis (%) of SRBC by yellowtail and flounder sera were 60-85% of that of RaRBC. In addition, neuraminidase-treatment of SRBC raised the degree of hemolysis of the cells. This indicates that the sialic acid content of the membrane of a target cell influences the activation of the fish ACP.