Characterization of the Effects of Opsonins in Normal Hagfish Serum on the Ingestion of Rabbit Erythrocytes by Hagfish Macrophages

Abstract Normal serum from the hagfish Eptatretus burgeri strongly enhanced the phagocytic activity of homologous peritoneal macrophages against rabbit erythrocytes (RRBC). Such phagocytosis was effectively but incompletely inhibited by rabbit antibodies against hagfish C3 (HC3). However, when RRBC were pretreated with purified HC3 or normal hagfish serum in the presence of EDTA, little enhancement of ingestion was observed. The results indicate that HC3 binds to RRBC, in collaboration with other serum factor(s), and that bound HC3 functions as an opsonin. Additionally, we obtained evidence for the possible presence of another opsonin (HOP) that was resistant to EDTA in C3-depleted hagfish serum. HOP prepared from plasma had a molecular mass of approximately 1,000 kDa, consisting of various small polypeptides that were able to form aggregates via disulfide bonds and/or noncovalent interactions. The functions of HC3 and HOP might be essential to immunity in hagfish because no clear evidence for the production of antibodies in cyclostomes has been reported to date.

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