A cell‐surface opsonic receptor on leucocytes from the phylogenetically primitive vertebrate, Eptatretus stouti

A humoral recognition molecule that is homologous to the mammalian complement components C3, C4 and C5 has recently been identified in the Pacific hagfish, Eptatretus stouti. One function of this complement‐like protein (CLP) is to opsonize foreign material for phagocytosis by hagfish leucocytes. Here, we demonstrate that CLP's opsonic activity can be abrogated by pre‐incubating phagocytes with an anti‐hagfish leucocyte mAb (IBI). Moreover, antigen‐activated CLP can block the binding of the IBI antibody to hagfish leucocytes. Flow cytometry and immunoprecipitation indicate that IBI recognizes a 105 kDa cell‐surface, monomeric protein that is expressed exclusively on phagocytic hagfish leucocytes. It is concluded that this 105 kDa protein represents the cell surface receptor by which CLP mediates the phagocytosis of opsonized targets.