A CELL-SURFACE OPSONIC RECEPTOR ON LEUKOCYTES FROM THE PHYLOGENETICALLY PRIMITIVE VERTEBRATE, EPTATRETUS-STOUTI

A humoral recognition molecule that is homologous to the mammalian com plement components C3, C4 and C5 has recently been identified in the P acific hagfish, Eptatretus stouti. One function of this complement-lik e protein (CLP) is to opsonize foreign material for phagocytosis by ha gfish leucocytes. Here, we demonstrate that CLP's opsonic activity can be abrogated by pre-incubating phagocytes with an anti-hagfish leucoc yte mAb (1B1). Moreover, antigen-activated CLP can block the binding o f the 1B1 antibody to hagfish leucocytes. Flow cytometry and immunopre cipitation indicate that 1B1 recognizes a 105 kDa cell-surface, monome ric protein that is expressed exclusively on phagocytic hagfish leucoc ytes. It is concluded that this 105 kDa protein represents the cell su rface receptor by which CLP mediates the phagocytosis of opsonized tar gets.