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

A humoral recogllltion molecule that is homologous to the mammalian complement components C3, C4 and C5 has recently been identified In the Pacific hagfish, EptalrelllS 5101/11. One function of this complement-like protein (ClP) IS to opsonize foreIgn matenal 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 (I B I). Moreover. antIgen-activated ClP can block the binding of the I B I antibody to hagfish leucocytes. Flow cytometry and Immunopreclpltation Indicate that I B I recognizes a 105 kDa cell-surface. monomenc 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 0psolllzed targets.