Human FcγRII, in the absence of other Fcγ receptors, mediates a phagocytic signal

Abstract

Fcγ receptors are important components in the binding and phagocytosis of IgG-sensitized cells. Studies on the role of these receptors have been limited by the fact that most hematopoietic cells express more than one Fcγ receptor. We studied the role of FcγRIIA in isolation on a human erythroleukemia cell line (HEL) which expresses FcγRIIA as its only Fcγ receptor. HEL cells were observed to bind and phagocytose IgG-sensitized red blood cells (RBCs) in a dose-dependent manner. We then examined the role of FcγRI and FcγRII in isolation and in combination, in transfected COS-1 cells. FcγRIIA-rransfected COS cells also mediated both the binding and phagocytosis of IgG-sensitized RBCs. In contrast, phagocytosis was not observed in FcγRI-transfected cells, although these cells avidly bound IgG-sensitized RBCs. Furthermore, coexpression of both receptors by doubly transfected cells did not affect the phagocytic efficiency of FcγRHA. These studies establish that FCγRIIA can mediate phagocytosis and suggest that transfected COS-1 cells provide a model for examining this process. Since HEL cells exhibit characteristics of cells of the megakaryocyte-platelet lineage, including expression of FcγRII as the only Fcγ receptor, FcγRIIA on megakaryocytes and platelets may be involved in the ingestion of IgG-containing immune complexes. Furthermore, these studies indicate that FcγRI and FcγRIIA differ in their requirements for transduction of a phagocytic signal.