HUMAN FC-GAMMA-RII, IN THE ABSENCE OF OTHER FC-GAMMA RECEPTORS, MEDIATES A PHAGOCYTIC SIGNAL

Fc, 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 Fc, receptor. We studied the role of Fc-gamma-RIIA in isolation on a human erythroleukemia cell line (HEL) which expresses Fc-gamma-RIIA as its only Fc-gamma 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 Fc-gamma-RI and FC-gamma-RII in isolation and in combination, in transfected COS-1 cells. Fc-gamma-RIIA-transfected COS cells also mediated both the binding and phagocytosis of IgG-sensitized RBCs. In contrast, phagocytosis was not observed in Fc-gamma-RI-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 Fc-gamma-RIIA. These studies establish that Fc-gamma-RIIA 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 Fc-gamma-RII as the only Fc-gamma, receptor, Fc-gamma-RIIA on megakaryocytes and platelets may be involved in the ingestion of IgG-containing immune complexes. Furthermore, these studies indicate that Fc-gamma-RI and Fc-gamma-RIIA differ in their requirements for transduction of a phagocytic signal.