Analysis of structure and function of sugar chains included in Fc receptor

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A murine macrophage cell line, P388Dl, expresses Fc receptors which recognize Fc portion of IgG bound to antigen, but is not able to ingest the antigen-antibody complex. However, modification of cell surface sugar chains can induce Fc receptor-mediated phagocytosis without affecting nonspecific phagocytosis of latex beads. It is considered, therefore, that Fc receptors are structurally modified to express their function. To prove this possibility, the in vitro system for induction of phagocytosis by different kinds of reagents which modify structures of sugar chains was established. By using this system, the following results were obtained. (1)Altered glycosylation-induced phagocytosis was mediated by Fcr2b receptor. (2)The sugar chains of this receptor actually changed after induction of phagocytosis. (3)This structural change affected the ingestion of ligands bound to the receptor, but not the binding of ligands to the receptor. (4)I?inding of ligands to the structurally altered receptor induced phosphorylation of cellular proteins including Fc receptor itself, the release of arachidonic acid from the cells, and activation of phospholipase A2 responsible for arachidonic acid release. (5)Addition of inhibitors of arachidonic acid metabolism to the culture media of induced cells suppressed protein phosphorylation and phagocytosis.Thus, the results suggest that altered glycosylation of Fc receptor affects the ingestion process of phagocytosis by activating arachidonic acid metabolism and protein phosphorylation, resulting in the induction of Fc receptor-mediated phagocytosis.