The Molecular Dissection of Fcγ Receptor Mediated Phagocytosis

Because hematopoietic cells express multiple Fcγ receptor isoforms, the role of the individual Fcγ receptors in phagocytosis has been difficult to define. Transfection of Fcγ receptors into COS-1 cells, which lack endogeneous Fcγ receptors but have phagocytic potential, has proved valuable for the study of individual Fcγ receptor function. Using this model system, we have established that a single class of human Fcγ receptor mediates phagocytosis in the absence of other Fc receptors and that isoforms from each Fcγ receptor class mediate phagocytosis, although the requirements for phagocytosis differ. In investigating the relationship between structure and function for Fcγ receptor mediated phagocytosis, the importance of the cytoplasmic tyrosines of the receptor or its associated γ chain has been established. For example, two cytoplasmic YXXL sequences, in a configuration similar to the conserved tyrosine-containing motif found in Ig gene family receptors, are important for phagocytosis by the human Fcγ receptor, FcγRIIA. FcγRI and FcγRIIIA do not possess cytoplasmic tyrosines but transmit a phagocytic signal through interaction with an associated γ subunit that contains two YXXL sequences in a conserved motif required for phagocytosis. The human FcγRII isoforms FcγRIIB1 and FcγRIIB2 do not induce phagocytosis and have only a single YXXL sequence. Cross-linking the phagocytic Fcγ receptors induces tyrosine phosphorylation of either FcγRIIA or the γ chain, and treatment with tyrosine kinase inhibitors reduces both phagocytosis and phosphorylation of the receptor tyrosine residues. Activation of protein tyrosine kinases follows Fcγ receptor engagement of IgG-coated cells. The data indicate that coexpression of the protein tyrosine kinase Syk, which is associated with the γ chain in monocytes/macrophages, is important for phagocytosis mediated by FcγRI and FcγRIIIA. Furthermore, phosphatidylinositol-3 kinase is required for phagocytosis mediated by FcγRIIA as well as for phagocytosis mediated by FcγRI/γ and FcγRIIIA/γ.