Inhibition of Fcγ receptor-mediated phagocytosis by a nonphagocytic Fcγ receptor

There are three major classes of human Fc gamma receptors (Fc gamma RI, Fc gamma RII, and Fc gamma RIII) and various isoforms of each class are capable of mediating phagocytosis. Fc gamma RIIA is an unusual Fc gamma receptor in that it transmits a phagocytic signal in the absence of an additional receptor subunit. The cytoplasmic domain of Fc gamma RIIA contains a conserved motif containing two copies of the sequence YXXL. The tyrosines (Y) within the motif are phosphorylated after receptor crosslinking and the integrity of these conserved sequences is required for efficient phagocytosis. The Fc gamma RIIB receptors, Fc gamma RIIB1 and Fc gamma RIIB2, contain one copy of the cytoplasmic YXXL sequence and do not transmit a phagocytic signal. In B cells, Fc gamma IIB negatively regulates B-cell activation by the B-cell antigen receptor. Human macrophages express both Fc gamma RIIA and Fc gamma RIIB and while Fc gamma RIIA mediates phagocytosis, the function of Fc gamma RIIB in these cells is unknown. Using the epithelial/fibroblast-like cell line COS-1 as a model to examine the molecular events that regulate the phagocytosis of IgG-coated cells (EA), we investigated the effect of Fc gamma RIIB on Fc gamma RIIA signaling. Fc gamma RIIB inhibited phagocytosis mediated both by Fc gamma RIIA and by a chimeric Fc gamma RIIA receptor containing the extracellular domain of Fc gamma RI and the trans membrane and cytoplasmic domains of Fc gamma RIIA. This inhibition occurred at an early signaling stage because tyrosine phosphorylation of the Fc gamma RIIA cytoplasmic domain was inhibited after concurrent stimulation of these receptors with EA. Fc gamma RIIB mutations showed the importance of the Fc gamma RIIB YXXL for inhibition of Fc gamma RIIA-mediated phagocytosis. Deletion of the Fc gamma RIIB YXXL or conservative replacement of the YXXL tyrosine substantially reduced the inhibitory signal. Fc gamma RIIB had a lesser inhibitory effect on phagocytosis by the Fc gamma receptor Fc gamma RIIIA, which requires a gamma subunit to mediate a phagocytic signal. These results show that Fc gamma RIIB negatively regulates phagocytic signaling by Fc gamma RIIA and suggests that Fc gamma RIIB plays a role in modulating Fc gamma RIIA function in vivo. (C) 1998 by The American Society of Hematology.