The Src homology 2 domain–containing inositol 5-phosphatase negatively regulates Fc receptor–mediated phagocytosis through immunoreceptor tyrosine-based activation motif–bearing phagocytic receptors

Molecular mechanisms by which the Src homology 2 domain-containing inositol 5-phosphatase (SHIP) negatively regulates phagocytosis in macrophages are unclear. We addressed the issue using bone marrow–derived macrophages from Fc Ror SHIP-deficient mice. Phagocytic activities of macrophages from Fc RII(b) / and SHIP / mice were enhanced to a similar extent, relative to those from wild type. However, calcium influx was only marginally affected in Fc RII(b) / , but greatly enhanced in SHIP / macrophages. Furthermore, SHIP was phosphorylated on tyrosine residues upon Fc R aggregation even in macrophages from Fc RII(b) / mice or upon clustering of a chimeric receptor containing CD8 and the immunoreceptor tyrosinebased activation motif (ITAM)–bearing -chain or human-restricted Fc RIIa. These findings indicate that, unlike B cells, SHIP is efficiently phosphorylated in the absence of an immunoreceptor tyrosine-based inhibition motif (ITIM)– bearing receptor. We further demonstrate that SHIP directly bound to phosphorylated peptides derived from Fc RIIa with a high affinity, comparable to that of Fc RII(b). Lastly, Fc RIIa-mediated phagocytosis was significantlyenhanced inTHP-1cellsoverexpressing dominant-negative form of SHIP in the absence of Fc RII(b). These results indicate that SHIP negatively regulates Fc Rmediated phagocytosis through all ITAMcontaining IgG receptors using a molecular mechanism distinct from that in B cells. (Blood. 2002;100:3374-3382)