The Src homology 2 domain-containing inositol 5-phosphatase negatively regulates Fcγ 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 FcgammaR- or SHIP-deficient mice. Phagocytic activities of macrophages fro m FcgammaRII(b)(-/-) and SHIP-/- mice were enhanced to a similar extent, relative to those from wild type. However, calcium influx was only marginally. affected in FcgammaRII(b)(-/-), but greatly enhanced in SHIP-/- macrophages. Furthermore, SHIP was phosphorylated on tyrosine residues upon FcgammaR aggregation even in macrophages from FcgammaRII(b)(-/-) mice or upon clustering of a chimeric receptor containing CD8 and the immunoreceptor tyrosine-based Activation motif (ITAM)-bearing gamma-chain or human-restricted FcgammaRIIa. 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 FcgammaRIIa with a high Affinity, comparable,to that of FcgammaRII(b). Lastly, FcgammaRIIa-mediated phagocytosis was significantly enhanced in THP-1 cells overexpressing dominant-negative form of SHIP in the absence of FcgammaRII(b). These results indicate that SHIP negatively regulates FcgammaR-mediated phagocytosis through all ITAM-containing IgG receptors using a molecular mechanism distinct from that in B cells.