Selective in vivo recruitment of the phosphatidylinositol phosphatase SHIP by phosphorylated FcγRIIB during negative regulation of IgE-dependent mouse mast cell activation

Abstract

We demonstrated previously that the low-affinity IgG receptors FcγRIIB, which are coexpressed with the high-affinity IgE receptors FcεRI in mouse mast cells, can inhibit IgE-induced release of inflammatory mediators and cytokines by these cells. Inhibition was found to require the coaggregation of the two receptors and to depend on the presence of a tyrosine-based inhibition motif (ITIM) in the intracytoplasmic domain of FcγRIIB. We report here that the coaggregation with FcγRIIB does not prevent FcεRI from triggering activation signals in BMMC and induces the tyrosine phosphorylation of FcγRIIB. Phosphorylated ITIM peptides bound in vitro to three SH2 domain-containing phosphatases present in BMMC lysates: the phosphotyrosine phosphatases SHP-1 and SHP-2, and the inositolphosphate phosphatase SHIP. Using BMMC generated from the SHP-1-deficient motheaten mice, SHP-1 was found to be dispensable for inhibition of mast cell activation. When analyzed for in vivo association, SHIP coprecipitated with phosphorylated FcγRIIB, whereas SHP-1 or SHP-2 did not. These observations altogether indicate that FcεRI actively participates in its own regulation and that the mechanisms by which FcγRIIB inhibit cell activation might be different in mast cells and in B-cells.