Fcε Receptor I-Associated lyn-Dependent Phosphorylation of Fcγ Receptor IIB During Negative Regulation of Mast Cell Activation

Fc gamma RIIB are low-affinity receptors for IgG whose intracytoplasmic domain contains an immunoreceptor tyrosine-based inhibition motif (ITIM). Fc gamma RIIB inhibit cell activation triggered by receptors that signal via immunoreceptor tyrosine-based activation motifs. This inhibition requires ITIM tyrosyl phosphorylation and is correlated with the binding of SH2 domain-containing phosphatases that may mediate the inhibitory signal. In the present work, we investigated the mechanism of Fc gamma RIIB phosphorylation and its consequences in mast cells. We demonstrate that the phosphorylation of Fc gamma RIIB requires coaggregation with Fc epsilon RI and that, once phosphorylated, Fc gamma RIIB selectively recruit the inositol polyphosphate 5 phosphatase SHIP, in vivo. In vitro, however, the phosphorylated Fc gamma RIIB ITIM binds not only SHIP, but also the two protein tyrosine phosphatases, SHP-1 and SHP-2. We show that the coaggregation of Fc gamma RIIB with Fc epsilon RI does not prevent Fc epsilon RI-mediated activation of lyn and syk. Both kinases can phosphorylate Fc gamma RIIB in vitro. However, when coaggregated with Fc epsilon RI, Fc gamma RIIB was in vivo phosphorylated in syk-deficient mast cells, but not in lyn-deficient mast cells. When Fc epsilon RI are coaggregated with Fc gamma RIIB by immune complexes, Fc epsilon RI-associated lyn may thus phosphorylate Fc gamma RIIB. By this mechanism, Fc epsilon RI initiate ITIM-dependent inhibition of intracellular propagation of their own signals.