Abstract
Aggregation of IgE bound to high affinity IgE receptor (FcεRI) by multivalent antigen induces mast cell activation. Reportedly, disaggregation of aggregated FcεRI immediately terminated degranulation, and formation of co-ligated FcεRI and low affinity IgG receptor FcγRIIB blocked degranulation by inhibitory signal via SH2-containing inositol 5'-phosphatase 1 (SHIP1) phosphorylation. However, their molecular mechanisms to inhibit mast cell activation have been unclear in detail. Herein, we found that addition of excess monomeric hapten (TNP-alanine) to multivalent antigen (TNP-OVA)-activated rat basophilic leukemia cells and mouse bone marrow-derived mast cells induced immediate and transient Syk dephosphorylation, which was previously phosphorylated by TNP-OVA addition. Syk dephosphorylation correlated to rapidly decreased intracellular Ca2+ concentrations ([Ca2+ ]i ), terminated degranulation, and suppressed cytokine production through inhibition of Akt and ERK phosphorylation. Addition of hapten-specific IgG monoclonal antibody (anti-TNP IgG1) to activated mast cells induced translocation of SHIP1 to the plasma membrane and its phosphorylation, indicating that co-ligation of FcεRI and FcγRIIB after FcεRI aggregation can lead to SHIP1 activation. SHIP1 phosphorylation led to gradually decreased [Ca2+ ]i , weak inhibition of degranulation, and strong inhibition of cytokine production. Our findings clearly show the inhibitory mechanism of cell function in activated mast cells by operating Fc receptor crosslinking.