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Monomer hapten and hapten‐specific IgG inhibit mast cell activation evoked by multivalent hapten with different mechanisms

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.