Regulation of development of type I allergy by the signal transducer specific for the high-affinity IgE receptor

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MIST is the third member of SLP-76/BASH family signaling proteins, acting downstream of the immunoreceptors. These adaptor proteins are differentially expressed among hematopoietic cells, and very uniquely MIST is only expressed in mast cell lines and cytokine-dependent cell lines. We have demonstrated in this project that MIST is tyrosine phosphorylated upon high-affinity IgE receptor cross linking in mast cells, which is mainly mediated by Lyn kinase. Tyrosine-phosphorylated MIST can inter act with other signaling proteins including PLCy, Vav and Grb 2.The significant reduction of IgE recep-tor-mediated degranulation response was observed when the tyrosine phosphorylation-deficient MIST mutant was expressed in RBL-2H3 rat mast cell line, suggesting that MIST functions as an important scaffold protein for IgE receptor-degranulation. To understand MIST functions in vivo, we generated MIST-deficient mice by gene targeting using ES cell lines. MIST homozygous mice expressed no MIST protein in IL-3-induced bone marrow-derived mast cells and IL-2-induced spleen NK cells. IgE recep tor-mediated degranulation responses as well as cytokine production were profoundly reduced in MIST deficient mast cells, as compared with those in wild-type mast cells. Furthermore, IgE-mediated passive and systemic anaphylaxis in vivo were also attenuated in MIST-deficient mice, indicating that MIST is indeed involved in IgE receptor-mediated mast cell functions in vivo. Based on these findings, we be lieve that MIST would be an excellent target for anti-allergic drugs.