Role of the FceRI b-chain ITAM as a signal regulator for mast cell activation with monomeric IgE

The b-chain of the high-affinity receptor for IgE (FceRI) plays a crucial role for amplification of the intracellular signaling in mast cells upon FceRI cross-linking by IgE antigen complexes (IgE Ag). Some monomeric IgE as well as IgE Ag stimulate FceRI-signaling pathways, leading to cell activation, whereas the biological functions of the b-chain in the monomeric IgE-mediated mast cell signaling and responses are largely unknown. In the present study, FceRI is reconstituted with either wild-type b-chain or mutated b-chain immunoreceptor tyrosine-based activation motif (ITAM) employing retrovirus-mediated gene transfer into the FceRI b-chain / mast cells. We demonstrated that the transfectants with mutated b-chain ITAM stimulated with monomeric IgE sufficiently produce inflammatory cytokines, although degranulation, intracellular Ca mobilization and leukotriene C4 synthesis are significantly reduced. Furthermore, analyses of molecular mechanisms of the signaling revealed that the expression of cytokine genes and activation of extracellular signal-regulated kinase 1/2 and protein kinase C were significantly delayed in the b-chain ITAM mutant cells stimulated with monomeric IgE, suggesting that the b-chain ITAM regulates kinetics of gene transcriptions and signaling pathways for cytokine production. These findings for the first time revealed the unique functions of the b-chain ITAM in both chemical mediator release and cytokine production of mast cells upon monomeric IgE stimulation.