SIRPα Negatively Regulates IgE-induced Mast Cell Degranulation And Cytokine Production

Mast cells elicit allergic responses through degranulation and release ofproinflammatory mediators after antigen crosslinking of the immunoglobulin E (IgE)receptor FcεRI. FcεRI contains α-β-and γ-chains. The α-chain has an IgE-binding domain,while β-chain and γ-chains have immunoreceptor tyrosine-based activation motif (ITAM),mediating the downstream activation signals. Activated mast cells secrete preformedmediators (for example, histamine) stored in cytoplasmic granules, de novo synthesis ofproinflammatory lipid mediators, cytokines and chemokines as well. These result invasodilatation, increased vascular permeability and smooth muscle contraction.Signal regulatory protein (SIRP) α is especially abundant in innate immune cells,including macrophages and dendritic cells. It is reported that functional SIRPα alsoexpressed in mast cells. The cytoplasmic region of SIRPα contains2immunoreceptortyrosine-based inhibitory motifs (ITIMs) with4tyrosine residues that are phosphorylatedin response to a variety of growth factors and integrin-mediated cell adhesion. Thisphosphorylation enables recruitment and activation of Src homology-containingphosphotyrosine phosphatase2(SHP-2) that in turn dephosphorylates specific proteinsubstrates involved in mediating various physiological effects. Intracellular signalinginitiated either by tyrosine kinase-coupled receptors for growth factors or by cell adhesionto extracellular matrix proteins. Moreover, the expression of the dominant-negative form ofSIRPα stimulates NF-κB activity and makes the cells resistant to TNF-specific apoptosis.Recently, we have found a role of SIRPα in controlling anti-bacterial innate immuneresponses in macrophage. However, the biochemical nature and functional properties ofnative SIRPα molecules on the regulating of mast cells activation have not been examinedpreviously.In the present study, we have used stable transfection or RNA interference techniqueto obtain SIRPα-overexpressed basophiles or knockdown bone marrow-derived mast cells(BMMCs). We provide evidences here that elimination of SIRPα results in both increasedmast cell degranulation and cytokine gene expression upon FcεRI stimulation.SIRPα-overexpressed mast cells had impaired Ca2+influx, actin depolymerization and activation of the transcription factors NF-κB and NFAT mediated by FcεRI aggregation. Inaddition, alterations in SIRPα expression affected the sensitivity of IgE–mediatedimmediate-phase anaphylactic responses in vivo. Rather, absence of SIRPα results inincreased activation of the small GTPase Rac-1and in enhanced microtubulepolymerization upon FcεRI engagement. Coimmunoprecipitation experiments in ratbasophilic leukemia line(RBL2H3) show that SIRPα markedly decreased IgE inducedassociation of PI3Kp85or IKKβ with SHP-2. SIRPα may accomplish this partially throughits association and sequestration of the signal transducer SHP-2. Thus, SIRPα functions asa biologically important modulator of FcεRI signaling and anaphylaxis.