Regulation of mast cell signaling through high-affinity IgE receptor by CD45 protein tyrosine phosphatase

The transmembrane tyrosine phosphatase CD45 regulates the activity ofsrc family protein tyrosine kinases (PTK) and thereby influences the signaling via such receptors as T and B cell antigen receptors associated with these PTK, However, its implication in signaling through the mast cell receptor with high affinity for IgE (Fc epsilon RI) is less clear, although Lyn, a member of the src family, plays an important role in Fc epsilon RI-mediated signaling. To define a role for CD45 in Fc epsilon RI signal transduction, we established CD45 high expressing rat basophilic leukemia cell lines (RBL-CD45H) and cell lines expressing trace amounts of CD45 (RBL-CD4BL). We demonstrate that although all RBL-CD45L cell lines degranulate following IgE- and antigen-induced Fc epsilon RI aggregation, the response is significantly reduced at a low dose of antigen. The cells show a delayed and slowed Ca2+ mobilization even though at a higher dose where the cells degranulate to a similar extent as RBL-CD45H, This diminished Ca2+ response is restored by reconstitution of RBL-CD45L with a chimeric molecule containing the cytoplasmic phosphatase domains of rat CD45, Furthermore, tyrosine phosphorylation of Fc epsilon RI, association of Fc epsilon RI with Lyn and PTK activity associated with Fc epsilon RI, all of which are enhanced upon Fc epsilon RI aggregation in RBL-CD45H, are impaired in RBL-CD45L, Finally, we show that Fc epsilon RI is physically associated with CD45 in RBL-CD45H prior to receptor aggregation. Thus, we propose that, although not indispensable in mast cell degranulation, CD45 positively regulates the signaling through Fc epsilon RI by promoting the activation of Fc epsilon RI-associated Lyn.