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

Abstract

The transmembrane tyrosine phosphatase CD45 regulates the activity of src 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 (FcεRI) is less clear, although Lyn, a member of the src family, plays an important role in FcεRI-mediated signaling. To define a role for CD45 in FcεRI signal transduction, we established CD45 high expressing rat basophilic leukemia cell lines (RBL-CD45H) and cell lines expressing trace amounts of CD45 (RBL-CD45L). We demonstrate that although all RBL-CD45L cell lines degranulate following IgE- and antigen-induced FcεRI 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 FcεRI, association of FcεRI with Lyn and PTK activity associated with FcεRI, all of which are enhanced upon FcεRI aggregation in RBL-CD45H, are impaired in RBL-CD45L. Finally, we show that FcεRI 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 FcεRI by promoting the activation of FcεRI-associated Lyn.