Immobilization of the Type I Receptor for IgE Initiates Signal Transduction in Mast Cells

Clustering of the type I receptor for IgE (FcεRI) on mast cells initiates a cascade of biochemical processes that results in the secretion of inflammatory mediators. We have studied this clustering process in order to obtain information about receptor density and mobility required for initiating that cascade. Specifically, we examined the role of new cluster formation in sustaining the secretory response and the minimal cluster density required for initiating secretion. The experimental protocol adopted for these studies employed photoactivatable antigens and antigen-carrying solid surfaces which enabled us to control the density and mobility of the FcεRI within the cluster. Our results show that recruitment of new FcεRI into clusters, either by antigen exchange among FcεRI-bound IgE molecules or by IgE-bound FcεRI exchange with vacant receptors, is not required for sustaining the cellular secretory response. Furthermore, we find that the cell's secretory response is very sensitive to the density of immobilized FcεRIs, increasing steeply above a density of ca. 1000 immobilized molecules/μm2. Taken together, these findings suggest that immobilization of a fraction of the randomly distributed FcεRIs that are in sufficient proximity on the surface of mucosal-type mast cells of the RBL-2H3 line initiates a degranulation signal, and that this is maintained as long as these receptors are kept within this distance. The above conclusions and the experimental protocol presented in this study are expected to have wider applications for the study and understanding of signaling by immuno (as well as other) receptors.