Small, Mobile FcεR1 Receptor Aggregates Are Signaling Competent

Crosslinking of IgE-bound Fc epsilon R1 triggers mast cell degranulation. Previous fluorescence recovery after photobleaching (FRAP) and phosphorescent anisotropy studies suggested that Fc epsilon R1 must immobilize to signal. Here, single quantum dot (QD) tracking and hyperspectral microscopy methods were used for defining the relationship between receptor mobility and signaling. QD-IgE-Fc epsilon R1 aggregates of at least three receptors remained highly mobile overextended times at low concentrations of antigen that induced Syk kinase activation and near-maximal secretion. Multivalent antigen, presented as DNP-QD, also remained mobile at low doses that supported secretion. Fc epsilon R1 immobilization was marked at intermediate and high antigen concentrations, correlating with increases in cluster size and rates of receptor internalization. The kinase inhibitor PP2 blocked secretion without affecting immobilization or internalization. We propose that immobility is a feature of highly crosslinked immunoreceptor aggregates and a trigger for receptor internalization, but is not required for tyrosine kinase activation leading to secretion.