Cultured Human Mast Cells Are Heterogeneous for Expression of the High-Affinity IgE Receptor FcεRI

Objective: We determined the density of FcΕRI on mast cells cultured from cord (CBMC) and peripheral blood (PBMC) and studied the kinetics of the response through FcΕRI. Methods: Mast cells were cultured from CD133+ progenitors from peripheral or cord blood. FcΕRI was stabilized by culture with 2 µg/ml IgE. Cells were activated by addition of anti-FcΕRI antibody (1 ng/ml–10 µg/ml). Maximal activation, sensitivity, and cooperativity were determined. Results: All cultures were homogeneous for tryptase and metachromasy. All cells expressing FcΕRI could be activated by cross-linking FcΕRI to upregulate CD63. PBMC bind 203,000 molecules of IgE/cell. Stabilization of FcΕRI with IgE doubled the number of CD63+ cells (p = 0.0001) and increased the sensitivity (from 0.083 to 0.013 µg/ml anti-FcΕRI) and the slope factor (from 10.8 to 68) of PBMC but not of CBMC. Anti-IgE reversed these effects (p = 0.0002) but did not reduce activation levels below that of cell lines not stabilized with IgE. Conclusion: Baseline expression of FcΕRI is independent of anti-IgE. The fraction of PBMC that binds high levels of IgE can be activated through FcΕRI.