Demonstration of specific high-affinity Fc?-receptors on the human basophil-like leukemia cell line KU812 by flow cytometry

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Abstract
Specificity of IgE binding to a human basophil‐like cell line (KU812) was studied by flow cytometry. Four IgE myeloma proteins, representing both light‐chain types, one chimeric IgE protein, and polyclonal serum IgE blocked the direct binding of FITC‐labeled IgE(DES) myeloma protein to KU812 cells in a dose‐dependent and nearly equimolar way. Although not as efficiently as human IgE (from five to eight times less on a molar basis), both rat and mouse IgE blocked IgE(DES)‐FITC binding to KU812 cells. In sharp contrast, all four human IgG subclasses, both IgA subclasses, and IgM myeloma proteins, as well as monomeric and heat‐aggregated polyclonal human IgG, were unable to block significantly IgE(DES)‐FITC binding to KU812 cells (≤0.5/0 on a molar basis). The cytophilic epitope on IgE was heat‐susceptible (56 °h), lost after reduction alkylation, and resident in the papain‐derived Fcɛ ‐fragment, but not in the papain‐derived F(ab') 2ɛ.‐ and Fcɛ‐fragments nor in the pepsin‐derived F(ab')2ɛ‐ and Fcɛ.'‐fragments. Washing and displacement experiments indicated that a major part of IgE reacted with high affinity to KU812 cells. The results indicate that the binding of IgE to KU812 cells is highly specific and involves the classical high‐affinity FcɛRI‐receptor. Although the density of receptors is low, this human cell line offers a unique model to study IgE/FcɛRI interactions.