Establishment of a Sensitive Radioimmunoassay for the Detection of Human IgE-Binding Factor (Soluble CD23)

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Abstract
A monoclonal antibody (mAb) specific to low-affinity receptor for IgE (FcεRII/CD23) was established by the fusion of spleen cells of BALB/c mice immunized with the FcεRII+ human B lymphoblastoid cell line (RPMI 8866) with mouse myeloma P3U1. Four mAbs, 10/3 (IgG1), 11/4 (IgG1), 12/2 (IgG2b) and 15/6 (IgM), almost completely inhibited the IgE binding to FcεRII+ cells but not to FcεRII–– cells. More directly, they were demonstrated to react only with 43-kD component/FcεRII of the cell lysate of RPMI 8866 cells by sodium dodecyl sulfate polyacrylamide gel electrophoresis and Western blot analysis. Since they have a different epitope specificity, a solid-phase radioimmunossay (RIA) for the measurement of IgE-binding factor (IgE-BF) was established. It was found that the RIA with the use of 10/3 and 125I-labeled 11/4 or 12/2 gave good results in the detection of IgE-BF derived from B cells and monocytes as well as of T-cell-derived IgE-BF. More importantly, serum IgE-BF was also quantitatively measured by this RIA. Although increased serum levels of IgE-BF were observed in atopic patients, serum IgE-BF was decreased rather than increased in patients with very high serum IgE. This phenomenon may be explained by the decreased ability of the patients’ B cells to spontaneously release IgE-BF in vitro.

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