Necessity of the stalk region for immunoglobulin E interaction with CD23

Previously, a soluble mouse CD23 chimera, composed of an N-terminal trimeric isoleucine zipper motif (lz) followed by the entire extracellular region (amino acids 48-331) of CD23 (lz-CD2348-331), was prepared and exhibited strong binding to rodent immunoglobulin E (IgE). In the current study, we report the construction of a similar human chimeric protein (lz-huCD2345-321), as well as a series of murine chimeric lz-CD23 mutants with incremental portions of stalk deleted, to further investigate the role of the stalk region in mediating the CD23-IgE interaction. All chimeric proteins were designed such that the predicted heptad structure of the stalk was retained. IgE binding, as determined by the capacity to inhibit 125I-IgE from binding to FcepsilonRI-bearing RBL-2H3 cells, and by surface plasmon-resonance analysis using an IgE-coated sensor chip, was unchanged from the original lz chimera and the binding parameters were similar to those of cell-surface CD23. The minimal murine chimera that retained IgE-binding activity was lz-CD23139-331, which still contains 35 amino acids of the stalk region. When the lz motif was linked to CD23 amino acid 157 (or higher), significant IgE-binding capacity was lost. With human lz-CD23, as with mouse, deletion of the stalk greatly reduced IgE-binding ability. In summary, the data support the concept that at least a portion of the stalk region of CD23 plays a crucial role in maintaining high-affinity/avidity interaction with IgE. The lz-CD23 constructs represent a possible alternative for both blocking the IgE/FcepsilonRI interaction and inhibiting IgE production by B lymphocytes.