Production of a chimeric form of CD23 that is oligomeric and blocks IgE binding to the Fc epsilonRI.

The low affinity receptor for IgE (Fc epsilonRII/CD23) has previously been shown to interact with IgE with a dual affinity. Three chimeric constructs were created containing the lectin domain (amino acids 172-188) or the "neck" and lectin domain (amino acids 157-188) attached to subunits of oligomeric proteins. All chimeras were incapable of interacting with IgE with either a high or low affinity, indicating that the alpha-helical stalk of CD23 is important for orienting the lectin heads such that an interaction with IgE can occur. This concept received further support in that a chimeric CD23 composed of the human CD23 stalk and the mouse CD23 lectin head bound mouse IgE with a dual affinity, but could only bind rat IgE with a low affinity. Effort was next concentrated on a construct consisting of the entire extracellular (EC) region of CD23. A mutation to the first cleavage site of CD23 (C1M) resulted in a more stable molecule as determined by a decrease of soluble CD23 release. A soluble chimeric EC-C1M was prepared by attaching an isoleucine zipper to the amino terminus (lzEC-C1M). The interaction with IgE by lzEC-C1M was found to be superior to that seen with EC-CD23. The lzEC-C1M could inhibit binding of IgE to both CD23 and the high affinity receptor for IgE, Fc epsilonRI, providing further evidence for a strong interaction with IgE. Fc epsilonRI inhibition (approximately 70%) was seen at equimolar concentrations of lzEC-C1M, implying the effectiveness of this chimera and suggesting its potential therapeutic value.