SUMMARYPreviously, a solublemouse CD23 chimera,composedofanN-terminaltrimeric isoleucinezippermotif (lz) followed by the entire extracellular region (amino acids 48-331) of CD23 (lz-CD23 48À331

A chimeric soluble CD23, consisting of the extracellular domain of mouse CD23 and a modified leucine zipper (lz-CD23), has been shown to inhibit IgE binding to the FcεRI. A similar human CD23 construct was also shown to inhibit binding of human IgE to human FcεRI. In both systems, the inhibition was found to be temperature dependent; a 10-fold molar excess of lz-CD23 gave 90–98% inhibition at 4°C, dropping to 20–30% inhibition at 37°C. Surface plasmon resonance analysis of lz-CD23 binding to an IgE-coated sensor chip suggested that the effective concentration of lz-CD23 was lower at the higher temperatures. Analysis of 125I-IgE binding to CD23+-Chinese hamster ovary cells also indicated that increased temperature resulted in a lower percentage of IgE capable of interacting with CD23. In contrast, IgE interacts more effectively with FcεRI+-rat basophilic leukemia cells at 37°C compared with 4°C. The results support the concept that the open and closed IgE structures found by crystallography interact differently with the two IgE receptors and suggest that temperature influences the relative percentage of IgE in the respective structural forms. Changes in CD23 oligomerization also plays a role in the decreased binding seen at physiological temperatures.

Bryostatin-1, a macrocyclic lactone, is an antineoplastic agent that potently activates protein kinase C. Bryostatin-1 (Bryo) had an immunomodulatory effect on murine B cells in that it specifically inhibited IgE production. IgE levels were inhibited in a B cell dose-response curve, whereas IgM and IgG1 were induced by Bryo treatment. Taken together, ELISPOT and surface Ig staining data suggested that Bryo inhibition occurred at the level of class switching. RT-PCR and real time PCR data showed that this inhibition was achieved at an early step in switch recombination, namely, the appearance of Iε germline transcripts. Although Bryo caused a delay in the proliferative response of IL-4/CD40 ligand trimer-stimulated B cells, CFSE studies revealed that the Bryo-mediated inhibition of class switching to IgE occurred independently of the number of division cycles. Notably, Bryo showed the same specific IgE inhibition in human B cells. This study provides evidence for a unique mechanism regulating IgE production possibly downstream of PKC by specifically modulating Iε germline transcription.