BINDING OF ANTI-CD23 MONOCLONAL-ANTIBODY TO THE LEUCINE-ZIPPER MOTIF OF FC-EPSILON-RII CD23 ON B-CELL MEMBRANE PROMOTES ITS PROTEOLYTIC CLEAVAGE - EVIDENCE FOR AN EFFECT ON THE OLIGOMER/MONOMER EQUILIBRIUM/

In the present study we have compared the binding of two monoclonal an tibodies to CD23, EBVCS1 and mAb25, which recognize the stalk and the lectin domain, respectively, on the CD23 molecule. At 4 degrees C, EBV CS1 binds to about 10% of the receptors recognized by mAb25 on the B c ell surface, At 37 degrees C, whereas mAb25 reaches its maximal bindin g within a few seconds, EBVCS1 requires 60 min to bind to the same ext ent. Stabilization of the oligomeric structure of CD23 with IgE strong ly affects in a dose-dependent fashion the number of binding sites see n by EBVCS1 but not the t(1/2) to reach them, suggesting that EBVCS1 b inds to the coiled coil region through an allosteric mechanism, EBVCS1 rapidly down-modulates the membrane CD23 expression with a coincident increase of CD23-soluble fragments in the culture medium, an effect t hat is inhibited by IgE, In contrast, mAb25, as well as IgE, protects CD23 from proteolytic cleavage and stimulates its endocytosis, These r esults suggest that EBVCS1 unravels the coiled coil structure of CD23, rendering it more susceptible to proteolytic attack. This supports th e oligomeric model proposed previously (Gould, H,, Sutton, B,, Edmeade s, R,, and Beavil, A. (1991) Monogr. Allergy 29, 28-49), The biologica l significance of these observations is discussed.