FUNCTIONAL IMPLICATION FOR THE TOPOGRAPHICAL RELATIONSHIP BETWEEN MHC CLASS-II AND THE LOW-AFFINITY IGE RECEPTOR - OCCUPANCY OF CD23 PREVENTS LYMPHOCYTES-B FROM STIMULATING ALLOGENEIC MIXED LYMPHOCYTE-RESPONSES

Following the observation of Bonnefoy et al. (J. Exp. Med. 1988. 167: 57), that the low‐affinity IgE receptor (CD23) on B lymphocytes can be coupled (with the use of chemical cross‐linking reagents) to major histocompatibility complex (MHC) class II DR molecules, we now report that ligands binding within the lectin‐homology region of CD23 prevent B cells from stimulating allogeneic mixed lymphocyte responses. Ligands capable of blocking mixed lymphocyte responses include the anti‐CD23 antibodies MHM6 and EBVCS 4 but not EBVCS 1 and 5, IgE itself, and small peptides representing sequences within the CH3 domain of IgE. The detailed topographical relationship between CD23 and MHC class II on the B lymphocyte surface was examined using dual immuno‐fluorescence labeling of cells and direct visualization of the staining by confocal laser scanning microscopy. On transformed B lymphoblasts, the two antigens were seen to co‐localize in discrete patches; on normal B cells which had been cultured for 2 days with interleukin 4, CD23 and MHC class II converged at a single pole which exhibited a tendency to pseudopod formation and provided a focus for homotypic cell‐cell interactions. The possibility that CD23 could serve as a co‐stimulatory‐adhesion molecule in antigen presentation by B lymphocytes is discussed with special reference to a potential role in the regulation of IgE synthesis. Copyright © 1990 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim