CD21 IS A LIGAND FOR CD23 AND REGULATES IGE PRODUCTION

THE molecule CD23, a low-affinity receptor for IgE (Fc-epsilon-R2)1,2, is a type II transmembrane molecule expressed on many haemopoietic cell types3,4. CD23 has pleiotropic roles in the control of lymphocyte behaviour5-9, suggesting that CD23 may interact with another ligand in addition to IgE. To identify such a CD23 ligand, we expressed and purified full-length recombinant CD23, incorporated it into fluorescent liposomes and used these as a probe. We report here that fluorescent liposomes carrying CD23 interact specifically with the cell-surface protein CD21, identified as the receptor for Epstein-Barr virus and the complement receptor-2 on B cells, some T cells and follicular dendritic cells. In addition, fluorescent CD23-liposomes were shown to bind to hamster kidney cells (BHK-21) transfected with CD21 complementary DNA. The interaction between fluorescent CD23-liposomes and B cells or CD21-transfected BHK-21 cells was specifically inhibited by anti-CD21 and anti-CD23 monoclonal antibodies. Western blotting analysis revealed that C-14-labelled liposomes carrying CD23, in contrast to anti-CD21 antibodies, reacted with a subtype of CD21 molecules. Triggering of CD21 either with an anti-CD21 antibody or with recombinant soluble CD23 was shown to increase specifically interleukin-4-induced IgE production from blood mononuclear cells. These results demonstrate that the cell-surface protein CD21 is a ligand for CD23 and that the pairing of these molecules may participate in the control of IgE production.