CD23 (FC-EPSILON-RII) RELEASE FROM CELL-MEMBRANES IS MEDIATED BY A MEMBRANE-BOUND METALLOPROTEASE

CD23 (low-affinity IgE receptor, Fc epsilon RII) is expressed as a Typ e II extracellular protein on a variety of cells such as B-cells, mono cytes and macrophages and is cleaved from the cell surface to generate several distinct fragments. The expression of CD23 on the cell surfac e as well as the generation of soluble fragments of CD23 has been show n to be involved in the regulation of IgE synthesis. Here we report th at the release of CD23 from the cell surface is mediated by a metallop rotease. An assay utilizing purified CD23 and an neo-epitope antibody specific for one of the known cleavage products is described and used to demonstrate unambiguously the cleavage of CD23 by a distinct protea se. Characterization of the mechanism of CD23 processing shows that th e protease exists as an integral membrane protein with a functional mo lecular mass of approx. 63 kDa as determined by gel-filtration chromat ography. The CD23-cleaving activity found in enriched plasma membranes from RPMI 8866 cells is inhibited by the metalloprotease inhibitors 1 ,10-phenanthroline and imidazole and by the matrix metalloprotease inh ibitor batimastat, but not by inhibitors of cysteine proteases, serine proteases or acid proteases. The same or a similar activity that clea ves CD23 to the known 33 kDa fragment and is inhibited by batimastat i s present in diverse cell types such as unstimulated fibroblasts and m onocytic cell lines not expressing CD23, as well as in the Epstein-Bar r virus-transformed B-cell line, RPMI 8866, which constitutively expre sses CD23.