MECHANISM OF FORMATION OF HUMAN IGE-BINDING FACTORS (SOLUBLE CD23) .3. EVIDENCE FOR A RECEPTOR (FC-EPSILON-RII)-ASSOCIATED PROTEOLYTIC ACTIVITY

There is mounting evidence that FceRII (CD23) and its soluble fragments (IgE-binding factors [BFs] or soluble CD23) have pleiotropic activities. IgE-BFs are formed mainly by the proteolytic cleavage of surface FceRII; they are first released as 37- and 33-kD unstable molecules that are subsequently transformed into 25-kD IgE-BFs. In this study, purified and radioiodinated 37-kD IgE-BFs as well as 45-kD FceRII were used as substrates to identify the proteases leading to the formation of 25-kD IgE-BFs. These substrates generate 25-kD IgE-BFs when incubated with several Fcc-RII-bearing cells, including CH01-7 cells (transfected with FcεRII cDNA); by contrast FceRII - cells, including CHO control cells, have no effect. Highly purified unlabeled native 37-kD and recombinant 29-kD IgE-BFs also cleave labeled 45-kD FcεRII into 25-kD IgE-BFs. The proteolytic activity of these purified IgE-BFs is specifically removed by immunoprecipitation with an antibody against IgE-BFs. These data strongly suggest that FceRII and some of its soluble fragments play an active role in the proteolytic mechanism generating IgE-BFs. They are supported by the observation that IgE-BFs released by CHOl-7 cells are cleaved exactly at the same sites as B cell-derived IgE-BFs. Taken collectively, the results are compatible with an autoproteolytic process. © 1990, Rockefeller University Press., All rights reserved.