An enzyme-linked immunosorbent assay specific for transient (29-37-kDa) fragments of soluble CD23/IgE-binding factors

The low-affinity IgE receptor (Fc epsilon RII) of B cells and monocytes--also known as CD23--is released from the cell surface by proteolytic cleavage to yield a series of soluble fragments which can accumulate in cell culture supernatants and body fluids. Of these, the most stable is a 25-kDa molecule which is generated from transient intermediates ranging in size from 29 to 37 kDa. It has been claimed that these latter species act as IgE-promoting factors while the 25-kDa molecule is endowed with various cytokine-like activities which are independent of IgE binding. We describe here a novel enzyme-linked immunosorbent assay (ELISA) which allows for the distinction between these two classes of soluble CD23. It is based on the observation that the CD23 antibody EBVCS1 can capture recombinant 29-kDa and 37-kDa fragments of CD23 but does not bind to the 25-kDa species: when EBVCS5 is used as the capture antibody, all three fragments are bound. The availability of these differential ELISA should facilitate investigations on the biological properties of CD23 fragments in health and disease.