CD23-MEDIATED HOMOTYPIC CELL-ADHESION - THE ROLE OF PROTEOLYSIS

CD23 is a multifunctional molecule expressed by cells of lymphoid, mye loid and hematopoietic lineages. As a cell surface molecule CD23 acts both as a low-affinity receptor for IgE (FcepsilonRII) and as a cell a dhesion molecule. CD23 can undergo autoproteolysis to release soluble 37-25-kDa CD23 (s-CD23) molecules with a range of cytokine activities. Here we show a causal link between the two apparently disparate funct ions of autoproteolysis and cell adhesion. The Epstein-Barr virus-tran sformed B cell line RPMI-8866 formed macroscopic cell clusters solely via CD23. Cell adhesion was inhibited by mAb to CD23 and by IgE. Cell adhesion was also dependent on serum as cells grown in serum-free medi a failed to form clusters. In serum-free conditions cell adhesion coul d be induced by the addition of not only 10 % FCS but also s-CD23. As s-CD23 is reported to possess proteolytic activity we screened a range of proteases to determine whether they also could induce cell adhesio n in serum-free medium. It was found that chymotrypsin and elastase in duced cell: cell adhesion in RPMI-8866 cells. The same panel of protea ses were screened against a range of CD23-positive (Jijoye, AF-10, T2, U937, ICH-1) and CD23-negative (RPMI-8226, 8226, MOLT-4, Ramos) cell lines. It was found that chymotrypsin and elastase induce cell adhesio n only in cells expressing CD23. Peptide mapping studies showed that c hymotrypsin and elastase cleaved immunoprecipitated CD23 near the same site by which 37-kDa s-CD23 is released (Ala 80). Serum demonstrated no proteolytic activity towards CD23. However, it was found that cells grown in serum-free medium released 25-kDa s-CD23 without the need fo r prior cleavage at the 37-kDa cleavage site. To confirm the role of p roteolysis in CD23-mediated cell adhesion we screened a range of prote ase inhibitors for their ability to antagonize this process. It was fo und that tosyl-lysine chloromethyl ketone inhibited CD23-mediated cell adhesion. Lactoperoxidase treatment, which inhibits CD23 cleavage, al so inhibited cell adhesion. Addition of chymotrypsin and elastase to l actoperoxidase-treated cells induced cell adhesion. From these data we propose that intact CD23 has no demonstrable role in cell adhesion; i nstead, the portion of CD23 remaining on the cell surface following cl eavage appears to mediate cell adhesion.