SECRETION OF SOLUBLE PRE-B-CELL RECEPTORS BY PRE-B CELLS

Pre-B cells can express secretory mu (mu(s))- as well as membrane mu ( mu(m))-chains. We evaluated the ability of mu(s)-chains to associate w ith surrogate light chains and assemble into a pre-B cell receptor (BC R) complex in pre-B cells, and explored whether mu(s)-chains could be exploited to generate a secreted soluble pre-BCR. We demonstrate that mu(s)-chains can associate with SLC internally. The mu(s)-containing c omplexes form higher order polymeric structures, but these are never a ssembled into completed covalent structures. Instead, the complexes ar e efficiently retained and rapidly degraded. Alteration of the intrace llular redox state by incubation with 2-ME resulted in the secretion o f mu(s)-chains, suggesting that they are retained by a thiol-mediated retention mechanism. To identify the sequences on mu(s)-chains respons ible for their retention, we generated stable transfectants of a mu-ne gative pre-B cell line expressing either wild-type or mutant mu(s) con structs. Mutation of a single cysteine (Cys575) in the mu(s) tailpiece resulted in the release and secretion of the mu(s) H chains. These we re associated with the surrogate light chain proteins lambda 5 and Vpr eB, and thus appear to constitute an authentic secreted soluble pre-BC R. The soluble pre-BCR has a specificity distinct from Ab consisting o f the same heavy chain V region paired with conventional light chains.