POLYMER IGM ASSEMBLY AND SECRETION IN LYMPHOID AND NONLYMPHOID CELL-LINES - EVIDENCE THAT J-CHAIN IS REQUIRED FOR PENTAMER IGM SYNTHESIS

The requirements for IgM assembly and secretion were evaluated by intr oducing a constitutively expressed J-chain cDNA into lymphoid and nonl ymphoid cell lines expressing the secretory form of monomer IgM. Assay s of cell lysates and supernatants showed that only secretory monomer IgM is required for the synthesis and secretion of hexamer IgM, wherea s J chain, as well as the secreted form of monomer, is required for th e synthesis and secretion of pentamer IgM. Moreover, J chain facilitat es the polymerization process so that pentamer IgM is preferentially s ynthesized. Other components of the polymerization process were found to be shared by all the cell lines examined, whether the cells were of lymphoid or nonlymphoid origin and had a rudimentary or developed sec retory apparatus. These results identify monomer IgM and J chain as th e two components that determine the B-cell-specific expression of IgM antibodies and, thus, as the appropriate targets for therapeutic regul ation of IgM responses.