Expression of mu and gamma 1 membrane forms of immunoglobulin segregate in somatic cell hybrids.

In the present investigation, we have utilized the somatic cell hybridization technique to generate an experimental model for studying the differential expression of membrane (mIg) and secreted (sIg) forms of immunoglobulin that characterize different stages of B cell development. We describe here that fusion of the dextran-binding myeloma, MOPC 104E (mu, lambda 1) and the phthalate-binding B cell hybridoma, 2C3E1 (gamma 1, kappa) results in the formation of antigen-specific, double hybrids (tribrids) that coexpress both parental secreted forms of Ig but express only one of the two possible membrane forms of immunoglobulin (Ig). This segregated expression of membrane Ig is a new and unexpected finding that has been substantiated here by both immunological and biochemical methods. Analysis by SDS-containing polyacrylamide gels (SDS-PAGE) reveals distinct and characteristic migration patterns for each of the four Ig heavy chains in the tribrids (mu membrane, mu secreted, gamma 1 membrane and gamma 1 secreted). Immunochemical analysis of the immunoglobulin from the tribrids confirms the coexpression of both secreted forms of immunoglobulin in most of the tribrid lines tested and indicates that about 30% of the tribrids express only phthalate-specific gamma 1 membrane Ig, while 38% express only dextran-binding mu membrane Ig. About 30% of the tribrids secrete both antibodies but express no membrane form and less than 1% are non-secretors. Approximately 2% initially express both membrane forms of Ig, as determined by immunocytoadherence assay using appropriate target cells but subsequently express only one membrane form during propagation in vitro. SDS-PAGE analysis of surface labeled tribrids confirms that in tribrids expressing membrane Ig, only a single mIg is synthesized. These results suggest that the expression of the secreted and membrane forms of immunoglobulin are separately regulated and the tribrids represent a model with which to study the mechanisms involved in the regulation of each structurally distinct immunoglobulin form.