Both immunoglobulin promoter and enhancer sequences are targets for suppression in myeloma-fibroblast hybrid cells.

When immunoglobulin (Ig)-producing B cells are fused with fibroblastic cells, expression of Igs is suppressed by a mechanism that selectively abolishes transcription of Ig genes. The suppression is also maintained in proliferating hybrids. We have used gene transfer followed by cell fusion to study this phenomenon further. Here we report that expression of a rearranged Ig heavy chain gene, stably integrated into a myeloma genome, is completely suppressed upon fusion with fibroblasts by a mechanism that is equally active on the endogenous myeloma lambda light chain gene. To define regulatory sequences within the Ig transcriptional unit that are involved in this down-regulation, we examined the transcriptional contributions of the IgH chain gene enhancer and the kappa light chain gene promoter individually by linking them to a heterologous reporter gene. Mouse myeloma cells were stably transformed with such test constructs and subsequently fused with mouse fibroblasts. To avoid any significant loss of chromosomes, hybrid cells were isolated shortly after fusion by fluorescence-activated cell sorting, and proliferating hybrids were harvested within 2-3 weeks. On the basis of RNase protection mapping of cytoplasmic RNA, and of nuclear run-on assays we showed that both the kappa light chain promoter and the IgH chain enhancer contain regulatory information that is made redundant or is suppressed in the hybrid environment.