Demethylation of the constant region genes of immunoglobulins reflects the differentiation state of the B cell.

Previous results showed a developmentally regulated, strong linkage between demethylation and transcriptional activity for the light chain kappa locus in the mouse (Kelley et al., Molec. cell. Biol. 8, 930-937, 1988). These results indicate the existence of a stage of development of the B cell in which permanent expression (which may be enhancer independent) of a gene is associated with its demethylation. According to this result, demethylation could mirror terminal differentiation of a cell. We tested this hypothesis by analyzing the methylation status of immunoglobulin (Ig) genes in normal B cells before and after their activation with lipopolysaccharide (LPS) to induce IgM secretion and an immunoglobulin class switch. This pattern of methylation has been compared with that of Ig genes in nonlymphoid tissues and in transformed cell lines. In general, transformed cells are terminally differentiated cells. Our results show, that in normal splenic B cells only regions proximal to the heavy chain enhancer are demethylated. The coding regions of the c mu, c delta and the c gamma 1 genes remain methylated regardless of transcription. Demethylation of the coding regions is only detectable in transformed cell lines. Hence demethylation of immunoglobulin genes may reflect a stage of terminal differentiation in which the transcription pattern of the cell is fixed. Methylation of the genes before terminal differentiation may be necessary to allow controlled expression of genes on the transcriptional level, such as by splicing and differential termination.