CH gene rearrangements in IgM-bearing B cells and in the normal splenic DNA component of hybridomas making different isotypes of antibody.

To probe mechanisms operating at the CH gene locus during normal B lymphocyte differentiation, we have used cloned probes for the constant region genes Cmu, C gamma 1 and C alpha to analyze the immunoglobulin heavy chain genes of three kinds of cell populations in successive stages of B cell development. These are IGM-bearing B lymphocytes from the normal spleen of unprimed mice, hybridomas prepared by fusing spleen cells from antigen-primed mice with the SP2/O permanent cell line and selected to secrete one of five different isotypes (IgM, IgG3, IgG1, IgG2 and IgA) and a set of plasmacytoma lines. The IgM-bearing B cells carry Cmu genes with rearrangements between VH and JH genes on both chromosomes even though only one chromosome is expressed; clearly, allelic exclusion cannot be explained by the lack of CH gene rearrangement on the nonexpressed chromosome. The normal splenic DNA component of antibody-secreting hybridomas displays rearrangements between JH and Cmu genes as well as among CH genes other than Cmu, with concomitant deletion of CH genes 5' to those expressed. These CH rearrangements and deletions are likely to accompany the isotype switching process and may occur on both expressed and nonexpressed chromosomes. We used hybridomas (spleen-derived), which secrete primarily IgM, and plasmacytomas (gut-derived), which secrete primarily IgA, to represent plasma cells in early and late stages of differentiation, respectively. A direct comparison of hybridomas and plasmacytomas making the same products (IgG3 or IgG1) indicates that hybridomas display a low frequency (2/12) of nonexpressed C alpha gene rearrangements in contrast to the high frequency (7/10) displayed by plasma-cytomas. We propose that CH gene switching rearrangements and deletions may occur successively along the CH gene locus, involving any of the undeleted genes at each step. These can occur on both expressed and nonexpressed chromosomes during the normal clonal outgrowth of a B cell line in vivo, and would result in the accumulation of both productive and nonproductive rearrangements of the presumed last CH gene, C alpha.