B cell receptor engagement and T cell contact induce bcl-6 somatic hypermutation in human B cells: Identity with Ig hypermutation

The human bcl-6 proto-oncogene has been found to be mutated in both neoplastic and normal B cells. We used CL-01 cells, our monoclonal model of germinal center differentiation, and normal human B cells to explore the induction requirements and the modalities of bcl-6 hypermutation. As we have previously shown, CL-01 cells are IgM⁺ IgD⁺ and effectively mutate the expressed Ig V(H)DJ(H) and VλJλ genes and switch to IgG, IgA, and IgE upon B cell receptor engagement and contact with CD4⁺ T cells through CD40:CD154 and CD80:CD28 coengagement. In this paper we showed that the same stimuli induce somatic hypermutation of bcl-6 in CL-01 and normal IgM⁺ IgD⁺ B cells. bcl-6 hypermutation was not accompanied by translocation of this proto-oncogene or hypermutation of the β-actin gene, and it did mimic Ig hypermutation. It was associated with transcription initiation, in that it targeted the first exon and a 696-bp sequence immediately downstream (~0.6 kb) of the transcription initiation site while sparing further downstream (~2.5 kb) and upstream (~0.1 kb) areas. bcl-6 hypermutation displayed an overall rate of 2.2 x 10^-4 changes/base/cell division with characteristic nucleotide preferences and showed strand polarity. These findings show that B cell receptor engagement promotes hypermutation in genes other than Ig, and suggest that cis-regulating elements similar to those of the Ig locus exist in bcl-6.