Enforced BCL6 Expression Inhibits B Cell Development in Vivo.

The B-Cell Lymphoma 6 (BCL6) gene encodes for a zinc finger motifs containing transcriptional repressor that is frequently dysregulated by chromosomal translocations in germinal center lymphomas. A putative protooncogene, its transforming ability in vivo was reported in I-mu-HA-BCL6 knock-in mice by Cattoretti et al last year. We also tested this assumption in transgenic mice expressing BCL6 in B cells under the control of kappa light chain regulatory elements. We replaced the murine C-kappa locus with the 16kb human BCL6 genomic locus in a construct containing the murine kappa light chain regulatory elements (Vk, EiK, 3′RR).

While control transgenics were readily obtained (5/32 founders), only 3/68 founders were positive for the BCL6 transgene, of which only one (bearing a single copy of the transgene) was able to transmit the transgene to its progeny, thus suggesting embryonal toxicity of exogenous BCL6.

In the bone marrow, flow cytometry revealed a nearly complete block of B cell development at the pro-B to pre-B transition. This was also the stage at which we first detected expression of EGFP in control reporter mice that were generated in parallel. Spleens of transgenic mice weighed about 50% of control spleens and less than 5% of splenocytes were CD19+ B cells. These were IgM high, IgD intermediate, corresponding to an immature B cell phenotype. Lymph nodes were smaller and B cells barely detected. Peyers’ patches were not visible.

Combined, our analysis of 6–8 weeks old VkHABCL6 transgenic mice reveals that enforced expression of BCL6 early in development results in a profound block of B lymphocyte differentiation. How transgenic BCL6 modulates this effect at the transcriptional level remains to be investigated. To test the oncogenic potential of BCL6 in B cells, it will be interesting to precisely turn on this gene in the germinal center.