Direct Repression of prdm1 by Bcl-6 Inhibits Plasmacytic Differentiation

We have identified two intronic regions of mouse prdm1, the gene encoding B lymphocyte-induced maturation protein-1 (Blimp-1), which confer transcriptional repression in response to Bcl-6. The Bcl-6 response element in intron 5, which is conserved between mice and humans, was studied in detail. It binds Bcl-6 in vitro and was shown by chromatin immunoprecipitation to be occupied by Bcl-6 in vivo. Neither Bcl-6 response element functions as a STAT3-response element, showing that STAT3 does not compete with Bcl-6 at these sites. Bcl-6(-/-) mice confirm the biological importance of Bcl-6-dependent repression of prdm1. These mice have elevated Ab response, increased Ig-secreting cells, and increased Blimp-1(+) cells in spleen following immunization and their splenic B cells show accelerated plasmacytic development in vitro.