A putative silencer element in the IL-5 gene recognized by Bcl6

The Bcl6 gene is ubiquitously expressed in adult murine tissues and its product functions as a sequence-specific transcriptional repressor. Bcl6-deficient mice displayed eosinophilic inflammation caused by overproduction of Th2 cytokines. The regulatory mechanism of those cytokine productions by Bcl6 is controversial. When CD4(+) T cells from Bcl6-deficient and lck-Bcl6-transgenic mice were stimulated with anti-CD3 Abs, production of IL-5 among Th2 type cytokines was preferentially affected by the amount of Bcl6 in the T cells. We found a putative Bcl6-binding sequence (IL5BS) on the 3' untranslated region in the murine and human IL-5 genes, and specific binding of Bcl6 protein to the sequence was confirmed by gel retardation assay and chromatin immunoprecipitation assay. The binding activity of endogenous Bc16 was transiently diminished in Th2 but not in Th1 clones after anti-CD3 stimulation. The exogenous Bc16 repressed expression of the reporter gene with the 1L5BS in K562 cells and the repressor activity was lost by a point mutation of the IL5BS. Furthermore, the IL5BS was required for Bc16 to repress expression of the IL-5 cDNA. Thus, the IL5BS may act as a silencer element for Bcl6 to repress expression of the IL-5 gene.