<p>The transcription factor Bcl-6 orchestrates germinal center (GC) reactions through its actions in B cells and T cells and regulates inflammatory signaling in macrophages. Here we found that genetic replacement with mutated Bcl6 encoding Bcl-6 that cannot bind corepressors to its BTB domain resulted in disruption of the formation of GCs and affinity maturation of immunoglobulins due to a defect in the proliferation and survival of B cells. In contrast, loss of function of the BTB domain had no effect on the differentiation and function of follicular helper T cells or that of other helper T cell subsets. Bcl6-null mice had a lethal inflammatory phenotype, whereas mice with a mutant BTB domain had normal healthy lives with no inflammation. The repression of inflammatory responses by Bcl-6 in macrophages was accordingly independent of the repressor function of the BTB domain. Bcl-6 thus mediates its actions through lineage-specific biochemical functions.</p>