The Journal of Experimental Medicine ARTICLE Vol. 203, No. 1, January 23, 2006 63–72 www.jem.org/cgi/doi/10.1084/jem.20051450 63 The response of peripheral B cells to T- dependent antigen involves a highly orches- trated series of changes. These are manifested by successive alterations in responsiveness to external signals, anatomic relocalization with passage through the germinal center (GC), mod- iications in Ig genes by somatic hypermutation (SHM) and class switch recombination (CSR), and the assumption of alternative fates as long- lived memory cells or Ig-secreting plasma cells (1). This multistep process of diferentiation is governed by an interlocking chain of transcrip- C.H. Lee and M. Melchers contributed equally to this work. The online version of this article contains supplemental material. C.H. Lee and M. Melchers contributed equally to this work. The online version of this article contains supplemental material. tion factors, some of which function as “master regulators” of developmental decision points, each with its own set of target genes (2–4). Among these factors, activation of NF-κB downstream of CD40 is required for initia- tion of the GC reaction (5) and induction of activation-induced cytidine deaminase (AICDA) (6), a protein required for SHM and CSR. BCL6 is also required for the formation of GC, where it acts in a B cell–autonomous manner to repress genes involved in the control of lymphocyte activation and cell cycle progres- sion, allowing rapid cell proliferation (4). BCL6 also arrests further development by acting in concert with PAX5 (BSAP) to repress genes Regulation of the germinal center gene program by interferon (IFN) regulatory factor 8/IFN consensus sequence-binding protein Chang Hoon Lee, 1 Mark Melchers, 2 Hongsheng Wang, 1 Ted A. Torrey, 1 Rebecca Slota, 2 Chen-Feng Qi, 1 Ji Young Kim, 4 Patricia Lugar, 2 Hee Jeong Kong, 4 Lila Farrington, 2 Boris van der Zouwen, 2 Jef X. Zhou, 1 Vassilios Lougaris, 2 Peter E. Lipsky, 3 Amrie C. Grammer, 2 and Herbert C. Morse III 1 1 Laboratory of Immunopathology, National Institute of Allergy and Infectious Diseases, 2 B Cell Biology Group and 3 Autoimmunity Branch, National Institute of Arthritis and Musculoskeletal and Skin Diseases, and 4 Laboratory of Molecular Growth Regulation, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892 Interferon (IFN) consensus sequence-binding protein/IFN regulatory factor 8 (IRF8) is a transcription factor that regulates the differentiation and function of macrophages, granu- locytes, and dendritic cells through activation or repression of target genes. Although IRF8 is also expressed in lymphocytes, its roles in B cell and T cell maturation or function are ill deined, and few transcriptional targets are known. Gene expression proiling of human tonsillar B cells and mouse B cell lymphomas showed that IRF8 transcripts were expressed at highest levels in centroblasts, either from secondary lymphoid tissue or transformed cells. In addition, staining for IRF8 was most intense in tonsillar germinal center (GC) dark- zone centroblasts. To discover B cell genes regulated by IRF8, we transfected puriied primary tonsillar B cells with enhanced green luorescent protein–tagged IRF8, generated small interfering RNA knockdowns of IRF8 expression in a mouse B cell lymphoma cell line, and examined the effects of a null mutation of IRF8 on B cells. Each approach identiied activation-induced cytidine deaminase (AICDA) and BCL6 as targets of transcriptional activation. Chromatin immunoprecipitation studies demonstrated in vivo occupancy of 5′ sequences of both genes by IRF8 protein. These results suggest previously unappreciated roles for IRF8 in the transcriptional regulation of B cell GC reactions that include direct regulation of AICDA and BCL6. CORRESPONDENCE Amrie C. Grammer: grammera@mail.nih.gov OR Herbert C. Morse III: hmorse@niaid.nih.gov Abbreviations used: AICDA, activation-induced cytidine deaminase; CB, centroblastic; ChIP, chromatin immunopre- cipitation; CSR, class switch recombination; EGFP, en- hanced GFP; EICE, Ets/IRF composite element; GC, germi- nal center; ICSBP, IFN consen- sus sequence-binding protein; IRF, IFN regulatory factor; PBB, peripheral blood B cell; qPCR, quantitative RT-PCR; SHM, somatic hypermutation; siRNA, small interfering RNA. CORRESPONDENCE Amrie C. Grammer: grammera@mail.nih.gov OR Herbert C. Morse III: hmorse@niaid.nih.gov Abbreviations used: AICDA, activation-induced cytidine deaminase; CB, centroblastic; ChIP, chromatin immunopre- cipitation; CSR, class switch recombination; EGFP, en- hanced GFP; EICE, Ets/IRF composite element; GC, germi- nal center; ICSBP, IFN consen- sus sequence-binding protein; IRF, IFN regulatory factor; PBB, peripheral blood B cell; qPCR, quantitative RT-PCR; SHM, somatic hypermutation; siRNA, small interfering RNA.