http://immunol.nature.com • may 2002 • volume 3 no 5 • nature immunology Kenji Tanigaki 1, *, Hua Han 1, *, Norio Yamamoto 1 , Kei Tashiro 2 , Masaya Ikegawa 2 , Kazuki Kuroda 1 , Akira Suzuki 3 ,Toru Nakano 4 and Tasuku Honjo 1 Published online: 22 April 2002, DOI: 10.1038/ni793 RBP-J is a key mediator of Notch signaling that regulates cell fate determination in various lineages. To investigate the function of Notch–RBP-J in mature B cell differentiation, we generated mice that selectively lacked B cell RBP-J expression using conditional mutagenesis.Absence of RBP-J led to the loss of marginal zone B (MZB) cells with a concomitant increase in follicular B cells; in contrast, B1 cells in the peritoneal cavity were unaffected. Lack of RBP-J caused no defects in B cells maintenance, survival, plasma cell differentiation or activation. It is therefore likely that Notch–RBP-J signaling regulates the lineage commitment of mature B cells into follicular versus MZB cells. In addition, in mice with RBP-J–deficient B cells, had no obvious changes in immunoglobulin production in response to Ficoll, lipopolysaccharide or chicken gammaglobulin. In contrast, these mice exhibited increased mortality rates after blood-borne bacterial infection, which indicates that MZB cells play pivotal roles in the clearance of these bacteria. 1 Department of Medical Chemistry and 2 Center for Molecular Biology and Genetics, Graduate School of Medicine, Kyoto University,Yoshida-Konoe, Sakyo-Ku, Kyoto, 606-8501, Japan. 3 Department of Biochemistry, Akita University School of Medicine, Akita 010-8543, Japan. 4 Department of Molecular Cell Biology, Research Institute for Microbial Diseases, Osaka University, Suita 565-0871, Japan. *These authors contributed equally to this work. Correspondence should be addressed to T. H. (honjo@mour.med.kyoto-u.ac.jp). Notch–RBP-J signaling is involved in cell fate determination of marginal zone B cells The mammalian Notch family consists of four highly conserved trans- membrane receptors that regulate cell fate determination in various tis- sues 1 . Ligand binding to the Notch receptors leads to proteolytic pro- cessing within the transmembrane domains that results in release of the intracellular domains 2,3 . The Notch intracellular domain translocates to the nucleus and acts as a transcriptional activator in association with a DNA-binding protein, RBP-J, through the RBP-J–association molecule (RAM) domain 4 . RBP-J is ubiquitously expressed and associates with the intracellular regions of all four types of Notch 5 . Association of the Notch intracellular domain with RBP-J replaces corepressors from RBP-J 6 and up-regulates transcription of a limited number of target genes, including Hes1 and Hes5, that play a critical role in differentia- tion regulation 7–9 . In the hematopoietic system, Notch functions at various stages, including the maintenance and proliferation of hematopoietic stem cells 10 and regulation of myeloid progenitor cell development 11,12 . Induced deletion of Notch1 in adult mouse bone marrow results in impaired early T cell development and ectopic differentiation of B cells in the thymus 13 . Conversely, retroviral expression of constitutively active Notch-1 in bone marrow progenitors causes the suppression of early B cell development and the ectopic development of immature T cells in the bone marrow 14 . Thus, Notch signaling determines T cell fate and inhibits B cell development. This observation was confirmed by induced deletion of the gene encoding RBP-J (Rbpsuh) in adult bone marrow cells 15 . In vitro studies that used the overexpression of activated Notch sug- gest that Notch may be involved in B cell function. In an avian imma- ture B cell line, DT40, expression of activated Notch induces G1 arrest and apoptosis 16 . In human and chicken mature B cell lines, activated Notch expression down-regulates immunoglobulin (Ig) heavy chain expression and up-regulates CD21, a B cell receptor (BCR) coreceptor that enhances signals from the BCR 17,18 . Thus, Notch signaling may also affect differentiation and/or activation of mature B cells. B-lineage cells differentiate from common lymphoid precursor cells in the bone marrow. Mature B cells that have undergone successful DNA recombination in the Ig variable region genes leave the bone mar- row after selection by self-antigen 19 . Two types of transitional mature B cell precursors exist in the spleen 20 . Type 1 (T1) transitional B cells (IgM hi IgD – CD21 – CD23 – ), which are recent immigrants from the bone marrow, develop into type 2 (T2) transitional B cells (IgM + IgD + CD21 + CD23 + ) in the spleen. In spleen, immature transitional B cells can fur- ther differentiate into follicular B and marginal zone B (MZB) cells, which are characterized by the surface markers CD21 int CD23 hi and CD21 hi CD23 lo , respectively. Follicular B cells respond to thymus- dependent (TD) antigens and proliferate extensively to form germinal centers, in which somatic hypermutation and class switch recombina- tion take place. However, the function of MZB cells is not clear. In addition, molecular mechanisms that regulate the differentiation of MZB versus follicular B cells remain to be determined. MZB cell num- bers are reduced in genetically manipulated mice with loss-of-function A RTICLES 443 © 2002 Nature Publishing Group http://immunol.nature.com