(14). BSAP is also important for proliferation and isotype switching in germinal center B cells (4,11,16,20,30). BSAP is expressed throughout B-cell development until the terminally differentiated plasma cell stage (1, 4). BSAP can either activate or repress transcription (33). Targets of BSAP activation include VpreB, 5, CD19, and blk (B lymphoid kinase) (8,15,17,22,35). BSAP represses J chain, the immunoglobulin heavy-chain 3ЈC␣ enhancer and XBP-1 (12,25,28).B-lymphocyte-induced maturation protein 1 (Blimp-1, encoded by the prdm1 gene) is a critical regulator of plasma cell differentiation, induced during cytokine-dependent differentiation of a B-cell lymphoma line (BCL-1) (29) and after lipopolysaccharide (LPS) treatment of primary murine splenocytes (2). Blimp-1 is expressed in all plasma cells and in a subset of germinal center B cells with a partial plasma cell phenotype but not in memory B cells (3). Ectopic expression of Blimp-1 in BCL-1 cells and in primary splenic B cells is sufficient to cause terminal differentiation and immunoglobulin M (IgM) secretion (2,19,26,29).Blimp-1 is a transcriptional repressor. Its DNA-binding activity is conferred by five zinc-finger motifs (7), whereas association with histone deacetylases (34) and hGroucho (24) is required for transcriptional repression. One important target of Blimp-1 repression is c-myc (10). Although repression of c-myc is necessary for terminal differentiation of BCL-1 cells, it is not sufficient, suggesting the existence of additional Blimp-1 targets (9). Indeed, MHC2A, encoding CIITA, a coactivator for major histocompatibility class II (MHC-II) transcription, was recently identified as a Blimp-1 target, providing a mechanism for extinction of MHC-II expression during plasma cell differentiation (19). We demonstrate here that Blimp-1 represses Pax-5 and show that Blimp-1-dependent repression of Pax-5 is required for plasma cell differentiation. MATERIALS AND METHODSCell culture. BCL-1 (CW13.20-3B3, ATCC CRL 1669), P3X (P3X63Ag8), 18-81 Raji, and primary splenocytes were cultured in RPMI medium supplemented with 10% heat-inactivated fetal bovine serum (FBS; Gemini Bio-Products, Inc.), 20 g of gentamicin (Gemini)/ml, and 50 M ␤-mercaptoethanol. To induce differentiation of BCL-1, cells (5 ϫ 10 5 cells/ml) were stimulated with interleukin-2 (IL-2) and IL-5, as described previously (29), for various times. 3T3 and Phoenix cells (G. Nolan, Stanford University) were cultured in Dulbecco modified Eagle medium supplemented with 10% FBS and 20 g of gentamicin/ ml. WI-L2 transfectants were cultured in the phenol red-free RPMI medium supplemented with 10% charcoal-dextran-treated FBS (HyClone) and penicillin-streptomycin (Gibco-BRL) and cultured in the presence of the selection antibiotic, hygromycin B (500 g/ml; Gibco-BRL). 4-Hydroxytomaxifen was dissolved in 70% ethanol (1 M) and CdSO 4 (5 M) from Sigma.Plasmids. To generate a Blimp-1 binding site mutated reporter, a wild-type luciferase reporter dependent on the Pax-5 promoter (BSAP-Luc) (18) was used as ...

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.

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B lymphocyte-induced maturation protein-1 (Blimp-1) is a transcriptional repressor that is sufficient to trigger terminal differentiation in the B cell lymphoma BCL-1. In this study, we have determined the expression pattern of Blimp-1 in vivo in primary and secondary lymphoid organs of humans and immunized mice. Blimp-1 is expressed in plasma cells derived from either a T-independent or T-dependent response in plasma cells that have undergone isotype switching and those resulting from secondary immunization. Blimp-1 is also present in long-lived plasma cells residing in the bone marrow. However, Blimp-1 was not detected in memory B cells. This expression pattern provides further evidence of a critical role for Blimp-1 in plasma cell development, supporting earlier studies in cultured lines. Significantly, Blimp-1 was also found in a fraction (4–15%) of germinal center B cells in murine spleen and human tonsils. Blimp-1 expression in the germinal center is associated with an interesting subset of cells with a phenotype intermediate between germinal center B cells and plasma cells. In the mouse, Blimp-1+ germinal center B cells peak at day 12 postimmunization and disappear soon thereafter. They are not apoptotic, some are proliferating, they express germinal center markers peanut agglutinin or CD10 but not Bcl-6, and most express CD138 (syndecan-1), IRF4, and cytoplasmic Ig. Together, these data support a model in which B cell fate decisions occur within the germinal center and Blimp-1 expression is critical for commitment to a plasma cell, rather than a memory cell, fate.