Differential effect of B lymphocyte-induced maturation protein (Blimp-1) expression on cell fate during B cell development

Influence of nutrients and metabolites on the differentiation of plasma cells and implications for autoimmunity

The modulation of inflammatory (auto)immune reactions by nutrients and gut bacterial metabolites is of great interest for potential preventive and therapeutic strategies. B cell-derived plasma cells are major players in inflammatory (auto)immune responses and can exhibit pro- or anti-inflammatory effects through (auto)antibody-dependent and -independent functions. Emerging evidence indicates a key role of nutrients and microbial metabolites in regulating the differentiation of plasma cells as well as their differentiation to pro- or anti-inflammatory phenotypes. These effects might be mediated indirectly by influencing other immune cells or directly through B cell-intrinsic mechanisms. Here, we provide an overview of nutrients and metabolites that influence B cell-intrinsic signaling pathways regulating B cell activation, plasma cell differentiation, and effector functions. Furthermore, we outline important inflammatory plasma cell phenotypes whose differentiation could be targeted by nutrients and microbial metabolites. Finally, we discuss possible implications for inflammatory (auto)immune conditions.

BLMP-1 promotes developmental cell death in C. elegans by timely repression of ced-9 transcription

Programmed cell death (PCD) is a common cell fate in metazoan development. PCD effectors are extensively studied, but how they are temporally regulated is less understood. Here, we report a mechanism controlling tail-spike cell death onset during Caenorhabditis elegans development. We show that the zinc-finger transcription factor BLMP-1, which controls larval development timing, also regulates embryonic tail-spike cell death initiation. BLMP-1 functions upstream of CED-9 and in parallel to DRE-1, another CED-9 and tail-spike cell death regulator. BLMP-1 expression is detected in the tail-spike cell shortly after the cell is born, and blmp-1 mutations promote ced-9-dependent tail-spike cell survival. BLMP-1 binds ced-9 gene regulatory sequences, and inhibits ced-9 transcription just before cell-death onset. BLMP-1 and DRE-1 function together to regulate developmental timing, and their mammalian homologs regulate B-lymphocyte fate. Our results, therefore, identify roles for developmental timing genes in cell-death initiation, and suggest conservation of these functions.

Precocious expression of Blimp1 in B cells causes autoimmune disease with increased self-reactive plasma cells

The transcription factor Blimp1 is not only an essential regulator of plasma cells, but also a risk factor for the development of autoimmune disease in humans. Here, we demonstrate in the mouse that the Prdm1 (Blimp1) gene was partially activated at the chromatin and transcription level in early B cell development, although mature Prdm1 mRNA did not accumulate due to posttranscriptional regulation. By analyzing a mouse model that facilitated ectopic Blimp1 protein expression throughout B lymphopoiesis, we could demonstrate that Blimp1 impaired B cell development by interfering with the B cell gene expression program, while leading to an increased abundance of plasma cells by promoting premature plasmablast differentiation of immature and mature B cells. With progressing age, these mice developed an autoimmune disease characterized by the presence of autoantibodies and glomerulonephritis. Hence, these data identified ectopic Blimp1 expression as a novel mechanism, through which Blimp1 can act as a risk factor in the development of autoimmune disease.

Human PRDM2: Structure, function and pathophysiology

PRDM2/RIZ is a member of a superfamily of histone/protein methyltransferases (PRDMs), which are characterized by the conserved N-terminal PR domain, with methyltransferase activity and zinc finger arrays at the C-terminus. Similar to other family members, two main protein types, known as RIZ1 and RIZ2, are produced from the PRDM2 locus differing by the presence or absence of the PR domain. The imbalance in their respective amounts may be an important cause of malignancy, with the PR-positive isoform commonly lost or downregulated and the PR-negative isoform always being present at higher levels in cancer cells. Interestingly, the RIZ1 isoform also represents an important target of estradiol action downstream of the interaction with hormone receptor. Furthermore, the imbalance between the two products could also be a molecular basis for other human diseases. Thus, understanding the molecular mechanisms underlying PRDM2 function could be useful in the pathophysiological context, with a potential to exploit this information in clinical practice.

Hypermethylation of PRDM1/Blimp-1 promoter in extranodal NK/T-cell lymphoma, nasal type: an evidence of predominant role in its downregulation

The loss of PRDM1 expression is common in extranodal NK/T-cell lymphoma, nasal type (EN-NK/T-NT), but the role of promoter methylation in silencing PRDM1 expression remains unclear. Hence, we performed pyrosequencing analysis to evaluate the promoter methylation of PRDM1 gene in vivo and in vitro, to analyze the association between methylation and its expression, and to assess cellular effects of PRDM1 reexpression. The promoter hypermethylation of PRDM1 gene was detected in 11 of 25 EN-NK/T-NT cases (44.0%) and NK92 and NKL cells. The promoter hypermethylation of PRDM1 was significantly correlated with PRDM1 expression in vivo and in vitro, predominantly contributing to the loss of PRDM1 expression compared with genetic deletion and aberrant expression of miR-223 in EN-NK/T-NT. PRDM1 expression was significantly restored by demethylation treatment, which induced cell proliferation suppression, cell cycle arrest, and apoptosis increase. We also found that PRDM1 reexpression could downregulate the expression of Ets-1, T-bet, granzyme B, and c-myc. Our findings demonstrated that the promoter hypermethylation of PRDM1 harbored a predominant role in the downregulation of PRDM1 expression, significantly affecting the biological behavior of tumor cells in EN-NK/T-NT.

Factors That Regulate the Generation of Antibody-Secreting Plasma Cells

The generation of antigen-specific neutralizing antibodies and memory B cells is one of the most important immune protections of the host and is the basis for successful vaccination strategies. The protective antibodies, secreted by preexisting long-lived plasma cells and reactivated antigen-experienced memory B cells, constitute the main humoral immune defense. Distinct from the primary antibody response, the humoral memory response is generated much faster and with greater magnitude, and it produces antibodies with higher affinity and variable isotypes. Humoral immunity is critically dependent on the germinal center where high-affinity memory B cells and plasma cells are generated. In this chapter, we focus on recent advances in our understanding of the molecular mechanisms that govern fate decision for memory B cells and plasma cells and the mechanisms that maintain the long-lived plasma-cell pool, with emphasis on how the transcription factor Blimp-1 (B lymphocyte-induced maturation protein-1) helps regulate the above-mentioned immunoregulatory steps to ensure the production and maintenance of antibody-secreting plasma cells as well as how it directs memory cell vs plasma-cell fate. We also discuss the molecular basis of Blimp-1 action and how its expression is regulated.

PRDM1/BLIMP1: a tumor suppressor gene in B and T cell lymphomas

The gene encoding the human BLIMP1, prdm1, is located on chromosome 6q21, a locus frequently deleted in lymphoid tumors. BLIMP1 is able to silence its target genes in a context-dependent manner through different mechanisms. BLIMP1 is expressed in both B and T cells, in which it plays important functions. In B cells, BLIMP1 acts as the master regulator of plasma cell differentiation, repressed by BCL6 and repressing both BCL6 and PAX5. In T cells, BLIMP1 is a critical factor for most terminal effector cell differentiation in both CD4+ and CD8+ T cells. BLIMP1 is frequently inactivated in a variety of lymphomas, including diffuse large B cell lymphomas, Natural Killer cell lymphoma and anaplastic large T cell lymphoma. In this review, we will summarize the role of BLIMP1 in normal cells, focusing on lymphoid cells, and on its function as tumor suppressor gene in lymphomas.

Redundant roles of PRDM family members in zebrafish craniofacial development

BACKGROUND

PRDM proteins are evolutionary conserved Zn-Finger transcription factors that share a characteristic protein domain organization. Previous studies have shown that prdm1a is required for the specification and differentiation of neural crest cells in the zebrafish.

RESULTS

Here we examine other members of this family, specifically prdm3, 5, and 16, in the differentiation of the zebrafish craniofacial skeleton. prdm3 and prdm16 are strongly expressed in the pharyngeal arches, while prdm5 is expressed specifically in the area of the forming neurocranium. Knockdown of prdm3 and prdm16 results in a reduction in the neural crest markers dlx2a and barx1 and defects in both the viscerocranium and the neurocranium. The knockdown of prdm3 and prdm16 in combination is additive in the neurocranium, but not in the viscerocranium. Injection of sub-optimal doses of prdm1a with prdm3 or prdm16 Morpholinos together leads to more severe phenotypes in the viscerocranium and neurocranium. prdm5 mutants have defects in the neurocranium and prdm1a and prdm5 double mutants also show more severe phenotypes.

CONCLUSIONS

Overall, our data reveal that prdm3, 5, and 16 are involved in the zebrafish craniofacial development and that prdm1a may interact with prdm3, 5, and 16 in the formation of the craniofacial skeleton in zebrafish.

BLIMP1α, the master regulator of plasma cell differentiation is a tumor supressor gene in B cell lymphomas

AIMS

The aim of this review was to summarize recent knowledge of the structure and function of a transcriptional repressor, B lymphocyte induced maturation protein 1 (BLIMP1) and its participation in the pathogenesis of B lymphomas.

METHODS AND RESULTS

This review summarizes the structure and function of BLIMP1, its major target genes and its role as a tumour suppressor in B cell lymphomas. We review our recent data implicating the loss of BLIMP1α as an important step in the pathogenesis of the Epstein-Barr virus (EBV) associated B cell lymphomas.

CONCLUSIONS

BLIMP1 is a transcriptional repressor essential for the differentiation of germinal centre (GC) B cells to plasma cells. The loss of BLIMP1 in GC B cells could contribute to the pathogenesis of EBV-associated lymphomas by preventing plasma cell differentiation and viral replication.

Cyclic AMP signaling inhibits megakaryocytic differentiation by targeting transcription factor 3 (E2A) cyclin-dependent kinase inhibitor 1A (CDKN1A) transcriptional axis

Signaling via the intracellular second messenger cyclic AMP (cAMP) has long been implicated in the repression of megakaryocytic differentiation. However, the mechanisms by which cAMP signaling impairs megakaryopoiesis have never been elucidated. In a human CD34(+) cell culture model, we show that the adenylyl cyclase agonist forskolin inhibits megakaryocytic differentiation in a protein kinase A-dependent manner. Using this system to screen for downstream effectors, we identified the transcription factor E2A as a key target in a novel repressive signaling pathway. Specifically, forskolin acting through protein kinase A-induced E2A down-regulation and enforced expression of E2A overrode the inhibitory effects of forskolin on megakaryopoiesis. The dependence of megakaryopoiesis on critical thresholds of E2A expression was confirmed in vivo in haploinsufficient mice and ex vivo using shRNA knockdown in human progenitors. Using a variety of approaches, we further identified p21 (encoded by CDKN1A) as a functionally important megakaryopoietic regulator residing downstream of E2A. These results thus implicate the E2A-CDKN1A transcriptional axis in the control of megakaryopoiesis and reveal the lineage-selective inhibition of this axis as a likely mechanistic basis for the inhibitory effects of cAMP signaling.

PRDM1/Blimp1 downregulates expression of germinal center genes LMO2 and HGAL

Human germinal center-associated lymphoma (HGAL) and LIM domain only-2 (LMO2) are proteins highly expressed in germinal center (GC) B lymphocytes. HGAL and LMO2 are also expressed in GC-derived lymphomas and distinguish biologically distinct subgroups of diffuse large B-cell lymphomas (DLBCL) associated with improved survival. However, little is known about their regulation. PRDM1/Blimp1 is a master regulator of terminal B cell differentiation and may also function as a tumor suppressor in the pathogenesis of DLBCL, where it is frequently inactivated by mutations and deletions. We now demonstrate that both HGAL and LMO2 are directly regulated by the transcription repressor PRDM1. In vivo studies demonstrate that PRDM1 directly binds to the recognition sites within the upstream promoters of both HGAL and LMO2. PRDM1 binding suppresses endogenous protein and mRNA levels of HGAL and LMO2. In addition, promoter analysis reveals that site-specific binding of PRDM1 to the promoters is capable of repressing transcriptional activity. This inhibitory effect of PRDM1 suggests that it has a key role in the loss of HGAL and LMO2 expression upon differentiation of GC B cells to plasma cells and may also contribute to absence of HGAL and LMO2 expression in post-GC lymphoid tumors.

Blimp1: driving terminal differentiation to a T

B lymphocyte maturation-induced protein-1 (Blimp1) is a transcriptional repressor expressed in diverse cell types. In the adaptive immune system, Blimp1 is expressed in lymphocytes that have undergone effector differentiation. Blimp1 is a master regulator of plasma cell differentiation and plays important roles in controlling T cell homeostasis and effector differentiation. Blimp1 can be induced by a variety of cytokines including IL-2, IL-4, IL-12, and IL-21 in addition to TCR and co-stimulatory signals. Blimp1-deficient mice develop spontaneous inflammatory disease mediated by infiltration of activated T cells into tissues. During immune responses Blimp1 is required for the differentiation of plasma cells as well as short-lived CD8(+) cytotoxic T cells. Mounting evidence suggests that Blimp1 plays a common role in the terminal differentiation of multiple cell subsets.

BLIMP1 against lymphoma: The verdict is reached

BLIMP1, a master regulator of plasma-cell differentiation, is implicated in the pathogenesis of Activated B cell (ABC)-like Diffuse Large B cell Lymphoma (DLBCL). In this issue of Cancer Cell, Mandelbaum and colleagues and Calado and colleagues unequivocally demonstrate that BLIMP1 functions as a tumor suppressor and guardian of ABC-like DLBCL lymphomagenesis.

PRDM1 is required for mantle cell lymphoma response to bortezomib

Mantle cell lymphoma (MCL) is an aggressive form of B-cell lymphoma with a poor disease-free survival rate. The proteasome inhibitor bortezomib is approved for the treatment of relapsed and refractory MCL and has efficacy in about 30% of patients. However, the precise mechanism of action of bortezomib is not well understood. This report establishes a requirement for the transcription repressor PR domain zinc finger protein 1 (PRDM1, Blimp1) in the response to bortezomib. Bortezomib rapidly induces transcription of PRDM1 as part of the apoptotic response in both cell lines and primary MCL tumor cells. Knockdown of PRDM1 blocks activation of NOXA and inhibits apoptosis, whereas ectopic expression of PRDM1 alone leads to apoptosis in MCL. Two novel direct targets of PRDM1 were identified in MCL cells: MKI67 (Ki67) and proliferating cell nuclear antigen (PCNA). Both MKI67 and PCNA are required for proliferation and survival. Chromatin immunoprecipitation and knockdown studies reveal that specific repression of MKI67 and PCNA is mediated by PRDM1 in response to bortezomib. Furthermore, promoter studies and mutation/deletion analysis show that PRDM1 functions through specific sites in the PCNA proximal promoter and an MKI67 distal upstream repression domain. Together, these findings establish PRDM1 as a key mediator of bortezomib activity in MCL.

PU.1 regulates positive regulatory domain I-binding factor 1/Blimp-1 transcription in lymphoma cells

The human positive regulatory domain I-binding factor 1 (PRDI-BF1) and its murine homolog Blimp-1 promote differentiation of mature B cells into Ab-secreting plasma cells. In contrast, ectopic expression of PRDI-BF1 in lymphoma cells can lead to inhibition of proliferation or apoptosis. However, little is currently known about the regulation of PRDM1, the gene encoding PRDI-BF1. This report establishes that in lymphoma cells stimulation through the BCR rapidly induces endogenous PRDM1 at the level of transcription with minor changes in mRNA stability. The induced PRDM1-encoded protein localizes to its target genes in vivo and suppresses their expression. In vivo genomic footprinting of the PRDM1 promoter in unstimulated lymphoma and myeloma cells reveals multiple common in vivo occupied elements throughout the promoter. Further functional and structural analysis of the promoter reveals that the promoter is preloaded and poised for activation in the B cell lines. The transcription factor PU.1 is shown to be required for the BCR-induced expression of PRDM1 in lymphoma cells and in PU.1-positive myeloma cells. Activation of PRDM1 is associated with loss of the corepressor transducin-like enhancer of split 4 from the PU.1 complex. These findings indicate that PRDM1 is poised for activation in lymphoma cells and therefore may be a potential therapeutic target to inhibit lymphoma cell proliferation and survival.

Involvement of histone demethylase LSD1 in Blimp-1-mediated gene repression during plasma cell differentiation

Plasma cell differentiation is orchestrated by the transcriptional repressor B lymphocyte-induced maturation protein-1 (Blimp-1), which silences the gene expression program of mature B cells. The molecular mechanism underlying Blimp-1 suppression of mature B-cell gene expression is not fully understood. Here we report that a proline-rich domain in Blimp-1 directly interacts with LSD1, a histone lysine demethylase. Both LSD1 knockdown and expression of Blimp-1 lacking the proline-rich domain derepressed the activities of Blimp-1-dependent luciferase reporters. Disruption of the Blimp-1 interaction with LSD1 or reduced LSD1 expression attenuated antibody production, demonstrating the biological significance of this interaction. Finally, using chromatin immunoprecipitation, we showed that Blimp-1 binding to its target sites is accompanied by LSD1 binding to those same sites and that LSD1 binding correlates with histone modifications of accessible chromatin. These findings provide further insights into the molecular mechanism of the silencing of mature B-cell genes by Blimp-1 in plasma cell differentiation.

Role of beta-catenin in B cell development and function

beta-Catenin is a central mediator of Wnt signaling pathway, components of which have been implicated in B cell development and function. B cell progenitors and bone marrow stromal cells express Wnt ligands, Frizzled receptors and Wnt antagonists, suggesting fine tuned regulation of this pathway in B cell development. In particular, deletion of Frizzled 9 gene results in developmental defects at the pre-B stage of development and an accumulation of plasma cells. Furthermore, Wnt signals regulate B cell proliferation through lymphocyte enhancer-binding factor-1. However, it is not known whether Wnt signaling in B cell development is mediated by beta-catenin and whether beta-catenin plays a role in mature B cell function. In this report, we show that mice bearing B cell-specific deletion of beta-catenin have normal B cell development in bone marrow and periphery. A modest defect in plasma cell generation in vitro was documented, which correlated with a defective expression of IRF-4 and Blimp-1. However, B cell response to T-dependent and T-independent Ags in vivo was found to be normal. Thus, beta-catenin expression was found to be dispensable for normal B cell development and function.

Regulation and functions of Blimp-1 in T and B lymphocytes

B lymphocyte-induced maturation protein-1 (Blimp-1), discovered 16 years ago as a transcriptional repressor of the IFNbeta promoter, plays fundamentally important roles in many cell lineages and in early development. This review focuses on Blimp-1 in lymphocytes. In the B cell lineage, Blimp-1 is required for development of immunoglobulin-secreting cells and for maintenance of long-lived plasma cells (LLPCs). Direct targets of Blimp-1 and the transcriptional cascades Blimp-1 initiates to trigger plasmacytic differentiation are described. Blimp-1 also affects the homeostasis and function of CD4(+), CD8(+), and regulatory CD4(+) T cells, and Blimp-1 levels are highest in antigen-experienced T cells. Blimp-1 attenuates T cell proliferation and survival and modulates differentiation. Roles for Blimp-1 in Th1/Th2 specification, regulatory T cell function, and CD8 differentiation and function are under investigation. Signals that induce Blimp-1 in B cells include Toll-like receptor ligands and cytokines; in T cells, T cell receptors and cytokines induce Blimp-1. In spite of some commonalities, different targets and regulators of Blimp-1 in B and T cells suggest intriguing evolutionary divergence of this regulatory machinery.

STAT3-mediated up-regulation of BLIMP1 Is coordinated with BCL6 down-regulation to control human plasma cell differentiation

STAT family members have been implicated in regulating the balance between B cell lymphoma (BCL)6 and B lymphocyte induced maturation protein (BLIMP)1 to control plasma cell differentiation. We previously showed that STAT5 induces BCL6 to block plasma cell differentiation and extend the life span of human B cells. The heterogeneity in STAT activation by cytokines and their effects on B cell differentiation prompted us to investigate the effect of STAT3 activation in plasma cell differentiation. First stimulation with IL-21, which promotes plasma cell differentiation, induced robust and prolonged STAT3 activation in primary human B cells. We then investigated effects of direct STAT3 activation on regulation of plasma cell genes, cellular phenotype, and Ig production. Activation of a tamoxifen-regulated STAT3-estrogen receptor fusion protein triggered BLIMP1 mRNA and protein up-regulation, plasma cell phenotypic features, and Ig secretion. When STAT3 was activated by IL-21 in B cells ectopically expressing BCL6, BLIMP1 was up-regulated, but only partial plasma cell differentiation was achieved. Lastly, through coexpression of BCL6 and STAT3-ER, we verified that STAT3 activation functionally mimicked IL-21 treatment and that STAT3-mediated BLIMP1 up-regulation occurred despite high BCL6 expression levels indicating that BCL6 is not the dominant repressor of BLIMP1. Thus, up-regulation of BLIMP1 alone is not sufficient for differentiation of primary human B cells into plasma cells; concomitant down-regulation of BCL6 is absolutely required for completion of the plasma cell differentiation program.

Genome-wide survey and developmental expression mapping of zebrafish SET domain-containing genes

SET domain-containing proteins represent an evolutionarily conserved family of epigenetic regulators, which are responsible for most histone lysine methylation. Since some of these genes have been revealed to be essential for embryonic development, we propose that the zebrafish, a vertebrate model organism possessing many advantages for developmental studies, can be utilized to study the biological functions of these genes and the related epigenetic mechanisms during early development. To this end, we have performed a genome-wide survey of zebrafish SET domain genes. 58 genes total have been identified. Although gene duplication events give rise to several lineage-specific paralogs, clear reciprocal orthologous relationship reveals high conservation between zebrafish and human SET domain genes. These data were further subject to an evolutionary analysis ranging from yeast to human, leading to the identification of putative clusters of orthologous groups (COGs) of this gene family. By means of whole-mount mRNA in situ hybridization strategy, we have also carried out a developmental expression mapping of these genes. A group of maternal SET domain genes, which are implicated in the programming of histone modification states in early development, have been identified and predicted to be responsible for all known sites of SET domain-mediated histone methylation. Furthermore, some genes show specific expression patterns in certain tissues at certain stages, suggesting the involvement of epigenetic mechanisms in the development of these systems. These results provide a global view of zebrafish SET domain histone methyltransferases in evolutionary and developmental dimensions and pave the way for using zebrafish to systematically study the roles of these genes during development.

BLIMP1 guides the fate of effector B and T cells

B-lymphocyte-induced maturation protein 1 (BLIMP1) is a transcriptional repressor, and its importance in controlling the terminal differentiation of antibody-secreting cells (ASCs) is well established. However, as we discuss in this Progress article, it has now become evident that the ASC programme consists of a discrete BLIMP1-independent initiation phase, followed by a second step in which BLIMP1 is absolutely required for the differentiation of fully mature ASCs. In addition, an important role for BLIMP1 in maintaining the homeostasis of effector T cells is emerging, suggesting intriguing parallels between the control of effector-cell fates in both B and T cells.

Ets-1 regulates plasma cell differentiation by interfering with the activity of the transcription factor Blimp-1

Development of immunoglobulin-secreting plasma cells from B cells is a tightly regulated process controlled by the action of a number of transcription factors. In particular, the transcription factor Blimp-1 is a key positive regulator of plasmacytic differentiation via its ability to suppress expression of genes involved in the mature B cell program. The transcription factor Ets-1 is a negative regulator of plasmacytic differentiation, as indicated by the development of increased numbers of IgM-secreting plasma cells in Ets-1 knock-out mice. We have previously shown that Ets-1-deficient B cells undergo enhanced differentiation into IgM-secreting plasma cells in response to Toll-like receptor 9 (TLR9) signaling. We now explore the mechanism by which Ets-1 limits differentiation downstream of TLR9. Our results indicate that Ets-1 physically interacts with Blimp-1, which leads to a block in Blimp-1 DNA binding activity and a reduction in the ability of Blimp-1 to repress target genes without interfering with Blimp-1 protein levels. In addition, we show that Ets-1 induces the expression of several target genes that are repressed by Blimp-1, including Pax-5. These results reveal a previously unknown mechanism for the control of Blimp-1 activity by Ets-1 and suggest that expression of Ets-1 must be down-regulated before plasmacytic differentiation can occur.

The Blimp-1 gene regulatory region directs EGFP expression in multiple hematopoietic lineages and testis in mice

The B lymphocyte-induced maturation protein-1 (Blimp-1) is a transcriptional repressor involved in B cell terminal differentiation. Recent studies suggest that it also plays roles in the differentiation of myeloid and T cells. Using reporter gene analysis, we found that the mouse Blimp-1 promoter constructs of 4.4 kb, 2.5 kb and 1.6 kb exerted similar transcription activity in vitro. To study the Blimp-1 promoter function in vivo, we generated a mouse transgenic line in which the expression of EGFP is driven by the 4.4 kb Blimp-1 promoter fragment. EGFP expression was detected in multiple hematopoietic lineages including hematopoietic progenitor cells (c-kit+), B cells at various developmental stages (B220+, IgM+, IgD+ and CD138+), myeloid cells (Gr1+ and CD11b+) and T cells (CD4+ and CD8+). Furthermore, we demonstrated, for the first time, that this Blimp-1 promoter fragment also drives EGFP expression in erythroid lineage (Ter119+) and embryonic livers. Finally, EGFP was expressed in the testes of transgenic mice in a cell-specific manner with expression weakly in primary spermatocytes and strongly in spermatids, but not in spermatogonia or spermatozoa. Our studies collectively suggest that Blimp-1 may play a general role in the development of hematopoietic lineages and a stage-specific role in spermatogenesis.

Epidermal terminal differentiation depends on B lymphocyte-induced maturation protein-1

The cornified layer is a compacted lattice of lipid-embedded corneocytes that provides an organism's barrier to the external environment. Cornification is the final differentiative step for epidermal keratinocytes and involves dramatic cell condensation before death. Using conditional gene deletion in mice, we identified the transcriptional repressor Blimp-1 (B lymphocyte-induced maturation protein-1) as an important regulator of keratinocyte transition from the granular to the cornified layer. More than 250 genes are misregulated in conditional knockout epidermis, including those encoding transcription factors, signal transduction components, proteinases, and enzymes involved in lipid metabolism. Steady-state mRNA and ChIP analyses of a subset of these genes provide evidence that nfat5, fos, prdm1, and dusp16 are novel direct targets of Blimp-1. Identifying nfat5 as a target of Blimp-1 repression indicates that cornification involves suppression of normal osmotic regulation in granular cells. Consistently, conditional knockout mice have delayed barrier formation as embryos, enlarged granular layer cells and corneocytes, and a morphologically abnormal cornified layer. These studies provide insight into cornification, identifying transcriptional regulatory circuitry and indicating the importance of blocking osmotic homeostasis.

A library of gene expression signatures to illuminate normal and pathological lymphoid biology

Genomics has provided a lever to pry open lymphoid cells and examine their regulatory biology. The large body of available gene expression data has also allowed us to define the of coordinately expressed genes, termed gene expression signatures, which characterize the states of cellular physiology that reflect cellular differentiation, activation of signaling pathways, and the action of transcription factors. Gene expression signatures that reflect the action of individual transcription factors can be defined by perturbing transcription factor function using RNA interference (RNAi), small-molecule inhibition, and dominant-negative approaches. We have used this methodology to define gene expression signatures of various transcription factors controlling B-cell differentiation and activation, including BCL-6, B lymphocyte-induced maturation protein-1 (Blimp-1), X-box binding protein-1 (XBP1), nuclear factor-kappaB (NF-kappaB), and c-myc. We have also curated a wide variety of gene expression signatures from the literature and assembled these into a signature database. Statistical methods can define whether any signature in this database is differentially expressed in independent biological samples, an approach we have used to gain mechanistic insights into the origin and clinical behavior of B-cell lymphomas. We also discuss the use of genomic-scale RNAi libraries to identify genes and pathways that may serve as therapeutic targets in B-cell malignancies.

Terminal differentiation of lymphocytes depends on Blimp-1

B lymphocyte induced maturation protein 1 (Blimp-1) has long been considered a master regulator of the terminal differentiation of B cells into antibody-secreting plasma cells. Gene-targeting experiments have now demonstrated that quantitative changes in Blimp-1 expression define plasma cell ontogeny--a process that requires the continual function of Blimp-1. Recently, new roles for Blimp-1 have been revealed, as a suppressor of diffuse large B cell lymphoma and as a key regulator of T-cell differentiation. Blimp-1 is expressed in differentiated effector T cells and controls their homeostasis. These new findings suggest that Blimp-1 has a conserved function in the final differentiation of both the cellular and the humoral arm of the adaptive immune response.

Cytokine-dependent Blimp-1 expression in activated T cells inhibits IL-2 production

After Ag activation of naive T cells in vitro, extensive growth and differentiation into effector cells depend upon IL-2. DNA microarray analysis was used to identify IL-2-dependent molecules regulating this process. In this study, we show that the transcriptional repressor B lymphocyte-induced maturation protein 1 (Blimp-1) is expressed by a cytokine-dependent pathway in activated T lymphocytes. IL-2 production by activated CD4(+) and CD8(+) T cells inversely correlated with Blimp-1 levels as higher IL-2 production was associated with lower Blimp-1 expression. Furthermore, ectopic expression of Blimp-1 by activated T cells inhibited IL-2 production but enhanced granzyme B and CD25 expression. Collectively, these findings indicate that there is a negative feedback regulatory loop in activated T cells such that IL-2 inhibits its own production through induction of Blimp-1 while promoting an effector cell phenotype.

Pax5--a critical inhibitor of plasma cell fate

Paired box protein 5 (Pax5) is essential for early B cell commitment as well as for B cell development, and continuous expression of Pax5 is required throughout the B cell lineage to maintain the functional identity of B cells. During B cell activation, Pax5 is downregulated before terminal differentiation into antibody-secreting plasma cells, and enforced expression of Pax5 prevents plasmacytic development. Recently, loss of Pax5 was shown to result in the substantial transition to a plasma cell state, demonstrating a functionally significant role for Pax5 in the regulation of terminal B cell differentiation. Here we elucidate the current understanding about the function of Pax5 as a key inhibitor of plasma cell differentiation.

Experimental immunization with Thalassophryne nattereri fish venom: Striking IL-5 production and impaired of B220+ cells

Murine experimental model have been useful to understanding the toxic as well as the pharmacological properties of the Thalassophryne nattereri venom. However, the specific immune response to T. nattereri venom in mice is yet unclear. Our results showed that the venom elicited in BALB/c mice high levels of specific IgG1 and total IgE isotype with high affinity, accompanied by a striking IL-5 production, what point out to a Th2-like response. Meanwhile, the production of IFN-gamma by lymphocytes pool expanded upon mitogen stimulus, suggests that the venom was also able to activate Th1 clones. Elevated number of antigen-presenting cells expressing CD11c or CD11b from day 4 to 6 supported ongoing antigen presentation process in the primary response and efficient T-cell expansion (increase of CD4(+) cells). In contrast, decreased B220 expression was observed, suggesting that the formation of memory long lived cell compartment. In conclusion, T. natterri venom stimulates an association of cytokine of both Th1 and Th2 profile, with a notable IL-5 production and specific IgG1 and total IgE isotypes secretion. Furthermore, our finding showed that T. natterri venom can affect the B cell fate and induce a memory antibody response through the secretion of protective IgG subclasses. Further studies with the venom protein toxins may provide clues to molecular mechanism regulating proliferation and differentiation of antibody-secreting cells in our model. A better understating of how T. natterri venom can modulate immune response could be useful in therapeutic strategies.

STAT5 represses BCL6 expression by binding to a regulatory region frequently mutated in lymphomas

Deregulated expression of BCL6 is a pathogenic event in many lymphomas. BCL6 blocks cellular differentiation by repressing transcription of its target genes, and this may promote tumorigenesis. Conversely, the transcription factor signal transducers and activators of transcription (STAT)5 promotes differentiation in many systems. STAT5 upregulates a number of genes repressed by BCL6, raising the possibility that STAT5 and BCL6 have opposing roles in transcriptional regulation. Therefore, we sought to determine the effects of STAT5 activation on BCL6 expression and function. We found that activation of STAT5 downregulates BCL6 expression in B-lymphoma cells and other hematopoietic cell lines. We identified two potential STAT-binding regions in the first exon and first intron of BCL6 that fell within regions of high inter-species homology, suggesting conservation of regulatory function. STAT5 can bind inducibly and regulate transcription at one of these regions, identifying BCL6 as a STAT5 target gene. Additionally, STAT5-mediated downregulation of BCL6 results in loss of BCL6 repression of its target genes, confirming that STAT5 is a negative regulator of BCL6 function. The STAT5 responsive region of the BCL6 gene is mutated frequently in B-cell lymphomas, suggesting that loss of the repressive effects of STAT5 on BCL6 might contribute to the pathogenesis of these cancers.

T cells climb on board Blimp-1

B lymphocyte-induced maturation protein (Blimp)-1, a transcriptional repressor, has long been known to drive the terminal maturation of B cells into plasma cells. However, two recent studies reveal an unanticipated role for Blimp-1 in T-cell homeostasis. Blimp-1-deficient T cells exhibit enhanced proliferative and functional responses following activation, and induce spontaneous colitis. Hence, aside from its role in B-cell differentiation, Blimp-1 directs restraint among T cells.

Loss of Pax5 promotes plasma cell differentiation

Pax5 is indispensable for the commitment of early lymphoid progenitors to the B cell lineage as well as for the development of B cells. To better understand the functional importance of Pax5 at the later stages of B cell differentiation, we established a Pax5-deficient DT40 B cell line. The Pax5(-/-) cells exhibited slower growth, decreased surface IgM expression, and total loss of B cell receptor signaling. Moreover, the expression of the plasma cell-characteristic transcription factors Blimp-1 and XBP-1 were significantly upregulated and the expression of Bcl-6 diminished in the Pax5(-/-) cells, and this alteration was normalized by restored Pax5 expression. The Pax5-deficient cells further manifested substantially elevated secretion of IgM into the supernatant, another characteristic of plasma cells. These results indicate that downregulation of Pax5 function promotes the plasma cell differentiation of B cells.

Inactivation of the PRDM1/BLIMP1 gene in diffuse large B cell lymphoma

PR domain containing 1 with zinc finger domain (PRDM1)/B lymphocyte–induced maturation protein 1 (BLIMP1) is a transcriptional repressor expressed in a subset of germinal center (GC) B cells and in all plasma cells, and required for terminal B cell differentiation. The BLIMP1 locus lies on chromosome 6q21-q22.1, a region frequently deleted in B cell lymphomas, suggesting that it may harbor a tumor suppressor gene. We report here that the BLIMP1 gene is inactivated by structural alterations in 24% (8 out of 34) activated B cell–like diffuse large cell lymphoma (ABC-DLBCL), but not in GC B cell–like (n = 0/37) or unclassified (n = 0/21) DLBCL. BLIMP1 alterations included gene truncations, nonsense mutations, frameshift deletions, and splice site mutations that generate aberrant transcripts encoding truncated BLIMP1 proteins. In all cases studied, both BLIMP1 alleles were inactivated by deletions or mutations. Furthermore, most non–GC type DLBCL cases (n = 20/26, 77%) lack BLIMP1 protein expression, despite the presence of BLIMP1 mRNA. These results indicate that a sizable fraction of ABC-DLBCL carry an inactive BLIMP1 gene, and suggest that the same gene is inactivated by epigenetic mechanisms in an additional large number of cases. These findings point to a role for BLIMP1 as a tumor suppressor gene, whose inactivation may contribute to lymphomagenesis by blocking post–GC differentiation of B cells toward plasma cells.

Transcriptional control of B cell activation

The developmental program that commits a hematopoietic stem cell to the B lymphocyte lineage employs transcriptional regulators to enable the assembly of an antigen receptor complex with a useful specificity and with signalling competence. Once a naive IgM+ B cell is generated, it must correctly integrate signals from the antigen receptor with those from cytokine receptors and co-receptors delivering T cell help. The B cell responds through the regulated expression of genes that implement specific cell expansion and differentiation, secretion of high levels of high-affinity antibody, and generation of long-term memory. The transcriptional regulators highlighted in this chapter are those for which genetic evidence of function in IgM+ B cells in vivo has been provided, often in the form of mutant mice generated by conventional or conditional gene targeting. A critical developmental step is the maturation of bone marrow emigrant "transitional" B cells into the mature, long-lived cells of the periphery, and a number of the transcription factors discussed here impact on this process, yielding B cells with poor mitogenic responses in vitro. For mature B cells, it is clear that not only the nature, but the duration and amplitude of an activating signal are major determinants of the transcription factor activities enlisted, and so the ultimate outcome. The current challenge is the identification of the target genes that are activated to implement the correct response, so that we may more precisely and safely manipulate B cell behavior to predictably and positively influence humoral immune responses.

Centrocytoid Plasma Cells of the Germinal Center

Germinal centers within the lymph node follicles are T-cell-dependent, antigen-driven B-cell proliferations that develop from the rapid clonal expansion of a few founder cells. The end results of this B-cell expansion are memory B cells or plasma cells. Two morphologic forms of plasma cell can be recognized in the germinal center: classic plasma cells, characterized morphologically by peripherally clumped arrangement of nuclear chromatin, and cells with a nuclear morphology more resembling that of the centrocytes, which the authors have termed "centrocytoid plasma cells." In this study the authors examined the distribution and immunohistochemical characteristics of these two populations of germinal center plasma cells. The centrocytoid plasma cells were arranged in a band stretching from the junction of the dark and light zone to the periphery of the germinal centers, while the classic plasma cells were mainly present at the germinal center periphery. Both marked with CD38, CD138, and VS38c, recognized markers for plasma cells; however, EMA and CD31 were present only in the classic form of plasma cell. The proliferation marker Ki67 was present in less than 1% of the cells labeling with CD138. Others have demonstrated Ki67 in 50% of the cells labeled with Blimp-1, which is consistent with Blimp-1 appearing earlier than CD138 in ontogeny. CD10 is co-expressed with CD138 in about 10% of cells and CD45 with CD138 in about 5% of cells. Their topographic features, together with the progressive acquisition of plasma cell markers, suggest that the centrocytoid plasma cells may be the precursors of the classic plasma cells. Of note, both the forms of plasma cell were absent in follicles of follicular lymphoma, which supports the concept that in this disease, lymphocytes fail to differentiate and mature beyond the centrocyte stage.

Regulation of plasma-cell development

Plasma cells are the terminally differentiated, non-dividing effector cells of the B-cell lineage. They are cellular factories devoted to the task of synthesizing and secreting thousands of molecules of clonospecific antibody each second. To respond to microbial pathogens with the necessary specificity and rapidity, B cells are exquisitely regulated with respect to both development in the bone marrow and activation in the periphery. This review focuses on the terminal differentiation of B cells into plasma cells, including the different subsets of B cells that become plasma cells, the mechanism of regulation of this transition, the transcription factors that control each developmental stage and the characteristics of long-lived plasma cells.

Blimp-1; immunoglobulin secretion and the switch to plasma cells

The transcription factor Blimp-1 governs the generation of plasma cells and immunoglobulin secretion. Recent microarray experiments indicate that Blimp-1 regulates a large set of genes that constitute a significant part of the plasma cell expression signature. The variety of differentially expressed genes indicates that Blimp-1 affects numerous aspects of plasma cell maturation, ranging from migration, adhesion, and homeostasis, to antibody secretion. In addition, Blimp-1 regulates immunoglobulin secretion by affecting the nuclear processing of the mRNA transcript and by affecting protein trafficking by regulating genes that impact on the activity of the endoplasmic reticulum. Interestingly, the differentiation events that Blimp-1 regulates appear to be modulated depending on the activation state of the B cell. This modulation may be due at least in part to distinct regions of Blimp-1 that regulate unique sets of genes independently of each other. These data hint at the complexity of Blimp-1 and the genetic program that it initiates to produce a pool of plasma cells necessary for specific immunity.

Plasma cell ontogeny defined by quantitative changes in blimp-1 expression

Plasma cells comprise a population of terminally differentiated B cells that are dependent on the transcriptional regulator B lymphocyte–induced maturation protein 1 (Blimp-1) for their development. We have introduced a gfp reporter into the Blimp-1 locus and shown that heterozygous mice express the green fluorescent protein in all antibody-secreting cells (ASCs) in vivo and in vitro. In vitro, these cells display considerable heterogeneity in surface phenotype, immunoglobulin secretion rate, and Blimp-1 expression levels. Importantly, analysis of in vivo ASCs induced by immunization reveals a developmental pathway in which increasing levels of Blimp-1 expression define developmental stages of plasma cell differentiation that have many phenotypic and molecular correlates. Thus, maturation from transient plasmablast to long-lived ASCs in bone marrow is predicated on quantitative increases in Blimp-1 expression.

Regulation of B Cell Differentiation and Plasma Cell Generation by IL-21, a Novel Inducer of Blimp-1 and Bcl-6

IL-21 is a type I cytokine whose receptor is expressed on T, B, and NK cells. Within the B cell lineage, IL-21 regulates IgG1 production and cooperates with IL-4 for the production of multiple Ab classes in vivo. Using IL-21-transgenic mice and hydrodynamics-based gene delivery of IL-21 plasmid DNA into wild-type mice as well as in vitro studies, we demonstrate that although IL-21 induces death of resting B cells, it promotes differentiation of B cells into postswitch and plasma cells. Thus, IL-21 differentially influences B cell fate depending on the signaling context, explaining how IL-21 can be proapoptotic for B cells in vitro yet critical for Ag-specific Ig production in vivo. Moreover, we demonstrate that IL-21 unexpectedly induces expression of both Blimp-1 and Bcl-6, indicating mechanisms as to how IL-21 can serve as a complex regulator of B cell maturation and terminal differentiation. Finally, BXSB-Yaa mice, which develop a systemic lupus erythematosus-like disease, have greatly elevated IL-21, suggesting a role for IL-21 in the development of autoimmune disease.

Modular Nature of Blimp-1 in the Regulation of Gene Expression during B Cell Maturation

The transcription factor Blimp-1 induces the maturation of B cells into Ab-secreting plasma cells. DNA microarrays were used to analyze the transcription profiles of both Blimp-1-transduced murine B cell lines and the inducible B cell line BCL(1). Hundreds of genes were differentially regulated, showing how Blimp-1 both restricts affinity maturation and promotes Ab secretion, homeostasis, migration, and differentiation. Strikingly, when different modes of plasma cell induction are used, very different genetic programs are used, suggesting that the transition from a B cell to plasma cell can occur in multiple ways, perhaps accounting for the different types of Ab-secreting cells observed in vivo. Furthermore, mutagenesis of Blimp-1 reveals multiple effector domains, which regulate distinct genes. This indicates that Blimp-1 subdivides the maturation program into select and tunable pathways.

Transcriptional repressor BCL-6 immortalizes germinal center-like B cells in the absence of p53 function

Chromosomal rearrangements in non-Hodgkin's B-cell lymphoma implicate BCL-6 as an oncogene, yet direct evidence for BCL-6 acting as an oncogene in B cells has been lacking. Here, we show that BCL-6 can immortalize primary B cells, but only in the absence of p53 tumor suppressor function. The expression of BCL-6 led to greatly increased B-cell proliferation, particularly in response to CD40 stimulation. Furthermore, BCL-6-infected p53-deficient B cells gave rise to immortalized cell lines that could be maintained by CD40 stimulation. We found that in primary mouse B cells, BCL-6 repressed expression of the Blimp-1, p27kip1, and cyclin D2 target genes. BCL-6 did not markedly repress the PDCD2 and BCL-XL target genes. The BCL-6 immortalized cell lines had a phenotype consistent with germinal center B cells, they expressed the germinal center-specific M17 gene, and a significant fraction of the cells stained positive with PNA. Our data indicate that BCL-6 may act to maintain B cells in a germinal center-like state, and repression of Blimp-1 by BCL-6 may be particularly crucial for stabilization of the germinal center phenotype. Our data also suggest that disruption of the p53 pathway may be crucial for the development of BCL-6-expressing B-cell lymphomas.

PRDI-BF1 recruits the histone H3 methyltransferase G9a in transcriptional silencing

PRDI-BF1, the human ortholog of mouse Blimp-1, is a DNA-binding protein involved in postinduction repression of interferon-beta gene transcription in response to viral infection. PRDI-BF1 also has an essential function in driving terminal differentiation of B lymphocytes and therein silences multiple genes. Here we show PRDI-BF1 assembles silent chromatin over the interferon-beta promoter in the osteosarcoma cell line U2OS through recruitment of the histone H3 lysine methyltransferase G9a. G9a is recruited only when in a complex with PRDI-BF1. G9a catalytic activity is required for the accumulation of methylated histone H3 and transcriptional silencing mediated by PRDI-BF1 in vivo. This establishes a mechanism for the recruitment of G9a, the main mammalian euchromatic methyltransferase, and defines nonembryonic targets of G9a.

Galectin-3 Mediates IL-4-Induced Survival and Differentiation of B Cells: Functional Cross-Talk and Implications during Trypanosoma cruzi Infection

The role of transcription factors in B cell survival and differentiation has been delineated during the last years. However, little is known about the intermediate signals and the intracellular pathways that control these events. In this study, we provide evidence both in vitro and in vivo, showing that galectin-3 (Gal-3), a beta-galactoside-binding protein, is a critical mediator of B cell differentiation and survival. Although Gal-3 is not expressed in resting B cells from normal mice, its expression is markedly induced after activation with stimuli such as IL-4 and CD40 cross-linking. These signals promote survival and block the final differentiation of these cells, thus allowing the rising of a memory B cell phenotype. In addition, Gal-3 is expressed in B cells from Trypanosoma cruzi-infected mice, which received signals for activation and differentiation in vivo. By using an antisense strategy, we determined that Gal-3 is a critical signal mediating the effects of IL-4 on B cell fate. Blockade of intracellular Gal-3 in vitro abrogated IL-4-induced survival of activated B cells, favoring the differentiation toward a plasma cell pathway. Moreover, B cells with restrained endogenous Gal-3 expression failed to down-regulate the Blimp-1 transcription factor after IL-4 stimulation. Finally, inhibition of Gal-3 in vivo skewed the balance toward plasma cell differentiation, which resulted in increased Ig production and parasite clearance during T. cruzi infection. Thus, the present study provides evidence of a novel role for Gal-3 as an intracellular mediator of B cell survival and a checkpoint in IL-4-induced B cell commitment toward a memory phenotype.

Regulatory mechanisms that determine the development and function of plasma cells

Plasma cells are terminally differentiated final effectors of the humoral immune response. Plasma cells that result from antigen activation of B-1 and marginal zone B cells provide the first, rapid response to antigen. Plasma cells that develop after a germinal center reaction provide higher-affinity antibody and often survive many months in the bone marrow. Transcription factors Bcl-6 and Pax5, which are required for germinal center B cells, block plasmacytic differentiation and repress Blimp-1 and XBP-1, respectively. When Bcl-6-dependent repression of Blimp-1 is relieved, Blimp-1 ensures that plasmacytic development is irreversible by repressing BCL-6 and PAX5. In plasma cells, Blimp-1, XBP-1, IRF4, and other regulators cause cessation of cell cycle, decrease signaling from the B cell receptor and communication with T cells, inhibit isotype switching and somatic hypermutation, downregulate CXCR5, and induce copious immunoglobulin synthesis and secretion. Thus, commitment to plasmacytic differentiation involves inhibition of activities associated with earlier B cell developmental stages as well as expression of the plasma cell phenotype.

Activation of terminal B cell differentiation by inhibition of histone deacetylation

A role for histone acetylation, which can alter the accessibility of DNA to transcriptional regulatory proteins and contribute to gene expression, in regulating terminal B cell differentiation was investigated in the mature B lymphoma L10A and mouse splenic B cells. Incubation of the L10A cells with the histone deacetylase (HDAC) inhibitors trichostatin A (TSA) and butyrate increased expression of Blimp-1, J chain, and mad genes, decreased expression of c-myc and BSAP/Pax-5 genes, increased the expression of surface CD43 and Syndecan-1, and decreased surface IgM. Incubation of splenic B cells with TSA and dextran conjugated anti-IgD Ab increased Blimp-1 gene and Syndecan-1 surface expression. The alteration in gene expression and cell surface markers was consistent with induction of the onset of terminal B cell differentiation. Co-incubation of L10A cells with TSA and cycloheximide (CHX) abrogated the up-regulation of Blimp-1 expression, indicating that TSA-activated Blimp-1 expression required synthesis of a transcriptional activator. In contrast, mad expression was increased in L10A cells cultured with TSA and cycloheximide or cycloheximide alone, suggesting mad expression may occur independent of Blimp-1 expression and is regulated by a labile, HDAC associated transcriptional repressor. The results demonstrate that histone acetylation regulates transcription of genes controlling terminal B cell differentiation.

Identification of a functionally impaired positive regulatory domain I binding factor 1 transcription repressor in myeloma cell lines

B cell differentiation into a plasma cell requires expression of the positive regulatory domain zinc finger protein 1 gene (PRDM1) that encodes the positive regulatory domain I binding factor 1 (PRDI-BF1 or Blimp-1) protein. It represses the transcription of specific target genes, including c-myc, the MHC class II trans-activator, Pax-5, and CD23b. In this study we demonstrate the presence of an alternative protein product of the PRDM1 gene. The new protein, PRDI-BF1 beta, has a disrupted PR domain and lacks the amino-terminal 101 aa of the originally described protein. PRDI-BF1 beta has a dramatic loss of repressive function on multiple target genes, but maintains normal DNA-binding activity, nuclear localization, and association with histone deacetylases and deacetylase activity. Myeloma cell lines express the highest levels of PRDM1 beta mRNA relative to the full-length form, while primary cells and several other cell lines have very low, but detectable, levels of PRDM1 beta. RNA analysis and analysis of the PRDM1 promoters demonstrate that PRDI-BF1 beta is generated from the same gene by alternative transcription initiation using an internal promoter. These newly described features of the PRDM1 gene are highly analogous to the PRDM2 (RIZ) and PRDM3 (MDS1-EVI1) genes, in which each express a truncated protein missing the PR domain. The expression of each of the truncated proteins is elevated in cancerous cells and may play an important role in the disease.