Expression of transcription factors Pu.1, Spi-B, Blimp-1, BSAP and oct-2 in normal human plasma cells and in multiple myeloma cells

Regulatory network of BLIMP1, IRF4, and XBP1 triad in plasmacytic differentiation and multiple myeloma pathogenesis

Antibody secreting plasma cell plays an indispensable role in humoral immunity. As activated B cell undergoes germinal center reaction and develops into plasma cell, it gradually loses B cell characteristics and embraces functional changes associated with immunoglobulins production. Differentiation of B cell into plasma cell involves drastic changes in cell structure, granularity, metabolism, gene expression and epigenetic regulation that couple with the mounting capacity for synthesis of a large quantity of antigen-specific antibodies. The interplay between three hallmark transcriptional regulators IRF4, BLIMP1, and XBP1, is critical for supporting the cellular reprograming activities during B to plasma cell transition. IRF4 promotes plasma cell generation by directing immunoglobulin class switching, proliferation and survival; BLIMP1 serves as a transcriptional repressor that extinguishes B cell features; whereas XBP1 controls unfolded protein response that relieves endoplasmic reticulum stress and permits antibody release during terminal differentiation. Intriguingly, high expression of IRF4, BLIMP1, and XBP1 molecules have been reported in myeloma cells derived from multiple myeloma patients, which negatively impact treatment outcome, prognosis, and relapse frequency. Despite the introduction of immunomodulatory drugs in recent years, multiple myeloma is still an incurable disease with poor survival rate. An in-depth review of IRF4, BLIMP1, and XBP1 triad molecules in plasma cell generation and multiple myeloma tumorigenesis may provide clues to the possibility of targeting these molecules in disease management.

The regulation and function of CD20: an "enigma" of B-cell biology and targeted therapy

The introduction of anti-CD20 monoclonal antibodies such as rituximab, ofatumumab, or obinutuzumab improved the therapy of B-cell malignancies even though the precise physiological role and regulation of CD20 remains unclear. Furthermore, CD20 expression is highly variable between different B-cell malignancies, patients with the same malignancy, and even between intraclonal subpopulations in an individual patient. Several epigenetic (EZH2, HDAC1/2, HDAC1/4, HDAC6, complex Sin3A-HDAC1) and transcription factors (USF, OCT1/2, PU.1, PiP, ELK1, ETS1, SP1, NFκB, FOXO1, CREM, SMAD2/3) regulating CD20 expression (encoded by MS4A1) have been characterized. CD20 is induced in the context of microenvironmental interactions by CXCR4/SDF1 (CXCL12) chemokine signaling and the molecular function of CD20 has been linked to the signaling propensity of B-cell receptor (BCR). CD20 has also been shown to interact with multiple other surface proteins on B cells (such as CD40, MHCII, CD53, CD81, CD82, and CBP). Current efforts to combine anti-CD20 monoclonal antibodies with BCR signaling inhibitors targeting BTK or PI3K (ibrutinib, acalabrutinib, idelalisib, duvelisib) or BH3-mimetics (venetoclax) lead to the necessity to better understand both the mechanisms of regulation and the biological functions of CD20. This is underscored by the observation that CD20 is decreased in response to the "BCR inhibitor" ibrutinib which largely prevents its successful combination with rituximab. Several small molecules (such as histone deacetylase inhibitors, DNA methyl-transferase inhibitors, aurora kinase A/B inhibitors, farnesyltransferase inhibitors, FOXO1 inhibitors, and bryostatin-1) are being tested to upregulate cell-surface CD20 levels and increase the efficacy of anti-CD20 monoclonal antibodies. Herein, we review the current understanding of CD20 function, and the mechanisms of its regulation in normal and malignant B cells, highlighting the therapeutic implications.

NonO Is a Novel Co-factor of PRDM1 and Regulates Inflammatory Response in Monocyte Derived-Dendritic Cells

Proper expression of the transcription factor, Positive regulatory domain 1 (PRDM1), is required for maintaining homeostasis of human monocyte derived-dendritic cells (MO-DCs). The molecular mechanisms and gene targets of PRDM1 in B and T lymphocytes have been identified. However, the function of PRDM1 in dendritic cells (DCs) remains unclear. We investigate co-regulators of PRDM1 in MO-DCs identified by mass spectrometry (MS) and co-immunoprecipitation (Co-IP). Notably, non-POU domain-containing octamer-binding protein (NonO) was found to be a PRDM1 binding protein in the nucleus of MO-DCs. NonO is recruited to the PRDM1 binding site in the promoter region of IL-6. Knockdown of NonO expression by siRNA lessened suppression of IL-6 promoter activity by PRMD1 following LPS stimulation. While NonO binding to PRDM1 was observed in human myeloma cell lines, an effect of NonO on IL-6 expression was not observed. Thus, loss of NonO interrupted the inhibitory effect of PRDM1 on IL-6 expression in MO-DCs, but not plasma cells. Moreover, MO-DCs with low expression of PRDM1 or NonO induce an increased number of IL-21-producing T_FH-like cells in vitro. These data suggest that low level of PRDM1 and NonO lead to enhanced activation of MO-DCs and the regulation of MO-DC function by PRDM1 is mediated through cell lineage-specific mechanisms.

Epstein-Barr virus (EBV) activates NKL homeobox gene HLX in DLBCL

NKL homeobox genes encode developmental transcription factors regulating basic processes in cell differentiation. According to their physiological expression pattern in early hematopoiesis and lymphopoiesis, particular members of this homeobox gene subclass constitute an NKL-code. B-cell specific NKL-code genes generate a regulatory network and their deregulation is implicated in B-cell lymphomagenesis. Epstein-Barr virus (EBV) infects B-cells and influences the activity of signalling pathways including JAK/STAT and several genes encoding developmental regulators. Therefore, EBV-infection impacts the pathogenesis and the outcome of B-cell malignancies including Hodgkin lymphoma and diffuse large B-cell lymphoma (DLBCL). Here, we isolated EBV-positive and EBV-negative subclones from the DLBCL derived cell line DOHH-2. These subclones served as models to investigate the role of EBV in deregulation of the B-cell specific NKL-code members HHEX, HLX, MSX1 and NKX6-3. We showed that the EBV-encoded factors LMP1 and LMP2A activated the expression of HLX via STAT3. HLX in turn repressed NKX6-3, SPIB and IL4R which normally mediate plasma cell differentiation. In addition, HLX repressed the pro-apoptotic factor BCL2L11/BIM and hence supported cell survival. Thus, EBV aberrantly activated HLX in DLBCL, thereby disturbing both B-cell differentiation and apoptosis. The results of our study appreciate the pathogenic role of EBV in NKL homeobox gene deregulation and B-cell malignancies.

PRDI-BF1 and PRDI-BF1P isoform expressions correlate with disease status in multiple myeloma patients

Human positive regulatory domain I binding factor 1 (PRDI-BF1 or BLIMP-1) is a transcription factor that acts as a master regulator and has crucial roles in the control of differentiation and in maintaining survival of plasma cells (PC). The PRDM1 gene, which codifies for PRDI-BF1, contains an alternative promoter capable of generating a PRDI-BF1 deleted protein (called PRDI-BF1β), which lacks 101 amino acids comprising most of the regulatory domain. PRDI-BF1β has been detected in relevant quantities especially in multiple myeloma cell lines (U266 and NCI- H929). The first aim of the study was to compare, using real time polymerase chain reaction (RT-PCR), the levels of PRDI-BF1 and PRDI-BF1β in myeloma patients and in normal human bone marrow. The second step was the examination of the expression of PRDI-BF1 and PRDI-BF1β isoform depending on disease status and treatment response. We demonstrate the correlation of PRDI-BF1 and the shorter PRDI-BF1β isoform protein levels with the clinical evolution and the management of myeloma patients.

Chronic lymphocytic leukaemia: could immunological tolerance mechanisms be the origin of lymphoid neoplasms?

Immunological tolerance theory in chronic lymphocytic leukaemia (CLL): we suggest that B cells that express B-cell receptors (BCR) that recognize their own BCR epitopes are viewed by immune system as 'dangerous cells'. BCR autonomous signalling may induce constant receptor editing and mistakes in allelic exclusion. The fact that whole BCR recognizes a self-antigen or foreing antigen may be irrelevant in early B cell development. In early B cells, autonomous signalling induced by recognition of the BCR's own epitopes simulates an antigen-antibody engagement. In the bone marrow this interaction is viewed as recognition of self-molecules and induces receptor editing. In mature B cells autonomous signalling by the BCR may promote 'reversible anergy' and also may correct self-reactivity induced by the somatic hypermutation mechanisms in mutated CLL B cells. However, in unmutated CLL B cells, BCR autonomous signalling in addition to self-antigen recognition augments B cell activation, proliferation and genomic instability. We suggest that CLL originates from a coordinated normal immunologic tolerance mechanism to destroy self-reactive B cells. Additional genetic damage induced by tolerance mechanisms may immortalize self-reactive B cells and transform them into a leukemia.

The transcription factor Spi-B regulates human plasmacytoid dendritic cell survival through direct induction of the antiapoptotic gene BCL2-A1

Plasmacytoid dendritic cells (pDCs) selectively express Toll-like receptor (TLR)-7 and TLR-9, which allow them to rapidly secrete massive amounts of type I interferons after sensing nucleic acids derived from viruses or bacteria. It is not completely understood how development and function of pDCs are controlled at the transcriptional level. One of the main factors driving pDC development is the ETS factor Spi-B, but little is known about its target genes. Here we demonstrate that Spi-B is crucial for the differentiation of hematopoietic progenitor cells into pDCs by controlling survival of pDCs and its progenitors. In search for Spi-B target genes, we identified the antiapoptotic gene Bcl2-A1 as a specific and direct target gene, thereby consolidating the critical role of Spi-B in cell survival.

Immune alterations in untreated and treated multiple myeloma

The incidence of second malignancies was shown to be increased in patients with multiple myeloma. Although a shared genetic predisposition or common environmental carcinogens may account for the occurrence of both myeloma and additional cancers, multiple immune defects encountered in myeloma might play an important role in this regard. This review explores the impairments in both cellular and humoral mediated immunity in multiple myeloma, linking them with increased susceptibility to infections and additional cancers. In addition, the recent therapeutic advances transformed myeloma into a chronic entity, with multiple relapses and salvage therapies, which may result in cumulative immunosuppression. Although recent reports have suggested an increased rate of second cancers in myeloma patients treated with lenalidomide, the true impact of this agent and other novel anti-myeloma therapies on the incidence of additional malignancies remains to be clarified.

Analysis of select members of the E26 (ETS) transcription factors family in colorectal cancer

The E-twenty-six (ETS) family of transcription factors is known to act as positive or negative regulators of the expression of genes that are involved in diverse biological processes, including those that control cellular proliferation, differentiation, hematopoiesis, apoptosis, metastasis, tissue remodeling, and angiogenesis. Identification of target gene promoters of normal and oncogenic transcription factors provides new insights into the regulation of genes that are involved in the control of normal cell growth and differentiation. The aim of the present investigation was to analyze the differential expression of 11 ETS (ELF-3, ESE3, ETS1, ETV3, ETV4, ETV6, NERF, PDEF, PU1, Spi-B, and Spi-C) as potential markers for prognostic of colorectal cancer. A series of paired tissue biopsies consisting of a tumor and a non-affected control sample were harvested from 28 individuals suffering from diagnosed colorectal lesions. Total RNA was isolated from the samples, and after reverse transcription, differential expression of the select ETS was carried out through real-time polymerase chain reaction. Tumor staging as determined by histopathology was carried out to correlate the degree of tumor invasiveness with the expression of the ETS genes. The results demonstrated a different quantitative profile of expression in tumors and normal tissues. ETV4 was significantly upregulated with further increase in the event of lymph node involvement. PDEF and Spi-B presented downregulation, which was more significant when lymph node involvement was present. These findings were supported by immunohistochemistry of tumoral tissues. The results suggest that select ETS may serve as potential markers of colorectal cancer invasiveness and metastasis.

Blimp1 is limiting for transformation in a mouse plasmacytoma model

Multiple myeloma (MM) and plasmacytomas are cancers of antibody-secreting cells (ASCs). PRDM1/BLIMP1 is an essential regulator of ASC development. Histologic evidence shows that 100% of MM expresses PRDM1/BLIMP1, indicating that PRDM1/BLIMP1 is important for the development or persistence of MM. In contrast, some diffuse large B-cell lymphomas (DLBCLs) lose PRDM1 expression, suggesting that PRDM1 may act as a tumor suppressor in DLBCL. Thus, the role of PRDM1/BLIMP1 in transformation of mature B cells is unclear. We have used a plasmacytoma-prone transgenic mouse model to study the effect of Blimp1 loss on plasmacytoma prevalence, latency, and phenotype. Two possible outcomes could be envisaged: loss of Blimp1 might decrease plasmacytoma prevalence, through reduction of plasma cells, and so the number of susceptible transformation targets. Alternatively, Blimp1 may participate in the transformation process itself. Our results support the latter scenario, showing that decreasing Blimp1 dosage does not change plasma cell number in nontransgenic mice in vivo, but it significantly reduces plasmacytoma prevalence in transgenic mice. Loss of functional Blimp1 completely prevents plasmacytoma formation in this tumor model. These observations suggest that Blimp1 is limiting for plasma cell transformation and thus has potential as a target for new therapies to combat MM.

Induction of apoptosis in plasma cells by B lymphocyte-induced maturation protein-1 knockdown

B lymphocyte-induced maturation protein-1 (Blimp-1) is a transcriptional repressor that plays an important role during plasmacytic differentiation and is expressed in normal and transformed plasma cells. We here investigated the importance of continuous Blimp-1 expression. We found that knockdown of Blimp-1 expression by lentiviral vector-delivered short hairpin RNA causes apoptosis in multiple myeloma cell lines and plasmacytoma cells, indicating that continued expression of Blimp-1 is required for cell survival. We examined the mechanism underlying Blimp-1 knockdown-mediated apoptosis and found that the Blimp-1 knockdown neither reversed the phenotypic markers of plasma cells nor caused cell cycle arrest. Instead, our results show that knockdown of Blimp-1 induced the proapoptotic protein Bim, reduced the antiapoptotic protein Mcl-1, and activated caspase-9 and caspase-3. We further link apoptosis in transformed plasma cells mediated by proteasome inhibitors, the effective therapeutic agent for multiple myeloma patients, with reduced expression of Blimp-1. Lastly, we show that Blimp-1-dependent cell survival may act downstream of IFN regulatory factor 4 (IRF4) because IRF4 knockdown leads to down-regulation of Blimp-1 and apoptosis in multiple myeloma cells and plasmacytoma cells. Together, our data suggest that Blimp-1 ensures the survival of transformed plasma cells.

Immunodeficiency and immunotherapy in multiple myeloma

Multiple myeloma is a malignant tumour of plasma cells that remains incurable for the vast majority of patients, with a median survival of 2-3 years. It is characterized by the patchy accumulation of tumour cells within bone marrow leading to variable anaemia, bone destruction, hypercalcaemia, renal failure and infections. Immune dysfunction is an important feature of the disease and leads to infections that are both a major cause of morbidity and mortality and may promote tumour growth and resistance to chemotherapy. Numerous defects of the immune system have been described in multiple myeloma although the relative clinical importance of these remains elusive. There has been considerable interest in the identification of an autologous response against myeloma. Although T cells and humoral responses directed against myeloma-associated antigens have been described, it is uncertain if the immune system plays a role in preventing or controlling myeloma cell growth. There is increasing interest in the potential role of immunotherapy but the success of these interventions is likely to be modified by the immunologically hostile environment associated with multiple myeloma. This review attempts to summarize the current knowledge relating to the immune defects found in multiple myeloma.

Down-regulation of PU.1 by methylation of distal regulatory elements and the promoter is required for myeloma cell growth

The transcription factor PU.1 is essential for myeloid and B-cell development. Down-regulation of PU.1 by disruption of its 14-kb 5' upstream regulatory element induced acute myeloid leukemia, T-cell lymphoma, and chronic lymphocytic leukemia-like disease in murine models. In the present study, we found that PU.1 was down-regulated in the majority of human myeloma cell lines and a subset of freshly isolated myeloma cells, in contrast to relatively high expression of PU.1 in normal plasma cells. Patients in this low PU.1 expression subset may have a poor prognosis. In human myeloma cell lines, the 17-kb 5' upstream enhancer and the promoter region of the PU.1 gene were highly methylated, and this is consistent with disappearance of DNase I-hypersensitive sites in these regions. To elucidate the significance of down-regulation of PU.1, we generated stable myeloma cell lines with an inducible PU.1 expression system. Exogenous expression of PU.1 in PU.1 null myeloma cell lines, U266 and KMS12PE, induced complete growth arrest and cell death. Up-regulation of PU.1 by 5-aza-2'-deoxycytidine also induced growth arrest of KMS12PE and KHM11 myeloma cells. These data suggest that down-regulation of PU.1 is an essential step for the survival of a subset of myeloma cells and that up-regulation of PU.1 by demethylation agents or other types of agents may represent a new therapeutic strategy for treatment of multiple myeloma patients.

MHC class II transactivator (CIITA) expression is upregulated in multiple myeloma cells by IFN-gamma

The MHC class II transactivator (CIITA) acts in the cell nucleus as the master regulator of MHC class II (MHC II) gene expression. It is important to study CIITA regulation in multiple myeloma since MHC expression is central to ability of myeloma cells to present antigen and to the ability of the immune system to recognize and destroy this malignancy. Regulation of CIITA by IFN-gamma in B lymphocytes occurs through the CIITA type IV promoter (pIV), one of the four potential promoters (pI-pIV) of this gene. To investigate regulation of CIITA by IFN-gamma in multiple myeloma cells, first the ability of these cells to respond to IFN-gamma was examined. RT-PCR analyses show that IFN-gammaR1, the IFN-gamma-binding chain of the IFN-gamma receptor, is expressed in myeloma cells and IRF-1 expression increases in response to IFN-gamma treatment. Western blotting demonstrates that STAT1 is activated by phosphorylation in response to IFN-gamma. RT-PCR and functional promoter analyses show that IFN-gamma upregulates the activity of CIITA pIV, as does ectopic expression of IRF-1 or IRF-2. In vivo protein/DNA binding studies demonstrate protein binding at the GAS, E box and IRF-E sites. In vitro studies confirm the binding of IRF-1 and IRF-2 to CIITA pIV. Although multiple myeloma cells express PRDI-BF1/Blimp-1, a factor that represses both the CIITA type III and IV promoters, they retain the capability to upregulate CIITA pIV and MHC II expression in response to IFN-gamma treatment. These findings are the first to demonstrate that although PRDI-BF1/Blimp-1 diminishes the constitutive ability of these cells to present antigen by limiting CIITA and MHC II expression, it is possible to enhance this expression through the use of cytokines, like IFN-gamma.

Stages of germinal center transit are defined by B cell transcription factor coexpression and relative abundance

The transit of T cell-activated B cells through the germinal center (GC) is controlled by sequential activation and repression of key transcription factors, executing the pre- and post-GC B cell program. B cell lymphoma (BCL) 6 and IFN regulatory factor (IRF) 8 are necessary for GC formation and for its molecular activity in Pax5+PU.1+ B cells. IRF4, which is highly expressed in BCL6- GC B cells, is necessary for class switch recombination and the plasma cell differentiation at exit from the GC. In this study, we show at the single-cell level broad coexpression of IRF4 with BCL6, Pax5, IRF8, and PU.1 in pre- and post-GC B cells in human and mouse. IRF4 is down-regulated in BCL6+ human GC founder cells (IgD+CD38+), is absent in GC centroblasts, and is re-expressed in positive regulatory domain 1-positive centrocytes, which are negative for all the B cell transcription factors. Activated (CD30+) and activation-induced cytidine deaminase-positive extrafollicular blasts coexpress Pax5 and IRF4. PU.1-negative plasma cells and CD30+ blasts uniquely display the conformational epitope of IRF4 recognized by the MUM1 Ab, an epitope that is absent from any other IRF4+PU.1+ lymphoid and hemopoietic subsets. Low grade B cell lymphomas, representing the malignant counterpart of pre- and post-GC B cells, accordingly express IRF4. However, a fraction of BCL6+ diffuse large B cell lymphomas express IRF4 bearing the MUM1 epitope, indicative of a posttranscriptional modification of IRF4 not seen in the normal counterpart.

New transcription factors in diagnostic hematopathology

The transcription factors (TFs) that controls the intricate machinery of multistep differentiation and activation programs of the lymphoid system, represent a complex array of proteins, whose identification and function has only in part been completed. TFs are usually expressed during specific differentiation or activation cellular programs, making them interesting tools in diagnostic immunohistochemistry. In fact, the specificity of some of these TFs for lineage or activation/differentiation passages or their abnormal expression in specific disease entity, represents a feature that has been exploited in diagnostic/prognostic immunohistochemistry. Bcl-6 was the prototype of this class of markers. Currently, the expanding knowledge of the TFs involved in the differentiation programs and in the activation processes of T-lymphocyte and B-lymphocyte in normal and neoplastic conditions and the availability of antibodies able to efficiently recognize these TFs in histologic material, represent a powerful tool in diagnostic hematopathology. In this review we will consider the basic biologic aspects and the applications in hematopathology of some of the lymphocyte-related TFs, including Pax5/BSAB, MUM1/IRF4, BOB1, Oct-2, T-bet, and FOXP3. This field is rapidly evolving, as witnessed by the ongoing growing number of novel TFs with possible diagnostic applications appearing in the literature.

Oct transcription factors mediate t(14;18) lymphoma cell survival by directly regulating bcl-2 expression

Oct-1 and Oct-2 are members of the POU homeodomain family of transcriptional regulators and are critical for normal embryonic development. Gene-targeting studies showed that Oct-1 and Oct-2 are largely dispensable for B-cell development and immunoglobulin production, although both Oct-2 and Bob-1 are required for a proper immune response and germinal center formation. In these studies, we investigated the role of Oct factors in B-cell lymphomas. Recent investigations have shown increased expression of Oct-2 and Bob-1 in lymphomas, and we observed greatly increased levels of Oct-2 in lymphoma cells with the t(14;18) translocation. Decreased expression of Oct-1, Oct-2, or Bob-1 by RNA interference resulted in apoptosis and down-regulation of bcl-2 expression. Furthermore, Oct-2 induced bcl-2 promoter activity and mediated this effect through three regions in the bcl-2 P2 promoter. Although these regions did not contain canonical octamer motifs, we observed the direct interaction of Oct-2 with all three sites both in vitro by EMSA and in vivo by chromatin immunoprecipitation assay. Moreover, by mutation analysis we found that the ability of Oct-2 to activate bcl-2 required C/EBP, Cdx, and TATA-binding sites. Oct-2, therefore, acts as a cell survival factor in t(14;18) lymphoma cells by directly activating the antiapoptotic gene bcl-2.

Epigenetic processes play a major role in B-cell-specific gene silencing in classical Hodgkin lymphoma

Many B-lineage-specific genes are down-regulated in Hodgkin and Reed-Sternberg (HRS) cells of classical Hodgkin lymphoma (cHL). We investigated the involvement of epigenetic modifications in gene silencing in cHL cell lines and in microdissected primary HRS cells. We assessed the expression and methylation status of CD19, CD20, CD79B, SYK, PU.1, BOB.1/OBF.1, BCMA, and LCK, all of which are typically down-regulated in cHL. We could reactivate gene expression in cHL cell lines with the DNA demethylating agent 5-aza-deoxycytidine (5-aza-dC). Using methylation-specific polymerase chain reaction (MSP), bisulfite genomic sequencing, and digestion with methylation-sensitive endonuclease followed by polymerase chain reaction (PCR), we determined the methylation status of promoter regions of PU.1, BOB.1/OBF.1, CD19, SYK, and CD79B. Down-regulation of transcription typically correlated with hypermethylation. Using bisulfite genomic sequencing we found that in microdissected HRS cells of primary cHL SYK, BOB.1/OBF.1, and CD79B promoters were also hypermethylated. Ectopic expression of both Oct2 and PU.1 in a cHL cell line potentiated endogenous PU.1 and SYK expression after 5-aza-dC treatment. These observations indicate that silencing of the B-cell-specific genes in cHL may be the consequence of a compromised regulatory network where down-regulation of a few master transcription factors results in silencing of numerous genes.

Vitamin A and immune function: retinoic acid modulates population dynamics in antigen receptor and CD38-stimulated splenic B cells

Vitamin A and its active metabolite, all-trans retinoic acid (RA), regulate the antibody response in vivo, although the underlying mechanisms are not well understood. We have investigated the regulation by RA of B cell population dynamics and Ig gene expression in purified splenic mouse B cells stimulated through the B cell antigen receptor (BCR) and/or CD38, a BCR coreceptor. After ligation of the BCR and/or CD38, B cells became more heterogeneous in size. RA substantially restrained this change, concomitant with inhibition of cell proliferation. To examine B cell heterogeneity more closely, we categorized stimulated B cells by size (forward angle light scatter) and determined cell division dynamics, germ-line Ig heavy chain gene transcription and surface IgG1 (sIgG1) expression. Flow cytometric analysis of carboxyfluorescein diacetate succinimidyl ester-labeled B cells costained for sIgG1 showed that the more proliferative groups of B cells were smaller, whereas cells expressing more sIgG1 were larger. RA enriched the latter population, whereas cell division frequency in general and the number of smaller B cells that had undergone division cycles were reduced. Although RA significantly inhibited Ig germ-line transcript levels in the total B cell population, CD19(-)IgG1(+) B cells, which represent a more differentiated phenotype, were enriched. Furthermore, pax-5 mRNA was decreased and activation-induced cytidine deaminase mRNA was increased in RA-treated stimulated B cells. Thus, RA regulated factors known to be required for Ig class switch recombination and modulated the population dynamics of ligation-stimulated B cells, while promoting the progression of a fraction of B cells into differentiated sIgG-expressing cells.

Targeting Positive Regulatory Domain I-Binding Factor 1 and X Box-Binding Protein 1 Transcription Factors by Multiple Myeloma-Reactive CTL

Growing evidence indicates that multiple myeloma (MM) and other malignancies are susceptible to CTL-based immune interventions. We studied whether transcription factors inherently involved in the terminal differentiation of mature B lymphocytes into malignant and nonmalignant plasma cells provide MM-associated CTL epitopes. HLA-A*0201 (A2.1) transgenic mice were used to identify A2.1-presented peptide Ag derived from the plasma cell-associated transcriptional regulators, positive regulatory domain I-binding factor 1 (PRDI-BF1) and X box-binding protein 1 (XBP-1). A2.1-restricted CTL specific for PRDI-BF1 and XBP-1 epitopes efficiently killed a variety of MM targets. PRDI-BF1- and XBP-1-reactive CTL were able to recognize primary MM cells from A2.1(+) patients. Consistent with the expression pattern of both transcription factors beyond malignant and nonmalignant plasma cells, PRDI-BF1- and XBP-1-specific CTL activity was not entirely limited to MM targets, but was also associated with lysis of certain other malignancies and, in defined instances, with low-to-intermediate level recognition of a few types of normal cells. Our results also indicate that the A2.1-restricted, PRDI-BF1- and XBP-1-specific human CD8(+) T cell repertoire is affected by partial self tolerance and may thus require the transfer of high-affinity TCR to break tolerance. We conclude that transcription factors governing terminal cellular differentiation may provide MM- and tumor-associated CTL epitopes.

Increased expression of CD27 on activated human memory B cells correlates with their commitment to the plasma cell lineage

Plasma cells (PC) or Ig-secreting cells (ISC) are terminally differentiated B cells responsible for the production of protective Ig. ISC can be generated in vitro by culturing human B cells with the T cell-derived stimuli CD40L, IL-2, and IL-10. ISC have traditionally been identified by the increased expression of CD38, analogous to primary human PC, and the acquired ability to secrete Ig. By tracking the proliferation history of activated B cells, we previously reported that the differentiation of memory B cells into CD38(+) B cells is IL-10 dependent, and increases in frequency with cell division. However, <50% of CD38(+) cells secreted Ig, and there was a population of CD38(-) ISC. Thus, the PC phenotype of CD38(+) cells generated in vitro did not correlate with PC function. To address this, we have examined cultures of activated memory B cells to accurately identify the phenotype of ISC generated in vitro. We found that CD27 is also up-regulated on memory B cells in an IL-10-dependent and division-dependent manner, and that ISC segregated into the CD27(high) subset of activated memory B cells irrespective of the acquired expression of CD38. The ISC generated in these cultures expressed elevated levels of the transcription factors Blimp-1 and X box-binding protein-1 and reduced levels of Pax-5, and exhibited selective migration toward CXCL12, similar to primary PC. We propose that the differentiation of memory B cells into PC involves a transitional stage characterized by a CD27(high)CD38(-) phenotype with the acquired ability to secrete high levels of Ig.

PU.1 protein expression has a positive linear association with protein expression of germinal centre B cell genes includingBCL-6, CD10, CD20 andCD22: identification of PU.1 putative binding sites in theBCL-6 promotor

The transcription factor PU.1 has been shown to be crucial for the early stages of B cell development but its function at later stages of B cell development is less well known. We observed previously that PU.1 is expressed uniformly throughout the mature pre-plasma cell B cell population, the only exception being a subpopulation of germinal centre (GC) cells which showed exceptionally high expression of PU.1. This suggested that PU.1 may also have a role in GC B cell biology. To test this hypothesis and to screen for possible genes regulated by PU.1, we first evaluated semi-quantitatively the possible co-expression of PU.1 with proteins known to be upregulated or downregulated during GC B cell development. Normal lymphoid tissues and 255 B cell non-Hodgkin lymphomas of putative GC B cell origin were evaluated. PU.1 expression was positively associated with CD10 (p < 0.0001), CD20 (p = 0.043), CD22 (p = 0.005), CD79a (p = 0.024) and Bcl-6 (p < 0.0001) and negatively associated with cytoplasmic immunoglobulin light-chain expression (p = 0.036) in diffuse large B cell lymphoma. Identical or nearly identical associations were found in follicular lymphoma. Since CD20 is known to be partly regulated by PU.1 and putative PU.1-binding sites have been described in the regulatory regions of the CD22, CD79a and CD10 genes, we looked for putative PU.1 binding sites in the BCL6 promotor. Four such putative PU.1 binding sites were identified. Further analysis by gel-shift electromobility essay showed that PU.1 protein binds to three of the four putative binding sites in the BCL6 promotor. PU.1 and Bcl-6 were also found to be upregulated in centroblasts in the normal GC, but jointly downregulated in a subpopulation of centrocytes. Our findings support the contention that PU.1 may also have an important role in GC B cell development.

Expression of PAX5 in CD20-positive multiple myeloma assessed by immunohistochemistry and oligonucleotide microarray

Silencing of PAX5 gene by upregulation of B-lymphocyte-induced maturation protein-1 (PRDM1) is essential for terminal differentiation of B cells to plasma cells. To investigate PAX5 gene expression and its protein product, B-cell-specific activator protein (BSAP), in a subgroup of multiple myeloma characterized by CD20 expression, we studied PAX5/BSAP by immunohistochemistry in 25 cases of myeloma, all expressing moderate to strong CD20 by flow cytometric analysis, and correlated the results with PAX5 and PRDM1 mRNA levels analyzed by the Affymetrix HuGeneFL GeneChip microarray in 17 cases. Using paraffin-embedded bone marrow biopsy sections, we found PAX5/BSAP was expressed in 72% (18/25) of cases overall with an intensity ranging from weak (10, 56%) to strong (8, 44%). PAX5/BSAP was negative in 10 randomly selected CD20-negative myelomas included as negative controls. PAX5 mRNA levels correlated inversely with that of PRDM1 in both CD20-positive and CD20-negative myelomas and failed to predict the expression levels of PAX5/BSAP, suggesting that detected PAX5/BSAP likely represents remnant of earlier stage of development. We conclude that CD20-positive myelomas expressing PAX5/BSAP can present as a diagnostic pitfall mimicking B-cell neoplasms with plasmacytoid differentiation.

Plasma cell differentiation and multiple myeloma

Microarray analyses and gene targeting have recently enhanced the understanding of factors involved in normal plasma cells and multiple myeloma. Plasma cells develop from marginal zone or germinal center B cells following stimulation by antigen, microbial products, TNF family signals and cytokines. Transcription factors, B-lymphocyte-induced maturation protein 1 (Blimp-1) and X-box binding protein 1 (XBP-1) are required for plasma cell development. They regulate sets of genes that induce immunoglobulin secretion, halt proliferation and block alternative B-cell fates. In multiple myeloma, transforming events lead to proliferation and survival, but programs for plasma cell differentiation and the inhibition of B-cell genes appear to be largely intact.

NF-κB and Oct-2 Synergize to Activate the Human 3′ Igh hs4 Enhancer in B Cells

In B cells, the Igh gene locus contains several DNase I-hypersensitive (hs) sites with enhancer activity. These include the 3' Igh enhancers, which are located downstream of the Calpha gene(s) in both mouse and human. In vivo experiments have implicated murine 3' enhancers, hs3B and/or hs4, in class switching and somatic hypermutation. We previously reported that murine hs4 was regulated by NF-kappaB, octamer binding proteins, and Pax5 (B cell-specific activator protein). In this study we report that human hs4 is regulated differently. EMSAs and Western analysis of normal B cells before and after stimulation with anti-IgM plus anti-CD40 showed the same complex binding pattern formed by NF-kappaB, Oct-1, and Oct-2 (but not by Pax5). A similar EMSA pattern was detected in mature human B cell lines (BL-2, Ramos, and HS-Sultan) and in diffuse large B cell lymphoma cell lines, although yin yang 1 protein (YY1) binding was also observed. We have confirmed the in vivo association of these transcription factors with hs4 in B cells by chromatin immunoprecipitation assays. The diffuse large B cell lymphoma cell lines had a distinctive slow-migrating complex containing YY1 associated with Rel-B. We have confirmed by endogenous coimmunoprecipitation an association of YY1 with Rel-B, but not with other NF-kappaB family members. Transient transfection assays showed robust hs4 enhancer activity in the mature B cell lines, which was dependent on synergistic interactions between NF-kappaB and octamer binding proteins. In addition, human hs4 enhancer activity required Oct-2 and correlated with expression of Oct coactivator from B cells (OCA-B).

The rise and fall of long-lived humoral immunity: terminal differentiation of plasma cells in health and disease

Long-lived humoral immune responses are a hallmark of thymus-dependent immunity. The cellular basis for enduring antibody-mediated immunity is long-lived memory B cells and plasma cells (PCs). Both of these cell populations acquire longevity as a result of antigen-specific, CD40-dependent, cognate interactions with helper T cells within germinal centers (GCs). At the molecular level, defined functional domains of CD40 control the post-GC fate of B cells. PC precursors that emerge from these GC reactions are highly proliferative and terminally differentiate to end-stage cells within the bone marrow (BM). The striking phenotypic similarities between the PC precursors and the putative malignant cell in multiple myeloma (MM) suggests that MM may result from the transformation of PC precursors. Within the domain of autoimmune disease, recent studies have shown that dysregulated migration of PCs to the BM may impact immune homeostasis and the development of lupus. Understanding the processes of normal PC differentiation will provide strategic insights into identifying therapeutic targets for the treatment of differentiated B-cell disorders.

Altered mRNA expression of Pax5 and Blimp-1 in B cells in multiple myeloma

Multiple myeloma (MM) is a plasma cell disorder that potentially initiates during an early stage of B-cell development. We encountered an unidentified isoform of B cell-specific activator protein (BSAP, or Pax5) in MM cells while performing differential analyses to compare mRNA expression in malignant and normal plasma cells. Pax5 is a transcription factor that plays a central role throughout B-cell development until the point of terminal differentiation. Our finding of this unique isoform prompted us to investigate Pax5 isoform usage in plasma cells and B-cell populations in other MM and healthy subjects. In contrast to normal Pax5 expression, we observed multiple isoforms of Pax5 in conjunction with low levels of expression of the full-length Pax5 in B cells from MM patients. The expressed isoforms in MM varied considerably from patient to patient, with no clear pattern. We also performed semiquantitative analyses of the mRNA expression levels of B lymphocyte-induced maturation protein (Blimp-1), because expression levels of Pax5 and Blimp-1 have been shown to be inversely correlated. We observed the expression of Blimp-1 in the B-cell populations in all 11 MM patients but in none of 11 healthy subjects. We hypothesize that premature Blimp-1 expression coupled to altered and deficient Pax5 expression causes some proliferating B cells to prematurely differentiate to plasma cells in MM.