Loss of B cell identity correlates with loss of B cell-specific transcription factors in Hodgkin/Reed-Sternberg cells of classical Hodgkin lymphoma

Aberrant expression of ID2 protein and its correlation with EBV-LMP1 and P16(INK4A) in classical Hodgkin lymphoma in China

BACKGROUND

The relationships between the expression of ID2, EBV-LMP1 and P16(INK4A) in Chinese classical Hodgkin lymphoma are unknown and need exploring.

METHODS

Samples of classical Hodgkin lymphoma from 60 Chinese patients were analyzed for the expression of ID2, EBV-LMP1 and p16(INK4A) proteins by immunohistochemistry.

RESULTS

ID2 protein was expressed in 83.3% of this group of classical Hodgkin lymphoma, staining strongly in both cytoplasm and nucleus of the Hodgkin and Reed-Sternberg (HRS) cells. EBV-LMP1 and P16(INK4A) were overexpressed in 85.0% and 71.7% of Hodgkin lymphoma, respectively. EBV-LMP1 was noted in the cytoplasm, membrane and nucleus of HRS cells; P16(INK4A) was in the nucleus and cytoplasm. Microscopically, ID2, EBV-LMP1 and P16(INK4A) staining distinguished the HRS cells from the complex background of lymphocytes. ID2 was positively correlated with EBV-LMP1(P < 0.01), but P16(INK4A) was inversely related to EBV-LMP1 (P < 0.05).

CONCLUSION

It is suggested that ID2, EBV-LMP1 and P16(INK4A) could play an important role in the evolution of classical Hodgkin lymphoma, and be considered as potential adjunct markers to identify HRS cells in diagnosis.

Epstein-Barr virus latent membrane protein 2A exploits Notch1 to alter B-cell identity in vivo

Expression of latent membrane protein 2 (LMP2A) during B-cell development leads to global alterations in gene transcription similar to those seen in Hodgkin Reed-Sternberg cells of Hodgkin lymphoma (HL). Along with the consistent detection of LMP2A in Epstein-Barr virus-associated HL, this implicates a role for LMP2A in the pathogenesis of HL. We have shown that LMP2A constitutively activates the Notch1 pathway to autoregulate the LMP2A promoter. To determine whether constitutive activation of the Notch pathway is important for LMP2A-mediated alterations in B-cell development in vivo, TgE-LMP2A-transgenic mice were intercrossed with mice expressing loxP-flanked Notch1 genes and Cre recombinase. B cells from TgE Notch1(lox/lox)-CD19(+/Cre) mice have an increase in immunoglobulin M and CD43 and a decrease in CD5 expression in the bone marrow compared with TgE Notch1(lox/lox) mice, indicating the LMP2A signal for developmental aberrations is impaired in the absence of Notch1. Real-time reverse-transcribed polymerase chain reaction analysis reveals that LMP2A requires the Notch1 pathway to alter levels of B cell-specific transcription factors, E2A and EBF. Interestingly, Notch1 appears to be important for LMP2A-mediated survival in low interleukin-7. We propose that LMP2A and the Notch1 pathway may cooperate to induce the alterations in B-cell identity seen in Hodgkin Reed-Sternberg cells.

HLA-G protein expression as a potential immune escape mechanism in classical Hodgkin's lymphoma

Classical Hodgkin's lymphoma (cHL) is characterized by the presence of an abundant reactive infiltrate, lacking effective cytotoxic responses. Especially in Epstein-Barr virus (EBV)-negative cHL, the neoplastic Hodgkin-Reed-Sternberg (HRS) cells have lost protein expression of major histocompatibility complex (MHC) class I, enabling escape from cytotoxic T lymphocyte (CTL) responses. However, downregulation of MHC class I generally induces natural killer (NK) cell activation. The paucity of NK cells in the reactive infiltrate of cHL and the systemic NK cell deficiency observed in cHL patients led us to investigate the expression of human leukocyte antigen (HLA)-G, which is known to inhibit NK-cell- and CTL-mediated cytotoxicity. By immunohistochemistry, HLA-G protein was expressed by HRS cells in 54% (95/175) of cHL cases. This expression was associated with absence of MHC class I on the cell surface of HRS cells (P < 0.001) and EBV-negative status (P < 0.001). Previously, genetic markers located in the proximity of the HLA-A and HLA-G genes had been shown to be associated with susceptibility to EBV-positive cHL. In the present study, these markers associated with MHC class I protein expression but not with presence of HLA-G. Our results suggest that induction of HLA-G protein expression in HRS cells contributes to the modulation of immune responses observed in cHL.

Aberrant expression of Notch1 interferes with the B-lymphoid phenotype of neoplastic B cells in classical Hodgkin lymphoma

Plasticity of committed mouse B cells has been demonstrated by inactivation of the B-cell commitment transcription factor PAX5, resulting in loss of the B-cell phenotype and differentiation into various hematopoietic lineages. Furthermore, mature mouse B cells could be reprogrammed into macrophages by overexpression of myeloid-specific transcription factors. Here, we report that aberrant activity of the transmembrane receptor, Notch1, interferes with the B-lymphoid phenotype of mature human germinal center-derived B cells in Hodgkin lymphoma, so called Hodgkin and Reed-Sternberg cells. They have lost the B-cell phenotype despite their mature B-cell origin. Notch1 remodels the B-cell transcription factor network by antagonizing the key transcription factors E2A and early B-cell factor (EBF). Through this mechanism, B lineage-specific genes were suppressed and B lineage-inappropriate genes were induced. We provide evidence that absence of the Notch inhibitor Deltex1 contributes to deregulated Notch activity in Hodgkin and Reed-Sternberg cells. These data suggest that Notch activation interferes with dedifferentiation of neoplastic B cells in Hodgkin lymphoma.

Repression of B-cell linker (BLNK) and B-cell adaptor for phosphoinositide 3-kinase (BCAP) is important for lymphocyte transformation by rel proteins

Persistent Rel/nuclear factor-kappaB (NF-kappaB) activity is a hallmark of many human cancers, and the Rel proteins are implicated in leukemia/lymphomagenesis but the mechanism is not fully understood. Microarray analysis to identify transformation-impacting genes regulated by NF-kappaB's oncogenic v-Rel and c-Rel proteins uncovered that Rel protein expression leads to transcriptional repression of key B-cell receptor (BCR) components and signaling molecules like B-cell linker (BLNK), the B-cell adaptor for phosphoinositide 3-kinase (BCAP) and immunoglobulin lambda light chain (Ig lambda), and is accompanied by a block in BCR-mediated activation of extracellular signal-regulated kinase, Akt, and c-Jun-NH(2)-kinase in response to anti-IgM. The BLNK and BCAP proteins were also down-regulated in lymphoid cells expressing a transformation-competent chimeric RelA/v-Rel protein, suggesting a correlation with the capacity of Rel proteins to transform lymphocytes. DNA-binding studies identified functional NF-kappaB-binding sites, and chromatin immunoprecipitation (ChIP) data showed binding of Rel to the endogenous blnk and bcap promoters in vivo. Importantly, restoration of either BLNK or BCAP expression strongly inhibited transformation of primary chicken lymphocytes by the potent NF-kappaB oncoprotein v-Rel. These findings are interesting because blnk and other BCR components and signaling molecules are down-regulated in primary mediastinal large B-cell lymphomas and Hodgkin's lymphomas, which depend on c-Rel for survival, and are consistent with the tumor suppressor function of BLNK. Overall, our results indicate that down-regulation of BLNK and BCAP is an important contributing factor to the malignant transformation of lymphocytes by Rel and suggest that gene repression may be as important as transcriptional activation for Rel's transforming activity.

IL-21 is expressed in Hodgkin lymphoma and activates STAT5: evidence that activated STAT5 is required for Hodgkin lymphomagenesis

Classical Hodgkin lymphoma (HL) is a malignant disorder characterized by the presence of neoplastic mononucleated Hodgkin and multinucleated Reed-Sternberg cells. Here, we show that both the interleukin (IL)-21 receptor as well as IL-21 are expressed by HL cells. IL-21 activates signal transducer of activation and transcription 3 (STAT3) and STAT5 in HL cell lines and activated human B cells. Ectopic expression of constitutively active STAT5 in primary human B cells resulted in immortalized B cells that have lost the B-cell phenotype and strongly resembled HL cells, which could partially be rescued by ectopic expression of the B cell-determining transcription factor E47. Data from experiments using reporter assays and overexpression of constitutively active IKK2 support the hypothesis that the STAT5 and nuclear factor-kappaB (NF-kappaB) pathways collaborate in HL genesis.

World Health Organization and beyond: new aspects in the pathology of an old disease

Hodgkin's lymphoma is a highly enigmatic lymphoma disease that still covers most of its secrets up to now. Much effort has been made to successfully wrest at least some of the pathogenetic particularities. The current diagnostic criteria are well established allowing hemato-pathologists to make a clear-cut distinction from other lymphomas in almost all cases. Although classic Hodgkin's lymphoma is curable in the vast majority of cases by treatment with highly aggressive drugs with or without radiotherapy, further molecular studies may lead to the identification of therapeutic targets that enable a more tailored treatment with fewer side effects.

The Pathogenesis of Classical Hodgkin's Lymphoma: A Model for B-Cell Plasticity

It has been shown that differentiated lymphoid cells can display a broad developmental potential and might even differentiate into other cell types. Recent data implicate such processes in the pathogenesis of classical Hodgkin's lymphoma (HL). In the malignant, B cell-derived Hodgkin's and Reed-Sternberg (HRS) cells of HL the expression of B cell-specific genes is lost, and B lineage-inappropriate genes are up-regulated. Experimental evidence has been presented in recent years that functional disruption of the B lineage-specific transcription factor program contributes to this process. HRS cells might be reprogrammed into cells resembling undifferentiated progenitor cells, which might offer an explanation for the unique HL phenotype and demonstrate a high degree of plasticity of human lymphoid cells.

The utility of PAX5 immunohistochemistry in the diagnosis of undifferentiated malignant neoplasms

PAX5 is a B-cell transcription factor whose expression at the protein level is reliably detected by immunohistochemistry in routine biopsies. The purpose of this study was to investigate whether PAX5 immunohistochemistry has diagnostic benefit as a B-cell marker in the work-up of undifferentiated malignant neoplasms. Twenty-five cases previously diagnosed as undifferentiated malignant neoplasms were selected. In addition, 59 hematolymphoid and 884 non-hematolymphoid malignancies were studied such that the specificity of PAX5 immunohistochemistry could be addressed. Two of the 25 (8%) undifferentiated neoplasms showed diffuse staining for PAX5, which indicated a B-cell derivation for these neoplasms that was not appreciated at the time of initial diagnosis. PAX5 staining was detected in the vast majority of hematolymphoid tumors of B-cell derivation but only in 5 of 884 (less than 1%) non-hematolymphoid tumors. Our results further show that PAX5 may be the only detectable marker of B lineage in lymphomas that lack or show equivocal CD45RB and CD20 expression. We conclude that the addition of PAX5 to a panel of immunohistologic markers used in the interrogation of undifferentiated neoplasms is of diagnostic benefit. Its expression can also facilitate the diagnosis of classical and nodular lymphocyte-predominant Hodgkin lymphoma with atypical morphologic and immunohistologic features. Lastly, we have shown that the lack of its expression at the protein level in many epithelial and mesenchymal neoplasms renders PAX5 expression an extremely specific marker of the B lineage.

Epstein-Barr virus infection in humans: from harmless to life endangering virus-lymphocyte interactions

After the primary infection, that may or may not cause infectious mononucleosis, the ubiquitous Epstein-Barr virus (EBV) is carried for lifetime. The great majority of adult humans are virus carriers. EBV was discovered in a B-cell lymphoma (Burkitt lymphoma). EBV infection in humans is the example for the power of immune surveillance against virus transformed, potentially malignant cells. Although the virus can transform B lymphocytes in vitro into proliferating lines, it induces malignancy directly only in immunosuppressed hosts. EBV-induced growth transformation occurs only in B lymphocytes. It is the result of a complex interaction between virally encoded and cellular proteins. Different forms of the virus-cell and the cell-host interactions have evolved during a long period of coexistence between the virus and all Old World (but not New World) primates. The asymptomatic carrier state is based on a viral-strategy that downregulates the expression of the transforming proteins in the virus-carrying cell. In addition to the silent viral-gene carriers and the expressors of the nine virus-encoded genes that drive the growth program, virus carrying cells exist that show other patterns of gene expression, depending on the differentiated state of the host cell. Certain combinations contribute to malignant transformation, but only in conjunction with additional cellular changes. These are induced by direct or cytokine-mediated interactions with normal cells of the immune system.

Bob-1 is expressed in classic Hodgkin lymphoma

Background

Almost all researchers agree on the lack of Bob-1 expression in Hodgkin/Reed-Sternberg (H/RS) cells in classic Hodgkin lymphoma (CHL), and utilize this marker as a diagnostic tool in conjunction with other markers to differentiate between lymphocyte predominance Hodgkin lymphoma (LPHL) and CHL.

Aim

To study the immunohistochemical (IHC) expression of Bob-1 in Egyptian CHL and to correlate this expression with Epstein-Barr virus (EBV) viral load.

Materials and methods

Paraffin sections of randomly selected 18 CHL cases were included: 2 lymphocyte rich (LR), 4 mixed cellularity (MC), 10 nodular sclerosis (NS) and 2 lymphocyte depletion (LD). All cases were immunostained for Bob-1. EBV was evaluated by EBV early RNA transcripts in situ hybridization (EBER ISH) and immunostaining for EBV latent membrane protein-1 (LMP-1).

Results

Sixty seven percent of cases (12/18) were positive for EBV by ISH and/or immunostaining for LMP-1. Moderate to strong nuclear Bob-1 was observed in 94% of cases. The positivity ranged between 25–100%. Bob-1 immunoreactivity was strongly associated with EBV positivity (p < 0.001).

Conclusion

This study proves nuclear IHC expression of Bob-1 on H/RS in CHL implying the difficulties in applying this marker to differentiate between LPHL and CHL. Does this difference between Western and Egyptian CHL reflect genetic and/or environmental factors, or simply no difference exists as most researchers are concentrated on the Western population and no comparative studies have been done. Studies from other countries might answer this question.

EBV-associated diseases in the AIDS patient

EBV-associated malignancies remain a considerable problem in HIV-infected individuals, even in the era of HAART. Although EBV is a common factor, each disease has a unique pathogenesis. Study of these diseases reveals the viral proteins expressed in the malignancies that might contribute to the development of the disease as well as the molecular basis for pathogenesis. It is likely that this knowledge will contribute to the development of novel therapeutics that will result in more favorable outcomes in the future.

Expression of the B cell-associated transcription factors PAX5, OCT-2, and BOB.1 in acute myeloid leukemia: associations with B-cell antigen expression and myelomonocytic maturation

The aberrant expression of the B-cell transcription factor PAX5 has been described in a subset of acute myeloid leukemia (AML) with t(8;21)(q22;q22) in association with B-cell antigen expression. However, the expression of other B cell-associated transcription factors, particularly OCT-2 and its B cell-specific coactivator BOB.1, has not been described in AML. In this study, expression of PAX5, OCT-2 and BOB.1 was evaluated by immunohistochemical staining of bone marrow samples from 83 cases of AML. The expression patterns were correlated with t(8;21)(q22;q22), B cell-associated antigen expression, and AML subtype. We confirmed the expression of PAX5 in AML with t(8;21)(q22;q22), but also demonstrated its expression in cases that express B-cell antigens but lack this translocation. Although OCT-2 and BOB.1 were not associated with PAX5 expression, we report expression of OCT-2 in AML with myelomonocytic/monocytic maturation and BOB.1 in normal hematopoietic elements.

Pathobiology of Classical Hodgkin Lymphoma

The World Health Organization has acknowledged the malignant nature of classical Hodgkin lymphoma (cHL), which encompasses four histological subtypes. The diagnosis of cHL is based on the detection of malignant Hodgkin and Reed-Sternberg cells (HRSC) confirmed by immunophenotyping and the detection of growth patterns specific to each histological subtype. The pathologic HRSC arise from germinal center or immediate postgerminal cells that lack detectable immunoglobulin/B-cell antigen receptor expression, with a consequent loss of B-cell identity; very few cHL cases are of T-cell origin. To escape apoptosis, which normally occurs in B cells with nonfunctioning antigen receptor machinery, HRSC develop concurrent antiapoptotic mechanisms by activation of nuclear factor-kappaB or are rescued by Epstein-Barr virus infection. HRSC are characterized by a variable and inconstant immunophenotype, with a remarkable loss of lineage-specific cell antigens and expression of antigens of other cell lineages. The master plan of B-cell identity in HRSC is disturbed not only at the immunoglobulin expression level, but also at the transcriptional factor level. HRSC are further characterized by profound cell cycle deregulation with futile replication, multinucleation and poly- and aneuploidy. Here, we review pathobiological aspects of cHL with respect to lymphomagenesis and routine diagnostics.

Modulation of Chemokine Gene Expression in CD133+Cord Blood-Derived Human Mast Cells by Cyclosporin A and Dexamethasone

We have recently developed a protocol for generating huge numbers of mature and functional mast cells from in vitro differentiated umbilical cord blood cells. Using CD133 as a positive selection marker to isolate haematopoietic progenitors we routinely expand the number of recovered cells at least 150-fold, which vastly exceeds the yields of conventional protocols using CD34+ cells as a source of progenitors. Taking advantage of the large quantities of in vitro differentiated mast cells, here we assess at the levels of transcription and translation the kinetics of chemokine gene induction following receptor mediated mast cell activation or following pharmacological activation of specific signal transduction cascades that become activated upon classical FcepsilonRI receptor crosslinking. We demonstrate that chemokine genes encoding IL-8, MCP-1, MIP-1alpha, and MIP-1beta are induced with different kinetics and with different amplitudes in a receptor activation dependent manner, and that these events can be mimicked using pharmacological agents which activate distinct signal transduction pathways. These findings were corroborated by adding immunomodulators such as cyclosporin A and dexamethasone prior to mast cell activation. Finally, we demonstrate that the same modulators added after mast cell activation can differentially quench ongoing chemokine gene induction. Thus, considering the vast yields of mast cells, our protocol is valuable not only for studying regulation of gene expression in mast cells in general, but also as an experimental tool to develop better and more balanced treatments of mast cell related disorders.

Aberrant expression of ID2, a suppressor of B-cell-specific gene expression, in Hodgkin's lymphoma

The global loss of B-cell-specific gene expression is a distinctive feature of the Hodgkin-Reed/Sternberg (HRS) cells of classical Hodgkin's lymphoma (HL). The reasons for this loss remained largely unknown as transcription factors with pleiotropic effects on B-cell-specific gene expression, namely E2A, EBF, and PAX5, are present in primary HRS cells. We show here that ID2, which can inactivate E2A and perhaps PAX5, is not detectable in normal B cells but is strongly and uniformly expressed in HRS cells of all cases of classical HL. Recurrent chromosomal gains of the ID2 gene might contribute to this aberrant expression. Co-immunoprecipitation of E2A with ID2 from HRS-derived cell lines together with the high amount of ID2 relative to the B-cell transcription factors E2A and PAX5 in HRS-derived cell lines and primary HRS cells indicated that aberrant ID2 expression contributes significantly to the loss of the B-cell-specific gene expression in HRS cells. ID2 was also expressed in lymphocyte-predominance HL, mediastinal large B-cell, diffuse large B-cell, and Burkitt's lymphoma, where lower amounts of ID2 relative to E2A and PAX5 compared with HRS cells might prevent a global down-regulation of B-cell-specific genes and ID2 may contribute to lymphomagenesis in other ways.

Expression profiling of transcription factors Pax-5, Oct-1, Oct-2, BOB.1, and PU.1 in Hodgkin's and non-Hodgkin's lymphomas: a comparative study using high throughput tissue microarrays

Analysis of B-cell-specific transcription factors is useful in understanding of the differentiation-linked phenotype in Hodgkin's as well as in non-Hodgkin's lymphomas. We analyzed the expression profiling of transcription factors Pax-5, Oct-1, Oct-2, BOB.1, and PU.1 in 109 cases, including non-Hodgkin's lymphomas of B- and T-lineage, classical Hodgkin's lymphomas, and nodular lymphocyte predominant Hodgkin's lymphomas. Our study revealed that all transcription factors were universally expressed in all cases of nodular lymphocyte predominant Hodgkin's and variably expressed in non-Hodgkin's lymphomas of B-lineage. Cases of classical Hodgkin's lymphoma variably expressed the Pax-5, Oct-1, Oct-2, and BOB.1. However, in contrast to nodular lymphocyte predominant Hodgkin's lymphoma, the transcription factor PU.1 was consistently absent in all cases of classical Hodgkin's lymphoma. Transcription factors Pax-5, BOB.1, and PU.1 were not detectable in cases of anaplastic large cell lymphoma. However, the Oct-1 was detected in all anaplastic large cells lymphoma cases, indicating that expression of this transcription factor was not restricted to B-lineage lymphoid malignancies. Our findings suggest that inclusion of the PU.1 antibody may prove useful in separating classical Hodgkin's lymphomas from nodular lymphocyte predominant Hodgkin's lymphomas in problematic cases.

Mediastinal gray zone lymphoma: the missing link between classic Hodgkin's lymphoma and mediastinal large B-cell lymphoma

In recent years, overlap in biologic and morphologic features has been identified between classic Hodgkin lymphoma (cHL) and B-cell non-Hodgkin lymphoma. Nevertheless, the therapeutic approaches for these diseases remain different. We undertook a study of "mediastinal gray zone lymphomas" (MGZL), with features transitional between cHL nodular sclerosis (NS) and primary mediastinal large B-cell lymphoma (MLBCL) to better understand the morphologic and immunophenotypic spectrum of such cases. Twenty-one MGZL cases were identified over a 20-year period. We also studied 6 cases of composite or synchronous lymphoma with two distinct components at the same time (cHL-NS and MLBCL) and 9 sequential cases with MLBCL and cHL-NS at different times. All patients had a large mediastinal mass. Immunohistochemical studies focused on markers known to discriminate between cHL and MLBCL, including B-cell transcription factors. VJ-PCR was performed in 8 cases to look at clonality of the immunoglobulin heavy chain gene (IgH). Of the gray zone cases, 11 had morphology reminiscent of cHL-NS, but with unusual features, including a large number of mononuclear variants, diminished inflammatory background, absence of classic Hodgkin phenotype, and strong CD20 expression (11 of 11). Ten cases had morphology of MLBCL, but with admixed Hodgkin/Reed-Sternberg and lacunar cells, absent (3 of 10) or weak (7 of 10) CD20 expression, and positivity for CD15 in 7 cases. B-cell transcription factor expression in the gray zone cases more closely resembled MLBCL than cHL with expression of Pax5, Oct2, and BOB.1 in all but 1 case studied (14 of 15). MAL staining was found in 7 of 10 MGZL, and in at least one component of 6 of 7 evaluable composite or sequential MLBCL/cHL cases. Two cases of sequential lymphoma showed rearrangements of the IgH gene of identical size: one in which MLBCL was the first diagnosis and one in which MLBCL was diagnosed at relapse, indicating clonal identity for the two components of cHL and MLBCL. There is accumulating evidence that MLBCL and cHL are related entities. Further support for a relationship between MLBCL and cHL-NS is provided by composite and metachronous lymphomas in the same patient, as well as the existence of MGZL with transitional morphology and phenotype.

Intrinsic inhibition of transcription factor E2A by HLH proteins ABF-1 and Id2 mediates reprogramming of neoplastic B cells in Hodgkin lymphoma

B cell differentiation is controlled by a complex network of lineage-restricted transcription factors. How perturbations to this network alter B cell fate remains poorly understood. Here we show that classical Hodgkin lymphoma tumor cells, which originate from mature B cells, have lost the B cell phenotype as a result of aberrant expression of transcriptional regulators. The B cell-specific transcription factor program was disrupted by overexpression of the helix-loop-helix proteins ABF-1 and Id2. Both factors antagonized the function of the B cell-determining transcription factor E2A. As a result, expression of genes specific to B cells was lost and expression of genes not normally associated with the B lineage was upregulated. These data demonstrate the plasticity of mature human lymphoid cells and offer an explanation for the unique classical Hodgkin lymphoma phenotype.

Cytokine mediated induction of the major Epstein-Barr virus (EBV)-encoded transforming protein, LMP-1

In the in vitro infected B-cells six EBV-encoded nuclear antigens (EBNA-1-6) and three latent membrane proteins (LMP-1, -2A, -2B) are expressed (type III latency). In addition, other restricted forms of latency occur in the EBV-carrying malignancies. In Burkitt lymphoma (BL) only EBNA-1 is expressed (type I), while in Hodgkin lymphoma (HL), T-, and NK-lymphoma, and nasopharyngeal carcinoma EBNA-1 and LMPs are expressed (type II). B-cells with these three expression patterns have been detected in healthy virus carriers. While in type III latency two viral transcriptional activators, EBNA-2 and -5, are responsible for LMP-1 expression, the mechanism that controls the expression of LMP-1 in type II latent cells is not known. In order to study the interaction of EBV- and HL-derived cells, we studied the in vitro EBV-converted subline of the KMH2 cells that express only EBNA-1 and LMP-2A. Interestingly, exposure of the KMH2-EBV cells to CD40-ligand and IL-4 induced LMP-1 expression, in the absence of EBNA-2. In BL cell lines lacking EBNA-2 another cytokine, IL-10, could induce LMP-1 expression. IL-10 induced LMP-1 also in tonsillar B-cells infected with the EBNA-2-deleted virus strain P3HR-1. Our results show that cytokines are responsible for the expression of LMP-1 in type II latent B-cells. These signals are available in the germinal center environment and in the granulation tissue of HLs. Based on these results we propose that LMP-1 expression is induced by extracellular signals and is not a constitutive characteristic of the EBV-carrying type II B-cells. Cytokine mediated induction of LMP-1 may also explain the heterogeneous expression of this viral gene seen in normal and malignant cells.

Rare expression of T-cell markers in classical Hodgkin's lymphoma

Hodgkin's and Reed-Sternberg cells of classical Hodgkin's lymphoma are primarily of B-cell origin, although there are instances of T-cell antigen expression suggesting T-cell origin. We comprehensively analyzed expression of various T-cell antigens in 259 classical Hodgkin's lymphoma cases using the tissue microarray technique. Expression of the T-cell antigens CD2, CD3, CD4, CD5, CD7 and CD8 was assessed by immunohistochemistry. Hodgkin's and Reed-Sternberg cells of T-cell marker-positive cases were microdissected and analyzed by a multiplex polymerase chain reaction for clonal immunoglobulin heavy chain- and T-cell receptor gamma gene rearrangements. In all, 12 cases (5%) expressed at least one T-cell marker in the following order: CD2 in 11 cases, CD4 in five, CD3 in two, and CD5 and CD8 in one case each; there were no CD7-positive cases, and five cases (2%) expressed more than one T-cell antigen. In positive cases, a mean fraction of 40% of the Hodgkin's and Reed-Sternberg cells (range 20-100%) expressed the analyzed T-cell markers. Two cases (<1%) evidenced clonal T-cell receptor gamma gene rearrangement. Phenotypic expression of T-cell antigens in Hodgkin's and Reed-Sternberg cells of classical Hodgkin's lymphoma is rare (5%), while genotypically, less than 1% of classical Hodgkin's lymphomas are of possible T-cell origin. Therefore, T-cell antigen expression on Hodgkin's and Reed-Sternberg cells is aberrant in the majority of cases and only infrequently classical Hodgkin's lymphomas are of T-cell origin.

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.

Evidence of abortive plasma cell differentiation in Hodgkin and Reed-Sternberg cells of classical Hodgkin lymphoma

Hodgkin and Reed-Sternberg (HRS) cells of classical Hodgkin lymphoma (cHL) show genotypic features of germinal centre-derived B-cells in most cases. Nevertheless, these cells typically lack expression of B-cell antigens. Previous studies have suggested that plasma cell differentiation may occur in HRS cells and that this may account for the down-regulation of B-cell antigens. However, these results are controversial. We have addressed this question using immunohistochemistry and a panel of antibodies directed against antigens which are differentially expressed during terminal B-cell differentiation. Pax-5, a transcription factor required for B-lineage commitment, and IRF4/Mum1, which is physiologically expressed in germinal centre cells and plasma cells, were consistently detectable in HRS cells. Bcl-6, a transcription factor expressed in germinal centre B-cells, was present in HRS cells of approximately 25% of cHL cases. Expression of the B-lymphocyte-induced maturation protein-1 (Blimp-1), a key regulator of plasma cell differentiation, was observed in HRS cells of 23% of cHL cases. In these cases, Blimp-1 expression was restricted to a small proportion of HRS cells. HRS cells were consistently negative for the plasma cell marker CD138. These results suggest that plasma cell differentiation may be initiated in a small subset of HRS cells but remains abortive. Thus, terminal differentiation is unlikely to explain the lack of B-cell antigen expression in HRS cells.

Patterned CpG methylation of silenced B cell gene promoters in classical Hodgkin lymphoma-derived and primary effusion lymphoma cell lines

Hodgkin and Reed-Sternberg (HRS) cells of classical Hodgkin lymphoma (cHL) and primary effusion lymphoma (PEL) are derived from germinal center (GC) and post-GC B cells, respectively. Neither express many of the B cell genes or surface markers typically expressed by other GC-derived B cell lymphomas or normal B cells. This loss of B cell gene expression is not due to a lack of essential transcription factors, as studies have shown that the ectopic expression of missing transcription factors failed to reactivate endogenous target genes. These results implicate epigenetic mechanisms extinguishing B cell gene expression. Silenced endogenous B cell genes representing a surface receptor, B29 (Igbeta, CD79b), a signaling molecule, TCL1, and a transcription factor, Bob1 (OCA-B, OBF-1), were reactivated by 5-aza-2'-deoxycytidine, indicating that gene silencing in HRS and PEL cells is due to DNA methylation. Genomic bisulfite sequencing corroborated this prediction and revealed three distinct patterns of methylation for the silenced B29 and TCL1 promoters. These distinct patterns consisted of 5' promoter CpG methylation alone, 5' and 3' promoter CpG methylation sparing sites in the central cores, and complete CpG methylation throughout the promoter regions. The silenced Bob1 promoter showed one pattern of dense CpG methylation at essentially all sites. These consistent patterns predict that, although gene silencing in many HRS and PEL cells mimics appropriate gene silencing, in some cases of complete CpG methylation throughout entire promoters both the activation and targeting of methylation is abnormal.

Differential requirement for OBF-1 during antibody-secreting cell differentiation

Resting B cells can be cultured to induce antibody-secreting cell (ASC) differentiation in vitro. A quantitative analysis of cell behavior during such a culture allows the influences of different stimuli and gene products to be measured. The application of this analytical system revealed that the OBF-1 transcriptional coactivator, whose loss impairs antibody production in vivo, has two effects on ASC development. Although OBF-1 represses early T cell–dependent (TD) differentiation, it is also critical for the completion of the final stages of ASC development. Under these conditions, the loss of OBF-1 blocks the genetic program of ASC differentiation so that Blimp-1/prdm1 induction fails, and bcl-6, Pax5, and AID are not repressed as in control ASC. Retroviral complementation confirmed that OBF-1 was the critical entity. Surprisingly, when cells were cultured in lipopolysaccharide to mimic T cell–independent conditions, OBF-1–null B cells differentiated normally to ASC. In the OBF-1^−/− ASC generated under either culture regimen, antibody production was normal or only modestly reduced, revealing that Ig genes are not directly dependent on OBF-1 for their expression. The differential requirement for OBF-1 in TD ASC generation was confirmed in vivo. These studies define a new regulatory role for OBF-1 in determining the cell-autonomous capacity of B cells to undergo terminal differentiation in response to different immunological signals.

Alterations of loci encoding PU.1, BOB1, and OCT2 transcription regulators do not correlate with their suppressed expression in Hodgkin lymphoma

Neoplastic cells of Hodgkin lymphoma (HL) originating from germinal or postgerminal center B cells lose their capacity to transcribe and to express surface immunoglobulins (Ig). This defect correlates with the absence of expression of B-cell-specific transcription regulators, including PU.1, BOB1, and OCT2. These findings suggest that Ig impairment in HL is caused by the defective transcription machinery. The mechanism or mechanisms underlying failure of Hodgkin cells to express PU.1, BOB1, and OCT2 remain unclear. The genes encoding for these three respective transcription factors have been mapped at 11p11.2 (SPI1), 11q23.1 (POU2AF1), and 19q13.2 (POU2F2); these are chromosomes recurrently affected in HL. To check the genomic status of PU.1, BOB1, and OCT2 in HL, we performed metaphase fluorescence in situ hybridization (FISH) analysis of 10 HL cases using locus-specific bacterial artificial chromosome clones. FISH signal pattern was correlated with the ploidy level of each analyzed cell and showed recurrent imbalances of the studied loci. The underrepresentation of one or two analyzed regions was detected in five cases; the remaining five cases showed either random losses, a ploidy-equivalent FISH pattern, or overrepresented signals. Neither a constant loss nor genomic aberration of at least one of these genes could be observed in studied cases. These findings indicate that genomic imbalances or rearrangements are not a cause of PU.1, BOB1, and OCT2 deficiency in cHL and argue for another mechanism underlying this phenomenon.

Accumulation of miR-155 and BIC RNA in human B cell lymphomas

We show that the microRNA miR-155 can be processed from sequences present in BIC RNA, a spliced and polyadenylated but non-protein-coding RNA that accumulates in lymphoma cells. The precursor of miR-155 is likely a transient spliced or unspliced nuclear BIC transcript rather than accumulated BIC RNA, which is primarily cytoplasmic. By using a sensitive and quantitative assay, we find that clinical isolates of several types of B cell lymphomas, including diffuse large B cell lymphoma (DLBCL), have 10- to 30-fold higher copy numbers of miR-155 than do normal circulating B cells. Similarly, the quantities of BIC RNA are elevated in lymphoma cells, but ratios of the amounts of the two RNAs are not constant, suggesting that the level of miR-155 is controlled by transcription and processing. Significantly higher levels of miR-155 are present in DLBCLs with an activated B cell phenotype than with the germinal center phenotype. Because patients with activated B cell-type DLBCL have a poorer clinical prognosis, quantification of this microRNA may be diagnostically useful.

Expression of the T-cell transcription factors, GATA-3 and T-bet, in the neoplastic cells of Hodgkin lymphomas

Since Hodgkin and Reed-Sternberg (HRS) cells of Hodgkin lymphoma (HL) generally have immunoglobulin gene rearrangements, they are considered to be of B-cell origin. One of the characteristics of HRS cells is a prominent production of various cytokines and chemokines. Cytokine production is generally driven by expression of T-cell transcription factors (TFs). Only limited information is available on the expression of T-cell TFs in HL. Expression of four T-cell TFs and the target cytokine spectrum of these TFs were analyzed in six HL and three large B-cell lymphoma (LBCL) cell lines using quantitative PCR. ERM expression was observed in all HL and LBCL cell lines. Out of HL cell lines, T-bet was expressed in five, GATA-3 in four, and c-Maf in two cell lines. Immunohistochemistry in HL tissues revealed that in 11 of 12 (92%) of the classical HL cases HRS cells were GATA-3 and/or T-bet positive. In three of six cases of nodular lymphocyte predominance type of HL, the neoplastic cells were T-bet positive. Overall, the T-cell TF and cytokine profiles of the HL cell lines showed a considerable degree of consistency. The expression of T-cell TFs may explain the production of various cytokines by HL cell lines and HRS cells.

Epigenetic silencing of the immunoglobulin heavy-chain gene in classical Hodgkin lymphoma-derived cell lines contributes to the loss of immunoglobulin expression

Immunoglobulin production is impaired in Hodgkin and Reed-Sternberg (HRS) cells of classical Hodgkin lymphoma (cHL) in spite of functional clonal rearrangements. The presence of "crippling" mutations in coding and regulatory regions, as well as down-regulation of B-cell-specific transcription factors, has been suggested as a potential reason for the lack of immunoglobulin (Ig) chain gene transcription. We have investigated the impact of epigenetic silencing in suppressing Ig heavy (H)-chain expression. Chromatin immunoprecipitation (ChIP) was used to analyze transcription factor binding to octamer motifs present in the IgH regulatory regions. Transcription factors were bound to these motifs in control cell lines, however, they were absent in the cHL-derived cell lines KMH2, L1236, and L428. Ectopic expression of octamer-binding transcription factor (Oct2) and/or B-cell Oct binding protein/Oct-binding factor (BOB.1/OBF.1) did not result in any measurable binding to these sites. Increased histone 3 Lysine 9 (H3-K9) methylation was observed in the promoter region of the IgH locus in L428 and L1236 cells. This is a typical feature of heterochromatic, transcriptionally silent regions. Treatment of cHL-derived cell lines with the DNA demethylating agent 5-aza-2'-deoxycytidine (5-aza-dC) partially reactivated IgH transcription and affected chromatin modifications. Our results suggest an important role of epigenetic silencing in the inhibition of IgH transcription in HRS cells.

Silencing of the p18INK4c gene by promoter hypermethylation in Reed-Sternberg cells in Hodgkin lymphomas

p18INK4c is a cyclin-dependent kinase (CDK) inhibitor that interferes with the Rb-kinase activity of CDK6/CDK4. Disruption of p18INK4c in mice impairs B-cell terminal differentiation and confers increased susceptibility to tumor development; however, alterations of p18INK4c in human tumors have rarely been described. We used a tissue-microarray approach to analyze p18INK4c expression in 316 Hodgkin lymphomas (HLs). Nearly half of the HL cases showed absence of p18INK4c protein expression by Reed-Sternberg (RS) cells, in contrast with the regular expression of p18INK4c in normal germinal center cells. To investigate the cause of p18INK4c repression in RS cells, the methylation status of the p18INK4c promoter was analyzed by methylation-specific polymerase chain reaction (PCR) and bisulfite sequencing. Hypermethylation of the p18INK4c promoter was detected in 2 of 4 HL-derived cell lines, but in none of 7 non-Hodgkin lymphoma (NHL)–derived cell lines. We also detected p18INK4c hypermethylation, associated with absence of protein expression, in 5 of 26 HL tumors. The correlation of p18INK4c immunostaining with the follow-up of the patients showed shorter overall survival in negative cases, independent of the International Prognostic Score. These findings suggest that p18INK4c may function as a tumor suppressor gene in HL, and its inactivation may contribute to the cell cycle deregulation and defective terminal differentiation characteristic of the RS cells.

In vitro EBV-infected subline of KMH2, derived from Hodgkin lymphoma, expresses only EBNA-1, while CD40 ligand and IL-4 induce LMP-1 but not EBNA-2

In about 50% of classical Hodgkin lymphomas, the Hodgkin/Reed Sternberg (H/RS) cells carry Epstein-Barr virus (EBV). The viral gene expression in these cells is restricted to EBNA-1, EBERs, LMP-1 and LMP-2 (type II latency). The origin of H/RS cells was defined as crippled germinal center B cells that escaped apoptosis. In spite of numerous attempts, only few typical Hodgkin lymphoma (HL) lines have been established. This suggests that the cells require survival factors that they receive in the in vivo microenvironment. If EBV is expected to drive the cells for growth in culture, the absence of EBNA-2 may explain the incapacity of H/RS cells for in vitro proliferation. In EBV carrying B lymphocytes, functional EBNA-2 and LMP-1 proteins are required for in vitro growth. For analysis of the interaction between EBV and the H/RS cells, we infected the CD21-positive HL line KMH2 with the B958 and Akata viral strains. Only EBNA-1 expression was detected in a few cells in spite of the fact that all cells could be infected. Using a neomycin-resistance-tagged recombinant EBV strain (Akata-Neo) we established an EBV-positive subline that was carried on selective medium. In contrast to the type II EBV expression pattern of H/RS cells in vivo, the KMH2 EBV cells did not express LMP-1. The EBV expression pattern could be modified in this type I subline. LMP-1 could be induced by the histone deacetylase inhibitors TSA and n-butyrate, by 5-AzaC, a demethylating agent, and by phorbol ester. None of these treatments induced EBNA-2. Importantly, exposure to CD40 ligand and IL-4 induced LMP-1 without EBNA-2 expression and lytic replication. The KMH2 EBV cells expressed LMP-2A, but not LMP-2B mRNAs. This result is highly relevant for the type II expression pattern of H/RS cells in vivo, since these stimuli can be provided by the surrounding activated T lymphocytes.

Part I: Hodgkin's lymphoma—molecular biology of Hodgkin and Reed-Sternberg cells

Classic Hodgkin's lymphoma is characterised by Hodgkin and Reed-Sternberg cells and in most cases are derived from germinal-centre B cells. Despite progress in basic research showing the natural precursor cells of Hodgkin's lymphoma, most key questions still remain unanswered. Among these are the basic transforming events, the involvement of oncogenic viruses, the mechanisms enabling Hodgkin and Reed Sternberg cells to resist apoptosis in the germinal centre, and the molecular causes of their characteristic phenotype. Beyond the disclosure of these issues, the detection of changes in gene expression, gene mutations, and chromosomal imbalances specific of Hodgkin's lymphoma are central to recent research that may allow one a better understanding of the natural history of this type of lymphoma.

Epstein-Barr Virus (EBV) LMP2A induces alterations in gene transcription similar to those observed in Reed-Sternberg cells of Hodgkin lymphoma

Epstein-Barr virus (EBV) is associated with the development of a variety of malignancies, including Hodgkin lymphoma. One of the few viral transcripts expressed in EBV-positive Hodgkin/Reed-Sternberg (HRS) cells of Hodgkin lymphoma is latent membrane protein 2A (LMP2A). This viral protein blocks B-cell receptor (BCR)-signaling in vitro. Furthermore, expression of LMP2A in developing B cells in vivo induces a global down-regulation of genes necessary for proper B-cell development. In this study we have analyzed gene transcription in primary B cells from LMP2A transgenic mice, LMP2A-expressing human B-cell lines, and LMP2A-positive and -negative EBV-infected lymphoblastoid cell lines (LCLs). We demonstrate that LMP2A increases the expression of genes associated with cell cycle induction and inhibition of apoptosis, alters the expression of genes involved in DNA and RNA metabolism, and decreases the expression of B-cell-specific factors and genes associated with immunity. Furthermore, many alterations in gene expression induced by LMP2A are similar to those recently described in HRS cells of Hodgkin lymphoma and activated, proliferating germinal center centroblasts/centrocytes. These correlations suggest that LMP2A expression in EBV-infected B cells may lead to the induction and maintenance of an activated, proliferative state that could ultimately result in the development of Hodgkin lymphoma.

OCA-B regulation of B-cell development and function

The transcriptional co-activator OCA-B [for Oct co-activator from B cells, also known as OBF-1 (OCT-binding factor-1) and Bob1] is not required for B-cell genesis but does regulate subsequent B-cell development and function. OCA-B deficient mice show strain-specific, partial blocks at multiple stages of B-cell maturation and a complete disruption of germinal center formation in all strains, causing humoral immune deficiency and susceptibility to infection. OCA-B probably exerts its effects through the regulation of octamer-motif controlled gene expression. The OCA-B gene encodes two proteins of distinct molecular weight, designated p34 and p35. The p34 isoform localizes in the nucleus, whereas the p35 isoform is myristoylated and is bound to the cytoplasmic membrane. p35 can traffic to the nucleus and probably activates octamer-dependent transcription, although this OCA-B isoform might regulate B cells through membrane-related signal transduction.

Expression of SH2D1A in five classical Hodgkin's disease-derived cell lines

The Src homology 2 domain protein 1A (SH2D1A) is a small, 128-amino acid protein consisting of a single SH2 domain; it is probably involved in signal regulation. It is expressed in activated T and natural killer (NK) cells, but not in B lymphocytes. It was discovered in studies on the rare hereditary condition X-linked lymphoproliferative disease (XLP). Individuals with this condition either lack or carry an altered protein. The serious symptoms (fatal mononucleosis) present almost exclusively at the first encounter with Epstein-Barr virus (EBV). The absence of SH2D1A in B cells, which are the targets of EBV, has to be reconciled with this clinical situation. In an earlier search for B lymphocytes expressing SH2D1A, we detected it in EBV-carrying type I Burkitt's lymphoma (BL) lines. We now show SH2D1A in 5 EBV-negative classical Hodgkin's disease (HD)-derived cell lines. Two lines belong to the T lineage and 3 to the B lineage. One B-HD line, which originated from nodular lymphocyte-predominant Hodgkin's lymphoma and differed in phenotype, was SH2D1A-negative. This finding is in accordance with the previously reported abundant SH2D1A mRNA in Hodgkin and Reed-Sternberg (HRS) cells. We thus found SH2D1A expression in lines of malignant origin assigned to the B lineage. Its presence in HRS cells may lead us closer to an understanding of the pathophysiology of the serious syndrome connected with EBV infection in XLP patients, because HRS-like cells have been detected in the lymphoid tissue of patients with infectious mononucleosis. It is likely therefore that in addition to the demonstrated functional defect of T and NK cells imposed by the SH2D1A mutation, the behavior of certain EBV-infected B lymphocytes is also modified.

Disruption of the B-cell specific transcriptional program in HHV-8 associated primary effusion lymphoma cell lines

Primary effusion lymphoma (PEL) is a lymphoproliferative disease of B-cell origin that is associated with HHV-8 infection. PEL cells harbor a non-B, non-T phenotype and lack significant surface immunoglobulin (Ig) expression, a characteristic that has not been fully explained. In the present study, we demonstrate that PEL cells constitutively express interferon regulatory factor (IRF)-4, a transcription factor that regulates the activity of the immunoglobulin light-chain enhancer elements lambdaB and kappaE3' through binding to a composite Ets-IRF site. IRF-4 activity requires its physical interaction with PU.1, an Ets family member involved in the activation of genes essential for B-cell development. However, in PEL-derived B-cell lines, PU.1 expression was completely abrogated; expression of the B cell specific transcription factor Oct-2, which is known to regulate PU.1 expression, was also abolished. Moreover, the B-cell-specific coactivator of octamer factors, BOB-1/OcaB, was expressed at very decreased levels in PEL cells. Ectopic expression of Oct-2 was able to fully restore PU.1 promoter activity in the PEL cell line BCBL-1, while PU.1 expression also reconstituted the activity of the lambdaB Ets-IRF site. In addition, protein levels of BSAP/Pax-5 and IRF-8/ICSBP were undetectable in PEL cells. The pattern of transcription factor ablation observed in PEL was found to be comparable to that observed in classical Hodgkin's disease-derived cell lines, which also lack B-cell-specific surface markers. These observations indicate that disruption of the B-cell-specific transcriptional program is likely to contribute to the incomplete B-cell phenotype characteristic of PEL cells.