T-cell/histiocyte-rich large B-cell lymphoma: a heterogeneous entity with derivation from germinal center B cells

Flow Cytometry for B-Cell Non-Hodgkin and Hodgkin Lymphomas

Multi-parametric flow cytometry is a powerful diagnostic tool that permits rapid assessment of cellular antigen expression to quickly provide immunophenotypic information suitable for disease classification. This chapter describes the classification of B-cell non-Hodgkin lymphoma (B-NHL) by flow cytometry suitable for the clinical and research environment. In addition to describing the immunophenotypic patterns of the most common B-NHL (including examples of common B-NHL), the effect of anti-CD19, -CD20, and -CD38 therapies on the evaluation of flow cytometric data is also discussed. Over the last 15 years, our laboratory has developed flow cytometry combinations that can immunophenotype classic Hodgkin lymphoma (CHL), nodular lymphocyte predominant Hodgkin lymphoma (NLPHL), and T-cell/histocyte-rich large B-cell lymphoma (THRLBCL) and the use of these assays will be presented. The CHL assay combination is also particularly well suited to immunophenotype primary mediastinal large B-cell lymphoma (PMLBCL) and our experience immunophenotyping PMLBCL by flow cytometry will be discussed. Finally, an approach to the evaluation of the reactive infiltrate of CHL, NLPHL, and THRLBCL that can provide diagnostic information will also be provided.

Flow Cytometry for Non-Hodgkin and Hodgkin Lymphomas

Multiparametric flow cytometry is a powerful diagnostic tool that permits rapid assessment of cellular antigen expression to quickly provide immunophenotypic information suitable for disease classification. This chapter describes a general approach for the identification of abnormal lymphoid populations by flow cytometry, including B, T, NK, and Hodgkin lymphoma cells suitable for the clinical and research environment. Knowledge of the common patterns of antigen expression of normal lymphoid cells is critical to permit identification of abnormal populations at disease presentation and for minimal residual disease assessment. We highlight an overview of procedures for processing and immunophenotyping non-Hodgkin B- and T-cell lymphomas and also describe our strategy for the sensitive and specific diagnosis of classic Hodgkin lymphoma, nodular lymphocyte predominant Hodgkin lymphoma, and T-cell/histiocyte-rich large B-cell lymphoma.

Role of Novel Quantitative Imaging Techniques in Hematological Malignancies

Hematological malignancies exhibit a widespread distribution, necessitating evaluation of disease activity over the entire body. In clinical practice, visual analysis and semiquantitative parameters are used to assess 18F-FDGPET/CT imaging, which solely represents measurements of disease activity from limited area and may not adequately reflect global disease assessment. An efficient method for assessing the global disease burden of hematological malignancies is to employ PET/computed tomography based novel quantitative parameters. In this article, we explored novel quantitative parameters on PET/CT imaging for assessing global disease burden and the potential role of artificial intelligence (AI) to determine these parameters in evaluation of hematological malignancies.

A 2020 Vision Into Hodgkin Lymphoma Biology

Hodgkin lymphomas (HLs) are lymphoid neoplasms uniquely characterized by a paucity of neoplastic cells embedded in a supportive heterogenous cellular microenvironment. Although first described in the 19th century, systematic biological understanding of HLs has been hindered due to the challenges presented in studying the complex tumor microenvironment and scarce tumorigenic cells. Recent advances in single-cell isolation and characterization, sensitive mutational analytic tools, and multiplex immunohistochemical strategies have allowed further advances in understanding the development and progression of HL. Here we provide a current update on the chromosomal and mutational abnormalities seen in HL, the impact of Epstein-Barr virus infection on driving a subset of HLs, and the possibility of disease monitoring via high-sensitivity detection of genetic aberrations. We also discuss recent developments in understanding the intricate microenvironment through intercellular cross-talk, and describe novel potential biomarkers to aid in distinction of HL from other overlapping entities.

Challenges in the Diagnosis of Gray Zone Lymphomas

The diagnosis of B-cell lymphoma unclassifiable with features intermediate between diffuse large B-cell lymphoma and classical Hodgkin lymphoma, also called gray zone lymphoma (GZL), is frequently challenging. Incorrect diagnosis as either classic Hodgkin lymphoma or diffuse large B-cell lymphoma has significant implications for choice of upfront therapy based on recent large multi-institutional series from the United States and Europe. These studies have clarified some diagnostic challenges and provided guidance on the spectrum of morphologic features in this entity. This article clarifies some of the diagnostic conundrum surrounding GZL and provides an evidence-based approach to GZL diagnosis using morphology and immunohistochemistry.

Immunophenotypic Characterization and Purification of Neoplastic Cells from Lymph Nodes Involved by T-Cell/Histiocyte-rich Large B-cell Lymphoma by Flow Cytometry and Flow Cytometric Cell Sorting

BACKGROUND

T-cell/histiocyte rich large B-cell lymphoma (THRLBCL) is B-cell lymphoma in which rare neoplastic cells are embedded in a reactive infiltrate. We describe the first characterization of the neoplastic cells by flow cytometry (FC).

METHODS

Using FC, we immunophenotyped the neoplastic cells of 11 cases of THRLBCL and 11 cases of DLBCL, NOS (controls). Neoplastic THRLBCL cells were also purified by flow cytometric cell sorting (FCCS).

RESULTS

A neoplastic THRLBCL population was detected by FC in 9 of 11 cases (82%). Neoplastic THRLBCL cells demonstrated an aberrant germinal center B-cell immunophenotype (bright CD20, bright CD40; positive for Bcl-6 and CD75; weakly positive for CD32; negative for IgH). With regard to adhesion molecules, CD54 was overexpressed, CD58 expression varied between cases, and CD50 expression was intermediate. Evaluation of immunomodulatory receptors demonstrated that PD-L2 was weakly expressed and PD-L1 was variably expressed. Finally, FCCS of two cases showed large multi-lobated cells with morphology consistent with neoplastic cells of THRLBCL.

CONCLUSIONS

The immunophenotype identified and the morphology of the FCCS purified cells confirms the FC defined populations are neoplastic cells from THRLBCL. While the cohort is small, neoplastic THRLBCL cells lack surface immunoglobulins. CD40, CD50, and CD54 were overexpressed in THRLBCL relative to DLBCL, NOS, perhaps contributing to the predominance of T cells in THRLBCL. Expression of CD32, PD-L1, and PD-L2 may be useful in distinguishing THRLBCL and NLPHL. Finally, the FC assays will be useful for purifying neoplastic cells of THRLBCL and for diagnostic immunophenotyping of THRLBCL. © 2019 International Clinical Cytometry Society.

Diagnosis of Hodgkin lymphoma in the modern era

The Hodgkin lymphomas are a family of unique lymphoma subtypes, in which the nature of the neoplastic cell was enigmatic for many years. Much of the mystery has been solved, with all forms now considered to be of B-cell origin, in most cases of germinal centre derivation. Today we recognize Hodgkin lymphoma as an eponym that encompasses multiple entities. One of the unifying themes is the major contribution from the tumour microenvironment. Both the character of the neoplastic cells and the nature of the immune environment are critical to accurate diagnosis. Moreover, an understanding of the molecular alterations that characterize both the neoplastic cells and their microenvironment have led to therapeutic advances, targeting both neoplastic and reactive components. Other conditions may foster a similar inflammatory milieu and lead to lymphoproliferations that mimic the Hodgkin lymphomas. In this review we provide an update on the diagnostic features of the various subtypes and include additional information relevant for prognostic evaluation and investigation of potential therapeutic targets. Additionally, we also discuss those conditions that often cause confusion in diagnosis and need to be distinguished from the Hodgkin lymphomas.

T cell-rich lymphoid infiltrates with large B cells: a review of key entities and diagnostic approach

Accurate diagnostic interpretation of a lymphoid population composed predominantly of small T cells, together with smaller numbers of large B cells, with or without a nodular architecture, is a common problem faced by the histopathologist. The differential diagnosis of this histological pattern is wide, ranging from reactive conditions such as drug reactions and viral infections, through borderline entities such as immunodeficiency-related lymphoproliferative disorders to lymphomas. The latter includes entities where the large B cells are primarily neoplastic (classical and nodular lymphocyte-predominant Hodgkin lymphomas and T cell/histiocyte-rich large B cell lymphoma) as well as T cell lymphomas such as angioimmunoblastic T cell lymphoma where the large B cells represent an epiphenomenon and may or may not be neoplastic. Several rare variants of these conditions, and the fact that treatment can significantly modify appearances, add to the diagnostic difficulty of these pathological entities. Unlike monomorphic lymphoid infiltrates, the histological pattern of T cell-rich proliferation with large B cells requires close evaluation of the inter-relationship between B cells and T cells, follicular dendritic cells and sometimes other inflammatory cells. Epstein-Barr virus plays a key role in several of these scenarios, and interpreting not only its presence but also its distribution within cellular subgroups is essential to accurate diagnosis and the avoidance of some important diagnostic pitfalls. An understanding of normal immunoarchitecture and lymphoid maturational pathways is also fundamental to resolving these cases, as is a knowledge of their common patterns of spread, which facilitates correlation with clinical and radiological findings.

Recent Advances in Aggressive Large B-cell Lymphomas: A Comprehensive Review

Diffuse large B-cell lymphoma (DLBCL) is an aggressive disease with considerable heterogeneity reflected in the 2008 World Health Organization classification. In recent years, genome-wide assessment of genetic and epigenetic alterations has shed light upon distinct molecular subsets linked to dysregulation of specific genes or pathways. Besides fostering our knowledge regarding the molecular complexity of DLBCL types, these studies have unraveled previously unappreciated genetic lesions, which may be exploited for prognostic and therapeutic purposes. Following the last World Health Organization classification, we have witnessed the emergence of new variants of specific DLBCL entities, such as CD30 DLBCL, human immunodeficiency virus-related and age-related variants of plasmablastic lymphoma, and EBV DLBCL arising in young patients. In this review, we will present an update on the clinical, pathologic, and molecular features of DLBCL incorporating recently gained information with respect to their pathobiology and prognosis. We will emphasize the distinctive features of newly described or emerging variants and highlight advances in our understanding of entities presenting a diagnostic challenge, such as T-cell/histiocyte-rich large B-cell lmphoma and unclassifiable large B-cell lymphomas. Furthermore, we will discuss recent advances in the genomic characterization of DLBCL, as they may relate to prognostication and tailored therapeutic intervention. The information presented in this review derives from English language publications appearing in PubMed throughout December 2015. For a complete outline of this paper, please visit: http://links.lww.com/PAP/A12.

FDG-PET imaging in hematological malignancies

The majority of aggressive lymphomas is characterized by an up regulated glycolytic activity, which enables the visualization by F-18 FDG-PET/CT. One-stop hybrid FDG-PET/CT combines the functional and morphologic information, outperforming both, CT and FDG-PET as separate imaging modalities. This has resulted in several recommendations using FDG-PET/CT for staging, restaging, monitoring during therapy, and assessment of treatment response as well as identification of malignant transformation. FDG-PET/CT may obviate the need for a bone marrow biopsy in patients with Hodgkin's lymphoma and diffuse large B cell lymphoma. FDG-PET/CT response assessment is recommended for FDG-avid lymphomas, whereas CT-based response evaluation remains important in lymphomas with low or variable FDG avidity. The treatment induced change in metabolic activity allows for assessment of response after completion of therapy as well as prediction of outcome early during therapy. The five-point scale Deauville Criteria allows the assessment of treatment response based on visual FDG-PET analysis. Although the use of FDG-PET/CT for prediction of therapeutic response is promising it should only be conducted in the context of clinical trials. Surveillance FDG-PET/CT after complete remission is discouraged due to the relative high number of false-positive findings, which in turn may result in further unnecessary investigations. Future directions include the use of new PET tracers such as F-18 fluorothymidine (FLT), a surrogate biomarker of cellular proliferation and Ga-68 CXCR4, a chemokine receptor imaging biomarker as well as innovative digital PET/CT and PET/MRI techniques.

Hodgkin lymphoma variant of Richter transformation: morphology, Epstein-Barr virus status, clonality, and survival analysis-with comparison to Hodgkin-like lesion

Hodgkin/Reed-Sternberg (HRS) cells in the setting of chronic lymphocytic leukemia (CLL) exist in 2 forms: type I with isolated HRS cells in a CLL background (Hodgkin-like lesion) and type II with typical classic Hodgkin lymphoma, a variant of Richter transformation (CHL-RT). The clinical significance of the 2 morphological patterns is unclear, and their biological features have not been compared. We retrospectively reviewed 77 cases: 26 of type I and 51 of type II CHL-RT; 3 cases progressed from type I to type II. We examined clinical features, Epstein-Barr virus (EBV) status, and clonal relatedness after microdissection. Median age for type I was 62 years versus 73 years for type II (P=.01); 27% (type I) versus 73% (type II) had a history of CLL. HRS cells were positive for EBV in 71% (55/77), similar in types I and II. Clonality analysis was performed in 33 cases (type I and type II combined): HRS cells were clonally related to the underlying CLL in 14 and unrelated in 19. ZAP-70 expression of the CLL cells but not EBV status or morphological pattern was correlated with clonal relatedness: all 14 clonally related cases were ZAP-70 negative, whereas 74% (14/19) of clonally unrelated cases were ZAP-70 positive. Overall median survival (types I and II) after diagnosis was 44 months. Advanced age was an adverse risk factor for survival, but not histologic pattern, type I versus type II. HRS-like cells in a background of CLL carries a similar clinical risk to that of CHL-RT and may progress to classic Hodgkin lymphoma in some cases.

The Role of EBV in the Pathogenesis of Diffuse Large B Cell Lymphoma

Epstein-Barr virus (EBV) infection is a common feature of B cell lymphoproliferative disorders (LPDs), including diffuse large B cell lymphoma. Approximately 10 % of DLBCLs are EBV-positive, with the highest incidence in immunocompromised and elderly patients. Here, we review the clinical, genetic, and pathologic characteristics of DLBCL and discuss the molecular role of EBV in lymphoma tumorigenesis. Using EBV-positive DLBCL of the elderly as a model, we describe the key features of EBV-positive DLBCL. Studies of EBV-positive DLBCL of the elderly demonstrate that EBV-positive DLBCL has a distinct biology, related to both viral and host factors. The pathogenic mechanisms noted in EBV-positive DLBCL of the elderly, including enhanced NFκB activity, are likely to be a generalizable feature of EBV-positive DLBCL. Therefore, we review how this information might be used to target the EBV or its host response for the development of novel treatment strategies.

CD30 as a therapeutic target for lymphoma

Hodgkin's lymphoma (HL) and ALK(+) anaplastic large-cell lymphoma (ALCL) have become highly curable due to the success of modern regimens of chemotherapy and radiotherapy. However, up to one-third of the patients experience relapse or do not respond to first-line therapy, and half of them relapse again after secondary therapy with limited options for further treatment. In the last 15 years, monoclonal antibodies (mAbs) directed to surface receptors became a new and valuable therapeutic option in many hematologic malignancies. Due to its restricted expression on normal activated lymphocytes and its high expression on malignant cells, CD30 represents an attractive target molecule for HL and ALCL therapy. However, unconjugated CD30 mAbs have demonstrated a lack of objective clinical responses in patients with recurrent HL. CD30 exhibits complex signaling pathways, and binding of its natural ligand or anti-CD30 mAbs can induce apoptosis but may also promote proliferation and activation depending on the cellular context. Moreover, CD30 rapidly internalizes after crosslinking, which counteracts efficient recruitment of immunologic effectors but also provides the opportunity to transfer cytotoxic payloads coupled to CD30-specific mAbs into the tumor cells. Several tumor targeting approaches have been studied, including radio-immunoconjugates, immunotoxins, immunoRNases, immunokinases, and antibody drug conjugates (ADCs). In 2011, the ADC brentuximab-vedotin, consisting of the CD30-specific chimeric mAb cAC10 and the potent tubulin toxin monomethyl auristatin E, gained regulatory approval as a well tolerated and highly active drug in patients with refractory and relapsed HL and ALCL. SGN-35 is on the way to being incorporated in the standard management of CD30(+) lymphoma with significant therapeutic impact. This review gives a critical overview about anti-CD30 therapies with unconjugated, engineered, and conjugated mAbs and the therapeutic challenges of treatment of CD30(+) lymphoma.

EBV-positive large B-cell lymphomas in young patients: a nodal lymphoma with evidence for a tolerogenic immune environment

Few studies have reported Epstein-Barr virus-positive (EBV(+)) large B-cell lymphomas (LBCLs) in young patients without immunodeficiency. We identified 46 such cases in patients ≤45 years of age and analyzed the clinical and pathological characteristics. EBV(+) LBCLs affected predominantly males (male:female = 3.6:1), with a median age of 23 years (range, 4-45 years). All patients presented with lymphadenopathy and 11% also had extranodal disease. Morphologically, 3 patterns were identified: T-cell/histiocyte-rich large B-cell lymphoma-like (n = 36), gray zone lymphoma (n = 7), and diffuse LBCL-not otherwise specified (n = 3). Tumor cells (EBV(+) in >90% of cells) expressed B-cell antigens, were often CD30 and PD-L1 positive, and showed a nongerminal center immunophenotype. A total of 93% expressed EBV latency type II and 7% latency type III. Indoleamine 2,3-dioxygenase was expressed on background accessory cells. The most common treatment regimen was rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone (58%), with local radiation therapy added in 21%. With a median follow-up of 22 months, 82% of patients are in clinical remission and only 8% died of disease. Younger patients achieved a significantly higher overall survival than prior series of EBV(+) LBCLs reported in the elderly (P < .0001). In conclusion, EBV(+) LBCLs are not restricted to the elderly. Young patients present with nodal disease and have a good prognosis.

The histological classification of diffuse large B-cell lymphomas

Diffuse large B-cell lymphomas (DLBCLs) are aggressive B-cell neoplasms with considerable clinical, biologic, and pathologic diversity, in part reflecting the functional diversity of the B-cell system and multiple pathways of transformation. In recent years, the advent of new high-throughput genomic technologies has provided new insights into the biology of DLBCL, leading to the identification of distinct molecular identities and novel pathogenetic pathways. This increasing complexity had led to an expanding number of entities in the World Health Organization classification. Using a multi-modality approach, the updated 2008 classification delineated some new subgroups, including DLBCLs associated with particular age groups or specific anatomic sites, as well as two borderline categories (tumors at the interface between classical Hodgkin lymphoma and DLBCL as well as between Burkitt lymphoma and DLBCL). This article reviews the histopathologic features of the various aggressive B-cell lymphoma subtypes included in the 2008 classification, with emphasis on some of the new entities as well as areas of diagnostic challenge.

Diagnosis of primary pulmonary T- cell/histiocyte-rich large B cell lymphoma with tissue eosinophilia via clinicopathological observation and molecular assay

Background

Primary pulmonary lymphoma (PPL) is rare and easily misdiagnosed because of the lack of typical clinical features. It most commonly involves elderly patients aged between 60 and 70 years, and pathological diagnosis depends mainly on chest surgery rather than bronchial mucosal biopsy. Via percutaneous needle aspiration biopsy of the lung of a 33-year-old woman, which had distinct tissue eosinophilia, we diagnosed a rare case of rapidly growing large B cell lymphoma.

Methods

Bronchial mucosal biopsy and computed tomography–guided percutaneous needle aspiration biopsy were performed to determine the nature of the lesion, and we identified its immunophenotype using immunohistochemistry. We used BIOMED-2 gene rearrangement PCR to determine lymphocyte clonality; laser microdissection was used to confirm the clonality of suspicious malignant lymphocytes.

Results

Morphologically, the lesion was composed of a large number of eosinophilic cells and a few lymphoid cells. Immunohistochemical staining revealed a few CD1α-positive cells, but they were S-100–negative. The small lymphoid cells predominantly expressed CD3; the large lymphoid cells expressed CD20 and some scattered large lymphoid cells expressed Pax5. However, molecular studies confirmed clonal immunoglobulin heavy chain (IGH)-D gene rearrangement in Pax5–positive large B lymphocytes.

Conclusions

This is the first recorded case of T- cell/histiocyte-rich large B cell lymphoma with tissue eosinophilia of the lung. It highlights the unusual morphological features of PPL that might be mistaken for eosinophilic granuloma or parasitic infection. In addition, IGH and T cell receptor gene rearrangement play important roles in differentiating rare B cell lymphoma from lung space–occupying lesions with abundant eosinophils or T cell infiltration.

Virtual Slides

The virtual slide(s) for this article can be found here: http://med.motic.com/MoticGallery/Slides/AC5C9A6F-46EC-4C71-A448-1312F6900C65?user=2C69F0D6-A478-4A2B-ABF0-BB36763E8025

Macrophages in T cell/histiocyte rich large B cell lymphoma strongly express metal-binding proteins and show a bi-activated phenotype

Abundant macrophage infiltration in tumors often correlates with a poor prognosis. T cell/histiocyte rich large B cell lymphoma (THRLBCL) is a distinct aggressive B cell lymphoma entity showing a high macrophage content. To further elucidate the role of tumor-associated macrophages in THRLBCL, we performed gene expression profiling of microdissected histiocyte subsets of THRLBCL, nodular lymphocyte predominant Hodgkin lymphoma (NLPHL), Piringer lymphadenitis, sarcoidosis, nonspecific lymphadenitis and monocytes from peripheral blood. In a supervised principal component analysis, histiocytes from THRLBCL were most closely related to epithelioid cells from NLPHL, with both types of cells expressing genes related to proinflammatory and regulatory macrophage activity. Moreover, histiocytes from THRLBCL strongly expressed metal-binding proteins like MT2A, by which histiocytes of THRLBCL can be distinguished from the other histiocyte subsets investigated. Interestingly, the validation at the protein level showed a strong expression of TXN, CXCL9, MT2A and SOD2 not only in macrophages of THRLBCL but also in the tumor cells of NLPHL and classical Hodgkin lymphoma (cHL). Overall, the present findings indicate that macrophages in the microenvironment of THRLBCL have acquired a distinct gene expression pattern that is characterized by a mixed M1/M2 phenotype and a strong expression of several metal binding proteins. The microenvironments in NLPHL and THRLBCL appear to have a similar influence on the macrophage phenotype. The high expression of metal binding proteins in histiocytes of THRLBCL may be diagnostically useful, but a potential pathophysiological role remains to be identified.

The histological and biological spectrum of diffuse large B-cell lymphoma in the World Health Organization classification

Diffuse large B-cell lymphomas (DLBCLs) are aggressive B-cell lymphomas that are clinically, pathologically, and genetically diverse, in part reflecting the functional diversity of the B-cell system. The focus in recent years has been toward incorporation of clinical features, morphology, immunohistochemistry, and ever evolving genetic data into the classification scheme. The 2008 World Health Organization classification reflects this complexity with the addition of several new entities and variants. The discovery of distinct subtypes by gene expression profiling heralded a new era with a focus on pathways of transformation as well as a promise of more targeted therapies, directed at specific pathways. Some DLBCLs exhibit unique clinical characteristics with a predilection for specific anatomic sites; the anatomic site often reflects underlying biological distinctions. Recently, the spectrum of Epstein-Barr virus (EBV)-driven B-cell proliferations in patients without iatrogenic or congenital immunosuppression has been better characterized; most of these occur in patients of advanced age and include Epstein-Barr virus (EBV)-positive large B-cell lymphoma of the elderly. Human herpesvirus 8 is involved in the pathogenesis of primary effusion lymphoma, which can present as a "solid variant." Two borderline categories were created; one deals with tumors at the interface between classic Hodgkin lymphoma and DLBCL. The second confronts the interface between Burkitt lymphoma and DLBCL, so-called "B-cell lymphoma, unclassifiable, with features intermediate between DLBCL and Burkitt lymphoma" in the 2008 classification. Most cases harbor both MYC and BCL2 translocations and are highly aggressive. Another interesting entity is anaplastic lymphoma kinase-positive DLBCL, which renders itself potentially targetable by anaplastic lymphoma kinase inhibitors. Ongoing investigations at the genomic level, with both exome and whole-genome sequencing, are sure to reveal new pathways of transformation in the future.

Analyzing primary Hodgkin and Reed-Sternberg cells to capture the molecular and cellular pathogenesis of classical Hodgkin lymphoma

The pathogenesis of classical Hodgkin lymphoma (cHL), the most common lymphoma in the young, is still enigmatic, largely because its Hodgkin and Reed-Sternberg (HRS) tumor cells are rare in the involved lymph node and therefore difficult to analyze. Here, by overcoming this technical challenge and performing, for the first time, a genome-wide transcriptional analysis of microdissected HRS cells compared with other B-cell lymphomas, cHL lines, and normal B-cell subsets, we show that they differ extensively from the usually studied cHL cell lines, that the lost B-cell identity of cHLs is not linked to the acquisition of a plasma cell-like gene expression program, and that Epstein-Barr virus infection of HRS cells has a minor transcriptional influence on the established cHL clone. Moreover, although cHL appears a distinct lymphoma entity overall, HRS cells of its histologic subtypes diverged in their similarity to other related lymphomas. Unexpectedly, we identified 2 molecular subgroups of cHL associated with differential strengths of the transcription factor activity of the NOTCH1, MYC, and IRF4 proto-oncogenes. Finally, HRS cells display deregulated expression of several genes potentially highly relevant to lymphoma pathogenesis, including silencing of the apoptosis-inducer BIK and of INPP5D, an inhibitor of the PI3K-driven oncogenic pathway.

Diagnostic Impact of Molecular Lineage Analysis on Paraffin-Embedded Tissue in Hematolymphoid Neoplasia Reclassified by Current WHO Criteria

BACKGROUND AND OBJECTIVE

By current WHO criteria, most - though not all - cases of hematolymphoid neoplasm can be diagnosed immunomorphologically, diminishing the role of molecular tests for lymphoid antigen receptor clonality in lymphoma diagnosis. Hence, our objective was to glean immunomorphological and molecular correlates from hematolymphoid neoplasms that had remained unresolvable without diagnostic molecular input.

METHODS

Thirty-five such cases were reviewed histologically and with standard immunoperoxidases. In situ hybridization for Epstein-Barr virus (EBV)-encoded RNAs (EBER) was performed on selected cases. PCR amplification of genes encoding T-cell receptors (TcR) and immunoglobulin heavy chains (IgH) [TR and IGH genes, respectively] was performed on whole tissue in all cases, and on microdissected cells in two cases.

RESULTS

Twenty-five cases (71%) requiring diagnostic molecular genotyping had some form of peripheral T-cell lymphoma (PTCL). Twenty (80%) of these were complicated by a proliferation of B-lineage cells, either within the same tissue ('syntopic') as large B cells (LBC) or Reed-Sternberg (RS)-like cells (17 cases), florid lymphoid hyperplasia (two cases, one also with syntopic LBC) or monotypic plasma cells (one case), or at a separate ('metatopic') site as a B-cell lymphoma (two cases, one of which also had syntopic LBC) or Hodgkin lymphoma (HL; one case, also showing syntopic LBC). Fifteen (75%) of these 20 PTCLs with B-lineage proliferation yielded monoclonal TR gene rearrangements, and only two (10%) showed IGH monoclonality, which was transient in one case. Three (18%) of the PTCLs with LBC had originally been misinterpreted as some form of HL. Conversely, of the remaining cases, three of four (75%) that had been diagnosed initially as some form of large cell non-HL (NHL), including two of three that were called 'anaplastic', had to be revised to grade II/syncytial nodular sclerosing (NS) HL, yielding polyclonal TcRgamma gene (TRG) rearrangements, with one case, in addition, disclosing a biallelic clonal IGH gene rearrangement that excluded anaplastic large cell lymphoma.

DISCUSSION/CONCLUSION

Paradoxically, monoclonality of TR rather than IGH gene rearrangement may more often be detectable in a predominantly dispersed ('hodgkinoid'), large B-lineage cell proliferation, consistent with release from immune regulation in the milieu of impaired immunosurveillance within a PTCL. This is compounded by the difficulty in ascertaining clonal IGH gene rearrangements resulting from (1) poor consensus primer hybridization due to somatic hypermutations, and (2) 'dilution' in a T-cell-rich milieu. These same difficulties also account for the long-elusive identification of the RS cell lineage. Conversely, anaplastic lymphoma, which is of non-B lineage, may be mimicked by NSHL, which is of B lineage.

T cell/histiocyte-rich large B-cell lymphoma: an update on its biology and classification

T cell/histiocyte-rich large B-cell lymphoma (THRLBCL), originally considered an uncommon variant of Diffuse Large B-Cell Lymphoma (DLBCL), is recognized by the World Health Organisation as a separate clinicopathological entity since 2008. It predominantly affects middle aged men often presenting with advanced stage disease frequently involving spleen, liver and bone marrow at time of diagnosis. According to the WHO, this lymphoma is morphologically characterized by less than 10% of large neoplastic B cells in a background of abundant T cells and frequently histiocytes. Differentiating THRLBCL from other lymphoproliferative disorders such as Nodular Lymphocyte Predominant Hodgkin Lymphoma (NLPHL) and Lymphocyte-Rich classical Hodgkin lymphoma (LRcHL) is important from a clinical point of view and can be achieved in most cases, given adequate biopsy specimens, by careful morphological and immunohistochemical evaluation of both the neoplastic cells as well as the nonneoplastic stromal component. According to this WHO definition, THRLBCL is still considered a clinically heterogeneous entity, though it is noted that especially the cases containing numerous histiocytes behave aggressively and show resistance to current therapies for DLBCL. Gene expression profiling studies of THRLBCL provided evidence for a prominent role for this histiocytic component that is important for a tolerogenic host immune response in which they may assist neoplastic cells in escaping the T cell-mediated immune surveillance. Therefore, reserving the diagnosis of THRLBCL to cases containing a large proportion of histiocytes might be relevant, as modulating their activity could provide new therapeutic options.

Cutaneous T-cell/histiocyte-rich B-cell lymphoma: a case report and review of the literature

BACKGROUND

T-cell/histiocyte-rich large B-cell lymphoma (THRLBCL) primarily presenting on the skin is an extremely rare entity with only sporadic cases reported in the literature.

METHODS

We here report an extraordinary case of primary cutaneous THRLBCL with self-healing and 24 months of follow-up.

RESULTS

The lesion was a dermohypodermal/subcutaneous circumscribed ulcerated nodosity. Histological examination with immunohistochemical, molecular analysis and comparative genomic hybridization were performed. A complete staging was negative for secondary involvement.

CONCLUSION

Our case is remarkable because it is the second well-documented primary cutaneous THRLBCL in which we observed a complete self-regression of skin lesions without recurrences or dissemination of the disease. According to the literature, we highlight that the tumoral microenvironment, in our case, could play a relevant role in stopping lymphoma growth. Furthermore, this case supports the observation that THRLBCL primarily presenting on the skin shows an overall good prognosis.

Age-related EBV-associated lymphoproliferative disorders in the Western population: a spectrum of reactive lymphoid hyperplasia and lymphoma

We investigated age-related EBV(+) B-cell lymphoproliferations in the Western population. The clinical features, histology, immunophenotype, EBV-encoded RNA in situ hybridization, and clonality by PCR of T-cell receptor gamma and immunoglobulin genes were categorized in 122 EBV(+) lesions as follows: (1) reactive lymphoid hyperplasia; (2) polymorphic extranodal or (3) polymorphic nodal lymphoproliferative disease (LPD); and (4) diffuse large B-cell lymphoma (DLBCL). Interphase FISH for IG and PAX5 gene rearrangements was performed on 17 cases of DLBCL. The overall median age was 75 years (range, 45-101 years; 67 men, 55 women), and 67, 79, 73, and 77 years, respectively, for groups 1 through 4. Sixteen of 21 cases of polymorphic extranodal LPD were classified as EBV(+) mucocutaneous ulcer. PCR for immunoglobulin genes was polyclonal in reactive lymphoid hyperplasia (84%) and monoclonal in 33%, 63%, and 56% of polymorphic extranodal and nodal LPD cases and DLBCL, respectively. All groups showed restricted/clonal T-cell receptor responses (27%-70%). By FISH, 19% of DLBCLs showed IGH@ rearrangements, but PAX5 was unaffected. Disease-specific 5-year survival was 100%, 93%, 57%, and 25% for groups 1-4, respectively, and 100% for patients with EBV(+) mucocutaneous ulcer. Disease volume was predictive of therapy response (P = .0002), and pathologic subtype was predictive of overall outcome (P = .001). Age-related EBV(+) B-cell LPD encompasses a wider disease spectrum than previously recognized and includes both reactive and neoplastic conditions. Reduction in the T-cell repertoire may contribute to decreased immune surveillance.

Recent Advances in the Pathobiology of Hodgkin's Lymphoma: Potential Impact on Diagnostic, Predictive, and Therapeutic Strategies

From its first description by Thomas Hodgkin in 1832, Hodgkin's disease, now called Hodgkin's lymphoma, has continued to be a fascinating neoplasm even to this day. In this review, historical aspects, epidemiology, diagnosis, tumor biology, new observations related to host-microenvironment interactions, gene copy number variation, and gene expression profiling in this complex neoplasm are described, with an exploration of chemoresistance mechanisms and potential novel therapies for refractory disease.

Aggressive B-cell lymphomas: a review of new and old entities in the WHO classification

Aggressive B-cell lymphomas are clinically and pathologically diverse and reflect multiple pathways of transformation. The 2008 World Health Organization (WHO) classification reflects this complexity with the addition of several new entities and variants. Whereas MYC translocations have long been associated with Burkitt lymphoma (BL), deregulation of MYC has been shown to occur in other aggressive B-cell lymphomas, most often as a secondary event. Lymphomas with translocations of both MYC and BCL2 are highly aggressive tumors, with a high failure rate with most treatment protocols. These "double-hit" lymphomas are now separately delineated in the WHO classification as B-cell lymphoma, unclassifiable, with features intermediate between diffuse large B-cell lymphoma (DLBCL) and BL. A MYC translocation is also found uncommonly in DLBCL, but the clinical consequences of this in the absence of a double hit are not yet fully delineated. Most recently, MYC translocations have been identified as a common secondary event in plasma cell neoplasms, seen in approximately 50% of plasmablastic lymphoma. Another area that has received recent attention is the spectrum of EBV-driven B-cell proliferations in patients without iatrogenic or congenital immunosuppression; most of these occur in patients of advanced age and include the EBV-positive large B-cell lymphomas of the elderly.

The histopathologic and molecular basis for the diagnosis of histiocytic sarcoma and histiocyte-associated lymphoma of mice

Histiocytic sarcoma (HS) and histiocyte-associated lymphoma (HAL) of mice are difficult to distinguish histologically. Studies of multiple cases initially diagnosed as HS or HAL allowed us to define HS as round, fusiform, or mixed cell types that were F4/80+, Mac-2+, and PAX5-; that lacked markers for other sarcomas; and that had immune receptor genes in germline configuration. Two other subsets had clonal populations of lymphocytes. The first, HAL, featured malignant lymphocytes admixed with large populations of normal-appearing histiocytes. The second appeared to be composites of lymphoma and HS. Several cases suggestive of B myeloid-lineage plasticity were also observed.

Small intestinal presentation of nodular lymphocyte-predominant Hodgkin lymphoma with T cell/histiocyte-rich B cell lymphoma-like areas-with review of literature on extranodal presentation of this disease

Nodular lymphocyte-predominant Hodgkin lymphoma (NLPHL), accounts for ∼5% of all cases of Hodgkin lymphoma and is characterized by involvement of the peripheral lymph nodes. NLPHL occurs in young adults and is associated with frequent relapses. In 3% to 7% of cases, NLPHL progresses to a diffuse large B cell lymphoma. Furthermore, a proportion of NLPHL also have areas with features of T cell/histiocyte-rich large B cell lymphoma (THRLBCL), either at presentation or on follow-up. Here, we describe a 32-year-old man who presented to the emergency department with small bowel perforation. The resected small bowel showed full-thickness mural ulceration and involvement by a lymphoma with features of NLPHL that also had areas resembling THRLBCL. The patient had axillary lymphadenopathy, biopsy of which showed NLPHL with focal THRLBCL-like areas. Such a lymphoma presenting as small intestinal lesion/perforation has not been reported in the literature before. We take this opportunity to review the literature on extranodal presentations of NLPHL and discuss the natural history of this disease.

Activation-induced cytidine deaminase expression in diffuse large B-cell lymphoma with a paracortical growth pattern: a lymphoma of possible interfollicular large B-cell origin

CONTEXT

Activation-induced cytidine deaminase, necessary for immunoglobulin somatic hypermutation and class switch recombination, is usually expressed within the follicular dendritic network but is also expressed in a population of interfollicular large B cells outside the germinal center.

OBJECTIVE

To report 7 cases of diffuse large B-cell lymphoma with a distinct paracortical distribution. Expression of activation-induced cytidine deaminase, previously described in interfollicular large B cells, was evaluated.

DESIGN

A panel of immunohistochemical markers, including double staining for activation-induced cytidine deaminase and CD20, was used to illustrate the cases. Molecular studies were performed by polymerase chain reaction in the paraffin-embedded tissue for t(14;18) chromosomal translocation and immunoglobulin heavy chain and T-cell receptor rearrangements.

RESULTS

Patients included 3 males and 4 females ranging in age from 11 to 59 years (mean, 39 years). All specimens were lymph nodes (4 from the groin, 2 from the neck, and 1 from the axilla). Malignant lymphocytes were positive for CD20 and negative for CD5 and CD10. Staining for CD30, CD43, and BCL-2 was variable. The malignant cells showed at least focal staining with activation-induced cytidine deaminase. All cases were found to be monoclonal by immunoglobulin heavy-chain gene rearrangement or showed light-chain restriction. None of the tested cases showed t(14;18).

CONCLUSIONS

Diffuse large B-cell lymphoma with a paracortical distribution is unusual and may be a distinct morphologic variant. More study is necessary to determine the stage of B-cell development and the cell of origin of these tumors. However, activation-induced cytidine deaminase expression suggests they may arise from a putative interfollicular large B cell.

Morphologic, immunphenotypic and clinical discriminators between T-cell/histiocyte-rich large B-cell lymphoma and lymphocyte-predominant Hodgkin lymphoma

BACKGROUND

Features of T-cell/histiocyte rich large B-cell lymphoma (THRLBCL) overlap with those of lymphocyte predominant Hodgkin lymphoma (LPHL). The two lymphomas may represent a spectrum of the same disease, and differentiation between the two can sometimes be difficult. We looked at histomorphologic, immunophenotypic and clinical information that may help differentiate the two entities.

METHODS

Cases of THRLBCL and LPHL were blindly reviewed and studied for histological pattern (nodular vs. diffuse), nuclear features and pattern of expression of CD20, CD30, CD57, epithelial membrane antigen (EMA) and Epstein-Barr virus (EBV). A score encompassing diffuse histology, high nuclear grade, CD20 single-cell pattern, CD30+, CD57-, EMA-, and EBV+ was estimated for the diagnosis of TCHRLBCL.

RESULTS

There were 58 cases, including 30 cases of TCHRLBL and 28 cases of LPHL. The median age was 36 years for TCHRLBCL and 21 years for LPHL (P = 0.0001). Three types of nuclei were identified (lymphocytic/histocytic, Reed-Sternberg and centroblast-like). The latter two high-grade nuclei were suggestive of TCHRLBCL. TCHRLBCL and LPHL, respectively, showed diffuse histology, 90% vs. 4% (P = 0.001), single CD20+ cells, 93% vs. 3.5% (P = 0.00004), CD30+ cells, 30% vs. 0% (P = 0.01), CD57+ cells, 41% vs. 93% (P = 0.008), EMA+ cells, 27% vs. 60% (P = 0.113), EBV+ cells, 24% vs. 0% (P = 0.117), high nuclear grade, 70% vs. 0% (P = 0.001), total score 2-7 (mean 4.68) vs. 0-2 (mean 0.72) (P = 0.001), high stage, 86% vs. 7% (P = 0.0001).

CONCLUSION

Our findings indicate that a combination of multiple parameters can help differentiate between the two diseases. Two cases originally diagnosed as LPHL were re-assigned the diagnosis of THRLBCL.

Irradiated Blm-deficient mice are a highly tumor prone model for analysis of a broad spectrum of hematologic malignancies

Mutations in the BLM gene cause human Bloom syndrome (BS), an autosomal recessive disorder of growth retardation, immunodeficiency and cancer predisposition. Homozygous null Blm(m3/m3) mice are cancer prone with a 5-fold increased risk of cancer compared with Blm(m3/+) and Blm(+/+) mice. Irradiation of Blm(m3/m3) mice increased the risk to 28-fold. Tumors occurred mainly in the hematopoietic system and were similar to those in BS based on detailed histologic and immunohistochemical analyses. Irradiated Blm-deficient mice thus provide a novel model for understanding accelerated malignancies in BS and a new platform for investigating the molecular basis for a wide range of hematopoietic neoplasms.

Diverse hematological malignancies including hodgkin-like lymphomas develop in chimeric MHC class II transgenic mice

A chimeric HLA-DR4-H2-E (DR4) homozygous transgenic mouse line spontaneously develops diverse hematological malignancies with high frequency (70%). The majority of malignancies were distributed equally between T and B cell neoplasms and included lymphoblastic T cell lymphoma (LTCL), lymphoblastic B cell lymphoma (LBCL), diffuse large B cell lymphoma (DLBCL), the histiocyte/T cell rich variant of DLBCL (DLBCL-HA/T cell rich DLBCL), splenic marginal zone lymphoma (SMZL), follicular B cell lymphoma (FBL) and plasmacytoma (PCT). Most of these neoplasms were highly similar to human diseases. Also, some non-lymphoid malignancies such as acute myeloid leukemia (AML) and histiocytic sarcoma were found. Interestingly, composite lymphomas, including Hodgkin-like lymphomas, were also detected that had CD30(+) Hodgkin/Reed-Sternberg (H/RS)-like cells, representing a tumor type not previously described in mice. Analysis of microdissected H/RS-like cells revealed their origin as germinal center B cells bearing somatic hypermutations and, in some instances, crippled mutations, as described for human Hodgkin lymphoma (HL). Transgene integration in an oncogene was excluded as an exclusive driving force of tumorigenesis and age-related lymphoma development suggests a multi-step process. Thus, this DR4 line is a useful model to investigate common molecular mechanisms that may contribute to important neoplastic diseases in man.

Gray zones around diffuse large B cell lymphoma. Conclusions based on the workshop of the XIV meeting of the European Association for Hematopathology and the Society of Hematopathology in Bordeaux, France

The term "gray-zone" lymphoma has been used to denote a group of lymphomas with overlapping histological, biological, and clinical features between various types of lymphomas. It has been used in the context of Hodgkin lymphomas (HL) and non-Hodgkin lymphomas (NHL), including classical HL (CHL), and primary mediastinal large B cell lymphoma, cases with overlapping features between nodular lymphocyte predominant Hodgkin lymphoma and T-cell/histiocyte-rich large B cell lymphoma, CHL, and Epstein-Barr-virus-positive lymphoproliferative disorders, and peripheral T cell lymphomas simulating CHL. A second group of gray-zone lymphomas includes B cell NHL with intermediate features between diffuse large B cell lymphoma and classical Burkitt lymphoma. In order to review controversial issues in gray-zone lymphomas, a joint Workshop of the European Association for Hematopathology and the Society for Hematopathology was held in Bordeaux, France, in September 2008. The panel members reviewed and discussed 145 submitted cases and reached consensus diagnoses. This Workshop summary is focused on the most controversial aspects of gray-zone lymphomas and describes the panel's proposals regarding diagnostic criteria, terminology, and new prognostic and diagnostic parameters.

T-cell/histiocyte-rich large B-cell lymphoma shows transcriptional features suggestive of a tolerogenic host immune response

BACKGROUND

Gene expression profiling has successfully identified the prognostic significance of the host response in lymphomas. The aggressive T-cell/histiocyte-rich large B-cell lymphoma and the indolent nodular lymphocyte-predominant Hodgkin's lymphoma are both characterized by a paucity of tumor cells embedded in an overwhelming background. The tumor cells of both lymphomas share several characteristics, while the cellular composition of their microenvironment is clearly different.

DESIGN AND METHODS

We collected 33 cases of T-cell/histiocyte-rich large B-cell lymphoma and 56 cases of nodular lymphocyte-predominant Hodgkin's lymphoma and performed microarray gene expression profiling on ten cases of each lymphoma, to obtain a better understanding of the lymphoma host response. By quantitative reverse transcriptase polymerase chain reaction we verified that these 20 selected cases were representative of the entire population of T-cell/histiocyte-rich large B-cell and nodular lymphocyte-predominant Hodgkin's lymphomas.

RESULTS

We observed that the microenvironment in nodular lymphocyte-predominant Hodgkin's lymphoma is molecularly very similar to a lymph node characterized by follicular hyperplasia, while the microenvironment in T-cell/histiocyte-rich large B-cell lymphoma is clearly different. The T-cell/histiocyte-rich large B-cell lymphoma signature is hallmarked by up-regulation of CCL8, interferon-gamma, indoleamine 2,3 dioxygenase, VSIG4 and Toll-like receptors. These features may be responsible for the recruitment and activation of T cells, macrophages and dendritic cells, characterizing the stromal component of this lymphoma, and may point towards innate immunity and a tumor tolerogenic immune response in T-cell/histiocyte-rich large B-cell lymphoma.

CONCLUSIONS

The gene expression profile of T-cell/histiocyte-rich large B-cell lymphoma, in comparison with that of nodular lymphocyte-predominant Hodgkin's lymphoma, shows features suggestive of a distinct tolerogenic host immune response that may play a key role in the aggressive behavior of this lymphoma, and that may serve as a potential target for future therapy.

Bone marrow involvement by hodgkin and non-hodgkin lymphomas

Bone marrow evaluation plays a critical role in staging and predicting prognosis in patients with Hodgkin lymphoma or non-Hodgkin lymphoma. Bone marrow can be the initial site of detection of lymphoma in patients with unexplained symptoms or cytopenias. A comprehensive evaluation of bone marrow includes complete blood counts, blood morphology, bone marrow aspirate, and generous core biopsy sections. Specialized testing should be used in a logical fashion on a case by case basis.

Diffuse Large B-Cell Lymphoma—More Than a Diffuse Collection of Large B Cells: An Entity in Search of a Meaningful Classification

Context.—Diffuse large B-cell lymphoma is a heterogenous group of lymphomas. In this review, we present a brief description of the large number of entities recognized in the recently published (2008) World Health Organization classification of tumors of hematopoietic and lymphoid tissues.

Objective.—We highlight the unique clinicopathologic and molecular genetic features of these new and previously recognized entities, to illustrate the rational for the development of this classification. To help simplify the understanding of this now large and complex group of diseases, we have attempted to create broader subgroups of related entities. We discuss large B-cell lymphoma that are not otherwise specified, those that are based on anatomic site, those that have unique histology or phenotype or genotype, those that are associated with Epstein-Barr virus or Kaposi sarcoma–associated herpesvirus and herpesvirus 8, and those that are unclassifiable.

Data Sources.—World Health Organization classification of tumors of hematopoietic and lymphoid tissues (2008), published literature from PubMed (National Library of Medicine), and primary material from the authors' institution were reviewed.

Conclusions.—Recognition of the different subtypes of diffuse large B-cell lymphoma as described in the World Health Organization classification scheme will lead to improved understanding of the unique clinicopathologic and genetic features associated with these subtypes of lymphoma.

Gene expression profile of multipotent mesenchymal stromal cells: Identification of pathways common to TGFbeta3/BMP2-induced chondrogenesis

Multipotent mesenchymal stromal cells (MSC) display a high potential for the development of novel treatment strategies for cartilage repair. However, the pathways involved in their differentiation to functional non hypertrophic chondrocytes remain largely unknown, despite the work on embryologic development and the identification of key growth factors including TGFbeta, Hh, Wnt and FGF. In this study, we asked if we could identify specific biological networks common to the growth factors used (TGFbeta3 or BMP-2). To address this question, we used DNA microarrays and performed large-scale expression profiling of MSC at different time points during their chondrogenic differentiation. By comparing these data with those obtained during the differentiation of MSC into osteoblasts and adipocytes, we identified 318 genes specific for chondrogenesis and developed a new algorithm to classify the genes according to their kinetic profile. We distributed the selected genes in five classes according to their kinetic of expression. We could reconstruct three phases characterized by functional pathways. The first phase corresponds to cell attachment and apoptosis induction; the second phase is characterized by a proliferation/differentiation step, and the third phase is characterized by a differentiation/hypertrophy pathway. Indeed, these data propose new pathways to understand the complexity of MSC differentiation to chondrocytes.

Origin and pathogenesis of nodular lymphocyte-predominant Hodgkin lymphoma as revealed by global gene expression analysis

The pathogenesis of nodular lymphocyte–predominant Hodgkin lymphoma (NLPHL) and its relationship to other lymphomas are largely unknown. This is partly because of the technical challenge of analyzing its rare neoplastic lymphocytic and histiocytic (L&H) cells, which are dispersed in an abundant nonneoplastic cellular microenvironment. We performed a genome-wide expression study of microdissected L&H lymphoma cells in comparison to normal and other malignant B cells that indicated a relationship of L&H cells to and/or that they originate from germinal center B cells at the transition to memory B cells. L&H cells show a surprisingly high similarity to the tumor cells of T cell–rich B cell lymphoma and classical Hodgkin lymphoma, a partial loss of their B cell phenotype, and deregulation of many apoptosis regulators and putative oncogenes. Importantly, L&H cells are characterized by constitutive nuclear factor κB activity and aberrant extracellular signal-regulated kinase signaling. Thus, these findings shed new light on the nature of L&H cells, reveal several novel pathogenetic mechanisms in NLPHL, and may help in differential diagnosis and lead to novel therapeutic strategies.