Kikuchi–Fujimoto disease type lymph node reaction with increased plasmacytoid dendritic cells may appear as a side effect following COVID-19 vaccination: Report of a case and literature review / Linfadenitis de tipo Kikuchi-Fujimoto con expansión de células dendríticas plasmocitoides como evento adverso asociado a la vacunación por COVID-19. Reporte de un caso y revisión de la literatura

Subsequent to mass vaccination programs against COVID-19, diverse side effects have been described, both at the injection site, such as pain, redness and swelling, and systemic effects such as fatigue, headache, muscle or joint pain. On rare occasions, a lymphadenopathic syndrome may develop, raising the clinical suspicion of a lymphoproliferative disorder. We present the case of a 30-year-old woman who developed self-limiting left axillary lymphadenopathy following COVID-19 vaccination. To date, only seven similar cases with a complete clinicopathological description have been published, and fourteen cases have been notified to the European adverse events databases (Eudravigilance) in relationship with vaccination against COVID-19. It is important to be aware of this potential complication when a lymphadenopathic syndrome develops following vaccination, to avoid unnecessary treatment. (AU)

Tras la vacunación masiva frente a la COVID-19 se han comenzado a describir diversos efectos adversos incluyendo efectos locales en el lugar de la inyección, como dolor, enrojecimiento, hinchazón, etc., y efectos sistémicos como fatiga, dolor de cabeza, dolor muscular o articular. Más infrecuentemente se pueden desarrollar cuadros linfadenopáticos sospechosos clínicamente de proceso linfoproliferativo. Presentamos el caso de una mujer de 30 años que desarrolló linfadenopatía axilar izquierda tras la vacunación contra la COVID-19 con hallazgos histopatológicos de linfadenopatía necrotizante de tipo Kikuchi y resolución espontánea. Hasta el momento se han publicado 7 casos con descripción clinicopatológica completa en la literatura y notificado 14 casos en la Red Europea de Farmacovigilancia en relación con la vacunación. Es importante tener en cuenta esta entidad en linfadenopatías sospechosas de procesos linfoproliferativos en este contexto, para evitar un tratamiento innecesario. (AU)

Kikuchi–Fujimoto disease type lymph node reaction with increased plasmacytoid dendritic cells may appear as a side effect following COVID-19 vaccination: Report of a case and literature review / Linfadenitis de tipo Kikuchi-Fujimoto con expansión de células dendríticas plasmocitoides como evento adverso asociado a la vacunación por COVID-19. Reporte de un caso y revisión de la literatura

Subsequent to mass vaccination programs against COVID-19, diverse side effects have been described, both at the injection site, such as pain, redness and swelling, and systemic effects such as fatigue, headache, muscle or joint pain. On rare occasions, a lymphadenopathic syndrome may develop, raising the clinical suspicion of a lymphoproliferative disorder. We present the case of a 30-year-old woman who developed self-limiting left axillary lymphadenopathy following COVID-19 vaccination. To date, only seven similar cases with a complete clinicopathological description have been published, and fourteen cases have been notified to the European adverse events databases (Eudravigilance) in relationship with vaccination against COVID-19. It is important to be aware of this potential complication when a lymphadenopathic syndrome develops following vaccination, to avoid unnecessary treatment. (AU)

Tras la vacunación masiva frente a la COVID-19 se han comenzado a describir diversos efectos adversos incluyendo efectos locales en el lugar de la inyección, como dolor, enrojecimiento, hinchazón, etc., y efectos sistémicos como fatiga, dolor de cabeza, dolor muscular o articular. Más infrecuentemente se pueden desarrollar cuadros linfadenopáticos sospechosos clínicamente de proceso linfoproliferativo. Presentamos el caso de una mujer de 30 años que desarrolló linfadenopatía axilar izquierda tras la vacunación contra la COVID-19 con hallazgos histopatológicos de linfadenopatía necrotizante de tipo Kikuchi y resolución espontánea. Hasta el momento se han publicado 7 casos con descripción clinicopatológica completa en la literatura y notificado 14 casos en la Red Europea de Farmacovigilancia en relación con la vacunación. Es importante tener en cuenta esta entidad en linfadenopatías sospechosas de procesos linfoproliferativos en este contexto, para evitar un tratamiento innecesario. (AU)

Kikuchi-Fujimoto disease type lymph node reaction with increased plasmacytoid dendritic cells may appear as a side effect following COVID-19 vaccination: Report of a case and literature review.

Subsequent to mass vaccination programs against COVID-19, diverse side effects have been described, both at the injection site, such as pain, redness and swelling, and systemic effects such as fatigue, headache, muscle or joint pain. On rare occasions, a lymphadenopathic syndrome may develop, raising the clinical suspicion of a lymphoproliferative disorder. We present the case of a 30-year-old woman who developed self-limiting left axillary lymphadenopathy following COVID-19 vaccination. To date, only seven similar cases with a complete clinicopathological description have been published, and fourteen cases have been notified to the European adverse events databases (Eudravigilance) in relationship with vaccination against COVID-19. It is important to be aware of this potential complication when a lymphadenopathic syndrome develops following vaccination, to avoid unnecessary treatment.

CTNNB1 somatic mutations drive Wnt pathway activation in a case of incidental intranodal palisaded myofibroblastoma / Mutaciones somáticas de CTNNB1 inducen la activación de la vía Wnt en un caso incidental de miofibroblastoma en empalizada de localización intraganglionar

Intranodal palisaded myofibroblastoma (IPM) is a rare stroma-derived spindle-cell neoplasm of the lymph node with myofibroblastic differentiation and CTNNB1 (β-catenin gene) somatic mutations. We present a case of IPM found incidentally in the staging of lung adenocarcinoma. We describe the major histopathological and phenotypic features, including a palisaded bland spindle cell proliferation with myofibroblastic differentiation and Wnt pathway activation by immunohistochemistry, including β-catenin expression. Production of osteoid-like collagen directly from tumor cells was observed. We confirmed p.Gly34Arg CTNNB1 mutation by direct sequencing. We also reviewed the literature for similar cases.(AU)

El miofibroblastoma en empalizada intraganglionar linfático (MEIG) es una neoplasia infrecuente de células fusiformes del estroma del ganglio linfático con diferenciación miofibroblástica y mutaciones en CTNNB1 (gen de la β-catenina). Aquí mostramos el caso de un paciente con MEIG encontrado incidentalmente en la estadificación por un adenocarcinoma de pulmón. Se describen las características histopatológicas principales de la entidad, incluyendo una proliferación de células fusiformes con escasa atipia, empalizadas celulares y diferenciación miofibroblástica con activación de la vía Wnt, incluyendo expresión inmunohistoquímica de β-catenina. Se observó producción de colágeno de tipo osteoide por parte de las células tumorales. Se confirmó la presencia de la mutación p.Gly34Arg de CTNNB1 mediante secuenciación directa. Se recogen adicionalmente publicaciones de casos similares al nuestro.(AU)

Primary cutaneous CD30+ lymphoproliferative disorders with DUSP22 translocation.

Primary cutaneous CD30+ lymphoproliferative disorders (LPD) encompass a broad category of clonal T cell proliferations with varied clinical presentations. Classically, lymphomatoid papulosis (LyP) and primary cutaneous anaplastic large cell lymphoma (ALCL) have been recognized as distinct clinicopathological entities according to their differing clinical features. Recently, a subset of LyP and both cutaneous and systemic ALCL have been shown to carry a DUSP22 translocation [1-3], a defining molecular feature for the novel entity "LyP with DUSP22t" [1]. In cutaneous biopsies, both primary cutaneous DUSP22-translocated ALCL and LyP with DUSP22 rearrangements are characterized by a biphasic pattern with significant small cell epidermotropism. A distinct protein expression profile with preserved T Cell Receptor (TCR) expression, positivity for CD30, LEF1, HLA, and CD58, and negativity for cytotoxic marker expression as well as phospho-STAT3 protein is consistently found in these cases.

CTNNB1 somatic mutations drive Wnt pathway activation in a case of incidental intranodal palisaded myofibroblastoma.

Intranodal palisaded myofibroblastoma (IPM) is a rare stroma-derived spindle-cell neoplasm of the lymph node with myofibroblastic differentiation and CTNNB1 (ß-catenin gene) somatic mutations. We present a case of IPM found incidentally in the staging of lung adenocarcinoma. We describe the major histopathological and phenotypic features, including a palisaded bland spindle cell proliferation with myofibroblastic differentiation and Wnt pathway activation by immunohistochemistry, including ß-catenin expression. Production of osteoid-like collagen directly from tumor cells was observed. We confirmed p.Gly34Arg CTNNB1 mutation by direct sequencing. We also reviewed the literature for similar cases.

Expert consensus on the integrated diagnosis of idiopathic multicentric Castleman disease / Consenso de expertos sobre el diagnóstico integrado de la enfermedad de Castleman idiopática multicéntrica

Idiopathic multicentric Castleman disease (iMCD) is rare. The differential diagnosis includes inflammatory, autoimmune and neoplastic disease. The identification of the histopathological features of Castleman disease in the lymph node is the main diagnostic criterion.Fifty-three experts from three medical societies (SEMI, SEHH and SEAP) have created a multi-disciplinary consensus document in order to standardise the diagnosis of Castleman disease. Using the Delphi method, specific recommendations for the initial clinical, laboratory and imaging studies have been made for an integrated diagnosis of iMCD as well as for the best way to obtain samples for histopathological confirmation, correct laboratory procedure and interpretation and reporting of results.(AU)

La enfermedad de Castleman multicéntrica idiopática (ECMi) es una patología infrecuente. El diagnóstico diferencial incluye patología inflamatoria, autoinmune y neoplásica. El estudio anatomopatológico del ganglio linfático y la identificación de las características histopatológicas de la enfermedad de Castleman constituyen un criterio principal para el diagnóstico.Con el objetivo de estandarizar el proceso diagnóstico de esta patología, se ha desarrollado un documento de consenso multidisciplinario con la participación de 53 expertos de tres sociedades médicas (Sociedad Española de Medicina Interna [SEMI], Sociedad Española de Hematología y Hemoterapia [SEHH] y Sociedad Española de Anatomía Patológica [SEAP]). Mediante el método Delphi se han validado las recomendaciones específicas para el diagnóstico integrado de la ECMi con respecto a los estudios clínicos, de laboratorio y de imagen necesarios en el abordaje inicial del paciente y las recomendaciones acerca de la mejor obtención de muestras para confirmación histopatológica, así como procedimientos de laboratorio para el estudio de las muestras, interpretación de resultados y emisión de un informe anatomopatológico.(AU)

Systemic Mastocytosis with Associated Hematological Neoplasms. Diagnostic features and unique response pattern to tyrosine kinase inhibitors and allo-bone marrow transplantation therapy / Mastocitosis sistémica con neoplasias hematológicas asociadas. Características diagnósticas y patrón de respuesta único a los inhibidores de la tirosina cinasa y al tratamiento de trasplante de médula ósea

Systemic Mastocytosis is a clonal proliferation of mast cells; in a significant fraction of cases it is associated with another concurrent hematological neoplasm. Molecular analysis of KIT mutations and other associated genetic alterations suggest a common origin in the stem cell compartment. Mast cell infiltration patterns in bone marrow biopsy may be subtle in cases associated with t (8;21) AML. Here we report three cases of clonally related SM-AHN, two cases with SM-CMML and one case with SM- t (8;21) AML. We describe in detail the bone marrow infiltration pattern at diagnosis and during the course of treatment with allogeneic stem cell transplant and novel TK inhibitors, showing the unique dynamics of mast cell clearance after therapy.(AU)

La mastocitosis sistémica es una proliferación clonal de mastocitos que puede asociarse con otra neoplasia hematológica concurrente en una fracción significativa de los casos. El análisis molecular de mutaciones de KIT y otras alteraciones genéticas asociadas indican un origen común en el compartimento de células madre. Los patrones de infiltración de mastocitos en la biopsia de médula ósea pueden ser sutiles en los casos asociados con AML t (8; 21). Aquí se describen 3 casos de MS-NHA, 2 casos con MS-LMMC y un caso con MS-t (8; 21) LMA. Describimos en detalle el patrón de infiltración de la médula ósea en el momento del diagnóstico y durante el curso del tratamiento con alotrasplante de células madre y nuevos inhibidores de TK, mostrando la dinámica de la depuración de mastocitos después de la terapia.(AU)

Expert consensus on the integrated diagnosis of idiopathic multicentric Castleman disease.

Idiopathic multicentric Castleman disease (iMCD) is rare. The differential diagnosis includes inflammatory, autoimmune and neoplastic disease. The identification of the histopathological features of Castleman disease in the lymph node is the main diagnostic criterion. Fifty-three experts from three medical societies (SEMI, SEHH and SEAP) have created a multi-disciplinary consensus document in order to standardise the diagnosis of Castleman disease. Using the Delphi method, specific recommendations for the initial clinical, laboratory and imaging studies have been made for an integrated diagnosis of iMCD as well as for the best way to obtain samples for histopathological confirmation, correct laboratory procedure and interpretation and reporting of results.

Systemic Mastocytosis with Associated Hematological Neoplasms. Diagnostic features and unique response pattern to tyrosine kinase inhibitors and allo-bone marrow transplantation therapy.

Systemic Mastocytosis is a clonal proliferation of mast cells; in a significant fraction of cases it is associated with another concurrent hematological neoplasm. Molecular analysis of KIT mutations and other associated genetic alterations suggest a common origin in the stem cell compartment. Mast cell infiltration patterns in bone marrow biopsy may be subtle in cases associated with t (8;21) AML. Here we report three cases of clonally related SM-AHN, two cases with SM-CMML and one case with SM- t (8;21) AML. We describe in detail the bone marrow infiltration pattern at diagnosis and during the course of treatment with allogeneic stem cell transplant and novel TK inhibitors, showing the unique dynamics of mast cell clearance after therapy.

From Castleman disease histopathological features to idiopathic multicentric Castleman disease: a multiparametric approach to exclude potential iMCD histopathological mimickers.

AIMS: International consensus diagnostic criteria for idiopathic multicentric Castleman disease (iMCD) includes lymph node Castleman disease (CD) histopathological features as major criteria. Our aim was to apply those criteria in a series of 42 cases with CD to find differences among unicentric CD, iMCD, HHV-8+multicentric CD (HHV-8+MCD) and POEMS/plasma cell neoplasia (PCN)-associated CD. METHODS: Available clinical and laboratory criteria were collected. Histopathological features (germinal centre hyperplasia/regression, plasmacytosis, hypervascularity and follicular dendritic cell (FDC) prominence) were graded and immunohistochemistry with antibodies against CD20, CD3, CD138, HHV-8, Ig isotype (IgG, IgG4, IgA, IgM, IgD), kappa, lambda was performed in all cases. RESULTS: Fourteen cases had hyaline-vascular type unicentric CD, 15 were HHV-8+MCD, 7 cases PCN/POEMS-associated CD and 5 cases were iMCD. One case was consistent with systemic lupus erythematosus (SLE) lymphadenopathy. Differences in grading of the CD-associated histopathological features showed that FDC proliferation was prominent in unicentric CD, hypervascularity was increased in HHV-8 positive MCD and germinal centre hyperplasia was restricted to iMCD cases and SLE. Monotypic plasma cells were readily identifiable in the lymph node biopsies in 43% of PCN/POEMS-associated CD. All three cases had lambda light chain restriction with IgA (two cases) and IgG (one case) isotypes. CONCLUSIONS: HHV-8+ MCD and PCN/POEMS-related CD are the major mimickers of iMCD in lymph node biopsies. Grading of the five histopathological features for CD might be useful to, in conjunction with complete ancillary testing, suggest for specific disease entities.

An integrated prognostic model for diffuse large B-cell lymphoma treated with immunochemotherapy.

Diffuse large B-cell lymphoma (DLBCL), the most frequent non-Hodgkin's lymphoma subtype, is characterized by strong biological, morphological, and clinical heterogeneity, but patients are treated with immunochemotherapy in a relatively homogeneous way. Here, we have used a customized NanoString platform to analyze a series of 197 homogeneously treated DLBCL cases. The platform includes the most relevant genes or signatures known to be useful for predicting response to R-CHOP (Rituximab, Cyclophosphamide, Doxorubicin, Vincristine, and Prednisone) in DLBCL cases. We generated a risk score that combines the International Prognostic Index with cell of origin and double expression of MYC/BCL2, and stratified the series into three groups, yielding hazard ratios from 0.15 to 5.49 for overall survival, and from 0.17 to 5.04 for progression-free survival. Group differences were highly significant (p < 0.0001), and the scoring system was applicable to younger patients (<60 years of age) and patients with advanced or localized stages of the disease. Results were validated in an independent dataset from 166 DLBCL patients treated in two distinct clinical trials. This risk score combines clinical and biological data in a model that can be used to integrate biological variables into the prognostic models for DLBCL cases.

Mycosis fungoides: A rare cause of giant cardiac mass.

Mycosis fungoides is the most common cutaneous T-cell lymphoma. Although myocardial infiltration is frequent in advanced stages, symptomatic cardiac involvement is rare. We report an unusual case of rapidly progressing acute heart failure due to cardiac affection by mycosis fungoides manifested as an intracavitary mass in the right atrium. The patient received cytoreductive surgery and adjuvant radiotherapy, presenting an excellent cardiovascular outcome during follow-up. Learning objectives: •Recognize mycosis fungoides as a potential cause of cardiac mass due to its visceral progression.•Note the role of cytoreductive surgery with adjuvant radiotherapy in order to relieve heart failure symptoms and improve the prognosis of the disease in the exceptional case of mycosis fungoides presenting as an isolated cardiac mass.

CD229 (Ly9) a Novel Biomarker for B-Cell Malignancies and Multiple Myeloma.

CD229 (Ly9) homophilic receptor, which belongs to the SLAM family of cell-surface molecules, is predominantly expressed on B and T cells. It acts as a signaling molecule, regulating lymphocyte homoeostasis and activation. Studies of CD229 function indicate that this receptor functions as a regulator of the development of marginal-zone B cells and other innate-like T and B lymphocytes. The expression on leukemias and lymphomas remains poorly understood due to the lack of CD229 monoclonal antibodies (mAb) for immunohistochemistry application (IHC). In this study, we used a new mAb against the cytoplasmic region of CD229 to study the expression of CD229 on normal tissues and B-cell malignancies, including multiple myeloma (MM), using tissue microarrays. We showed CD229 to be restricted to hematopoietic cells. It was strongly expressed in all cases of MM and in most marginal-zone lymphomas (MZL). Moderate CD229 expression was also found in chronic lymphocyte leukemia (CLL), follicular (FL), classic mantle-cell (MCL) and diffuse large B-cell lymphoma. Given the high expression on myeloma cells, we also analyzed for the presence of soluble CD229 in the sera of these patients. Serum levels of soluble CD229 (sCD229) at the time of diagnosis in MM patients could be useful as a prognostic biomarker. In conclusion, our results indicate that CD229 represents not only a useful biomarker but also an attractive therapeutic target.

BCL2 translocation in high grade B cell lymphoma (NOS, DH/TH) is associated with reduced progression free survival.

High Grade B Cell Lymphoma, NOS, and High Grade B Cell Lymphoma with Dual Hit or Triple Hit have been recently recategorized in the 2016 revision of the WHO classification of lymphoid neoplasms. In this study we have characterized the genetic, histopathological, and clinical features of a series of this type of lymphoid neoplasia (17 HGBCL NOS and 53 HGBCL DH/TH).HGBCL NOS showed better response to first line treatment than HGBCL with DH/TH but no significant differences in PFS or OS were found between the two categories. Survival analysis in the whole cohort of cases found that only the presence of BCL2 translocation was significantly associated with PFS. Other clinical features such as IPI, LDH or stage were equivalent in both categories. Furthermore, both high grade and DLBCL morphological patterns showed equivalent PFS and OS in this set of High grade BCL NOS/DH/TH.Key pointsBCL2 translocation in High Grade B Cell Lymphoma NOS and High Grade B Cell Lymphoma with DH/TH is associated with reduced progression free survival.Both high grade and DLBCL morphological patterns showed equivalent outcome regarding PFS and OS in HGBCL.

ARID2 deficiency promotes tumor progression and is associated with higher sensitivity to chemotherapy in lung cancer.

The survival rate in lung cancer remains stubbornly low and there is an urgent need for the identification of new therapeutic targets. In the last decade, several members of the SWI/SNF chromatin remodeling complexes have been described altered in different tumor types. Nevertheless, the precise mechanisms of their impact on cancer progression, as well as the application of this knowledge to cancer patient management are largely unknown. In this study, we performed targeted sequencing of a cohort of lung cancer patients on genes involved in chromatin structure. In addition, we studied at the protein level the expression of these genes in cancer samples and performed functional experiments to identify the molecular mechanisms linking alterations of chromatin remodeling genes and tumor development. Remarkably, we found that 20% of lung cancer patients show ARID2 protein loss, partially explained by the presence of ARID2 mutations. In addition, we showed that ARID2 deficiency provokes profound chromatin structural changes altering cell transcriptional programs, which bolsters the proliferative and metastatic potential of the cells both in vitro and in vivo. Moreover, we demonstrated that ARID2 deficiency impairs DNA repair, enhancing the sensitivity of the cells to DNA-damaging agents. Our findings support that ARID2 is a bona fide tumor suppressor gene in lung cancer that may be exploited therapeutically.

Genetic lesions in MYC and STAT3 drive oncogenic transcription factor overexpression in plasmablastic lymphoma.

Plasmablastic lymphoma mutational profile is undescribed. Here we performed a targeted exonic NGS analysis of 30 plasmablastic lymphoma cases with a B cell lymphoma dedicated panel and FISH for the detection of MYC rearrangements. A complete phenotyping of the neoplastic and microenvironment cell populations was also performed. We have identified an enrichment in recurrent genetic events in MYC (69% with MYC translocation or amplification and 3 cases with missense point mutations), PRDM1/Blimp1 and STAT3 mutations. These gene mutations were more frequent in EBV positive disease. Other genetic events included mutations in BRAF, EP300, BCR (CD79A and CD79B), NOTCH pathway (NOTCH2, NOTCH1 and SGK1) and MYD88pL265P. Immunohistochemical analysis showed consistent MYC expression, higher in cases with MYC rearrangements together with phospho-STAT3 (Tyr705) overexpression in cases with STAT3 SH2 domain mutations. Microenvironment populations were heterogeneous and unrelated with EBV, with an enrichment of Tumor Associated Macrophages (TAM) and PD1 positive T cells. PD-L1 was expressed in all cases in the TAM population but only in 5 cases in the neoplastic cells (4 out of 14 EBV positive cases). HLA expression was absent in the majority of PBL cases. In summary, Plasmablastic lymphoma mutational profile is heterogeneous and related with EBV infection. Genetic events in MYC, STAT3 and PRDM1/Blimp1 are more frequent in EBV positive disease. An enrichment in TAM and PD1 reactive T lymphocytes is found in the microenvironment of PBL cases, that express PD-L1 in the neoplastic cells in a fraction of cases.

Prognostic factors, therapeutic approaches, and distinct immunobiologic features in patients with primary mediastinal large B-cell lymphoma on long-term follow-up.

Primary mediastinal large B-cell lymphoma (PMBCL) is a rare and distinct subtype of diffuse large B-cell lymphoma (DLBCL) without prognostic factors or a single standard of treatment clearly defined. In this study we performed retrospective analysis for clinical outcomes of 166 patients with PMBCL. In overall PMBCL, higher International Prognostic Index, stage, Ki-67 proliferation index, and positron emission tomography (PET) maximum standardized uptake values (SUVmax) at diagnosis were significantly associated with poorer survival, whereas MUM1 expression and higher peripheral blood lymphocyte/monocyte ratios were significantly associated with better survival. Patients who received R-HCVAD or R-EPOCH had better clinical outcome than did those who received the standard treatment R-CHOP. Treatment response and end-of-treatment PET SUVmax had remarkable correlations with survival outcome. In patients with refractory or relapsed PMBCL, stem cell transplant significantly improved overall survival. PMBCL had distinct gene expression signatures compared with overall DLBCL-NOS but not with DLBCL with PD-L1/PD-L2 amplification. PMBCL also showed higher PD-L2 expression in B-cells, lower PD-1 expression in T-cells, and higher CTLA-4 expression in T-cells and distinct miRNA signatures compared with DLBCL-NOS. The prognostic factors, effectiveness of treatment, transcriptional and epigenetic signatures, and immunologic features revealed by this study enrich our understanding of PMBCL biology and support future treatment strategy.

High-mobility group box (TOX) antibody a useful tool for the identification of B and T cell subpopulations.

Thymocyte selection-associated high-mobility group box (TOX) is a DNA-binding factor that is able to regulate transcription by modifying local chromatin structure and modulating the formation of multi-protein complexes. TOX has multiple roles in the development of the adaptive immune system including development of CD4 T cells, NK cells and lymph node organogenesis. However very few antibodies recognizing this molecule have been reported and no extensive study of the expression of TOX in reactive and neoplastic lymphoid tissue has been performed to date. In the present study, we have investigated TOX expression in normal and neoplastic lymphoid tissues using a novel rat monoclonal antibody that recognizes its target molecule in paraffin-embedded tissue sections. A large series of normal tissues and B- and T-cell lymphomas was studied, using whole sections and tissue microarrays. We found that the majority of precursor B/T lymphoblastic, follicular and diffuse large B-cell lymphomas, nodular lymphocyte-predominant Hodgkin lymphomas and angioimmunoblastic T-cell lymphomas strongly expressed the TOX protein. Burkitt and mantle cell lymphomas showed TOX expression in a small percentage of cases. TOX was not found in the majority of chronic lymphocytic leukemia, myelomas, marginal zone lymphomas and classical Hodgkin lymphomas. In conclusion, we describe for the first time the expression of TOX in normal and neoplastic lymphoid tissues. The co-expression of TOX and PD-1 identified in normal and neoplastic T cells is consistent with recent studies identifying TOX as a critical regulator of T-cell exhaustion and a potential immunotherapy target. Its differential expression may be of diagnostic relevance in the differential diagnosis of follicular lymphoma, the identification of the phenotype of diffuse large B-cell lymphoma and the recognition of peripheral T-cell lymphoma with a follicular helper T phenotype.