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T-cell acute lymphoblastic leukemia/lymphoma (T-ALL/LBL) is usually diagnosed based on the presence of immature lymphoid marker terminal deoxynucleotidyl transferase (TdT), and T-cell specific markers, specifically CD3, by immunohistochemistry (IHC) staining on bone marrow and/or extramedullary tissue. We present a novel, TdT and CD3 negative, aggressive early T-cell precursor LBL (ETP-LBL) initially misdiagnosed as a high grade B-cell lymphoma due to expression of CD79a and the erroneous detection of BCL2/IGH fusion. The patient was eventually evaluated using molecular diagnostic techniques, including fluorescence in situ hybridization (FISH) and next generation sequencing (NGS) assays that demonstrated PICALM-MLLT10 fusion and a NOTCH1 mutation in the absence of BCL2/IGH fusion. The use of NGS, specifically mate-pair sequencing (MPseq), subsequently confirmed an in-frame PICALM-MLLT10 fusion. Our retrospective analysis showed that PICALM-MLLT10 fusion has no association with CD3/TdT negativity, as 6/49 T-ALL/LBL cases from Mayo Clinic database (01/1998-09/2018), including this case, were noted to have PICALM-MLLT10 fusion; however, none of the other cases were associated with CD3/TdT negativity. We emphasize the importance of a comprehensive hematopathologic evaluation including multiple molecular studies for the appropriate interrogation and classification of a difficult acute leukemia diagnosis, and to prevent potential diagnostic errors of clinical significance., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Chronic lymphocytic leukemia (CLL) is characterized by the monoclonal expansion of small mature-looking B cells that accumulate in the blood, marrow, and lymphoid organs, and have a remarkable phenotypic homogeneity. By definition, CLL cells co-express CD5 and CD23 with faint to undetectable amounts of monoclonal surface immunoglobulins (sIg). The concept of phenotypic homogeneity has been reinforced by gene expression profiling data, which suggest that the pathogenesis of CLL has to be associated with a fairly common mechanism of transformation. In recent years the biology of CLL has been enriched by an unprecedented flurry of new observations that are leading to a better understanding of the natural history of the disease. Still CLL cells have so far defied any attempt to satisfactorily answer the simple time-honored question of what their cell of origin is. It is the purpose of this review to discuss the features a cell must possess to be considered with reasonable approximation the normal counterpart of a CLL B cell.

Intestinal tissue samples were examined from 32 cats in which a histopathological diagnosis of alimentary lymphoma or multicentric lymphoma affecting the gastrointestinal tract had been made. These samples were re-evaluated histopathologically and serial sections were examined immunohistochemically with antisera specific for the lymphoid markers CD3, CD79a and BLA-36 and for class II molecules of the major histocompatability complex. The cats ranged in age from 4-16 years (median 10.5 years). The main presenting clinical signs were vomiting, diarrhoea and weight loss. The majority of alimentary lymphomas were of the B-cell type (n=15), whereas cases of T-cell lymphoma were fewer in number (n=8). Four cats had lymphoma of a mixed T-and B-cell phenotype. In five of the cats, immunohistochemistry suggested an inflammatory process, in contradiction to the original histopathological diagnosis of lymphoma. Immunolabelling would appear to be a useful adjunct to histopathology in classifying cases of feline alimentary lymphoma, and may help in distinguishing lymphoma from severe intestinal inflammation.

CD24 is a surface marker expressed in immature and mature B cells and involved in cellular adhesion and apoptosis. There are no data, which delineate the stage in early development of human B cells, which marks the expression of CD24. We studied lymphopoiesis in normal pediatric bone marrow (BM) and found that 1.5+/-0.2% of WBC were CD24(+) lymphocytes which did not express CD19. A significant fraction of these cells expressed low levels of CD45 (CD19- CD24+ CD45low cells). Small numbers of CD19- CD24+ CD45low cells were found in the regenerating BM of children with acute lymphoblastic leukemia after the completion of chemotherapy and in normal adult BM. Flow cytometric analyses have shown that CD19- CD24+ CD45low lymphocytes express CD10, CD34, CD79a, CD179a (VpreB), and TdT markers, i.e., displayed antigenic properties of early B-cell progenitors. Our data indicate that CD19- early B-cell progenitors in human BM express CD24, and that the expression of CD24 in human B-cell development precedes the expression of CD19.