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From the Programa de Patología Molecular, Centro Nacional de Investigaciones Oncológicas, Madrid; and the Hospital Virgen de la Salud, Toledo, Spain
Monocytoid B cells (MBCs) are a subset of B cells that may be recognized in several reactive and tumoral lymph node conditions, including toxoplasmic lymphadenitis, infectious mononucleosis, and Hodgkin’s lymphoma. Although this is a commonly observed cell population, which has even given its name to a type of lymphoma, MBC lymphoma, scarcely any information is available about the function and characteristics of this cell type. A relationship with marginal zone (MZ) B lymphocytes has been claimed for MBCs, but this has not yet been fully proven. Indeed, specific markers for MBCs are still lacking, which has made it difficult to analyze their relationship with other B cell subpopulations and confirm the existence of tumors deriving from this B cell subset. We used a panel of cell cycle markers to explore the characteristics of MBCs and their relationship with MZ B cells, nodal MZ lymphoma, and splenic MZ lymphoma. We therefore compared the phenotypic profile of MBCs in different conditions with normal MZ B cells within the spleen and mesenteric lymph nodes, with a group of seven cases of nodal MZ/MBC lymphoma and another group of five cases of splenic MZ lymphoma. MBCs were mainly in the G0 to G1 phases, as deduced from the presence of a proportion of between 10 and 35% Ki67-positive cells, whereas very low expression was observed with cyclin A and cyclin B staining. Nests of MBCs were clearly labeled by the expression of p21WAF1, a cyclin-dependent kinase inhibitor (CKI), rarely detectable in benign lymphocytes, and by cyclin E. Basically all MBCs were bcl-2-negative, and high cyclin D2 and cyclin D3 were also detected in these cells, at proportions and intensities above expected levels, when the percentage of proliferating cells was taken into account. p27KIP1 expression was characterized by homogeneous reactivity, higher than that observed in other B cell populations with a relatively high-growth fraction. Immunoglobulin staining showed undetectable light and heavy chains. However, splenic MZ cells, nodal MZ lymphoma, and splenic MZ lymphoma showed a distinct expression of IgM and bcl-2, with high p27 KIP1 nuclear expression and undetectable or low levels of cyclin A, B, E, or D, or p21WAF1 expression. The data from this study show an unexpected immunophenotype in MBCs, different from the one observed in splenic and lymph node MZ B cells. This suggests that either MBCs are a unique B cell population from a distinct cell lineage, or if related to MZ cells, they would represent a definite differentiation stage characterized by a distinctive immunophenotype. They also show so-called MZ/MBC lymphoma to be more closely related to lymph node and splenic MZ B cells, as they do not share the most distinctive features of MBCs.
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  B. Falini, E. Tiacci, A. Pucciarini, B. Bigerna, J. Kurth, G. Hatzivassiliou, S. Droetto, B. V. Galletti, M. Gambacorta, A. Orazi, et al. Expression of the IRTA1 receptor identifies intraepithelial and subepithelial marginal zone B cells of the mucosa-associated lymphoid tissue (MALT) Blood, November 15, 2003; 102(10): 3684 - 3692. [Abstract] [Full Text] [PDF]
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