Journal of Pathology J Pathol 2006; 209: 250–257 Published online 28 February 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/path.1961 Original Paper Immunoglobulin class-switch recombination occurs in mantle cell lymphomas W Klapper, 1 * M Szczepanowski, 1 K Heidorn, 1 M M¨ uschen, 2 S Liedtke, 2 A Sotnikova, 1 NS Andersen, 3 J Greeve 4 and R Parwaresch 1 1 Department of Haematopathology and Lymph Node Registry Kiel, Schleswig-Holstein University Hospitals, Campus Kiel, Niemannsweg 11, 24105 Kiel, Germany 2 Laboratory for Molecular Stem Cell Biology, Heinrich-Heine University, 40225 D¨ usseldorf, Germany 3 Department of Haematology, Rigshospitalet, Copenhagen, Denmark 4 Department of Internal Medicine, Inselspital-University Hospital Bern, 3010 Bern, Switzerland *Correspondence to: W Klapper, Department of Hematopathology and Lymph Node Registry Kiel, Niemannsweg 11, 24105 Kiel, Germany. E-mail: wklapper@path.uni-kiel.de Received: 29 June 2005 Revised: 6 December 2005 Accepted: 20 December 2005 Abstract Mantle cell lymphoma (MCL) is an IgM-expressing B cell lymphoma that originates from naive B cells and responds poorly to chemotherapy. We show here that several MCLs harbour isotype-switched subclones. Similar to the situation in normal B cells, in vitro stimulation of MCL cell lines with CD40 ligand (CD40L) and interleukin-4 induced expression of activation-induced cytidine deaminase (AID) and germline transcription at the immunoglobulin heavy chain gene locus. Additionally, the occurrence of switch-circle transcripts and mature IgG transcripts after stimulation indicated ongoing class-switch recombination in mantle cell lymphoma cell lines. Furthermore, stimulation of primary MCL cells in vitro induced expression of class-switched IgG mRNA in the tumour cells. Our data indicate that mantle cell lymphomas have retained the ability to undergo class-switch recombination if appropriate stimuli, such as the CD40 ligand, are provided. Copyright  2006 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd. Keywords: class-switch; mantle cell lymphoma; immunoglobulin; activation-induced cyti- dine deaminase Introduction Mantle cell lymphoma (MCL) is a rare B cell non-Hodgkin’s lymphoma that responds poorly to chemotherapy. MCL is characterized by the t(11;14) translocation, which juxtaposes the gene coding for cyclin D1 with the immunoglobulin heavy chain locus on one allele. The other intact immunoglobulin heavy chain allele is found to be unswitched and it pro- duces IgM, which is expressed on the cell surface [1]. Recent studies have revealed that, contrary to naive B cells being the supposed cellular origin of MCL [2], a subset of mantle cell lymphomas har- bour mutated variable regions of the immunoglobulin heavy chain gene (V H ) [3–5]. Since Babbage et al [6] reported the detection of switch-circle transcripts and class-switched subclones in samples of leukaemic MCL cells, it has become evident that a subset of MCL has initiated cellular processes corresponding to a germinal centre reaction. Normal B cells and most B cell lymphomas express CD40, a surface receptor that regulates B cell pro- liferation and apoptosis. The physiological ligand of CD40 is CD154 (CD40L), which is expressed by T lymphocytes. In vitro stimulation of B cells by CD40L and appropriate cytokines, such as interleukin 4 (IL-4), induces proliferation and up-regulation of activation-induced cytidine deaminase (AID) followed by class-switch recombination (CSR) [7]. By analogy with normal B cells, CD40L and IL-4 are strong pro- liferative stimuli in primary MCL cells [8–10]. Previous reports failed to demonstrate class- switching in primary MCL cells after stimulation with CD40L and IL-4. However, these studies analysed protein expression by FACS analysis only [9,10]. We demonstrate here that MCL samples contain class- switched subclones. Furthermore, we show that class- switch recombination can be induced in vitro in MCL cell lines and primary MCL cells by stimulation with CD40L and IL-4. Since CD40L seems to stimulate class-switching in MCL, interaction with T cells might influence the biology of the lymphoma. Material and methods Tissue samples and primary MCL cells The study was performed in accordance with the local ethical requirements for samples stored at the Lymph Node Registry Kiel. Fresh frozen tumour samples of MCL from the tissue collection of the Lymph Node Registry Kiel were cut with a cryostat and Copyright  2006 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.