The Open Leukemia Journal, 2010, 3, 9-15 9 1876-8164/10 2010 Bentham Open Open Access Malignant T Cells Exhibit CD45 Resistant Stat3 Activation and Proliferation in Cutaneous T-Cell Lymphoma Thorbjørn Krejsgaard 1,2 , Rikke Helvad 1,2 , Elisabeth Ralfkiaer 3 , Karen Astvad 3,4 , Kirsten Grønbæk 4 , Karsten W. Eriksen 2 , Carsten Geisler 2 , Katharina Kopp 1,2 , Qian Zhang 5 , Niels Odum 1,2 and Anders Woetmann *,1,2 1 Department of Biology and 2 Institute of International Health, Immunology and Microbiology, University of Copenhagen, Copenhagen, Denmark 3 Department of Pathology, University Hospital of Copenhagen, Copenhagen, Denmark 4 Department of Haematology, Rigshospitalet, Copenhagen, Denmark 5 Department of Pathology and Laboratory Medicine, University of Pennsylvania, Philadelphia, PA 19104, USA Abstract: CD45 is a protein tyrosine phosphatase, which is well-known for regulating antigen receptor signalling in T and B cells via its effect on Src kinases. It has recently been shown that CD45 can also dephosphorylate Janus kinases (Jaks) and thereby regulate Signal transducer and activator of transcription (Stat) activation and cytokine-induced proliferation in lymphocytes. Consequently, CD45 dysregulation could be implicated in aberrant Jak/Stat activation and proliferation in lymphoproliferative diseases. Despite high expression of the CD45 ligand, Galectin-1, in skin lesions from cutaneous T-cell lymphoma (CTCL), the malignant T cells exhibit constitutive activation of the Jak3/Stat3 signalling pathway and uncontrolled proliferation. We show that CD45 expression is down-regulated on malignant T cells when compared to non-malignant T cells established from CTCL skin lesions. Moreover, CD45 cross-linking does not suppress the constitutive activation of Stat3 in the malignant T cells and there is no correlation between the level of activated Stat3 and the level of CD45 expression on the malignant T cells. Furthermore, in contrast to non-malignant T cells, the malignant T cells are protected against CD45-mediated inhibition of proliferation. In conclusion, our data suggest that CD45 dysregulation might play a role in the aberrant proliferation and Jak3/Stat3 activation in CTCL. keywoards: CD45, T cell lymphoma, CTCL, STAT, galectin. INTRODUCTION Cutaneous T-cell lymphomas (CTCLs) are the most frequent primary lymphomas of the skin [1]. They comprise a wide spectrum of heterogeneous lymphoproliferative disorders characterised by clonal accumulation of neoplastic T lymphocytes in the epidermis. Mycosis fungoides (MF) is the most common representative of CTCL [1, 2]. The etiology is unknown but it has been shown that the Janus kinase 3 (Jak3)/Signal transducer and activator of transcription 3 (Stat3) [3] pathway is constitutively active in tumour cell lines obtained from independent skin biopsies and peripheral blood of patients suffering from CTCL [4-6]. Importantly, evidence that the Jak3/Stat3 pathway is constitutively active in vivo has also been provided [7, 8]. Inhibitors of Jak3 block the constitutive activation of Stat3 and inhibit the proliferation of the tumour cells [4, 5, 9]. Furthermore, a dominant negative form of Stat3 blocks spontaneous cytokine production and triggers apoptosis in the malignant T cells [7, 10]. Collectively, these findings suggest that Jak3/Stat3 activation plays a critical role in the tumorigenesis of CTCL. However, the molecular mechanisms underlying the aberrant Jak3/Stat3 activation remain unknown. *Address correspondence to this author at the Department of Biology and Institute of International Health, Immunology and Microbiology, University of Copenhagen, Copenhagen, Denmark; Tel: +45 35 32 79 00; Fax: +45 35 32 78 68; E-mail: awoetmann@sund.ku.dk In non-malignant T cells, Stat activation is only transient and strictly controlled by positive and negative regulators. An important class of negative regulators of Stat activation are protein tyrosine phosphatases (PTPs). Some PTPs dephosphorylate and inactivate Jaks, whereas others dephosphorylate specific tyrosine residues in cytokine /growth factor receptors, while yet others inactivate Stats through direct dephosphorylation of critical tyrosine residues [11]. Hence, abnormal expression of PTPs is likely to be involved in the constitutive Stat activation. Accordingly, it has been shown that malignant T cells from CTCL patients often have a deficient expression of the PTP SHP-1 due to hypermethylation of the SHP-1 promoter [12,13]. Interestingly, treatment of the malignant T cells with methylation inhibitors restored the SHP-1 expression and decreased the constitutive phosphorylation of Jak3 [12]. However, not all CTCL cells/patients have deficient expression of SHP-1 suggesting a deficient function of other negative regulators of Jak3/Stat3 signalling [14]. CD45, also known as leukocyte-common antigen (LCA), is a transmembrane PTP expressed on all nucleated cells in the haematopoietic system and it is one of the most abundant glycoproteins on the surface of lymphoid cells [15]. It has long been recognised that CD45 has an important positive regulatory effect on T and B cell antigen receptor mediated signalling through its ability to dephosphorylate negative