Functional Properties of Lymphocytes in Idiopathic Thrombocytopenic Purpura Nicholas P. Webber, John O. Mascarenhas, Mary K. Crow, James Bussel, and Elaine J. Schattner ABSTRACT: Idiopathic or immune thrombocytopenic purpura (ITP) is characterized by antibody-mediated de- struction of platelets. The etiology is unknown. We pos- tulated that increased autoantibody production in ITP might be attributable to either increased or prolonged expression of CD40 ligand (CD40L, CD154) in T or B lymphocytes, as has been previously observed in systemic lupus erythematosus (SLE). In addition, we hypothesized that ITP is characterized by increased levels of interleukin 4 (IL-4), a prototypic Th2 cytokine which, along with CD40 ligation, is required for B cell differentiation and production of several IgG subclasses. Cell surface CD154 expression was measured in freshly-isolated and in vitro- activated peripheral blood lymphocytes of sixteen ITP patients and eight healthy volunteers. Plasma levels of IL-4 and the prototypic Th1 cytokine interferon-gamma (IFN) were determined. We observed that CD154 ex- pression in unstimulated and in vitro-activated lympho- cytes did not differ between ITP patients and healthy controls. Plasma levels of the Th2 cytokine IL-4 were significantly higher in the ITP patients. These studies indicate that overexpression of CD154 in lymphocytes is unlikely to be a primary pathophysiological defect in most patients with ITP. The data support that in addition to cell membrane antigens such as CD154, soluble cytokines such as IL-4 should be considered as potential targets for therapy in this disease. Human Immunology 62, 1346 –1355 (2001). © American Society for Histocom- patibility and Immunogenetics, 2001. Published by Elsevier Science Inc. KEYWORDS: idiopathic or immune thrombocytopenia; CD40 ligand (CD40L, CD154); lymphocytes; cytokines; interleukin-4 ABBREVIATIONS ITP idiopathic thrombocytopenic purpura Ig immunoglobulin CD40L CD40 ligand, CD154 IL-4 interleukin-4 IFN interferon gamma Th1 T-helper cell type 1 Th2 T-helper cell type 2 APC antigen presenting cell SLE systemic lupus erythematosus CLL chronic lymphocytic leukemia FITC fluoroscein isothiocyanate PE phycoerythrin PCR polymerase chain reaction ELISA enzyme linked immunosorbent assay ANOVA analysis of variance INTRODUCTION CD154 (CD40 ligand, CD40L) is a tumor necrosis factor (TNF) related inflammatory mediator that plays an es- sential role in T-cell dependent humoral immunity [1]. CD4+ T cells express CD154 for a period of hours upon crosslinking of the T-cell receptor (TCR), during which the T cells interact with antigen presenting cells (APCs) or other target cells expressing CD40, the receptor for CD154. Activated CD4+ T cells in the secondary lym- phoid organs provide cognate “help” via CD40 ligation to mature B cells binding the same antigen, resulting in B-cell survival, proliferation, and differentiation [2– 4]. In the normal host, CD154 expression in T cells is tightly regulated, such that activation and differentiation signals are provided to B cells and other APCs for only a limited time in the context of a specific response to a particular antigen. Some effects of CD40 signaling, such as isotype switching and the production of mature Ig From the Departments of Pediatrics and Medicine, Weill Medical College of Cornell University, and the Immunology Program, Weill Graduate School of Medical Sciences of Cornell University, New York, NY, USA. Address reprint requests to: Dr. Elaine Schattner, Division of Hematol- ogy-Oncology, Weill Medical College of Cornell University, 1300 York Avenue, New York, NY 10021, USA; Tel: (212) 746-2027; Fax: (212) 746-8866; E-mail: ejsch@med.cornell.edu. Received January 24, 2001; revised May 14, 2001; accepted August 31, 2001. Human Immunology 62, 1346 –1355 (2001) 0198-8859/01/$–see front matter © American Society for Histocompatibility and Immunogenetics, 2001 Published by Elsevier Science Inc. S0198-8859(01)00348-2