Expression of X-Linked Inhibitor of Apoptosis Protein in Reactive Lymphoid Tissues, Non-Hodgkin’s Lymphomas, and Hodgkin’s Disease.

X-linked inhibitor of apoptosis protein (XIAP) is an important regulator of apoptosis which binds to and inhibits caspases-3, -7 and -9, blocking the caspase 9-mediated apoptosis pathway. This pathway is activated by p53 and DNA damage and may be an important determinant of responsiveness to chemotherapy. Apoptosis also plays a major role in the regulation of follicle center B-cell proliferation and BCL2-mediated inhibition of apoptosis is a key factor in B-cell lymphomagenesis. In this study we examined the expression of XIAP in 65 reactive and neoplastic lymphoid proliferations utilizing a monoclonal antibody to XIAP (#610763 BD Biosciences, San Jose, CA) and immunohistochemistry with avidin-biotin-complex immunoperoxidase technique on formalin-fixed, paraffin-embedded sections. In reactive lymph nodes and tonsils, expression of XIAP was limited to large noncleaved cells in follicle centers (5 of 6 cases). XIAP was absent in plasmacytoma (3 cases) and small lymphocytic lymphoma/chronic lymphocytic leukemia (1 case). XIAP was expressed in follicular lymphoma, predominantly in large noncleaved cells (6 of 9 cases) and in diffuse large B cell lymphoma (11 of 16 cases), including cases of T-cell/histiocyte-rich diffuse large B cell lymphoma (2 cases), primary mediastinal large B cell lymphoma (1 case), and posttransplantation diffuse large B cell lymphoma (1 case). XIAP was consistently expressed in Burkitt and Burkitt-like lymphoma (3 of 3 cases) and anaplastic large cell lymphoma (3 of 3 cases) and in one case of adult T cell leukemia/lymphoma. XIAP was variably expressed in marginal-zone B cell lymphoma, predominantly in large blasts (2 of 4 cases) and in mantle cell lymphoma (2 of 3 cases). XIAP was not detected in peripheral T cell lymphoma, unspecified (1 case), extranodal NK/T cell lymphoma, nasal type (1 case), precursor B cell lymphoblastic leukemia (1 case), or granulocytic sarcoma (1 case). XIAP was consistently expressed in the Reed-Sternberg and mononuclear Reed-Sternberg-variant cells of classical Hodgkin disease (9 of 9 cases) and the L+H Reed-Sternberg-variant cells of nodular lymphocyte predominance Hodgkin disease (3 of 3 cases). XIAP is expressed across a broad range of lymphoproliferative disorders, including classical and nodular lymphocyte predominance Hodgkin disease, diffuse large B cell lymphoma, follicular lymphoma, Burkitt lymphoma, marginal-zone and mantle cell lymphoma, and anaplastic large cell lymphoma. XIAP appears to be selectively expressed in the proliferating elements of these lymphomas. The possible prognostic and therapeutic significance of XIAP expression needs to be determined.