ORIGINAL ARTICLE TRAF1 Expression and c-Rel Activation Are Useful Adjuncts in Distinguishing Classical Hodgkin Lymphoma From a Subset of Morphologically or Immunophenotypically Similar Lymphomas Scott J. Rodig, MD, PhD,* Kerry J. Savage, MD,† Vuong Nguyen, BS,* Geraldine S. Pinkus, MD,* Margaret A. Shipp, MD,† Jon C. Aster, MD, PhD,* and Jeffery L. Kutok, MD, PhD* Abstract: We demonstrate that the expression of TRAF1 and activated c-Rel, two proteins that function in signaling events down- stream of activated CD30 in Reed-Sternberg cells, reliably dis- tinguish classical Hodgkin lymphoma from anaplastic large cell lymphoma, nodular lymphocyte predominant Hodgkin lymphoma, and nonmediastinal diffuse large B-cell lymphoma. By immunohis- tochemistry, we found strong TRAF1 staining in 21 of 25 cases of classical Hodgkin lymphoma. In contrast, strong TRAF1 staining was present in only 1 of 17 cases of anaplastic large cell lymphoma, 0 of 15 cases of lymphocyte predominant Hodgkin lymphoma, and 2 of 36 cases of nonmediastinal diffuse large B-cell lymphoma. Nuclear staining for c-Rel, a pattern consistent with NFkB activation, was observed in the Reed-Sternberg cells in 23 of 25 cases of classical Hodgkin lymphoma but only in 1 of 15 cases of anaplastic large cell lymphoma and 3 of 15 cases of nodular lymphocyte predominant Hodgkin lymphoma. A heterogeneous pattern of subcellular c-Rel localization was found in nonmediastinal diffuse large B-cell lymphoma. Taken together, the combination of strong cytoplasmic TRAF1 expression and nuclear c-Rel was present in 80% of cases of classical Hodgkin lymphoma (n = 25) but in only 3% of cases of the other malignant lymphomas tested (n = 62). Thus, the differential expression patterns of downstream components in the CD30 signal- ing pathway may prove a useful adjunct in distinguishing cases of classical Hodgkin lymphoma from other malignant lymphomas in routine clinical practice. Key Words: TRAF1, c-Rel, Hodgkin lymphoma, CD30, immuno- histochemistry (Am J Surg Pathol 2005;29:196–203) C lassical Hodgkin lymphoma (cHL) is diagnosed, in part, on recognized morphologic characteristics that include the identification of Reed-Sternberg (RS) cells and variants in a background of reactive small lymphocytes, plasma cells, neutrophils, eosinophils, and macrophages. However, because RS-like cells can be seen in other lymphomas, including ana- plastic large cell lymphoma (ALCL), nodular lymphocyte pre- dominant Hodgkin lymphoma (NLPHL), and diffuse large B-cell lymphoma (DLBCL), the diagnosis of cHL often re- quires a panel of confirmatory immunohistochemical studies. The RS cells and variants of cHL are typically positive for CD30, CD15, and fascin, and negative for CD3, CD20, and CD45. 18,29 While CD30 is perhaps the most sensitive marker for diagnosing cHL, it is not specific, as the neoplastic cells of ALCL, a subset of DLBCL, and on occasion, other non- Hodgkin lymphomas also express it. In addition, approxi- mately 15% of cases of cHL are negative for CD15 and 10% to 20% of cases of cHL express detectable levels of CD20. 4,6,12,25,27,28,33,35 This variable immunohistochemical profile, together with overlapping morphologic features, can result in diagnostic uncertainty in a subset of cases of cHL. CD30 is a member of the tumor necrosis factor family of cell surface receptors (TNFRs) that includes at least 20 ligand-receptor pairs critical for the growth, survival, and dif- ferentiation of a variety of cell types. 2,11 An intriguing feature of the TNFR superfamily is that engagement of individual receptors by their respective ligands can lead to diametrically opposed outcomes, such as either apoptosis or enhanced growth and survival, in different cellular contexts. This diver- gence in phenotypic response is attributable in part to the varied expression of specific signal transduction proteins in different cell types. 32 One type of signaling protein that governs events down- stream of CD30 and other TNFRs that lack a death domain are the TNFR-associated factors (TRAFs). 3 TRAFs couple re- ceptor ligation with activation of the anti-apoptotic nuclear factor kappa B (NFkB) family of transcription factors. 8,14 In unstimulated cells, NFkB family members localize to the cytosol due to association with the inhibitor of NFkB (IkB). However, ligation of TNFRs, in the proper cellular context, activates TRAF-mediated signals that promote the ubiquitina- tion and proteosomal processing of IkB. Degradation of IkB permits NFkB to translocate to the nucleus, bind DNA, and induce the transcription of genes that promote cellular pro- liferation and differentiation. 5 Additionally, the presence of increased TRAF proteins and activated NFkB appears to inhibit competing apoptotic signals that result from other li- gands such as TNF-a. 1 From the *Department of Pathology, Brigham & Women’s Hospital and †Department of Medicine, Dana Farber Cancer Institute, Boston, MA. The current address of Dr. Savage is British Columbia Cancer Agency, Vancouver, British Columbia, Canada. Supported by NIH/NCI PO1CA092625-01 (to M.A.S., J.C.A., and J.L.K.). Reprints: Jeffery L. Kutok, MD, PhD, Department of Pathology, Amory 3 rd Floor, Brigham and Women’s Hospital, 75 Francis Street, Boston, MA 02115 (e-mail: Jkutok@partners.org). Copyright Ó 2005 by Lippincott Williams & Wilkins 196 Am J Surg Pathol  Volume 29, Number 2, February 2005