Hodgkin and Reed-Sternberg cells of classical Hodgkin lymphoma arise from B-lymphocytes. However, classical markers of the B-cell phenotype, such as CD20, are present only in about 25% of the cases. The aim of the present study was to assess expression of the B-cell-related antigens CD20, CD79a, and CD138 in classical Hodgkin lymphoma using a tissue microarray consisting of 330 classical Hodgkin lymphoma cases. Expression of CD15, CD20, CD30, CD79a, CD138, and latent membrane protein 1 of Epstein-Barr virus was assessed by immunohistochemistry, and the methodology was validated by direct comparison of CD20 expression on the tissue microarray cores with corresponding large sections. The influence of the number of arrayed sample cores on the obtained expression levels of CD20 was analyzed by comparing the results from single, duplicate, and triplicate cores. Two-hundred fifty-three (77%) of the 330 cases were morphologically representative. CD20 was expressed in 84 cases (33%), CD79a in 26 (10%), and CD138 in 2 (1%), respectively. CD20 and CD79a were co-expressed in 16 cases (P <.005), and expression of CD20 correlated inversely with CD15 (P <.01). Comparing the tissue microarray results with those from conventional sections for expression of CD20 yielded a concordance of 94% (63/67). Examining one, two, and three cores from individual cases revealed positivity for CD20 at 24% (61/253), 32% (82/253), and 33% (84/253), respectively. We conclude that B-cell markers are expressed in 38% of classical Hodgkin lymphoma in the following rank order: CD20>CD79a>>CD138. The use of two cores per tissue sample renders the tissue microarray technology effectively representative and thus very useful for high-throughput evaluation of heterogeneously expressed markers in classical Hodgkin lymphoma.

Because of marrow fibrosis, bone marrow aspirations are often nonconclusive in patients with hairy cell leukemia (HCL). Therefore, histologic examination is important in HCL but often difficult in cases with low numbers of tumor cells. A combined immunohistochemical positivity for DBA.44 and tartrate-resistant phosphatase was previously found in 100% of HCL and suggested to be specific for this diagnosis. To further assess the diagnostic specificity and sensitivity of this immunohistochemical approach in a higher number of cases, we analyzed 56 HCLs and lymphoma tissue microarrays, including 840 cases of the most frequent non-Hodgkin lymphomas. All HCLs showed combined positivity for these two proteins (100% sensitivity). Both antibodies were often positive in other lymphoma types. DBA.44 reactivity was especially frequent in follicular lymphomas (46%), whereas tartrate-resistant acid phosphatase (TRAP) expression was often seen in mantle cell lymphomas (57%), primary mediastinal large B-cell lymphomas (54%), and chronic lymphocytic leukemia/small lymphocytic lymphoma (41%). A combined DBA.44/TRAP positivity was seen in only 3% of non-HCL non-Hodgkin lymphomas, including cases of diffuse large B-cell lymphomas, follicular lymphomas, chronic lymphatic leukemia/small lymphocytic leukemias, and mantle cell lymphomas. Overall, these data confirm the utility of combined immunohistochemical DBA.44/TRAP expression analysis in confirming the diagnosis of HCL. However, combined positivity for these markers is highly sensitive but not absolutely specific for HCL.

KIT expression is a very rare event in NHL and virtually absent in HL. In the few positive cases, the aberrant expression is not caused by a mutation in the 'hot-spots' of the kit gene, indicating that treatment of these tumours with Imatinib may be ineffective.

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