Variable expression of B-cell transcription factors in reactive immunoblastic proliferations: a potential mimic of classical Hodgkin lymphoma.

Reactive immunoblastic proliferations can histologically mimic classical Hodgkin lymphoma (CHL), and show diffuse CD30 expression in large cells. The lack of expression of CD15 in a subset of CHL further complicates their separation from immunoblastic proliferations. Loss of expression of B-cell transcription factors is frequently exploited in making a diagnosis of CHL; however, the staining patterns of B-cell transcription factors in immunoblastic proliferations have not been extensively studied. Thirty-three cases of reactive immunoblastic proliferations were evaluated using a panel of immunohistochemistry for CD30, CD15, CD20, CD3, κ, λ, CD45RB, MUM1, PAX5, OCT2, and BOB.1, as well as Epstein-Barr virus (EBV)/EBV-encoded ribonucleic acid in situ hybridization. A newly developed dual-color chromogenic in situ hybridization technology for detection of κ/λ mRNAs was also used. The majority of immunoblasts expressed CD30 in 14 of 33 (42%) cases; none expressed CD15. Loss or weak expression of at least 1 transcription factor in B immunoblasts, most commonly PAX5, was noted in 24 of 29 (83%) cases. A polytypic light chain expression pattern was detected by immunohistochemistry in 14 of 22 (63.6%) cases and by dual-color chromogenic in situ hybridization in 9 of 10 (90%) cases studied. EBV-encoded ribonucleic acid was detected in 8 of 33 (24.2%) cases, 5 of which were clinically unrelated to infectious mononucleosis. We conclude that B-cell transcription factors can show loss or weak expression in a significant proportion of reactive immunoblastic proliferations, and, therefore, staining for B-cell transcription factors together with CD30 should be interpreted with caution before a diagnosis of CHL is made.