Aims: The B-cell-specific transcription factor PAX-5 is physiologically expressed in normal B cells and silenced in plasma cells. The aim of this study was to determine whether PAX-5 expression is universal among B-cell malignancies.
Methods and results: A wide spectrum of B-cell malignancies were subjected to immunohistochemical analysis for PAX-5 expression. The study was especially focused on cases lacking CD20, such as precursor B-cell acute lymphoblastic leukaemia (preB-ALL), CD20− B-cell lymphomas, classical Hodgkin’s lymphoma (CHL) and B-cell lymphomas with significant plasmacytic differentiation. Strong PAX-5 expression was identified, without exception, in all cases of CD20+ B-lymphoproliferative disorders. It was also invariably detected in 31/31 cases of preB-ALL, 14/14 cases of CD20− diffuse large B-cell lymphoma without plasmacytic differentiation and 26/26 CD20− B-cell lymphoma status post rituximab treatment. The vast majority of CHLs had unequivocal PAX-5 expression of varying intensity (80/86). However, variants of B-cell malignancies with characteristic plasmacytic differentiation exhibited no detectable PAX-5 expression (0/17).
Conclusions: PAX-5 is the most sensitive and reliable immunohistochemical marker for B-cell malignancies. Lack of PAX-5 expression correlates with the presence of marked plasma cell differentiation.