PAX8 and PAX5 are differentially expressed in B‐cell and T‐cell lymphomas

Aims:  The purpose of this study was to evaluate the expression patterns of B‐cell specific activator protein (BSAP)/PAX5 and PAX8 in a wide variety of B‐cell and T‐cell neoplasms. Methods and results:  A wide range of B‐cell and T‐cell neoplasms were subjected to immunohistochemical staining with antibodies against BSAP/PAX5 and PAX8 (polyclonal, pPAX8; monoclonal, mPAX8). Ten non‐neoplastic lymph node specimens were examined with the same panel. All of the tested neoplastic and non‐neoplastic B‐cells reacted with the BSAP/PAX5 and pPAX8 antibodies, but did not show reactivity with the mPAX8 antibody. All tested T‐cell neoplasms were negative using the BSAP/PAX5, pPAX8 and mPAX8 antibodies. Conclusions:  This is the first study to show the absence of reactivity to an mPAX8 antibody in an expanded panel of B‐cell lymphomas as well as in a variety of T‐cell neoplasms. In contrast to the mPAX8 antibody, the pPAX8 antibody shows nuclear positivity in non‐neoplastic B cells and mature B‐cell neoplasms; however, this expression is probably a result of cross‐reactivity with PAX5. Given that many laboratories use the pPAX8 antibody, a clear understanding of the differential staining patterns is necessary. The differential diagnosis of a B‐cell lymphoma should be entertained when a pPAX8‐positive, epithelial marker‐negative neoplasm of uncertain primary origin is encountered.