PAX5

PAX5 (Paired box gene 5), also known as BSAP (B-cell–specific activator protein), is a member of the paired box (Pax) family of transcription factors, which play a central role in cell differentiation and tissue development. The gene is located on the short arm of human chromosome 9 (9p13.2). PAX5 is a lineage-defining transcription factor for B-cells. It is essential for pro-B cell development and remains active throughout B-cell maturation, until it is downregulated during plasma-cell differentiation. On immunohistochemistry, PAX5 demonstrates a nuclear staining pattern, reflecting its function as a nuclear transcription factor.

PAX5 plays a primarily is a diagnostic role marker in hematolymphoid neoplasms. It is a robust nuclear marker of B-cell lineage, expressed in most B-cell lymphomas and B-lymphoblastic neoplasms. The expression is typically weak to moderate in Reed–Sternberg /Hodgkin cells of classical Hodgkin lymphoma and absent in most T-cell lymphomas as well as most plasma-cell neoplasms. 

• Mature B-cell lymphomas Mature B-cell lymphomas (such as DLBCL, FL, MZL, and MCL) typically show a strong nuclear staining reaction pattern in tumor cells, confirming their B-cell lineage and helping to distinguish them from other lymphomas, carcinomas or sarcomas. PAX5 is also particularly useful for excluding T-cell lymphomas, which are usually negative. Furthermore, inclusion of PAX5 staining can be valuable in the assessment of recurrent B-cell lymphomas treated with anti-CD20 therapy (e.g. Rituximab), as persistent loss of CD20 expression may occur, making PAX5 an important marker for confirming B-cell origin in such cases.
• Hodgkin lymphoma Classical Hodgkin lymphoma (cHL) typically shows weak nuclear PAX5 staining in Hodgkin/Reed–Sternberg cells, whereas the background B-cells display a strong staining reaction. This pattern supports a diagnosis of cHL (in combination with CD30 and CD15) and helps distinguish it from anaplastic large cell lymphoma (ALCL), which is usually PAX5-negative. In contrast, nodular lymphocyte-predominant Hodgkin lymphoma (NLPHL) typically exhibits strong nuclear PAX5 expression in the lymphocyte-predominant (LP or “popcorn”) cells. This feature assists in differentiating NLPHL, characterized by LP cells with strong PAX5, positivity for CD20 and BCL6, and negativity for CD30 and CD15, from cHL, in which Hodgkin/Reed–Sternberg cells generally show weak PAX5 expression, variable BCL6, frequent loss of CD20 and strong positivity for CD30 and CD15.
• Plasma-cell neoplasms Plasma-cell neoplasms (multiple myeloma, plasmacytoma, plasmablastic lymphoma) are typically negative for PAX5. Lack of PAX5 expression supports a plasma-cell phenotype when correlated with morphology and other markers such as CD138 and MUM1.
• B-lymphoblastic leukemia/lymphoma (B-LBL/L) B-LBL/L shows nuclear PAX5 positivity in most cases, confirming B-lineage in the blasts. Importantly, PAX5 expression must always be interpreted in the context of other markers. Integration with myeloid and T-cell immunohistochemical markers is essential for correct lineage assignment and accurate classification.
• Merkel cell carcinoma (MCC) MCC, cutaneous neuroendocrine carcinoma, shows PAX5 expression in about 65–90% of cases. In small round blue cell tumors of the skin, PAX5 positivity therefore favors MCC over a metastasis from, for example, small-cell lung carcinoma. In this context, a differential diagnosis using an immunohistochemical panel including PAX5, CK20, TdT, SATB2 and TTF-1 is recommended. The typical profiles are primary cutaneous MCC (PAX5+, CK20+, SATB2+, TdT+, TTF1-) versus pulmonary SCLC (PAX5-, TTF1+, TdT-, CK20-, SATB2-).

Tonsil and colon/appendix are recommended as basic positive and negative tissue controls for PAX5. In tonsil, protocols should be calibrated to produce a distinct and strong nuclear staining reaction in virtually all mantle zone B-cells, germinal center B-cells and interfollicular B-cells. In colon and appendix, dispersed B-cells in the lamina propria must a show strong nuclear staining reaction in some B-cells as germinal center centrocytes/centroblasts a weak cytoplasmic reaction is acceptable. No staining reaction should be observed in other cell types, including T-cells, stromal cells or epithelial cells of the tonsil and colon/appendix. As a supplement to these tissues both during the technical calibration and validation phase but also importantly for daily control of IHC assay reproducibility it is recommended to include classical Hodgkin lymphoma (Reed-Sternberg cells) to verify the performance of the assay with focus on low level of detection (LLOD).