Pax 5 Test

Pax5, or B-cell-specific activator protein, is a nuclear protein in the paired-box containing (PAX) family of transcription factors involved in control of organ development and tissue differentiation. Pax genes are a family of developmental control genes that encode nuclear transcription factors and have been implicated in the control of mammalian development. PAX-5 is a B cell specific transcription factor that is expressed in pro B-cells, pre-B and mature B cells, and subsequently in all stages of B cell development until that plasma cell stage in which it is down regulated. The test is done by Immunohistochemical Stain.

Paired Box 5 (PAX5) is a B-cell specific activator protein (BSAP). In early stages of B-cell development, PAX5 influences the expression of several B-cell specific genes, such as CD19 and CD20. PAX5 is expressed primarily in pro-, pre-, and mature B-cells, but not in plasma cells. There is an excellent correlation between CD20 and PAX5 expression; however, anti-PAX5 exceeds the specificity and sensitivity of L26 (CD20) because of its earlier expression in B-cell differentiation and its ability to detect all committed B-cells, including classic Hodgkin lymphoma. It is very specific to B-cell lineage and does not stain T-cells.

Pax5 is mostly expressed in B lymphocytes and B-cell lymphomas, although recent data have shown expression in the developing central nervous system, some neuroendocrine tumours, and occasional myeloid leukaemia. Pax5 immunohistochemistry shows robust nuclear staining, and has become a valuable tool in the diagnosis and subclassification of lymphomas. Pax5 staining is positive in most Hodgkin and B-cell non-Hodgkin lymphomas, and also precursor B-cell lymphoblastic neoplasms. Plasma cell neoplasms, multiple myeloma, and plasmablastic lymphomas typically are negative. T-cell lymphomas are, to date, consistently negative. Recently, Pax5 expression has been described in the majority of small cell carcinomas and Merkel cell carcinomas. Rare cases of Pax5 expression in other carcinomas have been reported. With these exceptions, Pax5 immunohistochemistry is fairly specific for B-cell lineage and is a valuable addition to the armamentarium of markers available for lymphoma subtyping.

A formalin-fixed, paraffin-embedded (FFPE) tissue block is preferred specimen type or One unbaked, unstained slide cut at 4-5 microns for H&E staining (required) and two to three positively charged unstained slides cut at 3-4 microns for each test/antibody ordered. Block and slide identifiers should be clearly written and match exactly with the specimen ID and specimen labelling as noted on the requisition.