title: PAX-5: a valuable immunohistochemical marker in the differential diagnosis of lymphoid neoplasms.
abstract: OBJECTIVE
  Undifferentiated tumors and hematolymphoid neoplasms can be diagnostically challenging due to potential overlap of morphologic features and variant antigen expression. PAX-5, a transcription factor expressed throughout B-cell maturation, is detected in most B-cell neoplasms including those that lack expression of mature B-cell markers, such as classical Hodgkin lymphoma (cHL), B-lymphoblastic leukemia and B-cell lymphomas following rituximab therapy. The lack of PAX-5 expression in most CD30-positive non-hematopoietic malignancies (embryonal carcinoma and seminoma) and T-cell lymphomas, such as anaplastic large cell lymphoma (ALCL), suggests that the absence of PAX-5 may be used to confirm non-B-cell lineage. The goal of this study was to retrospectively assess PAX-5 immunoreactivity in diagnostic samples of hematolymphoid and other non-hematopoietic malignancies.
  
  
  METHODS
  Diagnostic lymph node, decalcified core bone marrow biopsies and tissue sections from 111 archived paraffin-embedded tissue blocks and a tissue lymphoma microarray were immunostained using a monoclonal antibody to PAX-5. The corresponding hematoxylin and eosin stained tissue sections and additional immunostains were simultaneously evaluated. PAX-5 immunoreactivity in neoplastic cells was scored as positive or negative. This study was exempted by the Institutional Review Board for Human Research.
  
  
  RESULTS
  Nuclear PAX-5 immunoreactivity was detected in 88% (36/41) of Hodgkin lymphoma, all cases of diffuse large B-cell lymphoma (n=72), small B-cell lymphomas (n=5), B-lymphoblastic leukemia/lymphoma and mixed phenotype acute leukemia with B-cell lineage (n=5). PAX-5 was not detected in ALCL (n=22), T-cell lymphoblastic leukemia/lymphoma, mixed phenotype acute leukemia with T-cell lineage (n=5), acute myeloid leukemia (n=4), carcinoid tumors with typical morphology (n=5), melanoma (n=3), and undifferentiated/metastatic tumors (n=8). Non-neoplastic bone marrow sections showed scattered nuclear staining in small B-cell lymphocytes/hematogones. The detection of PAX-5 immunoreactivity resulted in the reclassification of two cases of ALCL to cHL.
  
  
  CONCLUSIONS
  Overall, our results demonstrate that including PAX-5 in a panel with other immunomarkers helps establish B-cell lineage and increases diagnostic yield.
authors.affiliation: Department of Pathology & Laboratory Medicine, Medical University of South Carolina; Charleston, South Carolina 29425, USA. lazarj@musc.edu
authors.forename: Mohamed M
authors.initials: MM
authors.lastname: Desouki
authors.forename: Ginell R
authors.initials: GR
authors.lastname: Post
authors.forename: Daniel
authors.initials: D
authors.lastname: Cherry
authors.forename: John
authors.initials: J
authors.lastname: Lazarchick
mesh_terms: D014408:Biomarkers, Tumor / Q000032:analysis*; D003937:Diagnosis, Differential; D006689:Hodgkin Disease / Q000175:diagnosis; D006801:Humans; D007150:Immunohistochemistry; D008223:Lymphoma / Q000175:diagnosis*; D016403:Lymphoma, Large B-Cell, Diffuse / Q000175:diagnosis; D017728:Lymphoma, Large-Cell, Anaplastic / Q000175:diagnosis; D051757:PAX5 Transcription Factor / Q000032:analysis*
chemical_list: D014408:Biomarkers, Tumor; D051757:PAX5 Transcription Factor; C497281:PAX5 protein, human