HAIRY-CELL IDENTIFICATION BY IMMUNOHISTOCHEMISTRY OF TARTRATE-RESISTANT ACID-PHOSPHATASE

Tartrate-resistant acid phosphatase (TRAcP) has been an indispensible marker for hairy cell leukemia (HCL) for over two decades. However, th e traditional TRAcP cytochemical stain cannot be performed effectively on sections of paraffin-embedded tissues that are important resources for histopathologic evaluation in diagnosis and treatment of HCL, Wid e variation in expression of TRAcP activity by hairy cells (HCs) withi n and among patients is an interesting biologic phenomenon that has no t been explained and can cause some diagnostic uncertainty as well. To solve the problem of staining TRAcP in paraffin sections and to begin to address the questions of variable TRAcP expression in HCL, we deve loped a monoclonal antibody to TRAcP, 9C5, for immunohistochemical ide ntification of HCs. In smears of blood and bone marrow, immunocytochem istry of TRAcP using 9C5 was as specific but slightly less sensitive t han direct cytochemical staining of enzymatic activity. In paraffin se ctions of spleen and bone marrow from HCL patients, immunohistochemist ry with 9C5 stained the HCs with high sensitivity and specificity and clearly showed the characteristic diffuse infiltration by HCs. Other c ells noted to stain strongly with 9C5 were occasional macrophages in b one marrow smears and osteoclasts and occasional tissue macrophages in paraffin sections. These are cells known to express abundant TRAcP ac tivity, Immunohistochemistry with anti-TRAcP monoclonal antibody 9C5 m ay have utility as an added option in the diagnosis of HCL, as a means to evaluate residual disease in HCL patients undergoing new treatment s, and as a way to address questions regarding variable expression of TRAcP activity by HCs within and among patients with HCL. Also, 9C5 ha s potential as a reagent for the immunoassay of bone-derived serum TRA cP in patients with certain bone diseases and cancers with bane metast asis. (C) 1995 by The American Society of Hematology.