THE VALUE OF IMMUNOHISTOCHEMISTRY
IN THE ASSESSMENT OF BONE MARROW DISORDERS

Attilio Orazi, M.D., FRCPath. and Dennis P. O'Malley, M.D.

SYSTEMIC MASTOCYTOSIS Systemic mastocytosis is a rare disease characterized by a generalized infiltration of mast cells involving many organs, most notably the bone marrow, spleen, liver and lymph nodes; cutaneous involvement may or may not be present. An association with malignant hematopoietic disorders has been recognized. In the bone marrow, characteristic findings include the presence of clusters of pale cells with oval or reniform nuclei, eosinophilia, and the presence of fibrosis. Although the diagnosis is usually established after special stains with Giemsa or toluidin blue, the mast cells can also be immunostained with c-KIT and CD68/KP-1 monoclonal antibodies. Special stain for CAE, which is frequently used to identify mast cells in extramedullary sites (e.g. skin), is inhibited by nitric acid decalcification and is therefore not used to stain marrow biopsies that are so processed . Mast cells are also positive, in most cases, with an antibody to tartrate resistant acid phosphate (TRAP). Anti-tryptase, a recently available antibody, has also been shown to be of value in the diagnosis of mast cell disorders (Horny et al, 1998). In cases of malignant mast cell disease, the anti-tryptase antibody can correctly identify as mast cells a higher proportion of the malignant cells than CAE or Giemsa. Chymase is another recently proposed marker for mast cells. We have had yet no direct experience with these reagents. References Buckley et al: The detection of mast cell subpopulations in formalin-fixed human tissues using a new monoclonal antibody specific for chymase. J Pathol. 1999;189:138-43. Escribano L, Orfao A, Villarrubia J, Martin F, Madruga JI, Cuevas M, Velasco JL, Rios A, San Miguel JF. Sequential immunophenotypic analysis of mast cells in a case of systemic mast cell disease evolving to a mast cell leukemia. Cytometry 1997;30:89-102. Horny HP, Ruck P, Krober S, Kaiserling E. Systemic mast cell disease (mastocytosis). General aspects and histopathological diagnosis. Histol Histopathol 1997;12:1081-9. Horny et al: Diagnostic value of immunostaining for tryptase in patients with mastocytosis. Am J Surg Pathol. 1998; 22:1132-40. Li WV, Kapadia SB, Sonmez-Alpan E, Swerdlow SH. Immunohistochemical characterization of mast cell disease in paraffin sections using tryptase, CD68, myeloperoxidase, lysozyme, and CD20 antibodies. Mod Pathol 1996;9:982-8.