A Histiocyte-Specific Marker in the Diagnosis of Malignant Fibrous Histiocytoma: Use of Monoclonal Antibody KP-1 (CD68)

KP-1 (CD68) is a recently described monoclonal antibody to a cytoplasmic epitope present on tissue histiocytes and macrophages. To determine the specificity and sensitivity of this marker in the evaluation of cases of malignant fibrous histiocytoma (MFH), this reagent and a panel of commercially antibodies were used to stain formalin-fixed paraffin sections from 25 cases of MFH and 25 other tumors, including a variety of soft-tissue sarcomas. Eighteen of 25 cases of MFH stained for KP-1 (72%), whereas all other tumors were negative, including 12 cases of pleomorphic soft-tissue sarcoma other than MFH. The percentage of tumor cells staining for KP-1 varied. In 11 cases KP-1 was only focally present, but staining was of a high intensity and associated with minimal nonspecific or background staining. Pleomorphic histiocytic cells and spindle cells from storiform tumors were strongly decorated with antibodies to KP-1 in most cases, and antigen also was present on tumor giant cells. Although alpha-1-antitrypsin and alpha-1-chymotrypsin stained a higher percentage of cases of MFH (92%), immunoreactivity for these markers also was noted in other tumors. Because of its specificity as a histiocyte marker, KP-1 is a useful component in a panel of antibodies for the characterization of soft-tissue sarcomas and the diagnosis of MFH.