UNUSUAL EXPRESSION OF S-100 PROTEIN IN HISTIOCYTIC NEOPLASMS

Twenty-nine cases of histiocytic neoplasms, some resembling juvenile x anthogranuloma (JXG) and others resembling reticulohistiocytoma (RH),w ere evaluated. Immunohistochemical stains were performed. In this seri es, seven cases were identified that expressed S-100 protein positive cells. The S-100 positive cells were predominantly large mononuclear a nd multinucleated histiocytes with eosinophilic cytoplasm, but also in some cases xanthomatous cells and Touton giant cells. These cells als o expressed a positive reaction for vimentin, KP-1, and Factor XIIIa. There was no reactivity observed for monoclonal antibody 010(CD1a). A positive reaction for S-100 protein is conventionally accepted as a us eful differentiating feature between histiocytosis X and non-X histioc ytosis such as JXG and RH. The conflicting results of the immunohistoc hemical stains in the lesions we studied could be potential pitfalls i n diagnosing histiocytic neoplasms.