Unusual expression of S-100 protein in histiocytic neoplasms

Twenty-nine cases of histiocytic neoplasms, some resembling juvenile xanthogranuloma (JXG) and others resembling reticulohistiocytoma (RH), were evaluated. Immunohistochemical stains were performed. In this series, seven cases were identified that expressed S-100 protein positive cells. The S-100 positive cells were predominantly large mononuclear and multinucleated histiocytes with eosinophilic cytoplasm, but also in some cases xanthomatous cells and Touton giant cells. These cells also 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 useful differentiating feature between histiocytosis X and non-X histiocytosis such as JXG and RH. The conflicting results of the immunohistochemical stains in the lesions we studied could be potential pitfalls in diagnosing histiocytic neoplasms.