Immunoelectronmicroscopic demonstration of S-100 protein in hairy cell leukemia cells

Hairy cell leukemia (HCL) is generally considered a neoplastic proliferation of B-cell lineage. Among immunocompetent cells, however, S-100 protein is also demonstrated in "dendritic" cells (i.e., interdigitating reticulum cells, Langerhans cells, follicular dendritic cells) and in a small percentage of T8-cells. This study reports the surprising detection of S-100 protein in peripheral blood HCL cells of B-cell type, using immunofluorescence and immunoelectronmicroscopy. However, negative results for S-100 protein staining were obtained in paraffin splenic sections of HCL cases. This report stresses the need for caution in the interpretation of immunocytochemical methods and results, and emphasizes the diagnostic ambiguity of the S-100 protein.