Immunohistochemical comparison of cutaneous histiocytoses and related skin disorders: diagnostic and histogenetic relevance of MS-1 high molecular weight protein expression.

Twenty-nine cases of Langerhans cell histiocytosis (LCH), non-Langerhans cell histiocytoses (N-LCH), non-infectious granulomas, and fibroblast-related lesions were examined with a panel of monoclonal and polyclonal antibodies on freshly frozen tissue sections to characterize the macrophage phenotype of N-LCH syndromes. MS-1 high molecular weight extracellular protein, specific for sinusoidal endothelial cells and dendritic perivascular macrophages in normal human organs, was expressed by N-LCH cells but was not found in LCH cells, epithelioid cells in sarcoidosis, or palisading histiocytes in granuloma annulare. The subcellular location of MS-1 protein, i.e., cytoplasmic vs. peripheral/extracellular, allowed discrimination of small and large (foamy or multinucleated) N-LCH cells. MS-1-positive cells, which were found intermingled in cellular dermatofibromas but not in fibrous dermatofibromas, differed from MS-1-positive N-LCH cells by their dendritic morphology, and thus rather resembled their normal dermal counterparts. A preserved functional relationship of these two MS-1-positive cell types was indicated by the fact that N-LCH and cellular dermatofibromas were the only lesions found to be highly vascularized. As expected, CD1a showed high specificity for LCH, while CD34 was predominantly expressed by fibroblast-related lesions; in cellular dermatofibromas, CD34 and MS-1 expression partially overlapped. The other antigens tested showed non-specific or overlapping patterns of expression. In conclusion, assessment of MS-1 protein expression (in addition to assessment of CD1a and CD34) promises to be of diagnostic value in the discrimination of N-LCH from related skin disorders, and it may indicate a common differentiative pathway for most N-LCH disease entities.