Cathepsin D and E co-expression in sinus histiocytosis with massive lymphadenopathy (Rosai-Dorfman disease) and Langerhans' cell histiocytosis: further evidences of a phenotypic overlap between these histiocytic disorders.

Nosological classification of sinus histiocytosis with massive lymphadenopathy (SHML; Rosai-Dorfman disease) is difficult, and the normal cellular counterpart of Rosai-Dorfman (RD) cells is uncharacterised. The peculiar S-100+ phenotype of RD cells suggests a relationship with the dendritic cell family. Recent investigations have revealed cathepsin E to be selectively concentrated in antigen-presenting cells, whereas cathepsin D was found to be expressed in cells of macrophage lineage. Cathepsin D and E distribution was investigated by immunohistochemistry in a series of SHML biopsies and in two types of dendritic cell proliferative lesions: dermatopathic lymphadenitis (DL) and Langerhans' cell histiocytosis (LCH). In SHML biopsies, RD cells and monocyte-related elements of the sinuses and pulp coexpressed cathepsin D and E. LCH cells also stained for both these aspartic proteinases. Conversely, in DL cathepsin E and D were localised to separate cells that resembled Langerhans' cells (LC) or macrophages, respectively, in morphology and distribution. Our data outline the peculiar immunophenotype of RD and LCH cells and suggest that caution should be exercised in the identification of their normal cellular counterpart. The common expression of cathepsin D and E and of S-100 protein suggests some phenotypic overlap between SHML and LCH cells, despite their striking morphological divergence.