Sinus histiocytosis with massive lymphadenopathy. A nodal and extranodal proliferation of S-100 protein positive histiocytes?

Four cases of sinus histiocytosis with massive lymphadenopathy (SHML) from Finland were studied clinically and immunohistologically. Three patients had histologically verified extranodal manifestations that often caused the most prominent clinical symptoms. Upper respiratory tract obstruction resulting from SHML lesions was present in two cases, multiple bone lesions in one case, and skin lesions in one case. Clinical remission was observed in all cases, but the course was often protracted. Immunohistochemical studies revealed S-100 protein and alpha-1-antichymotrypsin (ACT) positivity in most large pale cytoplasmic histiocyte-like cells in all cases, whereas lysozyme was not present in these cells. Sinus cells in ordinary sinus histiocytosis were constantly positive for ACT and variably positive for lysozyme and were generally negative for S-100 protein. The large histiocyte-like cells in SHML resemble interdigitating reticulum cells and Langerhans' cells in their S-100 protein positivity but differ from these cells by morphologic characteristics and the presence of alpha-1-antichymotrypsin, a marker constantly seen in ordinary histiocytes. These results suggest that SHML is a proliferative condition of histiocyte-related cells, which share some properties of histiocytes and some of interdigitating reticulum cells. Further studies are necessary to clarify the nature of the peculiar histiocyte-like cells in SHML.