Evidence for a hybrid macrophage phenotype in erythrophagocytic histiocytosis

PURPOSE: The phenotype of the proliferating cells in two patients with erythrophagocytic histiocytosis is described. These 6- and 18-month-old female patients presented with fever, anemia, hepatosplenomegaly, and lymphadenopathy. MATERIALS AND METHODS: Clinical histories were reviewed, and pathological specimens of both patients were studied by histology, and electron microscopy/immunohistochemistry using antibodies against macrophage and Langerhans cell (LC) antigens. RESULTS: Histology revealed prominent erythrophagocytosis of proliferating histiocytes. By immunohistochemistry, conventional macrophage (HAM-56, alpha 1-antitrypsin, alpha 1-antichymotrypsin, lisozyme, CD68, and alpha-subunit of S-100 protein) and LC (CD1a and S-100 protein) markers were positive, as well as double labeling for CD1a and alpha 1-antichymotrypsin, in a majority of proliferating cells. Ultrastructural examination revealed Birbeck granules and prominent phagolysosomes frequently in the same cell. CONCLUSIONS: The hybrid ultrastructural and immunohistochemical phenotype between phagocytic macrophage and LC of proliferating histiocytes supports the common origin of these different histiocyte subtypes. This unusual phenotype might be the expression of the proliferating (hybrid) precursor or be the effect of unknown stimuli. Additional cases of childhood erythrophagocytic histiocytosis should be studied with immunophenotyping and ultrastructure to determine whether the hybrid phenotype represents a specific entity or an epiphenomenon.