Evidence for a hybrid macrophage phenotype in erythrophagocytic histiocytosis

Purpose: The phenotype of the proliferating cells in two patients with eryt hrophagocytic histiocytosis is described. These 6- and 18-month-old female patients presented with fever, anemia, hepatosplenomegaly, and lymphadenopa thy. Materials and Methods: Clinical histories were reviewed, and pathological s pecimens of both patients were studied by histology, and electron microscop y/immunohistochemistry using antibodies against macrophage and Langerhans c ell (LC) antigens. Results: Histology revealed prominent erythrophagocytosis of proliferating histiocytes. By immunohistochemistry, conventional macrophage (HAM-56, alph a 1-antitrypsin, al-antichymotrypsin, lisozyme, CD68, and alpha-subunit of S-100 protein) and LC (CD1a and S-100 protein) markers were positive, as we ll as double labeling for CD1a and alpha 1-antichymotrypsin, in a majority of proliferating cells. Ultrastructural examination revealed Birbeck granul es and prominent phagolysosomes frequently in the same cell. Conclusions: The hybrid ultrastructural and immunohistochemical phenotype b etween phagocytic macrophage and LC of proliferating histiocytes supports t he common origin of these different histiocyte subtypes. This unusual pheno type might be the expression of the proliferating (hybrid) precursor or be the effect of unknown stimuli. Additional cases of childhood erythrophagocy tic histiocytosis should be studied with immunophenotyping and ultrastructu re to determine whether the hybrid phenotype represents a specific entity o r an epiphenomenon.