Extracellular deposition of eosinophil major basic protein in orbital histiocytosis X

Although eosinophils are prominent in orbital lesions of patients with histiocytosis X (Langerhans' cell histiocytosis), little is known of their pathogenic significance in the disease. To determine whether eosinophils degranulate and deposit toxic proteins in orbital histiocytosis X, the authors examined lesions by indirect immunofluorescence for localization of the core granule protein (major basic protein) outside of eosinophils. Four patients with histiocytosis X were studied: three with eosinophilic granuloma and one with Hand-Schüller-Christian disease. Tissue eosinophilia was prominent in all specimens; striking extracellular deposition of eosinophil major basic protein was noted in three patients, and focal deposition was present in the fourth patient. Orbital specimens obtained at autopsy from patients without orbital disease were studied as control specimens; no tissue eosinophilia or deposition of eosinophil major basic protein was observed. These findings indicate that eosinophils likely degranulate in lesions of orbital histiocytosis X and may participate in the pathogenesis of the disease.