Bone marrow monocytes in histiocytosis X acquire some phenotypic features of Langerhans cells in long term bone marrow cultures

Bone marrow cells of a patient with Letterer-Siwe disease were cultured for three weeks in long-term bone marrow culture (LTBMC) conditions and examined at one-week intervals with a large panel of monoclonal antibodies by immunohistochemistry and by the immunogold transmission electron microscopy (immunoTEM) technique. Although at diagnosis the bone marrow showed a slight increase of monocytes with a normal phenotype, a rapid expansion of cells expressing CD1a and CD1c was observed already after 1 week of culture. A progressive increase in CD4, CD11b and CD11c expression was also observed. ImmunoTEM of cultured cells demonstrated that CD1a⁺ cells had macrophage-like morphology, and did not contain Birbeck granules. These findings indicate that bone marrow monocytes acquire some phenotypical features of Langerhans cells in LTBMC and support the hypothesis that these cells may derive directly from a bone marrow monocytic precursor.