Clonal relationship between langerhans cell histiocytosis and myeloid sarcoma

Langerhans cells (LCs) represent the main antigen capturing, processing and presenting dentritic cells (DCs) resident in epithelial surfaces including the skin or the oral mucosa, thus leading to either the induction of immune responses or tolerance. They are classically considered as myeloid-derived DCs sharing initial steps of their differentiation with monocytes. LC histiocytosis (LCH) and the more aggressive variant LC sarcoma (LCS) are considered as the neoplastic counterparts of this DC cell type. The immunoreactivity for macrophage-associated antigens in conjunction with CD1a, S-100 protein and langerin (CD207) represents a useful tool to establish the diagnosis of LCH in paraffin-embedded material.1 Rare cases of neoplasms with LC differentiation features developing in patients previously diagnosed with follicular lymphoma or acute lymphoblastic lymphoma/leukaemia (ALL) of T- and B-cell lineage have been reported. In these cases, identical clonal IgH- or TCRγ-gene rearrangements were detected in the H/DC neoplasms and the lymphomas consistent with a common clonal origin.2, 3, 4, 5 The development of the histiocytic dentritic/cell (H/DC) neoplasm was associated with progressive disease in most of these cases. Thus, it has been suggested that these tumours may represent a part of the clonal evolution of the underlying lymphomas.4 The development of a LCS in a patient with an acute B-ALL in complete remission for 9 years has been attributed to a transcription factor-induced transdifferentiation.5 Both neoplasms showed an identical IgH rearrangement, but different expressions of PAX5 and ID2.5