Aims: Proliferation of plasmacytoid dendritic cells (PDCs) occurs in both reactive lymphoid hyperplasia and myeloproliferative disorders, especially chronic myelomonocytic leukaemia. PDCs in the former appear reactive, but in the latter are reported to be clonally related to the underlying myeloid neoplasm. Langerhans cells (LCs), another type of dendritic cell, also proliferate in both reactive dermatoses and, rarely, myeloproliferative disorders, such as acute leukaemia. Methods and results: We report a rare case of tumorous proliferation of PDCs and LCs in the systemic lymph nodes in a 55-year-old man with acute myeloid leukaemia. A microsatellite instability assay showed identical patterns of short tandem repeats in both microdissected PDC and LC components, along with blood blasts. Conclusions: We hypothesize that the combined proliferations of PDCs and LCs derive from the same haematopoietic stem cells, but that they differentiate divergently under the effect of different microenvironments.
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