The utility of PU.1 as an immunohistochemical marker for histiocytic and dendritic lesions of the skin.

PU.1 is a transcription factor restricted to the hematopoietic system. It is expressed in myeloid lineage and B lymphocytes but is absent in mature T cells and nonhematopoietic cells. Among myeloid lineage-derived cells, PU.1 is overexpressed in monocytes, histiocytes, and dendritic cells. We evaluated PU.1 expression in 78 cases of primary skin neoplasms, including 9 reticulohistiocytomas, 9 Langerhans cell histiocytoses, 7 juvenile xanthogranulomas, 9 fibrous papules, 8 dermatofibromas, 12 dermatofibrosarcoma protuberans, 9 Spitz nevi, and 15 malignant melanomas. Strong nuclear staining for PU.1 was seen in all cases of histiocyte and dendritic cell origin, including 9/9 reticulohistiocytomas, 9/9 Langerhans cell histiocytoses, and 7/7 juvenile xanthogranulomas. No staining for PU.1 was seen in any studied cases of fibrous papules, dermatofibrosarcoma protuberans, dermatofibromas, Spitz nevi, or malignant melanomas. This study indicates that PU.1 is a valuable immunohistochemical marker for identifying cutaneous histiocyte- and dendritic cell-derived lesions. PU.1 staining is easily interpreted due to the sharp nuclear staining as compared with the irregular and often variable cytoplasmic staining seen with CD68.