Original StudyPromyelocytic Differentiation in Infiltrates of Prurigo Pigmentosa An Analogy to Histiocytoid Sweet SyndromePirrone, Johanna MD*; Böer-Auer, Almut MD*,†Author Information *Dermatologikum Hamburg, Department of Dermatopathology, Hamburg, Germany; and †Department of Dermatology, Munster University, Münster, Germany. Correspondence: Johanna Pirrone, MD, Dermatologikum Hamburg, Stephansplatz 5, 20354 Hamburg, Germany (e-mail: email@example.com). The authors declare no conflicts of interest. The American Journal of Dermatopathology: February 2020 - Volume 42 - Issue 2 - p 86-91 doi: 10.1097/DAD.0000000000001449 Buy Metrics Abstract Prurigo pigmentosa (PP) is a rare inflammatory dermatosis of unknown etiology. Young women are affected most commonly. Clinically, heavily itchy papules erupt mainly on the trunk healing with residual reticulate pigmentation. Histopathologic descriptions of PP are somewhat controversial. First, PP was reported as lichenoid-interface dermatitis, and later, neutrophils were recognized as the characteristic feature, and the variation in histopathologic patterns was interpreted as a time-dependent phenomenon. Immunohistochemical studies on PP are rare. Biopsies of 5 patients with clinically typical PP were examined histopathologically, and infiltrates were characterized immunohistochemically: myeloperoxidase, CD11c, CD68, CD4, CD8, tryptase, and langerin. In all cases, myeloperoxidase-positive cells with band forms of nuclei and with histiocytoid cytomorphology were identified. They were seen in the epidermis (4/5) and in the dermal infiltrate (5/5). On staining with CD11c, myeloid dendritic cells could be demonstrated in the infiltrate (5/5). In conclusion, myeloid progenitor cells are part of the infiltrate in PP, and they may sometimes be more numerous than mature neutrophils, akin to the situation in histiocytoid Sweet syndrome. This supports the classification of PP as a “neutrophilic dermatosis.” In biopsies of suspected PP in which mature neutrophils are sparse, the section should be searched for neutrophilic band forms and histiocytoid promyelocytic cells. Immunohistochemical staining with myeloperoxidase helps to identify such cells and may enable a diagnosis of PP even when mature neutrophils are few. Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved.