Promyelocytic Differentiation in Infiltrates of Prurigo Pigmentosa: An Analogy to Histiocytoid Sweet Syndrome

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.

Vemurafenib-induced histiocytoid neutrophilic panniculitis simulating myeloid leukaemia cutis

Neutrophilic panniculitis is an infrequent but characteristic adverse event under therapy with BRAF inhibitors (BRAFi). Since the approval of vemurafenib for treatment of metastatic melanoma in 2011, only two cases of neutrophilic panniculitis in malignancies other than melanoma have been published. Histiocytoid infiltrates of immature neutrophils resembling histiocytes or myelocytes have been reported in Sweet's syndrome and rarely in other neutrophilic dermatoses. We describe a novel variant of neutrophilic panniculitis with histiocytoid myeloid cells in an early lesion from a patient treated with vemurafenib in combination with an anti-EGFR (epidermal growth factor receptor) agent for metastatic colon carcinoma, three weeks after initiation of therapy. Recognizing this variant of panniculitis associated to BRAFi can avoid misinterpretation of the atypical subcutaneous infiltrate as myeloid leukaemia cutis.

Idiopathic histiocytoid Sweet syndrome: a case report with clinical and histopathological considerations

BACKGROUND

Histiocytoid Sweet syndrome is characterized by a predominant neutrophilic dermal infiltrate. Usual clinical differential diagnosis includes erythema multiforme, drug eruption, and erythema nodosum. Histiocytoid Sweet syndrome is considered an uncommon histopathological variant of the disease.

METHODS

We evaluated clinical, histopathological, and immunohistochemical findings of a case categorized as idiopathic histiocytoid Sweet syndrome in which clinical-epidemiological data raised the possibilities of Sweet syndrome, leprosy, and drug reaction.

RESULTS

Positive reaction to myeloperoxidase (MPO) in histiocytoid cells of the dermal infiltrate, response to oral corticosteroids, clinical and laboratory investigation, and absence of cutaneous lesions or clinical complaints within 1 year of follow-up are consistent with the diagnosis of idiopathic histiocytoid Sweet syndrome. CD68 (PG-M1) and CD15 positive cells were also present among dermal cells.

CONCLUSIONS

Epidemiological data are relevant while considering a clinical differential diagnosis of Sweet syndrome that can be further expanded, from a histopathological point of view, when dealing with the histiocytoid variant since neutrophils, macrophages, and immature myelomonocytic cells with histiocytoid morphology are present. The significance of the MPO positive mononuclear dermal cells are not completely established.