Monocytic papulosis: A clonal mature monocyte-derived dendritic cell histiocytopathy of potentially diverse monocyte-derived dendritic cell phenotypes as a reflection of underlying chronic myeloproliferative disease.

Clonal histiocytopathy involving non-Langerhans mature, monocyte-derived dendritic cells is a poorly characterized cutaneous manifestation of chronic myeloproliferative diseases. We describe the clinical and histopathological features of a well-differentiated histiocytopathy composed of mature, CD1a-negative, S100-negative, langerin-negative monocyte-derived dendritic cells, proposing it as a potential dermatologic marker for acute and/or chronic myeloproliferative diseases. This case series includes two cases and a review of eight others from the Weill Cornell Medicine Dermatopathology database from 2016 to 2024. Formalin-fixed tissue underwent immunohistochemical and histologic analyses for monocyte-derived dendritic cell markers. Primarily middle-aged to older men, the patients presented with papular-nodular eruptions associated with chronic myeloproliferative disease, either predating or after diagnosis, except in one case related to acute myeloid leukemia. The majority of cases exhibited a protracted, waxing-waning course. Histologic analysis revealed perivascular and periadnexal well-differentiated monocytic infiltrates with a dendritic cell phenotype expressing CD4, CD14, CD11c, and occasionally CD123, myxovirus resistance protein A, and granzyme, suggesting a myeloid dendritic cell phenotype in select cases. We termed this eruption "monocytic papulosis." The limited sample size and followup require further research to evaluate the prognostic value of this eruption; it does not appear to correlate with an early indicator of aggressive myeloproliferative disease. Monocytic papulosis appears to represent a distinctive dermatologic manifestation associated with clonal acute and/or chronic myeloproliferative diseases.