Histiocytoid Sweet syndrome is infiltrated predominantly by M2-like macrophages.

BACKGROUND: Histiocytoid Sweet syndrome (HSS) is a rare variant of Sweet syndrome (SS). The nature of histiocytoid cells is still uncertain. OBJECTIVE: We sought to offer a comprehensive overview on clinical features of HSS and further information on immunohistochemical phenotype of the infiltrate. METHODS: The clinical, histologic, and immunohistochemical features of 12 of our patients with HSS and all cases retrieved through a PubMed search were analyzed. RESULTS: Lesions consisted of erythematous-violaceous papules and plaques, randomly distributed mostly on the trunk and the limbs. Three patients had myelodysplastic syndrome and 1 had a monoclonal gammopathy. The infiltrate was mainly composed of CD68(+)CD163(+)myeloperoxidase(+)myeloid cell nuclear differentiation antigen(+)CD117(-)CD15(-)CD34(-), a phenotype suggestive of M2-like macrophages. A few mature neutrophils and lymphocytes were also present. Review of all HSS cases showed no sex predominance and no extracutaneous infiltrates; inconstant presence of fever and blood neutrophilia; association with hematologic or solid neoplasms (26%), autoimmune conditions (12%), and infectious diseases (10%); and good response to steroid treatment, with rare relapses or recurrences. LIMITATIONS: The study includes a limited case series. The pathogenesis of the disease remains to be clarified. CONCLUSIONS: HSS lesions are infiltrated mostly by M2-like macrophages. The clinical features present more similarities than differences with SS.