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.