Cutaneous S100-negative, CD1a-positive histiocytosis successfully treated with combination therapy of oral...

S100-negative, CD1a-positive histiocytosis is a rare histiocytic disorder characterized by proliferation of histiocytic cells possessing a phenotype of epidermal Langerhans cells except for the lack of S100 expression and Birbeck granules. We report the case of a Japanese man suffering from S100-negative, CD1a-positive histiocytosis. The patient showed numerous smooth erythematous 5-10-mm papules/nodules on most of his body. The key histopathological feature was the presence of dermal infiltrates of non-epidermotropic S100-negative CD1a-positive mononuclear cells. No systemic involvement was detected. Initially bath-psoralen plus ultraviolet A therapy was effective, but the lesions became recalcitrant to this treatment. Methylprednisolone pulse therapy followed by low-dose methotrexate (up to 30 mg/week) in combination with prednisolone (15 mg/day) effectively controlled the skin lesions.