Cutaneous S100‐negative, CD 1a‐positive histiocytosis successfully treated with combination therapy of oral methotrexate and corticosteroid

S 100‐negative, CD 1a‐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 S 100 expression and Birbeck granules. We report the case of a Japanese man suffering from S 100‐negative, CD 1a‐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 S 100‐negative CD 1a‐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.