A distinct subtype of idiopathic myelofibrosis with bone marrow features mimicking hairy cell leukemia: evidence of an autoimmune pathogenesis.

A young female patient presented with severe anemia and myelofibrosis. The spleen was not enlarged on clinical examination and no leukoerythroblastosis or tear-drop poikilocytosis was found. A bone marrow biopsy disclosed severe myelofibrosis and many megakaryocytes, which appeared morphologically normal. A striking feature was a heavy spongy infiltration by large lymphocytic cells, which were tartrate-resistant acid phosphatase negative and without hairy projections. This bone marrow appearance highly resembled the clear cell appearance of the hairy cell infiltrate in hairy cell leukemia. A diagnosis of acute autoimmune myelofibrosis was made. During high-dose glucocorticoid therapy the bone marrow fibrosis completely resolved and there was an improvement of peripheral blood values. The disorder is supposed to feature a distinct clinicopathological entity within the syndrome of idiopathic myelofibrosis. The disease should be considered in the differential diagnosis of patients with myelofibrosing disorders, including acute myelofibrosis and hairy cell leukemia.