Therapeutic advances in the treatment of hairy cell leukemia

Hairy cell leukemia (HCL) is a rare chronic lymphoproliferative disorder typified by the presence of circulating monoclonal B-lymphocytes that have prominent cytoplasmic projections, and which display distinctive infiltration patterns in the bone marrow and spleen. In the USA it represents 2–3% of all adult leukaemias. There is a 4:1 male to female predominance with the median age at presentation being 52 years. The clinical presentation often includes a history of repeated infections, symptoms related to low peripheral blood counts, and splenomegaly on physical examination. Circulating hairy cells, which may not be present in large numbers, appear slightly larger than regular lymphocytes and display irregular, ruffled, pale blue cytoplasm (Fig. 1). The morphologic finding on the bone marrow core biopsy consists of a diffuse infiltration by lymphocytes that are spaced wider than expected — the ‘fried-egg’ appearance (Fig. 2). Reticulin staining is increased reflecting the fibrosis, which accounts for the difficulty usually encountered when attempting to aspirate the bone marrow (Fig. 3). Hairy cells exhibit tartrate-resistant acid phosphatase (TRAP) activity although this may also occasionally be seen in other lymphoproliferative disorders. Immunophenotypic analysis of the abnormal cells in HCL reveals CD11c, CD19, CD20, CD22, CD25, CD103, and kappa or lambda light chain positivity [1], and the absence of CD5 and CD21 antigen expression.