Hairy cell leukemia (HCL) is a rare chronic lymphoproliferative disorder characterized by circulating B cells with cytoplasmic projections, pancytopenia, splenomegaly, and a typical flow cytometry pattern. Recently, the BRAF V600E mutation was uniformly identified in one HCL series, which may provide insights into the pathogenic mechanisms. The disease course is usually indolent but inexorably progressive. Patients require treatment when they have significant cytopenia or occasionally recurrent infections from immunocompromise. In the mid-1980s, interferon replaced splenectomy as the initial treatment. A few years later, 2 purine nucleoside analogs, cladribine and pentostatin, showed promising activity in HCL. Complete response rates approached 95% with cladribine given as a single 7-day intravenous infusion. Newer methods of cladribine administration and modified dosing schedules have since been studied. Pentostatin response rates are comparable. We generally prefer cladribine because of its ease of administration, single infusion schema, and favorable toxicity profile. Since the introduction of these drugs, which have never been randomly compared, long-term follow-up studies have confirmed impressive and durable response durations. However, roughly 40% of patients with HCL eventually relapse. In this setting, patients can be re-treated with purine analogs. Rituximab also has a reasonable response rate in relapsed HCL; it can be given as a single agent sequentially after purine nucleosides or concurrently. Immunotoxins have robust responses but remain in development. Targeting the BRAF pathway will be an exciting future area of research. Many patients have minimal residual disease after initial treatment, but the clinical significance of this remains unknown. H airy cell leukemia (HCL) is a rare adult B-cell lymphoid leukemia characterized by pancytopenia, splenomegaly, and absolute monocytopenia. Morphologically, HCL is characterized by circumferential cytoplasmic projections (Figure 1). Bone marrow biopsy will reveal hypercellularity in most cases, with hairy cells having nuclei widely separated by abundant cytoplasm, giving a "fried-egg" appearance ( Figure 2). Classically, tartrate-resistant acid phosphatase activity confirmed the diagnosis of HCL.1 However, immunophenotyping by flow cytometry is now considered standard practice. Hairy cell leukemia is characterized by the B-cell antigens CD19, CD20, and CD22. In addition, they coexpress the surface antigens CD11c, CD25, and CD103. Hairy cells generally lack CD5, CD10, CD21, and CD23. Immunohistochemical stains for DBA44 and annexin A1 can also help confirm the diagnosis.
PATHOGENESISFor decades, the cellular event leading to HCL has evaded scientists and clinical researchers. Recently, 47 patients with HCL in Italy had wholeexome gene sequencing performed on leukemic cells and matched nonleukemic cells. 2 In every patient, the well-known BRAF V600E mutation was identified. This mutation, more commonly known for its presence in melanoma, was a surpr...