Hairy cell leukemia (HCL) is an uncommon form of adult chronic B-cell leukemia. Whereas the cell of origin is uncertain, at diagnosis the characteristic leukemic cells are found in the marrow, the blood, and the spleen. Patients present with fatigue and infections, and many have splenomegaly. They are often pancytopenic or may have isolated cytopenias and usually have monocytopenia. The leukemic cell in classic hairy cell leukemia (HCL-c) has a characteristic immunophenotypic profile (CD11c+, CD19+, CD20+bright, CD22+, CD25+, CD103+, and CD123+ and CD27−). A variant of hairy cell leukemia (HCL-v), which occurs less frequently, has been identified as a separate entity. A genetic mutation, BRAF V600E, has been identified in the majority of patients with the classic form of this disease but is not present in the variant. This mutation is also present in the hematopoietic stem cells of patients with HCL-c.
HCL is characterized by impaired marrow function and immunity, leading to a high incidence of infectious complications. Both pentostatin and cladribine are effective in achieving durable complete remissions. Long-term studies demonstrate prolonged survival of patients, but the disease-free survival curves do not plateau, suggesting that the disease is not cured but subject to relapse. Survival has been markedly improved with the introduction of purine nucleoside analogues and is estimated to be 90% at five-year follow-up. When patients relapse, high-quality remissions can be achieved with salvage therapy.
Acronyms and Abbreviations
HCL, hairy cell leukemia; HCL-c, classic HCL; HCL-v, variant of HCL; IHC, immunohistochemical stains; IL-2, interleukin-2; MRD, minimal residual disease; SEER, Surveillance, Epidemiology, and End Results Program; SIR, standardized incidence ratio; TGF, transforming growth factor; TRAP, tartrate-resistant acid phosphatase; WHO, World Health Organization.
DEFINITION AND HISTORY
Cases of malignant diseases involving marrow that probably represented examples of hairy cell leukemia (HCL) were reported through the first half of the 20th century and designated by such terms as “lymphoid fibrosis.” They had features characteristic of HCL, including marrow replacement by mononuclear cells, marrow fibrosis, splenomegaly, anemia, and thrombocytopenia. In 1958, Bouroncle and coworkers described this constellation of findings in a group of patients.1 At that time, there was no means to characterize the immunophenotype of malignant lymphoid cells, and they called the disease “leukemic reticuloendotheliosis.” In 1966, Schrek and Donnelly, described the distinctive feature of cytoplasmic projections that were evident on the blood cells in two cases of this disorder. They called these “hairy” cells.2 The designation HCL has become universally accepted as the name of this lymphocytic neoplasm characterized by infiltration of the marrow by malignant B lymphocytes of a specific immunophenotype (see “Laboratory Features” later), often accompanied by reticular fibrosis, splenomegaly, anemia, thrombocytopenia, neutropenia, monocytopenia, and usually pancytopenia. Occasionally, there is an elevated total white cell count because of the abundance of malignant B lymphocytes in the blood. Splenectomy was the sole therapeutic approach until the early observations of responses ...