Evidence of colony suppressor activity and deficiency of hematopoietic growth factors in hairy cell leukemia

The Biology of Classic Hairy Cell Leukemia

Classic hairy cell leukemia (HCL) is a rare mature B-cell malignancy associated with pancytopenia and infectious complications due to progressive infiltration of the bone marrow and spleen. Despite tremendous therapeutic advances achieved with the implementation of purine analogues such as cladribine into clinical practice, the culprit biologic alterations driving this fascinating hematologic disease have long stayed concealed. Nearly 10 years ago, BRAF V600E was finally identified as a key activating mutation detectable in almost all HCL patients and throughout the entire course of the disease. However, additional oncogenic biologic features seem mandatory to enable HCL transformation, an open issue still under active investigation. This review summarizes the current understanding of key pathogenic mechanisms implicated in HCL and discusses major hurdles to overcome in the context of other BRAF-mutated malignancies.

The biology of hairy-cell leukaemia

PURPOSE OF REVIEW

The biology of hairy-cell leukaemia is reviewed, focussing first on the hairy cell itself and then on its interactions with the microenvironment.

RECENT FINDINGS

Hairy cells are highly activated clonal B cells related to memory cells, normally resident in the marginal zone of the spleen. Their activation results from multiple stimuli arising from the microenvironment, autocrine cytokines and the still unknown transforming oncogenic event(s) responsible for the disease. Protein kinase Cepsilon is a central player in the activation process.

SUMMARY

The activation of hairy cells makes them unusually sensitive to interferon and nucleosides. Future important research topics include characterization of the oncogenic events responsible for the disease and for its associated differentiation block.

Clinical manifestations and infectious complications of hairy-cell leukaemia

Hairy-cell leukaemia is an indolent lymphoproliferative malignancy characterized by infiltration of the bone marrow, liver, spleen, and occasionally lymph nodes with a malignant B cell with hair-like cytoplasmic projections. This involvement leads to splenomegaly with secondary consumption of red cells, platelets and neutrophils as well as other complications of an enlarged spleen, including infarction-or-rarely rupture. The common haematological complications of anaemia, neutropenia and thrombocytopenia are due not only to the enlarged spleen but probably also to hairy cells in the bone marrow inducing cytokine-mediated suppression of haematopoiesis. Hepatic involvement, although frequent, only occasionally leads to liver dysfunction. Infections are a major cause of morbidity and mortality in patients with hairy-cell leukaemia, presumably owing to neutropenia and monocytopenia in these patients. The infections seen may be due to unusual pathogens, including Mycobacterium and Listeria. Autoimmune disease, including polyarthitis and vasculitis, occurs frequently and does not correlate with the severity of the disease. Other rare complications include bone involvement, meningitis and ascites. A wide range of secondary malignancies have been reported in patients with hairy-cell leukaemia, but it is still unclear whether the incidence is increased and whether they are related to the disease or treatment.

T-cell dysfunction in hairy cell leukemia: an updated review

Hairy cell leukemia (HCL) is clinically associated with severe T-cell dysfunction. Several new observations have given more insight into the abnormal T-cell responses seen in this disease. T-lymphocytes in the spleen of patients with HCL seem to be abnormally activated. On the other hand, they are non-responsive, possibly as a result of monocytopenia which may lead to inadequate antigen presentation. This, together with the lack of CD28 on T-cells, may cause T-cell dysfunction. Furthermore, there is a very restricted repertoire of the T-cell receptor-beta family, which may also result in non-responsiveness. Otherwise, T-cell clonal excess may be indicative for activated, possibly autoreactive T-cells.