Hairy Cell Leukemia - Pathophysiology

Pathophysiology

Pancytopenia in HCL is caused primarily by marrow failure and splenomegaly. Bone marrow failure is caused by the accumulation of hairy cells and reticulin fibrosis in the bone marrow, as well as by the detrimental effects of dysregulated cytokine production. Splenomegaly reduces blood counts through sequestration, marginalization, and destruction of healthy blood cells inside the spleen.

Hairy cells are nearly mature B cells, which are activated clonal cells with signs of VH gene differentiation. They may be related to pre-plasma marginal zone B cells or memory cells.

Cytokine production is disturbed in HCL. Hairy cells produce and thrive on TNF-alpha. This cytokine also suppresses normal production of healthy blood cells in the bone marrow.

Unlike healthy B cells, hairy cells express and secrete an immune system protein called Interleukin-2 receptor (IL-2R). In HCL-V, only part of this receptor is expressed. As a result, disease status can be monitored by measuring changes in the amount of IL-2R in the blood serum. The level increases as hairy cells proliferate, and decreases when they are killed. Although uncommonly used in North America and northern Europe, this test correlates better with disease status and predicts relapse more accurately than any other test.

Hairy cells respond to normal production of some cytokines by T cells with increased growth. Treatment with Interferon-alpha suppresses the production of this pro-growth cytokine from T cells. A low level of T cells, which is commonly seen after treatment with cladribine or pentostatin, and the consequent reduction of these cytokines, is also associated with reduced levels of hairy cells.

In June 2011, E Tiacci et al discovered that 100% of hairy-cell leukaemia samples analysed had the oncogenic BRAF mutation V600E, and proposed that this is the disease's driver mutation. Until this point, only a few genomic imbalances had been found in the hairy cells, such as trisomy 5 had been found. The expression of genes is also dysregulated in a complex and specific pattern. The cells underexpress 3p24, 3p21, 3q13.3-q22, 4p16, 11q23, 14q22-q24, 15q21-q22, 15q24-q25, and 17q22-q24 and overexpress 13q31 and Xq13.3-q21. It has not yet been demonstrated that any of these changes have any practical significance to the patient.