Leukemia & Lymphoma, October 2009; 50(S1): 8–11
The Biology of Hairy Cell Leukemia
JOHN CAWLEY
Division of Haematology, University of Liverpool, UK
Abstract
As in all malignancies, the biology of HCL reflects both the behaviour of the malignant cells (hairy cells) themselves and their
two-way interaction with the microenvironment. However, the tissue interactions of HCs are particularly striking and involve
extensive remodelling of bone marrow, spleen and liver, with relative sparing of lymph nodes. The mechanistic basis of this
remodelling is now largely understood and is described herein. Regarding HCs themselves, they are late clonal B cells which
have often undergone heavy-chain-isotype class switching and whose VH genes are usually mutated. HCs are highly activated
cells in which a number of signalling pathways are constitutively active. This activation determines many of the specific
features of HCs, but its cause remains unknown.
Keywords: Activation, adhesion, HCs, homing, memory cells, tissue remodelling
and CD22). The expression by HCs of SIg of a single
Introduction
light-chain type and the clonal rearrangement of their
When hairy-cell leukemia (HCL) was initially defined immunoglobulin genes confirm the clonal nature of
clearly as a distinct entity in 1958, it was termed the disorder. The frequent expression of SIgG and
leukemic reticuloendotheliosis [1] to reflect the dis- the absence of early B-cell markers indicate that HCs
tinctive pattern of tissue involvement in the disease. are arrested at a late stage of B-cell development.
Subsequently, the entity soon came to be known by its However, the malignant cells lack plasma-cell fea-
present,moredescriptive,andattention-grabbingname tures and show little or no propensity to plasmacy-
‘HCL’.Thisname,ofcourse,reflectsthepresenceinthe toid differentiation either in-vitro or in-vivo.
disease of highly distinctive malignant cells with
numerous projections hairy cells (HCs). These cells
Hairy cells as activated cells
are not only diagnostically important, but also their
behaviorandinteractionswithtissuesdeterminemost,if The malignant cells express several antigens (e.g.
not all, the biological features of HCL. FMC7, CD25, CD72, and CD40L) that are
associated with the activation of B cells and other
cell types. Furthermore, HCs lack antigens which
Hairy cells are often reduced during activation of other cells
(e.g. CD21 and 24). In addition, HCs possess a
Their nature
number of disease-restricted antigens (e.g. CD11c,
HCs are activated, late clonal B cells which may be CD68, CD103, HC2) which probably reflect their
related to memory cells [2,3]. activated nature. Similarly, the specific and the
diagnostic expression by HCs of the enzyme tar-
trate-resistant acid phosphatase (TRAP) is probably
Hairy cells as late clonal B cells
a manifestation of this activation. The cause of the
HCs consistently express a range of B-cell antigens activated phenotype of HCs is still not known, but it
(e.g. surface immunoglobulin (SIg), CD19, CD20, persists during in-vitro culture and is likely to
Correspondence: John Cawley, Head of Division of Haematology & The Ronald Finn Professor of Experimental Medicine, Division of Haematology,
School of Cancer Studies, University of Liverpool, 3rd Floor Duncan Building, Daulby Street, Liverpool, L69 3GA. Tel/Fax: 0151-706-4311.
E-mail: haem@liv.ac.uk
ISSN 1042-8194 print/ISSN 1029-2403 online C211 2009 Informa Healthcare USA, Inc.
DOI: 10.3109/10428190903142208
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