Hairy cell leukaemia variant is a very rare B-lineage lymphoproliferative disorder characterized by a clonal proliferation of B cells that morphologically resemble hairy cells but have a prominent nucleolus, resembling that of a prolymphocyte [1-3]. This disease occurs in the elderly without any male predominance.
Splenomegaly is characteristic while lymphadenopathy is usually minor.
Haematological and pathological features
In contrast to hairy cell leukaemia, the white cell count is moderately elevated (5-300, median around 80 × 109/l) [4-7]. There may be mild anaemia and thrombocytopenia. The monocyte count is preserved. The neoplastic cells have moderately plentiful cytoplasm with irregular margins and a round nucleus with a large prominent nucleolus and some chromatin condensation (Figure 12.1). There may be some binucleated cells. The tartrate-resistant acid phosphatase reaction is usually negative. Bone marrow infiltration is usually interstitial and often intrasinusoidal  (Figure 12.2). Cells may be spaced, as in hairy cell leukaemia, but this feature is not so consistently present. Reticulin deposition is increased but not to the extent that is usual in hairy cell leukaemia so that it is usually possible to aspirate bone marrow. Splenic infiltration is preferentially in the red pulp and may be indistinguishable from that of hairy cell leukaemia.
Peripheral blood film of a patient with hairy cell leukaemia variant showing characteristic cells. Romanowsky, x 100 objective.
Section of a trephine biopsy specimen showing, in the centre of the photograph, intra-sinusoidal infiltration. H&E, x 100 objective.
The immunophenotype is useful in distinguishing these cases from hairy cell leukaemia. CD11c is positive in the majority of patients and CD103 is positive in approaching two-thirds but CD25 and CD123 are usually negative [9, 10] (Figure 12.3). If these four markers are used, cases of hairy cell variant score 0-2 whereas cases of hairy cell leukaemia score 3-4. In contrast to most B-lineage lymphoproliferative disorders, CD79b is more often negative than positive. On immunohistochemistry there is expression of B-lineage markers such as CD20 (Figure 12.4) and DBA44 (Figure 12.5).
Flow cytometry immunophenotyping in a patient with hairy cell leukaemia variant. Sideways scatter and CD19 have been used for gating. In addition to CD19, the leukaemic cells express CD20, CD22 (strong), CD79b, FMC7 and strong κ light chain. There is partial expression of CD11c. The case is unusual in also expressing CD23. There is no expression of CD5, CD25, CD103 or CD123. The CLL score and the hairy cell leukaemia score are both 1. With thanks to Mr Ricardo Morilla.