Hairy cell leukaemia, negative for conventional cell markers, diagnosed using antibodies to Annexin A1 and T-bet

Abstract

An 88-year-old male presented with mild neutropenia (1AE3 and 1AE7 · 10/l) and thrombocytopenia (85 and 100 · 10/l). Haemoglobin concentration and lymphocyte count were normal whilst monocytopenia was noted retrospectively. Clinical examination was unremarkable although radiology showed an enlarged spleen. A diagnosis of hairy cell leukaemia (HCL) was considered. A bone marrow aspirate showed increased numbers of lymphocytes with hairy projections (top left). Flow cytometry analysis of the sample was positive for CD10 and CD20 but negative for CD5, CD23, CD25, CD103 and CD138. A trephine biopsy section (top right) showed dense infiltrates of small lymphocytes with hairy cell features. The cells expressed CD20, CD10, bcl-2 and IgD but were negative for CD5, CD23, tartrate-resistant acid phosphatase, CD72 (DBA.44) and CD138 with no immunoglobulin lightchain predominance. The combination of typical morphology but atypical immunostaining prompted further tests including use of antibodies to annexin A1 and T-bet (nuclear staining) (bottom left and right). The malignant cells were positive for both these markers. The blood count, distribution of lymphocytic infiltrate in the trephine biopsy section and the special immunostaining confirmed the diagnosis of HCL. Immunostaining for anti-annexin A1 is considered a highly sensitive and specific assay for the diagnosis of HCL. The aberrant expression of annexin A1 in HCL is thought to be due to the neoplastic transformation of a memory B cell accounting for this specificity. However annexin A1 is strongly expressed in normal haematopoietic cells, making it unsuitable for a reliable recognition of minimal infiltrates of hairy cells. Minimal hairy cell infiltration can also mimic interstitially distributed infiltrates of reactive B cells and other small B-cell lymphomas. T-bet, a T-box transcription factor, is expressed mainly in T cells and also in the cells of some B-cell neoplasms, but strongly and consistently expressed in all hairy cells. In summary, this case highlights some interesting features. It suggests bone marrow morphology is still a key feature in the diagnostic work up for lymphoproliferative disorders. Special staining should be considered if the morphology fits in with the clinical picture even if the conventional cell markers are negative. Annexin A1 and T-bet antibody stains may be useful in difficult cases of HCL diagnosis.