Diagnostic application of two-color flow cytometry in 161 cases of hairy cell leukemia

Recent immunophenotypic studies of hairy cell leukemia (HCL) have suggested specific patterns of immunoreactivity that may aid in diagnosis. We studied peripheral blood (PB) from 161 cases of HCL using two-color direct immunofluorescence flow cytometry and an extended panel of antibody combinations. Circulating hairy cells were identified by immunophenotypic features in 92% of the cases and could be detected even when representing < or = 1% of circulating lymphocytes. The 133 cases with > or = 2% detectable hairy cells were analyzed in detail. HCL showed a uniform and unique B-cell phenotype, with each of the following features identified in 99% to 100% of cases: (1) positive staining for B-ly7, coexpressed with CD19; (2) very intense, uniform expression of CD11c, with CD19; (3) moderately intense staining for CD25, with CD19; (4) very intense staining for CD22; (5) moderate to very intense staining for CD20; and (6) moderately intense monoclonal surface Ig. Phenotypic variability existed in expression of CD10 (26%) and CD5 (4%). Based on these features, HCL was easily distinguished from 50 cases of chronic lymphocytic leukemia (CLL). Although CLL exhibited frequent expression of CD11c (74%) and CD25 (68%), the intensity of staining was significantly less than HCL. Furthermore, CLL was uniformly positive for CD5 and showed weak staining for CD20, CD22, and surface Ig. B-ly7 proved to be the most specific marker, reacting with 100% of HCL cases, but absent in all cases of CLL. We conclude that two-color flow cytometry with specific antibody combinations is an efficacious method for characterization and sensitive detection of hairy cells in PB. Application of the phenotypic criteria described should help to increase accuracy in diagnosis of HCL.     

Flow cytometry of peripheral blood and bone marrow cells from patients with hairy cell leukemia: phenotype of hairy cells, lymphocyte subsets and detection of minimal residual disease after treatment

By flow cytometry (FC) and an extensive panel of markers we characterized leukemia cells from the peripheral blood (PB) and bone marrow (BM) of 13 symptomatic patients with hairy cell leukemia (HCL). Hairy cells (HCs) identified in the large cell gate always expressed B-cell markers - CD19, CD20, CD22, HLA-DR, and 'HCL-restricted' markers - CD22+CD11c, CD25 and CD103. Other markers, not followed regularly, were occasionally expressed, such as CD34, CD38, CD71, CD15, CD10 and kappa/lambda light chains. Furthermore, in one patient with suspect but not proved HCL in PB or BM, neither morphologically nor immunologically, we confirmed the diagnosis of HCL. Only the immunophenotyping of splenic cells after splenectomy confirmed HCL diagnosis. Flow cytometry was repeated at 3-5 month intervals, after treatment with 2-Chlorodeoxyadenosine (CdA) or less frequently alpha-interferon (IFN). We investigated serially lymphocyte subsets after treatment and we found profound and persistent CD4+ lymphopenia in majority of studied patients after CdA treatment. Simultaneously we investigated the value of FC to detect minimal residual disease (MRD) and to establish, whether MRD+ could predict relapse. Detection of MRD in our series predicted hematological relapse only in one case with persistent MRD+, in majority of cases with occasionally found MRD+ phenotype, did not. Using quantitative immunophenotyping we observed significantly higher values of molecule numbers of hairy cell B-cell markers, comparing to B-cells in nonleukemic gate of the same sample. Our study showed 1) the diagnostic value of FC in management of HCL patients, 2) long-lasting response in the majority of patients after CdA, 3) a profound and persistent CD4+ lymphopenia in CdA treated patients, 4) some correlation between persistent MRD staining and hematological relapse, and 5) further, till now not described activated feature of HCs, given by the increased values of molecular numbers (molecules of equivalent soluble fluoresceine - MESF) in B-cell antigens of HCL.     

Surface immunoglobulins on hairy cells of 55 patients with hairy cell leukemia

Surface immunoglobulins (SIg) were determined on peripheral blood samples from 55 patients with hairy cell leukemia (HCL) and on hairy cells from spleen preparations of 14 of these 55 patients. The patterns of SIg for HCL was compared to the patterns on peripheral blood leukemic cells from 39 patients with chronic lymphocytic leukemia (CLL) and 15 patients with poorly differentiated lymphocytic (PDL) lymphoma. Of the 55 HCL patients, 42 could be scored for individual heavy and light chains; 16 had only IgG, 14 had two or three heavy chains, 7 had only IgD, and 5 cases had no SIg and were E-rosette negative. This pattern was different from the B-cell pattern in CLL and PDL where there were few cases of IgG alone (5%) and many cases of IgM alone (50%). Surface marker profile did not correlate with survival in any of the sub-groups tested. HCL appears to be a B-cell lymphoproliferative disease in greater than 90% of cases; many combinations of heavy chains with only a single light chain can be demonstrated.     

Homeostatic chemokines drive migration of malignant B cells in patients with non-Hodgkin lymphomas

This study investigated the role of several chemokines and their receptors on malignant B lymphocytes recovered from 13 patients with chronic lymphocytic leukemia (CLL), 9 with hairy cell leukemia (HCL), 5 with mantle cell lymphoma (MCL), 5 with marginal zone B-cell lymphoma (MZL), 6 with small lymphocytic lymphoma (SLL), and 5 with follicular cell lymphoma (FCL). Flow cytometry analysis demonstrated that CXCR4 and CXCR5 were expressed on all malignant and normal B cells. Considering CC receptors, CCR1 was expressed in 70% of patients with CLL and 40% of those with HCL but was lacking in patients with MCL, MZL, SLL, and normal B cells. CCR2 showed a heterogeneous pattern of expression. CCR3 was found in almost all patients with CLL and in the majority of those with HCL, whereas it was usually lacking in patients with MZL and SLL and in healthy subjects. CCR5 was expressed in patients with HCL and MCL. Migration assays showed that different chemokines, mainly CXCL12 and CXCL13, are able to trigger migration of malignant B lymphocytes. Some of these chemokines induce calcium mobilization. These data indicate that different patterns of chemokine receptor expression identify different malignant B-cell subsets and that these receptors are functional and might play a role in malignant B-cell circulation.     

Immunological and molecular biological identification of a true case of T-hairy cell leukaemia

A hairy cell leukaemia (HCL) patient is presented in whom the peripheral blood mononuclear cells (PBMCs) carried suppressor T-cell markers (CD3+, CD2+, CD8+/CD4-, CD38+). Analysis of genomic DNA of PBMNC showed the presence of a monoclonal population of T cells, the T-cell receptor (TCR) beta-chain gene being rearranged on both alleles (DR/DR), while the immunoglobulin (Ig) heavy chain-genes were in germline configuration. The neoplastic cells were found to react with the monoclonal antibody RAB-1 - originally described as belonging to the B lineage-restricted monoclonal antibodies - and to carry RAB-1/CD-8 in a double marker assay. Natural killer activity of PBMNCs against K562 target cells was severely reduced, while the cells were found to exert strong antibody-dependent cellular cytotoxicity.     

Monoclonal antibody to the interferon-inducible protein Leu-13 triggers aggregation and inhibits proliferation of leukemic B cells

Interferon (IFN)-alpha inhibits DNA synthesis stimulated by low molecular weight B-cell growth factor (BCGF) in hairy cells in vitro, suggesting that the therapeutic efficacy of IFN-alpha in hairy cell leukemia (HCL) involves growth inhibition of malignant B cells. Evidence that the 16-Kd cell surface protein Leu-13 mediates an antiproliferative signal in T lymphocytes and is IFN-inducible in endothelial cells prompted us to examine the expression and functional role of this molecule in leukemic B cells. Leu-13 density, determined by flow cytometry, was upregulated in vitro and in vivo by IFN-alpha on malignant B cells from patients with HCL, chronic lymphocytic leukemia, and prolymphocytic leukemia. Monoclonal anti-Leu-13 triggered homotypic aggregation of leukemic B cells via an adhesion pathway that was not inhibited by antibodies to leukocyte function associated antigen-1 (LFA-1) or intercellular adhesion molecule-1 (ICAM-1). Moreover, anti-Leu-13 potentiated the inhibitory effects of IFN-alpha on BCGF-stimulated DNA synthesis, assessed by [3H]-thymidine and [3H]-deoxyadenosine incorporation into DNA. These results indicate that Leu-13 is part of a novel IFN-inducible signaling pathway which may modify the growth and adhesive properties of leukemic B cells under physiologic or therapeutic conditions.     

Involvement of CD44-hyaluronan interaction in malignant cell homing and fibronectin synthesis in hairy cell leukemia

The tissue homing of malignant hematic cells has both diagnostic and pathogenetic importance. Although such homing is incompletely understood, it generally involves cell adhesion and migration mediated by a number of adhesion receptors and cytokines. In this article, the potential importance of hyaluronan (HA) is examined for the tissue homing of hairy cells (HCs) in hairy cell leukemia (HCL). It is shown that HCs readily adhere to, and spontaneously move on, HA-coated surfaces using CD44. This indicates that activated CD44 and spontaneous movement on HA form part of the intrinsically activated phenotype of HCs. Interleukin-8 (IL-8) inhibited HC movement on HA, and this cell arrest was accompanied by increased actin polymerization and a more pronounced association of CD44 with the cytoskeleton. All of these findings are in sharp contrast to our previous observations with chronic lymphocytic leukemia cells, which are nonmotile on HA, but in response to IL-8 become polarized and motile using the receptor for HA-mediated motility rather than their apparently inactive CD44. Immunohistochemical examination of HCL tissues showed the ubiquitous presence of IL-8 and the prominence of HA in bone marrow stroma and hepatic portal tracts. This suggests that CD44-HA interactions are important in HC homing to these sites, but not to splenic red pulp or hepatic sinusoids, where HA is largely absent. Moreover, engagement of CD44 on HCs stimulates fibronectin synthesis, an observation that is likely to be relevant to the restriction of fibrosis in the disease to HC-infiltrated areas containing HA.     

Phorbol ester induces abnormal chronic lymphocytic leukemia cells to express features of hairy cell leukemia

We have investigated the relationship between chronic lymphocytic leukemia (CLL), hairy cell leukemia (HCL), and different normal B cell subsets: Mrbc+, T1+ and slgM+ tonsil cells; germinal center; mantle zone; and peripheral blood B lymphocytes. Both malignant and normal cells were incubated in vitro with the phorbol ester 12-O-tetradecanoyl- phorbol-13-acetate (TPA) for 72 hours and the morphology, cytochemical profile, and surface markers were evaluated. The results show that CLL cells TPA-induced become indistinguishable from HCL by four independent criteria: the morphology; the cytoplasmic tartrate resistant acid phosphatase (TRAP) enzyme activity; the membrane positivity with anti- Leu M5 (SHCL3); and anti-Tac monoclonal antibodies which, in the uninduced state, react only with HCL. The features of TRAP and Tac positivity are also expressed (though in variable degree) by different normal B cell populations activated with TPA or pokeweed mitogen (PWM). It is concluded that HCL might represent an aberrantly activated variant of CLL (or of a CLL-related disorder).     

An antigen common to chronic lymphocytic and hairy cell leukemia cells not shared by normal lymphocytes or by other leukemic cells

Rabbit xenoantisera and mouse monoclonal antibodies prepared against human chronic lymphocytic leukemia (CLL) B cells were found to react against a single polypeptide chain with a mol wt of 69 kd found on leukemic cells of all CLL (N = 40) and B type hairy cell leukemia (HCL) patients (N = 9) examined. This common CLL-associated antigen (cCLLa) was not detectable on circulating T or B lymphocytes, thymocytes, lymph node and splenic lymphocytes, or bone marrow leukocytes from normal persons. In addition, the cCLLa was not detectable on cultured T or B lymphoblastoid cell lines or on malignant cells from other forms of lymphocytic or myelocytic leukemia. Non-Hodgkin's lymphoma cells also failed to express the antigen. Autologous cultured lymphoblastoid cell lines were established from residual normal B cells from a CLL patient whose cells were used to generate one of the antisera. Absorption of the antibody with these cultured polyclonal B cells did not affect the anti-CLL activity, which suggests that the cCLLa is not HLA related. Unlike the T cell differentiation complex gp65-71, the cCLLa was not expressed on fetal or cord blood lymphocytes or on mitogen-stimulated normal lymphocytes and was distinct from the antigen recognized by the LEU-1 antibody in spite of their similar mol wt. The cCLLa was also determined to be unrelated to the human T cell leukemia lymphoma virus (HTLV-1). One of the monoclonal antibodies generated against the cCLLa was a complement binding IgG which exhibited highly selective cytotoxic activity against 100% of cells bearing the cCLLa. Such an antibody might prove clinically useful in early diagnosis and treatment of CLL and HCL.     

Hairy cell leukemia and other "hairy-cell" lymphoproliferative disorders (LPD-HC): the significance of morphologic, histologic and immunologic studies

In this report the clinical, morphologic, histologic and immunologic findings of 41 patients with hairy lymphoid cells in peripheral blood and/or bone marrow are analyzed. In 27 patients the diagnosis of hairy cell leukemia was established. 14 patients had other variants of lymphoproliferative disorders: malignant lymphoma with hairy cells--7, chronic lymphocytic leukemia with hairy cells--5, and T cell lymphoproliferative disorder with hairy cells--2 patients, respectively. Several variants of malignant lymphoma with hairy cells were defined: lymphocytic, centrocytic and lymphoplasmacytic. The importance of combined use of bone marrow biopsy and immunophenotyping for the correct diagnosis of hairy cell leukemia and other "hairy-cell" lymphoproliferative disorders is stressed. The obtained data suggest relationship between characteristic clinical manifestation (isolated splenomegaly), presence of hairy cells and CD11c-antigen expression.     

Reactivity of the monoclonal antibody RFB1 in chronic B-cell leukaemias

The monoclonal antibody RFB1, which reacts with early haemopoietic precursors in human bone marrow and peripheral T-lymphocytes, but not with pre-B cells and mature peripheral blood and marrow B lymphocytes, was tested in blood from 60 patients with chronic B cell leukaemias. All 37 cases of B chronic lymphocytic leukaemia (B-CLL) and 9 of hairy cell leukaemia (HCL) were positive. The antibody showed particularly strong reaction in HCL. On the other hand, RFB1 did not react with peripheral blood and B lymphocytes from 9 patients with non-Hodgkin lymphoma (NHL) in leukaemic phase and was negative or weakly expressed in 5 with prolymphocytic leukaemia (B-PLL). These findings may be significant for investigations on the cell origin of B-CLL and HCL. The reactivity of RFB1 was different from that of two anti-T1 monoclonal antibodies. The combined use of these reagents gave distinct patterns in B-CLL (RFB1+, T1+), HCL (RFB1++, T1-) and NHL and B-PLL (RFB1-/±, T1-/+), suggesting they may be of value in the differential diagnosis of these B-lymphoproliferative disorders.     

Hairy cell identification by immunohistochemistry of tartrate-resistant acid phosphatase

Tartrate-resistant acid phosphatase (TRAcP) has been an indispensible marker for hairy cell leukemia (HCL) for over two decades. However, the traditional TRAcP cytochemical stain cannot be performed effectively on sections of paraffin-embedded tissues that are important resources for histopathologic evaluation in diagnosis and treatment of HCL. Wide variation in expression of TRAcP activity by hairy cells (HCs) within and among patients is an interesting biologic phenomenon that has not been explained and can cause some diagnostic uncertainty as well. To solve the problem of staining TRAcP in paraffin sections and to begin to address the questions of variable TRAcP expression in HCL, we developed a monoclonal antibody to TRAcP, 9C5, for immunohistochemical identification of HCs. In smears of blood and bone marrow, immunocytochemistry of TRAcP using 9C5 was as specific but slightly less sensitive than direct cytochemical staining of enzymatic activity. In paraffin sections of spleen and bone marrow from HCL patients, immunohistochemistry with 9C5 stained the HCs with high sensitivity and specificity and clearly showed the characteristic diffuse infiltration by HCs. Other cells noted to stain strongly with 9C5 were occasional macrophages in bone marrow smears and osteoclasts and occasional tissue macrophages in paraffin sections. These are cells known to express abundant TRAcP activity. Immunohistochemistry with anti-TRAcP monoclonal antibody 9C5 may have utility as an added option in the diagnosis of HCL, as a means to evaluate residual disease in HCL patients undergoing new treatments, and as a way to address questions regarding variable expression of TRAcP activity by HCs within and among patients with HCL. Also, 9C5 has potential as a reagent for the immunoassay of bone-derived serum TRAcP in patients with certain bone diseases and cancers with bone metastasis.     

Selection of a panel of monoclonal antibodies for monitoring residual disease in peripheral blood and bone marrow of interferon-treated hairy cell leukaemia patients

A panel of monoclonal antibodies (mAbs) directed against B-cell and hairy cell leukaemia (HCL)-associated antigens was used to identify residual hairy cells in the peripheral blood and/or bone marrow samples from 20 patients with HCL, following treatment with interferon-alpha (IFN-alpha) or interferon-beta (IFN-beta). In all cases, hairy cells retained their characteristic phenotype, e.g. positivity for CD22, CD11c, CD25, CD32, and the HCL-associated trimeric protein (t-GP) recognized by the mAbs HML-1, B-ly7, LF61 and Ber-Act8. The most specific marker for identifying a small percentage of hairy cells in peripheral blood cytospins, was t-GP. In alkaline phosphatase/anti alkaline phosphatase (APAAP) stained preparations, t-GP+ hairy cells (provided with large cytoplasm and hairy surface) could be usually distinguished from t-GP+ normal lymphocytes (small-sized cells with smooth surface). In doubtful cases the percentage of residual hairy cells could exactly be estimated by double immunofluorescence staining for CD22 (B-cell marker) and t-GP. The rationale of the test is based on the finding that the small percentage (about 1%) of t-GP+ lymphocytes circulating in the peripheral blood of normal individuals are T-cells of the CD8 subset and not B-cells. The best markers for identifying residual hairy cells in routine bone marrow biopsies were CD45RA (mAb 4KB5) and CD20 (mAb L26). Immunohistological labelling was superior to morphological examination in picking up scattered hairy cells in bone marrow biopsies showing either severe hypoplasia or exuberant hyperplasia of normal haemopoietic series.     

Detection of hairy cell leukaemia in blood and bone marrow using multidimensional flow cytometry with CD45-PECy5 and SS gating

CD45 and right angle light scatter (SS) gating are used commonly in clinical flow cytometry to differentiate cells of various lineages (Stelzer et al., 1993). We have used CD45-PECy5 (Clone J33) since 1998 and have noticed that malignant lymphoid cells such as hairy cells can form distinct populations. Previous studies indicate that hairy cells reside where normal monocytes are usually found in CD45/SS scatter plots (Wells et al., 1998). We studied six patients with hairy cell leukaemia (HCL) and found that hairy cells have a higher CD45 mean cell fluorescence than normal lymphocytes and monocytes. Two of the six patients presented with mild unexplained cytopenias, without the usual clinical, morphological and cytochemical findings. In both cases, CD45/SS gating of bone marrow cells showed a small population with strong expression of CD45. The presence of hairy cells was confirmed using a panel of monoclonal antibodies. In one patient with HCL variant, CD45 expression was indistinguishable from that of normal lymphocytes. We conclude that CD45-PECy5 (Clone J33) is useful for screening peripheral blood and bone marrow and for the detection of HCL without obvious morphological involvement.     

Detection of hairy cell leukaemia in blood and bone marrow using multidimensional flow cytometry with CD45‐PECy5 and SS gating

CD45 and right angle light scatter (SS) gating are used commonly in clinical flow cytometry to differentiate cells of various lineages ( 10 ). We have used CD45‐PECy5 (Clone J33) since 1998 and have noticed that malignant lymphoid cells such as hairy cells can form distinct populations. Previous studies indicate that hairy cells reside where normal monocytes are usually found in CD45/SS scatter plots ( 14 ). We studied six patients with hairy cell leukaemia (HCL) and found that hairy cells have a higher CD45 mean cell fluorescence than normal lymphocytes and monocytes. Two of the six patients presented with mild unexplained cytopenias, without the usual clinical, morphological and cytochemical findings. In both cases, CD45/SS gating of bone marrow cells showed a small population with strong expression of CD45. The presence of hairy cells was confirmed using a panel of monoclonal antibodies. In one patient with HCL variant, CD45 expression was indistinguishable from that of normal lymphocytes. We conclude that CD45‐PECy5 (Clone J33) is useful for screening peripheral blood and bone marrow and for the detection of HCL without obvious morphological involvement.     

The degree of bone marrow infiltration in patients with hairy cell leukemia treated with splenectomy compatible with long-term hematological remission

To determine the degree of bone marrow infiltration with hairy cells which is compatible with long-term hematological remission in patients treated with splenectomy, we have investigated 7 patients surviving in hematological remission 61 to 255 months (median 184 months) after splenectomy. As hematological remission has been considered absence of hairy cells (HCs) in the peripheral blood, normalization of peripheral blood cell counts (hemoglobin 120 g/l, white cell count 4.0 x 10(9)/l, absolute granulocyte count 1.5 x 10(9)/l, platelet count 100 x 10(9)/l) and absence of lymfadenopathy and any other activity of the disease. For detection of HCs a very sensitive method of immunostaining with monoclonal antibody DBA.44 in our own modification has been used. Low values of sIL-2R which is considered to be a non invasive marker of tumor burden and activity in HCL were in agreement with the opinion that the bone marrow was the only locality of tumor involvement in splenectomized patients. Infiltration up to 20% with HCs (range 4% to 20%, median 10%) was found to be compatible with long-term hematological remission and long-term overall survival. Patient (No 1) with 30% infiltration of bone marrow with HCs, still normal peripheral blood cell counts, but a very high level of sIL-2R represents extraordinary finding which has been discussed in details.     

Hairy cell interactions with extracellular matrix: expression of specific integrin receptors and their role in the cell's response to specific adhesive proteins

Integrin/extracellular-matrix interactions are central to the migration, localization, and subsequent function of lymphocytes within tissues. In hairy cell leukemia (HCL) the malignant cells display a highly characteristic tissue distribution in which interactions with extracellular matrix (ECM) are often prominent. Therefore, we used HCL as a model in which to investigate the poorly understood integrin/ECM interactions that underlie the migratory behavior of malignant B lymphocytes. Using a combined approach involving immunocytochemistry, flow cytometry, and immunoprecipitation analysis, hairy cells (HCs) were shown to have a consistent and distinctive phenotype (mainly alpha 4 beta 1, alpha 5 beta 1, alpha v beta 1, and alpha v beta 3). Furthermore, functional studies utilising adhesion assays, time-lapse video-microscopy and image analysis showed that the HCs displayed very specific adhesive behaviour in response to relevant adhesive protein ligands. HCs were able to adhere to different extents on all the adhesive proteins examined, but, on laminin and collagen, binding was weak with little cytoplasmic spreading. In contrast, the cells showed strong adhesion both to fibronectin (FN) and to vitronectin (VN). On FN, the cells spread extensively with nonpolarized cytoplasmic projections, whereas on VN cytoplasmic projections were markedly polarized. This polarized morphology was shown to reflect cell motility. Investigation of the role of individual integrin receptors in the cell movement response suggested that alpha v beta 3 is the major integrin responsible for this motile behavior. These results are discussed in relation to the limited previous data on leukemic and activated B-cell integrins, and we suggest that the HC integrins play a significant role in the characteristic behavior of HCs within tissues.     

The diagnosis of hairy cell leukemia can be established by flow cytometric analysis of peripheral blood, even in patients with low levels of circulating malignant cells.

The diagnosis of hairy cell leukemia (HCL) has traditionally been based on microscopic means. Immunophenotypic analysis of peripheral blood by flow cytometry is not widely recognized as a method for diagnosing HCL, perhaps due to the expectation of low yield of neoplastic cells in patients who are characteristically leukopenic. The abnormal coexpression of CD103, CD25, and intense CD11c and CD20 on monotypic, slightly large B-lymphocytes has previously been shown to be highly characteristic of HCL. We wished to determine if this pattern was valuable in the diagnosis of HCL in leukopenic patients with low levels of neoplastic cells in the peripheral blood. The abnormal immunophenotype above was observed in 25 peripheral blood specimens from patients with unexplained cytopenias or suspected lymphoproliferative processes. Ten of the 25 blood samples exhibited this abnormal phenotype in less than 5% of circulating leukocytes (ranging from <1% to 4%). All 10 patients had other manifestations of HCL, including cytopenias (mean white blood cell count, 1.8 x 10(3)/mm(3); hemoglobin, 11.0 gm/dl; platelets, 74 x 10(3)/mm(3)), splenomegaly, and typical bone marrow morphologic changes. Eight of the 10 patients achieved an excellent response to one course of 2-CDA, with significant improvement of cytopenias (mean white blood cell count: 5.3 x 10(3)/mm(3); hemoglobin: 14.4 g/dl; platelets: 181 x 10(3)/mm(3)) and regression of splenomegaly. One patient had a partial response to alpha interferon and a subsequent complete response to 2-CDA, and one died during treatment. In conclusion, flow cytometric immunophenotyping of peripheral blood is capable of detecting low levels of circulating malignant cells in HCL, even in leukopenic patients. As such, it can be a very useful, non-invasive tool in the diagnosis of this disorder.     

Absence of surface CD27 distinguishes hairy cell leukemia from other leukemic B-cell malignancies

Surface expression of CD27 was evaluated in 75 mature leukemic B-cell neoplasms. All cases other than hairy cell leukemia (HCL) expressed CD27. Intensity was significantly higher in chronic lymphocytic leukemia. Lack of CD27 in 17/17 HCL contrasted with expression of this marker in 5/5 splenic lymphomas with villous lymphocytes. Lack of CD27 is a new distinctive feature of HCL among B-cell malignancies.