Quantitative expression of thrombopoietin receptor on leukaemia cells from patients with acute myeloid leukaemia and acute lymphoblastic leukaemia

A newly devised flow cytometric antibody binding assay helps evaluation of dithiothreitol treatment for the inactivation of CD38 on red blood cells

BACKGROUND AND OBJECTIVES

Anti-CD38 monoclonal antibodies, including daratumumab and isatuximab, often interfere with pretransfusion testing. Dithiothreitol (DTT) treatment of red blood cells (RBCs) negates this interference. However, the optimum DTT concentration and treatment time have not been well defined. Here, we quantified CD38 on RBCs before and after DTT treatment using a flow cytometric antibody binding assay (FABA) to specify the optimum conditions for CD38 inactivation.

MATERIALS AND METHODS

For FABA, untreated or DTT-treated RBCs were incubated with fluorescein isothiocyanate-labelled anti-CD38 antibody, in the presence or absence of 100-fold or more excess of unlabelled anti-CD38 antibody, and then analysed by flow cytometry (FCM). Dissociation of CD38-positive and control histograms was determined from the D-value using the Kolmogorov-Smirnov test. The results from FABA were compared with those from conventional FCM, indirect antiglobulin test (IAT) and Western blotting.

RESULTS

The results from FABA were more consistent than those from conventional FCM. The D-value was found to be reliable in the analysis of difference between CD38 before and after DTT treatment. Our data showed that 0·0075 mol/l DTT for 30 min is sufficient to inactivate CD38 on RBCs. These results were stable and consistent with the findings from IAT.

CONCLUSION

Flow cytometric antibody binding assay is an objective way of evaluating the efficacy of DTT treatment for CD38 on RBCs. This approach allows the detection of a small number of cell surface antigens and will be useful for assessing the various chemical treatments to denature RBC antigens.

Agonist antibody that induces human malignant cells to kill one another

An attractive, but as yet generally unrealized, approach to cancer therapy concerns discovering agents that change the state of differentiation of the cancer cells. Recently, we discovered a phenomenon that we call "receptor pleiotropism" in which agonist antibodies against known receptors induce cell fates that are very different from those induced by the natural agonist to the same receptor. Here, we show that one can take advantage of this phenomenon to convert acute myeloblastic leukemic cells into natural killer cells. Upon induction with the antibody, these leukemic cells enter into a differentiation cascade in which as many as 80% of the starting leukemic cells can be differentiated. The antibody-induced killer cells make large amounts of perforin, IFN-γ, and granzyme B and attack and kill other members of the leukemic cell population. Importantly, induction of killer cells is confined to transformed cells, in that normal bone marrow cells are not induced to form killer cells. Thus, it seems possible to use agonist antibodies to change the differentiation state of cancer cells into those that attack and kill other members of the malignant clone from which they originate.

Thrombopoietin/MPL signaling confers growth and survival capacity to CD41-positive cells in a mouse model of Evi1 leukemia

Ecotropic viral integration site 1 (Evi1) is a transcription factor that is highly expressed in hematopoietic stem cells and is crucial for their self-renewal capacity. Aberrant expression of Evi1 is observed in 5% to 10% of de novo acute myeloid leukemia (AML) patients and predicts poor prognosis, reflecting multiple leukemogenic properties of Evi1. Here, we show that thrombopoietin (THPO) signaling is implicated in growth and survival of Evi1-expressing cells using a mouse model of Evi1 leukemia. We first identified that the expression of megakaryocytic surface molecules such as ITGA2B (CD41) and the THPO receptor, MPL, positively correlates with EVI1 expression in AML patients. In agreement with this finding, a subpopulation of bone marrow and spleen cells derived from Evi1 leukemia mice expressed both CD41 and Mpl. CD41(+) Evi1 leukemia cells induced secondary leukemia more efficiently than CD41(-) cells in a serial bone marrow transplantation assay. Importantly, the CD41(+) cells predominantly expressing Mpl effectively proliferated and survived on OP9 stromal cells in the presence of THPO via upregulating BCL-xL expression, suggesting an essential role of the THPO/MPL/BCL-xL cascade in enhancing the progression of Evi1 leukemia. These observations provide a novel aspect of the diverse functions of Evi1 in leukemogenesis.

The thrombopoietin/MPL pathway in hematopoiesis and leukemogenesis

Hematopoietic stem cells (HSC) comprise a small percentage of total hematopoietic cells. Their ability to self-renewal is key to the continuous replenishment of the hematopoietic system with newly formed functional blood cell types while maintaining their multipotential capacity. Understanding the extrinsic signals that are essential to HSC maintenance will provide insights into the regulation of hematopoiesis at its most primitive stage, and with the knowledge applied, will potentially lead to improved clinical transplantation outcomes. In this review, we will summarize the current understanding of the role of the thrombopoietin/MPL signaling pathway in HSC maintenance during adult and fetal hematopoiesis. We will also speculate on the downstream key players in the pathway based on published data, and summarize the role of this pathway in leukemia.

Reduced proliferation of non-megakaryocytic acute myelogenous leukemia and other leukemia and lymphoma cell lines in response to eltrombopag

Leukemia cell lines were treated with eltrombopag or thrombopoietin and their proliferative response was determined. Eltrombopag did not increase proliferation of cell lines that did not express high levels of megakaryocyte markers. Instead, treatment with eltrombopag alone inhibited proliferation of many cell lines (IC(50) range=0.56-21 microg/mL). The addition of other cytokines, such as G-CSF, Epo or Tpo, did not affect the decrease in proliferation. The decrease in proliferation appears to be through a TpoR-independent, nonapoptotic mechanism. These findings suggest that eltrombopag does not enhance, but rather inhibits, proliferation of leukemia cell lines in vitro.

A prototype nonpeptidyl, hydrazone class, thrombopoietin receptor agonist, SB-559457, is toxic to primary human myeloid leukemia cells

Biologic characterization of SB-559457 (SB), a nonpeptidyl hydrazone class of thrombopoietin receptor (Mpl) agonist, revealed toxicity toward human leukemia cells. Antiproliferative effects followed by significant, nonapoptotic, cell death within 72 hours occurred in 24 of 26 acute myeloid leukemia, 0 of 6 acute lymphoblastic leukemia, and 3 of 6 chronic myeloid leukemia patient samples exposed to SB, but not recombinant human thrombopoietin (rhTpo), in liquid suspension culture. Further investigation revealed increased phosphorylation of p70S6/S6 kinases in SB-, but not in rhTpo-, treated cells. Expression profiling of cells exposed to SB versus rhTpo revealed statistically significant, more than 2-fold changes in GAPDH and REDD1 gene expression, confirmed by quantitative reverse-transcribed polymerase chain reaction. These genes, induced in energy or hypoxia stressed cells, have been implicated in cell death pathways, and may provide important clues to the mechanism of SB-induced, leukemic cell death. These results suggest that nonpeptidyl, hydrazone class Mpl agonists may be clinically useful antileukemic agents by virtue of their combined thrombopoietic and antileukemic effects.

Endogenous thrombopoietin levels during the clinical management of acute myeloid leukaemia

Thrombocytopenia represents a major problem in the management of acute myeloid leukaemia (AML). The data regarding the alterations of endogenous thrombopoietin (TPO) regulation during the clinical course of AML are limited. The aim of this study was to investigate endogenous TPO dynamics in association with platelets during the clinical course of AML. We serially measured both TPO and platelets concurrently over the entire treatment period of newly diagnosed patients receiving both remission induction and consolidation chemotherapies. The median concentration of TPO in AML patients at the initial diagnosis was 469.71 pg/ml and increased significantly during the aplastic period due to remission induction chemotherapy (median: 1085.33 pg/ml) but then decreased to a level (median: 45.26 pg/ml) encountered in the healthy control subjects (median: 56.90 pg/ml). In the cytopenic period due to consolidation treatment, TPO level again increased significantly to a high level (median: 891.38 pg/ml) during the platelet nadir, but decreased toward normal (median: 100.75 pg/ml) after the thrombocytopenic period had elapsed. In conclusion, endogenous TPO levels exhibit an inverse fluctuation in relation to platelet counts during the clinical course of AML. Pharmacological stimulation of thrombopoiesis in AML with novel molecules, including the recombinant thrombopoietins and the small peptide agonists, should be based on a critical administration strategy that must consider the endogenous levels of TPO. TPO levels in distinct AML disease states may explain the unsuccessful recombinant TPO trials and could help to design better strategies for 'pharmacological stimulation of thrombopoiesis' in AML.

Differentiation of hematopoietic progenitor cells towards the myeloid and B-lymphoid lineage by hepatocyte growth factor (HGF) and thrombopoietin (TPO) together with early acting cytokines

OBJECTIVES

The effect of stem cell factor (SCF), flt3-ligand (FL), and interleukin (IL)-3 (SF3) in combination with hepatocyte growth factor (HGF), thrombopoietin (TPO), and Hyper-IL-6 on maintenance and differentiation of early human peripheral blood-derived progenitor cells was investigated.

METHODS

Single sorted CD34(+) 38(-) cells were cultured with various combinations of these growth factors in order to identify the most effective cytokine combination. Then, lineage-depleted cells were stimulated for 7 d in bulk culture before they were assessed by flow cytometry and in functional assays.

RESULTS

The highest number of clones in the single-cell assay was obtained after culture with SF3 + TPO + HGF. Cell expansion with SF3 + TPO + HGF yielded an increase of the total cell number (11-fold), the number of CD34(+) cells (sevenfold), colony forming cells (CFC; 13-fold), granulocytes (CD15/66b(+); 45-fold) and B-cells (CD19/20(+); 55-fold). However, the number of long-term culture initiating cells (LTC-IC) decreased from 779 +/- 338 per 1 x 10(5) CD34(+) cells on day 0 to 253 +/- 115 on day 7. In parallel, the number of pluripotent mouse repopulating cells decreased by the factor 11, and no significant change in the proportion of human myeloid or lymphoid cells found in the mouse bone marrow was noted.

CONCLUSION

The observation that mature cells of different lineages are generated and that transplantable multipotent hematopoietic cells are lost during culture suggests the differentiation of early hematopoietic progenitors toward lineage committed cells by the tested cytokines. The detection of cells expressing B-lymphoid markers after culture indicates a possible role in the propagation of B-cells.

Circulating thrombopoietin as an in vivo growth factor for blast cells in acute myeloid leukemia

Thrombopoietin (TPO), the major growth factor for cells of the megakaryocytic lineage, is removed from circulation by binding to c-mpl receptors present on platelets and megakaryocytes. We studied patients with acute lymphoblastic leukemia (ALL) or acute myeloblastic leukemia (AML) and used TPO-induced c-fos protein up-regulation as a marker of c-mpl functionality and observed that c-mpl-presenting blast cells were present in 62% (37 of 60) of patients with ALL but that c-mpl was nonfunctional in 0 of 28 patients and that they were present in 56% (22 of 39) of patients with AML and were functional in 43% (12 of 28). Adequate increases in serum TPO level in response to thrombocytopenia were seen in patients with ALL and with c-mpl-deficient (c-mpl-) AML. In contrast, in patients with c-mpl-proficient (c-mpl+) AML, TPO levels were found to be inappropriately low but increased to expected values during induction chemotherapy as blasts disappeared. In vitro significant TPO-associated blast cell proliferation or decreased apoptosis was observed only in patients with c-mpl+ AML compared with ALL or c-mpl- AML and was highly correlated with low in vivo TPO levels (P < .001). These data suggest that, in patients with AML, inadequate TPO levels are secondary to TPO clearing by functional c-mpl receptor myeloid blast cells and that TPO may serve as an in vivo myeloid leukemic growth factor in a significant number of patients. (Blood. 2006; 107:2525-2530)

Leukemic cells and the cytokine patchwork

BACKGROUND

In leukemia, the clonal population is characterized by a hierarchical organization. Although the majority of the leukemic population is generated after post-determinic divisions, a subset of cells retain undifferentiated "blast" morphology. In addition, leukemic cells often have numerical or structural chromosomal abnormalities, aberrant gene expression patterns, and abnormal cell surface marker profiles. Despite these differences when compared to normal bone marrow and blood cells, leukemic cell survival and proliferation, just like that of normal progenitor cells, is influenced by hematopoietic growth factors. A major issue is whether differential regulation of normal and leukemic hematopoietic cells by cytokines can be exploited in antileukemic treatment or, in contrast, whether in vivo cytokine therapy may even be harmful to the patients.

PROCEDURE

Here we review the results of recent experimental and clinical observations that investigated the influence of cytokines on leukemic cell growth and differentiation in vitro and in vivo.

RESULTS

The majority of studies indicate that hematopoietic growth factors are involved in the regulation of proliferation and terminal differentiation of leukemic blast cells. Genetic aberrations involving cytokines or their receptors may contribute to leukemogenesis. Abundant interactions, cross-lineage stimulation, and aberrant response patterns seem to transform the complex cytokine network regulation of normal hematopoiesis into an even more interlaced "patchwork" that controls leukemic hematopoiesis.

CONCLUSIONS

Since hematopoietic growth factors are present in high serum concentrations in patients with acute leukemia and myelodysplastic syndromes, consequences of possible interactions should be kept in mind even when well-defined human recombinant factors in single application are to be involved in antileukemic protocols.

Erythropoietin receptor in myelodysplastic syndrome and leukemia

Erythropoietin (Epo) is one of the main regulators of growth and differentiation of hematopoietic cells. In normal bone marrow cells, the amount of erythropoietin receptor (EpoR) was highest in the CD34+ CD38- subset, and decreased on lineage committed progenitor cells expressing CD38 antigens. Among the erythroid cells expressing GpA antigens, CD34-positive fractions expressed more EpoR than CD34-negative fractions. Although the amounts of EpoR of bone marrow cells from patients with refractory anemia (RA) were less than those of normal bone marrow cells in all phenotypes examined, there was no statistical significant difference. EpoR was detected on leukemia cells from 60% of acute myeloblastic leukemia (AML) cases and 29% of acute lymphoblastic leukemia (ALL) cases, and distributed widely among all FAB-subtypes. In spite of the presence of EpoR, in vitro proliferative response to Epo was not observed in a large proportion of AML. And there was no correlation between the amount of EpoR and the in vitro response to EPO. Patients with both EpoR expression and in vitro response to Epo had shorter remission duration than those without EpoR.

Frequent gain of chromosome 19 in megakaryoblastic leukemias detected by comparative genomic hybridization

Acute megakaryocytic leukemia is a rare subtype of AML that is often difficult to diagnose; it is most commonly associated with Down syndrome in children. To identify chromosomal imbalances and rearrangements associated with acute megakaryocytic leukemia, we used G-banding, comparative genomic hybridization (CGH), and whole chromosome painting (WCP) on a variety of primary patients' samples and leukemia cell lines. The most common abnormality was gain of chromosome 19 or arm 19q, which was detected by CGH in four of 12 (33.3%) primary samples and nine of 11 (81.8%) cell lines. In none of the primary samples was this abnormality detected by G-banding analysis. WCP was used to define further the nature of the chromosome 19 gain in the cell lines, which was found to be due to the presence of additional 19q material on marker chromosomes or to cryptic translocations involving 19q. The most common chromosomal loss--detected only in the cell lines--was deletion of chromosomal band 13q14, which was seen in six of 11 (54.5%) cell lines. Other recurrent changes included gains of 1p, 6p, 8q, 11q, 15q, 17q, and 21q and losses of 2, 4q, 5q, 7q, 9p, and 11p. Combining conventional and molecular cytogenetic analyses defined recurrent clonal chromosomal abnormalities, which will aid in the identification of critical genes that are abnormal in acute megakaryocytic leukemia cells.

Decreased amount of mpl and reduced expression of glycoprotein IIb/IIIa and glycoprotein Ib on platelets from patients with refractory anemia: analysis by a non-isotopic quantitative ligand binding assay and immunofluorescence

Using a non-isotopic ligand binding assay and immunofluorescence, we examined the amount of mpl, glycoprotein IIb/IIIa (gpIIb/IIIa) and glycoprotein Ib (gpIb) on platelets from healthy volunteers and patients with refractory anemia (RA). For the analysis of mpl expression, we applied both a non-isotopic ligand binding assay and immunofluorescence using anti-mpl monoclonal antibody, and compared the results from both methods. The non-isotopic ligand binding assay has been developed in our laboratory and is suitable for the quantitative analysis of a small amount of cytokine receptors such as mpl on platelets. In platelets from patients with RA, the amount of mpl expressed by the D value was 0.05+/-0.03 (mean+/-standard deviation), and was significantly lower than that in healthy volunteers (0.15+/-0.05, p<0.0001). The mean fluorescence intensities (MFI) of gpIIb/IIIa and gpIb on platelets from RA patients were 28.8+/-8.8 and 20.8+/-7.7, respectively, and were significantly lower than those on normal subjects (93.2+/-22.6 and 67.4+/-9.1, p<0.0001 and p<0.0001, respectively). There was a good correlation between the amount of mpl and the MFI of gpIIb/IIIa (p=0.794, p<0.0001) or gpIb (p=0.774, p<0.0001), and between those of gpIIb/IIIa and gpIb (p=0.728, p<0.0001). We demonstrated a decreased amount of mpl as well as a reduced expression of gpIIb/IIIa and gpIb on platelets from RA patients.

Expression of thrombopoietin receptor and its functional role in human B-precursor leukemia cells with 11q23 translocation or Philadelphia chromosome

Thrombopoietin (TPO) is a hematopoietic growth factor which plays a central role in normal megakaryocytopoiesis and thrombopoiesis. Although the interaction between TPO and its receptor c-Mpl encoded by the c-mpl gene is now known to be implicated in the proliferation and/or differentiation of abnormal myeloid cells and normal hematopoietic stem cells, little is known about a role of the TPO/c-Mpl system in lymphoid leukemia cells. In the present study, we first examined the expression of c-mpl/c-Mpl in 23 human lymphoid leukemic cell lines (T-lineage 4, B-lineage 19) using three distinct methods. The c-mpl mRNA was detectable in as many as 20 cell lines (T-lineage 3, B-lineage 17) by reverse transcriptase-polymerase chain reaction, but its translated product, c-Mpl, was demonstrable by Western blot only in B-lineage cell lines. Flow cytometric analysis revealed the surface c-Mpl expression in 13 of 17 B-lineage cell lines, but its higher expression (>40%) was restricted in nine B-precursor cell lines, eight of which had 11q23 translocation or Philadelphia chromosome (Ph1). We also demonstrated that two of eight cell lines with 11q23 translocation or Ph1 exhibited a significant proliferative response to TPO in the 3H-thymidine uptake and colony-forming assays. Triggering of these cell lines by TPO transiently up-regulated tyrosine phosphorylation of JAK-2 and Shc, indicating that their receptor is functional. Primary leukemia cells separated from patients with B-precursor acute lymphoblastic leukemia with Ph1 or 11q23 translocation also showed the surface c-Mpl expression and a significant responsiveness to TPO. These results suggest that the TPO/c-Mpl interaction may play a physiological role in the growth regulation of B-precursor leukemia cells particularly with specific chromosomal abnormalities.

Effect of thrombopoietin on proliferation of blasts from patients with myelodysplastic syndromes

Thrombopoietin (TPO), a major cytokine involved in megakaryocytopoiesis/thrombopoiesis, may be effective for treatment of the thrombocytopenia associated with myelodysplastic syndromes (MDS). However, it has been unclear whether TPO stimulates proliferation of MDS blasts, as observed in de novo acute myeloid leukemia. This study examined this concern. When marrow cells from 37 MDS cases were cultured with or without recombinant human PEGylated TPO, TPO increased the blast number (stimulation index > or =1.5) in 9 of 16 high-risk MDS cases (refractory anemia with excess blasts [RAEB] and RAEB in transformation) and 4 of 10 cases with MDS transformed to acute leukemia (MDS-AL), but none of 11 cases with low-risk MDS (RA and RA with ringed sideroblasts). When the cell cycle of cultured cells was determined by three-color flow cytometry, TPO activated the cell cycle of MDS cells (causing a decrease in G(0)-phase cells) in most of the cases whose blast number increased in response to TPO. Reverse transcriptase-polymerase chain reaction analysis detected TPO receptor messenger RNA in purified blasts from all six cases examined, irrespective of the response of their blasts to TPO in culture. Analysis of the patients' characteristics identified a high-serum lactate dehydrogenase (LDH) value as being associated with blast proliferation in high-risk MDS cases (p = 0.0036). We conclude that TPO stimulates in vitro proliferation of blasts from a fraction of MDS patients. High-risk MDS patients, especially those who have a high-serum LDH value, and MDS-AL patients should be monitored with particular care in clinical trials of TPO for MDS.

Analysis of thrombopoietin receptor (c-mpl) mRNA expression in de novo acute myeloid leukemia

Expression of the thrombopoietin receptor, c-mpl, has been recently suggested to represent an adverse prognostic factor in myelodysplasia and acute myeloid leukemia (AML). To further evaluate this putative correlation, we assessed the c-mpl mRNA expression in blast samples of 53 AML patients. Overall, c-mpl mRNA expression was observed in 27 (51%) patients. No significant difference between c-mpl(+) and c-mpl(-) patients was found with respect to established prognostic factors such as age (50 vs. 53 years) or karyotype, whereas a significant correlation was observed between c-mpl and CD34 expression (P = 0.026). Among 40 patients who completed standard-/high-dose cytarabine-containing induction/consolidation treatment and were evaluable for treatment response, a higher complete remission (CR) rate was achieved in c-mpl- than in c-mpl(+) patients (95 vs. 68%; P = 0.026). Upon multivariate analysis, this relationship was independent from CD34 expression. CR duration was not significantly longer in c-mpl(-) than in c-mpl(+) patients (median: 14 vs. 10 months, P = 0.262). In conclusion, our data strongly support the previously suggested notion that c-mpl expression is of prognostic relevance for CR induction in de novo AML patients, and suggest determination of c-mpl expression within larger prospective studies in the attempt to develop risk-adapted AML treatment strategies.

Effect of thrombopoietin on acute myelogenous leukemia blasts

It has been shown that the expression of c-mp1, which is a specific receptor for thrombopoietin (TPO), is restricted to the surface of megakaryocytes, platelets, human CD34+ progenitor cells and human erythroid/megakaryocytic leukemic cell lines. Recently, however, it has been reported that some acute myelogenous leukemia (AML) blasts expressed c-mp1 on their cell surface and proliferated in response to TPO. We therefore investigated the effect of thrombopoietin on the growth of leukemic blasts from patients with CD7-positive acute myelogenous leukemia (AML), which is a distinct biological and clinical subtype of AML. Significant growth responses of leukemic blasts to TPO were seen in 10/10 CD7+ and 7/20 CD7- AML cases using 3H-thymidine incorporation, while synergistic stimulatory effects of TPO with stem cell factor (SCF), interleukin-3 (IL-3), granulocyte colony-stimulating factor, and granulocyte-macrophage colony-stimulating factor were observed in both groups. In a leukemic blast colony assay, significant growth response to TPO was observed in 5/6 CD7+ and 4/17 CD7- AML cases examined. Furthermore, the expression of c-mp1 seemed to be higher in CD7+ AML cases than in CD7- cases, suggesting a relationship between the expression of c-mp1 and the proliferative response to TPO. These findings imply that CD7+ leukemic blasts express functional TPO receptors and proliferate in response to TPO. Thus CD7 expression on AML blasts may indicate the involvement of leukemic progenitors at an early stage of multipotent hemopoietic stem cells. In this review, we discuss the effect of TPO on AML blasts, especially in CD7+ AML cases.

Expression of thrombopoietin and its receptor (c-mpl) in chronic myelogenous leukemia: correlation with disease progression and response to therapy

BACKGROUND

Chronic myelogenous leukemia (CML) represents a paradigm of the stepwise increment in disease aggressiveness, resistance to therapy, and transformation. Thrombopoietin (TPO) and its receptor, c-mpl, support the proliferation of multiple types of immature hematopoietic progenitor cells, and induce clonal growth of leukemic cells. The authors investigated whether TPO or c-mpl overexpression might correlate with progression of CML, disease aggressiveness, or response to therapy.

METHODS

Expression of c-mpl and TPO was measured in bone marrow samples from 208 patients with CML by Western blot analysis and solid-phase plate radioimmunoassay (used for quantification). Samples obtained from individuals without evidence of hematologic abnormalities were used as controls.

RESULTS

There were no significant differences in TPO or c-mpl expression among patients in different phases of CML or between patients with Philadelphia chromosome positive and negative CML. When TPO and c-mpl levels were analyzed in relation to prognostically important host and disease characteristics in early chronic phase CML, platelet and white blood cell counts demonstrated significant differences in both TPO and c-mpl expression, but age and spleen size demonstrated significant differences in TPO expression only. Responses to interferon (INF)-alpha-based therapy and survival were not influenced by TPO or c-mpl levels.

CONCLUSIONS

TPO or c-mpl overexpression did not correlate with different CML phases, suggesting that they were not involved in CML progression from early to advanced phase. Neither TPO nor c-mpl overexpression was particularly evident in any risk group, suggesting lack of correlation between their expression and disease aggressiveness. This was supported by the finding of similar response to IFN-alpha-based therapy and survival regardless of the level of TPO or c-mpl expression.

Quantitative expression of erythropoietin receptor (EPO-R) on acute leukaemia cells: relationships between the amount of EPO-R and CD phenotypes, in vitro proliferative response, the amount of other cytokine receptors and clinical prognosis. Japan Adult Leukaemia Study Group

Expression of erythropoietin (EPO) receptor (EPO-R) was analysed in leukaemia cells from 150 patients with acute myeloid leukaemia (AML) or acute lymphoblastic leukaemia (ALL). EPO-R was expressed in 81 (60%) out of 136 AML, and in vitro treatment with EPO led to proliferation of leukaemia cells in 13 (16%) out of 81 AML examined. EPO-R expression and in vitro response to EPO were observed in all subtypes of AML according to the French-American-British (FAB) classification. All eight patients with FAB-M6 expressed EPO-R, and one out of four showed an in vitro response to EPO. Although there was no significant correlation (r = 0.2522) between the amount of EPO-R and the in vitro response to EPO, all of the AML patients who showed in vitro response expressed EPO-R. Stem cell factor significantly enhanced both EPO-R expression and in vitro response to EPO. Interleukin-3 tended to increase in vitro response to EPO. CD phenotypes, the amount of granulocyte colony-stimulating factor (G-CSF) receptors and the amount of TPO receptors had no significant relationship with the amount of EPO-R. Patients with both EPO-R expression and in vitro response to EPO had shorter duration of complete remission than those without EPO-R (P = 0.0053). EPO-R was expressed in four (29%) out of 14 ALL, and none out of five ALL showed in vitro response to EPO.