Serum thrombopoietin level in various hematological diseases

Patients with Waldenström macroglobulinemia have impaired platelet and coagulation function

ABSTRACT

Clinical features in patients with the B-cell lymphoma, Waldenström macroglobulinemia (WM), include cytopenias, immunoglobulin M (IgM)-mediated hyperviscosity, fatigue, bleeding, and bruising. Therapeutics such as Bruton's tyrosine kinase inhibitors (BTKis) exacerbate bleeding risk. Abnormal hemostasis arising from platelet dysfunction, altered coagulation or vascular impairment have not yet been investigated in patients with WM. Therefore, this study aimed to evaluate hemostatic dysfunction in samples from these patients. Whole blood (WB) samples were collected from 14 patients with WM not receiving therapy, 5 patients receiving BTKis and 15 healthy donors (HDs). Platelet receptor levels and reticulation were measured by flow cytometry, plasma thrombin generation with or without platelets by fluorescence resonance energy transfer assay, WB clotting potential by rotational thromboelastometry, and plasma soluble glycoprotein VI (sGPVI) and serum thrombopoietin (TPO) by enzyme-linked immunosorbent assay. Donor platelet spreading, aggregation, and ability to accelerate thrombin generation in the presence of WM-derived IgM were assessed. WM platelet receptor levels, responses to physiological agonists, and plasma sGPVI were within normal ranges. WM platelets had reduced reticulation (P = .0012) whereas serum TPO levels were increased (P = .0040). WM plasma displayed slower thrombin generation (P = .0080) and WM platelets contributed less to endogenous thrombin potential (ETP; P = .0312). HD plasma or platelets incubated with IgM (50-60 mg/mL) displayed reduced spreading (P = .0002), aggregation (P < .0001), and ETP (P = .0081). Thus, alterations to thrombin potential and WB coagulation were detected in WM samples. WM IgM significantly impaired hemostasis in vitro. Platelet and coagulation properties are disturbed in patients with well-managed WM.

Interaction of S100A6 Protein with the Four-Helical Cytokines

S100 is a family of over 20 structurally homologous, but functionally diverse regulatory (calcium/zinc)-binding proteins of vertebrates. The involvement of S100 proteins in numerous vital (patho)physiological processes is mediated by their interaction with various (intra/extra)cellular protein partners, including cell surface receptors. Furthermore, recent studies have revealed the ability of specific S100 proteins to modulate cell signaling via direct interaction with cytokines. Previously, we revealed the binding of ca. 71% of the four-helical cytokines via the S100P protein, due to the presence in its molecule of a cytokine-binding site overlapping with the binding site for the S100P receptor. Here, we show that another S100 protein, S100A6 (that has a pairwise sequence identity with S100P of 35%), specifically binds numerous four-helical cytokines. We have studied the affinity of the recombinant forms of 35 human four-helical cytokines from all structural families of this fold to Ca2+-loaded recombinant human S100A6, using surface plasmon resonance spectroscopy. S100A6 recognizes 26 of the cytokines from all families of this fold, with equilibrium dissociation constants from 0.3 nM to 12 µM. Overall, S100A6 interacts with ca. 73% of the four-helical cytokines studied to date, with a selectivity equivalent to that for the S100P protein, with the differences limited to the binding of interleukin-2 and oncostatin M. The molecular docking study evidences the presence in the S100A6 molecule of a cytokine-binding site, analogous to that found in S100P. The findings argue the presence in some of the promiscuous members of the S100 family of a site specific to a wide range of four-helical cytokines. This unique feature of the S100 proteins potentially allows them to modulate the activity of the numerous four-helical cytokines in the disorders accompanied by an excessive release of the cytokines.

Identification of Plasma Thrombopoietin Level and Its Significance in Patients with Aplastic Anemia and Myelodysplastic Syndrome

Objective  Our objective was to investigate the concentration of plasma thrombopoietin (TPO) in patients with aplastic anemia (AA) and myelodysplastic syndrome (MDS), as well as its relationship with patients' responses to recombined human TPO (rhTPO) therapy. Methods  We detected the concentration of plasma TPO in 31 patients with AA, 27 patients with MDS, and 11 normal controls using enzyme-linked immunosorbent assay. Results  The median concentration of plasma TPO in patients with AA, MDS, and controls was (841.08 ± 768.64), (212.41 ± 338.93), and (35.09 ± 18.21) pg/mL, respectively. The TPO concentration in patients with AA and MDS was significantly higher than that in controls ( p  < 0.05). The median platelet (PLT) counts were (184 ± 34) ×10 9 /L in the control group and (24 ± 19) ×10 9 /L and (80 ± 71) ×10 9 /L in AA and MDS patients, respectively. Negative correlations were found between plasma TPO concentration and PLT counts as well as megakaryocytes in bone marrow ( p  < 0.05). In AA patients treated with rhTPO, a negative correlation was observed between increased PLT counts and pretreatment TPO levels ( p  < 0.05). Conclusion  Plasma TPO concentration in AA and MDS was significantly higher than that in normal controls. Plasma TPO was negatively correlated with peripheral blood PLT counts and bone marrow megakaryocyte counts. The pretreatment TPO level may serve as a prognostic indicator for the therapeutic effect of rhTPO in AA patients.

Mechanisms and therapeutic prospects of thrombopoietin receptor agonists

The second-generation thrombopoietin (TPO) receptor agonists eltrombopag and romiplostim are potent activators of megakaryopoiesis and represent a growing treatment option for patients with thrombocytopenic hematological disorders. Both TPO receptor agonists have been approved worldwide for the treatment of children and adults with chronic immune thrombocytopenia. In the EU and USA, eltrombopag is approved for the treatment of patients with severe aplastic anemia who have had an insufficient response to immunosuppressive therapy and in the USA for the first-line treatment of severe aplastic anemia in combination with immunosuppressive therapy. Eltrombopag has also shown efficacy in several other disease settings, for example, chemotherapy-induced thrombocytopenia, selected inherited thrombocytopenias, and myelodysplastic syndromes. While both TPO receptor agonists stimulate TPO receptor signaling and enhance megakaryopoiesis, their vastly different biochemical structures bestow upon them markedly different molecular and functional properties. Here, we review and discuss results from preclinical and clinical studies on the functional and molecular mechanisms of action of this new class of drug.

What is the role of novel thrombopoietic agents in the management of acute leukemia?

The role of novel thrombopoietic agents in the management of acute leukemia is a tale of two molecules, romiplostim and eltrombopag. Both are thrombopoietin (TPO) receptor agonists with somewhat different mechanisms of action. Romiplostim is a peptide TPO receptor agonist that activates the TPO receptor by binding to it just like TPO. Eltrombopag is a nonpeptide TPO receptor agonist that activates the TPO receptor by binding to the transmembrane domain. Both TPO receptor agonists increase platelet counts in healthy humans and in those with immune thrombocytopenia. This review focuses on the potential these agents may have in supportive care of patients with acute leukemia.

A Rare Case of Acute Myeloid Leukemia with a t(2;3) Chromosomal Translocation Characterized by Thrombophilia and Chemoresistance

We hereby report a case of acute myeloid leukemia with translocation t(2;3) and involvement of the ectopic virus integration site-1 (EVI1) gene. Like most other 3q26-related disorders reported thus far, we describe a phenotype with elevated platelet counts and dysmegakaryopoesis. The clinical course of our patient was complicated by symptomatic thrombophilia and chemoresistance. In addition, our case exhibited FLT3 (Fms-related tyrosine kinase 3) internal tandem duplication. Although anagrelide was successful in controlling elevated platelet counts, allogeneic stem cell transplantation failed to overcome chemoresistance due to simultaneous graft-versus-host-disease and relapse of acute myeloid leukemia. Given the dismal outcome of our case and previously reported cases, we propagate the implementation of targeted therapies to newly diagnosed patients with acute myeloid leukemia t(2;3). Preclinical models indicate drugs that plausibly target the EVI1-related molecular vulnerability as candidates for basket trials. Anagrelide exhibited a hopeful signal of activity in 3q26-related thrombocytosis and should be evaluated for implementation as supportive care.

Does thrombopoiesis in multiple myeloma patients depend on the stage of the disease?

PURPOSE

Infiltration of the bone marrow by neoplastic plasmocytes in multiple myeloma (MM) patients might impair megakaryocytopoiesis. The aim of the study was to evaluate stage-dependent platelet count (PLT) and thrombopoietin (TPO) concentration in comparison to the control group. We also wanted to establish whether TPO might be recognized as a marker of the stage of the disease.

MATERIAL/METHODS

The study group consisted of 41 patients (mean age 67.7) with newly diagnosed MM prior to treatment and categorized according to the Durie and Salmon diagnostic classification. The control group consisted of 30 healthy subjects (mean age 65.5). PLT, WBC, RBC and Hb were measured with the use of the haematological analyser. TPO was assayed with the use of ELISA and albumin with the use of the immunonephelometry method. The number of plasma cells in the bone marrow was evaluated in bone marrow smears under light microscopy.

RESULTS

PLT was not statistically different as compared the control groups, but was stage-dependent. Thrombocytopenia was observed in the III stage of MM. TPO median was significantly higher in study group than in healthy subjects and it was increasing considerably with the stage of the disease. TPO concentration was negatively correlated with albumin and PLT. AUC for TPO was 0.9764. The number of plasma cells in the bone marrow was considerably increasing with the stage of the disease.

CONCLUSIONS

PLT and TPO in MM patients were stage-dependent. Elevated TPO concentration in MM patients might be an unfavourable marker of the stage of the disease.

Involvement of thrombopoietin in acinar cell necrosis in L-arginine-induced acute pancreatitis in mice

Thrombopoietin (TPO) plays an important role in injuries of different tissues. However, the role of TPO in acute pancreatitis (AP) is not yet known. The aim of the study was to determine the involvement of TPO in AP. Serum TPO was assayed in necrotizing pancreatitis induced by L-arginine in mice. Recombinant TPO and anti-TPO antibody were given to mice with necrotizing pancreatitis. Amylase, lipase, lactate dehydrogenase, myeloperoxidase activity and pancreatic water content were assayed in serum and tissue samples. Pancreas and lung tissue samples were also collected for histological evaluation. Immunohistochemistry of amylase α and PCNA were applied for the study of acinar regeneration and TUNEL assay for the detection of apoptosis in the pancreas. Increased levels of serum TPO were found in necrotizing pancreatitis. After TPO administration, more severe acinar necrosis was found and blockade of TPO reduced the acinar necrosis in this AP model. Acinar regeneration and apoptosis in the pancreas were affected by TPO and antibody treatment in necrotizing pancreatitis. The severity of pancreatitis-associated lung injury was worsened after TPO treatment, but attenuated after Anti-TPO antibody treatment. In conclusion, serum TPO is up-regulated in the necrotizing pancreatitis induced by L-arginine in mice and may be a risk factor for the pancreatic acinar necrosis in AP. As a pro-necrotic factor, blockade of TPO can attenuate the acinar necrosis in AP and may be a possible therapeutic intervention for AP.

Management of chronic immune thrombocytopenic purpura: targeting insufficient megakaryopoiesis as a novel therapeutic principle

Traditionally, anti-platelet autoantibodies accelerating platelet clearance from the peripheral circulation have been recognized as the primary pathopysiological mechanism in chronic immune thrombocytopenia (ITP). Recently, increasing evidence supports the co-existence of insufficient megakaryopoiesis. Inadequate low thrombopoietin (TPO) levels are associated with insufficient proliferation and differentiation of megakaryocytes, decreased proplatelet formation, and subsequent platelet release. Recently two novel activators of TPO receptors have been made available: romiplostim and eltrombopag. In several phase III studies, both agents demonstrated increase of platelet counts in about 80% of chronic ITP patients within 2 to 3 weeks. These agents substantially broaden the therapeutic options for patients with chronic ITP although long-term results are still pending. This review will provide an update on the current conception of underlying mechanisms in ITP and novel, pathophysiologically based treatment options.

Thrombopoietic cytokines and platelet count in multiple myeloma

Cytokines like interleukin (IL)-6 and IL-1beta are both implicated in multiple myeloma (MM) pathogenesis and megakaryopoiesis. The dynamic interaction between thrombopoiesis and thrombopoietic cytokines in MM may affect platelet (PLT) counts. Sixty-eight patients with MM (30 female, 38 male; median age 58 (40-79), 38 newly diagnosed, 15 in plateau, and 15 relapse and/or refractory patients) and 21 controls were included in the study. Plasma levels of thrombopoietin (TPO), IL-1beta, IL-11 and IL-6 were measured by ELISA. PLT counts were not different between the control group and MM patients with various disease stages and activity. IL-6 and TPO levels were higher in MM patients than healthy subjects (p < 0.001). PLT counts were inversely correlated with TPO (r = -0.566; p < 0.001) and positively correlated with IL-6 (r = 0.263; p = 0.04) levels in MM patients. TPO and IL-6 levels were significantly correlated (r = 0.305; p < 0.001). Disease activity has no effect on plasma cytokine levels. TPO levels were higher in stage III than stage I (p = 0.05) and stage II (p = 0.03) patients in newly diagnosed MM. High TPO levels induced by IL-6 may sustain normal PLT counts despite bone marrow infiltration by plasma cells and decreased PLT half-life.

Thrombopoietic cytokine and P-selectin levels in patients with multiple myeloma undergoing autologous stem cell transplantation: decrease in posttransplantation P-selectin levels might predict the degree of maximum response

PURPOSE

This study was designed to determine the pretransplantation levels of thrombopoietic cytokines, which have a fundamental role in both megakaryopoiesis and myeloma pathogenesis and P-selectin in patients with multiple myeloma undergoing autologous hematopoietic stem cell transplantation (AHSCT) and to correlate the cytokine levels with time to platelet recovery. The effect of AHSCT on the levels of the cytokines and its correlation with maximum disease response was also investigated.

PATIENTS AND METHODS

The levels of thrombopoietin, interleukin (IL)-6, IL-11, IL-1beta, and P-selectin was measured before and 30 days after AHSCT in 32 patients with a median age of 55 years. The median time to platelet recovery was day +11 (range, 0-14 days) without any significant correlation with pretransplantation cytokine levels.

RESULTS

No significant change was observed in thrombopoietic cytokines after AHSCT, whereas serum P-selectin levels showed a significant decrease after AHSCT (P = .001). The decrease in P-selectin was found to be significant in patients who achieved complete remission (P1 = .008) and partial remission (P2 = .018) early after AHSCT. Our data suggest that the level of thrombopoietic cytokines does not have a role in time to platelet recovery.

CONCLUSION

The change in P-selectin levels early after transplantation could be a surrogate marker in determining the maximum posttransplantation response.

Pathogenesis of chronic immune thrombocytopenia: increased platelet destruction and/or decreased platelet production

Chronic immune thrombocytopenia (ITP) is a haematological disorder in which patients predominantly develop skin and mucosal bleeding. Early studies suggested ITP was primarily due to immune-mediated peripheral platelet destruction. However, increasing evidence indicates that an additional component of this disorder is immune-mediated decreased platelet production that cannot keep pace with platelet destruction. Evidence for increased platelet destruction is thrombocytopenia following ITP plasma infusions in normal subjects, in vitro platelet phagocytosis, and decreased platelet survivals in ITP patients that respond to therapies that prevent in vivo platelet phagocytosis; e.g., intravenous immunoglobulin G, anti-D, corticosteroids, and splenectomy. The cause of platelet destruction in most ITP patients appears to be autoantibody-mediated. However, cytotoxic T lymphocyte-mediated platelet (and possibly megakaryocyte) lysis, may also be important. Studies supporting suppressed platelet production include: reduced platelet turnover in over 80% of ITP patients, morphological evidence of megakaryocyte damage, autoantibody-induced suppression of in vitro megakaryocytopoiesis, and increased platelet counts in most ITP patients following treatment with thrombopoietin receptor agonists. This review summarizes data that indicates that the pathogenesis of chronic ITP may be due to both immune-mediated platelet destruction and/or suppressed platelet production. The relative importance of these two mechanisms undoubtedly varies among patients.

AKR-501 (YM477) in combination with thrombopoietin enhances human megakaryocytopoiesis

OBJECTIVE

AKR-501 (YM477) is an orally active thrombopoietin (TPO) receptor agonist that mimics the biological effect of TPO in vitro and in vivo. Here, we report that AKR-501 in combination with TPO has additive effect on megakaryocytopoiesis.

MATERIALS AND METHODS

Granulocyte colony-stimulating factor-mobilized human peripheral blood CD34+ cells were cultured with AKR-501, TPO, or a combination of the two in serum-free liquid culture system. The numbers of hematopoietic progenitor cells, megakaryocytic progenitor cells, and megakaryocytes were measured using flow cytometry. Further, the effect of AKR-501 on TPO binding to TPO receptor was examined.

RESULTS

Both AKR-501 and TPO alone increased the number of megakaryocytes, and the maximum activities of AKR-501 and TPO were similar. Interestingly, in the presence of TPO concentrations producing maximal stimulation, the addition of AKR-501 increased the number of megakaryocytes to about 200% of that generated with TPO only. In the time course experiment, the combination of AKR-501 and TPO augmented the numbers of hematopoietic progenitor cells and colony-forming unit in culture in the early stages. Thus, the combination of AKR-501 and TPO enhanced not only the differentiation into megakaryocytes, but also the expansion of human hematopoietic progenitor cells. Further, AKR-501 did not inhibit TPO binding to the TPO receptor. This result indicated the possibility that AKR-501 and TPO may act simultaneously on the TPO receptor, and this could be responsible for their additive effect of on megakaryocytopoiesis.

CONCLUSIONS

This study suggests that AKR-501 would be useful for the treatment of thrombocytopenia even at high plasma levels of endogenous TPO following chemotherapy.

Efficacy of romiplostim in patients with chronic immune thrombocytopenic purpura: a double-blind randomised controlled trial

BACKGROUND

Chronic immune thrombocytopenic purpura (ITP) is characterised by accelerated platelet destruction and decreased platelet production. Short-term administration of the thrombopoiesis-stimulating protein, romiplostim, has been shown to increase platelet counts in most patients with chronic ITP. We assessed the long-term administration of romiplostim in splenectomised and non-splenectomised patients with ITP.

METHODS

In two parallel trials, 63 splenectomised and 62 non-splenectomised patients with ITP and a mean of three platelet counts 30x10(9)/L or less were randomly assigned 2:1 to subcutaneous injections of romiplostim (n=42 in splenectomised study and n=41 in non-splenectomised study) or placebo (n=21 in both studies) every week for 24 weeks. Doses of study drug were adjusted to maintain platelet counts of 50x10(9)/L to 200x10(9)/L. The primary objectives were to assess the efficacy of romiplostim as measured by a durable platelet response (platelet count > or =50x10(9)/L during 6 or more of the last 8 weeks of treatment) and treatment safety. Analysis was per protocol. These studies are registered with ClinicalTrials.gov, numbers NCT00102323 and NCT00102336.

FINDINGS

A durable platelet response was achieved by 16 of 42 splenectomised patients given romplostim versus none of 21 given placebo (difference in proportion of patients responding 38% [95% CI 23.4-52.8], p=0.0013), and by 25 of 41 non-splenectomised patients given romplostim versus one of 21 given placebo (56% [38.7-73.7], p<0.0001). The overall platelet response rate (either durable or transient platelet response) was noted in 88% (36/41) of non-splenectomised and 79% (33/42) of splenectomised patients given romiplostim compared with 14% (three of 21) of non-splenectomised and no splenectomised patients given placebo (p<0.0001). Patients given romiplostim achieved platelet counts of 50x10(9)/L or more on a mean of 13.8 (SE 0.9) weeks (mean 12.3 [1.2] weeks in splenectomised group vs 15.2 [1.2] weeks in non-splenectomised group) compared with 0.8 (0.4) weeks for those given placebo (0.2 [0.1] weeks vs 1.3 [0.8] weeks). 87% (20/23) of patients given romiplostim (12/12 splenectomised and eight of 11 non-splenectomised patients) reduced or discontinued concurrent therapy compared with 38% (six of 16) of those given placebo (one of six splenectomised and five of ten non-splenectomised patients). Adverse events were much the same in patients given romiplostim and placebo. No antibodies against romiplostim or thrombopoietin were detected.

INTERPRETATION

Romiplostim was well tolerated, and increased and maintained platelet counts in splenectomised and non-splenectomised patients with ITP. Many patients were able to reduce or discontinue other ITP medications. Stimulation of platelet production by romiplostim may provide a new therapeutic option for patients with ITP.

Serum levels of stem cell factor and thrombopoietin are markedly decreased in fulminant hepatic failure patients with a poor prognosis

BACKGROUND AND AIM

Hematopoietic growth factors including stem cell factor (SCF), thrombopoietin (TPO) and granulocyte colony stimulating factor (G-CSF) have a potential role in inducing bone marrow hematopoietic stem cells to move into the circulation, and the association of these factors with liver regeneration has received a lot of attention recently. The aim of this study was to determine the serum levels of such factors in patients with acute liver injury.

METHODS

The subjects were 25 patients with acute hepatitis (AH) who had a favorable prognosis and 26 patients with fulminant hepatitis (FH), of whom 11 were alive and 15 had died. Sixty-six healthy subjects matched for age and sex served as controls. Serum samples were collected before treatment, and the levels of SCF, TPO and G-CSF were measured using enzyme-linked immunosorbant assays.

RESULTS

The levels of SCF and TPO were significantly lower in FH patients than in AH patients and the controls, and were also significantly lower in the FH patients who died, compared to the surviving patients. The G-CSF levels did not differ among them.

CONCLUSIONS

These results suggest that low serum levels of SCF and TPO may be linked to poor prognosis in patients with severe liver injury.

AMG 531, a thrombopoiesis-stimulating protein, for chronic ITP

BACKGROUND

Most current treatments for chronic immune thrombocytopenic purpura (ITP) act by decreasing platelet destruction. In a phase 1-2 study, we administered a thrombopoiesis-stimulating protein, AMG 531, to patients with ITP.

METHODS

In phase 1, 24 patients who had received at least one treatment for ITP were assigned to escalating-dose cohorts of 4 patients each and given two identical doses of AMG 531 (0.2 to 10 microg per kilogram of body weight). In phase 2, 21 patients were randomly assigned to receive six weekly subcutaneous injections of AMG 531 (1, 3, or 6 microg per kilogram) or placebo. The primary objective was to assess the safety of AMG 531; the secondary objective was to evaluate platelet counts during and after treatment.

RESULTS

No major adverse events that could be attributed directly to AMG 531 occurred during the treatment period; 4 of 41 patients had transient post-treatment worsening of thrombocytopenia. In phase 1, a platelet count that was within the targeted range (50,000 to 450,000 per cubic millimeter) and at least twice the baseline count was achieved in 4 of 12 patients given 3, 6, or 10 mug of AMG 531 per kilogram. Overall, a platelet count of at least 50,000 per cubic millimeter was achieved in 7 of 12 patients, including 3 with counts exceeding 450,000 per cubic millimeter. Increases in the platelet count were dose-dependent; mean peak counts were 163,000, 309,000, and 746,000 per cubic millimeter with 3, 6, and 10 microg of AMG 531 per kilogram [corrected], respectively. In phase 2, the targeted platelet range was achieved in 10 of 16 patients treated with 1 or 3 mug of AMG 531 per kilogram per week for 6 weeks. Mean peak counts were 135,000, 241,000, and 81,000 per cubic millimeter in the groups that received the 1-mug dose, the 3-mug dose, and placebo, respectively.

CONCLUSIONS

AMG 531 caused no major adverse events and increased platelet counts in patients with ITP. (ClinicalTrials.gov number, NCT00111475 [ClinicalTrials.gov].).

Multiple cycles of recombinant human thrombopoietin therapy in a patient with chronic refractory idiopathic thrombocytopenic purpura

We describe a 41-year-old woman with chronic idiopathic thrombocytopenic purpura who received recombinant human thrombopoietin (rhTPO) therapy. rhTPO was administrated subcutaneously at a dosage of 1.0 mug/kg daily for a maximum of 14 days until the platelet count was more than 50 x 10/l. The patient received three cycles (six, 13, and eight doses each) of rhTPO, each initiated when the platelet counts was less than 10 x 10/l. The platelet count increased to above 50 x 10/l on days 5, 11 and 8, and peaked at 456 x 10/l, 130 x 10/l and 82 x 10/l on days 9, 15 and 13 in the three respective cycles, each followed by a gradual decline. The durations of platelet counts at more than 50 x 10/l in the three cycles were 13, 7 and 10 days, respectively. rhTPO was well tolerated with no adverse event observed. Antibodies to rhTPO by enzyme-linked immunosorbent assay were not detected. Our observations suggested that rhTPO could transiently increase the peripheral platelet count in patients with chronic refractory idiopathic thrombocytopenic purpura. The reasons why the peak platelet counts decreased and the duration of response shortened after successive cycles of treatment were unclear.

Pharmacokinetics of pegylated recombinant human megakaryocyte growth and development factor in healthy volunteers and patients with hematological disorders

OBJECTIVES

The purpose of this study is to examine the pharmacokinetics of pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) in healthy volunteers with normal hematopoiesis and patients with idiopathic thrombocytopenic purpura (ITP), acute myeloid leukemia (AML), myelodysplastic syndromes (MDS), and aplastic anemia (AA).

METHODS

PEG-rHuMGDF was intravenously administered to healthy volunteers and patients with ITP, AML, MDS, and AA. The serum concentration of PEG-rHuMGDF was measured and the pharmacokinetics was investigated using non-linear mixed-effects modeling technique.

RESULTS

The systemic clearance (CL) and volume of distribution at steady-state (Vss) consistently decreased in the healthy subjects, when the dose increased. In AML patients, CL and Vss decreased when the dose increased, but the change of CL was not statistically significant. In contrast, no significant dose dependency of these parameter estimates was observed in MDS patients. In AA patients there was no significant change in Vss but the CL of the higher dose groups was slightly smaller than that of the lower dose groups. Relatively smaller CL and Vss in ITP patients than those of healthy volunteers at the same dose were observed.

CONCLUSIONS

This saturable pathway of CL may involve the receptor-mediated endocytosis and degradation by megakaryocyte lineage cells and platelets. The saturable distribution space can be also explained by the receptors on hematopoietic cells. The non-saturable distribution space corresponds to the value of plasma and interstitial fluid volume.

Prospective screening of 205 patients with ITP, including diagnosis, serological markers, and the relationship between platelet counts, endogenous thrombopoietin, and circulating antithrombopoietin antibodies

Immune thrombocytopenia purpura (ITP) is characterized by destruction of circulating platelets and the presence of antiplatelet antibodies. Many of the current immunomodulatory therapies act by reducing platelet destruction and usually do not have a lasting effect. This prospective, exploratory study characterized patients with ITP by identifying their demographic and comorbid clinical factors, use of treatments, serologic markers of autoimmunity, and possible relationships between platelet counts, concentrations of endogenous thrombopoietin (eTPO), and the presence of circulating anti-TPO antibodies. Data including medical history and laboratory evaluations were collected at a single patient visit on 205 patients (19 children, 186 adults). Reported histories revealed a 5% rate of thrombotic/ischemic events. Autoimmune markers including direct antiglobulin test and antinuclear antibodies were found more frequently than in the normal population; antiplatelet antibody testing was not done. eTPO concentrations were comparable to concentrations found in healthy volunteers. Our study confirmed that no significant inverse correlation occurred between circulating concentrations of eTPO and platelet counts in patients with ITP (Spearman r = -0.15). Two of the 205 patients tested (1%) had neutralizing activity of recombinant human TPO in a biological assay; however, this activity was confirmed to be anti-TPO antibody in only 1 patient.

Decrease of Smad4 gene expression in patients with essential thrombocythaemia may cause an escape from suppression of megakaryopoiesis by transforming growth factor-beta1

Essential thrombocythaemia (ET) is characterized by the abnormal and sustained proliferation of megakaryocytes. The mechanism for this lineage-specific expansion in ET, remains unclear. We have previously reported that transforming growth factor-beta1 (TGF-beta1) is involved in negative feedback regulation of megakaryopoiesis in both healthy volunteers (HV) and patients with idiopathic thrombocytopenic purpura (ITP). The present study found that megakaryocyte colony-forming units (CFU-MK) of ET patients were less sensitive to TGF-beta1 than those of HV. The expression of Smad4 (Sma- and Mad-related protein-4) in CFU-MK of ET patients was reduced in comparison with that of HV. Finally, to confirm that the impaired TGF-beta1 sensitivity was caused by reduced expression of Smad4, we examined Smad4-transfected CFU-MK from ET patients in the presence of TGF-beta1, and verified that the transfectants were indeed as susceptible as CFU-MK from HV to TGF-beta1. Thus it was surmised that one of the mechanisms for impaired sensitivity of CFU-MK to TGF-beta1 is the reduced expression of Smad4.

Pathophysiology and thrombokinetics in autoimmune thrombocytopenia

Studies that have measured platelet survival by autologous platelet labeling with (111)In and (51)Cr have differed in their results. Although all studies have revealed a significant decrease in platelet life span, the rates of platelets entering the circulation, a calculated and inferred determination, have been found to be moderately decreased to as much as five times normal. Several mechanisms have been proposed to explain an apparent decrease in platelet production, including a true decrease due to damage to the megakaryocytes by autoantibody, versus a decrease only in 'effective' production due to intramedullary destruction by the reticuloendothelial system. Recently, the identification of the cytokine, thrombopoietin, has allowed the evaluation of another aspect of the pathophysiology of thrombocytopenic states. Megakaryocyte growth factor levels are increased 10 to 20 times in patients who are thrombocytopenic due to chemotherapy or aplastic anemia, but may be decreased, normal, or only modestly raised in patients with immune platelet destruction. Autologous platelet survival measurements, prior to and while on therapy with a stable platelet count, reveal that removal of part of the reticuloendothelial system with splenectomy leads to increased platelet survival and platelet number. Similar studies reveal that corticosteroid treatment for immune thrombocytopenic purpura (ITP) effectively increases the rate of platelet production but does not change platelet survival. It may be that other therapies are effective by a combination of these mechanisms. Stimulation of thrombopoietin production or administering exogenous cytokine may have a role to play in future management of patients with ITP.

Mimicry between neurokinin-1 and fibronectin may explain the transport and stability of increased substance P immunoreactivity in patients with bone marrow fibrosis

Bone marrow (BM) fibrosis may occur in myeloproliferative diseases, lymphoma, myelodysplastic syndrome, myeloma, and infectious diseases. In this study, the role of substance P (SP), a peptide with pleiotropic functions, was examined. Some of its functions-angiogenesis, fibroblast proliferation, and stimulation of BM progenitors-are amenable to inducing BM fibrosis. Indeed, a significant increase was found in SP-immunoreactivity (SP-IR) in the sera of patients with BM fibrosis (n = 44) compared with the sera of patients with hematologic disorders and no histologic evidence of fibrosis (n = 46) (140 +/-12 vs 18 +/-3; P <.01). Immunoprecipitation of sera SP indicated that this peptide exists in the form of a complex with other molecule(s). It was, therefore, hypothesized that SP might be complexed with NK-1, its natural receptor, or with a molecule homologous to NK-1. To address this, 3 cDNA libraries were screened that were constructed from pooled BM stroma or mononuclear cells with an NK-1 cDNA probe. A partial clone (clone 1) was retrieved that was 97% homologous to the ED-A region of fibronectin (FN). Furthermore, sequence analyses indicated that clone 1 shared significant homology with exon 5 of NK-1. Immunoprecipitation and Western blot analysis indicated co-migration of SP and FN in 27 of 31 patients with BM fibrosis. Computer-assisted molecular modeling suggested that similar secondary structural features between FN and NK-1 and the relative electrostatic charge might explain a complex formed between FN (negative) and SP (positive). This study suggests that SP may be implicated in the pathophysiology of myelofibrosis, though its role would have to be substantiated in future research. (Blood. 2001;97:3025-3031)

A pilot study of rhuIL-11 treatment of refractory ITP

The objective of this research was to determine whether rhuIL-11 is an effective treatment in patients with refractory immune thrombocytopenic purpura (ITP). Platelet production is decreased in certain cases of refractory ITP. IL-11 stimulates megakaryocytopoiesis in vitro and was licensed for its clinical effects to ameliorate chemotherapy-induced thrombocytopenia. A pilot study was initiated, intending to enroll 12 patients with ITP. These patients were to receive rhuIL-11 (Neumega) at a dose of 50 microg/kg subcutaneously daily for 21 consecutive days and be observed afterward for 21 additional days. CBC with platelets were obtained twice weekly with visits and physical examinations weekly. The study was terminated after 7 patients were enrolled because of toxicity and lack of efficacy. All 7 patients had had ITP for >9 years and had failed splenectomy, intravenous gammaglobulin, corticosteroids, and a variety of other treatments. The patients at entry all had platelet counts <20,000/microl; 5 of 7 had counts <10,000/microl. The maximal median increase for any day of the study was 6,000/microl. No patient achieved a count of 30,000/microl, and only 3 patients achieved (once each) a platelet count >20,000/microl. Substantial toxicity was seen. The nadir hemoglobin decrease was a mean of 2 g/dl. rhuIL-11 was not effective at increasing the platelet count in any of these patients with refractory ITP. Toxicity was substantial. The lack of platelet response to rhuIL-11 in this study does not exclude the possibility of better effects at other doses and/or in less refractory patients.

Endogenous TPO (eTPO) levels in health and disease: possible clues for therapeutic intervention

The factor which is the primary regulator of megakaryocyte and platelet production has recently been identified as the ligand for the receptor Mpl. This discovery has resulted in substantial advances in our understanding of platelet homeostasis. The access to new experimental reagents has enabled studies of the endogenous circulating form of this ligand, endogenous thrombopoietin, in normal individuals and in patients with altered platelet numbers. The relationship of endogenous TPO in health and disease will be examined with consideration of the implications for successful therapeutic intervention with exogenous recombinant Mpl ligands in selected settings.

NF-kappa B as a central mediator in the induction of TGF-beta in monocytes from patients with idiopathic myelofibrosis: an inflammatory response beyond the realm of homeostasis

Immune-mediated mechanisms have been implicated in the etiology of idiopathic bone marrow fibrosis (IMF). However, the mechanism remains poorly defined. Compared with healthy controls, IMF monocytes are overactivated, with increased production of TGF-beta and IL-1. TGF-beta is central to the progression of fibrosis in different organs. In the lung, fibrosis is associated with up-regulation of TGF-beta-inducible genes. Because IL-1 and TGF-beta have pro- and antiinflammatory properties and neither appears to regulate the high levels of each other in IMF, we studied the mechanism of this paradigm. We focused on the role of RelA, a subunit of the transcription factor, NF-kappaB that is associated with inflammatory responses. We transiently knocked out RelA from IMF monocytes with antisense oligonucleotides and showed that RelA is central to IL-1 and TGF-beta production and to the adhesion of IMF monocytes. Because the NF-kappaB family comprises subunits other than RelA, we used aspirin and sodium salicylate to inhibit kinases that activate NF-kappaB and showed effects similar to those of the RelA knockout system. It is unlikely that RelA could be interacting directly with the TGF-beta gene. Therefore, we determined its role in TGF-beta production and showed that exogenous IL-1 could induce TGF-beta and adherence of IMF monocytes despite the depletion of NF-kappaB. The results indicate that IL-1 is necessary for TGF-beta production in IMF monocytes, but NF-kappaB activation is required for the production of endogenous IL-1. Initial adhesion activates NF-kappaB, which led to IL-1 production. Through autocrine means, IL-1 induces TGF-beta production. In total, these reactions maintain overactivation of IMF monocytes.

Hepatocyte growth factor/scatter factor enhances the thrombopoietin mRNA expression in rat hepatocytes and cirrhotic rat livers

BACKGROUND

Although thrombopoietin (TPO) is mainly produced in the liver, the regulatory mechanism of TPO gene expression in hepatocytes remains unclear. The role of TPO in thrombocytopenia associated with liver cirrhosis has not been identified.

METHODS

We examined the effects of various growth factors and cytokines on TPO mRNA expression in adult rat hepatocytes in primary cultures using a semiquantitative reverse transcription-polymerase chain reaction assay.

RESULTS

Among them, only hepatocyte growth factor/scatter factor (HGF/SF) enhanced TPO mRNA expression; other growth factors (epidermal growth factor and transforming growth factor-beta) and cytokines (erythropoietin, granulocyte-colony stimulating factor, granulocyte-macrophage-colony stimulating factor, interleukin (IL)-3, IL-6 and interferon-gamma) did not. Next, we examined TPO mRNA expression in the livers of rats with CCl4-induced cirrhosis, the effects of HGF/SF on hepatic TPO mRNA expression and peripheral platelet and bone marrow megakaryocyte counts in the cirrhotic rats. In the cirrhotic rats, both the peripheral platelet count and TPO mRNA expression in the livers were markedly decreased compared with those of the normal rats. The administration of HGF/SF to the cirrhotic rats stimulated TPO mRNA expression in the livers and resulted in significant increases of peripheral platelets and bone marrow megakaryocytes.

CONCLUSIONS

These results suggest that HGF/SF is a possible regulatory factor for TPO gene expression and that HGF/SF increases platelet production through an enhancement of TPO mRNA expression in the livers of cirrhotic rats.

Pathophysiological significance of simultaneous measurement of reticulated platelets, large platelets and serum thrombopoietin in non-neoplastic thrombocytopenic disorders

An automated reticulocyte counter using flowcytometric analysis, the R-3000 (Sysmex Inc. Kobe, Japan), has recently been modified to determine reticulated platelets (RPs) and large platelets (LPs). We measured frequencies of RPs, LPs in total platelet count and serum thrombopoietin concentration comprehensively in non-neoplastic thrombocytopenic patients with immune thrombocytopenic purpura (ITP, n = 23), aplastic anemia (AA, n = 21), liver cirrhosis (LC, n= 17), and hematologically normal subjects (control, n = 151). ITP was characterized as high frequencies of both RP and LP, AA as high RP frequency and elevated thrombopoietin concentration, and LC as no difference compared with control. Interestingly, the frequency of RP appeared to depend on total platelet count rather than the cause of thrombocytopenia, while the frequency of LP appeared to depend much less on total platelet count. Furthermore, significant positive correlations were observed between frequencies of RP and LP in control, ITP and LC subjects, in whom bone marrow stem cells are intrinsically normal. However, there was no such correlation in AA patients with stem cell deficiency, suggesting that this correlation might be a useful new parameter for detecting qualitatively abnormal platelets. Measurement of RP and LP is thus useful for elucidating the pathophysiology of thrombocytopenic disorders.

Thrombopoietin serum levels at the start of mobilization, collection, and transfusion of autologous peripheral blood stem cells

Thrombopoietin (TPO) serum levels in 14 patients (9 male and 5 female, mean age 36 years, range 16 to 55 years) with breast cancer (n = 5), testicular cancer (n = 7), or lymphoma (n = 2), undergoing high dose chemotherapy with peripheral blood stem cell (PBSC) transplantation, were evaluated at the first day of the mobilization chemotherapy (1), at the day of the first apheresis (2), and at the day of stem cell transfusion (3). All patients have been pretreated (one to four regimens) and received chemotherapy and granulocyte colony stimulating factor (G-CSF) or granulocyte-macrophage colony stimulating factor (GM-CSF) both at 5 microg/kg body weight (bw). for stem cell mobilization. TPO was measured with a human TPO immunoassay. Mean TPO serum levels were: (1) 274+/-248.8 pg/ml (range 0 to 953 pg/ml), (2) 518+/-399.1 pg/ml (range 118 to 1,283 pg/ml), and (3) 556+/-506.4 pg/ml (range 147 to 1,570 pg/ml). The CD34+ cell concentration in the peripheral blood at the time of apheresis was 65+/-48.2/microl (7 to 148/microl), and the number of transfused CD34+ cells was 3.0+/-1.0x10(6)/kg bw (1.7 to 5.5x10(6)/kg bw). TPO levels showed some weak inverse correlation (r = -0.64) with the platelet counts at the day of the first apheresis that increased to -0.70 if a semilog correlation was done (plt[log] vs. TPO). The number of platelet transfusions after HDCT correlated to some degree (r = 0.61) with the TPO serum level at the day of PBSC transfusion. There was no correlation between any TPO serum level and the CD34+ cell concentration in the peripheral blood or neutrophil and platelet engraftment. We conclude from this study that TPO serum levels do not seem to correlate with the CD34+ cell concentration in the peripheral blood and the time to engraftment, although there was some weak correlation with the number of platelet transfusions.

Characterization of defective megakaryocytic development in patients with myelodysplastic syndromes

Megakaryocytic differentiation of progenitor cells was investigated in nine patients with low-risk myelodysplastic syndromes (MDS) (eight refractor anemia [RA] and one RA with ringed sideroblasts [RARS] and five patients with high-risk MDS (two RA with excess of blasts [RAEB] and three RAEB in transformation [RAEB-T]). Bone marrow-derived CD34+ cells were enriched to a purity of 87% +/- 2% (mean +/- SEM) and assayed in short-term suspension cultures in the presence of 10 ng/mL of PEGylated recombinant human megakaryocyte (MK) growth and development factor (PEG-rHuMGDF) and in addition to 50 ng/mL stem cell factor and 10 ng/mL interleukin-3. Cells of the megakaryocytic lineage were identified by flow cytometric analysis of CD42b (GP1b) and mature MKs by morphologic criteria. Transcription of c-mpl receptor-specific mRNA in the CD34+ cells of these patients was investigated by full-length reverse transcriptase polymerase chain reaction of the p form of c-mpl as well as of the alternative splice product c-mpl k. CD34+ cells from seven healthy bone marrow donors served as controls. Differentiation along the MK pathway was stimulated in five patients with RA. C-mpl mRNA was expressed in the CD34+ cells in all cases. In three low-risk patients the capacity for in vitro MK growth was absent or minimal even though mRNA for c-mpl receptor was detected in the CD34+ cells of this group as well. In patients with high-risk MDS, PEG-rHuMGDF stimulated in vitro MK growth from CD34+ cells in only one of five cases. As in the patients with low-risk MDS, c-mpl mRNA for both c-mpl p and c-mpl k splicing products was detected. These results indicate that the in vitro response to stimulation with c-mpl ligand discriminates between two groups of patients with low-risk MDS and that the observed defect in megakaryocytic development is unrelated to the level of c-mpl expression in both low-risk and high-risk MDS.

Plasma thrombopoietin (TPO) levels and expression of TPO receptor on platelets in patients with myelodysplastic syndromes

Data on endogenous thrombopoietin (TPO) levels and their regulation in myelodysplastic syndromes (MDS) are sparse. We examined the plasma TPO level of 85 MDS patients by a sensitive enzyme immunoassay and the platelet expression of TPO receptor (TPO-R) protein, which metabolizes endogenous TPO, in 19 MDS patients with an equilibrium binding assay using 125I-TPO. The MDS patients had higher plasma TPO levels (7.0 +/- 9.3 fmol/ml) than 52 normal subjects (P < 0.0001). Refractory anaemia (RA) patients (n = 39) had higher plasma TPO levels than patients (n = 28) with RA with excess blasts (RAEB) or RAEB in transformation (RAEB-t) (P = 0.0002), irrespective of similar platelet counts in these groups. The plasma TPO level correlated inversely with the platelet count in RA patients (P = 0.0027) but not in RAEB and RAEB-t patients (P = 0.7865). These data suggest that the physiological pathway for TPO production and metabolism is conserved, at least partially, in RA, but deranged in RAEB/RAEB-t. The number of TPO-R per platelet was significantly smaller in 19 MDS patients (17.5 +/- 13.3) than in normals (P = 0.0014), but similar between RA patients and patients with RAEB and RAEB-t. Further, the bone marrow megakaryocyte count, determined in 31 MDS patients, was quite similar between RA patients and patients with RAEB or RAEB-t. Thus, in addition to thrombocytopenia, a reduced platelet TPO-R number may contribute to elevated plasma TPO levels in MDS, and a regulatory pathway for circulating TPO other than platelet TPO-R and marrow megakaryocytes, such as blasts expressing TPO-R, may operate in RAEB/RAEB-t.

Analysis of thrombopoietin and c-mpl expression in a child with essential thrombocythemia

The pathogenesis of the increased number of megakaryocytes and thrombocytosis in essential thrombocythemia (ET) is still unknown. We examined the expression of c-mpl, a receptor of thrombopoietin (TPO), and its signaling molecules in a patient with ET. An 8-year-old girl showed a high platelet count and an increased number of bone marrow megakaryocytes. Neither chromosomal abnormalities nor myelofibrosis was observed. Following the diagnosis of ET, aspirin therapy was begun for the patient, with only modest improvement of symptoms. Her platelet count ranged from 1,200,000/microL to 2,200,000/microL for more than 2 years. In the analyses, the serum TPO level in the patient was 420 attomoles/mL (normal, 760 +/- 320). The level of c-mpl expression in bone marrow mononuclear cells was higher in the patient than in healthy children, while there was no difference in the level of c-mpl expression in CD34+ cells, indicating an expanded pool of megakaryocytic lineage cells. The level of Janus kinase 2 (Jak2) expression was lower in the patient than in a healthy child. These findings indicate that the signal pathway mediated by c-Mpl after binding to TPO may be impaired in ET. Further analysis is needed to clarify the mechanism underlying the development of thrombocytosis in ET patients.

Plasma thrombopoietin levels in thrombocytopenic states: implication for a regulatory role of bone marrow megakaryocytes

To evaluate the diagnostic value of thrombopoietin (TPO, c-mpl ligand) measurements, and clarify the regulatory mechanisms of TPO in normal and in thrombocytopenic conditions, the plasma TPO concentration was determined in normal individuals (n = 20), umbilical cord blood (n = 40), chronic idiopathic thrombocytopenic purpura (ITP; n = 16), in severe aplastic anaemia (SAA; n = 3), chemotherapy-induced bone marrow hypoplasia (n = 10), myelodysplastic syndrome (MDS; n = 11), and sequentially during peripheral blood progenitor cell transplantation (n = 7). A commercially available ELISA and EDTA-plasma samples were used for the analysis. The plasma TPO concentration in the normals and umbilical cord blood were 52 +/- 12 pg/ml and 66 +/- 12 pg/ml, respectively. The corresponding values in patients with SAA and chemotherapy-induced bone marrow hypoplasia were 1514 +/- 336 pg/ml and 1950 +/- 1684 pg/ml, respectively, and the TPO concentration, measured sequentially after myeloablative chemotherapy and peripheral blood progenitor cell transplantation, was inversely related to the platelet count. In contrast, the plasma TPO recorded in patients with ITP (64 +/- 20 pg/ml) and MDS (68 +/- 23 pg/ml) were only slightly higher than normal levels. In conclusion, TPO levels were significantly elevated in patients in which bone marrow megakaryocytes and platelets in circulation were markedly reduced, whereas TPO levels were normal in ITP patients, and only slightly increased in the MDS patients. These latter patients displayed a preserved number of megakaryocytes in bone marrow biopsies. Our data support the suggestion that megakaryocyte mass affects the plasma TPO concentration. In thrombocytopenic patients a substantially increased plasma TPO implies deficient megakaryocyte numbers. However, TPO measurements do not distinguish between ITP and thrombocytopenia due to dysmegakaryopoiesis, as seen in MDS patients.

The Mpl ligand and platelet homeostasis

The classification of Mpl as a cytokine receptor present on cells of the platelet lineage has led to the identification and cloning of its ligand. This has resulted in a rapid accumulation of data advancing the understanding of the processes of megakaryopoiesis and thrombopoiesis and the regulation of endogenous Mpl ligand (thrombopoietin, eTPO). Highlights of in vitro human and non-human primate data will be discussed, as well as preclinical (in vivo) non-human primate studies. Two recombinant forms of Mpl ligands (rTPO) are currently being tested in clinical trials and early results will be reviewed. The preclinical and clinical studies will be summarized with consideration of the observations which provide insights into the biology of the response to exogenous rTPO. Understanding the biology of platelet production and the condition of target cells in treatment populations will facilitate the appropriate use of this potential therapeutic agent.

Thrombopoietin serum levels in patients with aplastic anaemia: correlation with platelet count and persistent elevation in remission

In an attempt to evaluate the role of thrombopoietin (TPO) in the pathobiology of aplastic anaemia (AA), we have examined TPO levels in sera from 54 AA patients and 119 healthy controls. A total of 92 samples were collected from AA patients: 43 samples were harvested at diagnosis, 23 samples in the cytopenic period after treatment, and 26 samples when patients were in partial (n=10) or complete remission (n=16) following immunosuppressive treatment. TPO serum levels were assessed by a sandwich-antibody ELISA that utilized a polyclonal rabbit antiserum for both capture and signal. Serum samples from normal donors revealed a mean TPO level of 95.3 +/- 54.0 pg/ml (standard deviation). Mean TPO levels in AA sera collected at diagnosis and before onset of treatment were 2728 +/- 1074 pg/ml (P<0.001 compared to normal controls: mean platelet count at that time: 27x10(9)/l). TPO serum levels of AA patients in partial or complete remission after immunosuppressive treatment were significantly lower than TPO levels at diagnosis (P<0.001). However, despite normal platelet counts (mean 167x10(9)/l), TPO levels remained significantly elevated in complete remission (mean TPO 1009 +/- 590 pg/ml, P<0.001 compared to normal controls). There was a significant inverse correlation between serum TPO levels and platelet counts in AA patients who were not transfused for at least 2 weeks prior to sample collection (coefficient of correlation (r) = -0.70, P<0.0001). In summary, TPO levels were highly elevated in sera of patients with AA. Thus there is no evidence to suggest an impaired TPO response contributing to thrombocytopenia in AA. Thrombopoietin did not return to normal levels in remission, indicating a persisting haemopoietic defect in remission of AA. We hypothesize that elevated levels of TPO may be required to maintain normal or near normal platelet counts in remission of AA.

Neutralization of biological activity and inhibition of receptor binding by antibodies against human thrombopoietin

Thrombopoietin (TPO) is a recently isolated cytokine that primarily regulates megakaryocytopoiesis and thrombopoiesis. We recently reported the development of a variety of antibodies (Abs) to synthetic peptides of human (h)TPO and to recombinant human TPO (rhTPO). In this study, we characterized the Abs and mapped immunologically distinct areas of the molecule. Among the five different antipeptide polyclonal Abs, only one, raised against synthetic peptide D8 to Q28, neutralized the TPO-dependent growth of FDCP-2 cells expressing human Mpl (FDCP-hMpl5 cells). One out of seven anti-rhTPO monoclonal Abs, designated as TN1, also showed neutralizing activity. TN1 was found to be specifically reactive with two proteolytic fragments, residues S1 to R117 and A60 to K122 of hTPO, indicating that the epitope(s) of TN1 is localized in residues A60 to R117 of the molecule. These two neutralizing Abs inhibited the binding of biotinylated rhTPO to FDCP-hMpl5 cells. On the other hand, the other Abs, which reacted with five polypeptides of S47 to D62, L108 to A126, N172 to A190, S262 to T284, and P306 to G332 of hTPO, did not show either the neutralizing activity or the ability to inhibit the binding of biotinylated rhTPO to the cell surface hMpl. These findings indicate that two regions, residues D8 to Q28 and A60 to R117 of hTPO, may contain the domains associated with its receptor, C-Mpl. These Abs characterized here are valuable for studying the structural analysis and the biological function of hTPO mediated by its receptor.

Genes for thrombopoietin and c-mpl are not responsible for familial thrombocythaemia: a case study

The underlying molecular basis for familial thrombocythaemia (FT). an extremely rare form of primary thrombocythaemia which occurs in an autosomal dominant manner, is currently unknown. We have investigated a family with FT and clarified whether we could detect alteration(s) in the genes coding for c-mpl and its ligand, thrombopoietin (TPO). There was no difference in platelet c-mpl mRNA expression levels between the affected and non-affected individuals in the family. Nucleotide sequence analysis of the c-mpl cDNA for the proband revealed no abnormality. We identified an intragenic tetranucleotide short tandem repeat system in the TPO gene and found non-linkage between the TPO locus and the FT phenotype. We conclude that genes for c-mpl and TPO are not responsible for thrombocythaemia in our FT family.

Thrombopoietin level in young patients is related to megakaryocyte frequency and platelet count

PURPOSE

To examine the relationships among platelet counts, bone marrow megakaryocyte frequency, and circulating thrombopoietin (TPO) levels.

PATIENTS AND METHODS

TPO levels in 17 children and one young adult with chronic or recurrent thrombocytopenia were measured by ELISA and megakaryocyte frequency was analyzed by light microscopy. Three groups of patients were studied: Group I patients had aplastic anemia and absent or decreased megakaryocytes; Group II patients had intermittent periods of chemotherapy-induced thrombocytopenia; and Group III patients had normal or increased megakaryocytes. Controls consisted of 77 healthy adults.

RESULTS

Patients in Group I had markedly increased TPO levels compared to normal controls. Their levels were significantly different (p = 0.03) from those of patients in Group III. The latter had normal or only mildly increased TPO levels except for one patient with myelodysplastic syndrome. Patients in Group II had markedly elevated TPO levels. After their bone marrow and platelet counts recovered from chemotherapy, their TPO levels decreased. In all three groups, a transient increase in platelet count (e.g., after platelet transfusion or anti-D immune globulin therapy) was associated with a moderate decrease in TPO.

CONCLUSIONS

From this study, three conclusions can be made: 1) TPO levels are inversely related to megakaryocyte frequency; 2) platelet counts have a modest influence on TPO level; and 3) TPO levels may have clinical utility in diagnosis and management and further our understanding of the pathobiology of the disorders that cause thrombocytopenia.