Binding and regulation of thrombopoietin to human megakaryocytes

The Birth and Death of Platelets in Health and Disease

Blood platelets are involved in a wide range of physiological responses and pathological processes. Recent studies have considerably advanced our understanding of the mechanisms of platelet production and clearance, revealing new connections between the birth and death of these tiny, abundant cells. Key insights have also been gained into how physiological challenges such as inflammation, infection, and chemotherapy can affect megakaryocytes, the cells that produce platelets.

Circulating thrombopoietin levels in normal healthy blood donors and in aplastic anemia patients in relation to disease severity

Background:

Thrombopoietin (TPO) is the key hematopoietic growth factor regulating the production of platelets from bone marrow megakaryocytes and maintaining platelet hemostasis. This study was done to find any relationship between the levels of thrombopoietin and the severity of disease in patients with aplastic anemia.

Materials and Methods:

Serum samples were collected from 52 patients with a confirmed diagnosis of aplastic anemia and 45 normal healthy blood donors of both sexes over a period of 2 years, and TPO was estimated by using commercially available TPO-specific-enzyme-linked immunosorbent assay.

Results:

The median TPO level of 1190 pg/ml (range 625-7651 pg/ml) in aplastic anemia patients was significantly higher than the median TPO level of 121.1 pg/ml (81.25-237.7 pg/ml) in normal healthy blood donors (P = 0.000). No significant difference was observed in TPO levels of male and female patients (P = 0.453). The median TPO concentrations observed in very severe aplastic anemia, severe aplastic anemia, and nonsevere aplastic anemia were 2765 pg/ml (range 625-6451 pg/ml), 1190 pg/ml (range 672.1-7651 pg/ml), and 1111.5 pg/ml (range 761.1-2289.2 pg/ml), respectively. TPO in patients of very severe aplastic anemia was significantly higher than patients of nonsevere aplastic anemia (P = 0.043), with no significant relation among rest of the groups.

Discussion:

TPO levels in aplastic anemia patients were significantly higher than in healthy blood donors; however, in aplastic anemia patients TPO levels were significantly higher only in patients with very severe disease.

Eliminative signaling by Janus kinases: role in the downregulation of associated receptors

Activation of cytokine receptor-associated Janus kinases (JAKs) mediates most, if not all, of the cellular responses to peptide hormones and cytokines. Consequently, JAKs play a paramount role in homeostasis and immunity. Members of this family of tyrosine kinases control the cytokine/hormone-induced alterations in cell gene expression program. This function is largely mediated through an ability to signal toward activation of the signal transducer and activator of transcription proteins (STAT), as well as toward some other pathways. Importantly, JAKs are also instrumental in tightly controlling the expression of associated cytokine and hormone receptors, and, accordingly, in regulating the cell sensitivity to these cytokines and hormones. This review highlights the enzymatic and non-enzymatic mechanisms of this regulation and discusses the importance of the ambidextrous nature of JAK as a key signaling node that integrates the combining functions of forward signaling and eliminative signaling. Attention to the latter aspect of JAK function may contribute to emancipating our approaches to the pharmacological modulation of JAKs.

GATA-dependent regulation of TPO-induced c-mpl gene expression during megakaryopoiesis

Thrombopoietin (TPO) and its receptor, c-Mpl, play the crucial role during megakaryocytopoiesis. Previously, we have shown that the promoter activity of c-mpl induced by TPO is modulated by transcription through a PKC-dependent pathway and that GATA(-77) is involved as a positive regulatory element in TPO-induced c-mpl gene expression in the megakaryoblastic CMK cells. In this research, to examine participating possibility of GATA promoter element in TPO- induced c-mpl gene expression through a PKC-independent pathway, the promoter activity of site-directed mutagenesis and the effect of potein kinase C modulator were measured by a transient transfection assay system. Together with our previous results on the TPO-induced c-mpl promoter, this study indicates destruction of -77GATA in c-mpl promoter decreased the activity by 47.3% under existence of GF109203. These results suggest that GATA promoter element plays significant role in TPO-induced c-mpl gene expression through a PKC-independent pathway.

Novel anti-c-Mpl monoclonal antibodies identified multiple differentially glycosylated human c-Mpl proteins in megakaryocytic cells but not in human solid tumors

Thrombopoietin and its cognate receptor, c-Mpl, are the primary molecular regulators of megakaryocytopoiesis and platelet production. To date the pattern of c-Mpl expression in human solid tumors and the distribution and biochemical properties of c-Mpl proteins in hematopoietic tissues are largely unknown. We have recently developed highly specific mouse monoclonal antibodies (MAb) against human c-Mpl. In this study we used these antibodies to demonstrate the presence of full-length and truncated human c-Mpl proteins in various megakaryocytic cell types, and their absence in over 100 solid tumor cell lines and in the 12 most common primary human tumor types. Quantitative assays showed a cell context-dependent distribution of full-length and truncated c-Mpl proteins. All forms of human c-Mpl protein were found to be modified with extensive N-linked glycosylation but different degrees of sialylation and O-linked glycosylation. Of note, different variants of full-length c-Mpl protein exhibiting differential glycosylation were expressed in erythromegakaryocytic leukemic cell lines and in platelets from healthy human donors. This work provides a comprehensive analysis of human c-Mpl mRNA and protein expression on normal and malignant hematopoietic and non-hematopoietic cells and demonstrates the multiple applications of several novel anti-c-Mpl antibodies.

Role of promoter element in c-mpl gene expression induced by TPO

Thrombopoietin (TPO) and its receptor, c-Mpl, play the crucial role for the development of megakaryocyte and considered to regulate megakaryocytopoiesis. Previously we reported that TPO increased the c-mpl promoter activity determined by a transient expression system using a vector containing the luciferase gene as a reporter and the expression of the c-mpl gene is modulated by transcription through a protein kinase C (PKC)-dependent pathway in the megakaryoblastic cells. In this research, to elucidate the required elements in c-mpl promoter, the promoter activity of the deletion constructs and site-directed mutagenesis were measured by a transient transfection assay system. Destruction of -77GATA in c-mpl promoter decreased the activity by 22.8%. Our study elucidated that -77GATA involved in TPO-induced c-mpl gene expression in a human megakaryoblastic cell line, CMK.

Pharmacokinetic and pharmacodynamic modeling of romiplostim in animals

PURPOSE

Romiplostim is a novel thrombopoiesis-stimulating peptibody that targets the thrombopoietin c-Mpl receptor, resulting in increased platelet production. The pharmacodynamic-mediated disposition (PDMDD) and its stimulatory effect on platelet production in Sprague-Dawley rats, rhesus monkeys, and cynomolgus monkeys following IV bolus and SC administration at various dose levels were determined.

METHODS

The pharmacokinetic (PK) profile was described by a PDMDD model that accounts for romiplostim binding to the c-Mpl receptor. The PD model contained a series of aging compartments for precursor cells in bone marrow and platelets. The stimulatory function was described by an on-and-off function operating on the fractional receptor occupancy (RO). The threshold effect, RO(thr), and K(D) parameters were determinants of drug potency, whereas S(max) reflected drug efficacy.

RESULTS

The model implicated that receptor-mediated clearance was negligible. RO(thr) estimated occupancies were 0.288, 0.385, 0.771 for rats, rhesus, and cynomolgus monkeys, respectively. The analogous estimated values of K(D) were 4.05, 2320, and 429 ng/mL, implying that romiplostim was much more potent in rats, which was confirmed by a dose-response (ratio of peak platelet count to baseline) relationship.

CONCLUSIONS

The model adequately described romiplostim serum concentrations and platelet counts in rats, rhesus monkeys, and cynomolgus monkeys, and quantified linear clearance, PDMDD, and potency of romiplostim.

Thrombopoietin factors

Megakaryopoiesis and thrombopoiesis are the central biological processes of platelet generation. Severe thrombocytopenia is a major morbidity and mortality factor in several diseases and represents a significant unmet medical need. Since the discovery of thrombopoietin (TPO) as the primary physiological regulator of megakaryopoiesis, a number of therapeutics have been developed for thrombocytopenia and been tested in preclinical models and human clinical trials. The TPO mimetics romiplostim (Nplate(®) or AMG531) and eltrombopag (Promacta(®)) have recently been approved for the treatment of adult chronic idiopathic (immune) thrombocytopenic purpura (ITP) and are successful examples of these endeavors. This chapter will review scientific progress over the last 20 years on various thrombopoietic factors with an emphasis on the biology, physiology, and pharmacology of TPO, its cognate receptor, c-Mpl, and various TPO mimetics.

PKC plays a crucial roles in c-mpl gene expression in megakaryoblastic cells

Thrombopoietin is the cytokine involved in megakaryopoiesis and its receptor (c-Mpl) is considered to regulate development of megakaryocyte. In this research, to elucidate the underlying mechanisms of c-mpl gene expression in megakaryoblastic cells, we investigated the effect of a protein kinase C (PKC) on c-mpl promoter activity in a time-dependent manner. PKC is a member of a family of serine/threonine protein kinases in the cytosol involved in cell growth and differentiation. Phorbol 12-myristate 13-acetate (PMA) is known as PKC activator, significantly enhanced the c-mpl promoter activity and PKC inhibitor, 2-methylpiperazine dihydrochloride (H-7) suppressed the up-regulation of PMA-induced promoter activity and this effect decreased in a time-dependent manner. These results clearly suggest that in megakaryoblastic cells, PKC plays the crucial role in the initiation of up-regulation of PMA-induced c-mpl promoter activity.

Pharmacodynamics-mediated drug disposition (PDMDD) and precursor pool lifespan model for single dose of romiplostim in healthy subjects

The objective of this study was to characterize the pharmacokinetics and pharmacodynamics (PK-PD) of romiplostim after single-dose administration in healthy subjects. The mean serum romiplostim concentrations (PK data) and mean platelet counts (PD data) collected from 32 subjects receiving a single intravenous (0.3, 1 and 10 μg/kg) or subcutaneous (0.1, 0.3, 1, and 2 μg/kg) dose were fitted simultaneously to a mechanistic PK-PD model based on pharmacodynamics-mediated drug disposition (PDMDD) and a precursor pool lifespan concept. The two-compartment PK model incorporated receptor-mediated endocytosis and linear mechanisms as parallel elimination pathways. The maximal concentration of receptors (assumed to be proportional to the platelet count), the equilibrium dissociation constant, and the first-order internalization rate constant for endocytosis of the drug-receptor complex were 0.022 fg/platelet, 0.131 ng/mL, and 0.173 h⁻¹, respectively. Romiplostim concentration stimulates the production of platelet precursors via the Hill function, where the SC₅₀ was 0.052 ng/mL and S (max) was 11.2. The estimated precursor cell and platelet lifespans were 5.9 and 10.5 days, respectively. Model-based simulations revealed that the romiplostim exposure and the platelet response are both dependent on the dose administered and the baseline platelet counts. Also, weekly dosing produced a sustained PD response while dosing intervals ≥2 weeks resulted in fluctuating platelet counts. Thus, the mechanistic PK-PD model was suitable for describing the romiplostim PK-PD interplay (PDMDD), the dose-dependent platelet stimulation, and the lifespans of thrombopoietic cell populations.

Modulation mechanism of c-Mpl promoter activity in megakaryoblastic cells

Thrombopoietin receptor (c-Mpl) is considered to regulate megakaryocytopoiesis. In this study, we investigated an effect of activation of a protein kinase C (PKC) on c-mpl promoter activity to elucidate the underlying mechanisms of c-mpl gene expression in megakaryoblastic cells. PKC is a member of a family of serine/threonine protein kinases in the cytosol involved in cell growth and differentiation. Phorbol 12-myristate 13-acetate (PMA) is known as PKC activator, significantly enhanced the c-mpl promoter activity and PKC inhibitors (H7, GF109203) suppressed the up-regulation of PMA-induced promoter activity and reduced the steady level of its activity. These results strongly suggest that PKC plays the essential role in the modulation of c-mpl promoter activity of megakaryoblastic cells.

Pharmacokinetic and pharmacodynamic modeling of pegylated thrombopoietin mimetic peptide (PEG-TPOm) after single intravenous dose administration in healthy subjects

Pegylated thrombopoietin mimetic peptide (PEG-TPOm) is a novel, potent thrombopoietin receptor agonist with low immunotoxicity potential that protects against chemotherapy-induced thrombocytopenia in preclinical animal models. The aim of this study was to develop a population pharmacokinetic and pharmacodynamic model of PEG-TPOm following single intravenous doses in healthy subjects. Data were obtained from a double-blind, randomized, placebo-controlled study. A model based on target-mediated drug disposition and precursor pool life spans was applied. Model evaluation was performed through predictive checks and bootstrap analysis. The half-life of PEG-TPOm ranged between 18 and 36 hours, and the estimated distributional volume was 5 L. The increase in platelet counts was observed after a 4-day delay, consistent with the megakaryocyte cell life span. The platelet life span was estimated to be 5 days. After maximum platelets counts were achieved on day 9, platelets returned back to baseline on day 29. Model-based simulations were undertaken to explore pharmacodynamic effects after multiple dosing. Weekly dosing produced a sustained pharmacodynamic response, whereas an interdosing interval >or=2 weeks resulted in fluctuating pharmacodynamic profiles. Thus, the mechanistic pharmacokinetic/pharmacodynamic model was suitable for describing the complex PEG-TPOm pharmacokinetics/pharmacodynamics, including target-mediated disposition, dose-dependent platelet stimulation, and mean life spans of thrombopoietic cell populations.

Internalization of the thrombopoietin receptor is regulated by 2 cytoplasmic motifs

Receptor-mediated internalization appears to be the primary mean of regulating the plasma level of thrombopoietin (TPO). However, the processes that regulate Mpl internalization have not previously been described. Using the cytokine-dependent cell line BaF3, we have identified 2 distinct motifs within the cytoplasmic domain of Mpl that underlie ligand-dependent internalization. Removal of the fourth cytoplasmic tyrosine residue by deletion or truncation results in a significant decrease in maximal internalization. The remaining receptor internalization is abrogated by deletion of cytoplasmic residues 54-69, which include the core box2 region (L54L55E56I57L58) and the only dileucine motifs (L54L55 and I57L58) within the cytoplasmic domain of Mpl. Receptor internalization mediated by this latter subdomain does not require Jak2 activation. Furthermore, TPO-stimulated cellular proliferation appears to be directly correlated with receptor internalization, indicating that internalization of the TPO/Mpl complex may be essential for normal signal transduction. Finally, we have demonstrated that upon removal of TPO from the supernatant, Mpl promptly reappears on the cell surface, suggesting that a pool of intracellular Mpl can be rapidly recycled to the cell surface. These data help identify the receptor motifs involved in TPO-induced internalization of Mpl and suggest that Mpl translocation may be necessary for normal cellular proliferation.

Thrombopoietin as a drug: biologic expectations, clinical realities, and future directions

After the cloning of thrombopoietin (c-mpl ligand, Tpo) in 1994, 2 recombinant thrombopoietic growth factors, full-length glycosylated recombinant human Tpo (reHuTPO) and polyethylene glycol conjugated megakaryocyte growth and development factor (PEG-reHuMGDF), have been studied in humans in a variety of clinical settings. Both thrombopoietins are generally well tolerated if administered intravenously (IV). The c-mpl ligands produce a dose-related enhancement of platelet levels, reduce nonmyeloablative chemotherapy-induced mild thrombocytopenia, and mobilize hematopoietic progenitors. On September 11, 1998, the development of PEG-reHuMGDF was suspended in the U.S., due to formation of the neutralizing anti-Tpo antibody. Those neutralizing antibodies lead to thrombocytopenia and pancytopenia in some patients receiving subcutaneous (SC) PEG-reHuMGDF. Japanese investigators indicate that the probability of antibody formation against PEG-reHuMGDF is low when the drug is administered IV instead of SC. reHuTPO has a more favorable safety profile from the point of antibody production. The c-mpl ligands can improve apheresis yields when administered to normal platelet donors. Preliminary data about the use of PEG-reHuMGDF in myelodysplasia, aplastic anemia, and immune thrombocytopenic purpura are promising. Tpo is usually not effective in myeloablative thrombocytopenia when bone marrow hematopoietic progenitors are not present. The major obstacle for the thrombopoietins is their delayed action for managing clinical thrombocytopenia. This review will focus on the biologic basis, current clinical experience, and future directions for the use of thrombopoietic molecules as drugs. The identification of a safe, effective, and potent pharmacologic platelet growth factor could significantly improve the management of thrombocytopenia-induced bleeding.

Pharmacodynamic modeling of thrombopoietin, platelet, and megakaryocyte dynamics in patients with acute myeloid leukemia undergoing dose intensive chemotherapy

A proposed model of thrombopoietin (TPO) regulation is that of a constitutive production of TPO with circulating levels being predominately regulated by changes in platelet and megackaryocyte mass. Using a pharmacodynamic (PD) approach, the authors examined the validity of this model for patients with acute myeloid leukemia (AML) undergoing dose-intensive postinduction chemotherapy (HDT). TPO and platelet values were assayed weekly in AML patients undergoing HDT. A parsimonious dynamic model was then applied to these experimental data. The results (1) support the proposed model of TPO regulation, (2) model and quantify the effects of HDT on the megakaryocyte compartment, (3) characterize variables not amenable to direct measurement, and (4) have clinical utility as this model predicted that TPO given after HDT would not have a significant effect on platelet recovery, a finding borne out in clinical trials. This model provides information relevant to the interpretation of clinical trials of hematopoietic growth factors.

Human plasma thrombopoietin levels are regulated by binding to platelet thrombopoietin receptors in vivo

BACKGROUND

Data from several studies support the hypothesis that thrombopoietin (TPO) plasma levels are regulated via circulating platelet (PLT) numbers by binding to PLT TPO receptors (TPO-Rs). In this study, PLT numbers and TPO plasma levels were measured following the transfusion of unmanipulated, sham-saturated, and TPO-R-saturated PLT preparations to provide additional in vivo evidence for this regulatory mechanism.

STUDY DESIGN AND METHODS

Following in vitro experiments to characterize pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) binding characteristics, PLT numbers and TPO plasma levels were measured following the transfusion of unmanipulated, sham-saturated, and TPO-R-saturated PLT preparations in thrombocytopenic patients. Sham-saturated and TPO-R-saturated PLTs were prepared by a 1-hour incubation without and with 40 ng per mL of PEG-rHuMGDF, respectively, and subsequent washing and resuspension.

RESULTS

In vitro, 2.72 +/- 0.8 ng of PEG-rHuMGDF per 1 x 10(8) PLTs was bound within 1 hour of incubation. No additional PEG-rHuMGDF was bound following a second incubation with PEG-rHuMGDF, and bound PEG-rHuMGDF was not released over time. In vivo, TPO plasma levels decreased significantly (p < 0.001), by 30.7 +/- 5.8 and 20.9 +/- 2.1 percent after transfusion of unmanipulated and sham-saturated PLT preparations, respectively. However, TPO plasma levels were unaffected after the transfusion of TPO-R-saturated PLTs despite comparable transfusion-induced PLT count increases.

CONCLUSION

These data strongly support the concept that binding to PLT TPO-R is directly involved in human TPO plasma level regulation in vivo.

Membrane localization is not required for Mpl function in normal hematopoietic cells

Cellular trafficking of growth factor receptors, including cross-talk among receptors at the cell surface, may be important for signal transduction in normal hematopoietic cells. To test this idea, the signaling domain of Mpl (the thrombopoietin receptor) was targeted to the plasma membrane, or to the cytoplasm of murine marrow cells, and the ability of the cells to proliferate and differentiate in response to Mpl dimerized at the plasma membrane or free in the cytoplasm was assessed. Constructs encoding the signaling domain of Mpl linked to an FK506 binding protein domain (to permit dimerization by the membrane-permeable ligand AP20187) with or without a myristylation sequence (to target the receptor to the plasma membrane) and a hemagglutinin epitope tag were generated and introduced into murine marrow cells using a murine stem cell virus (MSCV)-based retroviral vector. Both populations of transduced marrow cells proliferated in Iscoves modified Dulbecco medium-10% FCS-100 nM AP20187 without exogenous growth factors for more than 100 days and achieved greater than a 10(7)-fold expansion of cells by day 50 (n = 4 transductions). Growth was dimerizer dependent, and myeloid, erythroid, and megakaryocytic progenitors were generated. Activation of Mpl either at the plasma membrane or in the cytoplasm allowed for the terminal maturation of transduced progenitor cells. Introduction of membrane-targeted or cytoplasmic Mpl into fetal liver cells from homozygous JAK2 knock-out mice or wild-type littermates demonstrated that both forms of Mpl require JAK2 for signaling. These data show that the activation of Mpl independent of its normal plasma membrane location can support production of the full range of normal hematopoietic progenitor cells in vitro.

The role of the liver in the production of thrombopoietin compared with erythropoietin

The liver plays an important role in the production of haemopoietic hormones. It acts as the primary site of synthesis of erythropoietin (EPO) in the fetal stage, and it is the predominant thrombopoietin (TPO)-producing organ for life. In contrast to that of EPO and other liver proteins, the hepatic synthesis of TPO is influenced little by external signals. Hepatocytes express the TPO gene in a constitutive way, i.e. irrespective of the level of platelets in blood. Megakaryocytes and platelets remove the hormone from blood by means of their high-affinity TPO receptors. Normally, the plasma level of TPO is relatively low ( approximately 10(-12) mol/l). However, in thrombocytopenic states due to marrow failure or bleeding, the concentration of circulating TPO may increase greatly. The simple feedback regulation by TPO and its target cells is efficient in maintaining constant platelet numbers in healthy people. Persisting thrombocytopenia develops only in severe liver or marrow failure. On the other hand, an increase in circulating TPO and interleukin 6 (IL-6) may cause reactive thrombocytosis in inflammatory diseases, including cancer. The indications for recombinant human thrombopoietin (rHuTPO) therapy and its impact on transfusion medicine are still under investigation.

Thrombopoietin in the fetus and neonate

C-mpl ligand or thrombopoietin (Tpo) is increasingly recognised as the major regulator of platelet homeostasis in humans. Relatively little is known about Tpo in the fetus and neonate but no evidence has yet been found to suggest any fundamental difference in Tpo structure, function and regulation in the fetus and neonate compared to older age groups. Tpo mRNA transcripts have been detected in the fetus as early as 6 weeks post conception and the liver appears to be the main site of Tpo production in both the fetus and neonate. The vast majority of healthy newborns have detectable levels of circulating Tpo and raised Tpo levels are commonly, but not consistently, found in thrombocytopenic neonates. In adults receptor binding and subsequent metabolism of Tpo is proposed as the main method of regulation of the circulating Tpo level. Preliminary studies in neonates showing increased Tpo levels most often during thrombocytopenia accompanied by reduced megakaryocytopoiesis supports this concept. In addition to this demonstrable fetal and neonatal endogenous Tpo production megakaryocyte progenitor and precursor cells from the fetus and from preterm and term newborns proliferate and differentiate extensively in-vitro in response to exogenous Tpo. Furthermore a recent study has shown a marked rise in platelet count in newborn rhesus monkeys administered one form of recombinant Tpo. Although these studies remain at an early stage together these findings strongly suggest that, as in adults, Tpo is the major regulator of platelet homeostasis in the fetus and neonate. Thrombocytopenia is common in sick neonates and progress in understanding this important clinical problem is likely to be greatly enhanced by the current and future research into Tpo production, function and regulation in the healthy and thrombocytopenic fetus and neonate.

Determination of interactions between human thrombopoietin and its receptor MPL by yeast two-hybrid system and affinity biosensor

The binding of human thrombopoietin to the extracellular domain of its receptor MPL prompts a cascade transduction of intracellular signals, leading to the development of megakaryocyte precursors and the production of circulating platelets. We have used a yeast two-hybrid system to reveal, via in vivo interactions between different deletion constructs of MPL and thrombopoietin, that the extracellular subunit 1 of MPL is the ligand binding site and the N-terminal domain of thrombopoietin alone is sufficient for the binding. The extracellular portion of MPL was heterologously expressed in E. coli and its specific affinity with thrombopoietin was visualized in vitro by resonance mirror biosensor technique.

Elevated serum thrombopoietin and interleukin-6 concentrations in thrombocytosis associated with inflammatory bowel disease

Reactive thrombocytosis is a typical feature in inflammatory bowel disease (IBD). The question arose as to whether the normal negative feedback regulation of the concentration of thrombopoietin (TPO) in blood was altered in IBD patients. We measured serum immunoreactive TPO in 30 patients with active IBD, 29 patients with inactive IBD, and 56 healthy controls. The results were related to platelet and leukocyte counts and to the serum concentration of interleukin 6 (IL-6). Patients with active IBD exhibited significantly increased TPO levels (medians 112 pg/ml vs. 90 pg/ml in controls, p < 0.05) in association with thrombocytosis (428 platelets/nl blood vs. 241 platelets/nl blood in controls), leukocytosis, and increased IL-6 levels (12.9 pg/ml vs. 2.5 pg/nl in controls). In patients with inactive IBD, only platelets (322/nl) and leukocytes were above normal. Although the observation of increased TPO and IL-6 levels provides an explanation for the occurrence of thrombocytosis in IBD, the pathogenetic mechanisms underlying the elevated TPO level still need to be identified.