The structure, biology and potential therapeutic applications of recombinant thrombopoietin

Thrombopoietin-based CAR-T cells demonstrate in vitro and in vivo cytotoxicity to MPL positive acute myelogenous leukemia and hematopoietic stem cells

While targeting CD19+ hematologic malignancies with CAR T cell therapy using single chain variable fragments (scFv) has been highly successful, novel strategies for applying CAR T cell therapy with other tumor types are necessary. In the current study, CAR T cells were designed using a ligand binding domain instead of an scFv to target stem-like leukemia cells. Thrombopoietin (TPO), the natural ligand to the myeloproliferative leukemia protein (MPL) receptor, was used as the antigen binding domain to engage MPL expressed on hematopoietic stem cells (HSC) and erythropoietic and megakaryocytic acute myeloid leukemias (AML). TPO-CAR T cells were tested in vitro against AML cell lines with varied MPL expression to test specificity. TPO-CAR T cells were specifically activating and cytotoxic against MPL+ leukemia cell lines. Though the TPO-CAR T cells did not extend survival in vivo, it successfully cleared the MPL+ fraction of leukemia cells. As expected, we also show the TPO-CAR is cytotoxic against MPL expressing bone marrow compartment in AML xenograft models. The data collected demonstrate preclinical potential of TPO-CAR T cells for stem-like leukemia through assessment of targeted killing of MPL+ cells and may facilitate subsequent HSC transplant under reduced intensity conditioning regimens.

Retrospective analysis of the bleeding risk induced by oral antiplatelet drugs during radiotherapy

We conducted this retrospective analysis to assess whether oral antiplatelet drugs (APDs) during radiotherapy increase bleeding risk.Patients who underwent radiotherapy for esophageal cancer (EC) in the Third Affiliated Hospital of Soochow University from January 2015 to December 2019 were screened. After the differences in clinical parameters were eliminated by a propensity-score matched (PSM) analysis at a 1:1 ratio, the thrombocytopenia, consumption of platelet-increasing drugs, suspension of radiotherapy, and bleeding in patients taking APDs were compared with those in the control group.A total of 986 patients were included in the original dataset. Of these, 34 patients took APDs during radiotherapy. After matching, the APD and control groups each retained 31 patients. There was no significant difference in platelet concentrations between the two groups before radiotherapy (P = .524). The lowest platelet concentration during radiotherapy in the APD group was significantly lower (P = .033). The consumption of platelet-increasing drugs in the APD group was higher than that in the control group (P  < .05). However, there was no significant difference in the average number of days of radiotherapy suspension because of thrombocytopenia (P = .933) and no significant difference in the incidence of bleeding between the two groups (P = .605).Oral APDs during radiotherapy lead to a further decrease in platelet concentration, but timely and adequate application of platelet-increasing drugs can avoid the increased risk of bleeding and the reduced efficacy of radiotherapy.

The non-glycosylated N-terminal domain of human thrombopoietin is a molten globule under native conditions

Human thrombopoietin (hTPO) is a primary hematopoietic growth factor that regulates megakaryocytopoiesis and platelet production. The non-glycosylated form of 1-163 residues of hTPO (hTPO₁₆₃ ) including the N-terminal active site domain (1-153 residues) is a candidate for treating thrombocytopenia. However, the autoantigenicity level of hTPO₁₆₃ is higher than that of the full-length glycosylated hTPO (ghTPO₃₃₂ ). In order to clarify the structural and physicochemical properties of hTPO₁₆₃ , circular dichroism (CD) and differential scanning calorimetry (DSC) analyses were performed. CD analysis indicated that hTPO₁₆₃ undergoes an induced-fit conformational change (+19.0% for helix and -16.7% for β-strand) upon binding to the neutralizing antibody TN1 in a manner similar to the coupled folding and binding mechanism. Moreover, DSC analysis showed that the thermal transition process of hTPO₁₆₃ is a multistate transition; hTPO₁₆₃ is thermally stabilized upon receptor (c-Mpl) binding, as indicated with raising the midpoint (T_m ) temperature of the transition by at least +9.5 K. The conformational variability and stability of hTPO₁₆₃ indicate that hTPO₁₆₃ exists as a molten globule under native conditions, which may enable the induced-fit conformational change according to the type of ligands (antibodies and receptor). Additionally, CD and computational analyses indicated that the C-terminal domain (154-332 residues) and glycosylation assists the folding of the N-terminal domain. These observations suggest that the antibody affinity and autoantigenicity of hTPO₁₆₃ might be reduced, if the conformational variability of hTPO₁₆₃ is restricted by mutation and/or by the addition of C-terminal domain with glycosylation to keep its conformation suitable for the c-Mpl recognition.

Thrombopoietin induces hematopoiesis from mouse ES cells via HIF-1α-dependent activation of a BMP4 autoregulatory loop

Understanding the molecular mechanisms underlying hematopoietic differentiation of embryonic stem (ES) cells may help to ascertain the conditions for the in vitro generation of hematopoietic cells. Previously, we found that patients with congenital amegakaryocytic thrombocytopenia (CAMT), who develop pancytopenia early after birth, harbor mutations within the thrombopoietin (TPO) receptor, c-MPL. This knowledge, together with observations in vitro and in vivo, suggests that TPO/c-MPL signaling promotes early hematopoiesis. However, the mechanisms underlying TPO signaling are not fully elucidated. Here, we describe a direct connection between TPO and bone morphogenetic protein 4 (BMP4) signaling pathways in determining the hematopoietic fate of ES cells. Morphogen BMP4 is known to induce early hematopoietic differentiation of ES cells. Treatment of ES cells with TPO induced the autocrine production of BMP4 with concomitant upregulation of the BMP receptor BMPR1A, phosphorylation of SMAD1, 5, 8, and activation of specific BMP4 target genes; this was mediated by TPO-dependent binding of transcription factor HIF-1α to the BMP4 gene promoter. Treatment of ES cells with the BMP antagonist noggin substantially reduced TPO-dependent hematopoietic differentiation of ES cells. Thus, our findings contribute to the establishment of techniques for generating hematopoietic cells from ES cells.

Milestones in understanding platelet production: a historical overview

The discovery of thrombopoietin (TPO, also termed THPO) in 1994 was a major achievement in understanding the regulation of platelet production. In prior decades, physiological studies had demonstrated that platelets were produced from bone marrow megakaryocytes and that the megakaryocytes responded to thrombocytopenia by increasing their number, size and DNA ploidy. In 1958, it was proposed that a 'thrombopoietin' must exist that regulated this interaction between the circulating platelet mass and the bone marrow megakaryocytes. After over three decades of effort, TPO was finally purified by five independent laboratories. TPO stimulated megakaryocyte colony-forming cell growth and increased the number, size and ploidy of megakaryocytes. When the genes for TPO or TPO receptor were eliminated in mice, megakaryocytes grew and platelets were made, but at 15% of their normal number. A first generation of recombinant human (rh) TPO molecules [rhTPO and pegylated recombinant human megakaryocyte growth and development factor (PEG-rhMGDF)] rapidly entered clinical trials in 1995 and increased platelet counts in humans undergoing non-myeloablative chemotherapy but not in those undergoing stem cell transplantation. Antibodies developed against PEG-rhMGDF and development of these recombinant thrombopoietins ended. A second generation of TPO receptor agonists (romiplostim and eltrombopag) was then developed. Neither of these TPO receptor agonists demonstrated any significant untoward effects and both are now licensed in many countries for the treatment of immune thrombocytopenia. This review describes the significant experiments that have surrounded the discovery of TPO and its clinical development.

Romiplostim

Thrombocytopenia is a common clinical problem associated with a wide range of medical conditions including immune thrombocytopenia (ITP), chemotherapy-induced thrombocytopenia (CIT), hepatitis C-related thrombocytopenia, and myelodysplastic syndromes (MDS). Until recently, the only treatments for thrombocytopenia were to alleviate the underlying cause or to provide platelet transfusions. With the discovery and recent clinical availability of thrombopoietin (TPO) mimetics, a new treatment option has emerged. Two TPO mimetics are currently clinically available for treating ITP: romiplostim (an injectable peptide TPO mimetic) and eltrombopag (a non-peptide, orally available TPO mimetic). This chapter reviews the development, biology, and clinical trials with romiplostim. With few adverse effects, romiplostim is effective in raising the platelet count in over 80% of ITP patients, allowing them to discontinue other therapies, reduce the need for splenectomy, and improve their quality of life. Long-term theoretical side effects of romiplostim treatment include reticulin formation, thromboembolism, and antibody formation to romiplostim. A practical way of using romiplostim is provided: a higher starting dose of 3 mg/kg is recommended along with efforts to avoid withholding the dose. Future studies will assess the utility of romiplostim in CIT, hepatitis-C related thrombocytopenia, and MDS.

New thrombopoietic growth factors

Thrombopoietin (TPO) is the physiologic regulator of platelet production and works by binding to its receptor on megakaryocyte precursor cells, thereby activating a large number of antiapoptotic and cell maturation pathways. "First-generation" recombinant forms of TPO were developed over a decade ago and were found to increase the platelet count in patients undergoing nonmyeloablative chemotherapy, in patients with immune thrombocytopenic purpura (ITP) and myelodysplasia, as well as in platelet apheresis donors. Thrombopoietin did not improve platelet counts in patients undergoing stem cell transplantation or acute leukemia induction. Further development ended when antibodies formed against one of the recombinant proteins. Subsequently, 2 "second-generation" TPO mimetics have been developed and are entering clinical practice: romiplostim and eltrombopag. Romiplostim is an injectable peptide TPO mimetic that activates the TPO receptor just like native TPO. Eltrombopag is an oral nonpeptide TPO mimetic that activates the TPO receptor by binding to a different region of the TPO receptor that does not compete with TPO binding. Both increased the platelet counts in healthy subjects and in over two thirds of patients with ITP both before and after splenectomy; responses were maintained for at least 1 year. Romiplostim and eltrombopag are now US Food and Drug Administration approved for the second-line treatment of patients with ITP. Adverse events have been few, but long-term assessment for reticulin formation, increased bone marrow blasts, and thromboembolism is ongoing. Studies are under way to assess the efficacy of these drugs in the treatment of other thrombocytopenic disorders associated with chemotherapy, myelodysplasia, and chronic hepatitis.

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.

SH2-inositol phosphatase 1 negatively influences early megakaryocyte progenitors

Background

The SH2-containing-5′inositol phosphatase-1 (SHIP) influences signals downstream of cytokine/chemokine receptors that play a role in megakaryocytopoiesis, including thrombopoietin, stromal-cell-derived-Factor-1/CXCL-12 and interleukin-3. We hypothesize that SHIP might control megakaryocytopoiesis through effects on proliferation of megakaryocyte progenitors (MKP) and megakaryocytes (MK).

Methodology and Principal Findings

Herein, we report the megakaryocytic phenotype and MK functional assays of hematopoietic organs of two strains of SHIP deficient mice with deletion of the SHIP promoter/first exon or the inositol phosphatase domain. Both SHIP deficient strains exhibit a profound increase in MKP numbers in bone marrow (BM), spleen and blood as analyzed by flow cytometry (Lin⁻c-Kit⁺CD41⁺) and functional assays (CFU-MK). SHIP deficient MKP display increased phosphorylation of Signal Transducers and Activators of Transcription 3 (STAT-3), protein kinase B (PKB/AKT) and extracellular signal-regulated kinases (ERKs). Despite increased MKP content, total body number of mature MK (Lin⁻c-kit⁻CD41⁺) are not significantly changed as SHIP deficient BM contains reduced MK while spleen MK numbers are increased. Reduction of CXCR-4 expression in SHIP deficient MK may influence MK localization to the spleen instead of the BM. Endomitosis, process involved in MK maturation, was preserved in SHIP deficient MK. Circulating platelets and red blood cells are also reduced in SHIP deficient mice.

Conclusions/Significance

SHIP may play an important role in regulation of essential signaling pathways that control early megakaryocytopoiesis in vivo.

The prolonged half-lives of new erythropoietin derivatives via peptide addition

Erythropoietin, or Epo, is a hematopoietic cytokine that promotes erythropoiesis, and recombinant human Epo has been used in the treatment of anemia in various chronic diseases. Here, we have constructed novel Epo derivatives with prolonged half-lives by adding peptides to the carboxy terminus of Epo without using linkers. The fused peptides were selected from the carboxy terminal region of human chorionic gonadotropin (hCG) or human thrombopoietin (hTpo), which promote the proper folding, secretion, and stabilization of bioactive glycoproteins. Addition of these peptides did not interfere with secretion or receptor binding, and significantly increased the in vivo half-life of human Epo, as measured by intravenous administration in rats. The plasma half-life of the Epo constructs was longest when the carboxy terminal 28 aa of the beta subunit of hCG was added (Epo-CGC), a half-life that was slightly longer than NESP (Aranesp), which is the most effective Epo product in current clinical use. The transformation of four Ser glycosylation sites to Ala on the CGC sequence also lengthened the plasma half-life of Epo, indicating that the in vivo stabilizing effect of the hCG peptide was due to both structures within the peptide itself and its O-glycosylations. The application of the carboxy terminal half of hTpo also resulted in remarkably reduced elimination of the Epo chimera (Epo-TpC), possibly due to protection by the TpC sequence. The in vivo hematopoietic activity of Epo derivatives in mice was consistent with their pharmacokinetic profiles. Therefore, these derivatives with prolonged half-lives may provide opportunities for developing new Epo therapeutics with less frequent administration.

Ex Vivo Expansion of Megakaryocyte Progenitor Cells: Cord Blood Versus Mobilized Peripheral Blood

Thrombocytopenia is a problematic and potentially fatal occurrence after transplantation of cord blood stem cells. This problem may be alleviated by infusion of megakaryocyte progenitor cells. Here, we compared the ability of hematopoietic progenitor cells obtained from cord blood and expanded in culture to that of mobilized peripheral blood cells. The CD34(+) cells were plated for 10 days in presence of thrombopoietin (TPO) alone and combined with stem cell factor (SCF), Flt3-ligand (FL), interleukin-3 (IL-3), IL-6, and IL-11. Cells were analyzed for the CD41 and CD42b expression and for their ploidy status. Ex vivo produced platelets were enumerated. We show that (1) TPO alone was able to induce differentiation of CD34(+) cells into CD41(+) cells, with limited total leucocyte expansion; (2) the addition of SCF to TPO decreased significantly CD41(+) cell percentage in CB, but not in MPB; and (3) in CB, the addition of FL, IL-6, and IL-11 to TPO increased the leukocyte expansion with differentiation and terminal maturation into MK lineage. In these conditions, high numbers of immature CD34(+)CD41(+) MK progenitor cells were produced. Our results thereby demonstrate a different sensitivity of CB and MPB cells to SCF, with limited CB MK differentiation. This different sensitivity to SCF (produced constitutively by BM stromal cells) could explain the longer delay of platelet recovery after CB transplant. Nevertheless, in CB, the combination of TPO with FL, IL-6, and IL-11 allows generation of a suitable number of immature MK progenitor cells expressing both CD34 and CD41 antigens, which are supposed to be responsible for the platelet recovery after transplantation.

Colony stimulating factors for prevention of myelosupressive therapy induced febrile neutropenia in children with acute lymphoblastic leukaemia

BACKGROUND

Acute lymphoblastic leukaemia (ALL) is the most common cancer in childhood and febrile neutropenia is a potentially life-threatening side effect of its treatment. Current treatment consists of supportive care plus antibiotics. Clinical trials have attempted to evaluate the use of colony-stimulating factors (CSF) as additional therapy to prevent febrile neutropenia in children with ALL. The individual trials do not show whether there is significant benefit or not. Systematic review provides the most reliable assessment and the best recommendations for practice.

OBJECTIVES

To evaluate the safety and effectiveness of the addition of G-CSF or GM-CSF to myelosuppressive chemotherapy in children with ALL, in an effort to prevent the development of febrile neutropenia. Evaluation of number of febrile neutropenia episodes, length to neutrophil count recovery, incidence and length of hospitalisation, number of infectious disease episodes, incidence and length of treatment delays, side effects (flu-like syndrome, bone pain and allergic reaction), relapse and overall mortality (death).

SEARCH STRATEGY

The search covered the Cochrane Central Register of Controlled Trials, MEDLINE, EMBASE, CANCERLIT, LILACS, and SciElo. We manually searched records of conference proceedings of ASCO and ASH from 1985 to 2003 as well as databases of ongoing trials. We consulted experts and scanned references from the relevant articles.

SELECTION CRITERIA

We looked for randomised controlled trials (RCTs) comparing CSF with placebo or no treatment as primary or secondary prophylaxis to prevent febrile neutropenia in children with ALL.

DATA COLLECTION AND ANALYSIS

Two authors independently selected, critically appraised studies and extracted relevant data. The end points of interest were:* Primary end points: number of febrile neutropenia episodes and overall mortality (death) * Secondary end points: time to neutrophil count recovery, incidence and length of hospitalisation, number of infectious diseases episodes, incidence and length of treatment delays, side effects (flu-like syndrome, bone pain and allergic reaction) and relapse. We conducted a meta-analysis of these end points and expressed the results as Peto odds ratios. For continuous outcomes we calculated a weighted mean difference and a standardised mean difference. For count data, meta-analysis of the logarithms of the rate ratios using generic inverse variance was employed.

MAIN RESULTS

We scanned more than 5500 citations and included six studies with a total of 332 participants in the analysis. There were insufficient data to assess the effect on survival. The use of CSF significantly reduced the number of episodes of febrile neutropenia episodes (Rate Ratio = 0.63; 95% confidence interval (CI) 0.46 to 0.85; p =0.003, with substantial heterogeneity), the length of hospitalisation (weighted mean difference (WMD) = -1.58; 95% CI -3.00 to -0.15; p = 0.03), and number of infectious diseases episodes (Rate Ratio=0.44; 95%CI 0.24 to 0.80; p=0.002). In spite of these results, CSF did not influence the length of episodes of neutropenia (WMD = -1.11; 95% CI -3.55 to 1.32; p = 0.4) or delays in chemotherapy courses (Rate Ratio=0.77; 95%CI 0.49 to 1,23; p=0.28) .

AUTHORS' CONCLUSIONS

Children with ALL treated with CSF benefit from shorter hospitalisation and fewer infections. However, there was no evidence for a shortened duration of neutropenia nor fewer treatment delays, and no useful information about survival. The role of CSF regarding febrile neutropenia episodes is still uncertain. Although current data shows statistical benefit for CSF use, substantial heterogeneity between included trials does not allow this conclusion.

Inherited thrombocytopenias: toward a molecular understanding of disorders of platelet production

PURPOSE OF REVIEW

To review the defined syndromes of inherited thrombocytopenia and discuss new genetic data for several disorders that shed light on the process of megakaryopoiesis.

RECENT FINDINGS

The genes responsible for several inherited thrombocytopenias have been recently discovered, including congenital amegakaryocytic leukemia, amegakaryocytic thrombocytopenia with radio-ulnar synostosis, familial platelet syndrome with predisposition to acute myelogenous leukemia, Paris-Trousseau, Wiskott-Aldrich syndrome, and the May-Hegglin, Sebastian, Epstein, and Fechner syndromes. These clinical syndromes, combined with studies in mouse and in vitro models, reveal the importance of these genes for normal hematopoiesis.

SUMMARY

Although inherited syndromes of thrombocytopenia are rare, characterization of mutations in these disorders has contributed greatly to our understanding of megakaryocyte and platelet development. A systematic registry of congenitally thrombocytopenic individuals would almost certainly lead to new genetic discoveries.

CD41+ and CD42+ hematopoietic progenitor cells may predict platelet engraftment after allogeneic peripheral blood stem cell transplantation

The objective of this study was to quantify subpopulations of CD34+ cells such as CD41+ and CD42+ cells that might represent megakaryocyte (MK) precursors in peripheral blood stem cell (PBSC) collections of normal, recombinant human granulocyte-colony stimulating factor (rhG-CSF) primed donors and to determine whether there is a statistical association between the dose infused megakaryocytic precursors and the time course of the platelet recovery following an allogeneic PBSC transplantation. Twenty-six patients with various hematologic malignancies transplanted from their HLA identical siblings between July 1997 and December 1999 were used. All patients except one with severe aplastic anemia who had cyclophosphamide (CY) alone received busulfan-CY as preparative regimen and cyclosporine-methotrexate for GVHD prophylaxis. Normal healthy donors were given rhG-CSF 10 microg/kg/day subcutaneously twice daily and PBSCs were collected on days 5 and 6. The median number of infused CD34+, CD41+ and CD42+ cells were 6.61 x 10(6)/kg (range 1.47-21.41), 54.85 x 10(4)/kg (5.38-204.19), and 49.86 x 10(4)/kg (6.82-430.10), respectively. Median days of ANC 0.5 x 10(9)/L and platelet 20 x 10(9)/L were 11.5 (range 9-15) and 13 (8-33), respectively. In this study, the number of CD41+ and CD42+ cells infused much better correlated than the number of CD34+ cells infused with the time to platelet recovery of 20 x 10(9)/L in 26 patients receiving an allogeneic match sibling PBSC transplantation (r = -0.727 and P < 0.001 for CD41+ cells, r = -0.806 and P < 0.001 for CD42+ cells, r = -0.336 and P > 0.05 for CD34+ cells). There was an inverse correlation between the number of infused CD41+ and CD42+ cells and duration of platelet engraftment. Therefore, as the number of CD41+ and CD42+ cells increased, duration of platelet engraftment (time to reach platelet count of > or = 20 x 10(9)/L) shortened significantly. Based on this data we may conclude that flow cytometric measurement of CD41+ and CD42+ progenitor cells may provide an accurate indication of platelet reconstitutive capacity of the allogeneic PBSC transplant.

Evidence for a novel thrombopoietin signalling event: activation of protein kinase A in human megakaryoblastic CMK cells

Thrombopoietin (TPO) plays a crucial role in megakaryocyte development. TPO signalling, which is mediated by its receptor Mpl, includes Janus kinase, (JAK) signal transducer and activator of transcription (STAT) and Shc/Ras/mitogen-activated protein kinase (MAPK) pathways. The precise nature of these signalling routes has not been clarified in detail up until now. We investigated the effect of TPO on activation of cAMP-dependent protein kinase (PKA) and its involvement in MAPK signalling in human megakaryoblastic leukaemia CMK cells. For estimation of PKA activity, phosphorylation of a PKA-specific peptide substrate was assayed in CMK cell lysates. Since activation of PKA is associated with translocation of its catalytic subunit alpha (C-PKA) into the cell nucleus, Western blot analysis of nuclear fractions with an anti-C-PKA antibody was additionally performed. The activation of TPO-induced MAPK activation and the effect of the PKA inhibitor H-89 was measured using immunoblotting with a monoclonal anti-pERK antibody. TPO enhanced cAMP and induced activation of PKA in CMK cells. In addition, H-89 partly blocked TPO-induced MAPK activation in CMK cells. Our results indicate a novel TPO-triggered signalling event, activation of the cAMP/PKA pathway in human megakaryoblastic CMK cells. This signal transduction route seems to be involved in TPO-induced MAPK signaling.

Protein characteristics of thrombopoietin

Thrombopoietin (TPO) was purified from irradiated thrombocytopenic rat plasma. In the process of purification, some biochemical and biological characteristics were investigated. Rat plasma TPO was extremely hydrophobic and exhibited multiple peaks of activity on gel filtration. Both the low and high molecular weight fractions were separately subjected to further purification. Consequently, a rat TPO cDNA was cloned based on the amino acid sequences of purified rat plasma TPO. It revealed that each final purified rat plasma TPO was not a full-length form. In addition, rat hepatocytes and three rat hepatoma cell lines were found to produce rat TPO. Each native TPO derived from cultured cells was also partially purified, and hepatocyte-derived TPOs were shown to be heterogeneous in molecular weight. To study the structure of TPO, various recombinant TPO molecules were generated. Two disulfide bonds (Cys7-Cys151 and Cys29-Cys85) located in the N-terminal domain of TPO have an important effect on its biological activity. The human TPO muteins, sequentially deleted from the C-terminal, were expressed in COS-1 cells. TPO (1-151) was active, but TPO (1-150), which lacks Cys151, did not exhibit TPO activity. These findings indicate that the region essential for TPO activity is the N-terminal domain, which contains two disulfide bonds. Although the role(s) of the C-terminal domain is not clear at present, the potential N-glycosylation in the C-terminal domain is not directly required for exhibiting TPO activity.

Gene expression and transcriptional regulation of thrombopoietin

Thrombopoietin (TPO) is predominantly expressed in the liver among various tissues that express TPO transcripts. To investigate the transcriptional regulation of the human TPO gene in the liver, we determined the major transcription initiation site by means of 5'-RACE and Northern blotting. From these analyses, we concluded that TPO gene transcription started at various points, and the transcription initiation sites of the human TPO gene were localized downstream, close to a point we determined by S1 nuclease mapping. The human TPO promoter region contains consensus sequences of GATA, Evi-1, and Ets binding sites. We used the hepatocellular carcinoma cell line, HepG2, that expresses TPO mRNA to analyze its promoter activity by transfecting various reporter plasmids containing a sequentially 5'-deleted human TPO promoter. Although GATA binding factors increased the promoter activity, their effect was independent of the GATA binding consensus sequence. On the other hand, Evi-1 did not affect transcription. Moreover, we defined the core promoter region, in which an Ets binding consensus sequence was located. The deletion or mutation of the Ets binding site resulted in a loss of the promoter activity. These results suggested that TPO is regulated by the Ets family of transcription factors.

Ex vivo expansion of early and late megakaryocyte progenitors

Our goal is to produce ex vivo-expanded human megakaryocytes (MK) cells from peripheral blood progenitor cell (PBPC) harvests for use in supplementing conventional autografts. In this paper we show the megakaryocytopoietic productivity of small-scale in vitro serum-free cultures of human CD34+ cells containing MK growth and development factor (MGDF) and stem cell factor (Kit ligand; SCF) +/- granulocyte colony-stimulating factor (G-CSF). Cultures were characterized after 3, 6, 9, and 13 days by flow cytometry and clonogenic assays. CD34+ cells expanded 5.2- and 3.4-fold, and produced 2.2 and 2.4 CD34+/41(+) cells per seeded CD34+ cell after 6 and 9 days in culture, respectively. None were detected at day 13. CD41+ cells expanded exponentially over 13 days. Colony-forming unit-megakaryocyte (CFU-MK) also expanded exponentially, but the proportion of the most primitive CFU-MK dropped from 45% to 1.5% and to <1% after 6 and 9 days, respectively. G-CSF increased total cell expansion, but decreased CD41+ frequency, yielding no gain in MK production. We also found that PB CD34+ cells cultured for 3-6 days are richer in primitive MK progenitors, while those cultured for 9-13 days have greater numbers of more differentiated MKs. Overall, the combination of MGDF+SCF proved sufficient for expanding CD34+/CD41+ cells. As the stage of ex vivo MK differentiation most conducive to optimal platelet production in vivo is not known, we are planning a clinical trial to determine the efficacy of ex vivo-expanded MKs on platelet recovery in relation to MK maturity.

Leukocyte behavior in a free-flap model following chemotherapy and application of granulocyte colony-stimulating factor (GCSF)

Free-flap reconstruction following tumor resection and chemotherapy is used increasingly in sarcoma patients. Granulocyte colony-stimulating factor (GCSF) is used to stimulate polymorphnuclear leukocytes (PMNs). In this study we used a free-flap model to investigate PMN behavior after chemotherapy and GCSF stimulation. Eighteen Lewis rats were divided into three groups: (1) chemotherapy + vehicle; (2) chemotherapy + GCSF; and (3) control. Group 1 received vinblastine; group 2 received vinblastine plus 300 microg rhGCSF. Blood was drawn for total white cell and PMN counts for 9 days. Free-flap surgery was simulated by isolating the cremaster muscle on its pedicle and anastomosing the artery. Leukocyte-endothelium interaction was assessed by observation of leukocytes rolling and sticking. Leukocyte sequestration was measured by counting leukocytes in the lungs and the abdomen. We found that leukocyte rolling and sticking were significantly increased, while sequestration was decreased in the chemotherapy + GCSF group. We conclude that leukocytes-endothelium interaction after chemotherapy and GCSF administration is increased in the microcirculation. This augments the risk of microvascular compromise and subsequent flap failure, as capillary flow may be altered.

Identification of megakaryocyte precursors in peripheral blood stem cell collections from normal donors

Platelet engraftment, the time course and magnitude of platelet recovery (PR) post-transplant, is imprecisely defined but is most often reported as the time to transfusion (tx) independence and/or a platelet count > or = 20,000/microl. While correlations between engraftment time for granulocytes (PMN) and the dose of CD34-positive cells per kilogram are established, such associations have not been established for platelet engraftment. The objective of this study was to quantify subpopulations of CD34-positive cells in peripheral blood stem cell (PBSC) collections of normal, colony-stimulating factor-granulocyte) (G-CSF) primed donors that might represent megakaryocyte (MK) precursors, and to determine whether there is a statistical association between the dose transfused and the time course of the recovery. Based on previously published data of the sequential expression of CD34, HLA-DR, and CD61, among others, during MK maturation, a combination of corresponding antibodies for the detection of various antigen coexpressions by flow cytometry fluorescence-activated cell sorting [FACS] was chosen. CD34-positive cells were further subdivided into CD34++ (bright) and + (dim). Ploidy of density-gradient separated cells was examined in subsequent donor samples by FACS. For the entire group of patients, there was no strong correlation between any of the studied subpopulations and time to PR. Only in a selected groups of patients whose platelet counts showed a sustained increase during the first 6 days after engraftment, there was a weak correlation between the time to PR and the quantity of CD34+/+CD61+ (r = -0.57) and CD34++HLA-DR-CD61+ (r = -0.62) cells infused. The magnitude of platelet production in these pt., a product of the peripheral blood platelet count and the patient's blood volume, was correlated with the time to PR (r = -0.73). We conclude from this study that subpopulations within CD34+ cells are making some contribution to PR in allogeneic peripheral blood stem cell transplantation, but the correlations are not sufficiently strong because there are probably too many unpredictable and unknown variables in the allogeneic setting that influence the pattern of engraftment.

Effects of Recombinant Human Megakaryocyte Growth and Development Factor (rHuMGDF) on Platelet Production, Platelet Aggregation, and Thrombosis

Recombinant human megakaryocyte growth and development factor (rHuMGDF) is a c-mpl ligand that promotes the differentiation of CD34+ precursor cells into megakaryocyte, and then platelets. In experimental animals, injection of this and other c-mpl ligands leads to profound increases in the circulating platelet count in a matter of days. However, c-mpl ligands have also been shown to sensitize platelets to aggregating agents in vitro, raising the possibility that c-mpI ligands may have prothrombotic effects in vivo. Therefore, characterizing rHuMGDF in an in vivo model of thrombosis is a necessary and critical step in defining the in vivo pharmacology of this novel and important hernatopoietic factor, a pegylated form of which is currently in clinical trials. To determine the biologically effective doses in the rabbit, daily subcutaneous injections of rHuMGDF at 0.1, 1.0, or 10 µg/kg were administered ever 7 days. Daily injection of 10 µ/kg produced an approximate fourfold increase in platelet count and 1.0 µ/kg doubled platelet count over the injection period, both of which were statistically significant. The serum concentrations of rHuMGDF were determined 10 minutes following a single intravenous injection with 0.1, 1.0, and 10 µ/kg, and were 0.05 +/- 0.02, 0.98 +/- 0.07, and 21.32 +/- 21.35 ng/ml. To determine whether rHuMGDF can sensitize platelets in vivo, platelet aggregometry was performed on platelets isolated from animals immediately before and 10 minutes after they had been injected intravenously with rHuMGDF (0.1, 1.0, and 10 µ/kg). Intravenous injection of 10 µ/kg produced measurable changes in platelet aggregometry ex vivo, as evidenced by an increased sensitivity of platelets to adenosine diphosphate (ADP). To assess. the in vivo prothrombotic potential of rHuMGDF, a rabbit carotid artery model of cyclic flow reduction (CFR) was used to measure the effect of intravenous rHuMGDF administration on the rate of thrombus formation as assessed by CFR slope and frequency. Intravenous administration of rHuMGDF had no effect on CFR slope or frequency when administered in doses ranging from 0.1 to 10 µ/kg. Control experiments demonstrated that CFR slope and frequency can be enhanced by intravenous infusion of epinephrine and can be abolished by the combined administration of aspirin and ketanserin, indicating that potentially prothrombotic and antithrombotic agents can be identified in this model. We conclude that biologically active doses of rHuMGDF used in this study (1.0 and 10 µ/kg) produce measurable serum levels, induce a thrombopoietic effect, and sensitize platelets in vivo, as determined by ex vivo aggregometry, at 10 µ/kg. Despite the sensitization of platelets to aggregation induced by ADP, it is clear that rHuMGDF does not alter the pattern of CFRs observed in the rabbit carotid artery, whereas agents known to sensitize platelets (epinepherine) and to inhibit platelets (aspirin and ketanserin) readily affected the CFR pattern. These findings indicate that intravenous rHuMIGDF administration, while capable of sensitizing pIatelets, does not enhance platelet-dependent thrombosis in vivo.

Expression and characterization of bioactive human thrombopoietin in the milk of transgenic mice

Human thrombopoietin (hTPO) is the primary physiological regulator of platelet production and plays a pivotal role in promoting the proliferation and maturation of megakaryocytic progenitor cells and megakaryocytes. In this study, transgenic mice were produced harboring either full-length or the erythropoietin (EPO)-like amino-terminal domain of hTPO cDNA sequences fused to the regulatory elements of the bovine beta-casein gene. The transgene RNA was expressed exclusively in the mammary glands of eight transgenic mice, and a trace amount of the transgene was also found in the lungs of one mouse. The full-length form induced efficient expression of the protein with the highest expression level of 1500 microg/ml; however, the EPO-like domain alone expressed the protein at <0.1 microg/ml. The proteins from the two recombinant cDNAs have apparent molecular weights of about 74 and 17 kDa, due to glycosylation in the case of the full-length cDNA. Cell proliferation assay in vitro indicated that both of the recombinant forms stimulated proliferation of the TPO-dependent BaF3-Mpl cells. A positive correlation appeared between the amount of TPO in the milk of lactating animals and their blood platelet levels. About a twofold increase in platelet numbers in the blood was observed after direct subcutaneous injection of the recombinant hTPO at the level of 30 microg/kg of body weight. On the basis of these results, we anticipate that the recombinant hTPO produced efficiently in milk of transgenic mice will have the same activities as the native hTPO in a few in vivo as well as in vitro biochemical aspects.

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.

Endogenous thrombopoietin serum levels during multicycle chemotherapy

Little is known about the behaviour of endogenous thrombopoietin (TPO) serum levels during rapid sequences of dose-intensified chemotherapy. To characterize the relationship between TPO levels and platelet counts in this setting we serially measured both parameters over the entire treatment period of patients receiving multicycle polychemotherapy. We found TPO and platelet responses to be generally antagonistic through all cycles. However, a cross-correlation analysis indicated that TPO responses preceded platelet responses by approximately one day in all patients. The cumulative severity of thrombocytopenia observed over successive cycles was accompanied by an increasing TPO response which tended to grow overproportionally in relation to the degree of peripheral thrombocytopenia. These findings are consistent with a model suggesting that both platelet and megakaryocyte mass contribute to a receptor-dependent consumption process regulating the endogenous TPO level. In order to develop optimal schedules for exogenous TPO administration it might be important to consider endogenous TPO response characteristics.

Tissue Uptake of Circulating Thrombopoietin Is Increased in Immune-Mediated Compared With Irradiated Thrombocytopenic Mice

We have previously demonstrated a significant inverse correlation between circulating thrombopoietin (TPO) levels and peripheral platelet (PLT) counts in patients with thrombocytopenia secondary to megakaryocytic hypoplasia but not in patients with immune thrombocytopenic purpura (ITP; Chang et al, Blood 88:3354, 1996). To test the hypothesis that the differences in the circulating TPO levels in these two types of thrombocytopenia are caused by differences in the total capacity of Mpl receptor-mediated TPO clearance, thrombocytopenia was induced in female CD-1 mice either by sublethal irradiation (irradiated) or rabbit antimouse PLT serum (RAMPS) for 1 day (1 d RAMPS) and 5 days (5 d RAMPS). A well-characterized murine model of autoimmune thrombocytopenic purpura, male (NZW × BXSB) F1 mice (W/B F1), was also included in this study. All thrombocytopenic mice and their controls received trace amounts of 125I-recombinant murine TPO (125I-rmTPO) intravenously and were killed 3 hours postinjection. Blood cell-associated radioactivity was significantly decreased in all 4 groups of thrombocytopenic mice. Significantly increased plasma and decreased whole spleen-associated radioactivity was observed in the irradiated group compared with controls (P &lt; .05). While a lesser but still significant increase in plasma and decrease in whole spleen-associated radioactivity was observed in the 1 d RAMPS mice (P &lt; .05), there were no significant differences between the 5 d RAMPS nor the W/B F1 male mice compared with controls, although whole spleen-associated radioactivity was higher in the W/B F1male. A significant inverse correlation of plasma and whole spleen-associated radioactivity was demonstrated in W/B F1male mice (r = −.91, n = 6, P &lt; .05). There was also a decrease in bone (femur)/blood-associated radioactivity in the irradiated group compared with controls (P &lt; .05), but a significant increase in 1 d and 5 d RAMPS mice (P &lt; .01). Furthermore, the 125I-rmTPO uptake capacity within the spleen and marrow of immune thrombocytopenic mice appeared to be associated with a higher megakaryocytic mass when tissue samples were examined by light microscopy. Internalization of 125I-rmTPO by megakaryocytes and PLTs in the spleens and marrows of ITP mice was also demonstrated directly using electron microscopic autoradiography. Labeled PLTs were also found within splenic macrophages. Additionally, the mean PLT volumes of RAMPS mice were significantly higher than those of the control and irradiated mice (P &lt; .05), as was the bound 125I-rmTPO (cpm) per million PLT (P &lt; .05). Finally, significantly decreased 125I-rmTPO degradation products were only found in the plasma of the irradiated mice compared with control animals (P &lt; .05). These data suggest that the lack of Mpl+ cells in the mice with thrombocytopenia secondary to megakaryocytic hypoplasia (irradiated) results in decreased uptake and degradation of TPO and higher circulating TPO levels. Furthermore, these data also suggest that, after a brief TPO surge in response to immune thrombocytopenia (1 d RAMPS), the lack of an inverse correlation of circulating TPO with PLT counts during steady-state immune thrombocytopenic mice (5 d RAMPS + W/B F1 male) is due, at least in part, to its uptake and degradation by the high PLT turnover and increased mass of megakaryocytes.

Tissue Uptake of Circulating Thrombopoietin Is Increased in Immune-Mediated Compared With Irradiated Thrombocytopenic Mice

We have previously demonstrated a significant inverse correlation between circulating thrombopoietin (TPO) levels and peripheral platelet (PLT) counts in patients with thrombocytopenia secondary to megakaryocytic hypoplasia but not in patients with immune thrombocytopenic purpura (ITP; Chang et al, Blood 88:3354, 1996). To test the hypothesis that the differences in the circulating TPO levels in these two types of thrombocytopenia are caused by differences in the total capacity of Mpl receptor-mediated TPO clearance, thrombocytopenia was induced in female CD-1 mice either by sublethal irradiation (irradiated) or rabbit antimouse PLT serum (RAMPS) for 1 day (1 d RAMPS) and 5 days (5 d RAMPS). A well-characterized murine model of autoimmune thrombocytopenic purpura, male (NZW × BXSB) F1 mice (W/B F1), was also included in this study. All thrombocytopenic mice and their controls received trace amounts of 125I-recombinant murine TPO (125I-rmTPO) intravenously and were killed 3 hours postinjection. Blood cell-associated radioactivity was significantly decreased in all 4 groups of thrombocytopenic mice. Significantly increased plasma and decreased whole spleen-associated radioactivity was observed in the irradiated group compared with controls (P < .05). While a lesser but still significant increase in plasma and decrease in whole spleen-associated radioactivity was observed in the 1 d RAMPS mice (P < .05), there were no significant differences between the 5 d RAMPS nor the W/B F1 male mice compared with controls, although whole spleen-associated radioactivity was higher in the W/B F1male. A significant inverse correlation of plasma and whole spleen-associated radioactivity was demonstrated in W/B F1male mice (r = −.91, n = 6, P < .05). There was also a decrease in bone (femur)/blood-associated radioactivity in the irradiated group compared with controls (P < .05), but a significant increase in 1 d and 5 d RAMPS mice (P < .01). Furthermore, the 125I-rmTPO uptake capacity within the spleen and marrow of immune thrombocytopenic mice appeared to be associated with a higher megakaryocytic mass when tissue samples were examined by light microscopy. Internalization of 125I-rmTPO by megakaryocytes and PLTs in the spleens and marrows of ITP mice was also demonstrated directly using electron microscopic autoradiography. Labeled PLTs were also found within splenic macrophages. Additionally, the mean PLT volumes of RAMPS mice were significantly higher than those of the control and irradiated mice (P < .05), as was the bound 125I-rmTPO (cpm) per million PLT (P < .05). Finally, significantly decreased 125I-rmTPO degradation products were only found in the plasma of the irradiated mice compared with control animals (P < .05). These data suggest that the lack of Mpl+ cells in the mice with thrombocytopenia secondary to megakaryocytic hypoplasia (irradiated) results in decreased uptake and degradation of TPO and higher circulating TPO levels. Furthermore, these data also suggest that, after a brief TPO surge in response to immune thrombocytopenia (1 d RAMPS), the lack of an inverse correlation of circulating TPO with PLT counts during steady-state immune thrombocytopenic mice (5 d RAMPS + W/B F1 male) is due, at least in part, to its uptake and degradation by the high PLT turnover and increased mass of megakaryocytes.

Megakaryocyte ex vivo expansion potential of three hematopoietic sources in serum and serum-free medium

Megakaryocytes (MK) were expanded from purified human CD34+ cells obtained from three sources, bone marrow (BM), mobilized peripheral blood progenitor cells (PB), and umbilical cord (UC) blood. CD34+-selected cells were cultured for 12 days with 10 ng/ml thrombopoietin (TPO), 10 ng/ml IL-3, 10 ng/ml TPO + 10 ng/ml IL-3, or 200 ng/ml promegapoietin (PMP), a chimeric dual agonist of the c-Mpl and human IL-3 receptors. MK production was compared in serum-free versus human serum-supplemented liquid media. PMP and the combination of TPO and IL-3 (TPO + IL-3) increased MK production similarly. Culturing CD34+ cells with PMP in serum-free medium resulted in a twofold increase in MK yield compared with serum-supplemented medium. CD34+ cells from UC proliferated more than those from either BM or PB in liquid culture, resulting in much greater MK production under all conditions. Phenotypic analysis of the uncultured CD34+ cells showed that BM had a higher frequency of CD34+/CD41+ cells than PB or UC. TPO + IL-3 or PMP produced larger and greater numbers of BFU-MK and CFU-MK per seeded CD34+/CD41+ cell from UC than from either BM or PB. Thus, although uncultured CD34+-selected BM cells contained a higher frequency of committed mature MK progenitors, UC CD34+ cells had a greater proliferative capacity and, therefore, were more productive. PMP induced megakaryocytopoietic activity comparable to that achieved with TPO + IL-3 and may be useful for ex vivo expansion of MK for clinical trials.

Human thrombopoietin structure-function relationships: identification of functionally important residues

Thrombopoietin (TPO) is a haematopoietic growth factor responsible for megakaryocyte progenitor proliferation and differentiation. It belongs to the four-helix-bundle cytokine family and exerts its biological effects through binding to a specific receptor, c-Mpl. With the use of site-directed mutagenesis we have generated 20 TPO mutants. Each of the TPO mutants was produced in a eukaryotic expression system and the mutants' ability to induce the proliferation of factor-dependent c-Mpl-expressing megakaryoblastic M-O7e cells was compared with that of wild-type TPO. Among the mutations studied, 10 lead to a significant decrease in TPO bioactivity. Of these ten residues, three are located in helix A of the protein (Arg10, Lys14 and Arg17) and four in helix D (His133, Gln132, Lys138 and Phe141), indicating that in TPO, as in other cytokines, these two helices are important for functional cytokine/receptor interactions. Surprisingly, mutant Arg10-->Ala (R10A) lacked any proliferative activity, despite the fact that this mutation was recently reported to have no effect on TPO/c-Mpl binding in a TPO phage ELISA [Pearce, Potts, Presta, Bald, Fendly and Wells (1997) J. Biol. Chem. 272, 20595-20602]. The lack of M-O7e proliferation is probably due to an inability of R10A mutant to promote receptor dimerization and thus receptor activation. Moreover we found that the Arg10 and Arg17 residues of TPO seem to be specific determinants for TPO/c-Mpl recognition. We also demonstrate that the O-glycosylation site located at position 110 of TPO is not necessary for the bioactivity of the cytokine.

Mpl ligand or thrombopoietin: biological activities

Thrombopoietin (TPO) or Mpl ligand is the primary physiological regulator of platelet production. This cytokine is the most potent stimulator of the proliferation and differentiation of MK progenitor and precursor cells in vitro. It also acts additively or synergistically with several cytokines on progenitor cells from various hematopoietic lineages, including the primitive stem cells. The factor is an extremely potent thrombocytopoietic agent when administrated to normal animals, and it accelerates platelet and erythropoietic recovery in several models of myelosuppression. Phase I/II clinical trials are ongoing with no detectable adverse effects. Mpl ligand does not induce platelet aggregation, but it lowers the platelet sensitivity to physiological dose of agonists. In experimental mouse models, high and chronic dose of Mpl ligand results in myelofibrosis. TPO is constantly produced by the liver and the kidney; its plasmatic clearance occurs by binding to its receptor expressed on megakaryocytes and platelets. However, the full spectrum of the biological effects of this new cytokine is not fully understood, in particular its the role in the terminal stage of platelet production. In the near future, it is likely that new insights will be obtained in the physiopathological mechanisms underlying abnormal platelet production in human.

Effects of recombinant human thrombopoietin alone and in combination with erythropoietin and early-acting cytokines on human mobilized purified CD34+ progenitor cells cultured in serum-depleted medium

The effects of recombinant thrombopoietin (TPO) alone and in combination with erythropoietin (EPO) and early-acting cytokines such as interleukin 3 (IL-3), stem cell factor (SCF) and GM-CSF on highly purified mobilized human CD34+ progenitor cells were studied in a serum-depleted culture system. Eight leukapheresis samples were cultured for seven days and analyzed; aliquots were replated and re-evaluated on day 12. Three-color flow cytometry was used together with morphologic analysis to determine proliferation and megakaryocytic or erythroid maturation. TPO alone was sufficient for cell survival and proliferation in serum-depleted medium. In the absence of other growth factors, almost all CD34+ cells differentiated along the megakaryocytic pathway within 12 days. Concomitantly, the progenitor cells gradually acquired the morphologic features of mature megakaryocytes. After exposure to TPO for one week, 50% of the cells still expressed CD34; by day 12 the remaining CD34+ cells (11%) were all coexpressing CD41. TPO alone did not support proliferation of glycophorin-A-positive cells. The addition of TPO to early-acting cytokines (EPO, GM-CSF, SCF and/or IL-3) not only increased the overall megakaryocyte expansion, but also generated a different maturation pattern of the CD41+ megakaryocyte progenitors. It further doubled the number of erythroid cells and c-kit+ cells in the second week of culture. Interestingly, the overall number of CD34+ cells was increased about fivefold when TPO was added to the early-acting cytokines, with a marked expansion of the CD34+/CD41+ and CD34+/CD117+ subpopulations. TPO can augment the pool of committed progenitors, thereby increasing the number of its own target cells and the number of EPO-responsive cells. These properties make TPO an interesting cytokine for the ex vivo expansion of human progenitor cells.

Phorbol ester PMA induces expression of the thrombopoietin receptor MPL in leukemia cells

Thrombopoietin (TPO) is a major regulator of megakaryocytopoiesis both in vivo and in vitro. TPO initiates its biological effects by binding to the c-MPL receptor, which is a member of the hematopoietin receptor superfamily. To define the regulation of the MPL receptor, six continuous human leukemia cell lines with megakaryocytic properties were treated with the phorbol ester 12-myristate 13-acetate (PMA), TPO and transforming growth factor (TGF)-beta 1, a cytokine known to possess inhibitory effects. We used Northern blotting and flow cytometry analysis to determine MPL mRNA and protein levels. An increase of MPL mRNA and protein expression was observed in 2/6 PMA-exposed cell lines. There is no evidence from this study that TPO or TGF-beta 1 cause any decrease or increase in MPL expression. MPL upregulation triggered by PMA was accompanied by signs of induced differentiation such as increase in CD41, CD42 and CD61 expression, increase in cell size and cessation of proliferation. These data demonstrate that MPL can be upregulated in differentiating megakaryocytic cells via stimulation of protein kinase C, the intracellular target of PMA and a key kinase in one of the second messenger signal transduction pathways. These findings further the understanding of the regulation of this molecule, a cytokine receptor that, together with its ligand TPO, appears to represent a crucial element in megakaryocytopoiesis.

Recombinant human ligand for MPL, megakaryocyte growth and development factor (MGDF), stimulates thrombopoiesis in vivo in normal and myelosuppressed baboons

Megakaryocyte growth and development factor (MGDF) is a ligand for c-mpl and a member of the hematopoietic growth factor superfamily. Recombinant murine MGDF specifically stimulates thrombopoiesis in mice. Recombinant human (rHu) MGDF stimulates megakaryocytic differentiation of baboon CD34+ marrow cells in vitro. Therefore, we determined the in vivo biological effects of rHuMGDF administered to normal baboons in the absence and presence of myelosuppression with 5-fluorouracil (5-FU). rHuMGDF was administered to normal baboons as a single s.c. injection at doses of 1, 10, 25 and 50 micrograms/kg/day for 10 days and, as a control, heat-inactivated MGDF was administered at a dose of 10 micrograms/kg/day. Platelet counts were markedly increased in all animals administered native rHuMGDF but not in animals given heat-inactivated rHuMGDF. Platelet counts began to increase between three and six days after starting rHuMGDF administration and the maximum average increases were 1.7-, 3.4-, 5.1- and 4.0-fold above baseline in animals administered 1, 10, 25 and 50 micrograms/kg/day, respectively. Maximum platelet counts were reached between 7 and 10 days after starting rHuMGDF and maintained for four days after the last dose. Thereafter, platelet counts decreased, reaching stable pretreatment values between 11 and 14 days after the last dose of rHuMGDF. No changes in red cell mass, peripheral blood white blood cell counts or differentials were observed during rHuMGDF treatment. For animals administered 10, 25 and 50 micrograms/kg/day of rHuMGDF, megakaryocytes increased more than threefold in marrow, were markedly enlarged, and had increased numbers of lobes. Overall marrow cellularity remained unchanged, as did red cell and white cell morphology. No marrow fibrosis was detected. Progenitor cells were not increased in marrow but did increase modestly in the peripheral blood, associated with increased numbers of CD34+ cells in the circulation. Following a single dose of 5-FU (120 mg/kg) animals were given either saline or pegylated (PEG) rHuMGDF (25 micrograms/kg/day) for 14 days. Platelet counts recovered to baseline by 13.8 +/- 1.8 days for PEG-rHuMGDF-treated baboons compared with 16.8 +/- 0.6 days for saline treated controls. Marrow biopsies revealed more rapid recovery of overall marrow cellularity and megakaryocytes in PEG-rHuMGDF-treated animals compared with controls. Thus, rHuMGDF specifically stimulates thrombopoiesis in normal and myelosuppressed baboons. rHuMGDF may be useful for stimulating thrombopoiesis in humans in clinical settings after myelosuppression.

Expression of thrombopoietin and thrombopoietin receptor MPL in human leukemia-lymphoma and solid tumor cell lines

Thrombopoietin (TPO) is the major regulator of platelet production in vivo and is the ligand for the MPL receptor. In an effort to determine the distribution of TPO and MPL in the different hematopoietic cell types and in various types of tissue, we examined the mRNA expression of this ligand-receptor pair in two series of human leukemia-lymphoma cell lines and of solid tumor cancer cell lines using northern blot and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. At the northern blot mRNA level, 8/15 (53%) megakaryocytic and 3/11 (27%) erythroid leukemia cell lines expressed MPL mRNA; except for one positive monocytic cell line, the remaining 78 pre B-cell, B-cell, plasma cell, T-cell, NK cell, myeloid, monocytic and Hodgkin/anaplastic large cell lymphoma (ALCL)-derived cell lines were negative. No MPL message was detected in any of the 23 solid tumor cell lines established from 21 different tumors. In order to examine whether a low level of MPL expression could be detected, 51 leukemia cell lines were investigated with the RT-PCR technique. By this technique, MPL message was seen in many more cell types: 13/26 (50%) of non-erythromegakaryocytic cell lines and in nearly all megakaryocytic (14/15, 93%) and erythroid (10/11, 91%) cell lines. Thus, the highest expression of MPL clearly occurs in cells with megakaryocytic differentiation; furthermore, expression of MPL appears to be restricted to hematopoietic cell types. TPO mRNA expression was examined by RT-PCR and found in 9/11 (82%) of the solid tumor cell lines (derived from colon, endometrium, kidney, liver, ovary, retinoblastoma and urinary bladder cancers). Among the leukemia-lymphoma cell lines, TPO mRNA was detected by RT-PCR in most plasma cell, myeloid, megakaryocytic and erythroid cell lines, but not in pre B-cell, B-cell or T-/NK-cell lines. The results reported here extend the observations of MPL and TPO expression in normal cells to the whole spectrum of hematological cell types and to an array of different tissue types, both exemplified by their malignant counterparts.

Platelets and coagulation. An update

Accepted mechanisms of coagulation are being challenged as new proteins and feedback mechanisms are discovered and evaluated. These new proteins, including tissue factor pathway inhibitor, not only change the way we look at the coagulation mechanism, but also give us potential alternatives in treatment of hemophilia. Gene therapy continues to be a challenging research area, with hemophilia serving as the prototypic hereditary disease target. As acquired disorders of coagulation secondary to immune-mediated events are studied and better understood, our approaches to management of these cases will also change to the benefit of our patients.

FLT3/FLK-2 (STK-1) Ligand does not stimulate human megakaryopoiesis in vitro

It has not yet been determined if the FLT3/FLK-2 or STK-1 Ligand (STK-1L)/FLT3/FLK-2 or STK-1 receptor (STK-1R) axis has the ability to regulate human megakaryopoiesis in vitro. To address this question, we exposed normal human CD34+ marrow mononuclear cells to recombinant human STK-1L alone, or in combination with other growth factors. Colony-forming unit-megakaryocytic/thrombocytes (CFU-Meg) and BFU-E-derived colonies were then enumerated, and effects on colony size and maturation noted. As assessed by these parameters, STK-1L had no demonstrable effect on megakaryocyte colony formation. Similarly, suppressing STK-1R expression with oligodeoxynucleotides also had no influence on CFU-Meg-derived colony formation. To begin to derive a physiologic explanation for these findings, we examined freshly isolated normal human megakaryocytes for the presence of STK-1L and STK-1R mRNA. In contrast to a growing number of growth factors and growth factor receptors which appear to be expressed by megakaryocytes, normal mature human megakaryocytes express neither STK-1R or STK-1L mRNA. Accordingly, our results led us to hypothesize that if STK-1/STK-1L have any effects on megakaryocyte development in vitro, they are likely subtle and of uncertain physiologic significance.

Beta 1 integrins mediate adherent phenotype of human erythroblastic cell lines after phorbol 12-myristate 13-acetate induction

We investigated the effects of phorbol ester (phorbol 12-myristate 13-acetate; PMA) treatment on the adhesive behaviour of three erythroleukaemia cell lines: HEL, LAMA-84 and AP217. In the three cell lines PMA induced an increase in expression of a megakaryocytic marker: alpha IIb beta 3 integrin, but did not promote activation of this receptor. Indeed, an antibody specific for the activated form of alpha IIb beta 3 failed to react with the three cell lines. PMA induction led to different adhesive phenotypes depending on the cell line; in fact LAMA-84 and HEL cells became adherent while AP217 cells remained non-adherent. By studying cell surface receptors we found that the major difference between the adherent and the non-adherent cells was the expression of beta 1 integrins. After PMA induction, beta 1 integrin expression was totally abolished in AP217 cells and the amount of beta 1 mRNA was reduced preventing new synthesis of the subunit. In HEL and LAMA-84 cells, PMA treatment did not alter the overall level of beta 1 integrin but induced a new pattern of alpha-subunit expression: up-regulation of alpha 2 and alpha v subunits and down-regulation of alpha 4 and alpha 5 subunits. Function-perturbing antibodies against beta 1, alpha 4, alpha 5, alpha v and alpha 2 reduced adhesion of HEL cells to fibronectin or collagen, whereas antibodies against beta 3 or alpha v beta 3 did not. Our results favour the involvement of beta 1 integrins in PMA-induced adhesion of erythroleukaemia cells.