In vivo biological effects of various forms of thrombopoietin in a murine model of transient pancytopenia

Secreted mutant calreticulins as rogue cytokines in myeloproliferative neoplasms

Mutant calreticulin (CALR) proteins resulting from a -1/+2 frameshifting mutation of the CALR exon 9 carry a novel C-terminal amino acid sequence and drive the development of myeloproliferative neoplasms (MPNs). Mutant CALRs were shown to interact with and activate the thrombopoietin receptor (TpoR/MPL) in the same cell. We report that mutant CALR proteins are secreted and can be found in patient plasma at levels up to 160 ng/mL, with a mean of 25.64 ng/mL. Plasma mutant CALR is found in complex with soluble transferrin receptor 1 (sTFR1) that acts as a carrier protein and increases mutant CALR half-life. Recombinant mutant CALR proteins bound and activated the TpoR in cell lines and primary megakaryocytic progenitors from patients with mutated CALR in which they drive thrombopoietin-independent colony formation. Importantly, the CALR-sTFR1 complex remains functional for TpoR activation. By bioluminescence resonance energy transfer assay, we show that mutant CALR proteins produced in 1 cell can specifically interact in trans with the TpoR on a target cell. In comparison with cells that only carry TpoR, cells that carry both TpoR and mutant CALR are hypersensitive to exogenous mutant CALR proteins and respond to levels of mutant CALR proteins similar to those in patient plasma. This is consistent with CALR-mutated cells that expose TpoR carrying immature N-linked sugars at the cell surface. Thus, secreted mutant CALR proteins will act more specifically on the MPN clone. In conclusion, a chaperone, CALR, can turn into a rogue cytokine through somatic mutation of its encoding gene.

Single thrombopoietin dose alleviates hematopoietic stem cells intrinsic short- and long-term ionizing radiation damage. In vivo identification of anatomical cell expansion sites

Hematopoietic stem cells (HSC) are essential for maintaining the integrity of complex and long-lived organisms. HSC, which are self-renewing, reconstitute the hematopoietic system through out life and facilitate long-term repopulation of myeloablated recipients. We have previously demonstrated that when mice are exposed to sublethal doses of ionizing radiation, subsets of the stem/progenitor compartment are affected. In this study we examine the role of thrombopoietin (TPO) on the regenerative capacities of HSC after irradiation and report the first demonstration of efficacy of a single injection of TPO shortly after in vivo exposure to ionizing radiation for reducing HSC injury and improving their functional outcome. Our results demonstrate that TPO treatment not only reduced the number of apoptotic cells but also induced a significant modification of their intrinsic characteristics. These findings were supported by transplantation assays with long-term HSC that were irradiated or unirradiated, TPO treated or untreated, in CD45.1/CD45.2 systems and by using luciferase-labeled HSC for direct bioluminescence imaging in living animals. Of particular importance, our data demonstrate the skull to be a highly favorable site for the TPO-induced emergence of hematopoietic cells after irradiation, suggesting a TPO-mediated relationship of primitive hematopoietic cells to an anatomical component. Together, the data presented here: provide novel findings about aspects of TPO action on stem cells, open new areas of investigation for therapeutic options in patients who are treated with radiation therapy, and show that early administration of a clinically suitable TPO-agonist counteracts the previously observed adverse effects.

Hematopoietic recovery following autologous bone marrow transplantation in a nonhuman primate: effect of variation in treatment schedule with PEG-rHuMGDF

Mathematical modeling of pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) pharmacokinetics (PK) and pharmacodynamics (PD) suggest that variations in the PEG-rHuMGDF treatment schedule could reduce the severity and duration of thrombocytopenia following myeloablation and bone marrow transplant (BMT). We tested this hypothesis in a rhesus monkey model of autologous (Au) bone marrow-derived mononuclear cell (BM-MNC) transplantation following lethal myeloablation. On day 0, animals were myeloablated by total body exposure to 920 cGy, 250 kVp x-irradiation (TBI). Four cohorts of animals were infused with 1 x 10(8) AuBM-MNC/kg body weight within 2 hours of TBI. The AuBMT-alone cohort received no cytokine, the daily dosage cohort received PEG-rHuMGDF (2.5 micro g/kg/day, s.c.) post TBI and AuBMT, and the pre/post-transplant cohort received PEG-rHuMGDF (2.5 micro g/kg/day, s.c.) pre (day -9 to day -5) and post TBI and AuBMT. The post-transplant PEG-rHuMGDF administration in the above cohorts was begun on day 1 post TBI and continued until platelet counts reached 200,000 micro l (range, 15-31 days). Another group received PEG-rHuMGDF (300 micro g/kg/day, s.c.) on days 1 and 3 only following TBI and AuBMT. The TBI controls received neither AuBMT nor cytokine therapy. In this model of AuBMT, with regard to the PEG-rHuMGDF administration schedule, the daily dosage of the post-transplant cohort did not significantly improve platelet recovery; the pre/post-transplant schedule and an abbreviated high-dosage, post-transplant schedule (days 1 and 3) significantly improved the duration and nadir of thrombocytopenia and platelet recovery. These data confirm predictions from PK/PD modeling of PEG-rHuMGDF that thrombocytopenia is preventable following AuBMT.

Promegapoietin-1a, an engineered chimeric IL-3 and Mpl-L receptor agonist, stimulates hematopoietic recovery in conventional and abbreviated schedules following radiation-induced myelosuppression in nonhuman primates

Promegapoietin-1a (PMP-1a), a multifunctional agonist for the human interleukin 3 and Mpl receptors, was evaluated for its ability to stimulate hematopoietic reconstitution in nonhuman primates following severe radiation-induced myelosuppression. Animals were total body x-irradiated (250 kVp) to 600 cGy total midline tissue dose. PMP-1a was administered s.c. in several protocols: A) daily (50 microg/kg) for 18 days; B) nine doses (5 microg/kg) every other day for 3 weeks; C) a single high dose (100 microg/kg) at 20 hours, or D) a single high dose (100 microg/kg) at 1 hour following TBI. The irradiation controls received 0.1% autologous serum for 18 consecutive days. Hematopoietic recovery was assessed by bone marrow clonogenic activity, peripheral blood cell nadirs, duration of cytopenias, time to recovery to cellular thresholds, and requirements for clinical support. PMP-1a, irrespective of administration schedule, significantly improved all platelet-related parameters: thrombocytopenia was eliminated, the severity of platelet nadirs was significantly improved, and recovery of platelet counts to > or =20,000/miccrol was significantly reduced in all PMP-1a-treated cohorts. As a consequence, all PMP-1a-treated cohorts were transfusion-independent. Neutrophil regeneration was augmented in all treatment schedules. Additionally, all PMP-1a-treated cohorts showed an improvement in red blood cell nadir and recovery. PMP-1a in conventional or abbreviated schedules induced significant thrombopoietic regeneration relative to the control cohort, whereas significant improvement in neutrophil recovery was schedule-dependent in radiation-myelosuppressed nonhuman primates.

The molecular control of hematopoiesis: progress and problems with gene manipulation

The in vitro-based discovery and characterization of hematopoietic regulators were of great value in identifying many of the agents active in controlling hematopoiesis. Subsequent in vivo studies have validated most of the information obtained from the in vitro studies, although the in vitro studies proved to be somewhat misleading in predicting which agents would exhibit the greatest quantitative effects in vivo. Establishing more clearly the actual situation in vivo has required a return to more complex, and often less satisfactory, studies on genetically-manipulated whole animals. Of the two possible general approaches, gene inactivation models have proved more informative than transgenic, overexpression models. Each model has raised multiple questions in need of further resolution and the deletion studies have also indicated that other regulators must exist for various lineages, but have yet to be discovered. Of particular interest is the finding from gene inactivation studies that both G-CSF and thrombopoietin are necessary for the maintenance of normal numbers of progenitor cells in multiple lineages, suggesting that each of these lineage-dominant regulators may have broader actions when operating on cells in the stem cell and progenitor cell compartments.

The efficacy of recombinant TPO in murine And nonhuman primate models for myelosuppression and stem cell transplantation

Radiation-induced pancytopenia proved to be a suitable model system in mice and rhesus monkeys to study thrombopoietin (TPO) target cell range and efficacy. TPO was highly effective in rhesus monkeys exposed to the midlethal dose of 5-Gy (300 kV x-rays) TBI, a model in which it alleviated thrombocytopenia, promoted red cell reconstitution, accelerated reconstitution of immature CD34+ bone marrow (BM) cells and potentiated the response to growth factors such as GM-CSF and G-CSF. The accelerated reconstitution of BM CD34+ cells appeared to be reflected by a similar rise in peripheral blood CD34+ cells, both being augmented by concomitant GM-CSF. However, TPO was ineffective following transplantation of limited numbers of autologous BM or highly purified stem cells in monkeys conditioned with 8-Gy TBI. In the 5-Gy model, a single dose of TPO 24 h after TBI was effective in preventing thrombocytopenia and was augmented by GM-CSF. The strong erythropoietic stimulation may result in iron depletion and TPO treatment should be accompanied by monitoring of iron status. In mice, similar observations were made and the importance of dose and dose schedule for stimulation of multilineage repopulating cells versus the lineage-dominant thrombopoietic response studied in detail.

The combined administration of daniplestim and Mpl ligand augments the hematopoietic reconstitution observed with single cytokine administration in a nonhuman primate model of myelosuppression

This study evaluated the ability of daniplestim, a high affinity interleukin 3 receptor agonist, to enhance the hematopoietic response of Mpl ligand (Mpl-L) administration in nonhuman primates following severe, radiation-induced myelosuppression. Rhesus monkeys were total body x-irradiated (TBI) to 600 cGy, midline tissue dose. Beginning on day 1 post-TBI, animals were s.c. administered daniplestim (100 microg/kg bid; n = 4), Mpl-L (10 microg/kg qd; n = 3), daniplestim (100 microg/kg bid) plus Mpl-L (10 microg/kg qd) (n = 4) or 0.1% autologous serum (AS) (n = 11) for 18 days. CBCs were monitored for 60 d after TBI. The duration of thrombocytopenia (platelet count; PLT <20,000/microl) was significantly decreased by the administration of daniplestim (6.5 d, p = .01), Mpl-L (3.0 d, p = .003) and the coadministered daniplestim/Mpl-L (1.3 d, p = .001) compared to controls (10.4 d). As monotherapy Mpl-L but not daniplestim significantly improved the PLT nadir (21,000/microl, p = .023 and 5,000/microl, p = .266, respectively) compared to the control (3,000/microl). The combined administration of daniplestim and Mpl-L significantly improved the PLT nadir (28,000/microl, p = .007) compared to both the control cohort (3,000/microl) and the daniplestim only cohort (5,000/microl, p = .043). Recovery of PLT to preirradiation values occurred earlier in the daniplestim only (d 21) or the daniplestim/Mpl-L cohorts (d 18) than in the Mpl-L only or control cohorts (d 28, d 29, respectively). The administration of daniplestim or Mpl-L alone neither shortened the duration of neutropenia (ANC<500/microl) compared to the controls (15.8 d, 16.0 d versus 16.2 d, respectively), nor improved the recovery time of neutrophils to baseline values (d 22, d 25, and d 23, respectively). The ANC nadir was significantly improved by daniplestim alone but not Mpl-L administration (76/microl, p = .001 and 50/microl, p = .093, respectively) compared to the controls (8/microl). Coadministration of daniplestim and Mpl-L significantly improved the ANC nadir (196/microl, p = .001) compared to either the AS- or the monotherapy-treated cohorts. Also the duration of neutropenia observed in the AS-controls (16.2 d) was significantly reduced in the daniplestim/Mpl-L cohort (10.8 d, p = .002). The combined administration of daniplestim and Mpl-L significantly improved hematopoietic recovery and further enhanced the stimulatory effect of cytokine monotherapy, as well as reducing clinical support requirements after radiation-induced bone marrow myelosuppression.

Single administration of thrombopoietin to lethally irradiated mice prevents infectious and thrombotic events leading to mortality

A sufficiently high dose of thrombopoietin to overcome initial c-mpl-mediated clearance stimulates hematopoietic reconstitution following myelosuppressive treatment. We studied the efficacy of thrombopoietin on survival after supralethal total body irradiation (9 Gy) of C57BL6/J mice and the occurrence of infectious and thrombotic complications in comparison with a bone marrow graft or prophylactic antibiotic treatment. Administration of 0.3 microg thrombopoietin, 2 hours after irradiation, protected 62% of the mice as opposed to no survival in placebo controls. A graft with a supraoptimal number of syngeneic bone marrow cells (10(6) cells) fully prevented mortality, whereas antibiotic treatment was ineffective. Blood cell recovery was observed in the thrombopoietin-treated mice but not in the placebo or antibiotic-treated group. Bone marrow and spleen cellularity as well as colony-forming unit granulocyte-macrophage and burst-forming unit erythroid were considerably increased in thrombopoietin-treated mice relative to controls. Histologic examination at day 11 revealed numerous petechiae and vascular obstructions within the brain microvasculature of placebo-treated mice, which was correlated with hypercoagulation and hypofibrinolysis. Thrombopoietin treatment prevented coagulation/fibrinolysis disorder and vascular thrombosis. High fibrinogen levels were related to bacterial infections in 67% of placebo-treated mice and predicted mortality, whereas the majority of the thrombopoietin-treated mice did not show high fibrinogen levels and endotoxin was not detectable in plasma. We conclude that thrombopoietin administration prevents mortality in mice subjected to 9-Gy total body irradiation both by interfering in the cascade leading to thrombotic complications and by amelioration of neutrophil and platelet recovery and thus protects against infections and hemorrhages.

Thrombopoietin promotes hematopoietic recovery and survival after high-dose whole body irradiation

PURPOSE

The therapeutic potential of thrombopoietin (TPO), the major regulator of platelet production, was evaluated for hematopoietic recovery and survival in mice following lethal and supralethal total body irradiation (TBI).

METHODS AND MATERIALS

Hematopoietic recovery was studied in C57BL6/J mice after 8 Gy TBI (gamma-rays). Survival experiments were performed with C57BL6/J and BCBA F1 mice. Two protocols of TPO administration were evaluated: treatment for 7 consecutive days (7 x 0.3 microg/mice) beginning 2 h after exposure, or a single dose (0.3 microg/mice) administered 2 h after irradiation.

RESULTS

TPO improved the platelet nadir and accelerated the platelet reconstitution of irradiated mice in comparison to placebo-treated mice. Recovery of neutrophils and erythrocytes was stimulated as well. TPO induced an accelerated recovery of hematopoietic progenitors and immature multilineage progenitors in bone marrow and spleen. In addition, TPO administration induced approximately 90% survival of 8 Gy irradiated C57BL6/J mice, a TBI dose which resulted in 100% mortality within 30 days for placebo-treated mice. Single TPO administration was as effective as repeated injections for hematopoietic recovery and prevention of mortality. Dose-effect survival experiments were performed in BCBA F1 mice and demonstrated that TPO shifted the LD50/30 from approximately 9.5 Gy to 10.5 Gy TBI given as a single dose, and from 14 Gy to as high as 17 Gy when TBI was given in three equal doses, each separated by 24 h.

CONCLUSION

These results demonstrate that the multilineage hematopoietic effects of TPO may be advantageously used to protect against lethal bone marrow failure following high dose TBI.

The efficacy of recombinant thrombopoietin in murine and nonhuman primate models for radiation-induced myelosuppression and stem cell transplantation

Radiation-induced pancytopenia proved to be a suitable model system in mice and rhesus monkeys for studying thrombopoietin (TPO) target cell range and efficacy. TPO was highly effective in rhesus monkeys exposed to the mid-lethal dose of 5 Gy (300 kV x-rays) TBI, a model in which it alleviated thrombocytopenia, promoted red cell reconstitution, accelerated reconstitution of immature CD34+ bone marrow cells, and potentiated the response to growth factors such as GM-CSF and G-CSF. In contrast to the results in the 5 Gy TBI model, TPO was ineffective following transplantation of limited numbers of autologous bone marrow or highly purified stem cells in monkeys conditioned with 8 Gy TBI. In the 5 Gy model, a single dose of TPO augmented by GM-CSF 24 h after TBI was effective in preventing thrombocytopenia. The strong erythropoietic stimulation may result in iron depletion, and TPO treatment should be accompanied by monitoring of iron status. This preclinical evaluation thus identified TPO as a potential major therapeutic agent for counteracting radiation-induced pancytopenia and demonstrated pronounced stimulatory effects on the reconstitution of immature CD34+ hemopoietic cells with multilineage potential. The latter observation explains the potentiation of the hematopoietic responses to G-CSF and GM-CSF when administered concomitantly. It also predicts the effective use of TPO to accelerate reconstitution of immature hematopoietic cells as well as possible synergistic effects in vivo with various other growth factors acting on immature stem cells and their direct lineage-committed progeny. The finding that a single dose of TPO might be sufficient for a clinically significant response emphasizes its potency and is of practical relevance. The heterogeneity of the TPO response encountered in the various models used for evaluation points to multiple mechanisms operating on the TPO response and heterogeneity of its target cells. Mechanistic mouse studies made apparent that the response of multilineage cells shortly after TBI to a single administration of TPO is quantitatively more important for optimal efficacy than the lineage-restricted response obtained at later intervals after TBI and emphasized the importance of a relatively high dose of TPO to overcome initial c-mpl-mediated clearance. Further elucidation of mechanisms determining efficacy might very well result in a further improvement, e.g., following transplantation of limited numbers of stem cells. Adverse effects of TPO administration to myelosuppressed or stem cell transplanted experimental animals were not observed.

The molecular control of hematopoiesis: progress and problems with gene manipulation

The in vitro-based discovery and characterization of hematopoietic regulators were of great value in identifying many of the agents active in controlling hematopoiesis. Subsequent in vivo studies have validated most of the information obtained from the in vitro studies, although the in vitro studies proved to be somewhat misleading in predicting which agents would exhibit the greatest quantitative effects in vivo. Establishing more clearly the actual situation in vivo has required a return to more complex, and often less satisfactory, studies on genetically manipulated whole animals. Of the two possible general approaches, gene inactivation models have proved more informative than transgenic, overexpression models. Each model has raised multiple questions in need of further resolution and the deletion studies have also indicated that other regulators must exist for various lineages, but have yet to be discovered. Of particular interest is the finding from gene inactivation studies that both G-CSF and thrombopoietin are necessary for the maintenance of normal numbers of progenitor cells in multiple lineages, suggesting that each of these lineage-dominant regulators may have broader actions when operating on cells in the stem cell and progenitor cell compartments.

A Single Dose of Thrombopoietin Shortly After Myelosuppressive Total Body Irradiation Prevents Pancytopenia in Mice by Promoting Short-Term Multilineage Spleen-Repopulating Cells at the Transient Expense of Bone Marrow–Repopulating Cells

Thrombopoietin (TPO) has been used in preclinical myelosuppression models to evaluate the effect on hematopoietic reconstitution. Here we report the importance of dose and dose scheduling for multilineage reconstitution after myelosuppressive total body irradiation (TBI) in mice. After 6 Gy TBI, a dose of 0.3 μg TPO/mouse (12 μg/kg) intraperitoneally (IP), 0 to 4 hours after TBI, prevented the severe thrombopenia observed in control mice, and in addition stimulated red and white blood cell regeneration. Time course studies showed a gradual decline in efficacy after an optimum within the first hours after TBI, accompanied by a replacement of the multilineage effects by lineage dominant thrombopoietic stimulation. Pharmacokinetic data showed that IP injection resulted in maximum plasma levels 2 hours after administration. On the basis of the data, we inferred that a substantial level of TPO was required at a critical time interval after TBI to induce multilineage stimulation of residual bone marrow cells. A more precise estimate of the effect of dose and dose timing was provided by intravenous administration of TPO, which showed an optimum immediately after TBI and a sharp decline in efficacy between a dose of 0.1 μg/mouse (4 μg/kg; plasma level 60 ng/mL), which was fully effective, and a dose of 0.03 μg/mouse (1.2 μg/kg; plasma level 20 ng/mL), which was largely ineffective. This is consistent with a threshold level of TPO required to overcome initial c-mpl–mediated clearance and to reach sufficient plasma levels for a maximum hematopoietic response. In mice exposed to fractionated TBI (3 × 3 Gy, 24 hours apart), IP administration of 0.3 μg TPO 2 hours after each fraction completely prevented the severe thrombopenia and anemia that occurred in control mice. Using short-term transplantation assays, ie, colony-forming unit–spleen (CFU-S) day 13 (CFU-S-13) and the more immature cells with marrow repopulating ability (MRA), it could be shown that TPO promoted CFU-S-13 and transiently depleted MRA. The initial depletion of MRA in response to TPO was replenished during long-term reconstitution followed for a period of 3 months. Apart from demonstrating again that MRA cells and CFU-S-13 are separate functional entities, the data thus showed that TPO promotes short-term multilineage repopulating cells at the expense of more immature ancestral cells, thereby preventing pancytopenia. The short time interval available after TBI to exert these effects shows that TPO is able to intervene in mechanisms that result in functional depletion of its multilineage target cells shortly after TBI and emphasizes the requirement of dose scheduling of TPO in keeping with these mechanisms to obtain optimal clinical efficacy.

© 1998 by The American Society of Hematology.

A Single Dose of Thrombopoietin Shortly After Myelosuppressive Total Body Irradiation Prevents Pancytopenia in Mice by Promoting Short-Term Multilineage Spleen-Repopulating Cells at the Transient Expense of Bone Marrow–Repopulating Cells

Thrombopoietin (TPO) has been used in preclinical myelosuppression models to evaluate the effect on hematopoietic reconstitution. Here we report the importance of dose and dose scheduling for multilineage reconstitution after myelosuppressive total body irradiation (TBI) in mice. After 6 Gy TBI, a dose of 0.3 μg TPO/mouse (12 μg/kg) intraperitoneally (IP), 0 to 4 hours after TBI, prevented the severe thrombopenia observed in control mice, and in addition stimulated red and white blood cell regeneration. Time course studies showed a gradual decline in efficacy after an optimum within the first hours after TBI, accompanied by a replacement of the multilineage effects by lineage dominant thrombopoietic stimulation. Pharmacokinetic data showed that IP injection resulted in maximum plasma levels 2 hours after administration. On the basis of the data, we inferred that a substantial level of TPO was required at a critical time interval after TBI to induce multilineage stimulation of residual bone marrow cells. A more precise estimate of the effect of dose and dose timing was provided by intravenous administration of TPO, which showed an optimum immediately after TBI and a sharp decline in efficacy between a dose of 0.1 μg/mouse (4 μg/kg; plasma level 60 ng/mL), which was fully effective, and a dose of 0.03 μg/mouse (1.2 μg/kg; plasma level 20 ng/mL), which was largely ineffective. This is consistent with a threshold level of TPO required to overcome initial c-mpl–mediated clearance and to reach sufficient plasma levels for a maximum hematopoietic response. In mice exposed to fractionated TBI (3 × 3 Gy, 24 hours apart), IP administration of 0.3 μg TPO 2 hours after each fraction completely prevented the severe thrombopenia and anemia that occurred in control mice. Using short-term transplantation assays, ie, colony-forming unit–spleen (CFU-S) day 13 (CFU-S-13) and the more immature cells with marrow repopulating ability (MRA), it could be shown that TPO promoted CFU-S-13 and transiently depleted MRA. The initial depletion of MRA in response to TPO was replenished during long-term reconstitution followed for a period of 3 months. Apart from demonstrating again that MRA cells and CFU-S-13 are separate functional entities, the data thus showed that TPO promotes short-term multilineage repopulating cells at the expense of more immature ancestral cells, thereby preventing pancytopenia. The short time interval available after TBI to exert these effects shows that TPO is able to intervene in mechanisms that result in functional depletion of its multilineage target cells shortly after TBI and emphasizes the requirement of dose scheduling of TPO in keeping with these mechanisms to obtain optimal clinical efficacy.

© 1998 by The American Society of Hematology.

Hematopoietic deficiencies in c-mpl and TPO knockout mice

Thrombopoietin (TPO) is the primary regulatory of megakaryocyte (Meg) and platelet production. Its receptor, c-mpl, is a member of the cytokine receptor superfamily. Major insight into the physiological role of this receptor/ligand pair came from the study of mice carrying disrupted alleles of these two genes. Both TPO and c-mpl knockout mice are viable, but have a 90% reduction in platelet counts. Their thrombocytopenia is caused by a reduction in progenitor cell numbers and a decrease in Meg ploidy. However, the Megs and platelets produced in the absence of TPO or c-mpl appear morphologically and functionally normal indicating that, in vivo, the main role of TPO is to control their numbers, rather than their maturation. In addition to its effect on the Meg lineage, TPO also affects hematopoietic stem cells as measured by a reduction of the repopulating capacity of bone marrow cells from c-mpl-deficient mice. Finally, analysis of these gene targeted mice provided substantial evidence to a model where the circulating TPO level is directly regulated by the platelet mass through binding to c-mpl receptors present at the platelet surface. This elegant feedback mechanism allows a tight regulation of the amount of TPO available to stimulate megakaryocytopoiesis.