Interleukin-11 stimulates multiple phases of erythropoiesis in vitro.

Interleukin-11 (IL-11), a pleiotropic cytokine originally isolated from a primate bone marrow stromal cell line, has been shown to stimulate T-cell-dependent B-cell maturation, megakaryopoiesis, and various stages of myeloid differentiation, but to inhibit adipogenesis. Because stromal cells are essential for the maintenance of early hematopoietic progenitor cells in long-term culture, we investigated the effects of IL-11 on multipotent and erythroid precursors from murine bone marrow in vitro in suspension and semisolid cultures. Our results show that in the presence of IL-3 or c-kit ligand (KL), IL-11 has profound stimulatory effects on primitive multilineage hematopoietic progenitors, pre-CFC(multi), as well as on precursors representing various stages of erythroid differentiation observable in vitro, including CFC(multi), BFU-E, and CFU-E. In addition, the combination of KL with IL-11 also stimulated highly proliferative erythroid progenitors that yield remarkable macroscopic erythroblast colonies in culture. These results indicate that IL-11 is likely to play a pivotal role in early hematopoiesis and at multiple stages of erythropoiesis.     

In vivo effects of recombinant interleukin-11 on myelopoiesis in mice

Purified recombinant human interleukin-11 (rhuIL-11) was assessed for its in vivo effects on the proliferation and differentiation of hematopoietic progenitors as well as its capacity to accelerate the recovery of a drug-suppressed hematopoietic system. Dosage and time sequence studies demonstrated that administration of IL-11 to normal mice resulted in increases in absolute numbers of femoral marrow and splenic myeloid (granulocyte-macrophage colony-forming unit [CFU-GM], burst-forming unit-erythroid [BFU-E], CFU-granulocyte, erythroid, macrophage, megakaryocyte) progenitor cells and in stimulation of these progenitors to a higher cell cycling rate. This was associated with increased numbers of circulating neutrophils. Administration of IL-11 to mice pretreated with cyclophosphamide decreased the time required to regain normal levels of neutrophil and platelet counts in peripheral blood. In addition, IL-11 accelerated reconstitution to normal range of myeloid progenitors from bone marrow and spleen of myelosuppressed mice. These data suggest that IL-11 may play an important role in the regulation of hematopoiesis, and the application of this novel cytokine may have clinical therapeutic benefits.     

Effects of recombinant human interleukin-11 on hematopoietic reconstitution in transplant mice: acceleration of recovery of peripheral blood neutrophils and platelets

We have examined the effects of recombinant human interleukin-11 (rhIL- 11) on the recovery of peripheral blood cell counts and proliferation of progenitors and hematopoietic stem cells (day 12 colony-forming units-spleen-CFU-S12) in vivo using a mouse bone marrow (BM) and spleen cell transplantation model. Recovery of leukocytes was accelerated in animals receiving daily administration of rhIL-11 (100 micrograms/kg/d) and reached normal levels by day 14 posttransplantation. This increased total leukocyte count reflected mainly an increase in neutrophils. Neutropenia (absolute neutrophil count [ANC] < 1,500) was present in control transplant mice for 14 to 15 days, while in the rhIL-11-treated group, neutrophils recovered to normal by days 8 to 10 and continued to increase until day 19. Animals treated with rhIL-11 had only 1 day with ANC demonstrated < 500. Correspondingly, rhIL-11 treatment increased granulocyte-macrophage progenitors (CFU-GM) derived from both spleen and BM cells. Higher doses of IL-11 increased CFU-GM nearly threefold and CFU-Mix fourfold to fivefold, while increasing burst-forming units- erythroid to a lesser degree. BM and spleen cellularity were both increased in IL-11-treated mice, but no increase in CFU-S12 was noted. In addition, in vivo daily administration of IL-11 increased peripheral platelet counts by threefold over control transplant mice at day 10 posttransplantation during the post-irradiation platelet nadir. Further treatment led to platelet counts higher than normal 18 days posttransplantation when control animals had just attained normal platelet counts. IL-11 can accelerate the recovery of the peripheral blood leukocytes, mainly neutrophils, and platelets in transplant mice, effects that may be clinically useful in future applications for BM transplantation and chemotherapy-related cytopenias.     

Interleukin-11 inhibits adipogenesis and stimulates myelopoiesis in human long-term marrow cultures

Interleukin-11 (IL-11) is a bone marrow (BM) stromal-derived growth factor that has been shown to stimulate murine myeloid and lymphoid cells both in vitro and in vivo and to inhibit adipogenesis in a murine fibroblast cell line. We have studied the effects of IL-11 on highly purified human BM stem and progenitor cells and on human long-term marrow cultures (LTMC). Adipocyte differentiation is an integral component of murine and human LTMC. IL-11 stimulates myeloid growth as a single cytokine when added to highly enriched CD34+, HLA-DR+ bone marrow cells. IL-11 stimulated no growth in the more primitive CD34+, HLA-DR- population even in the presence of additional cytokines. IL-11 addition to human LTMC resulted in the expansion of myeloid and mixed, but not erythroid, progenitor populations. IL-11 dramatically increased the adherent cell populations, including both stromal cells and macrophages. Treated cultures also showed marked inhibition of fat accumulation in the adherent cells due in part to a block in the differentiation of preadipocytes to adipocytes, as shown by RNA analysis using adipocyte-specific markers. These data show that IL-11 stimulates a more differentiated, although multipotential, progenitor cell in human BM and that LTMC provide a useful model for studying the effects of this cytokine in the context of the hematopoietic microenvironment.     

Cyclooxygenase-2 is essential for normal recovery from 5-fluorouracil-induced myelotoxicity in mice.

Cyclooxygenase (COX) plays a key regulatory role in prostaglandin synthesis. COX-2 is inducible and is the major isoform of inflammatory cells. COX-2-deficient mice were shown to have normal basal hematopoiesis and hematology. We hypothesized that COX-2 induction plays a role in the recovery phase of 5-fluorouracil (5-FU) induced bone marrow injury, because significant macrophage-driven phagocytic removal of necrotic debris and stromal cell reorganization of repopulating marrow occur after 5-FU induction of bone marrow necrosis. Hematologic recovery was markedly delayed with moderately severe leukopenia, thrombocytopenia and reticulocytopenia compared to heterozygotes on day 8 or 12 in Cox-2-/- mice. Mild anemia was present in 5-FU-treated Cox-2-/- and Cox-2+/- mice on days 8 and 12, which was more severe in Cox-2-/- mice. Cox-2-/- mice had markedly decreased bone marrow cell counts per femur and reduced numbers of erythroid and myeloid colony-forming cells compared to heterozygote mice on days 8 and 12 post 5-FU. Histologic examination of 5-FU-treated Cox-2-/- mice revealed a failure to repopulate the intact marrow stroma with hematopoietic cells. Accelerated erythropoiesis following phenylhydrazine-induced hemolytic anemia, however, was comparable between Cox-2-/- and Cox+/- mice, as were induced levels of renal erythropoietin mRNA. COX-2 induction is likely a central event in the accelerated hematopoiesis following myelotoxic injury, because recovery from 5-FU-induced myeloablation is markedly impaired in Cox-2-/- mice but is normal after phenylhydrazine induction of anemia.     

Prolactin exerts hematopoietic growth-promoting effects in vivo and partially counteracts myelosuppression by azidothymidine.

Prolactin (PRL) is a neuroendocrine hormone that influences immune and hematopoietic development. The mechanism of action of this hormone in vivo remains unclear; therefore, we assessed the effects of PRL on hematopoiesis in vivo and in vitro. Normal resting mice were treated with 0, 1, 10, or 100 microg of recombinant human prolactin (rhPRL) for 4 consecutive days and euthanized on the fifth day for analysis of myeloid and erythroid progenitors in the bone marrow and spleen. Both frequencies and absolute numbers of splenic colony-forming unit granulocyte-macrophage (CFU-GM) and burst-forming unit-erythroid (BFU-e) were significantly increased in mice receiving rhPRL compared to the controls that had received saline only. Bone marrow cellularities were not significantly affected by any dose of rhPRL, but the absolute numbers and frequencies of bone marrow CFU-GM and BFU-e were augmented by rhPRL. These results suggest that rhPRL can promote hematopoiesis in vivo. Because rhPRL augments myeloid development in vivo, we examined the potential of the hormone to reverse the anemia and myelosuppression induced by azidothymidine (AZT). Mice were given rhPRL injections concurrent with 2.5 mg/mL AZT in drinking water. rhPRL partially restored hematocrits in the animals after 2 weeks of treatment and increased CFU-GM and BFU-e in both spleens and bone marrow. The experiments with AZT and rhPRL support the conclusion that the hormone increases myeloid and erythroid progenitor numbers in vivo, and they suggest that the hormone is clinically useful in reversing myelosuppression induced by AZT or other myeloablative therapies.     

Synergy of interleukin 1 and granulocyte colony-stimulating factor: in vivo stimulation of stem-cell recovery and hematopoietic regeneration following 5-fluorouracil treatment of mice.

The human bladder carcinoma cell line 5637 produces hematopoietic growth factors [granulocyte and granulocyte/macrophage colony-stimulating factors (G-CSF and GM-CSF)] and hemopoietin 1, which synergizes with CSFs to stimulate colony formation by primitive hematopoietic stem cells in 5-fluorouracil-treated mouse bone marrow. Molecular and functional properties of hemopoietin 1 identified it as identical to interleukin 1 alpha (IL-1 alpha). When bone marrow cells from 5-fluorouracil-treated mice were cultured in suspension for 7 days with recombinant human IL-1 alpha and/or G-CSF, it was found that the two factors synergized to enhance recovery of myelopoietic cells and colony-forming cells of both high and low proliferative potential. G-CSF alone did not sustain these populations, but the combination had greater-than-additive stimulating capacity. In vivo, 5-fluorouracil (150 mg/kg) produced profound myelosuppression and delayed neutrophil regeneration for up to 2 weeks in C3H/HeJ mice. Daily administration of recombinant human G-CSF or recombinant human IL-1 alpha accelerated recovery of stem cells, progenitor cells, and blood neutrophils by up to 4 days in 5-fluorouracil-treated C3H/HeJ and B6D2F1 mice. The combination of IL-1 alpha and G-CSF acted synergistically, reducing neutropenia and accelerating recovery of normal neutrophil numbers by up to 7 days. This was accompanied by accelerated regeneration of spleen colony-forming units and erythroid, myeloid, and megakaryocytic progenitor cells in marrow and spleen, with enhanced erythroid and granulocytic differentiation. These results indicate the possible therapeutic potential of combination therapy with IL-1 and hematopoietic growth factors such as G-CSF in the treatment of chemotherapy- or radiation-induced myelosuppression.     

Recombinant glycosylated human interleukin-6 accelerates peripheral blood platelet count recovery in radiation-induced bone marrow depression in baboons.

This report was aimed at confirming the potential clinical use for a genetically engineered glycosylated human interleukin-6 (rhIL-6) in hematopoiesis. Its tolerance and efficacy were assessed on hematopoietic restoration after neutron radiation-induced bone marrow injury on baboons, which represent an adequate model of parallelism for studying hematology in the human. The particular neutron radiation absorption pattern in the body allows the preservation of underexposed bone marrow areas that mimics an autotransplantation-like situation. An initial dose finding study (1 microgram up to 20 micrograms/kg/d for 8 consecutive days) in normal baboons established a dose-dependent response regarding the peripheral platelet count (range of increase, 1.5- to 4-fold). A significant elevation in white blood cell (WBC) count, as well as a substantial reversible normochromic normocytic anemia, were observed for the highest doses only (10 and 20 micrograms/kg/d). All rhIL-6 administered doses were clinically well tolerated. In myelosuppressed baboons, a selected dose of 10 micrograms/kg/d of rhIL-6 for 13 consecutive days significantly lessened the degree of induced thrombocytopenia as compared with the control group (P = .01) and shortened the time to occurrence of the nadir, showing that the onset of recovery occurs much earlier, ie, an average of 5 days (P = .003), in the treated group. Moreover, this accelerated platelet recovery is evidenced by an 8-day shorter mean time back to baseline values (P = .03) in the rhIL-6--treated animals. At this dose no effect was observed on the WBC recovery pattern. Importantly rhIL-6 did not accentuate the radiation-induced anemia and was clinically well tolerated. All tested monkeys recovered from their induced pancytopenia and no animal loss was recorded. IL-6, tumor necrosis factor, and IL-1 blood measurements are reported. In conclusion, rhIL-6 is a potent thrombopoietic factor for the treatment of induced thrombocytopenia in nonhuman primates at a clinically well-tolerated dose.     

Hematologic abnormalities of the immunodeficient mouse mutant, viable motheaten (mev)

We have studied the hematopoietic system of the immunodeficient mouse mutant, viable motheaten (mev/mev). These mice usually die by 9 weeks of age from severe pneumonitis. The lungs at that time are infiltrated with granulocytes, macrophages, and lymphocytes. Granulocyte and macrophage precursor cells (CFU-GM) are dramatically increased in the spleens of mev/mev mice, whereas the bone marrow population of these precursors is decreased when compared with littermate control animals. The CFU-GM population retained its normal dependence on granulocyte-macrophage colony-stimulating factor (GM-CSF) for proliferation and differentiation. In contrast, the frequency of an erythroid precursor (CFU-E) was dramatically increased in spleen and showed increased sensitivity to erythropoietin (Epo). Moreover, a splenic CFU-E subpopulation formed normally appearing erythroid colonies in the absence of exogenous Epo. The bone marrow CFU-E population was significantly diminished in size when compared with either wildtype C57BL/6J mice or mice heterozygous for the mev allele. Unlike the CFU-E population, erythroid burst-forming unit (BFU-E) frequency in mev/mev mice was diminished both in bone marrow and in spleen, although the total number of splenic BFU-E was increased because of splenomegaly in these animals. BFU-E retained their dependence on the presence of both Epo and a source of interleukin 3 (IL-3) for proliferation and differentiation into erythroid bursts. Spleen cells from mev/mev mice, when stimulated in vitro with pokeweed mitogen, failed to produce significant quantities of IL-3. Comparison with medium or +/mev heterozygotes revealed that mev/mev spleen cell-conditioned medium showed a 40-fold reduction in burst-promoting activity. Thus, in viable motheaten mice, there is a major shift in hematopoiesis from bone marrow to spleen, which is accompanied by a diminished capacity of spleen cells to produce burst-promoting activity. These data and those from other studies suggest that the hematopoietic microenvironment of marrow may be impaired in this mutant.     

Exacerbation of autoimmune neutropenia to agranulocytosis in association with severe autoimmune thrombocytopenia and hemolytic anemia in a patient with Sjögren's syndrome

A 73-year-old woman with Sj?gren's syndrome and autoimmune neutropenia (AIN) associated with large granular lymphocytosis of the polyclonal T cell type, demonstrated autoimmune thrombocytopenia (AIT) at diagnosis of sigmoid colon cancer. Ten months later, both AIN and AIT had exacerbated to agranulocytosis and severe thrombocytopenia below 10×10(9)/L, respectively. There were no dysplastic features of bone marrow hematopoietic cells. Furthermore, an in vitro assay of hematopoietic progenitors showed normal granuloid and erythroid colony formation. Although we serially treated her with prednisolone (oral), filgrastim, intravenous high-dose immunoglobulin infusion, cyclophosphamide (oral), danazol, cyclosporine A (oral), and rituximab, number of neutrophils and platelets elevated only temporarily. During the course of agranulocytosis and severe thrombocytopenia, the patient also developed autoimmune hemolytic anemia (AIHA). She died of pneumonia 5 months after the onset of agranulocytosis. This case is very unique and novel in terms of autoimmune phenomena simultaneously directed to granulocytes, platelets, and red blood cells under the background of Sj?gren's syndrome.     

Further Examination of Various Administration Protocols of Pegylated Recombinant Human Megakaryocyte Growth and Development Factor on Thrombocytopenia in Myelosuppressed Mice

Abstract: Thrombopoietin (TPO) is the recently isolated lineage-dominant hematopoietic factor that plays a pivotal role in the regulation of megakaryocytopoiesis and throm-bopoiesis. In vivo studies have shown that daily multiple injections of pegylated human megakaryocyte growth and development factor (PEG-rHuMGDF), a truncated molecule related to human TPO, modified with polyethylene glycol, greatly improve thrombocytopenia and in most cases anemia and neutropenia in myelosuppressed animal models. In this study, we further examined various administration protocols of PEG-rHuMGDF on thrombocytopenia in mice treated with a combination of irradiation and carboplatin. After the myelosuppressive treatment on Day 0, mice received the same amount of PEG-rHuMGDF be ginning on Day 1 by a single, 3 times (on alternate days), or 7 day daily administration. A single injection of PEG-rHuMGDF significantly reduced the severity and duration of thrombocytopenia and anemia with a concomitant accelerated recovery of megakaryocytic and erythroid progenitors in the bone marrow, similar to the 2 other administration protocols. As the start of a single injection of PEG-rHuMGDF was delayed, its therapeutic effects were attenuated. These results indicate that an administration of PEG-rHuMGDF at an earlier time after the myelosuppressive treatment is necessary to improve thrombocytopenia and anemia.     

In vivo hematopoietic effects of recombinant interleukin-1 alpha in mice: stimulation of granulocytic, monocytic, megakaryocytic, and early erythroid progenitors, suppression of late-stage erythropoiesis, and reversal of erythroid suppression with erythropoietin

Interleukin-1 alpha (IL-1 alpha) is a macrophage-derived, multifunctional cytokine that broadly potentiates myelopoiesis and induces the synthesis of hematopoietic colony-stimulating factors (CSF) in vitro and in vivo. To evaluate the possibility for use of IL-1 alpha in ameliorating in vivo bone marrow suppression induced by drugs or radiation, we examined the in vivo effects of the cytokine on erythropoietic and other hematopoietic progenitor cells. Normal mice were treated with a single intraperitoneal (IP) injection of recombinant human IL-1 alpha at varying doses and were assayed at various times post-treatment. By six hours postinjection, a significant suppression of mature erythroid progenitors (CFU-E) was observed in animals treated with IL-1 alpha (0.5 micrograms/mouse), with maximum suppression of CFU-E and peripheral blood reticulocyte counts occurring at 24 hours. Decreases in peripheral blood hematocrit did not occur after a single IL-1 alpha injection but were observed after multiple injections of the cytokine. The suppressive effects of IL-1 alpha on late-stage erythropoiesis were abrogated by simultaneous administration of erythropoietin (EPO). At 48 hours post-treatment, a marked stimulation was observed in the numbers of spleen and marrow immature erythroid (BFU-E), macrophage (CFU-M), granulocyte (CFU-G), granulocyte- macrophage (CFU-GM), and megakaryocyte (CFU-meg) progenitor cells. These results demonstrate the potential use of IL-1 alpha as a generalized stimulator of hematopoiesis and show that the cytokine- induced suppression of late-stage erythropoiesis can be prevented by EPO.     

Synergistic interactions between interleukin-11 and interleukin-4 in support of proliferation of primitive hematopoietic progenitors of mice.

Interleukin-11 (IL-11) is a newly identified lymphohematopoietic cytokine originally derived from the primate bone marrow stromal cell line, PU-34. Separately, we reported that IL-11 augments IL-3-dependent proliferation of primitive murine hematopoietic progenitors in culture. We have now examined the synergistic interactions between IL-11 and IL-4 in support of colony formation from marrow cells of mice treated 2 days before with 150 mg/kg 5-fluorouracil. Neither recombinant human IL-11 nor murine IL-4 alone was effective in the support of colony formation. When the two factors were combined, there was major enhancement of colony formation, including that of multilineage colony-forming cells. Serial observations (mapping studies) of development of multipotential blast cell colonies indicated that the synergy between IL-11 and IL-4 is due in part to shortening of the dormant period of the stem cells, an effect very similar to that of IL-6 and granulocyte colony-stimulating factor. The combination of IL-11 and IL-4 may be useful in the stimulation of dormant hematopoietic stem cells in vivo.     

Mechanism of ranitidine associated anemia

Ranitidine, an H2 receptor antagonist, has been associated with hematotoxicity. The mechanism(s) underlying the toxicity is not well understood. The authors studied the mechanism of anemia in a patient with ranitidine associated anemia and thrombocytopenia. Clinical evaluation suggested drug-induced Coombs' positive reticulocytopenic hemolysis. In vitro, with the patient off ranitidine, the authors were able to induce Coombs' positivity by incubating patient's red cells with ranitidine and his serum. This process was inhibited by prior exposure of his red cells to histamine. In vitro studies using clonal assays for hematopoietic progenitors revealed that while the patient's serum or ranitidine alone did not affect the patient's or normal bone marrow hematopoiesis, the simultaneous presence of both agents significantly suppressed both patient's and normal erythroid progenitor (BFU-E) colony development. This suppressive effect was prevented by the prior exposure of marrow to histamine and was not observed when the patient's serum was heat inactivated. These studies suggest that the anemia may have resulted from complement-dependent autoimmune destruction/inhibition of progenitor/mature erythroid cells by a process critically dependent on the presence of ranitidine and possibly acting at or near the histamine receptor.     

Thrombopoietin signaling is required for in vivo expansion of IL-11--responsive hematopoietic progenitor cells in the steady state

OBJECTIVE: mpl(-/-) mice have a profound defect in platelets and megakaryocytes and a defect in hematopoietic progenitor cells and stem cells. However, no specific subset of the progenitor/stem cell compartment has been shown to be particularly affected by this deficiency in mpl(-/-) mice. In this article, we identified a specific subset of bone marrow progenitor/stem cells that was altered in mpl(-/-) mice. MATERIALS AND METHODS: In vitro and in vivo hematopoietic assays were utilized to examine the response to interleukin-11 in mice lacking the receptor for thrombopoietin (TPO) (mpl(-/-) mice). RESULTS: The interleukin (IL)-11-responsive subset of progenitor cells was not detected in clonal cultures of bone marrow cells from mpl(-/-) mice. However, mpl(-/-) mice responded to IL-11 in vivo as evidenced by a rise in platelet count and an increase in spleen weight. Experiments were performed to address this paradox: administration of 5-fluorouracil with consequent "expansion" of early hematopoietic cells resulted in the appearance of IL-11-responsive cells in mpl(-/-) mice when assayed in in vitro cultures. CONCLUSIONS: Thus, although mpl(-/-) mice have the capacity to produce IL-11-responsive progenitor cells, under steady state conditions their expansion is dependent on TPO. This is the first evidence that a specific subset of bone marrow progenitor/stem cells is altered in mpl(-/-) mice.     

Effect of stem cell factor on in vitro erythropoiesis in patients with bone marrow failure syndromes.

Stem cell factor (SCF) enhances normal hematopoiesis. We examined its effect in vitro on bone marrow and blood progenitors from patients with inherited bone marrow failure syndromes, including 17 patients each with Diamond-Blackfan anemia (DBA) and Fanconi's anemia (FA), 3 with dyskeratosis congenita (DC), and 1 each with amegakaryocytic thrombocytopenia (amega) and transient erythroblastopenia of childhood (TEC). Mononuclear cells were cultured with erythropoietin (Ep) alone or combined with SCF or other factors. SCF increased the growth of erythroid progenitors in cultures from 50% of normal controls, 90% of DBA, 70% of FA, 30% of DC, and the amega and TEC patients; normal numbers were reached in 25% of DBA studies. Improved in vitro erythropoiesis with SCF in all types of inherited marrow failure syndromes does not suggest a common defect involving kit or SCF, but implies that SCF may be helpful in the treatment of hematopoietic defects of varied etiologies.     

Bcl-2 Family Members As Prognostic Indicators in AML

It has been postulated that interleukin-lα (IL-lα) and interleukin-1β (IL-1β) play a role in the pathogenesis of the anemia of chronic disease by inhibiting the proliferation of human erythroid progenitor cells. In the course of investigating this hypothesis we found that IL-1 type I receptor (IL-1R) mRNA is expressed on erythroid progenitor enriched, primitive human hematopoietic cells (CD34+, c-kit-R^bright) and on cells isolated from human erythroid burst forming colonies (BFU-E). Nevertheless, when CD34+, c-kit-R^bright cells were exposed to IL-1α and IL-1β in vitro, cloning efficacy of BFU-E and CFU-E in a serum free culture system was not inhibited. Moreover, in apparent contradiction to the hypothesis being tested, we found that both IL-1 isoforms actually increased the survival of human BFU-E in serum free, growth factor free medium. Accordingly, these results suggest that if IL-1 plays a role in the pathogenesis of the anemia of chronic disease, it is not due to a direct suppressive effect on erythroid cell growth. Rather, our data support the hypothesis that IL-1 may cause the elaboration of another inhibitory cytokine (s) by cells of the marrow microenvironment.     

Ectopic expression of murine TPO receptor (c-mpl) in mice is pathogenic and induces erythroblastic proliferation

c-mpl, the cellular homologue of the v-mpl oncogene transduced in the myeloproliferative leukemia virus (MPLV), encodes the receptor for thrombopoietin, a cytokine involved in the proliferation and differentiation of cells of the megakaryocytic lineage. Here, we show that a retrovirus containing murine c-mpl cDNA (HSFmmpl) is pathogenic in vivo when inoculated in adult mice. All mice developed hepatosplenomegaly and died within 9 to 12 weeks after infection. Histological analysis showed that spleen, liver, and peripheral blood were invaded by erythroblasts at every stage of differentiation. In contrast to the myeloproliferative syndrome induced by MPLV, we did not observe an infiltration of these organs with cells from the granulocytic lineage nor a thrombocytosis. In fact, the platelet count of HSFmmpl mice progressively decreased and a severe thrombocytopenia was observed late in the course of the disease. Further characterization of the target progenitor of HSFmmpl virus in the spleen and bone marrow of diseased animals was accomplished using in vitro clonogenic progenitor cell assays. This analysis indicated that both late and early erythroid compartment (colony-forming unit- erythroid and burst-forming unit-erythroid) were largely increased in the spleens. The colony-forming unit-granulocyte-macrophage compartment was also increased but to a lesser extent. This study shows for the first time that ectopic expression of a member of the cytokine receptor superfamily promotes hematopoietic progenitor cell proliferation and could play a role in leukemogenesis.