[Show abstract][Hide abstract] ABSTRACT: Background
Bone marrow mononuclear cell (BMMC) transplantation for heart failure has shown inconsistent therapeutic efficacy.
We enrolled 104 ischemic heart failure patients scheduled for coronary bypass surgery (CABG). After 4-12-week pharmacotherapy optimization, 39 patients with left ventricular ejection fraction (LVEF) ≤45% received injections of BMMCs or vehicle intraoperatively into the myocardial infarction border area in a randomized and double-blind manner.
The median number of cells injected was 8.4x108 (interquartile range [IQR]: 5.2x108-13.5x108). We measured LV function and myocardial scar size by magnetic resonance imaging (MRI), and viability by positron emission tomography (PET) and single-photon emission computed tomography (SPECT), preoperatively and after one-year follow-up. LVEF, the predefined primary end-point measure, improved by a median of 5.6% in the control (IQR:0.2-10.1) and by 4.8% in the BMMC group (IQR:-0.5-8.2) (p=0.59). Wall thickening in injected segments rose by a median of 4.5% among controls (IQR:-18.1-23.9) and by 5.5% in the BMMC group (IQR:-6.6-26.5) (p=0.68). Changes in viability by PET and SPECT did not differ between groups. Myocardial scar size by MRI in injected segments rose by a median of 5.1% among controls (IQR:-3.3-10.8) but fell by 13.1% in the BMMC group (IQR:-21.4--6.5) (p=0.0002).
BMMC therapy combined with CABG failed to improve LV systolic function, or viability, despite reducing myocardial scar size.
The Journal of heart and lung transplantation: the official publication of the International Society for Heart Transplantation 04/2014; · 5.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We previously showed cell-cell contacts of human dermal fibroblasts to induce expression of the hepatocyte growth factor/scatter factor (HGF) in a process designated as nemosis. Now we report on nemosis initiation in bone marrow mesenchymal stem cells (BMSCs). Because BMSCs are being used increasingly in cell transplantation therapy we aimed to demonstrate a functional effect and benefit of BMSC nemosis for wound healing. Nemotic and monolayer cells were used to stimulate HaCaT keratinocyte migration in a scratch-wound healing assay. Both indicators of nemosis, HGF production and cyclooxygenase-2 expression, were induced in BMSC spheroids. When compared with a similar amount of cells as monolayer, nemotic cells induced keratinocyte in vitro scratch-wound healing in a concentration-dependent manner. The HGF receptor, c-Met, was rapidly phosphorylated in the nemosis-stimulated keratinocytes. Nemosis-induced in vitro scratch-wound healing was inhibited by an HGF-neutralizing antibody as well as the small molecule c-Met inhibitor, SU11274. HGF-induced in vitro scratch-wound healing was inhibited by PI3K inhibitors, wortmannin and LY294002, while LY303511, an inactive structural analogue of LY294002, had no effect. Inhibitors of the mitogen-activated protein kinases MEK/ERK1/2 (PD98059 and U0126), and p38 (SB203580) attenuated HGF-induced keratinocyte in vitro scratch-wound healing. We conclude that nemosis of BMSCs can induce keratinocyte in vitro scratch-wound healing, and that in this effect signaling via HGF/c-Met is involved.
Wound Repair and Regeneration 07/2009; 17(4):569-77. · 2.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The in vitro cultures of erythroid (BFU-E) and megakaryocytic (CFU-Meg) progenitors have been useful diagnostic tools in myeloproliferative disorders (MPD). However, after the discovery of the JAK2V617F mutation, their diagnostic role has been uncertain. In this single-centre retrospective study we analyzed JAK2V617F mutation in 58 ET and 42 PV patients diagnosed according to WHO criteria and compared the results with those of colony forming assays with special emphasis on CFU-Meg growth. 91% of PV and 57% of ET patients had JAK2V617F mutation and they all showed spontaneous BFU-E growth. However, endogenous BFU-E formation was also seen in nine JAK2V617F mutation negative patients displaying also a normal JAK2 exon 12 allele. Endogeneous CFU-Meg colony formation was found in 59% of PV and 53% of the ET patients. A subgroup of ET patients (n=7) displayed sole spontaneous CFU-Meg growth without spontaneous BFU-E growth. They all were JAK2 mutation negative, but one of them had MPL mutation. In conclusion, in vitro cultures of haematopoietic progenitors are sensitive diagnostic tools in the present group of 100 MPD patients revealing also JAK2 mutation negative ET and PV patients displaying sole spontaneous CFU-Meg or BFU-E growth.
Leukemia Research 09/2008; 33(1):54-9. · 2.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recent data indicate a role for angiogenesis in hematologic malignancies. In addition to promoting new vessel growth in the bone marrow microenvironment, angiogenic factors are regulators of both hematopoietic and leukemic cells. Activation of vascular endothelial growth factor receptor 3 (VEGFR-3) and Tie1 tyrosine kinase receptor are known to promote leukemia cell survival. The details of this complex angiogenesis-related interaction are still uncertain.
We studied bone marrow samples from 73 patients with acute lymphoblastic (ALL) or myelogenous (AML) leukemia by using immunological methods.
Vascular endothelial growth factor receptor 3 expression was found in 15% of the samples, particularly in samples with pediatric lymphoblastic leukemias and monocytic AMLs. Tie1 protein expression was found in 11% of the samples, all of which were from adult AML patients.
Our findings suggest that there are angiogenesis-related differences between pediatric and adult lymphoblastic leukemias as well as between lymphoid and myeloid leukemias.
Pediatric Blood & Cancer 05/2007; 48(4):387-92. · 2.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Podocalyxin-like protein (PCLP) is a sialomucin-type membrane protein structurally related to CD34 and endoglycan. It was first described in glomerular podocytes and endothelial cells. In mice, PCLP is present in haemangioblasts, and in both chicken and mice it is a marker of early haematopoietic stem cells and lineage-restricted haematopoietic progenitors. Its expression decreases during differentiation of haematopoietic cells. Of mature blood cells, only chicken and rat thrombocytes express PCLP protein. PCLP expression in human haematopoietic cells has not been studied. Here we demonstrate PCLP mRNA in human CD34+ cells, in lineage committed erythroid, megakaryocyte and myeloid progenitors, in K562 leukaemia cells, and in peripheral blood leucocytes. The mRNA expression level was higher in developing cells than in mature leucocytes. By Northern blotting and cDNA sequencing, the haematopoietic and renal PCLP mRNAs were identical. Of the mobilized CD34+ cells, 28% (mean; range 14-61%) expressed PCLP protein and the majority of PCLP+ cells were CD117+. Almost all of the K562 cells expressed PCLP protein. Surprisingly, PCLP protein was not detected in any mature blood cells. These results suggest that human PCLP may be a valuable marker for a subset of haematopoietic stem cells.
British Journal of Haematology 04/2004; 124(6):809-18. · 4.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Human CD133 (AC133)(+)CD34(+) stem and progenitor cells derived from fetal liver and from bone marrow and blood incorporate a functional population of circulating endothelial precursor cells. Vascular endothelial growth factor receptor 3 (VEGFR-3) regulates cardiovascular development and physiological and pathological lymphangiogenesis and angiogenesis. However, the origin of VEGFR-3(+) endothelial cells (ECs) and the mechanisms by which these cells contribute to postnatal physiological processes are not known, and the possible existence of VEGFR-3(+) lymphatic or vascular EC progenitors has not been studied. Using monoclonal antibodies to the extracellular domain of VEGFR-3, we show that 11% +/- 1% of CD34(+) cells isolated from human fetal liver, 1.9% +/- 0.8% CD34(+) cells from human cord blood, and 0.2% +/- 0.1% of CD34(+) cells from healthy adult blood donors are positive for VEGFR-3. CD34(+)VEGFR-3(+) cells from fetal liver coexpress the stem/precursor cell marker CD133 (AC133). Because mature ECs do not express CD133, coexpression of VEGFR-3 and CD133 on CD34(+) cells identifies a unique population of stem and progenitor cells. Incubation of isolated CD34(+)VEGFR-3(+) cells in EC growth medium resulted in a strong proliferation (40-fold in 2 weeks) of nonadherent VEGFR-3(+) cells. Plating of these cells resulted in the formation of adherent VEGFR-3(+)Ac-LDL(+) (Ac-LDL = acetylated low-density lipoprotein) EC monolayers expressing various vascular and lymphatic endothelial-specific surface markers, including CD34, VE-cadherin, CD51/61, CD105, LYVE-1, and podoplanin. These data demonstrate that human CD34(+)CD133(+) cells expressing VEGFR-3 constitute a phenotypically and functionally distinct population of endothelial stem and precursor cells that may play a role in postnatal lymphangiogenesis and/or angiogenesis.
[Show abstract][Hide abstract] ABSTRACT: Stanniocalcin-1 (STC) is a 56-kDa homodimeric glycoprotein hormone originally found in fish, in which it regulates calcium/phosphate homeostasis and protects against toxic hypercalcaemia. The recently characterized human STC is 80% similar to fish STC. We have earlier reported a high expression of STC in terminally differentiated human and rodent brain neurones, and found that STC contributes to the maintenance of their integrity. Here, we report that mature megakaryocytes and platelets display high STC content. K562 cells, induced to megakaryocytoid differentiation in vitro, acquired expression of STC, which was not seen in untreated K562 cells or cells induced to erythroid differentiation.
British Journal of Haematology 12/2002; 119(2):359-63. · 4.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Interaction of intercellular adhesion molecules (ICAMs) with their receptors has a key role in normal leucocyte adhesion and migration, whereas in leukaemia this has not been well established. In this study, we have evaluated the roles of different adhesion molecules in normal and leukaemia cell extravasation in a novel organotypic model for vessel wall and measured plasma ICAM-1 and -2 levels in acute leukaemia patients at diagnosis and during chemotherapy. We found that both normal mononuclear cells and blast cells from acute leukaemia patients, as well as retinoic acid-treated promyelocytic leukaemia cells, rapidly extravasated through endothelial cell layers into the underlying collagen matrix. ICAM-1 antibody prevented the extravasation, while antibodies to other adhesion molecules showed little (CD18, ICAM-2) or no inhibition (CD11a and ICAM-3). Soluble ICAM-1 (sICAM-1) protein had no effect. We also observed increased plasma sICAM-1 and -2 levels in leukaemia patients and found that they correlated only weakly with the white blood cell count. No correlation was found between sICAM-1 or -2 levels and the response to therapy. Although elevated sICAM-2 levels decreased rapidly during chemotherapy, sICAM-1 levels did not. Because sICAM-1 protein had no effect on leukaemia cell extravasation in vitro, it is probable that the increased plasma sICAM-1 levels in leukaemia patients may not play a role in leukaemia cell infiltration. However, as we showed that ICAM-1 mediated leukaemia cell extravasation on the cell surface, it is possible that cellular ICAM-1 has an important role in disease progression.
British Journal of Haematology 07/2001; 113(4):989-1000. · 4.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The importance of plasminogen activation, mediated by urokinase (uPA) and its receptor (uPAR), is well established in many physiologica and pathological processes, such as in cell migration and tumor-cell invasion. Recently, additional functions have been described for uPA and uPAR, particularly in cell adhesion and chemotaxis. The amounts of uPA and uPAR in various tumor types and in the plasma/serum samples of cancer patients have been shown to correlate with survival prognosis, indicating the relevance of these molecules in malignancy. We previously showed that in acute myeloid leukemia, a high level of plasma soluble uPAR (suPAR) at diagnosis correlates with poor response to chemotherapy. However, in this case, as in other cancers, the origin of suPAR is unknown. Therefore, we have now analyzed uPAR in cells, plasma, and urine of patients with acute leukemia (n = 35) at 0, 5, 14, 28, and 56 days after start of chemotherapy. In response to cytotoxic treatment, suPAR levels decreased rapidly, and the decreasing plasma suPAR (p-suPAR levels correlated highly with decreasing numbers of circulating tumor cells, suggesting that the elevated p-suPAR was produced by circulating tumor cells. Moreover, the p-suPAR level appeared to correlate with the amount of uPAR in tumor cell lysates at diagnosis. Our results also show for the first time that in lysates of circulating tumor cells, studied by immunoprecipitation and immunoblotting, uPAR was partly in fragmented form, whereas only full-length uPAR was found in normal leukocytes. We also detected fragmented suPAR in peripheral blood plasma, in urine, and especially in the plasma compartment of bone-marrow aspirates of acute myeloid leukemia patients, in a pattern differing considerably from that found in healthy individuals. Because proteolytic cleavage of uPAR induces a potent chemotactic response in vitro, it is possible that these fragments may play a role in the pathophysiology of acute leukemia.
Cancer Research 01/2001; 60(24):7126-32. · 9.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Plasminogen activation in leukemia has been less well characterized than in other malignancies. However, the increased tendency to bleeding and tissue infiltration by leukemic cells are processes in which plasminogen activation may be involved. We have examined plasma and the peripheral blood mononuclear cell fraction from 80 patients including 53 patients with newly diagnosed acute leukemia and 27 patients with other hematological disorders as well as 21 healthy controls. In 28 of 29 examined patients with acute myeloid leukemia (AML) and in two of three patients with hybrid leukemia we found urokinase receptor (uPAR) on the cell surface, while most (7/9) samples from patients with acute lymphoblastic leukemia (ALL) were negative for uPAR. The plasma mean value for soluble uPAR (suPAR) was significantly elevated in patients with AML and ALL. In AML the highest values were found in patients who had residual disease after several cycles of chemotherapy. Compared to controls the uPA antigen levels in patient plasmas were elevated and decreased along with uPAR during treatment. Our results suggest that cell surface uPAR may be a useful marker for leukemia classification and in our material a high level of plasma suPAR correlated with resistance to chemotherapy in AML.
Thrombosis and Haemostasis 06/1999; 81(5):705-10. · 5.76 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The active process of pericellular proteolysis is central in tumor invasion, and in particular the essential role of the urokinase-type plasminogen activator (uPA) is well established. uPA-mediated plasminogen activation facilitates cell migration and invasion through extracellular matrices by dissolving connective tissue components. uPA, its receptor (uPAR) and plasminogen activator inhibitor-1 (PAI-1) are enriched in several types of tumors. The importance of proteolysis and especially plasminogen activation is less clear in hematopoietic malignancies than in solid tumors. However, patients with leukemia have an increased tendency to bleeding, not always attributable to thrombocytopenia, and tissue infiltration by leukemic cells, processes in which plasminogen activation may be involved. Several studies have indicated that plasminogen activators (PAs) are highly expressed by cultured leukemia cells. Furthermore, differing from adherent tumor cells, leukemic cells have an enhanced capacity to activate pro-uPA and mainly the active form of uPA is released to culture medium. Ex vivo studies have shown that uPAR, uPA and its inhibitors can be found on the surface of normal blood cells and on the blast cell surfaces from patients with acute leukemia as well as from plasma samples. Elevated levels of PAs and their inhibitors have been detected in leukemic cell lysates. Few studies have tried to demonstrate a correlation between prognosis of leukemia and levels of plasminogen activators. More in vivo studies are needed to show, if any of the factors of the plasminogen activation process can be used as tools in subclassification or as markers for prognosis in leukemia. This review article will focus on the in vivo studies of plasminogen activation in leukemia and will present several in vitro findings on PAs in normal leukocytes and leukemic cell lines.
[Show abstract][Hide abstract] ABSTRACT: Lymphatic vessels have been difficult to study in detail in normal and tumor tissues because of the lack of molecular markers. Here, monoclonal antibodies against the extracellular domain of the vascular endothelial growth factor-C receptor that we have named VEGFR-3 were found to specifically stain endothelial cells of lymphatic vessels and vessels around tumors such as lymphoma and in situ breast carcinoma. Interestingly, the spindle cells of several cutaneous nodular AIDS-associated Kaposi's sarcomas and the endothelium around the nodules were also VEGFR-3 positive. The first specific molecular marker for the lymphatic endothelium should provide a useful tool for the analysis of lymphatic vessels in malignant tumors and their metastases and the cellular origin and differentiation of Kaposi's sarcomas.
Cancer Research 05/1998; 58(8):1599-604. · 9.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The class III receptor tyrosine kinase FLT3/FLK2 (FLT3; CD135) represents an important molecule involved in early steps of hematopoiesis. Here we compare cell-surface expression of FLT3 on bone marrow (BM) and cord blood (CB) cells using monoclonal antibodies (MoAbs) specific for the extracellular domain of human FLT3. Flow cytometric analysis of MACS-purified BM and CB cells showed that 63% to 82% of BM CD34+ and 88% to 95% of the CB CD34+ cells coexpress FLT3. Clonogenic assays and morphological characterization of FACS-sorted BM CD34+ cells demonstrate that colony-forming unit-granulocyte-macrophage (CFU-GM) and immature myelo-monocytic precursor cells are enriched in the subpopulation staining most brightly with the FLT3 MoAb whereas the majority of the burst-forming units-erythroid (BTU-E) and small cells with lymphoid morphology are found in the FLT3- population. In contrast, statistically indistinguishable proportions of CFU-granulocyte-erythrocyte-megakaryocyte-macrophage (CFU-GEMM) and more primitive cobblestone area forming cells (CAFC) were detected in both fractions, albeit the FLT3+ fraction consistently showed more CAFC activity than the FLT3- fraction. Although in both, BM and CB the majority of CD34+CD117+ (KIT+), CD34+CD90+ (Thy-1+), and CD34+CD109+ cells coexpress FLT3, three-color phenotypic analyses are consistent with the functional findings and suggest that the most primitive cells defined as CD34+CD38-, CD34+CD71low, CD34+HLA-DR-, CD34+CD117low, CD34+CD90+, and CD34+CD109+ express low levels of cell-surface FLT3 and were therefore not enriched to a statistically significant extent with the bright versus negative sorting scheme. Thus, clear segregation of the most primitive progenitors from BM CD34+ cells was confounded by low apparent levels of FLT3 cell-surface expression on these cells, whereas myeloid progenitors unambiguously segregated with the FLT3 brightest cells and erythroid progenitors with the FLT3 dimmest. Additional phenotypic analyses using MoAbs against progenitor/stem cell markers including the mucinlike molecule MGC-24v (CD164), the receptor tyrosine kinases TIE, FMS (CD115), and KIT (CD117) further illustrate the differences in surface antigen expression profiles of BM and CB CD34+ cells. Notably, CD115 is rarely detected on CB CD34+ cells, whereas 20% to 25% of the BM CD34+FLT3+ cells are CD115+. Furthermore, 80% to 95% of the CB CD34+CD117+ but only 60% to 75% of the BM CD34+CD117+ cells coexpress FLT3. Only a negligible amount of CD34+CD19+ are detected in CB, while in BM 20% to 30% of CD34+CD19+ presumed pro/pre-B cells coexpress FLT3. In contrast, the majority of the CD34+CD164+ and CD34+TIE+ subsets in both CB and BM coexpress FLT3. Analysis of unseparated cells showed that FLT3 expression is not restricted to CD34+ subsets. About 65% to 70% of lymphocyte-gated BM CD34-FLT3+ cells are positive for the monocytic marker CD115 whereas 25% to 30% of these cells consist of CD10 expressing B-cell precursors. Finally, CD34- monocytes in BM, CB, and PB express FLT3 whereas granulocytes are FLT3-. Our data show that detectable FLT3 appears first at low levels on the surface of primitive multilineage progenitor cells and disappears during defined stages of B-cell development, but is upregulated and maintained during monocytic maturation.
[Show abstract][Hide abstract] ABSTRACT: We generated a panel of monoclonal antibodies against the extracellular domain of the Tie receptor tyrosine kinase and studied its expression in human haemopoietic and tumour cell lines and in samples from leukaemia patients. Most of the erythroblastic/megakaryoblastic (6/8), 2/7 myeloid and 3/6 B-lymphoblastic leukaemia cell lines were Tie-positive. The erythroblastic/megakaryoblastic leukaemia cell lines also expressed the related Tie-2/Tek gene and, surprisingly, its recently cloned ligand gene angiopoietin-1, which was located in chromosome 8q23.1. In addition, 16% of freshly isolated leukaemia samples were Tie positive. Peripheral blood mononuclear cells were Tie negative, but a few Tie positive cells were found in immunoperoxidase staining of mobilized peripheral blood stem cells. Long-term culture of isolated umbilical cord blood CD34+ Tie+ and CD34+ Tie- cells indicated that the Tie+ fraction contained a slightly higher frequency of cobblestone area forming cells (CAFC). Thus, Tie is expressed on haemopoietic progenitor cells and some leukaemic blasts. The coexpression of Tie-2 and angiopoietin-1 in megakaryoblastic leukaemia cell lines suggests the existence of an autocrine ligand/receptor signalling loop in these cells.
British Journal of Haematology 08/1997; 98(1):195-203. · 4.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We generated a panel of monoclonal antibodies against the extracellular domain of the Tie receptor tyrosine kinase and studied its expression in human haemopoietic and tumour cell lines and in samples from leukaemia patients. Most of the erythroblastic/megakaryoblastic (6/8), 2/7 myeloid and 3/6 B-lymphoblastic leukaemia cell lines were Tie-positive. The erythroblastic/megakaryoblastic leukaemia cell lines also expressed the related Tie-2/Tek gene and, surprisingly, its recently cloned ligand gene angiopoietin-1, which was located in chromosome 8q23.1. In addition, 16% of freshly isolated leukaemia samples were Tie positive. Peripheral blood mononuclear cells were Tie negative, but a few Tie positive cells were found in immunoperoxidase staining of mobilized peripheral blood stem cells. Long-term culture of isolated umbilical cord blood CD34+Tie+ and CD34+Tie−cells indicated that the Tie+ fraction contained a slightly higher frequency of cobblestone area forming cells (CAFC). Thus, Tie is expressed on haemopoietic progenitor cells and some leukaemic blasts. The coexpression of Tie-2 and angiopoietin-1 in megakaryoblastic leukaemia cell lines suggests the existence of an autocrine ligand/receptor signalling loop in these cells.
British Journal of Haematology 06/1997; 98(1):195 - 203. · 4.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Myc oncoprotein is associated with cell proliferation and is often down-regulated during cell differentiation. The related Mad transcription factor, which antagonises Myc activity, is highly expressed in epidermal keratinocytes. Mad also inhibits cell proliferation in vitro. To study Mad expression in keratinocyte proliferation and differentiation, we have analysed Mad RNA expression in regenerating and hyperproliferative epidermal lesions and epidermal tumours of varying degrees of differentiation using the RNA in situ hybridisation and RNAase protection techniques. Mad was strongly expressed in differentiating suprabasal keratinocytes in healing dermal wounds and in benign hyperproliferative conditions, but also in squamous cell carcinomas, in which the keratinocytes retain their differentiation potential. However, Mad expression was lost in palisading basal carcinoma cells and poorly differentiated squamous cell carcinomas, which lacked the epithelial differentiation marker syndecan-1. We therefore suggest that Mad expression is closely associated with epithelial cell differentiation, and that this association is retained in epithelial tumours of the skin.
British Journal of Cancer 07/1996; 73(11):1347-55. · 4.82 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Growth factor receptors in human hematopoietic progenitor cells have become the focus of intense interest, because they may provide tools for the monitoring, enrichment, and expansion of stem cells. We have shown earlier that the Tie receptor tyrosine kinase is expressed in erythroid and megakaryoblastic human leukemia cell lines, in the blood islands of the yolk sac, and in endothelial cells starting from day 8.0 of mouse development. Here, the expression of Tie was studied in human hematopoietic cells of various sources. Peripheral blood mononuclear cells were Tie-. However, a large fraction of CD34+ cells from umbilical cord blood (UCB) and bone marrow (BM) expressed tie protein and mRNA. On average, 64% of the fluorescence-activated cell sorting-gated UCB CD34+ cells including CD38- cells and a fraction of cells expressing low levels of c-Kit were Tie+. Also, 30% to 60% of BM CD34+ cells were Tie+, including most of the BM CD34+CD38-, CD34+Thy-1+, and CD34+HLA-DR- cells. Under culture conditions allowing myeloid, erythroid, and/or megakaryocytic differentiation, purified UCB CD34+ cells lost Tie mRNA and protein expression concomitantly with that of CD34; however, a significant fraction of cells expressed Tie during megakaryocytic differentiation. These data suggest that, in humans, the Tie receptor and presumably its ligand may function at an early stage of hematopoietic cell differentiation.