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KDR receptor: A key marker defining hematopoietic stem cells
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of sickle events. The authors of this study investigated whether patients assigned to a chronic transfusion protocol could be safely switched to treatment with hydroxyurea without transfusion. The study was an uncontrolled cohort study in which 16 patients with sickle cell disease (ages 3 to 19), a history of stroke, and prior chronic transfusions were enrolled. Each patient was selected because he or she had a clinical reason to consider stopping chronic transfusion, including alloimmunization, recurrent stroke wkile receiving chronic transfusion therapy, or high serum ferritin levels. Each patient had previously received transfusions for an average of 56 months. After providing consent, patients were treated with hydroxyurea at 15 rag&g/d, and the dose was increased by 5 mg/kg/d every 8 weeks up to a maximal dose of 30 mg/kg/d. In addition, patients with laboratory evidence of iron overload underwent periodic phlebotomy to remove excess iron. After a median follow-up of 22 months, 3 patients (19%) had recurrent stroke. These recurrences occurred at 13 weeks, 16 weeks, and 11 weeks, respectively, after starting hydroxynrea and discontinuing chronic transfusion therapy. By comparison, the background rate of recurrent stroke among patients treated with chronic transfusion therapy is 11%. This preliminary report suggests an alternative treatment regimen for sickle cell patients who have experienced stroke. Although the absence of a concurrent control group and the small sample size limit the confidence of the findings of this paper, a subset of patients with sickle disease and stroke are very difficult or impossible to manage on a chronic blood transfusion program. For such patients, a trial of hydroxyrea and phlebotomy m~iy be an option to consider. (S.D.)
KDR receptor: A key marker defining hematopoietic stem cells. B.L. Ziegler, M. Valtieri, G.A. Porada, st al. Science 285:1553-1558, 1999. This article will be of interest to everyone with even a passing interest in stem cells. For years now, the CD34 antigen has been the most reliable marker to identify early hematopoietic progenitor cells. Despite its practical usefulness, studies have suggested that the CD34 antigen likely identifies a population of cells that contains stem cells but does not itself identify stem cells. CD34 is likely to represent cells that have already committed to myeloid cell production rather than true stem cells capable of both myeloid and lymphoid long-term repopulation. Thus, the
CURRENT LITERATURE
hunt has been on for identification of the true stem cell. The authors of this article think that the hunt is now over and that they have found the true marker for stem cells. They identified the KDR receptor (also called the vascular endothelial growth factor receptor 2, VEGFR2) as their prime candidate. This work was done using mice and sheep transplanted with human cells. The investigators used a monoclonal antibody to KDR and flow cytometry to purify CD34+ KDR+ cells versus CD34+ K D R - cells. Using long-term in vitro cultures (Dexter cultnres), they first showed that hematopoietic progenitor activity declined at 8 weeks among CD34+ K D R - cells but increased among CD34+ KDR+ cells. This suggested that the selfrenewing capacity of stem cells was more potent in the K D R + fraction. Next, the investigators transplanted mice with severe combined immunodeficiency disease (SCID). SCID mice were selected because these animals can be successfully transplanted with human ceils. Animals received either CD34+ cells (50,000 to 250,000 per mouse); CD34+ KDR+ ceils (5,000 to 10,000 per mouse); or CD34+ K D R - (10,000 to 250,000 per mouse). These cells were obtained from either human bone marrow, peripheral blood, mobilized blood, or cord blood. After 3 months, the animals were killed and exanained for human cells. There was consistent engraftment (successful u'ansplantation) with K D R + cells but none with CD34+ K D R - cells. Of note, engraftment was documented not only for myleoid elements (granulocytes, red cells, and platelets) but also for lymphoid elements, including T and B cell lineages. Thus, the K D R + fraction was responsible for lymphohematopoietic engraftment. Amazingly, in dosage studies, it was found that as few as 5 K D R + cells could result in complete lymphohematopoietic reconstitution in the transplanted mice. The authors then preformed preliminary studies showing similar transplant results in a model in which fetal sheep receive human cells. Although a great deal more confirmatory work needs to be done, this report is potentiaily a landmark paper identifying the tong-sought-after human stem cell. If these initial results are confirmed, the K D R + cell will have great potential application in transplantation and gene therapy. Only about 1 in 1,000 peripheral blood mononuclear cells is a CD34+ cell. Only about 1 in 1,000 CD34+ cells is a K D R + cell. The authors may have indeed found a cell that is truly one in a million. (S.D.)
of sickle events. The authors of this study investigated whether patients assigned to a chronic transfusion protocol could be safely switched to treatment with hydroxyurea without transfusion. The study was an uncontrolled cohort study in which 16 patients with sickle cell disease (ages 3 to 19), a history of stroke, and prior chronic transfusions were enrolled. Each patient was selected because he or she had a clinical reason to consider stopping chronic transfusion, including alloimmunization, recurrent stroke wkile receiving chronic transfusion therapy, or high serum ferritin levels. Each patient had previously received transfusions for an average of 56 months. After providing consent, patients were treated with hydroxyurea at 15 rag&g/d, and the dose was increased by 5 mg/kg/d every 8 weeks up to a maximal dose of 30 mg/kg/d. In addition, patients with laboratory evidence of iron overload underwent periodic phlebotomy to remove excess iron. After a median follow-up of 22 months, 3 patients (19%) had recurrent stroke. These recurrences occurred at 13 weeks, 16 weeks, and 11 weeks, respectively, after starting hydroxynrea and discontinuing chronic transfusion therapy. By comparison, the background rate of recurrent stroke among patients treated with chronic transfusion therapy is 11%. This preliminary report suggests an alternative treatment regimen for sickle cell patients who have experienced stroke. Although the absence of a concurrent control group and the small sample size limit the confidence of the findings of this paper, a subset of patients with sickle disease and stroke are very difficult or impossible to manage on a chronic blood transfusion program. For such patients, a trial of hydroxyrea and phlebotomy m~iy be an option to consider. (S.D.)
KDR receptor: A key marker defining hematopoietic stem cells. B.L. Ziegler, M. Valtieri, G.A. Porada, st al. Science 285:1553-1558, 1999. This article will be of interest to everyone with even a passing interest in stem cells. For years now, the CD34 antigen has been the most reliable marker to identify early hematopoietic progenitor cells. Despite its practical usefulness, studies have suggested that the CD34 antigen likely identifies a population of cells that contains stem cells but does not itself identify stem cells. CD34 is likely to represent cells that have already committed to myeloid cell production rather than true stem cells capable of both myeloid and lymphoid long-term repopulation. Thus, the
CURRENT LITERATURE
hunt has been on for identification of the true stem cell. The authors of this article think that the hunt is now over and that they have found the true marker for stem cells. They identified the KDR receptor (also called the vascular endothelial growth factor receptor 2, VEGFR2) as their prime candidate. This work was done using mice and sheep transplanted with human cells. The investigators used a monoclonal antibody to KDR and flow cytometry to purify CD34+ KDR+ cells versus CD34+ K D R - cells. Using long-term in vitro cultures (Dexter cultnres), they first showed that hematopoietic progenitor activity declined at 8 weeks among CD34+ K D R - cells but increased among CD34+ KDR+ cells. This suggested that the selfrenewing capacity of stem cells was more potent in the K D R + fraction. Next, the investigators transplanted mice with severe combined immunodeficiency disease (SCID). SCID mice were selected because these animals can be successfully transplanted with human ceils. Animals received either CD34+ cells (50,000 to 250,000 per mouse); CD34+ KDR+ ceils (5,000 to 10,000 per mouse); or CD34+ K D R - (10,000 to 250,000 per mouse). These cells were obtained from either human bone marrow, peripheral blood, mobilized blood, or cord blood. After 3 months, the animals were killed and exanained for human cells. There was consistent engraftment (successful u'ansplantation) with K D R + cells but none with CD34+ K D R - cells. Of note, engraftment was documented not only for myleoid elements (granulocytes, red cells, and platelets) but also for lymphoid elements, including T and B cell lineages. Thus, the K D R + fraction was responsible for lymphohematopoietic engraftment. Amazingly, in dosage studies, it was found that as few as 5 K D R + cells could result in complete lymphohematopoietic reconstitution in the transplanted mice. The authors then preformed preliminary studies showing similar transplant results in a model in which fetal sheep receive human cells. Although a great deal more confirmatory work needs to be done, this report is potentiaily a landmark paper identifying the tong-sought-after human stem cell. If these initial results are confirmed, the K D R + cell will have great potential application in transplantation and gene therapy. Only about 1 in 1,000 peripheral blood mononuclear cells is a CD34+ cell. Only about 1 in 1,000 CD34+ cells is a K D R + cell. The authors may have indeed found a cell that is truly one in a million. (S.D.)