Stem Cells in Umbilical Cord Blood: A Review

Transcription

1 Stem Cells in Umbilical Cord Blood: A Review December 13, 2006 Cellular and Molecular Biotechnology of Mammalian Systems Undergraduate Students: Andrew Fu Allen Fung Graduate Student: Aizhan Tastanova

2 Introduction Stem cells are the precursor cells to all types of cells that exist in a human body. These special cells are capable of self-renewal, the ability to undergo cell division while maintaining an undifferentiated state, and they are also capable of differentiating into a number of cells. Stem cells are highly desired for possible therapeutic use as they potentially can serve as a source for replacing damaged cells. At the top of the hierarchy are the embryonic stem cells (ES). ES cells are found in the inner cell mass of a blastocyst and are capable of differentiation into all three germ layers: ectoderm, endoderm, and mesoderm. The capability of differentiating into these three germ layers is defined as being pluripotent. Because they are pluripotent, ES cells are very desirable for research into treating diseases and conditions such as cancer, Parkinson s, diabetes, heart damage, etc. Although ES cells are potentially very powerful in therapeutic benefit, the ethics of using ES cells are highly questioned and research into ES cell lines using public funds has been limited. These restrictions arise from the sourcing of ES cells. Harvesting these cells require the destruction of a human embryo and it has been argued that by doing so, it is equivalent to the act of murder or abortion. Most ES cells come from embryos that have been created by in vitro fertility treatments, but were ultimately not used for implantation. Because of these issues, other sources for stem cells are currently being examined. One such source for less controversial stem cells is from umbilical cord blood. Hematopoietic Stem Cells After a baby is born, umbilical cord blood (UCB) can be collected from the remaining placenta and remaining attached umbilical cord. 1 Cord blood contains the normal components of blood (red blood cells, white blood cells, etc), in addition to hematopoietic stem cells (HSCs). These HSCs are the progenitors of all cell types contained in the myeloid and lymphoid lineages. Contained in these lineages are the macrophages, platelets, red blood cells, T-cells, B-cells, and other types of blood cells. HSCs are thus classified as multipotent stem cells. Hematopoietic stem cells are a rare heterogeneous population of immature hematopoietic precursor cells that occur postnatally at a frequency of about 1 in 10,000 to 1 in 100,000 cells. As stem cells, HSCs are capable of performing self-renewal to maintain their cell line, and also are able to differentiate. The HSCs are multipotent and can differentiate into about 11 different types of lineages hematopoietic lineages. From the HSCs, the entire hematopoietic system is created and sustained throughout a person s lifetime. Current research is interested in UCB as it has higher content of the HSCs than bone marrow and mobilized peripheral blood, both common sources of HSC for transplantation. One difference between the HCSs in UCB and bone marrow is that UCB cells are slower cycling, whereas bone marrow HSCs have a higher proportion in G2, M and S phases. 4 Hematopoietic cells are descendents from a common hematopoietic and endothelial stem cell known as a hemangioblast. This cell appears during development as a result from mesodermal commitment. The hemangioblasts are first found in the extra-embryoinic yolk sac blood islands during the third week of gestation. One week later, they can then be found on the ventral floor of

3 the dorsal aorta and other arteries. From there, HSCs are able to migrate using the circulation system to colonize the liver, thymus, GI tract, and bone marrow. Once of adult age, HSC production is centered at the bone marrow. One popular use of umbilical cord blood is in HSC transplantation. 2 The first allogenic umbilical cord blood transplant (UCBT) was performed in 1988 to treat a patient suffering from anemia. UCBTs require that the donor be Human Leukocytic Antigen (HLA) compatible with the patient. HLAs are a set of proteins situated on the surface of leukocytes that allow for self/non-self recognization. As with typical transplantations, identical or similar HLA types are required for patient acceptance of the donor tissue. The success of the 1988 procedure altered the landscape of allogeneic hematopoietic stem cell transplants by showing success with umbilical cord blood. This pioneering procedure showed that a single unit of UCB contained enough HSCs to reconstitute lymphohematopoietic cells. Also, the feasibility of collection of UCB at birth (without harm to a newborn infant) and that the blood could then be successfully used after cryoprservation and transplantation, was proved. Stem cell transplantation has been successfully used as treatment in high-risk or recurrent hematologic malignancies, marrow failure syndromes, severe congenital immunodeficiency states, and some metabolic disorders. 3 Umbilical cord blood with its associated HSCs has recently become more popular in sourcing for stem cell transplant procedures. Data collected post transplantation has suggested that using peripheral blood stem cells have similar risks of rejection as when using bone marrow stem cells. One of the main problems of bone marrow transplantation (BMT) as a source for HSCs is that only 30-40% of potential recipients have HLA-matched family donors. 3 In efforts to increase the pool for allogeneic BMT, the National Marrow Donor Program was established in The program has been successful in increasing the likelihood of treatment with a BMT, but there are still limitations of the procedure, in terms of finding suitable donors. Transplantations are generally done allogeneically as an autologous graft would not solve any genetic disease of the cells in the patient, the newly transplanted cells could possibly contain the same genetic defect. In a case of leukemia, the patient s cells could also become malignant. However, allogeneic transplants carry the risk of rejection. HLA typing is used to have the best possible match between the patient and donor to optimize chance the success of the procedure. UCB HSCs have several advantages over other HSC sources (BMT). Firstly, for patients with more rare HLA types, UCB may be the only source of allogenic HSCs. Next, HCSs can be easily collected from autologous, or related and unrelated donors. The HLA types can then be determined and either immediately used or banked for later use. Furthermore, allogeneic HSC transplant recipients received HLA-mismatched allografts have been shown to tolerate a higher level of HLA disparities in UCB than in other types of grafts (bone marrow, etc). 4 Autologous UCB transplantation has been suggested as a possible method towards gene therapy. BMTs have been previously used to treat a number of genetic diseases such as sickle cell anemia, Fanconi anemia, severe combined immune deficiency secondary to adenosine deaminase

4 deficiency (ADA0, and other types of metabolic/storage deficiencies.. BMT has been suggested as a method to treat conditions that were secondary to a single genetic deletion by Parkman. BMTs are capable of gene transduction because the BM contains HSCs which can renew themselves and differentiate into cells that contain the transduced gene for the lifetime of the patient. As such, UCB transplantation can possible be a suitable method to perform the same task as a BMT would. 4 Cord Blood-Stem Cells In 2006, Zhao et al. identified a novel type of stem cell from human umbilical cord blood. Designated cord blood-stem cells (CB-SC), it has been demonstrated that these cells share properties with human embryonic stem (ES) cells and hematopoietic cells. These cells have low immunogenicity and differentiation capability. ES cells are defined by two unique properties: they can self renew (as with all stem cells) and are pluripotent. With these abilities, they are highly sought after for possible therapeutic benefit towards human diseases. However, ES cells are highly controversial in their use, and adult stem cells do not appear to be as useful. CB-SC cells as characterized by Zhao expressed Esc specific markers such as Oct-4 and Nanog. In vitro, CB-SCs have been differentiated to create cells with the characteristics of cells from the mesoderm, ectoderm and endoderm. However, unlike ES cells, it was found that after 3 months in culture, their proliferation was decreased. 5 One notable application of the CB-SCs is that they are capable of producing a therapeutic glycemic effect in a mouse model of diabetes. In both type 1 and type 2 diabetic patients, there is a deficit of insulin-producing cells. An endogenous insulin source is preferred for these patients as exogenous insulin does not provide the same level of glycemic control nor does it present itself as a long-term solution. Islet cell transplantation has been explored, but is limited by a shortage of pancreas as a source of purified islet. An alternative to such therapy, stem-cell derived insulin-producing cells can possibly be an approach for endogenous insulin. With the ethical issues behind ES cells, CB-SCs may show to be helpful in this field. 5 In type I diabetes, the pancreatic islet beta-cells are selectively destroyed by auto-reactive T lymphocytes. Stem cell therapy is potentially capable of replacing these beta cells, but the preexisting T cells are likely to also attack the new stem cell derived insulin-producing cells. In such a treatment for type I diabetes, it would be prudent to protect the new cells from immune attack. Work by Zhao et al. has shown that CB-SC can significantly inhibit both PHA and IL-2 stimulated lymphocyte proliferations. IL-2 is produced by T cells after they have been activated to stimulate the activation of T cells. Flow analysis has demonstrated that lymphocyte migrationassociated molecules are decreased in co-culture with CB-SC. The results suggest that CB-SC can potentially suppress inflammation caused by lymphocyte aggregations. Additionally, there is evidence that CB-SC has an ability to promote self-limitation of activated T cells by up regulation of CD69 expression. 6 CD8+ T cells are capable of inducing the death of cells that are infected with viruses or other pathogens and cells that are damaged. There has been evidence that these cells also contribute to the development of several autoimmune diseases, including type 1 diabetes and multiple sclerosis. Therefore, when transplanting new beta cells into a type 1 diabetes patient, it would be

5 beneficial to suppress the CD8+ cells to improve the success of such transplantation. With coculture of CD8+ cells and CB-SC, studies have shown that the percentage of CD8+ T cells decreases. In addition, proliferation of activated CD8+ T cells is inhibited. These results could possibly be used in the future to assist in the treatment of type 1 diabetes. 6 Meesenchymal Stem Cells In addition to HSC and CB-SC, mesenchymal stem cells (MSC) can be derived from human umbilical cord blood. Like many types of stem cells, MSCs possess the dual capacity of selfrenewal and multilineage differentiation. Their versatile differentiation potential ranges from mesenchyme-related multipostency to neuroectodermal and endodermal competency. Such versatility makes MSCs a promising alternative to embryonic stem cells (ESCs) in the stem cellbased therapy and transplantation. One of the most studied subjects of stem cell therapy is the liver. Researchers hope to replace the diseased hepatocytes with stem cell-derived healthy cells. While the hematopoietic component of UCB has been proven to be useful in the treatment of liver disorders, the mesenchymal component has remained elusive till recently. Hong et al. showed in their 2005 paper that UCBderived MSCs are also capable of differentiating in vitro along a hepatic linearge. 7 Due to UCB s enormous advantage over other sources of stem cells in cell procurement, storage, and transplantation, USB-derived MSCs are likely to become the cell source of choice for stem cell therapy of the liver. MSCs can be isolated from the Ficoll layer of umbilical cord blood. They form an adherent population that allows their separation from haematopoietic progenitor cells by differential adherence selection. 8 MSCs from UCB are CD45-, CD34-, and do not express the endothelial marker, CD31. These properties match that of MSCs from bone marrow and in first trimester fetal-blood. Hong s group employed several different methods to confirm that MSCs are able to undergo hepatic differentiation. First, UCB-derived MSCs underwent morphological changes under prohepatogenic conditions as early as the third day. Second, RT-PCR analysis reveals temporal expression pattern of hepatocyte-specific genes such as, albumin, AFP, CK-18, GS, TAT, HGF, c-met, PEPCK, and CPS. Third, WB and IF analyses were performed to examine the protein expression of albumin, AFP, CK-18, and Ck-19 to confirm hepatic differentiation. Finally, to ensure that the proteins assembled are actually functional, Hong s team analyzed the LDL update and confirmed the functionality of the proteins. There are, however, some limitations to UCB-derived MSCs that has hindered their current use in scientific research. The major problem is that MSCs yield from UCB is much lower than that from the bone marrow even lower than that of first trimester blood. However, as researchers develop more effective and efficient methods to proliferate MSCs, MSCs are expected to be more useful for stem cell therapy. Current Challenges One of the major challenges in the study of HSCs is the difficulty of identification. The reason is that the molecular phenotypes of HSC can change depending on the activation status of the

6 precursor cell population and on the source of the cells. As a result, the best approach is to use a combination of markers. The best defined surface marker is CD34, which is expressed on both primitive and mature haemopoietic precursors. 9 Like many other markers, CD34 is not HSCs specific. Hence, other markers are used in addition to CD34: CD133+, CD38-, HLA-DR+. Recent studies have also shown that HSCs express aldehyde dehydrogenase. 10 Another challenge is to increase umbilical cord blood HSC numbers without reducing their "stemness" characteristic, or to promote HSC differentiation and use the differentiated cells for haematological support during transplantation. This is a critical matter since the low number of HSCs have been one of the major hindrances in advancing UCB research. The lack of reliable protocols to identify HSCs, as mentioned above, has been hindering researchers to find ways to proliferate HSCs. Recently, much attention has been drawn to the use of available cytokine cocktails that promote HSC survival and proliferation. Many researchers have shown that the read-out assays include CD133 and CD34 phenotypes, LTC-IC and CAFC assays in vitro, and functional engraftment assays in vivo. The cytokines that have proven to be effective include FLT3 ligand, Thrombopoietin (TPO), Stem Cell Factor (SCF), IL-II, and hyper-il-6. Large expansions of the CD133 and CD34 cells were observed. In contrast, there are lesser expansions of LTC-IC or CAFC cell subsets and either negligible or up to 4-5-fold expansions of cells that repopulate immunodeficient mice. However, it is uncertain whether these expansion protocols have negative effects on HSC self-renewal, or whether they affect the homing receptors for HSC engraftment. An alternative method to promote HSC cell cycling is to use drug discovery programmes to produce soluble transcription factors or transcription factor peptide mimetics. Such important transcription factors include NF-Y, which regulates genes such as telomerase, LEF-1 in the Wnt pathway and homeobox (HOX) genes, as well as the Wnt/LEFT-1, Bmi-1 and HOXB4 molecules. Researchers have also discovered that soluble Tat-HOXB4, when added to HSCs for a short period of time can increase murine HSC numbers by fold. Such research, however, must be practiced by extreme care. Before clinical use, it is critical to ensure that such proliferation methods do not promote cellular abnormalities or diminish HSC lifespans. Ethical Issues There are several important ethical issues involved with the collection of umbilical cord blood. First, the donor of umbilical cord blood is always a minor. Consent must be obtained from the infant's mother. One question is the timing of the consent. Consent cannot be obtained from the mother in active labor or immediately after delivery. Other questions include whether consent should also be obtained by infant's father. The situation becomes more complex when the infant turns 21 years of age. What rights do the donor have to his or her umbilical cord blood that was given to an umbilical cord blood bank? What if there is a surrogate mother. Such issues are complex matters. With the increasing amounts of umbilical cord blood collection and transplantation, many of such ethical issues need to addressed urgently. Umbilical cord blood could, alternatively, be considered as a discarded issue. This is not an illogical consideration since collection of umbilical cord blood poses no risk to the mother or the infant. In this case, no consent will be needed for the collection of umbilical cord blood.

7 However, this does not necessarily solve all the ethical dilemmas. What if a genetic disease, such as sickle cell disease, thalassemia, G6PD deficiency, hereditary spherocytosis, is detected upon screening? Many questions remain to be answered. Ethical issues are not limited to consent and disease screening. Other issues include the potential of deliberate conception and embryo cloning for the purpose of an HLA-identical tissue donor and the commercialization of umbilical cord blood collection. Conclusion Umbilical cord blood stem cells have been demonstrated through recent studies to be an alternative to stem cells from the bone marrow. UCB stem cells are versatile. Three major types have been identified: HSC, CB-SC, and MSC. These stem cells may give rise to a variety of cells including, macrophages, platelets, red blood cells, T-cells, B-cells, mesenchyme-related cells, and neuroectodermal and endodermal cells. The future of UBC research should be focused in the following areas. First, researcher must develop optimal methods for harvesting, processing, and storing umbilical cord blood units. Second, efforts should be made to minimize the number of banked umbilical cord blood units necessary to support the needs of recipients who lack matched family member donors. Third and finally, ethical and regulatory issues of collecting and storing umbilical cord blood units must be addressed. Medical ethicists and various members of the medical community need to reach a consensus opinion to guide clinicians and storage services in their practices. Clinical trials have suggested that umbilical cord blood from a single donor could serve as a source of autologous or major histocompatibility complex-matched allogeneic transplantable hematopoeitic repopulating cells. Many organized programs for the banking of unrealated donor umbilical cord blood are underway worldwide. The future is bright and the effects will be beneficial for millions the search time of donor stem cells will be greatly reduced; risks associated with unrelated donor BMT will be also reduced; and finally, such organized UCB banks will hopefully provide a remedy for the shortage of donors from ethnic and racial backgrounds that are underrepresented currently.

8 References Gluckman, E., Current status of umbilical cord blood hematopoietic stem cell transplantation. Experimental Hematology, 28 (2000), Cairo, MS., Wagner, JE., Placental and/or Umbilical Cord Blood: An Alternative Source of Hematopoietic Stem Cells for Transplantation. Blood, 90:12 (1997), Watt, SM., Conteras, M., Stem cell medicine: Umbilical cord blood and its stem cell potential. Seminars in Fetal & Neonatal Medicine, 10 (2005) Zhao, Y., Wang, H., Mazzone, T., Identification of stem cells from human umbilical cord blood with embryonic and hematopoietic characteristics. Experimental Cell Research (2006), Zhao Y, et al., Immune regulation of T lymphocyte by a newly characterized human umbilical cord blood stem cell. Immunol Lett (2006), doi: /j.imlet Hong SH, et al., In vitro differentiation of human umbilical cord blood-derived mesenchymal stem cells into hepatocyte-like cells. Biochemical and Biophysical Research Communications 330 (2005) Campagnoli C, Roberts IA, Kumar S et al. Identification of messenchymal stem progenitor cells in human first-trimester fetal blood, liver, and bone marrow. Blood 2001; 98(8): Martin-Rendon E, Watt SM. Exploitation of stem cell plasticity. Transfus Med 2003; 13: Hess DA, Meyerrose TE, Wirthlin L, Craft TP, Herrbich PE, Creer MH, et al. Functional characterization of highly purified human hematopoietic repopulating cells isolated according to aldehyde dehydrogenase activity. Blood 2004; 104: