Stem-cell transplantation holds great promise for the regeneration of diseased, damaged, or defective tissue. Hematopoietic stem cells are already used to restore hematopoietic cells, and their use is described in the clinic below. However, rapid advances in stem-cell research have raised the possibility that other stem-cell types, too, may soon be routinely employed for replacement of other cells and tissues. Two properties of stem cells underlie their utility and promise. They have the capacity to give rise to more differentiated cells, and they are self-renewing, because each division of a stem cell creates at least one stem cell. If stem cells are classified according to their descent and developmental potential, four levels of stem cells can be recognized: totipotent, pluripotent, multipotent, and unipotent.
Totipotent cells can give rise to an entire organism. A fertilized egg, the zygote, is a totipotent cell. In humans the initial divisions of the zygote and its descendants produce cells that are also totipotent. In fact, identical twins, each with its own placenta, develop when totipotent cells separate and develop into genetically identical fetuses. Pluripotent stem cells arise from totipotent cells and can give rise to most but not the entire cell types necessary for fetal development. For example, human pluripotent stem cells can give rise to all of the cells of the body but cannot generate a placenta. Further differentiation of pluripotent stem cells leads to the formation of multipotent and unipotent stem cells.
Multipotent stem cells can give rise to only a limited number of cell types, and unipotent cells to a single cell type. Pluripotent cells, called embryonic stem cells, or simply ES cells, can be isolated from early embryos, and for many years it has been possible to grow mouse ES cells as cell lines in the laboratory. Strikingly, these ES cells can be induced to generate many different types of cells. Mouse ES cells have been shown to give rise to muscle cells, nerve cells, liver cells, pancreatic cells, and, of course, hematopoietic cells.
Recent advances have made it possible to grow lines of human pluripotent cells. This is a development of considerable importance to the understanding of human development, and it has great therapeutic potential. In vitro studies of the factors that determine or influence the development of human pluripotent stem cells along one developmental path as opposed to another will provide considerable insight into the factors that affect the differentiation of cells into specialized types. There is also great interest in exploring the use of pluripotent stem cells to generate cells and tissues that could be used to replace diseased or damaged ones. Success in this endeavor would be a major advance because transplantation medicine now depends totally upon donated organs and tissues. Unfortunately, the need far exceeds the number of donations and is increasing. Success in deriving practical quantities of cells, tissues, and organs from pluripotent stem cells would provide skin replacement for burn patients, heart muscle cells for those with chronic heart disease, pancreatic islet cells for patients with diabetes, and neurons for use in Parkinson’s disease or Alzheimer’s disease.
The transplantation of hematopoietic stem cells (HSCs) is an important therapy for patients whose hematopoietic systems must be replaced. It has three major applications:
- Providing a functional immune system to individuals with a genetically determined immunodeficiency, such as severe combined immunodeficiency (SCID).
- Replacing a defective hematopoietic system with a functional one to cure some patients who have a lifethreatening nonmalignant genetic disorder in hematopoiesis, such as sickle-cell anemia or thalassemia.
- Restoring the hematopoietic system of cancer patients after treatment with doses of chemotherapeutic agents and radiation so high that they destroy the system. These high-dose regimens can be much more effective at killing tumor cells than are therapies that use more conventional doses of cytotoxic agents. Stem-cell transplantation makes it possible to recover from such drastic treatment. Also, certain cancers, such as some cases of acute myeloid leukemia, can be cured only by destroying the source of the leukemia cells, the patient’s own hematopoietic system.
Restoration of the hematopoietic system by transplanting stem cells is facilitated by several important technical considerations. First, HSCs have extraordinary powers of regeneration. Experiments in mice indicate that only a few—perhaps, on occasion, a single HSC—can completely restore the erythroid population and the immune system. In humans it is necessary to administer as little as 10% of a donor’s total volume of bone marrow to provide enough HSCs to completely restore the hematopoietic system. Once injected into a vein, HSCs enter the circulation and find their own way to the bone marrow, where they begin the process of engraftment. There is no need for a surgeon to directly inject the cells into bones. In addition, HSCs can be preserved by freezing. This means that hematopoietic cells can be “banked.” After collection, the cells are treated with a cryopreservative, frozen, and then stored for later use. When needed, the frozen preparation is thawed and infused into the patient, where it reconstitutes the hematopoietic system. This cell-freezing technology even makes it possible for individuals to store their own hematopoietic cells for transplantation to themselves at a later time. Currently, this procedure is used to allow cancer patients to donate cells before undergoing chemotherapy and radiation treatments and then to reconstitute their hematopoietic system from their own stem cells. Hematopoietic stem cells are found in cell populations that display distinctive surface antigens. One of these antigens is CD34, which is present on only a small percentage (~1%) of the cells in adult bone marrow. An antibody specific for CD34 is used to select cells displaying this antigen, producing a population enriched in CD34_ stem cells. Various versions of this selection procedure have been used to enrich populations of stem cells from a variety of sources.
Transplantation of stem cell populations may be autologous (the recipient is also the donor), syngeneic (the donor is genetically identical, i.e., an identical twin of the recipient), or allogeneic (the donor and recipient are not genetically identical). In any transplantation procedure, genetic differences between donor and recipient can lead to immune-based rejection reactions. Aside from host rejection of transplanted tissue (host versus graft), lymphocytes in the graft can attack the recipient’s tissues, thereby causing graft versus- host disease (GVHD), a life threatening affliction. In order to suppress rejection reactions, powerful immuno-suppressive drugs must be used. Unfortunately, these drugs have serious side effects, and immuno-suppression increases the patient’s risk of infection and further growth of tumors. Consequently, HSC transplantation has fewest complications when there is genetic identity between donor and recipient.
At one time, bone-marrow transplantation was the only way to restore the hematopoietic system. However, the essential element of bone-marrow transplantation is really stem-cell transplantation. Fortunately, significant numbers of stem cells can be obtained from other tissues, such as peripheral blood and umbilical-cord blood (“cord blood”). These alternative sources of HSCs are attractive because the donor does not have to undergo anesthesia and the subsequent highly invasive procedure that extracts bone marrow. Many in the transplantation community believe that peripheral blood will replace marrow as the major source of hematopoietic stem cells for many applications. To obtain HSC-enriched preparations from peripheral blood, agents are used to induce increased numbers of circulating HSCs, and then the HSCcontaining fraction is separated from the plasma and red blood cells in a process called leukopheresis. If necessary, further purification can be done to remove T cells and to enrich the CD34_ population.
Umbilical cord blood already contains a significant number of hematopoietic stem cells. Furthermore, it is obtained from placental tissue (the “afterbirth”) which is normally discarded. Consequently, umbilical cord blood has become an attractive source of cells for HSC transplantation. Although HSCs from cord blood fail to engraft somewhat more often than do cells from peripheral blood, grafts of cord blood cells produce GVHD less frequently than do marrow grafts, probably because cord blood has fewer mature T cells.
Beyond its current applications in cancer treatment, many researchers feel that autologous stem-cell transplantation will be useful for gene therapy; the introduction of a normal gene to correct a disorder caused by a defective gene. Rapid advances in genetic engineering may soon make gene therapy a realistic treatment for genetic disorders of blood cells, and hematopoietic stem cells are attractive vehicles for such an approach. The therapy would entail removing a sample of hematopoietic stem cells from a patient, inserting a functional gene to compensate for the defective one, and then reinjecting the engineered stem cells into the donor.
The advantage of using stem cells in gene therapy is that they are self renewing. Consequently, at least in theory, patients would have to receive only a single injection of engineered stem cells. In contrast, gene therapy with engineered mature lymphocytes or other blood cells would require periodic injections because these cells are not capable of self renewal.
(Adapted from Clinical Focus of Kuby’s Textbook of Immunology)