8536d_cho2024-056 8/5/02 4:02 PM Page 34 mac79 Mac 79: 45_BW: Go dsby et al./Immunology se CLINICAL FOCUS Stem Cells--Clinical Uses stem cells to generate cells and tissues that could be used to replace diseased or dam. and potential aged ones. Success in this endeavor would be a major advance because transplanta- nated organs and tissues. Unfortunately, Stem-cell es in the laboratory. Strikingly, these e need far exceeds the number of dona- transplanta- cells can be induced to generate many dif- tions and is increasing. Success in deriving tion holds great promise for the regener- ferent types of cells. Mouse ES cells have practical quantities of cells, tissues, and ation of diseased, damaged, or defective been shown to give rise to muscle cells, gans from pluripotent stem cells would pro- tissue Hematopoietic stem cells are al- nerve cells, liver cells, pancreatic cells, and, vide skin replacement for bum patients, ready used to restore hematopoietic of course, hematopoietic cells eart musde cells for those with chronic cells. and their use is described in the Recent advances have made it possible heart disease, pancreatic islet cells for pa- clinic below. However, rapid advances in to grow lines of human pluripotent cells. tients with diabetes, and neurons for use in stem-cell research have raised the possi- This is a development of considerable im- Parkinson 's disease or Alzheimer's disease. bility that other stem-cell types, too, may portance to the understanding of human The transplantation of hematopoietic on be routinely employed for replace- development, and it has great therapeutic stem cells(HSCs) is an important ther- ment of other cells and tissues. Two potential In vitro studies of the factors that apy for patients whose hematopoietic properties of stem cells underlie their determine or influence the development of systems must be replaced. It has three utility and promise. They have the capac. human pluripotent stem cells along one de- major application ity to give rise to more differentiated velopmental path as opposed to another cells, and they are self-renewing, because will provide considerable insight into the 1. Providing a functional immune each division of a stem cell creates at factors that affect the differentiation of cells system to individuals with a least one stem cell. If stem cells are clas- into specialized types. There is also great in- genetically determined sified according to their descent and de- terest in exploring the use of pluripoter immunodeficiency, such as severe velopmental potential, four levels of stem cells can be recognized: totipotent, pluripotent, multipotent, and unipotent. Totipotent cells can give rise to an en- organism. A fertilized egg, the zygo a totipotent cell. In humans the initial di- visions of the zygote and its descendants produce cells that are also totipotent. In fact, identical twins, each with its own pla centa, develop when totipotent cells sepa- Human pluripotent stem cells rate and develop into genetically identical fetuses. Pluripotent stem cells arise from totipotent cells and can give rise to most but not all of the cell types necessary for fe. tal 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 pluripo- tent stem cells leads to the formation of ltipotent and unipotent stem cells Multipotent stem cells can give rise to only Nerve cells Heart muscle cells Pancreatic islet cells a limited number of cell types, and unipo- Human pluripotent stem cells can differentiate into a variety of different cell types, tent cells to a single cell type. Pluripotent some of which are shown here. (Adapted from Stem Cells: A Primer, N/H web site cells,calledembryonicstemcellsorsimhttp://www.nih.gov/news/stemcell/primer.htm.Micrographs(leftorighty ply ES cells, can be isolated from early em- oto Associates/ Science Source/Photo Researchers; Biophoto Associates/Photo bryos, and for many years it has been hers; AFIP/Science Source/Photo Researchers; Astrid a Hanns-Frieder possible to grow mouse ES cells as cell r/ Science Photo Library/Photo Researchers. I
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: 1. Providing a functional immune system to individuals with a genetically determined immunodeficiency, such as severe 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 all of the 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 CLINICAL FOCUS Stem Cells—Clinical Uses and Potential Bone marrow Nerve cells Heart muscle cells Human pluripotent stem cells Pancreatic islet cells Human pluripotent stem cells can differentiate into a variety of different cell types, some of which are shown here. [Adapted from Stem Cells: A Primer, NIH web site http://www.nih.gov/news/stemcell/primer.htm. Micrographs (left to right): Biophoto Associates/Science Source/Photo Researchers; Biophoto Associates/Photo Researchers; AFIP/Science Source/Photo Researchers; Astrid & Hanns-Frieder Michler/Science Photo Library/Photo Researchers.] 34 PART I Introduction 8536d_ch02_024-056 8/5/02 4:02 PM Page 34 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e:
8536d_ch02024-056 8/7/02 8: 25 AM Page 35 mac79 Mac 79: 45_BW: Gotdsby et Immunology 5e: ans o mmune System CHAPTER 2 combined immunodeficiency sible for individuals to store their own donor does not have to undergo anesthesia hematopoietic cells for transplantation to and the subsequent highly invasive proce- 2. Replacing a defective hematopoieti themselves at a later time. Currently, this dure that extracts bone marrow. Many in the system with a functional one to cure procedure is used to allow cancer patients transplantation community believe that pe. some patients who have a life to donate cells before undergoing chemo- ripheral blood will replace marrow as the threatening nonmalignant genetic therapy and radiation treatments and then major source of hematopoietic stem cells disorder in hematopoiesis, such as reconstitute their hematopoietic system for many applications. To obtain HSC-en sickle-cell anemia or thalassemia from their own stem cells. Hematopoietic riched preparations from peripheral blood 3. Restoring the hematopoietic syster stem cells are found in cell populations that agents are used to induce increased display distinctive surface antigens. One of bers of circulating HSCs, and then the HSC. of cancer patients after treatmen these antigens is CD34, which is present on containing fraction is separated from the with doses of chemotherapeutic only a small percentage (-1%)of the cells plasma and red blood cells in a process agents and radiation so high that in adult bone marrow. An antibody specific called leukopheresis. If necessary, further they destroy the system. These for CD34 is used to select cells displaying purification can be done to remove T cells can this antigen, producing a population en- and to enrich the CD34 population more effective at killing tumor cells riched in CD34+ stem cells, various ver. Umbilical cord blood already contains a an are therapies that use more sions of this selection procedure have been significant number of hematopoietic stem conventional doses of cytotoxic used to enrich populations of stem cells cells. Furthermore, it is obtained from pla agents. Stem-cell transplantation from a variety of sources cental tissue( the"afterbirth")which is nor- makes it possible to recover from Transplantation of stem cell popula- mally discarded. Consequently, umbilical such drastic treatment. Also, certain tions may be autologous( the recipient is cord blood has become an attractive cancers such as some cases of also the donor), syngeneic(the donor is source of cells for HSC transplantation. Al- acute myeloid leukemia, can be genetically identical, i. e, an identical twin though HSCs from cord blood fail to en- cured only by destroying the source of the recipient), or allogeneic (the donor graft somewhat more often than do cells of the leukemia cells, the patient's and recipient are not genetically identical). from peripheral blood, grafts of cord blood own hematopoietic system In any transplantation procedure, genetic cells produce GVHD less frequently than Restoration of the hematopoietic sys. differences between donor and recipient do marrow grafts, probably because cord tem by transplanting stem cells is facili- can lead to immune-based rejection reac- blood has fewer mature T cells tated by several important technical tions. Aside from host rejection of trans- Beyond its current applications in can- considerations. First, HSCs have extraordi- planted tissue(host versus graft), cer treatment, many researchers feel that nary powers of regeneration. Experiments lymphocytes in the graft can attack the re- autologous stem-cell transplantation will in mice indicate that only a few-perhaps, cipient's tissues, thereby causing graf. be useful for gene therapy, the introduction on occasion, a single HSC--can com. versus- host disease (GVHD), a life. of a normal gene to correct a disorder pletely restore the erythroid population and threatening affiction. In order to suppress caused by a defective gene. Rapid ad e immune system. In humans it is neces- rejection reactions, powerful immunosup- vances in genetic engineering may soon sary to administer as little as 10% of a pressive drugs must be used. Unfortu- make gene therapy a realistic treatment for donor,s total volume of bone marrow to nately, these drugs have serious side genetic disorders of blood cells, and provide enough HSCs to completely re. effects, and immunosuppression in- hematopoietic stem cells are attractive store the hematopoietic system. Once in- creases the patient,'s risk of infection and hicles for such an approach. The therapy jected into a vein, HSCs enter the further growth of tumors. Consequently, would entail removing a sample of circulation and find their own way to the HSC transplantation has fewest complica. hematopoietic stem cells from a patient, bone marrow, where they begin the process tions when there is genetic identity be. inserting a functional gene to compensate of engraftment. There is no need for a sur- tween donor and recipient. for the defective one, and then reinjecting geon to directly inject the cells into bones At one time, bone-marrow transplanta- the engineered stem cells into the donor. In addition, HSCs can be preserved by tion was the only way to restore the The advantage of using stem cells in gene freezing. This means that hematopoietic hematopoietic system. However, the essen. therapy is that they are self renewing Con- cells can be"banked. "After collection, the tial element of bone-marrow transplanta. sequently, at least in theory, patients would cells are treated with a cryopreservative, tion is really stem-cell transplantation. have to receive only a single injection of en- frozen, and then stored for later use. When Fortunately, significant numbers of stem gineered stem cells. In contrast, gene ther- cells can be obtained from other tissue and infused into the patient, where it re- such as peripheral blood and umbilical-cord or other blood cells would require periodic constitutes the hematopoietic system. This blood (cord blood"). These alternative injections because these cells are not ca cell-freezing technology even makes it pos- sources of HSCs are attractive because the pable of self renewal
Cells and Organs of the Immune System CHAPTER 2 35 sible 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 graftversus-host disease (GVHD), a lifethreatening affliction. In order to suppress rejection reactions, powerful immunosuppressive drugs must be used. Unfortunately, these drugs have serious side effects, and immunosuppression 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. combined immunodeficiency (SCID). 2. 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. 3. 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 pos- 8536d_ch02_024-056 8/7/02 8:25 AM Page 35 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: