V. Human Stem Cells and the Treatment of Disease

A major goal of stem cell research is to provide healthy differentiated cells that, once transplanted, could repair or replace a patient’s diseased or destroyed tissues. In pursuit of this goal, one likely approach would start by isolating stem cells that could be expanded substantially in vitro. A large number of the cultivated stem cells could then be stored in the frozen state, extensively tested for safety and efficacy as outlined above, and used as reproducible starting material from which to prepare differentiated cell preparations that will express the needed beneficial properties when they are transplanted into patients with specific diseases or deficiencies.To make more concrete both the potential of this approach and the obstacles it faces, we will summarize, as a case study example, some current information on the properties of cells derived from human stem cell populations that have been used in an animal model of Type-1 diabetes. But before doing so, we discuss an obstacle to any successful program of stem cell-based transplantation therapy: the problem of immune rejection of the transplanted cells.
A. Will Stem Cell-Based Therapies Be Limited by Immune Rejection?
Much of the impetus for human stem cell research comes from the hope that stem cells (or, more likely, differentiated cells derived from them) will one day prove useful in cell transplantation therapies for a variety of human diseases. Such cell transplantation would augment the current practice of whole organ transplantation. To the extent that the healing process works with in vitro derived cells, the need for organ donors and long waiting lists for organ donation might be reduced or even eliminated.Will the recipient (patient) accept or reject the transplanted human cells? In principle, the problem might seem avoidable altogether: adult stem cells could be obtained from each individual patient needing treatment. They could then be grown or modified to produce the desired (autologous and hence rejection-proof) transplantable cells. But the logistical difficulties in processing separate and unique materials for each patient suggest that this approach may not be practical. The cost and time required to produce sufficient numbers of well-characterized cells suitable for therapy suggest that it will be cells derived from one or another unique stem cell line that will be used to treat many (genetically different) individual patients (allogeneic cell transplantation).When allogeneic organ or tissue transplantation is currently done using, for example, bone marrow, kidney, or heart, powerful immunosuppressive drugs—carrying undesirable side effects—must be used to prevent immunological rejection of the transplanted tissue.5 Without such immunosuppression, the patient’s T-lymphocytes and natural killer (NK) cells recognize surface molecules on the transplanted cells as “foreign” and attack and destroy the cells. Also, in whole organ transplantation, donor T-lymphocytes and NK cells, entering the recipient with the transplanted organ, can also destroy the tissues of the transplant recipient (called “graft versus host” disease).Are the differentiated derivatives of human stem cells as likely to incite immune rejection, when transplanted, as are solid organs? Do their surfaces carry those protein antigens that will be recognized as “foreign”? Experiments have been done to examine human ESC and MSC preparations growing in vitro for the expression of surface molecules known to play important roles in the immune rejection process. Drukker and coworkers39 showed that embryonic stem cells in vitro express very low levels of the immunologically crucial major histocompatibility complex class I (MHC-I) proteins on their cell surface. The presence of MHC-I proteins increased moderately when the ESCs became differentiated, whether in vitro or in vivo. A more pronounced increase in MHC-I antigen expression was observed when the ESCs were exposed to gamma-interferon, a protein produced in the body during immune reactions. Thus, under some circumstances, human ESC-derived cells can express cell surface molecules that could lead to immune rejection upon allogeneic transplantation.Similarly, Majumdar and colleagues showed that human mesenchymal stem cells in vitro express multiple proteins on their cell surfaces that would enable them to bind to, and interact with, T-lymphocytes. They also observed that gamma-interferon increased expression of both human leukocyte antigen (HLA) class I and class II molecules on the surface of these MSCs.40 These results indicate that it will probably not be possible to predict, solely on the basis of in vitro experiments, the likelihood that transplanted allogeneic MSCs would trigger immune rejection processes in vivo.Many further studies in this area are badly needed. At this time there is insufficient information to determine which, if any, of the approaches to get around the rejection problem will eventually prove successful.
B. Case Study: Stem Cells in the Future Treatment of Type-1 Diabetes?
1. The Disease and Its Causes.
The human body converts the sugar glucose into cell energy for heart and brain functioning, and indeed, for all bodily and mental activities. Glucose is derived from dietary carbohydrates, is stored as glycogen in the liver, and is released again when needed into the bloodstream. A protein hormone called insulin, produced by the beta cells in the islets of the pancreas, facilitates the entrance of glucose from the bloodstream into the cells, where it is then metabolized. Insulin is critical for regulating the body’s use of glucose and the glucose concentration in the circulating blood.The body’s failure to produce sufficient amounts of insulin results in diabetes, an extremely common metabolic disease affecting over 10 million Americans, often with widespread and devastating consequences. In some five to ten percent of cases, known as Type-1 diabetes (or “juvenile diabetes”), the disease is caused by “autoimmunity,” a process in which the body’s immune system attacks “self.”xiv T-lymphocytes attack the patient’s own insulin-producing beta cells in the pancreas. Eventually, this results in destruction of ninety percent or so of the beta cells, resulting in the diabetic state.With a deficiency or absence of insulin, the blood glucose becomes elevated and may lead to diabetic coma, a fatal condition if untreated. Chronic diabetes, both Type-1 and the much more common Type-2 diabetes (which is not autoimmune, but largely genetic), causes late complications in the retina, kidneys, nerves, and blood vessels. It is the leading cause of blindness, kidney failure, and amputations in the U.S. and a major cause of strokes and heart attacks.Type-1 diabetes is a devastating, lifelong condition that currently affects an estimated 550,000-1,100,000 Americans,41 including many children. It imposes a significant burden on the U.S. healthcare system and the economy as a whole, over and above the disabilities and impairments borne by individual sufferers. Recent estimates suggest that treatment of all forms of diabetes costs Americans a total of $132 billion per year.42 At 5-10 percent of all diabetes cases, the costs of Type-1 diabetes can be estimated as $6.5-$13 billion per year.
2. Current Therapy Choices and Outcomes.
The current treatment of Type-1 diabetes consists of insulin injections, given several times a day in response to repeatedly measured blood glucose levels. Although this treatment is life-prolonging, the procedures are painful and burdensome, and in many cases they do not adequately control blood glucose concentrations. Whole pancreas transplants can essentially cure Type-1 diabetes, but fewer than 2,000 donor pancreases become available for transplantation in the U.S. each year, and they are primarily used to treat patients who also need a kidney transplant. Like all recipients of donated organs, pancreas transplant recipients must continuously take powerful drugs to suppress the immunological rejection of the transplanted pancreas.In addition to treatment with whole pancreas transplantation, small numbers of Type-1 diabetes patients have been treated by transplantation of donor pancreatic islets into the liver of the patient coupled with a less intensive immunosuppressive treatment (the Edmonton protocol).43 Expanded clinical trials of this procedure are currently underway. Scientists are also evaluating methods of slowing the original autoimmune destruction of pancreatic beta cells that produces the disease in the first place.Whole pancreas and islet cell transplants ameliorate Type-1 diabetes, but there is nowhere near enough of these materials to treat all in need. To overcome this shortage, people hope that human stem cells can be induced—at will and in bulk—to differentiate in vitro into functional pancreatic beta cells, available for transplantation. Of course, it would still be crucial to prevent immunological destruction of the newly transplanted stem cell-derived beta cells.
indicate, cells derived from some human stem cells transplanted into specific strains of mice mimicking major aspects of Type-1 human diabetes51 were able to reverse high blood glucose concentrations. Although these results are encouraging, the transplant rejection question remains unanswered because the likely immune rejection of the transplanted human cells was prevented in these experiments by using special strains of immunodeficient mice that lack the capacity to recognize and attack foreign cells.No tumors were observed in the transplanted mice, but the experiments were terminated after about three months, an insufficient time for much tumor development to occur. Because many Type-1 diabetes patients are children and because a largely effective therapy (insulin injection) is currently available, the introduction of islet cell transplant therapy will need a high degree of certainty that the introduced cells or their derivatives will not become malignant over the course of the patient’s life. Stringent tests of the cancer-causing potential of candidate cell preparations will be required, including multi-year studies in animals that live longer than mice or rats. Long-term follow-up of children and adult patients who had received bone marrow transplants many years ago has revealed an increased risk of severe neurologic complications52 and a variety of types of cancer.53
C. Therapeutic Applications of Mesenchymal Stem Cells (MSCs)
Before stem cell based therapies are used to treat human diseases, they will have to gain approval through the Food and Drug Administration (FDA) regulatory process. The first step in this process is filing an Investigational New Drug (IND) application. As of July 2003, four IND applications have been filed for clinical applications of mesenchymal stem cells. The disease indications include: (1) providing MSC support for peripheral blood stem cell transplantation in cancer treatment, (2) providing MSC support for cord blood transplantation in cancer treatment, (3) using MSCs to stimulate regeneration of cardiac tissue after acute myocardial infarction (heart attack), and (4) using MSCs to stimulate regeneration of cardiac tissue in cases of congestive heart failure. The first two applications are currently in Phase II of the regulatory process, with pivotal Phase III trials scheduled to begin in 2004.54
D. Evaluating the Different Types of Stem Cells
A major unresolved issue at present involves the therapeutic potential of human adult stem cells compared with embryonic stem cells. The answer may well be different for different diseases and for patients of different ages. For example, in treating an elderly patient with Parkinson’s Disease, the use of adult stem cells may be appropriate even if these cells may have a more limited number of cell divisions remaining. On the other hand, treating a child with Type I Diabetes, one may want to use embryonic stem cells because of their potentially greater longevity, or other factors. The only valid way to resolve these questions is by instituting rigorous therapeutic trials which test the efficacy of the different types of stem cells in treating a variety of different diseases to determine their comparative efficacy. Clearly, such trials would be a long-term endeavor, since it would take years to obtain answers to these very critical questions.
VI. Private Sector Activity
In the United States, much of the basic research on animal stem cells and human adult stem cells has been publicly funded. Yet before 2001, research in the U.S., using human ESCs could only be done in the private sector (the locus also of much research on animal and human adult stem cells). The current state of knowledge about human ESCs (and also about human MSCs) reflects pioneering and on-going stem cell research funded by the private sector in the U.S.54,55 For example, the work that led to the 1998 reports of the first isolation of both ESCs and EGCs, was funded by Geron Corporation. Embryonic and adult stem cell research is today vigorously pursued by many companies and supported by several private philanthropic foundations,56 and the results of some of this research have been published in peer-reviewed journals.57 Private sector organizations have pursued and been awarded patents on the stem cells themselves and methods for producing and using them to treat disease. As noted above, at least one company (Osiris Therapeutics) has protocols under review at the FDA for clinical trials with MSCs. It seems likely that private sector companies will continue to play large roles in the future development of stem cell based therapies.
VII. Preliminary Conclusions
While it might be argued that it is too soon to attempt to draw any conclusions about the state of a field that is changing as rapidly as stem cell research, we draw the following preliminary conclusions regarding the current state of the field.Human stem cells can be reproducibly isolated from a variety of embryonic, fetal, and adult tissue sources. Some human stem cell preparations (for example, human ESCs, EGCs, MSCs, and MAPCs) can be reproducibly expanded to substantially larger cell numbers in vitro, the cells can be stored frozen and recovered, and they can be characterized and compared by a variety of techniques. These cells are receiving a large share of the attention regarding possible future (non-hematopoietic) stem cell transplantation therapies.Preparations of ESCs, EGCs, MSCs, and MAPCs can be induced to differentiate in vitro into a variety of cells with properties similar to those found in differentiated tissues.Research using these human stem cell preparations holds promise for: (a) increased understanding of the basic molecular process underlying cell differentiation, (b) increased understanding of the early stages of genetic diseases (and possibly cancer), and (c) future cell transplantation therapies for human diseases.The case study of developing stem cell-based therapies for Type-1 diabetes illustrates that, although insulin-producing cells have been derived from human stem cell preparations, we could still have a long way to go before stem cell-based therapies can be developed and made available for this disease. This appears to be true irrespective of whether one starts from human embryonic stem cells or from human adult stem cells. The transplant rejection problem remains a major obstacle, but only one among many.Human mesenchymal stem cells are currently being evaluated in pre-clinical studies and clinical trials for several specific human diseases.Much basic and applied research remains to be done if human stem cells are to achieve their promise in regenerative medicine.58 This research is expensive and technically challenging, and requires scientists willing to take a long perspective in order to discover, through painstaking research, which combinations of techniques could turn out to be successful. Strong financial support, public and private, will be indispensable to achieving success.