All blood cells arise from a type of cell called Hematopoietic stem cell. Stem cells have received a great deal of publicity as lately, due both to recent achievements in their isolation and cultivation and their potential widespread therapeutic uses. A simple definition of stem cells is that they have the capacity both to self-renew and to generate differentiated progeny. This means that they can generate undifferentiated daughter cells are committed along a certain developmental pathway. In this presentation, we would like to communicate about the different clinical uses of Hematopoietic stem cell transplantation in various disease conditions like SCID, Sickle cell anemia and other cancers.
Introduction:
Stem cells have two important characteristics that distinguish them from other types of cells. First, stem cells are unspecialized cells that are capable of self-renewal. Second, under certain physiologic or experimental conditions, stem cells can differentiate into cells with special functions. Scientists primarily work with two kinds of stem cells: embryonic stem cells and adult stem cells that each have different functions and characteristics [4-6]. Embryonic stem cells are a type of Pluripotent stem cell derived from the inner cell mass of the blastocyst. They are primitive and undifferentiated cells that have the potential to become a wide variety of specialized cell types. In some adult tissues, such as bone marrow, muscle, and brain, discrete populations of adult stem cells generate replacements for cells that are normally lost or damaged by injury or disease. Pluristem has decided to work with adult stem cells, focusing on the expansion of Pluripotent Hematopoietic Stem Cells (HSCs). Bone marrow is the spongy tissue found in the cavities of the body’s bones that contains special stem cells, called hematopoietic stem cells (HSC). Each type of blood cell begins its life as a hematopoietic stem cell. The stem cells divide and differentiate to form the various cells that are later found in our blood and immune systems. These cells include: leukocytes, lymphocytes, erythrocytes and Platelets that are responsible for blood clotting.
HSCs have a greater self-renewal and differentiation capacity than any other adult tissue and hold the promise of being able to repair or replace damaged cells and tissues. A cardinal property of hematopoietic stem cells is their ability to divide without significant alteration of their proliferative potential or differentiation state. The first demonstration that such stem cell self-renewal divisions can occur in vitro (albeit without a net increase in their numbers) made use of retroviral marking to identify the clonal progeny in multiple recipients of individual hematopoietic stem cells that had been amplified in 4-week-old long-term cultures (LTC)[1]. Subsequent attempts to improve the output of stem cells in these cultures by using various cell lines as feeders have allowed input numbers to be maintained, but a net expansion has not been achieved[2-4]. Within the hematopoietic system, Pluripotent HSCs are the only cells with extensive capacities to expand, differentiate and self-renew. These HSCs are exclusively required for hematopoietic reconstitution following transplantation and serve as a primary target for gene therapy. In spite of the key role of HSCs in maintaining the hematopoietic system, their extremely low frequency in the hematopoietic tissue, as well as the limited ability to maintain or expand undifferentiated stem cells under ex vivo conditions, not only remains a major drawback to essential clinical applications of these cells, but also reflects the current unavailability of, and the need for, novel stem cell regulators.
Sources of hematopoietic stem cells:
The primary source of hematopoietic stem and progenitor cells for use in autologous and allogeneic transplantation has been bone marrow. More recently, use of peripheral blood as a source of these cells has increased. This product is referred to variously as peripheral blood stem cells (PBSC), peripheral blood progenitor cells (PBPC), or stem cells. Although each cell source used for hematopoietic transplantation contains stem cells, when the term stem cell is used without a qualifier, it usually refers to PBSC. Table 1 lists the major sources of hematopoietic stem cells used in transplantation, with the cellular characteristics of each. The use of PBSC has advantages and disadvantages, and the relative balance between these differs, depending on the type of transplant being performed and the disease state being treated. The majority of transplantable HSCs in adults come from adult donor bone marrow or mobilized peripheral blood. A new and very effective source of transplantable and lasting HSCs comes from umbilical cord blood (CB). Today, there is a tremendous amount of focus on research, development and business in the area of cord blood innovations. The collection of cord blood is simple and non-invasive. CB is drawn from the umbilical cord after birth, before the placenta is discharged. As opposed to working with embryonic stem cells, CB is not ethically or controversially charged. It is also important to note that there is no difference in the functionality of hematopoietic stem cells from bone marrow, mobilized peripheral blood or umbilical cord blood. In the last few years, a new industry has evolved for the collection and storage of cord blood. With the increased development and use of this extraordinary source of HSC for BMT, the number of cord blood banks is rapidly increasing.
Hematopoietic Stem Cell Transplantation[1-3] is a procedure based on a very simple principle. First a patient is identified who has a potentially lethal disease (e.g. acute myeloid leukemia or beta thalassaemia major). This disease should be such that it can be eradicated by high dose chemoradiotherapy. Such treatment is highly toxic and will invariably lead to permanent destruction of hematopoietic stem cell - i.e. the body's blood forming cells. Thus the potentially curative therapy can only be used if new normal hematopoietic stem cells can be infused and made to work in the patient's body. Such normal blood stem cells are harvested from a donor and reinfused into the patient (after being given the potentially curative chemo-radiotherapy)[5,6,8]. HSCT is a unique mode of therapy in the field of oncology that
1. it is the most intense form of treatment followed by medical oncologists.
2. it exposes the entire human body to potentially life threatening consequences because this is the only hope of cure.
3. it is the only transplantation procedure which does not require a surgeon.
Who can have a transplant? To be eligible for any form of bone marrow transplant a patient's general health must be good enough to withstand very high dose treatment with anti- cancer drugs and radiation. One of the major factors predicting the risks of transplantation is the age of the patient. Younger patients tend to have a far better chance of success than older patients and less risk of a transplant related death. Most transplant centres will not carry out allogeneic transplants on patients over the age of about 55 years, in some centres the limit is set at 50 years. It is usually acceptable to perform autologous transplants on older patients because the risk is not so great.
Indications for transplant: Acute leukaemia, Acute lymphoblastic leukaemia, Acute myeloid leukaemia, Chronic lymphocytic leukaemia, Chronic myeloid leukaemia, Hodgkin's disease, Non-Hodgkin's lymphoma, Multiple myeloma, myelodysplastic syndromes and myelofibrosis.
The transplant procedure: Before a patient can receive a transplant their own marrow and immune system must be destroyed with high doses of drugs with or without total body irradiation (TBI). This is necessary even in patients with aplastic anaemia in whom the marrow appears to have already failed. This preparation is called conditioning treatment. There are several different conditioning regimens (a regimen is a specific combination of drug and radiation treatment). For autologous transplantation a regimen called BEAM is frequently used, this is a combination of several drugs given over a period of five days. For myeloma melphalan is frequently used, with or without TBI. Conditioning for allogeneic transplant is either with busulphan and cyclophosphamide or cyclophosphamide with TBI. Because the chemotherapy used is often at high dose and is given intravenously a tube called a Hickman line is placed under the skin of the chest and into a vein. This is often done in the operating theatre where an X-ray machine can be used to ensure it is correctly inserted. Local painkillers and a sedative are given and it is not painful.
Hematopoietic stem cells in circulation: Presence of circulating CD 34 +ve stem cells in humans has been known since 1971. However their number is too small to be of clinical use for transplantation (being less than 0.5% in adults). With the advent of recombinant human hematopoietic growth factors, we now have a method to mobilise them (make them come out of the marrow into circulation) in sufficient number for clinical use. Initially HSCTs were performed only in the autologous setting. Now many centres, including in India New Delhi India
Major risks after Hematopoietic stem cell transplantation: Patients who undergo these transplantation may experience short-term side effects such as nausea, vomiting, fatigue, loss of appetite, mouth sores, hair loss, and skin reactions. Additionally, patients receiving stem cells may experience nausea and vomiting while receiving the transplant, and chills and fever during the first 24 hours after the transplant. Potential long-term risks include infertility (the inability to produce children); cataracts (clouding of the lens of the eye, which causes loss of vision); secondary (new) cancers; and complications in the liver, kidneys, lungs, and/or heart. With allogeneic transplantation, a complication known as graft-versus-host disease (GVHD) sometimes develops. GVHD can generally be treated with steroids or another immunosuppressive agent. Clinical trials are being conducted to find ways to prevent GVHD from occurring.
Gene Therapy:
In gene addition the whole emphasis is on normal cells where a new gene is added to confer a survival advantage. For example the hematopoietic cells can be made more resistant to the therapeutic consequences of myelosuppression. Gene transfer is used for several indications like repair of mutations. For instance replacing mutated p53 with the normal functioning counterpart in lung cancer leads to a higher response rate (mechanisms involved in this and other potential beneficial effects include increased tumor sensitivity, host sensitivity and host response to therapy as well as gene marking). Certain drugs need activation by enzymes before they become active. The classical examples are gancyclovir, which requires phosphorylation with thymidine kinase. Cells that are transfected with genes for the corresponding enzymes will specifically become susceptible to the toxic effects of these drugs and can be targeted in vivo for selective elimination. In the hematopoietic transplantation setting its immediate application would be in the use of donor lymphocyte transfusions. Such DLT are commonly used for cytogenetic or clinical relapse following allogeneic BMT for its graft v/s leukemia effect. However it has the potential to cause life threatening GVHD and aplastic anemia. Such donor lymphocytes can be transfected with the thymidine kinase gene. Currently clinical trials using gene therapy and hematopoietic transplantation are ongoing for Gene marking (no therapeutic benefit to patient),ADA SCID,X SCID,Chronic Granulomatous Disease,Goucher's Disease, Fanconi's Anemia.
Conclusion:
Hematopoietic stem cell transplantation is by no means an ideal mode of therapy. It is currently the only available curative mode of treatment for several oncological ailments. However, in the future, Gene therapy and stem cell research is expected to change the way we cure patients completely changing the application of HSCT. Ethical problems are the main barriers to treat many diseases using Hematopoietic Stem Cell Transplantation.
Existing centres need to focus on the development of regimen/ strategies tailored to our needs. For instance, cancers are diagnosed very early in our population. They are in the most productive phase of their lives and are fit to tolerate dose intensive therapy. Hence they should be subject to strategies that lead to a curative outcome.
References:
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4. Ketterer N, Salles G, Raba M, et al: High CD34(+) cell counts decrease hematologic toxicity of autologous peripheral blood progenitor cell transplantation. Blood 1998 May 1; 91(9): 3148-55
5. Philip T, Guglielmi C, Hagenbeek A, et al: Autologous bone marrow transplantation as compared with salvage chemotherapy in relapses of chemotherapy-sensitive non-Hodgkin's lymphoma. N Engl J Med 1995 Dec 7; 333(23): 1540-5.
6. Powles R, Mehta J, Kulkarni S, et al: Allogeneic blood and bone-marrow stem-cell transplantation in haematological malignant diseases: a randomised trial. Lancet 2000 Apr 8; 355(9211): 1231-7.
7. Schmitz N, Linch DC
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