stem cells, also known as hematopoietic stem cells (HSCs), were the first stem cells to be identified. They are located in the bone marrow of the femur, pelvis, the vertebrae, and the rib cage. They can also be extracted from peripheral blood, umbilical cord blood (UCB), and the placenta. These stem cells differentiate into all types of blood cells in the body; they may be required to produce more than 500 billion of these cells daily. This demonstrates the multipotency as well as extensive self-renewal capacity of the blood stem cells. HSCs are located in a specific “stem cell niche” in the bone marrow.
The stem cells may go through various stages, such as quiescence, self-renewal, differentiation, or apoptosis, with these processes being controlled by the internal environment of the cells themselves. Experiments have identified GRP94, an endoplasmic reticulum chaperone, as an intrinsic factor present in the HSCs, which is required to maintain them within the niche. It also plays a role in regulating early T- and B-cell lymphopoiesis. However, it is now suspected, according to the niche hypothesis, that the microenvironment of the niche also plays a role in the maturation of the HSCs. In fact, the latest research suggests that bone cells (osteoblasts and osteocytes) may not only regulate the microenvironment of the niche but also modulate immune cell differentiation.
It is now possible to induce these stem cells to move into the bloodstream, from where they can be easily harvested and genetically modified. This strategy has been successfully used for the treatment of severe β-thalassemia, in which up to fourfold enrichment of HSCs was achieved. Peripheral blood stem cell mobilization was achieved by granulocyte colony-stimulating factor, followed by CD34+ HSC harvesting and lentiviral transduction, and eventual transplantation for the treatment of β thalassemia.
Development of Blood Stem Cells
HSCs have been shown to develop from embryonic mesodermal hemangioblast cells. Once the precursors have been specified from the mesoderm, they divide to form a pool of functional HSCs. This pool moves through embryonic niches, such as the yolk sac, placenta, and fetal liver, which provide signals for their establishment and self-renewal ability. The HSCs move to occupy the bone marrow at birth and a steady state is established. Most HSCs are quiescent (in the G0 phase) and division is tightly controlled to maintain the pool as well as the differentiated blood cells. When differentiation is required, cell division is asymmetric, giving rise to a long-term HSC daughter cell, which retains self-renewal capacity and a short-term HSC daughter cell/progenitor with limited self-renewal capacity. The progenitors may follow the erythroid (producing erythrocytes), lymphoid (producing B cells and T cells) or myeloid lineages (producing granulocytes, megakaryocytes, and macrophages).
Commitment to a specific lineage and further maturation has been found to be influenced by cytokines (cell-signaling proteins) and growth factors. The cytokines include those in the common beta chain, common gamma chain, and interleukin-6 families. The growth factors involved include epidermal growth factor, fibroblast growth factor, growth differentiation factor, insulin-like growth factor, platelet derived growth factor, and vascular endothelial growth factor. In addition, activins, bone morphogenetic proteins, and hedgehog molecules also determine the fate of the HSCs. These factors act in various ways, primarily controlling the expression of certain genes.
The erythroid, lymphoid, and myeloid lineages can be traced because of the presence of specific markers. Human HSCs can be identified by high levels of CD54 expression. Lymphoid cells show enhanced expression of MS4A1/CD20 and CD4 in cells differentiating into B cells and T cells, respectively. Antibodies are also available to detect the differentiation of the myeloid cells.
Plasticity of Blood Stem Cells
It is challenging to recreate the microenvironment of the stem cell niche for ex vivo multiplication of HSCs. Cultured HSCs lose their ability to engraft and divide in vivo, which severely limits the process of in vitro manipulation and growth for therapeutic purposes.
Nevertheless, it is a continuing endeavor to produce patient-specific stem cells, particularly HSCs in comparatively larger quantities in vitro. These are preferred for autologous stem cell therapy, and would circumvent the development of adverse clinical reactions and infections. A recent study reported successful dedifferentiation (reprogramming) of connective tissue cells such as fibroblasts into HSCs by using a combination of eight transcription factors. It indicates the generation of induced pluripotent cells (iPSCs) with the transduction of defined transcription factors. Interest has lately shifted to reprogramming of mononuclear cells from peripheral blood into iPSCs and their in vitro/in vivo conversion to mesenchymal stem cells, hepatocytes, or neural cells. This reprogramming has its challenges and is being achieved by genetic modification using lentiviral vectors, as well as the Sendai virus and other episomal factors.
Another experiment successfully demonstrated the plasticity of adult stem cells when purified HSCs restored the biochemical function of the liver in deficient mice. This phenomenon, also known as transdifferentiation, defines the pluripotent nature of blood stem cells. It is, however, not a regular phenomenon and the mechanisms behind it are not fully understood. HSCs, along with CD34 (adhesion factor), have also been used to treat spinal cord injury, liver cirrhosis, and peripheral vascular disease. Stem cells isolated from peripheral blood as well as UCB were observed to differentiate into endothelial-like cells, which could be incorporated into hypoxic tissues, as seen in ischemic cardiomyopathies. They promoted neovascularization and tissue repair following an enhanced oxygen and nutrient supply. This plasticity of stem cells can be exploited to plan therapeutic interventions in regenerative medicine.
Since they can generate parenchymatous cells, such as myocytes, hepatocytes, endothelial and myocardial cells, and neuronal and glial cells, HSCs can be used to repair damaged tissues in the body. Besides this, they secrete growth factors, chemokines, and cytokines, which encourage angiogenesis as well as suppress inflammation and cell death. It is expected that age-related functional defects; hematopoietic and immune system disorders; heart failures; chronic liver injuries; diabetes; neurodegenerative disorders such as Parkinson’s and Alzheimer’s diseases; arthritis; and muscular, skin, lung, eye, and digestive disorders, as well as aggressive and recurrent cancers, could be successfully treated by stem cell-based therapies. A pediatric population comprising adolescents, infants, and children with Down syndrome have also undergone HSC transplantation for leukemia, and the results are being evaluated for improvement of the outcome. They do show higher treatment-related toxicity and mortality.
A review of the ongoing clinical trials, as well as a literature search, revealed that UCB-based therapies are being increasingly used in nonhematopoietic diseases, particularly neurologic disorders. Cellular regeneration and immune modulation are the other targets of UCB transplantation.
Transplantation of Blood Stem Cells
Since the bone marrow is rich in blood stem cells, it is transplanted to treat disorders of the blood, such as leukemia, lymphoma, or multiple myeloma, as well as genetic disorders. Small quantities of blood stem cells obtained from the UCB can be used to treat leukemia, congenital immunodeficiencies, anemias, or sickle cell disease in children. This is known as stem cell therapy.
Transplantation of adult blood stem cells into a new environment is a complex process with several success-limiting steps. The transplanted HSCs must first migrate from the peripheral circulation to the bone marrow niches. This process is known as homing. They need to be successfully lodged at the site, a process which requires adherence and retention. The freshly transplanted cells interact with the extrinsic components of the niche, which will dictate their stability and future course of action. It is expected that homing and lodging in the appropriate microenvironment will dictate the clinical outcome of the bone marrow transplantation.
Studies have shown the necessity of the calcium-sensing receptor in the process of lodging and engraftment in the bone marrow niche. Stimulation of the receptor also leads to augmented homing. It follows that ex vivo modulation of the calcium-sensing receptors with calcimimetics can be used to increase the success rate of the transplants. Other cell-adhesion molecules, such as α4 integrins, hyaluronic acid, and osteopontin present on the cell surface