Key Points
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Expansion of haematopoietic stem cells (HSCs) would be useful for developing new strategies for clinical transplantation and gene therapy. Through an increased understanding of the molecular mechanisms that control HSC self-renewal, HSC expansion has been achieved in several experimental systems.
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Three general mechanisms can be modulated to achieve HSC expansion: cell-signalling pathways, transcription factors that are involved in HSC replication and genes that regulate cell-cycle progression.
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WNT proteins, homeobox B4 (HOXB4) and Notch ligands have been generated as soluble factors. Each has resulted in the expansion of HSCs in vitro when added to the culture medium.
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Retroviral vectors that express HOXB4 alone or together with a gene that confers drug resistance might also be useful for the selective expansion of transduced HSCs. The safety of this approach is not yet established and might be improved by creating a pharmacologically regulated HOXB4 fusion protein.
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Analysis of mice with homozygous disruptions of cell-cycle genes has identified p18 and p21 as potential targets for HSC expansion. Downregulation of these genes by antisense oligonucleotides or small interfering RNA molecules could be used for expanding HSCs in culture.
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HSC expansion could be used to increase the rate of immune reconstitution in patients who receive allogeneic transplants, to broaden the use of transplantation of cord blood and to simplify the collection of HSCs from normal donors. HSC expansion would also improve the outcome of gene therapy by potentially improving transduction rates and by limiting the 'dose' of insertion events to which the patient is exposed.
Abstract
Haematopoietic stem cells (HSCs) give rise to all blood and immune cells and are used in clinical transplantation protocols to treat a wide variety of diseases. The ability to increase the number of HSCs either in vivo or in vitro would provide new treatment options, but the amplification of HSCs has been difficult to achieve. Recent insights into the mechanisms of HSC self-renewal now make the amplification of HSCs a plausible clinical goal. This article reviews the molecular mechanisms that control HSC numbers and discusses how these can be modulated to increase the number of HSCs. Clinical applications of HSC expansion are then discussed for their potential to address the current limitations of HSC transplantation.
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Acknowledgements
I thank C. Sherr for his critical reading of this manuscript and for his many useful suggestions. This work was supported by grants from the National Institutes of Health (Bethesda, United States), a Program Project Grant (United States) and the American Lebanese Syrian Associated Charities (Memphis, United States).
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Glossary
- AUTOLOGOUS HSCs
-
A transplant with autologous haematopoietic stem cells (HSCs) is a treatment in which transplanted HSCs are obtained directly from the patient. This is typically used to support intensive treatment with cytotoxic drugs, but it can also be used for gene therapy of genetic disorders.
- GENE THERAPY
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In the context of haematopoietic disorders, this is a strategy for transducing autologous haematopoietic stem cells or lymphocyte progenitors with genetic vectors that express a therapeutic transgene. Genetically modified cells are then re-infused to reconstitute the haematopoietic system. The goal can be either to replace a defective gene, such as for treatment of sickle-cell anaemia, or to confer a new property to blood cells, such as resistance to cytotoxic drugs.
- ALLOGENEIC HSCs
-
A transplant with allogeneic haematopoietic stem cells (HSCs) is a treatment in which transplanted HSCs are obtained from a normal donor. This approach can be used to treat either malignant or non-malignant disorders. Mismatches between the histocompatibility antigens of the donor and patient can lead to adverse events, such as rejection of the transplanted graft or pathological immune responses to normal tissues in the patient.
- MATCHED SIBLING TRANSPLANTS
-
Transplants for which the donor is a sibling who has all of the same paternal and maternal major histocompatibility alleles as the patient. These cases are usually associated with the least transplant-related complications.
- MISMATCHED UNRELATED-DONOR TRANSPLANTS
-
Allogeneic transplants for which the donor is an unrelated individual. The donors are screened for phenotypic similarities in histocompatibility antigens; however, adverse immunological reactions are more problematic than for matched sibling transplants.
- β-THALASSAEMIA
-
An inherited disorder of erythrocytes caused by decreased or absent expression of β-globin and resulting in chronic anaemia. The most severe form of β-thalassaemia, sometimes called Cooley's anaemia, is characterized by the requirement for regular blood transfusions to sustain life. β-thalassaemia is a relatively common cause of anaemia in Africa, Europe and Asia.
- SICKLE-CELL ANAEMIA
-
An inherited disorder of erythrocytes, with a high prevalence in African and African American populations, that is caused by a mutation in the β-globin gene. A single nucleotide substitution (and the resultant amino-acid substitution) leads to the polymerization of haemoglobin when it is deoxygenated, ultimately resulting in the occlusion of small blood vessels. Disease manifestations include chronic anaemia, multiple painful crises, organ damage and increased susceptibility to bacterial infections.
- ACUTE MYELOID LEUKAEMIA
-
A clonal, malignant disease of blood cells that is characterized by proliferation of abnormal leukemic blast cells in the bone marrow. These blast cells express myeloid cell-surface markers and are usually present in high concentrations in the blood and bone marrow. Non-random cytogenetic abnormalities are usually present and define the causal genetic lesion underlying the malignancy.
- CHRONIC GRANULOMATOUS DISEASE
-
An inherited disorder caused by defective oxidase activity in the respiratory burst of phagocytes. It results from mutations in any of the four genes that are necessary to generate the superoxide radicals required for normal neutrophil function. Affected patients suffer from increased susceptibility to recurrent infections.
- LYSOSOMAL-STORAGE DISORDERS
-
A group of inherited disorders in which one or more tissues become progressively engorged with lipid. Mutations in lysosomal enzymes result in an accumulation of lipid-degradation products, which typically occurs in monocytes and macrophages derived from the bone marrow. Many of these disorders result in damage to the spleen, liver, brain and bone marrow.
- VIRAL 2A SEQUENCES
-
Peptide sequences found in picornaviruses and other viruses that mediate protein cleavage through a ribosomal skip mechanism. These relatively small sequences have been used in gene-therapy vectors to express multiple proteins from a single mRNA transcript.
- COMPETITIVE-REPOPULATION ASSAYS
-
Functional assays for measuring the number of mouse haematopoietic stem cells (HSCs). The HSC content of an undefined population of cells is determined by mixing the cells with a defined number of fresh bone-marrow cells from another source. After transplantation into lethally irradiated mice, genetic markers are used to distinguish progeny from the two HSC sources that are present in the blood and haematopoietic organs. These measurements allow a quantitative assessment of HSC activity.
- SMALL INTERFERING RNA (siRNA) MOLECULES
-
Synthetic double-stranded RNA molecules of 19–23 nucleotides, which are used to 'knockdown' (silence the expression of) a specific gene. This is known as RNA interference (RNAi) and is mediated by the sequence-specific degradation of mRNA.
- T-CELL-DEPLETED GRAFT
-
The removal of T cells from an allogeneic graft to prevent immunological complications, such as graft-versus-host disease. This is typically carried out for transplants from haploidentical and mismatched unrelated donors.
- GRAFT-VERSUS-HOST DISEASE
-
(GVHD). A potentially serious complication that arises when donor-derived T cells attack host tissues, typically resulting in hepatic, dermatological and gastrointestinal damage. Acute GVHD occurs within the first 100 days after transplantation, whereas chronic GVHD occurs later and has a different pathophysiology.
- HAPLOIDENTICAL TRANSPLANT
-
An allogeneic transplant in which the donor is matched for half of the major histocompatibility alleles of the recipient and is typically one of the parents of the patient. Because as many as half of the alleles are mismatched, specific treatment of the patient and processing of the haematopoietic stem-cell graft are required to avoid severe immunological consequences.
- CD34+ CELLS
-
Human haematopoietic cells that are immunopurified based on their expression of the CD34 antigen. These cells typically comprise 5% of the total bone-marrow cell population. Although this population is considerably enriched for haematopoietic stem cells (HSCs), most CD34+ cells are not HSCs.
- PERIPHERAL-BLOOD HSCs
-
Haematopoietic stem cells (HSCs) collected from the peripheral blood of the donor, usually after treatment with granulocyte colony-stimulating factor, which mobilizes HSCs to migrate from the bone marrow to the blood.
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Sorrentino, B. Clinical strategies for expansion of haematopoietic stem cells. Nat Rev Immunol 4, 878–888 (2004). https://doi.org/10.1038/nri1487
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DOI: https://doi.org/10.1038/nri1487
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