In vivo dynamics of human stem cell repopulation in NOD/SCID mice

Therapeutic Mesenchymal Stromal Cells for Immunotherapy and for Gene and Drug Delivery

Mesenchymal stromal cells (MSCs) possess several fairly unique properties that, when combined, make them ideally suited for cellular-based immunotherapy and as vehicles for gene and drug delivery for a wide range of diseases and disorders. Key among these are: (1) their relative ease of isolation from a variety of tissues; (2) the ability to be expanded in culture without a loss of functionality, a property that varies to some degree with tissue source; (3) they are relatively immune-inert, perhaps obviating the need for precise donor/recipient matching; (4) they possess potent immunomodulatory functions that can be tailored by so-called licensing in vitro and in vivo; (5) the efficiency with which they can be modified with viral-based vectors; and (6) their almost uncanny ability to selectively home to damaged tissues, tumors, and metastases following systemic administration. In this review, we summarize the latest research in the immunological properties of MSCs, their use as immunomodulatory/anti-inflammatory agents, methods for licensing MSCs to customize their immunological profile, and their use as vehicles for transferring both therapeutic genes in genetic disease and drugs and genes designed to destroy tumor cells.

Homology-driven genome editing in hematopoietic stem and progenitor cells using ZFN mRNA and AAV6 donors

Genome editing with targeted nucleases and DNA donor templates homologous to the break site has proven challenging in human hematopoietic stem and progenitor cells (HSPCs), and particularly in the most primitive, long-term repopulating cell population. Here we report that combining electroporation of zinc finger nuclease (ZFN) mRNA with donor template delivery by adeno-associated virus (AAV) serotype 6 vectors directs efficient genome editing in HSPCs, achieving site-specific insertion of a GFP cassette at the CCR5 and AAVS1 loci in mobilized peripheral blood CD34⁺ HSPCs at mean frequencies of 17% and 26%, respectively, and in fetal liver HSPCs at 19% and 43%, respectively. Notably, this approach modified the CD34⁺CD133⁺CD90⁺ cell population, a minor component of CD34⁺ cells that contains long-term repopulating hematopoietic stem cells (HSCs). Genome-edited HSPCs also engrafted in immune-deficient mice long-term, confirming that HSCs are targeted by this approach. Our results provide a strategy for more robust application of genome-editing technologies in HSPCs.

Mechanisms controlling hematopoietic stem cell functions during normal hematopoiesis and hematological malignancies

Hematopoiesis, the process by which all mature blood cells are generated from multipotent hematopoietic stem cells (HSCs), is a finely tuned balancing act in which HSCs must constantly decide between different cell fates: to proliferate, to self-renew or differentiate, to stay quiescent in the bone marrow niche or migrate to the periphery, to live or die. These fates are regulated by a complex interplay between cell-extrinsic cues and cell-intrinsic regulatory pathways whose function is to maintain a homeostatic balance between HSC self-renewal and life-long replenishment of lost blood cells. Improper regulation of these competing cellular programs can transform HSCs and progenitor cells into disease-initiating leukemic stem cells (LSCs). Strikingly, many of the mechanisms required for maintenance of normal HSC fate decisions are equally critical for the aberrant functions of LSCs. Because of the inherent complexities of these molecular mechanisms, a systematic approach to understanding the regulatory networks underlying HSC self-renewal is critical for uncovering the similarities and differences between HSCs and LSCs. In this review, we focus on recent developments in elucidating the regulatory networks governing normal HSC self-renewal programs and their implications for leukemic transformation. We describe the current technical and methodological limitations in isolating and characterizing HSCs and LSCs, and the emerging approaches that may afford a better understanding of the regulation of normal and leukemic hematopoiesis. Finally, we discuss how such basic mechanistic information may be of use for the design of novel therapies that will selectively reprogram and/or eliminate LSCs.

Mesenchymal stem cells in ex vivo cord blood expansion

Umbilical cord blood (CB) is becoming an important source of haematopoietic support for transplant patients lacking human leukocyte antigen matched donors. The ethnic diversity, relative ease of collection, ready availability as cryopreserved units from CB banks, reduced incidence and severity of graft versus host disease and tolerance of higher degrees of HLA disparity between donor and recipient, are positive attributes when compared to bone marrow or cytokine-mobilized peripheral blood. However, CB transplantation is associated with significantly delayed neutrophil and platelet engraftment and an elevated risk of graft failure. These hurdles are thought to be due, at least in part, to low total nucleated cell and CD34(+) cell doses transplanted. Here, current strategies directed at improving TNC and CD34(+) cell doses at transplant are discussed, with particular attention paid to the use of a mesenchymal stem cell (MSC)/CB mononuclear cell ex vivo co-culture expansion system.

Highly multiparametric analysis by mass cytometry

This review paper describes a new technology, mass cytometry, that addresses applications typically run by flow cytometer analyzers, but extends the capability to highly multiparametric analysis. The detection technology is based on atomic mass spectrometry. It offers quantitation, specificity and dynamic range of mass spectrometry in a format that is familiar to flow cytometry practitioners. The mass cytometer does not require compensation, allowing the application of statistical techniques; this has been impossible given the constraints of fluorescence noise with traditional cytometry instruments. Instead of "colors" the mass cytometer "reads" the stable isotope tags attached to antibodies using metal-chelating labeling reagents. Because there are many available stable isotopes, and the mass spectrometer provides exquisite resolution between detection channels, many parameters can be measured as easily as one. For example, in a single tube the technique allows for the ready detection and characterization of the major cell subsets in blood or bone marrow. Here we describe mass cytometric immunophenotyping of human leukemia cell lines and leukemia patient samples, differential cell analysis of normal peripheral and umbilical cord blood; intracellular protein identification and metal-encoded bead arrays.

Identification of a hierarchy of multipotent hematopoietic progenitors in human cord blood

Mouse hematopoiesis is initiated by long-term hematopoietic stem cells (HSC) that differentiate into a series of multipotent progenitors that exhibit progressively diminished self-renewal ability. In human hematopoiesis, populations enriched for HSC activity have been identified, as have downstream lineage-committed progenitors, but multipotent progenitor activity has not been uniquely isolated. Previous reports indicate that human HSC are enriched in Lin-CD34+CD38- cord blood and bone marrow and express CD90. We demonstrate that the Lin-CD34+CD38- fraction of cord blood and bone marrow can be subdivided into three subpopulations: CD90+CD45RA-, CD90-CD45RA-, and CD90-CD45RA+. Utilizing in vivo transplantation studies and complementary in vitro assays, we demonstrate that the Lin-CD34+CD38-CD90+CD45RA- cord blood fraction contains HSC and isolate this activity to as few as 10 purified cells. Furthermore, we report the first prospective isolation of a population of candidate human multipotent progenitors, Lin-CD34+CD38-CD90-CD45RA- cord blood.

Characterization of an immunodeficient mouse model of mucopolysaccharidosis type I suitable for preclinical testing of human stem cell and gene therapy

Mucopolysaccharidosis type I (MPS-I or Hurler syndrome) is an inherited deficiency of the lysosomal glycosaminoglycan (GAG)-degrading enzyme alpha-l-iduronidase (IDUA) in which GAG accumulation causes progressive multi-system dysfunction and death. Early allogeneic hematopoietic stem cell transplantation (HSCT) ameliorates clinical features and extends life but is not available to all patients, and inadequately corrects its most devastating features including mental retardation and skeletal deformities. To test novel therapies, we characterized an immunodeficient MPS-I mouse model less likely to develop immune reactions to transplanted human or gene-corrected cells or secreted IDUA. In the liver, spleen, heart, lung, kidney and brain of NOD/SCID/MPS-I mice IDUA was undetectable, and reduced to half in heterozygotes. MPS-I mice developed marked GAG accumulation (3-38-fold) in these organs. Neuropathological examination showed GM(3) ganglioside accumulation in the striatum, cerebral peduncles, cerebellum and ventral brainstem of MPS-I mice. Urinary GAG excretion (6.5-fold higher in MPS-I mice) provided a non-invasive and reliable method suitable for serially following the biochemical efficacy of therapeutic interventions. We identified and validated using rigorous biostatistical methods, a highly reproducible method for evaluating sensorimotor function and motor skills development. This Rotarod test revealed marked abnormalities in sensorimotor integration involving the cerebellum, striatum, proprioceptive pathways, motor cortex, and in acquisition of motor coordination. NOD/SCID/MPS-I mice exhibit many of the clinical, skeletal, pathological and behavioral abnormalities of human MPS-I, and provide an extremely suitable animal model for assessing the systemic and neurological effects of human stem cell transplantation and gene therapeutic approaches, using the above techniques to measure efficacy.

The science and ethics of making part-human animals in stem cell biology

The National Academy of Sciences recently issued voluntary guidelines to govern human embryonic stem cell research. Among other restrictions, these guidelines prohibit certain kinds of combinations of human and nonhuman animal cells, and call for ethics review and oversight of any protocol involving the transfer of human embryonic stem cells into nonhuman animals. In this essay, I discuss the history of and scientific rationales for combining human cells with cells of nonhuman animals, and critically assess the most recent attempts to limit such research on moral grounds--and find them lacking. Nonetheless, as I show, this research remains scientifically and morally contested. I then explore whether and how the NAS's recommended Embryonic Stem Cell Research and Oversight committees will allow for scientifically well-informed moral assessment of this controversial, but possibly important, research.

Clonal analysis of individual marrow-repopulating cells after experimental peripheral blood progenitor cell transplantation

Methods to analyze the clonality of an adverse event in preclinical or clinical retroviral stem cell gene therapy protocols are needed. We analyzed the progeny of retrovirally transduced human peripheral blood progenitor cells (PBPCs) after transplantation and engraftment in immune-deficient mice. The integration site of the provirus serves as a unique tag of the individual transduced PBPC. A plasmid library of junctions between proviral and human genomic DNA was generated. We were able to detect individual transduced cell clones that amounted to 0.14%-0.0001% of chimeric bone marrow cells. This is the first report in which the contribution of individual marrow-repopulating cells to human hematopoiesis is directly quantified.

Adult human hematopoietic cells provide functional hemangioblast activity

The murine adult hematopoietic stem cell is able to function as a hemangioblast, contributing both to blood reconstitution and to blood vessel repair in response to ischemic injury. We developed a novel mouse xenotransplantation model of retinal neovascularization to test human hematopoietic cell plasticity. Immunocompromised nonobese diabetic (NOD)/scid mice underwent myeloablative conditioning and transplantation with human CD34+ umbilical cord blood. After multilineage reconstitution was established, retinal ischemia was induced to promote neovascularization. Our results demonstrate human retinal neovascularization, thus revealing the functional hemangioblast activity of human hematopoietic cells.

Genetic modification of hematopoietic stem cells: recent advances in the gene therapy of inherited diseases

Hematopoietic stem cells constitute a rare population of precursor cells with remarkable properties for being used as targets in gene therapy protocols. The last years have been particularly productive both in the fields of gene therapy and stem cell biology. Results from ongoing clinical trials have shown the first unquestionable clinical benefits of immunodeficient patients transplanted with genetically modified autologous stem cells. On the other hand, severe side effects in a few patients treated with gene therapy have also been reported, indicating the usefulness of further improving the vectors currently used in gene therapy clinical trials. In the field of stem cell biology, evidence showing the plastic potential of adult hematopoietic stem cells and data indicating the multipotency of adult mesenchymal precursor cells have been presented. Also, the generation of embryonic stem cells by means of nuclear transfer techniques has appeared as a new methodology with direct implications in gene therapy.