Many multipotential gene-marked progenitor or stem cell clones contribute to hematopoiesis in nonhuman primates

Estimating the Number of Hematopoietic or Lymphoid Stem Cells Giving Rise to Clonal Chromosome Aberrations in Blood T Lymphocytes

Quantifying the proliferative capacity of long-term hematopoietic stem cells in humans is important for bone marrow transplantation and gene therapy. Obtaining appropriate data is difficult, however, because the experimental tools are limited. We hypothesized that tracking clonal descendants originating from hematopoietic stem cells would be possible if we used clonal chromosome aberrations as unique tags of individual hematopoietic stem cells in vivo. Using FISH, we screened 500 blood T lymphocytes from each of 513 atomic bomb survivors and detected 96 clones composed of at least three cells with identical aberrations. The number of clones was inversely related to their population size, which we interpreted to mean that the progenitor cells were heterogeneous in the number of progeny that they could produce. The absolute number of progenitor cells contributing to the formation of the observed clones was estimated as about two in an unexposed individual. Further, scrutiny of ten clones revealed that lymphocyte clones could originate roughly equally from hematopoietic stem cells or from mature T lymphocytes, thereby suggesting that the estimated two progenitor cells are shared as one hematopoietic stem cell and one mature T cell. Our model predicts that one out of ten people bears a non- aberrant clone comprising >10% of the total lymphocytes, which indicates that clonal expansions are common and probably are not health-threatening.

Side effects of retroviral gene transfer into hematopoietic stem cells

Recent conceptual and technical improvements have resulted in clinically meaningful levels of gene transfer into repopulating hematopoietic stem cells. At the same time, evidence is accumulating that gene therapy may induce several kinds of unexpected side effects, based on preclinical and clinical data. To assess the therapeutic potential of genetic interventions in hematopoietic cells, it will be important to derive a classification of side effects, to obtain insights into their underlying mechanisms, and to use rigorous statistical approaches in comparing data. We here review side effects related to target cell manipulation; vector production; transgene insertion and expression; selection procedures for transgenic cells; and immune surveillance. We also address some inherent differences between hematopoiesis in the most commonly used animal model, the laboratory mouse, and in humans. It is our intention to emphasize the need for a critical and hypothesis-driven analysis of "transgene toxicology," in order to improve safety, efficiency, and prognosis for the yet small but expanding group of patients that could benefit from gene therapy.

Genetic marking as an approach to studying in vivo hematopoiesis: progress in the non-human primate model

Retroviral insertion site analysis following transplantation of marked hematopoietic stem cells (HSCs) is a powerful method for studying hematopoiesis in vivo. High-level gene transfer efficiency was achieved in murine models in the late 1980s, but early human gene transfer protocols into hematopoietic stem and progenitor cells using the murine methodology showed consistently poor results. The utility of non-human primates as pre-clinical models has since become apparent. Modifications in retroviral transduction conditions have resulted in stable long-term gene transfer efficiency as high as 15-20% to primitive repopulating cells in non-human primate models. This has permitted, for the first time in a large animal model, tracking of individual stem and progenitor cell clones via insertion site analysis, an advantage over competitive transplantation studies, which cannot firmly evaluate the number or life span of individual clones contributing to hematopoiesis. Retroviral tracking studies in mice suggest that stable hematopoiesis may be dominated by a small number of clones, but these studies have been limited by insensitive detection methods, low numbers of transplanted stem cells, and limited life span of immunodeficient mice. Autologous transplantation studies in non-human primates have just begun and have the potential to shed light on controversial issues such as the number of clones contributing to stable hematopoiesis, clonal succession, and lineage commitment, as well as the effect of clinically relevant manipulations such as cytokines, chemotherapy, and radiation on hematopoiesis. These approaches will have significant impact in studying various aspects of stem cell biology including the phenomenon of stem cell plasticity.