Long-term regulation of genetically modified primary hematopoietic cells in dogs

Getting the Most: Enhancing Efficacy by Promoting Erythropoiesis and Thrombopoiesis after Gene Therapy in Mice with Hurler Syndrome

Novel strategies are needed to solve the conundrum of achieving clinical efficacy with high vector copy numbers (VCNs) in hematopoietic stem cells (HSCs) while attempting to minimize the potential risk of oncogenesis in lentiviral vector (LV)-mediated gene therapy clinical trials. We previously reported the benefits of reprogramming erythroid-megakaryocytic (EMK) cells for high-level lysosomal enzyme production with less risk of activating oncogenes in HSCs. Herein, using a murine model of mucopolysaccharidosis type I (MPS I) with a deficiency of α-L-iduronidase (IDUA), we sought to determine the transgene minimum effective doses (MEDs) in major organs, and if a transient increase of IDUA-containing red blood cells and platelets by repeated phlebotomy would provide further therapeutic benefits in diseased mice after EMK-restricted LV-mediated gene therapy. The MEDs for complete metabolic correction ranged from 0.1 to 2 VCNs in major visceral organs, which were dramatically reduced to 0.005–0.1 VCN by one cycle of stress induction and were associated with a further reduction of pathological deficits in mice with 0.005 VCN. This work provides a proof of concept that transiently stimulating erythropoiesis and thrombopoiesis can further improve therapeutic benefits in HSC-mediated gene therapy for MPS I, a repeatable and reversible approach to enhance clinical efficacy in the treatment of lysosomal storage diseases.

Optimizing autologous cell grafts to improve stem cell gene therapy

Over the past decade, stem cell gene therapy has achieved unprecedented curative outcomes for several genetic disorders. Despite the unequivocal success, clinical gene therapy still faces challenges. Genetically engineered hematopoietic stem cells are particularly vulnerable to attenuation of their repopulating capacity once exposed to culture conditions, ultimately leading to low engraftment levels posttransplant. This becomes of particular importance when transduction rates are low or/and competitive transplant conditions are generated by reduced-intensity conditioning in the absence of a selective advantage of the transduced over the unmodified cells. These limitations could partially be overcome by introducing megadoses of genetically modified CD34(+) cells into conditioned patients or by transplanting hematopoietic stem cells hematopoietic stem cells with high engrafting and repopulating potential. On the basis of the lessons gained from cord blood transplantation, we summarize the most promising approaches to date of increasing either the numbers of hematopoietic stem cells for transplantation or/and their engraftability, as a platform toward the optimization of engineered stem cell grafts.

Evolving insights into the synergy between erythropoietin and thrombopoietin and the bipotent erythroid/megakaryocytic progenitor cell

Although the synergy between erythropoietin and thrombopoietin has previously been pointed out, the clonal demonstration of a human bipotent erythroid/megakaryocytic progenitor (MEP) was first published in Experimental Hematology (Papayannopoulou T, Brice M, Farrer D, Kaushansky K. Exp Hematol. 1996;24:660-669) and later in the same year in Blood (Debili N, Coulombel L, Croisille L, et al. Blood. 1996;88:1284-1296). This demonstration, and the fact that both bipotent and monopotent erythroid or megakaryocytic progenitors co-express markers of both lineages and respond to both lineage-specific transcription factors, has provided a background for the extensive use of MEP assessment by fluorescence-activated cell sorting in many subsequent studies. Beyond this, the demonstration of shared regulatory elements and the presence of single mutations affecting both lineages have inspired further studies to decipher how the shift in transcription factor networks occurs from one lineage to the other. Furthermore, in addition to shared effects, erythropoietin and thrombopoietin have additional independent effects. Most notable for thrombopoietin is its effect on hematopoietic stem cells illustrated by in vitro and in vivo approaches.

Serial Activation of the Inducible Caspase 9 Safety Switch After Human Stem Cell Transplantation

Activation of the inducible caspase 9 (iC9) safety gene by a dimerizing drug (chemical inducer of dimerization (CID) AP1903) effectively resolves the symptoms and signs of graft-versus-host disease (GvHD) in haploidentical stem cell transplant (HSCT) recipients. However, after CID treatment, 1% of iC9-T cells remain and can regrow over time; although these resurgent T cells do not cause recurrent GvHD, it remains unclear whether repeat CID treatments are a safe and feasible way to further deplete residual gene-modified T cells should any other adverse effects associated with them occur. Here, we report a patient who received an infusion of haploidentical iC9-T cells after HSCT and subsequently received three treatments with AP1903. There was a mild (grade 2) and transient pancytopenia following each AP1903 administration but no non-hematological toxicity. Ninety five percent of circulating iC9-T cells (CD3(+)CD19(+)) were eliminated after the first AP1903 treatment. Three months later, the residual cells had expanded more than eightfold and had a lower level of iC9 expression. Each repeated AP1903 administration eliminated a diminishing percentage of the residual repopulating cells, but elimination could be enhanced by T-cell activation. These data support the safety and efficiency of repeated CID treatments for persistent or recurring toxicity from T-cell therapies.

Towards in vivo amplification: Overcoming hurdles in the use of hematopoietic stem cells in transplantation and gene therapy

With the advent of safer and more efficient gene transfer methods, gene therapy has become a viable solution for many inherited and acquired disorders. Hematopoietic stem cells (HSCs) are a prime cell compartment for gene therapy aimed at correcting blood-based disorders, as well as those amenable to metabolic outcomes that can effect cross-correction. While some resounding clinical successes have recently been demonstrated, ample room remains to increase the therapeutic output from HSC-directed gene therapy. In vivo amplification of therapeutic cells is one avenue to achieve enhanced gene product delivery. To date, attempts have been made to provide HSCs with resistance to cytotoxic drugs, to include drug-inducible growth modules specific to HSCs, and to increase the engraftment potential of transduced HSCs. This review aims to summarize amplification strategies that have been developed and tested and to discuss their advantages along with barriers faced towards their clinical adaptation. In addition, next-generation strategies to circumvent current limitations of specific amplification schemas are discussed.

A hyperactive Mpl-based cell growth switch drives macrophage-associated erythropoiesis through an erythroid-megakaryocytic precursor

Several approaches for controlling hematopoietic stem and progenitor cell expansion, lineage commitment, and maturation have been investigated for improving clinical interventions. We report here that amino acid substitutions in a thrombopoietin receptor (Mpl)--containing cell growth switch (CGS) extending receptor stability improve the expansion capacity of human cord blood CD34(+) cells in the absence of exogenous cytokines. Activation of this CGS with a chemical inducer of dimerization (CID) expands total cells 99-fold, erythrocytes 70-fold, megakaryocytes 0.5-fold, and CD34(+) stem/progenitor cells 4.4-fold by 21 days of culture. Analysis of cells in these expanded populations identified a CID-dependent bipotent erythrocyte-megakaryocyte precursor (PEM) population, and a CID-independent macrophage population. The CD235a(+)/CD41a(+) PEM population constitutes up to 13% of the expansion cultures, can differentiate into erythrocytes or megakaryocytes, exhibits very little expansion capacity, and exists at very low levels in unexpanded cord blood. The CD206(+) macrophage population constitutes up to 15% of the expansion cultures, exhibits high-expansion capacity, and is physically associated with differentiating erythroblasts. Taken together, these studies describe a fundamental enhancement of the CGS expansion platform, identify a novel precursor population in the erythroid/megakaryocytic differentiation pathway of humans, and implicate an erythropoietin-independent, macrophage-associated pathway supporting terminal erythropoiesis in this expansion system.

Isolation, genetic manipulation, and transplantation of canine spermatogonial stem cells: progress toward transgenesis through the male germ-line

The dog is recognized as a highly predictive model for preclinical research. Its size, life span, physiology, and genetics more closely match human parameters than do those of the mouse model. Investigations of the genetic basis of disease and of new regenerative treatments have frequently taken advantage of canine models. However, full utility of this model has not been realized because of the lack of easy transgenesis. Blastocyst-mediated transgenic technology developed in mice has been very slow to translate to larger animals, and somatic cell nuclear transfer remains technically challenging, expensive, and low yield. Spermatogonial stem cell (SSC) transplantation, which does not involve manipulation of ova or blastocysts, has proven to be an effective alternative approach for generating transgenic offspring in rodents and in some large animals. Our recent demonstration that canine testis cells can engraft in a host testis, and generate donor-derived sperm, suggests that SSC transplantation may offer a similar avenue to transgenesis in the canine model. Here, we explore the potential of SSC transplantation in dogs as a means of generating canine transgenic models for preclinical models of genetic diseases. Specifically, we i) established markers for identification and tracking canine spermatogonial cells; ii) established methods for enrichment and genetic manipulation of these cells; iii) described their behavior in culture; and iv) demonstrated engraftment of genetically manipulated SSC and production of transgenic sperm. These findings help to set the stage for generation of transgenic canine models via SSC transplantation.

Creating genetic resistance to HIV

HIV/AIDS remains a chronic and incurable disease, in spite of the notable successes of combination antiretroviral therapy. Gene therapy offers the prospect of creating genetic resistance to HIV that supplants the need for antiviral drugs. In sight of this goal, a variety of anti-HIV genes have reached clinical testing, including gene-editing enzymes, protein-based inhibitors, and RNA-based therapeutics. Combinations of therapeutic genes against viral and host targets are designed to improve the overall antiviral potency and reduce the likelihood of viral resistance. In cell-based therapies, therapeutic genes are expressed in gene modified T lymphocytes or in hematopoietic stem cells that generate an HIV-resistant immune system. Such strategies must promote the selective proliferation of the transplanted cells and the prolonged expression of therapeutic genes. This review focuses on the current advances and limitations in genetic therapies against HIV, including the status of several recent and ongoing clinical studies.

Hematopoietic-stem-cell-based gene therapy for HIV disease

Although combination antiretroviral therapy can dramatically reduce the circulating viral load in those infected with HIV, replication-competent virus persists. To eliminate the need for indefinite treatment, there is growing interest in creating a functional HIV-resistant immune system through the use of gene-modified hematopoietic stem cells (HSCs). Proof of concept for this approach has been provided in the instance of an HIV-infected adult transplanted with allogeneic stem cells from a donor lacking the HIV coreceptor, CCR5. Here, we review this and other strategies for HSC-based gene therapy for HIV disease.