Sylvia Fessler

Extensive Methylation of Promoter Sequences Silences Lentiviral Transgene Expression During Stem Cell Differentiation In Vivo

Lentiviral vectors (LV) are widely used to stably transfer genes into target cells investigating or treating gene functions. In addition, gene transfer into early murine embryos may be improved to efficiently generate transgenic mice. We applied lentiviral gene transfer to generate a mouse model transgenic for SET binding protein-1 (Setbp1) and enhanced green fluorescent protein (eGFP). Neither transgenic founders nor their vector-positive offspring transcribed or expressed the transgenes. Bisulfite sequencing of the internal spleen focus-forming virus (SFFV) promoter demonstrated extensive methylation of all analyzed CpGs in the transgenic mice. To analyze the impact of Setbp1 on epigenetic silencing, embryonic stem cells (ESC) were differentiated into cardiomyocytes (CM) in vitro. In contrast to human promoters in LV, virally derived promoter sequences were strongly methylated during differentiation, independent of the transgene. Moreover, the commonly used SFFV promoter (SFFVp) was highly methylated with remarkable strength and frequency during hematopoietic differentiation in vivo in LV but less in γ-retroviral (γ-RV) backbones. In summary, we conclude that LV using an internal SFFVp are not suitable to generate transgenic mice or perform constitutive expression studies in differentiating cells. Choosing the appropriate promoter is also crucial to allow stable transgene expression in clinical gene therapy.

Stable long-term blood formation by stem cells in murine steady-state hematopoiesis

Hematopoietic stem cells (HSCs) generate all mature blood cells during the whole lifespan of an individual. However, the clonal contribution of individual HSC and progenitor cells in steady‐state hematopoiesis is poorly understood. To investigate the activity of HSCs under steady‐state conditions, murine HSC and progenitor cells were genetically marked in vivo by integrating lentiviral vectors (LVs) encoding green fluorescent protein (GFP). Hematopoietic contribution of individual marked clones was monitored by determination of lentiviral integration sites using highly sensitive linear amplification‐mediated‐polymerase chain reaction. A remarkably stable small proportion of hematopoietic cells expressed GFP in LV‐injected animals for up to 24 months, indicating stable marking of murine steady‐state hematopoiesis. Analysis of the lentiviral integration sites revealed that multiple hematopoietic clones with both myeloid and lymphoid differentiation potential contributed to long‐term hematopoiesis. In contrast to intrafemoral vector injection, intravenous administration of LV preferentially targeted short‐lived progenitor cells. Myelosuppressive treatment of mice prior to LV‐injection did not affect the marking efficiency. Our study represents the first continuous analysis of clonal behavior of genetically marked hematopoietic cells in an unmanipulated system, providing evidence that multiple clones are simultaneously active in murine steady‐state hematopoiesis. Stem Cells2012;30:1961–1970

Hematopoietic activity of human short-term repopulating cells in mobilized peripheral blood cell transplants is restricted to the first 5 months after transplantation

Kinetics of hematopoietic recovery driven by different types of human stem and progenitor cells after transplantation are not fully understood. Short-term repopulating cells (STRCs) dominate early hematopoiesis after transplantation. STRCs are highly enriched in adult mobilized peripheral blood compared with cord blood, but the length of their contribution to hematopoiesis remains unclear. To understand posttransplantation durability and lineage contribution of STRCs, we compared repopulation kinetics of mobilized peripheral blood (high STRC content) with cord blood transplants (low STRC content) in long-lived NOD.Cg-Prkdc(scid)Il2rg(tm1Wjl)/SzJ (IL2RG(-/-)) mice. This comparison demonstrates that quantitative contribution of human STRCs to hematopoiesis is restricted to the first 5 months after transplantation. The ratio of STRCs to long-term repopulating cells dramatically changes during ontogeny. This model enables to precisely determine early and late engraftment kinetics of defined human repopulating cell types and to preclinically assess the engraftment kinetics of engineered stem cell transplants.

10-Year stability of clinical-grade serum-free γ-retroviral vector-containing medium

More than 10 years ago, we developed an efficient protocol for serum-free retroviral transduction of human hematopoietic stem cells derived from mobilized peripheral blood. After upscaling of the methodology, serum-free retroviral gibbon-ape leukemia virus (GALV) pseudotype PG13/LN vector supernatant produced under strict good manufacturing practice (GMP) conditions was used in the first clinical gene-marking trial in Germany. In this study, we analyzed the titer and transduction efficiency of this serum-free clinical-grade retroviral supernatant 10 years after production to evaluate the long-term stability. Long-term storage and transport on dry ice resulted in modestly decreased titers and levels of transduction efficiency in CD34+ cells ranging from 38.4 to 49.1%. We conclude that the stability of retroviral vectors in serum-free medium allows extended storage and distribution of approved clinical-grade retroviral vector stocks to distant sites in multicenter clinical trials.

The Inherent Differentiation Program of Short-Term Hematopoietic Repopulating Cells Changes During Human Ontogeny

Human umbilical cord blood (CB) could be an attractive source of hematopoietic repopulating cells for clinical stem cell therapy because of its accessibility and low propensity for unwanted immune reaction against the host. However, CB recipients suffer from severely delayed and often chronically deficient platelet recovery of unknown cause. Here we show that human short-term repopulating cells (STRCs), which predominantly carry early hematopoietic reconstitution after transplantation, display an intrinsically fixed differentiation program in vivo that changes during ontogeny. Compared to adult sources of hematopoietic cells, CB myeloidrestricted STRC-M showed a markedly reduced megakaryocytic and erythroid cell output in the quantitative xenotransplantation of human short-term hematopoiesis in NOD/SCID-beta2m(-/-) mice. This output in vivo was not altered by pre-treating CB cells before transplantation with growth factors that effectively stimulate megakaryocytopoiesis in vitro. Moreover, injecting mice with granulocyte colony-stimulating factor did not affect the differentiation of human STRC. These findings demonstrate that the differentiation capacity of human STRCs is developmentally regulated by mechanisms inaccessible to currently available hematopoietic growth factors, and explain why thrombopoiesis is deficient in clinical CB transplantation.