Peter Kurre

Prolonged Adherence of Human Immunodeficiency Virus-Derived Vector Particles to Hematopoietic Target Cells Leads to Secondary Transduction In Vitro and In Vivo

ABSTRACT Human immunodeficiency virus type 1-derived lentivirus vectors bearing the vesicular stomatitis virus G (VSV-G) envelope glycoprotein demonstrate a wide host range and can stably transduce quiescent hematopoietic stem cells. In light of concerns about biosafety and potential germ line transmission, they have been used predominantly for ex vivo strategies, thought to ensure the removal of excess surface-bound particles and prevent in vivo dissemination. Studies presented here instead reveal prolonged particle adherence after ex vivo exposure, despite serial wash procedures, with subsequent transduction of secondary target cells in direct and transwell cocultures. We explored the critical parameters affecting particle retention and transfer and show that attachment to the cell surface selectively protects virus particles from serum complement-mediated inactivation. Moreover, studies with nonmyeloablated murine recipients show that transplantation of vector-exposed, washed hematopoietic cells results in systemic dissemination of functional VSV-G/lentivector particles. We demonstrate genetic marking by inadvertent transfer of vector particles and prolonged expression of transgene product in recipient tissues. Our findings have implications for biosafety, vector design, and cell biology research.

Envelope fusion protein binding studies in an inducible model of retrovirus receptor expression and in CD34(+) cells emphasize limited transduction at low receptor levels.

Successful gene therapy for the treatment of heritable or acquired diseases typically requires high efficiency gene transfer and sustained transgene expression. Indirect evidence on the basis of RNA analysis and in vivo competitive repopulation experiments in animal models suggests a correlation between transduction efficiency and the abundance of retrovirus receptors on the hematopoietic target cell. However, transduction by oncoretroviral vectors is also subject to other factors such as target cell cycle status and the composition of the virus-containing medium, making it difficult to determine the level of receptor expression required for efficient transduction. In the present study we investigated the impact of receptor expression level on transduction by a vector with a gibbon ape leukemia virus (GALV) envelope protein in a tetracycline-inducible tissue culture model that allowed for the cell cycle-independent, regulated expression of the GALV receptor (Pit1) in otherwise non-susceptible NIH 3T3 cells. Up-regulation of receptor RNA expression by 4.5-fold resulted in a mean 150-fold increase in transduction efficiency. We then analyzed cell surface expression of the Pit1 receptor using a fusion protein consisting of GALV SU portion of the viral envelope protein linked to the human IgG Fc. These experiments showed that tetracycline-regulated receptor induction resulted in a dose-dependent increase in binding of fusion protein. At maximum induction fusion protein binding increased up to five-fold which paralleled the increase in RNA expression, and correlated with the improved transduction efficiency. Finally, studies of pseudotype-specific fusion protein binding to human CD34-enriched cells revealed increased expression of retrovirus receptors after cytokine stimulation, although overall receptor expression in CD34+cells remained lower than in fibroblast cell lines efficiently transduced by amphotropic and GALV vectors.

Ex vivo selection for oncoretrovirally transduced green fluorescent protein-expressing CD34-enriched cells increases short-term engraftment of transduced cells in baboons

In an effort to improve hematopoietic stem cell gene transfer rates using gibbon ape leukemia virus (GALV)-pseudotype retroviral vectors in baboons, we have studied preselection of transduced green fluorescent protein (GFP)-expressing CD34-enriched marrow cells. Three animals were transplanted with GFP-selected (GS) CD34-enriched marrow. To ensure engraftment, preselected GFP-positive cells were infused together with unselected neo-transduced cells. After transduction on fibronectin, cells were cultured for an additional 2 days to allow for expression of GFP. GFP-expressing cells were enriched by fluorescence-activated cell sorting and infused together with cells from the unselected fractions after myeloablative irradiation of the recipient. Three other animals were transplanted with GFP-transduced CD34-enriched cells without prior GFP selection (GU). At 4 weeks after transplant, the percentage of GFP-expressing white blood cells was significantly higher in the GS group (6.6%) than in the GU group (1.3%) (p < 0.002). The higher gene transfer levels in the animals transplanted with GS cells gradually declined, and by day 100 after transplant, gene transfer levels were similar in both groups. PCR analysis performed on genomic DNA isolated from peripheral blood cells demonstrated that the decline in GFP-positive cells was due to the loss of gene-marked cells and not due to loss of expression. These results show that transplantation of CD34-positive marrow cells selected for GFP-positive cells after transduction provides high levels of transduced granulocytes in the short term. However, using this experimental design with concomitant infusion of unselected cells and the use of oncoretroviral vectors, preenrichment of vector-expressing, transduced CD34-enriched cells does not improve long-term persistence and expression.

Reduced toxicity and prompt engraftment after minimal conditioning of a patient with Fanconi anemia undergoing hematopoietic stem cell transplantation from an HLA-matched unrelated donor.

Given the profound sensitivity of patients with Fanconi anemia to conventional conditioning regimens before hematopoietic stem cell transplantation (HSCT), we developed a minimally toxic regimen consisting of 2 Gy total body irradiation, 90 mg/m2 fludarabine, and postgrafting immunosuppression with cyclosporine and mycophenolate to treat FA patients undergoing HSCT from HLA-matched unrelated donors. With over 10 months follow-up, our first patient has complete and sustained engraftment. Graft-versus-host disease was limited to mild skin and liver and moderate gut manifestations. We conclude that the approach is well tolerated and ideally suited to reduce regimen-related toxicities while achieving sustained engraftment.

In vivo administration of interferon γ does not cause marrow aplasia in mice with a targeted disruption of FANCC

OBJECTIVE Hematopoietic cells from patients with Fanconi anemia (FA) and mice carrying a targeted disruption of the gene encoding complementation group C protein (FANCC(-/-)) demonstrate an apoptotic phenotype in response to alkylating agents and cytokines including interferon gamma (IFN-gamma) in vitro. The aim of this study was to explore these apoptosis-inducing effects of IFN-gamma on the bone marrow of FANCC(-/-) mice as a potential strategy to select gene-corrected cells in vivo. Following pharmacokinetic studies to determine if serum concentrations effective in vitro can be achieved in vivo, we injected FANCC(-/-) mice with recombinant murine IFN-gamma. Hematopoietic effects were investigated by serial determinations of blood counts, progenitor colony formation, and marrow cellularity. RESULTS Serial weekly intraperitoneal administrations of escalating doses of rmIFN-gamma did not affect peripheral blood counts in FANCC(-/-) mice, even after subsequent antibody-mediated fas ligation. Additionally, prolonged exposure after sequential daily administration of recombinant IFN-gamma did not impair progenitor cell clonogenicity in vitro. Pharmacokinetic data confirmed that the failure of IFN-gamma to induce marrow aplasia occurred in spite of peak serum levels greater than 100-fold in excess of those effective in vitro. CONCLUSION We conclude that in spite of the well-documented in vitro apoptotic tendency of FA-phenotype hematopoietic cells, the in vivo administration of IFN-gamma with and without subsequent fas ligation does not induce bone marrow failure in FANCC(-/-) (129SvJ strain) mice. Additional selective pressure may be necessary to achieve targeted ablation of uncorrected, FA-phenotype, marrow cells.

Endogenous DNA Damage Leads to p53-Independent Deficits in Replicative Fitness in Fetal Murine Fancd2−/− Hematopoietic Stem and Progenitor Cells

Summary Our mechanistic understanding of Fanconi anemia (FA) pathway function in hematopoietic stem and progenitor cells (HSPCs) owes much to their role in experimentally induced DNA crosslink lesion repair. In bone marrow HSPCs, unresolved stress confers p53-dependent apoptosis and progressive cell attrition. The role of FA proteins during hematopoietic development, in the face of physiological replicative demand, remains elusive. Here, we reveal a fetal HSPC pool in Fancd2−/− mice with compromised clonogenicity and repopulation. Without experimental manipulation, fetal Fancd2−/− HSPCs spontaneously accumulate DNA strand breaks and RAD51 foci, associated with a broad transcriptional DNA-damage response, and constitutive activation of ATM as well as p38 stress kinase. Remarkably, the unresolved stress during rapid HSPC pool expansion does not trigger p53 activation and apoptosis; rather, it constrains proliferation. Collectively our studies point to a role for the FA pathway during hematopoietic development and provide a new model for studying the physiological function of FA proteins.

Concise Review: Adaptation of the Bone Marrow Stroma in Hematopoietic Malignancies: Current Concepts and Models

The bone marrow stroma maintains hematopoiesis and coordinately regulates regenerative responses through dynamic interactions with hematopoietic stem and progenitor cells. Recent studies indicate that stromal components in the bone marrow of leukemia patients undergo a process of successive adaptation that in turn exerts dramatic effects on the hematopoietic stem cell compartment and promotes leukemic drug resistance. Therefore, functional changes in discrete marrow stromal populations can be considered an aspect of leukemia biogenesis in that they create an aberrant, self‐reinforcing microenvironment. In this review, we will describe the current understanding of the remodeling of the hematopoietic stem cell niche following invasion by leukemia cells. We place emphasis on existing evidence of how mesenchymal stem cells and their progeny facilitate neoplastic growth and describe available models and analytical techniques to understand the conversion of the niche toward disease persistence. Stem Cells 2018;36:304–312

Transmissible ER stress reconfigures the AML bone marrow compartment

Successive adaptation of the bone marrow (BM) from homeostatic hematopoietic microenvironment to a self-reinforcing niche is an integral aspect of leukemogenesis. Yet, the cellular mechanisms underlying these functional alterations remain to be defined. Here, we found that AML incursion precipitates compartmental endoplasmic reticulum (ER) stress and an unfolded protein response (UPR) in both leukemia and stromal cells. We observed that extracellular vesicles (EV) transmit ER stress in vivo from the AML xenograft to BM stroma, whereby the upregulation of core UPR components drives subsequent osteolineage differentiation of mesenchymal stem cells (MSC). Finally, we show that the underlying mechanism involves quantitative incorporation and cell–cell transfer of Bone Morphogenic Protein 2 (BMP2), a potent osteogenic signal, by AML-EVs. Corroborative studies in AML patient samples support the translational relevance of AML-EVs as a platform for BMP trafficking and source of compartmental crosstalk. Transmissible ER stress was previously identified as a source of chemoresistance in solid tumor models, and this work reveals a role in remodeling the BM niche in AML.

Is hematopoietic stem cell homing deficient in Fanconi anemia

To the editor: We read with interest the article by Zhang and colleagues comparing progenitor colony formation, chimerism after xenotransplantation into immunodeficient mice, and adhesive properties of whole bone marrow (WBM) cells from Fanconi anemia (FA) patients and healthy donors.[1][1] We