For successful gene transfer to primitive hematopoietic cells several requirements need to be achieved. These include identification of the desired target cell population, identification of the appropriate vector to be used, and achieving desired levels of gene expression. To date, successful gene transfer in human subjects remain problematic. To address these problems as well as important safety issues, studies in non-human primates are being undertaken to optimize gene transfer to nonhuman primate hematopoietic cells prior to human clinical studies. Vectors that have been evaluated include self-inactivating (SIN) retroviral vectors and adeno-associated viral vectors. These vectors have been constructed to express reporter genes, such as the enhanced green fluorescent protein (EGFP), or therapeutic genes, such as drug resistance genes, erythropoietin. Transduction conditions employed the RGD-containing fibronectin fragment, RetroNectin (CH-296) and a variety of recombinant hematopoietic growth factors, such as stem cell factor (SCF), interleukin-6, megakaryocyte growth and differentiation factor (MGDF or thrombopoietin) and the human Flt-3 (fms-like tyrosine kinase) ligand in either serum containing or serum free media. Viral vectors evaluated include adeno-associated virus (AAV) vectors, and third generation chimeric human immunodeficiency virus type-1 (HIV-1)-based lentiviral vectors. Third generation lentiviral vectors were pseudotyped with the vesicular stomatitis virus G-protein. Our efforts over the past year have resulted in publications evaluating novel chimeric vectors optimized to transduce rhesus CD34+ cells, the safety and efficacy of small molecules such as prostaglandin E2 and AMD3100 in the transplant model, to better define the molecular mechanism behind G-CSF administration and its impact on neutrophil adherence, to effectively mobilize both conventional and regulatory T lymphocytes with the bicyclam AMD3100, to identify a monoclonal antibody which specifically recognizes rhesus red blood cells, to develop a safe and efficacious pharmacological means to eradicate neutralizing antibody following AAV gene transfer, and to develop a safe and efficacious means to ablate gene marked cells using novel suicide gene technology. Efforts continue to be made to improve the level of gene marking and to derive stem cells from other tissues besides BM and cytokine mobilized PB, such as adult mesenchymal stem/progenitor cells and induced pluripotential stem cells. Despite these successes, questions remain. How can consistent high levels of expression be obtained using therapeutic genes? Can other stem cells either derived from bone marrow or other easily accessible tissues be targeted to assist in either the contribution or repair of other organs? How to best to evaluate stem cells and their progeny therapeutically? Future studies are aimed to evaluate therapeutic vectors, improve hematopoietic stem cell recovery and transduction efficiency, further delineate the nature and clonality of populations contributing to the reconstitution using genetic tracking methodologies, and to isolate or induce and characterize primitive cell populations which may contribute to organogenesis or the repair of damaged tissues.
Showing the most recent 10 out of 39 publications
