Prolonged adherence of human immunodeficiency virus-derived vector particles to hematopoietic target cells leads to secondary transduction in vitro and in vivo

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.

RabGEF1 regulates stem cell factor/c-Kit-mediated signaling events and biological responses in mast cells

We recently reported that RabGEF1 is a negative regulator of high-affinity Fc receptor for IgE (Fc epsilonRI)-dependent mast cell activation and that mice lacking RabGEF1 develop severe skin inflammation and increased numbers of dermal mast cells. To better understand how RabGEF1 can regulate signaling events and biological responses in mast cells, we examined the responses of bone marrow-derived cultured mast cells (BMCMCs) from wild-type (+/+) and Rabgef1 knockout (-/-) mice after stimulation with the c-Kit ligand, stem cell factor (SCF), an important regulator of mast cell development, survival, proliferation, and activation. We found that RabGEF1-deficient mast cells exhibited enhanced and prolonged activation of Ras and extracellular regulated kinase, and significantly elevated IL-6 secretion, after stimulation with SCF. SCF-induced activation of c-Jun N-terminal kinase was increased in Rabgef1-/- BMCMCs, but without corresponding significant increases in SCF-induced migration or adhesion. SCF-mediated activation of the survival-enhancing kinase, Akt, also was increased in Rabgef1-/- BMCMCs, and these cells had a survival advantage over their +/+ counterparts in vitro. Despite enhanced Ras activation in the absence of RabGEF1, SCF-induced proliferation was lower in Rabgef1-/- BMCMCs compared with their +/+ counterparts. Finally, we found that c-Kit internalization was delayed in the absence of RabGEF1, probably reflecting a positive role for RabGEF1 in the regulation of endocytic events, and that infection of Rabgef1-/- BMCMCs with a wild-type RabGEF1 lentiviral construct normalized c-Kit internalization to the levels seen in +/+ BMCMCs. Thus, RabGEF1 plays a critical role in the regulation of SCF/c-Kit-mediated signaling events and biological responses in mast cells.

Direct evidence for ex vivo expansion of human hematopoietic stem cells

To characterize human hematopoietic stem cells (HSCs), xenotransplantation techniques such as the severe combined immunodeficiency (SCID) mouse repopulating cell (SRC) assay have proven the most reliable methods thus far. While SRC quantification by limiting dilution analysis (LDA) is the gold standard for measuring in vitro expansion of human HSCs, LDA is a statistical method and does not directly establish that a single HSC has self-renewed in vitro. This would require a direct clonal method and has not been done. By using lentiviral gene marking and direct intra-bone marrow injection of cultured CD34+ CB cells, we demonstrate here the first direct evidence for self-renewal of individual SRC clones in vitro. Of 74 clones analyzed, 20 clones (27%) divided and repopulated in more than 2 mice after serum-free and stroma-dependent culture. Some of the clones were secondary transplantable. This indicates symmetric self-renewal divisions in vitro. On the other hand, 54 clones (73%) present in only 1 mouse may result from asymmetric divisions in vitro. Our data demonstrate that current ex vivo expansion conditions result in reliable stem cell expansion and the clonal tracking we have employed is the only reliable method that can be used in the development of clinically appropriate expansion methods.

Robust in vivo gene transfer into adult mammalian neural stem cells by lentiviral vectors

Stable genetic modification of adult stem cells is fundamental for both developmental studies and therapeutic purposes. Using in vivo marking studies, we showed that injection of lentiviral vectors (LVs) into the subventricular zone of the adult mouse brain enables efficient gene transfer into long-term self-renewing neural precursors and steady, robust vector expression in their neuronal progeny throughout the subventricular zone and its rostral extension, up to the olfactory bulb. By clonal and population analysis in culture, we proved that in vivo-marked neural precursors display self-renewal and multipotency, two essential characteristics of neural stem cells (NSCs). Thus, LVs efficiently target long-term repopulating adult NSCs, and the effect of the initial transduction is amplified by the continuous generation of NSC-derived, transduced progeny. LVs may thus allow novel studies on NSCs' physiology in vivo, and introduction of therapeutic genes into NSCs may allow the development of novel approaches for untreatable CNS diseases.

Cell therapy: filling the gap between basic science and clinical trials October 15-17, 2001, Rome, Italy

Summarized here, and in forthcoming issues of, are the concepts that emerged at a recent international workshop on cell therapy organized by The Istituto Superiore di Sanità in Rome in collaboration with Istituto Dermatopatico dell'Immacolata, Rome; Istituto Nazionale Ricerca Cancro-Centro Biotecnologie Avanzate, Genova; and University G. D'Annunzio, Chieti. The meeting intent was to provide an overview of the most recent developments in cell therapy, the future perspectives for these clinical trials, and the regulatory issues they involve, as well as a progress report on the clinical protocols that have been approved up to now in Italy. The meeting included six scientific sessions (Immunotherapy, Epithelium, Osteoregeneration, Hematopoiesis, Future Perspectives, and Overview of the National and International Regulations) and involved lectures from Italian and foreign scientists.