T lymphocytes as targets of gene transfer with Moloney-type retroviral vectors

High-titre retroviral vector system for efficient gene delivery into human and mouse cells of haematopoietic and lymphocytic lineages

Genetically modified cells of haematopoietic and lymphocytic lineages could provide potentially curative treatments for a wide range of inherited and acquired diseases. However, this application is limited in mouse models by the low efficiency of lentiviral vectors. To facilitate the rapid production of high-titre helper-free retroviral vectors for enhanced gene delivery, multiple modifications to a prototype moloney murine leukemia virus (MoMLV)-derived vector system were made including adaptation of the vector system to simian virus 40 ori/T antigen-mediated episomal replication in packaging cells, replacement of the MoMLV 5' U3 promoter with a series of stronger composite promoters and addition of an extra polyadenylation signal downstream of the 3' long terminal repeat. These modifications enhanced vector production by 2-3 logs. High-titre vector stocks were tested for their ability to infect a variety of cells derived from humans and mice, including primary monocyte-derived macrophage cultures. Whilst the lentiviral vector was significantly restricted at the integration level, the MoMLV-based vector showed effective gene transduction of mouse cells. This high-titre retroviral vector system represents a useful tool for efficient gene delivery into human and mouse haematopoietic and lymphocytic cells, with particular application in mice as a small animal model for novel gene therapy tests.

Improving the ex vivo retroviral-mediated suicide-gene transfer process in T lymphocytes to preserve immune function

The retroviral-mediated transfer of a suicide gene into donor T cells has been proposed as a method to control alloreactivity after hematopoietic stem cell (HSC) transplantation. Gene-modified cells (GMC) may be infused into the patient either at the time of transplantation, together with a T-cell depleted HSC graft, or after transplantation, as a donor lymphocyte infusion. Administration of a so-called pro-drug activating the "suicide" mechanism only after occurrence of GvHD should selectively destroy the alloreactive GMC in vivo, eventually leading to GvHD abrogation. Although phase I-II clinical trials provided vital proof of the principle of GvHD control by suicide-gene therapy, this approach is still suboptimal. Indeed, current gene transfer strategies rely on gamma-retroviral vectors that require extensive T-cell activation and expansion for efficient transduction. Both in vitro and in vivo studies have shown that the activation, cell expansion, transduction and selection steps lead to TCR repertoire alterations and impairment of crucial T-cell functions, such as alloreactivity and anti-EBV reactivity. Thus, improvements of the suicide-gene transfer processes are required in order to preserve T-cell function. This could be achieved by using CD3/CD28 co-stimulation and immunomagnetic selection of transduced cells. In future clinical trials, lentiviral vectors may prove to be a better alternative to gamma-retroviral-mediated gene transfer, by reducing the need for prolonged ex vivo culture.

Stem cell clonality and genotoxicity in hematopoietic cells: Gene activation side effects should be avoidable

Two serious adverse events involving activation of the LMO2 oncogene through retrovirus vector insertion in the otherwise extremely successful first gene therapy trial for X-linked severe combined immunodeficieny type 1 (SCID-X1) had initially caused widespread concern in the patient and research communities. Careful consideration 1 year after diagnosis of the second case still finds 12 of the treated patients clearly benefiting from gene therapy (freedom from treatment failure, 80%; survival 100%), a situation that should not portend the end of gene therapy for this disease, and is, in fact encouraging. While current approaches are justified to treat patients with otherwise life-threatening disorders, a broad consensus has developed that systematic basic research is required to further understand the pathophysiology of these serious adverse events and to provide new insights, enabling safer and more effective gene therapy strategies. With the continued success of SCID-X1 gene therapy in the majority of patients treated, it is of even greater importance to understand exactly which vector element or combination of elements predispose to toxicity. An in-depth study of the mechanisms behind the activation of the LMO2 and gammac genes will be highly instructive for the development of safer procedures and vectors. We summarize the central observations, ongoing experimental approaches, new concepts, and developments relevant to understanding, interpreting, and eventually overcoming the real and perceived obstacles posed by insertional mutagenesis due to gene transfer vectors.

A novel 'sort-suicide' fusion gene vector for T cell manipulation

Retroviral suicide gene vectors have successfully been used in clinical studies to improve the safety of adoptive immunotherapy with allogeneic T lymphocytes in the treatment of malignant and viral diseases. At the same time these studies have revealed several problems that are yet to be resolved including impaired T cell function due to long ex vivo culture. Here we present new retroviral vectors co-expressing truncated CD34, a gene transfer marker which ensures rapid enrichment of transduced cells using commercially available GMP-approved devices, and a splice-corrected variant of Herpes simplex virus thymidine kinase (scHSVtk) which confers high sensitivity to the prodrug ganciclovir. We show that a retroviral hybrid vector, MP71, based on the myeloproliferative sarcoma virus (MPSV) and the murine embryonic stem cell virus (MESV), encoding a tCD34/scHSVtk fusion protein mediates high expression of the 'sort-suicide' selection marker, thereby allowing for highly efficient purification and selective elimination of transduced cells.

Highly efficient retroviral gene transfer based on centrifugation-mediated vector preloading of tissue culture vessels

Efficient retroviral gene transfer into primary cells is a prerequisite for various gene therapeutic strategies. We have developed a transduction protocol based on the preloading of tissue culture vessels with retroviral particles by low-speed (1000g) centrifugation. We show that vector-preloaded tissue culture vessels allow highly efficient gene transfer into various target cells. We obtained transduction rates of up to 85% for primary T lymphocytes after just a single round of transduction. Under clinically relevant conditions using a vector developed for suicide gene therapy and produced under good manufacturing practice (GMP) conditions, the described method allowed generation of large numbers (>2x10(9)) of gene-modified T cells. The preloading concept ensures transduction of target cells in their optimal growth medium regardless of the medium used for vector production. This facilitated highly efficient gene transfer into quite different target cells such as CD34(+) and AC133(+) bone marrow progenitor as well as mesenchymal stem cells. The presented method combines high gene-transfer rates with a great potential for standardization in accordance with GMP guidelines and is consequently well suited for both research and clinical applications. (c)2002 Elsevier Science (USA).