maturity in the opinion of the parents and Pis. All patients 
12 years and older will give assent. 
Recently published work suggests the feasibility of in vivo 
HS-tk gene transfer for the treatment of malignant astrocytomas 
(5) . The central nervous system has several advantages of safety 
and efficacy for retroviral mediated in vivo gene transfer. First, 
retroviral vectors only integrate and therefore express vector 
genes proliferating cells. In the brain, the tumor contains the 
most mitotically active cells, with only macrophage-derived cells, 
blood cells and endothelial cells at minimal risk of transduction. 
In humans, all mitotic activity leading to the generation of 
neurons has ceased by birth, with the exception of the external 
granular cell layer of the cerebellum. By 2 years of age, all such 
mitoses are complete. Therefore, the possibility of specific 
transduction of the tumor is enhanced. Second, the brain is a 
partially immunologic privileged site, which may allow a somewhat 
longer survival of the xenogeneic murine cells in the brain and a 
greater transduction frequency of the growing tumor. This feature 
is further increased since human gliomas are known to further 
depress local immunity. This is thought to be secondary to a down 
regulation of IL-2 secretion and diminished expression of high 
affinity IL-2 receptors on T-lymphocytes (6). The murine cells 
should survive sufficiently long in the brain to allow for 
transduction of greater numbers of tumor cells. However, this 
period of survival will be limited since all cells that integrate 
and express HS-tk will be destroyed by the GCV or eventually 
destroyed by the immune system. 
Retroviral-mediated Gene Transfer (7, 8) 
In contrast to chemical and physical cellular transfection 
methods, murine retroviral vectors have proven to be extremely 
efficient for gene transfer into mammalian cells, with efficiencies 
as high as 90% in cultured murine fibroblast cell lines. Murine 
retroviral vectors differ from the adenoviruses and herpes simplex 
viruses in that the retroviruses will only integrate and 
subsequently express their genes in proliferating tissues (e.g. 
tumors) . This feature of the retroviral vectors may be 
particularly advantageous in the brain, where the tumor is the 
predominant mitotic cell type, maximizing specific transduction of 
tumor with minimal, or absent, transduction of normal brain. The 
Moloney murine leukemia virus-based (MoMLV) vectors have been 
designed to minimize the possibility of recombination resulting in 
regeneration of a replication-competent virus (9,10). 
In 1989, the first gene transfer experiment in humans was 
conducted at the NIH. This study involved the treatment of 10 
patients with autologous T-cells (TIL) that have been transduced 
with a retroviral-vector. None of these patient have demonstrated 
any untoward effects secondary to receiving the genetically- 
altered cells (11). 
Human gene therapy for adenosine deaminase deficiency has been 
performed on 2 children since the protocol opened in September, 
1990. Twenty-three intravenous infusions of genetically altered 
autologous T-cells have been administered to date (12, 13). 
Recombinant DNA Research, Volume 18 
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