Lymphocyte Gene Therapy for Mild Hunter Syndrome 
2.4.1 Animal Models 
The response of potential gene therapy systems has been assessed, not only at the level of in 
vitro expression of functional gene products, but also the extent to which gene transfer and 
expression can bring about amelioration of symptoms of the disease in animal models. In animal 
models of human diseases, a variety of gene transfer systems have demonstrated correction of 
growth defects (42), reproductive dysfunction (43), thallassemia (44), neurological defects (45) and 
even glycosaminoglycan storage disease (46). The Watanabe rabbit has been successfully used as 
an animal model for gene therapy of familial hypercholesterolemia by retroviral-mediated LDL 
receptor gene transfer into hepatocytes of the liver (47), demonstrating a long-term reduction in 
serum cholesterol. A mouse strain bred to homozygosity for the CFTR gene (disrupted by 
homologous recombination) has also been used for gene transfer experiments targeting the lung 
epithelium. Chloride ion defects were corrected as a model for gene therapy of cystic fibrosis (48). 
In animal studies most closely paralleling the currently-proposed clinical trial, retroviral- 
mediated insertion of a C-glucuronidase gene into hematopoietic cells resulted in a significant 
reduction in liver and spleen pathology in mice affected with mucopolysaccharidosis type VII (or 
"Sly disease") (49). In combination with the past experience of bone marrow transplantation in 
human subjects (30. 50 V such animal studies illustrate the feasibility and physiologic responses 
which are to be anticipated in patients with mild Hunter syndrome receiving ex vivo lymphocyte 
gene therapy . 
2.4.2 Clinical Trials Of Gene Therapy 
Retroviral-mediated gene transfer was first initiated in humans in 1989 for the purpose of 
tracking cells infused for therapeutic purposes (51) Gene therapy, where the therapeutic purpose 
involved actual expression of the inserted gene, was first initiated for adenosine deaminase (ADA) 
deficiency in the fall of 1990 (52). There are currently 16 human gene therapy protocols which 
have been initiated where expression of the inserted gene may be of therapeutic benefit, 10 
protocols for treatment of cancer or AIDS and 6 protocols for treatment of inherited human 
disease including ADA deficiency, familial hypercholesterolemia, hemophilia B and cystic fibrosis 
(53). In addition, there are numerous human gene therapy protocols currently under regulatory 
review which may be initiated in the near future. 
2.4.3 Gene Transfer Into Lymphocytes 
Lymphocytes have been investigated as a target cell population for several human gene 
transfer studies (51). Although genetically corrected lymphocytes would have a limited duration 
of action, lymphocytes do provide a cell population in which gene transfer has now been proven 
feasible. Human tumor-infiltrating lymphocytes (TIL) were initially transduced ex vivo with 
recombinant retroviruses on a large scale (54) and then introduced into patients for tracking 
purposes. Vector sequences were detected in the blood stream for three weeks and longer (51). 
The marked TIL studies formed the basis for more recently-initiated clinical trials of ADA- 
virus transduced autologous peripheral blood lymphocytes (PBL) in the treatment of ADA 
deficiency (55). In this trial, leukocytes were obtained by apheresis from the affected patient and 
cultured with anti-CD3 monoclonal antibody and recombinant interleukin-2 (IL2) to stimulate T- 
lymphocyte growth and transduction with ADA-retrovirus (56, 57). Infusions were initiated in 
September, 1990 (52). Two patients demonstrated a persistence of the ADA gene-corrected T cells 
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