Top of pageAbstract Mucopolysaccharidosis I (MPS I) is a lysosomal storage disease (LSD) due to deficient |[aacute]|-L-iduronidase (IDUA) activity. Although neonatal gene therapy with a retroviral vector (RV) expressing canine IDUA (cIDUA) resulted in stable expression in most mice and marked improvement in clinical manifestations, most patients with MPS I are not diagnosed at birth. It will therefore be necessary to determine if gene therapy can be effective in older animals. Adult MPS I mice were transiently treated with hepatocyte growth factor at 6 weeks of age to induce hepatocyte replication, which was followed by injection of hAAT-cIDUA-WPRE during the period of hepatocyte replication. hAAT-cIDUA-WPRE is an amphotropic gamma RV with the liver-specific human |[aacute]|1-antitrypsin promoter upstream of the canine IDUA cDNA, although expression can also derive from the long terminal repeat of the RV in non-hepatic cells. Although most of the IDUA made by hepatocytes is transported to the lysosome after modification with mannose 6-phosphate, some enzyme is secreted into blood where it can be taken up by other organs. Eight mice achieved serum IDUA activity of 97 |[plusmn]|19 U/ml at 1 week after transduction. However, activity fell to <1 U/ml at 3 weeks or later, which was associated with a cytotoxic T lymphocyte response. No anti-cIDUA antibodies were produced. A potent CTL response without an antibody response is different from our previous results with RVs expressing secreted coagulation proteins in adults, suggesting that lysosomal enzymes may be more likely to induce a CTL response than secreted proteins. In an attempt to block this CTL response to cIDUA, two mice were treated with 4 doses of human CTLA4-Ig of 25 mg/kg over 2 weeks starting just prior to gene transfer. CTLA4-Ig is an immunosuppressive agent that binds to CD80 and CD86 and prevents them from activating CD28, an important co-stimulatory molecule on the surface of lymphocytes. These mice achieved 104 |[plusmn]|24 U/ml of serum IDUA that was maintained longer, but fell to <10 U/ml at 3 months or later. This suggests that transient CTLA4-Ig can delay, but not prevent, a CTL response to cIDUA. The final experiment gave CTLA4-Ig for a prolonged period. Five mice received 25 mg/kg twice a week for a month, once a week for a month, and are currently receiving it once a month. All mice have maintained stable expression of cIDUA in serum at 94 |[plusmn]|10 U/ml for up to 4 months after gene transfer. These mice will be evaluated at 6.5 months after gene transfer (8 months of age) by echocardiography, electroretinogram, and auditory-evoked brainstem response to determine if gene transfer into adults can prevent the clinical manifestations of MPS I. These studies indicate that CTL responses can occur when using gene therapy to treat LSD. It is interesting to speculate that uptake of enzyme by antigen presenting cells via the mannose or mannose 6-phosphate receptor may potentiate antigen presentation by Class I MHC molecules, a hypothesis that is currently being tested. It may be necessary to modulate the immune system to achieve stable expression after gene transfer into older animals.
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Jul 1, 2005
Laurence D Fechter + 1
Open Access
