984. Liver Restricted Expression of Canine Iduronidase Does Not Prevent a CTL Response in Adult MPS I Mice after Retroviral Vector-Mediated Gene Therapy, but Transient Immunomodulation with CTLA4-Ig Combined with Anti-CD4 or Anti- CD40 Ligand Does

Top of pageAbstract Mucopolysaccharidosis I (MPS I) is due to deficient activity of the 70 KD lysosomal enzyme |[alpha]|-L-iduronidase (IDUA) and results in an inability to degrade glycosaminoglycans in many cells. Severe MPS I results in cardiac disease, skeletal deformities, hearing and vision abnormalities, and mental retardation. We have previously reported that neonatal gene therapy with a high dose of a retroviral vector (RV) expressing canine IDUA resulted in stable expression and marked clinical improvements. However, since most patients with MPS I are not diagnosed at birth, it is necessary to test if gene therapy can be effective in older mice. However, a CTL responseoccurred after gene therapy to adults using the RV designated hAAT-cIDUA-WPRE for which the long terminal repeat (LTR) or the liver-specific human |[alpha]|1-antitrypsin promoter (hAAT) are in the same orientation. Although the liver is the major site of expression with this vector, expression also occurs at early times in spleen cells of an unknown phenotype. We hypothesized that an RV with liver-specific expression might avoid expression in antigen-presenting cells, and thus be unable to induce a CTL response. An RV was developed that contained a cassette with the hAAT promoter, a splice site, the canine IDUA cDNA, and a polyadenylation site in the opposite orientation to the LTR. This vector expressed IDUA activity well in HepG2 (human hepatoma) cells, but not in NIH 3T3 cells. This vector was injected IV into adults after hepatocyte replication was induced with hepatocyte growth factor (HGF). Transduced cells can secrete IDUA into blood, and serum IDUA activity reflects the stability of expression. Although serum IDUA expression was high at 1 week, it fell to low levels at 1 month. These data suggest that liver-restricted expression cannot prevent a CTL response against this lysosomal enzyme. We speculate that APCs might take up enzyme with M6P, and present its peptides. An alternative approach to prolong expression was to inhibit the immune system at the time of transfer of 1|[times]|109 transducing units/kg of hAAT-cIDUA-WPRE. CTLA4-Ig is an immunosuppressive agent that binds to CD80 and CD86 and prevents them from activating CD28, anti-CD4 antibody can deplete T cells, and anti-CD40 ligand can block co-stimulation. Although short-term administration of any agent alone did not result in long-term expression, prolonged CTLA4-Ig led to long-term (6.5 month) expression. Alternatively, 2 weeks of CTLA4-Ig combined with 2 days of anti-CD4 or with 2 days of anti-CD40 ligand led to long term expression. Mice that received adult gene transfer with successful immunomodulation had an average of 88+/|[minus]|11 units (U)/ml of IDUA activity in serum. Bone mineral density and bone radiographs were normal (N=6), the a-wave of an electroretinograms was significantly improved (N=4), and 3 of 4 mice had improved echocardiograms. However, hearing was not improved (N=3). We conclude that adult gene therapy with short term immunomodulation can reduce some manifestations of disease in MPS I mice.