Canine alpha-L-iduronidase (iduronidase) deficiency is a model of the human lysosomal storage disorder mucopolysaccharidosis type I (MPS I). We used this canine model to evaluate the therapeutic potential of hematopoietic stem cell (HSC) gene therapy for enzyme deficiencies. In previous studies, iduronidase-deficient dogs infused with autologous marrow cells genetically modified to express iduronidase had long-term engraftment with provirally marked cells, but there was no evidence of proviral iduronidase expression or clinical improvement. The presence of humoral and cellular immune responses against iduronidase apparently abrogated the therapeutic potential of HSC gene therapy in these experiments. To evaluate HSC gene therapy for canine MPS I in the absence of a confounding immune response, we have now performed in utero adoptive transfer of iduronidase-transduced MPS I marrow cells into preimmune fetal pups. In three separate experiments, 17 midgestation fetal pups were injected with 0.5-1.5 x 10(7) normal or MPS I allogeneic long-term marrow culture (LTMC) cells transduced with neo(r)- or iduronidase-containing retroviral vectors. Nine normal and three MPS I pups survived the neonatal period and demonstrated engraftment of provirally marked progenitors at levels of up to 12% for up to 12 months. However, the proportion of provirally marked circulating leukocytes was approximately 1%. Neither iduronidase enzyme nor proviral-specific transcripts were detected in blood or marrow leukocytes of any MPS I dog. Humoral immune responses to iduronidase were not detected in neonates, even after "boosting" with autologous iduronidase-transduced LTMC cells. All MPS I dogs died at 8-11 months of age from complications of MPS I disease with no evidence of amelioration of MPS I disease. Our results suggest that iduronidase-transduced primitive hematopoietic progenitors can engraft in fetal recipients, contribute to hematopoiesis, and induce immunologic nonresponsiveness to iduronidase in MPS I dogs. However, the therapeutic potential of HSC gene transfer in this model of iduronidase deficiency appears to be limited by poor maintenance of proviral iduronidase gene expression and relatively low levels of genetically corrected circulating leukocytes.