Genetically Corrected Autologous Stem Cells Engraft, But Host Immune Responses Limit Their Utility in Canine α-L-iduronidase Deficiency

Canine α-L-iduronidase (α-ID) deficiency, a model of the human storage disorder mucopolysaccharidosis type I (MPS I), is an ideal system in which to evaluate the clinical benefit of genetically corrected hematopoietic stem cells. We performed adoptive transfer of genetically corrected autologous hematopoietic cells in dogs with α-ID deficiency. Large volume marrow collections were performed on five α-ID–deficient dogs. Marrow mononuclear cells in long-term marrow cultures (LTMCs) were exposed on three occasions during 3 weeks of culture to retroviral vectors bearing the normal canine α-ID cDNA. Transduced LTMC cells from deficient dogs expressed enzymatically active α-ID at 10 to 200 times the levels seen in normal dogs. An average of 32% of LTMC-derived clonogenic hematopoietic cells were provirus positive by polymerase chain reaction and about half of these expressed α-ID. Approximately 107autologous gene-modified LTMC cells/kg were infused into nonmyeloablated recipients. Proviral DNA was detected in up to 10% of individual marrow-derived hematopoietic colonies and in 0.01% to 1% of blood and marrow leukocytes at up to 2 to 3 years postinfusion. Despite good evidence for engraftment of provirally marked cells, neither α-ID enzyme nor α-ID transcripts were detected in any dog. We evaluated immune responses against α-ID and transduced cells. Humoral responses to α-ID and serum components of the culture media (fetal bovine and horse sera and bovine serum albumin) were identified by enzyme-linked immunosorbent assay. Cellular immune responses to autologous α-ID but not neor transduced cells were demonstrated by lymphocyte proliferation assays. To abrogate potential immune phenomena, four affected dogs received posttransplant cyclosporine A. Whereas immune responses were dampened in these dogs, α-ID activity remained undetectable. In none of the dogs engrafted with genetically corrected cells was there evidence for clinical improvement. Our data suggest that, whereas the α-ID cDNA may be transferred and maintained in approximately 5% of hematopoietic progenitors, the potential of this approach appears limited by the levels of provirally derived enzyme that are expressed in vivo and by the host's response to cultured and transduced hematopoietic cells expressing foreign proteins.