Progressive childhood diseases should be treated before pathological sequelae are evident. A technically promising but underdeveloped approach experimentally and clinically is in utero transplantation. Two major causes for unsuccessful human in utero therapy were identified by animal studies. They are micro- rather than macro-chimerism following congenic and allogeneic transfers and allogeneic donor cell rejection initiated by unexpected and inexplicable immune complications. It is critical that we understand the mechanisms behind these failures and develop alternative approaches to correct them. This is the goal of our current proposal. We investigate stem cell biology and curative interventions in Mucopolysaccharidosis type VII (MPS VII) mice, a model that mimics human lysosomal storage diseases. MPS VII mice lack the enzyme beta glucuronidase (GUSB). This allows us to track and enumerate GUSB+ donor cells by histochemistry and confirm our results by flow cytometry, biochemistry, and pathogenesis. Disease symptoms are ameliorated by enzyme replacement. We compared repopulation in genetically myeloablated and non-ablated MPS VII fetal recipients of congenic fetal GUSB+ hematopoietic cells. Results indicate that the donor cells do not have a selective advantage as previously hypothesized in the rapidly growing nonablated fetuses. Surprisingly, donor cells do amplify modestly between birth and one week of age and are maintained at a low percentage long term. Donor cells undergo rapid proliferation within 24 hours in the genetically myeloablated fetus and increase thereafter until they completely repopulate the host. We predict that donor cell amplification is directly linked to levels of stem cell implantation. Following allogeneic transfers, there are reasons to believe that the dam and fetus supply immunologically competent cells to one another and that allogeneic donor cell expansion after birth occurs in a hostile environment. We hypothesize that significant allogeneic donor cell implantation and expansion can be effected in utero without serious immune consequences. Our aims are to: Determine if donor stem cell implantation levels in utero affect subsequent stem and differentiated cell amplification; Identify procedures that increase corrective donor cells in the fetus; Define the role of dam and fetal immune response in allogeneic in utero transplantation; and Block immune responses to allogeneic cells.