Effect of erythropoietic stress on donor hematopoietic cell expression in chimeric rhesus monkeys transplanted in utero.

We have previously reported the successful development of hematopoietic chimerism after the in utero transplantation of fetal hematopoietic stem cells (HSC) in rhesus monkeys (Macaca mulatta). These animals exhibit sustained engraftment without immunosuppression or graft-versus-host disease (GVHD). To assess the functional response of the donor-derived erythropoietic population, we assayed the relative expression of donor and recipient hematopoietic progenitors in chimeric monkeys before and after anemic stress. Anemia in our chimeric animals resulted in increased erythropoietin (EPO) production comparable to controls. This was accompanied by changes in erythroid progenitor profiles, again similar to controls. Chimeric animals demonstrated normal reticulocytosis and reconstituted their hematocrit after hemorrhage at the same rate as controls. The donor-derived erythropoietic population exhibited normal responses to recipient regulatory signals and did not seem to expand at the expense of other hematopoietic lineages. Thus the proportions of engraftment for the myeloid and erythroid precursors in bone marrow and for blood lymphocytes remained stable. Our results demonstrate that the in utero transplantation of fetal HSC results in stable engraftment of donor erythropoietic progenitors, which appear to be fully integrated within the recipient's regulatory system. The abnormalities reported in the postnatal transplantation setting can then be attributed to immunologic reactions requiring conditioning myeloablative regimens. Fetal transplantation bypasses all these factors.