In utero transplantation of stem cells in cynomolgus monkeys

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To achieve human embryonic stem (ES) cell-based transplantation therapies, allogeneic transplantation models of non-human primates would be useful. We have prepared cynomolgus ES cells genetically marked with the green fluorescent protein (GFP). The cells were transplanted into the allogeneic fetus in utero, taking advantage of the fact that the fetus is immunologically too immature to induce immune responses to transplanted cells and that fetal tissue compartments are rapidly expanding and thus providing space for the engraftment. Cynomolgus ES cells were genetically modified to express the GFP gene using a simian immunodeficiency viral vector or electoroporation. These cells were transplanted in utero with ultrasound guidance into the cynomolgus fetus in the abdominal cavity (n=2) or liver (n=2) at the end of the first trimester. Three fetuses were delivered 1 month posttransplantation, and the other, 3 months posttransplantation. Fetal tissues were examined for transplanted cell progeny by quantitative PCR and in situ PCR of the GFP sequence. A fluorescent tumor, apparently derived from transplanted ES cells, was found in the thoracic cavity at 3 months posttransplantation in one fetus. However, transplanted cell progeny were also detected (〜1%) without teratomas in multiple fetal tissues. The cells were solitary and indistinguishable from surrounding host cells. Transplanted cynomolgus ES cells can be engrafted in allogeneic fetuses. The cells will, however, form a tumor if they "leak" into an improper space such as the thoracic cavity.