Human Embryonic Stem Cell (hESC)-Derived Hematopoietic Elements Are Capable of Engrafting Primary as well as Secondary Fetal Sheep Recipients.

We used transplantation into 1⁰ and 2⁰ pre-immune fetal sheep recipients (55–65 days-old, term: 145 days) to evaluate the in vivo potential of hematopoietic elements derived from hESC. The in utero human/sheep xenograft model has proven valuable in assessing the in vivo hematopoietic activity of stem cells from a variety of fetal and post-natal human sources. Five transplant groups were established. Non-differentiated hESC were injected in one group. In the second and third group, embroid bodies differentiated for 8 days were injected whole or CD34+ cells were selected for injection. In the fourth and fifth group, hESC were differentiated on S17 mouse stroma layer and injected whole or CD34+ cells were selected for injection. The animals were allowed to complete gestation and be born. Bone marrow and peripheral blood samples were taken periodically up to over 12 months after injection, and PCR and flowcytometry was used to determine the presence of human DNA/blood cells in these samples. A total of 30 animals were analyzed. One primary recipient that was positive for human hematopoietic activity was sacrificed and whole bone marrow cells were transplanted into a secondary recipient. We analyzed the secondary recipient at 9 months post-injection by PCR and found it to be positive for human DNA in its peripheral blood and bone marrow. This animal was further challenged with human GM-CSF and human hematopoietic activity was noted by flowcytometry analyses of bone marrow and peripheral blood samples. Further, CD34+ cells enriched from its bone marrow were cultured in methylcellulose and human colonies were identified by PCR. We therefore conclude that hESC are capable of generating hematopoietic cells that engraft in 1° sheep recipients. These cells also fulfill the criteria for long-term engrafting hematopoietic stem cells as demonstrated by engraftment and differentiation in the 2⁰ recipient.