NOD/SCID Repopulating Cells Contribute Only to Short-Term Repopulation in the Baboon.

The nonobese diabetic/severe combined immune deficiency (NOD/SCID) xenotransplantation model is widely used as an assay to study human candidate stem cells. However, in contrast to large animal or human studies, the NOD/SCID model is limited by the short life span of the recipient and the limited proliferative demand placed on transplanted cells and the inability to support differentiation into all hematopoietic lineages. We have previously performed a direct comparison in the repopulation ability of gene modified baboon CD34+ cells in an autologous transplantation versus a xenotransplant model in irradiated NOD/SCID mice. Baboon CD34-selected marrow cells were transduced with a gammaretrovirus vector and infused into irradiated baboons and NOD/SCID mice. Our results showed a significant decline in the percentage of transduced cells in the baboon 2–3 months following transplant, while the levels in the NOD/SCID mice remained high and stable up to 12 weeks. A limited integration site analysis could only detect 2 common retrovirus integration sites (RIS) in the NOD/SCID and monkey. Here we have extended this analysis and performed LAM-PCR and locus-specific PCR on 30 clones recovered from 12-week old NOD/SCID b2M mice reconstituted with transduced baboon CD34+ cells. We cloned the sequences of these RIS and generated specific primers for these sequences. We then performed PCR analysis on samples in the baboon to determine whether the clones were present in the baboon peripheral blood at different time points after transplantation. We analyzed samples early (2 to 6 weeks) and late (6 to 12 months) after transplantation. We were able to identify five common integrants in the baboon early after transplant (2 weeks), two of which could still be detected at 6 weeks. None of the clones present in the NOD/SCID b2M mice, however, was found in the baboon during the long-term follow-up (6 and 12 months post-transplant). These results confirm that repopulating cells in the NOD/SCID mouse model contribute only to short-term repopulation in a clinically relevant large animal model and suggest that distinct hematopoietic stem/progenitor cell populations are responsible for the hematopoietic reconstitution in NOD/SCID mice versus nonhuman primates.