To compensate for this shortcoming and to confirm the human specificity of our ��2m staining, we employed human specific lineage specific antibodies throughout the study. Alternatively, we have also recently described an alternative murine xenograft model based on the ��-glucuronidase (GUSB) deficient NOD/SCID/MPSVII FTY720 solubility mouse strain[17,23]. The lack of GUSB expression by the host tissue similarly allows rapid and precise identification of engrafting human cells by staining for donor GUSB activity. Using the NOD/SCID/MPSVII model, we demonstrated multi-organ engraftment of human UCB-derived ALDHhiLin- cells 10-12 weeks post transplantation[11]. Both the present model and the NOD/SCID/MPSVII model are thus ideally suited for pre-clinical evaluation of prospective cell populations and application strategies in cell-based regenerative therapy.

We and others have previously shown that ALDHhiLin- cells have a superior hematopoietic repopulating potential in the BM and spleen of NOD/SCID and NOD/SCID ��2m null mice, as compared to CD34+ or ALDHloLin- cells [7-10]. ALDHloLin- cells are, as verified in the present study, indeed virtually devoid of long term repopulation potential. In addition, we have recently shown that ALDHhiLin- sorted cells from human BM contained populations of functionally primitive mesenchymal progenitor populations[26]. UCB, as used in the present study, is, however, known to contain lower numbers of mesenchymal progenitors in comparison to BM[17]. We cultured the cells overnight under conditions that promote retention of primitive hematopoietic phenotypes[17].

The present AMI xenotransplantation study thus predominantly reflects the regenerative potential of highly purified hematopoietic stem and progenitor cells. Gentry et al. have previously shown that ALDHhi sorted cells contain subsets of primitive stem and progenitor cells of non-hematopoietic lineages, including mesenchymal stem cells and endothelial progenitor cells[6]. Although we did not assess the proportion of these non-hematopoietic cells in the present study, due to the cell source and isolation and culture method, it is unlikely that they contributed to the observed results in a substantial way. We found no evidence of a direct contribution of the transplanted cells to regenerated infarcted tissue although down regulation of ��2m expression by the donor cells as discussed above may have rendered some donor-derived cells types undetectable by our present methods. Engrafting human cells were predominantly of a hematopoietic phenotype, although non-hematopoietic cells were also identified. AV-951 These CD45 negative cells rarely appeared in the infarcted tissue and it is therefore unlikely that they represent primitive cardiomyocytes.