Adult Human Stem Cells Exert Therapeutic Effects To Repair Damaged Tissues in Xenograft Systems through Secretion of Trophic Factors Rather Than Direct Incorporation and Expansion.

Human stem cells from adult sources have been shown, in our laboratory and others, to promote tissue repair. Different populations of stem cells have been shown to contribute to the regeneration of muscle, liver, heart, and vasculature, although the mechanisms by which they accomplish this are still not well understood. Stem cells are known, however, to secrete a variety of factors that have both paracrine and autocrine activities. One theory of tissue repair and regeneration by adult stem cells is that they home to hypoxic and/or inflamed areas, and release trophic factors that hasten endogenous repair. These secreted bioactive factors suppress the local immune system, enhance angiogenesis, inhibit fibrosis and apoptosis, and stimulate recruitment, retention, mitosis and differentiation of tissue-residing stem cells. These effects, which are referred to as trophic effects, are distinct from the direct differentiation of stem cells into the tissue to be regenerated. We tested human umbilical cord blood (UCB) derived CD34+, ALDH^hiLin⁻ and ALDH^loLin⁻ cells following transplantation to NOD/SCID or NOD/SCID/B2M null mice with experimentally induced acute myocardial infarction (AMI). We used combined nanoparticle labeling and whole organ fluorescent imaging to detect homing of the cells to multiple organs 48 hours post transplantation. Long term engraftment and the regenerative effect of cell treatment was assessed four weeks post transplant. There was superior homing of ALDH^hiLin⁻ cells to the site of injury, as compared to CD34⁺ or ALDH^loLin⁻ cells at 48 hours post transplantation. At four weeks post transplantation, ALDH^hiLin⁻ cells engrafted multiple organs, including the heart, liver and kidney, at higher frequencies than ALDH^loLin⁻ cells. We found no donor derived cardiomyocytes and only few endothelial cells of donor origin. However, there was a significant increase in the density of large caliber vessels in the central infarct zone of ALDH^hiLin⁻ cell-transplanted mice, as compared to PBS and ALDH^hiLin⁻ cell treated groups. Tissue staining in the damaged regions revealed that the transplanted human cells had undergone very few cell divisions after homing to the area of hypoxia or inflammation. It is important to note that, with over 300 mice now analyzed in tissue repair experiments in our group, no adult human stem cell-derived tumors have ever been observed, even though the mouse strains used have no capacity to reject the human cells. The paucity of human cells remaining in the tissue after repair suggest that the tissue improvements that were observed were not the result of generation of transplanted cell-derived endothelial cells or cardiac tissue, but suggest that cytokines secreted from transplanted cells potentiated angiogenic activity and tissue repair by the endogenous murine cells. Our data indicate that adult human stem cells do not become a significant part of the damaged tissue, but rapidly home to and persist only temporarily at a site of hypoxia or inflammation to exert significant trophic effects on tissue repair, and to enhance recovery of the vasculature.