Transplanted Human Aldehyde Dehydrogenase Expressing Stem Cells Circulate and Engraft Nonhematopoietic Tissues.

Murine bone marrow (BM)-derived cells can engraft non-hematopoietic organs such as the heart, liver and pancreas, and induce cellular repair after tissue damage. Subsequent development of therapies using human cells has been delayed due to a lack of good in vivo models to carefully study tissue engraftment and repair. Transplantation of human hematopoietic stem cells (HSC) that express the enzyme glucuronidase-B (GUSB) into GUSB-deficient NOD/SCID/MPSVII mice allows the detection of xenotransplanted cells without reliance on the expression of human-specific cell surface markers or in situ hybridization (Hofling, Blood, 2003). In the current study, we utilized the NOD/SCID/MPSVII model to track the tissue distribution of transplanted human cells into hematopoietic and non-hematopoietic organs. We have characterized a novel population of reconstituting HSC from human umbilical cord blood (UCB) by lineage depletion (Lin-) and selection of cells with high aldehyde dehydrogenase (ALDHhi) activity (Hess, Blood, 2004). This population has enhanced long-term repopulating capacity in the NOD/SCID model when co-selected with CD133, a surface molecule expressed in progenitors from neural, endothelial, and hematopoietic lineages. Transplantation of 10⁵ ALDHhiLin- (n=10) or ALDHhiCD133+Lin- (n=6) cells produced robust engraftment of HLA ABC+/CD45+ cells in the BM (55.9±7.1%), spleen (6.5±2.1%) and peripheral blood (PB)(12.3±6.7%) of NOD/SCID/MPSVII mice. In contrast, transplantation of 10-fold greater ALDHloLin- (n=6) or ALDHhiCD133-Lin- (n=4) cells produced no hematopoietic engraftment. Human HLA ABC+/CD45- cells were also detected in the BM (2.5±1.1%) and circulating in the PB (3.9±2.1%) of highly engrafted mice transplanted with both ALDHhiLin- populations. By using a GUSB-specific histochemical assay, engraftment of human cells was detected with single-cell sensitivity in the liver, lung, pancreas, heart, kidney, eye, muscle, and other non-hematopoietic organs. NOD/SCID/MPSVII livers showed robust engraftment 8–12 weeks post-transplantation, containing small GUSBhi/CD45+ cells, as well as cells with hepatocyte-like morphology that did not co-express human CD45, and had intermediate GUSB expression. The presence of liver residing CD45- human cells was confirmed by flow cytometry using a fluorescent GUSB substrate, and were not observed after transplantation of control CD34-selected cells. NOD/SCID/MPSVII pancreata showed human (GUSBhi) cell engraftment 8-12 weeks post-transplantation in the peri-islet and ductal regions, a pattern previously reported after transplantation of eGFP+ murine BM cells (Hess, Nature Biotechnology, 2003). Thus, ALDHhiLin- and ALDHhiCD133+Lin- cells demonstrated a previously uncharacterized high level of engraftment in the tissues of the NOD/SCID/MPSVII mouse. These ALDHhiLin- populations contain or produce CD45- progeny that circulate and engraft non-hematopoietic organs. We are currently investigating the utilization of ALDHhiLin- cell populations for the induction of functional recovery after tissue-specific chemical injury, and after femoral artery ligation (ischemic injury). Due to their unique tissue seeding characteristics, ALDHhiLin- and ALDHhiCD133+Lin- cells could be highly useful for cellular therapy applications to repair damaged or diseased tissues.