Beta-glucuronidase (GUSB) is a lysosomal enzyme expressed in virtually all human and murine cell types. Transplantation of GUSB-expressing human hematopoietic stem cells (HSC) into the recently developed GUSB-deficient NOD/SCID/MPSVII mouse allows for the sensitive histochemical detection of xenotransplanted cells in hematopoietic and non-hematopoietic tissues. Most notably, human cells expressing GUSB can be detected without reliance on the expression of human cell surface markers. In addition, we have recently characterized a novel population of reconstituting HSC from human umbilical cord blood (UCB) by lineage depletion (Lin) followed by selection of cells with high aldehyde dehydrogenase (ALDH) activity. We have also demonstrated the robust hematopoietic engraftment of ALDHhiLin cells in the NOD/SCID model. Because donor cell surface protein expression may be altered in non-hematopoietic tissues due to cell fusion or other mechanisms, we have used the NOD/SCID/MPSVII model to study the tissue distribution of ALDHhiLin or ALDHloLin cells in multiple organs. Tail vein injection of 2x105–4x105 ALDHhiLin cells into sub-lethally irradiated (300cGy) NOD/SCID/MPSVII mice (n=3) demonstrated high levels of hematopoietic (human CD45+ cells) engraftment in the BM (70.5±15.1%), spleen (7.0±2.8%) and peripheral blood (17.0±10.7%). In contrast, injection of 2x105–106 ALDHloLin cells (n=3) did produce significant human cell engraftment in these hematopoietic tissues, with only 1 of 3 mice engrafting at 0.3% CD45+ cells in the murine BM. By using GUSB-specific histochemical staining, significant human engraftment was detected, with single cell sensitivity, in hematopoietic (BM and spleen) and non-hematopoietic (liver, pancreas, and lung) tissues of mice transplanted with ALDHhiLin cells. GUSB activity was colocalized with CD45 immunohistochemical staining in the BM, liver, and pancreas of mice transplanted with ALDHhiLin cells. Human cells were not detected in the muscle or brain of these engrafted mice. In contrast, mice transplanted with equal doses of ALDHloLin cells showed no GUSB expression in the BM, spleen, liver, pancreas, lung, muscle, and brain. We are currently developing a flow cytometric assay to enumerate the frequency of GUSB expressing cells in the tissues of the transplanted mice. These data indicate that ALDHhiLin UCB cells demonstrate previously uncharacterized engraftment in the liver, pancreas, and lung of NOD/SCID MPSVII mice. This novel model provides a unique opportunity to visualize donor human cells after xenotransplantation without reliance upon the expression of cell surface proteins, in situ hybridization, or cell labeling using viral transduction or membrane dyes.

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