Dogs are useful preclinical models for the translation of cell transplantation therapies from the bench to the bedside. In order for canine models to be utilized for stem cell transplantation research, it is necessary to advance discoveries in the fields of canine stem cell biology and transplantation. The use of side population hematopoietic stem cells (HSCs) has garnered much interest for the purification of mouse HSCs and has been translated to several other species, including human. In order to assess if this method of purification of HSCs could be useful for stem cell therapies in humans, safety and efficacy studies in a large animal model, such as the dog would be required. With this objective in mind, we isolated canine bone marrow-derived side population (SP) stem cells and assessed their multilineage differentiation in vitro and engraftment potential in vivo. Utilizing a pregating strategy to enrich for small, agranular SP cells we were able to enrich for blast cells, expressing the ABCG2 transmembrane pump known to be associated with murine and human SP cells. Canine SP cells were also enriched for C-KIT positive cells and lacked expression of CD34 as identified in other species. The small, agranular SP fraction had high CFU potential after long-term culture with canine bone marrow stromal cells and cytokine supplementation. Yet, canine SP cells demonstrated low-level engraftment within the NOD/SCID-β2m-/- xenotransplantation model as compared to unfractionated canine bone marrow, which was indicative of suboptimal activation of quiescent canine SP cells within the murine bone marrow niche. A second source of transplantable canine stem cells was examined through the derivation of canine embryonic stem cells (cESCs). The cESC lines described herein were determined to have similar pluripotent stem cell characteristics to human embryonic stem cells, in that they were maintained in an undifferentiated state upon extended passaging as determined by their expression of the human stem cell markers, OCT3/4, NANOG, SOX2, SSEA3, SSEA4, TRA1-60, TRA1-81 and alkaline phosphatase. In addition, cESCs could be induced to differentiate to cells of the three germ layers within in vitro embryoid body cultures and adherent differentiation cultures. Importantly, these cESC lines were the first reported to differentiate in vivo within teratomas. One method of transplanting stem cells to canine recipients involves the delivery of donor cells to the yolk sacs of developing fetuses in utero. Utilizing cells labeled with supraparamagnetic particles conjugated to a Dragon Green fluorophore and the intracellular fluorescent dye, CMTMR, donor cells were tracked from the yolk sac injection site to fetal tissues after transplantation in early (day-25) and mid (day-35) gestation canine fetuses. Labeled cells were localized primarily to the fetal liver and developing bone marrow cavities when examined at gestational day 32, and had been redistributed to not only the fetal liver and bone marrow by day 42, but also to nonhematopoietic tissues, including the lungs and hearts. No labeled cells were detected within the yolk sacs of transplanted fetuses at either time point. These studies demonstrated the efficacy of yolk sac in utero transplantation for the delivery of donor cells to fetal tissues. Collectively, these results indicate that canine stem cells with characteristics similar to human can be isolated and their engraftment, proliferation and differentiation may be assessed in future studies utilizing the canine in utero transplantation model employing yolk sac delivery.

Dogs are useful preclinical models for the translation of cell transplantation therapies from the bench to the bedside. In order for canine models to be utilized for stem cell transplantation research, it is necessary to advance discoveries in the fields of canine stem cell biology and transplantation. The use of side population hematopoietic stem cells (HSCs) has garnered much interest for the purification of mouse HSCs and has been translated to several other species, including human. In order to assess if this method of purification of HSCs could be useful for stem cell therapies in humans, safety and efficacy studies in a large animal model, such as the dog would be required. With this objective in mind, we isolated canine bone marrow-derived side population (SP) stem cells and assessed their multilineage differentiation in vitro and engraftment potential in vivo. Utilizing a pregating strategy to enrich for small, agranular SP cells we were able to enrich for blast cells, expressing the ABCG2 transmembrane pump known to be associated with murine and human SP cells. Canine SP cells were also enriched for C-KIT positive cells and lacked expression of CD34 as identified in other species. The small, agranular SP fraction had high CFU potential after long-term culture with canine bone marrow stromal cells and cytokine supplementation. Yet, canine SP cells demonstrated low-level engraftment within the NOD/SCID-β2m-/- xenotransplantation model as compared to unfractionated canine bone marrow, which was indicative of suboptimal activation of quiescent canine SP cells within the murine bone marrow niche. A second source of transplantable canine stem cells was examined through the derivation of canine embryonic stem cells (cESCs). The cESC lines described herein were determined to have similar pluripotent stem cell characteristics to human embryonic stem cells, in that they were maintained in an undifferentiated state upon extended passaging as determined by their expression of the human stem cell markers, OCT3/4, NANOG, SOX2, SSEA3, SSEA4, TRA1-60, TRA1-81 and alkaline phosphatase. In addition, cESCs could be induced to differentiate to cells of the three germ layers within in vitro embryoid body cultures and adherent differentiation cultures. Importantly, these cESC lines were the first reported to differentiate in vivo within teratomas. One method of transplanting stem cells to canine recipients involves the delivery of donor cells to the yolk sacs of developing fetuses in utero. Utilizing cells labeled with supraparamagnetic particles conjugated to a Dragon Green fluorophore and the intracellular fluorescent dye, CMTMR, donor cells were tracked from the yolk sac injection site to fetal tissues after transplantation in early (day-25) and mid (day-35) gestation canine fetuses. Labeled cells were localized primarily to the fetal liver and developing bone marrow cavities when examined at gestational day 32, and had been redistributed to not only the fetal liver and bone marrow by day 42, but also to nonhematopoietic tissues, including the lungs and hearts. No labeled cells were detected within the yolk sacs of transplanted fetuses at either time point. These studies demonstrated the efficacy of yolk sac in utero transplantation for the delivery of donor cells to fetal tissues. Collectively, these results indicate that canine stem cells with characteristics similar to human can be isolated and their engraftment, proliferation and differentiation may be assessed in future studies utilizing the canine in utero transplantation model employing yolk sac delivery.