Human mesenchymal stem cell engraftment in the chimeric sheep model

The human-sheep xenograft model is a system that can, be used for studies of engraftment of stem cells in a mammal that is physiologically similar to humans. In studies of engraftment of human mesenchymal stem cells (hMSC), this model has been used to demonstrate hMSC engraft in a wide range of tissues, including bone, liver, and kidney; engrafted human cells have been shown persist in these tissues. Engraftment of transplanted hMSC was examined in the fetal and adult heart of the chimeric sheep; we demonstrate that hMSC engraft in the fetal heart, predominantly in the Purkinje fiber (PF) system. To further understand these engraftment events, the timeline of engraftment events directly after transplantation was explored. Engraftment events were also investigated in the organs of the chimeric fetal sheep. We demonstrate that engraftment in the heart occurs at ∼40 hours after transplantation, and engraftment within other organs occurs at or before ∼40 hours; engrafted hMSC express tissue-specific proteins, suggesting transdifferentiation occurs after engraftment.; In our first studies, adult and fetal human mesenchymal stem cells were transplanted into sheep fetuses. Hearts at late fetal development were analyzed for engraftment of human cells. The majority of the engrafted cells formed Purkinje fibers containing areas of exclusively human cells. There were no differences in engraftment of hMSC from adult bone marrow, fetal brain and fetal liver. In a similar study, unrestricted somatic stem cells (isolated from human cord blood) transplanted into fetal sheep showed engraftment in the adult heart, persisting several months.; To further characterize result from our previous studies, we investigated the initial time course of engraftment of hMSC in fetal sheep heart. hMSC engrafted between 29 and 39 hours after injection; in this window the engrafted cells differentiated, and by 45 hours were found to transdifferentiate into cardiac cells. To characterize engraftment events, homing was investigated by labeling two groups of hMSC with two dyes with different spectral characteristics. No aggregates contained cells labeled with both dyes suggesting each aggregate of human PF cells in the fetal sheep heart is derived from a single hMSC engraftment event. From the sheep used in the time course experiments, the brain, liver, lung, pancreas, and thymus were examined at 40, 60, and 120 hours after injection. hMSC engraftment was demonstrated in all these organs, engrafting and transdifferentiating to express tissue specific markers within the time course examined.