Appendix L. 
363 
host. This approach has been tested in a variety of small and large 
animal settings and currently available data suggest that stable 
engraftment of donor bone marrow reliably renders the host tolerant 
to donor antigens and subsequently to any cellular or solid organ 
graft of the same donor. 
The two underlying mechanisms by which creation of a mixed- 
chimeric host results in tolerance induction are (1) thymic deletion of 
potentially donor-specific alloreactive T cells, and (2) nonthymic 
peripheral mechanisms, such as blocking costimulatory T cell 
activation, which facilitate the process of donor bone-marrow or stem 
cell engraftment. However, despite the efficacy of an approach using 
fully HLA-mis matched stem cells in an allogeneic host to induce 
tolerance to a subsequent organ allograft, the host is placed at a high 
risk of substantial morbidity and mortality due to toxicity of the 
myeloablative conditioning regimen and potential for graft-versus- 
host disease, or immune-mediated attack of the host by the 
implanted allogeneic stem cells. 
In an attempt to overcome these potential limiting toxicities, 
investigators have suggested the use of either adult bone marrow- 
derived mesenchymal stem cells or preimplantation-derived 
embryonic stem (ES) cells for induction of mixed chimerism. The 
theoretical advantages of these cell types is their low level of surface 
expression of HLA class I and II antigens, and reduced surface 
expression of co-stimulatory molecules important for T cell activation. 
Rat preimplantation stage derived embryonic-like stem cells have 
been shown to successfully engraft in the recipient bone marrow 
without the need for pre-conditioning therapies such as irradiation, 
cytotoxic drug regimens or T cell depletion. Long-term partial mixed 
chimerism by use of rat preimplantation stage derived embryonic - 
like stem cells did not trigger graft- versus -host reactions, in contrast 
to the high frequency of this complication in the clinical setting of 
allogeneic hematopoietic stem cell transplantation. Of most interest, 
the induced partial chimerism enabled the recipient animals to be 
tolerant to a subsequent heart allograft. Allograft acceptance 
required the presence of an intact thymus, and rat ES cells were 
present in the recipient thymus. 
Similar results have been reported following transplantation of 
human adult bone marrow-derived mesenchymal stem cells (MSG) 
into fetal sheep early in gestation, before and after the expected 
development of immunologic competence. In this xenogeneic system, 
human MSG engrafted, differentiated in a site-specific manner, and 
persisted in multiple tissues for as long as 13 months after 
transplantation, including the thymus. Since MSGs do not present 
alloantigen and do not require MHG expression to exert their 
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