The application of human embryonic stem (ES) cells in medicine and biology has an inherent reliance on understanding the starting cell population. Human ES cells differ from mouse ES cells and the specific embryonic origin of both cell types is unclear. Previous work suggested that mouse ES cells could only be obtained from the embryo before implantation in the uterus. Here we show that cell lines can be derived from the epiblast, a tissue of the post-implantation embryo that generates the embryo proper. These cells, which we refer to as EpiSCs (post-implantation epiblast-derived stem cells), express transcription factors known to regulate pluripotency, maintain their genomic integrity, and robustly differentiate into the major somatic cell types as well as primordial germ cells. The EpiSC lines are distinct from mouse ES cells in their epigenetic state and the signals controlling their differentiation. Furthermore, EpiSC and human ES cells share patterns of gene expression and signalling responses that normally function in the epiblast. These results show that epiblast cells can be maintained as stable cell lines and interrogated to understand how pluripotent cells generate distinct fates during early development.

By explanting tissues isolated microsurgically from implanting strain 129 mouse blastocysts individually on STO feeder cells we have established that embryonic stem (ES) cells originate from the epiblast (primitive ectoderm). Isolated early epiblasts yielded ES cell lines at a substantially higher frequency than intact blastocysts regardless of whether they were explanted whole or as strictly single-cell suspensions. When explanted from delayedimplanting 129 blastocysts, epiblasts gave lines consistently in 100% of cases. If primary embryonic fibroblasts rather than STO cells were used as feeders, germline-competent ES cell lines were obtained readily from epiblasts of delayedimplanting blastocysts of several hitherto refractory strains, particularly when recombinant leukemia inhibitory factor was included in the medium during the initial period of culture. Because lines were obtained from the nonpermissive CBA͞Ca strain at a rate of up to 56%, this approach to the derivation of germline-competent ES cell lines may not only prove generic for the mouse but also worth pursuing in other species of mammal.

Although human embryonic stem (ES) cells may one day provide a renewable source of tissues for cell replacement therapy (CRT), histoincompatibility remains a significant barrier to their clinical application. Current estimates suggest that surprisingly few cell lines may be required to facilitate rudimentary tissue matching. Nevertheless, the degree of disparity between donor and recipient that may prove acceptable, and the extent of matching that is therefore required, remain unknown. To address this issue using a mouse model of CRT, we have derived a panel of ES cell lines that differ from CBA/Ca recipients at defined genetic loci. Here, we show that even expression of minor histocompatibility (mH) antigens is sufficient to provoke acute rejection of tissues differentiated from ES cells. Nevertheless, despite their immunogenicity in vivo, transplantation tolerance may be readily established by using minimal host conditioning with nondepleting monoclonal antibodies specific for the T cell coreceptors, CD4 and CD8. This propensity for tolerance could be attributed to the paucity of professional antigen-presenting cells and the expression of transforming growth factor (TGF)-␤2. Together, these factors contribute to a state of acquired immune privilege that favors the polarization of infiltrating T cells toward a regulatory phenotype. Although the natural privileged status of ES cell-derived tissues is, therefore, insufficient to overcome even mH barriers, our findings suggest it may be harnessed effectively for the induction of dominant tolerance with minimal therapeutic intervention.cell replacement therapy ͉ acquired immune privilege ͉ regulatory T cell

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