In this report we demonstrated that undifferentiated hESCs

In this report, we demonstrated that undifferentiated hESCs expressed moderate levels of HLA-class Ia, no class Ib, and low levels of class II molecules on the cell surface. HLA class Ia processing and presentation are required for effective T cell recognition and impact graft rejection. This finding supported the idea that undifferentiated hESCs are immunogenic and subject to allorejection after transplantation into wild-type, fully HLA-mismatched recipients (Swijnenburg et al., 2005; Wu et al., 2008; Nussbaum et al., 2007). Furthermore, during EB differentiation, HLA class Ia and II molecule levels decreased significantly. This may be due to the decreased levels of β2m protein, as pluripotent stem dopamine receptor antagonist (Shef-1, NT2 and MSUH-002 cell lines) previously shown to express low to no β2M light chain also showed limited cell surface levels of trimeric HLA class I (Suarez-Alvarez et al.). Derivation of iPSC by nuclear reprogramming of patient-specific cells (Carvajal-Vergara et al., 2010; Narsinh et al., 2011) brought much enthusiasm for the possibility of patient specific cell therapy without immune rejection. However, the assumption that differentiated cells derived from patient specific iPSC can be transplanted in the same patient without immune rejection has been disputed by studies performed by Zhao et al. (2011). This study demonstrated that teratomas formed from iPSCs derived from C57BL/6 (B6) mouse are immunogenic in syngeneic B6 recipient. On the other hand, autologous ESC derived teratomas did not show any evidence of immune rejection in B6 mice. Furthermore, although initial studies indicated that human iPSCs are similar to hESCs (Guenther et al., 2010; Yu et al., 2007), recent studies indicate the presence of considerable differences with regard to in vitro differentiation potential (Feng et al., 2010), gene expression (Chin et al., 2009), and DNA methylation (Doi et al., 2009). Thus, generation of immune tolerant hESCs and their derivatives is critical for the advancement of allogenic transplantation. Our study demonstrated a proof of concept on engineering immune tolerant hESCs that could be used as a pluripotent universal cell source. Nonetheless, it remains to be determined whether HLA-G transgenic hESC and their epidermal derivatives are immune-tolerant in vivo. Humanized mice harboring human immune system cells (Brehm and Shultz, 2012) have recently been developed and may serve as useful tools to assess the immunogenicity of mHLA-G transgenic hESC and hEEP in vivo.
Acknowledgments Basil M. Hantash, MD, PhD was supported by a Stem Cell Transplantation Immunology Award (RM1-01711) from California Institute for Regenerative Medicine.
Introduction Human embryonic stem (ES) and induced pluripotent stem (iPS) cells are expected to revolutionise regenerative medicine and are potentially powerful tools for therapeutic drug screening (Inoue and Yamanaka, 2011; Rowntree and McNeish, 2010). Despite recent rapid advances in our ability to expand these cells as pluripotent cells (Ungrin et al., 2008) and direct their differentiation into a range of desired cell lineages (Keller, 2005), much is to be learned about the extrinsic and endogenous factors produced that control these processes, and how these are modulated by cell–cell contact signalling and other features of the microenvironment. The ability to genetically manipulate mouse ES cells and the generation of mouse models with constitutive or inducible tissue-specific, cell-specific or gene-specific fluorescent reporters has greatly accelerated our understanding of pluripotent stem cell biology. However, in the case of human pluripotent cells the development of such tools has been much slower, mainly because of the inefficient nature of homologous recombination in human pluripotent cells, the difficulty in obtaining clonal populations of cells, and their inherent genetic and epigenetic instability in culture. The derivation of transgenic, clonal, and karyotypically normal human pluripotent stem cell lines is therefore a non-trivial endeavour. Furthermore, there is the obvious inability to perform human blastocyst injection of geneticallytagged ES cells, an assay that is widely used in mice to exemplify the efficacy of ES and iPS cell reporters. Given the intrinsic differences between mouse and human ES cells in terms of molecular pathways maintaining pluripotency and inducing lineage-specific differentiation, and the need to isolate pure populations of lineage-specific progenitors (or the identification of specific cell surface markers thereof), a number of laboratories have undertaken the arduous task of genetically tagging promoters and genes of interest using either classical homologous recombination or zinc finger/TALEN-based genome editing techniques (Liu et al., 2011; Zou et al., 2009; Zwaka and Thomson, 2003). It is, however, becoming increasingly clear that human embryonic stem cell lines differ intrinsically in their ability to grow and respond to pluripotent cell culture conditions and also possess different biases in their propensity to undergo differentiation into specific cell types (Lepski, 2012). This is most evident when comparing human iPS cell lines derived from different cell types or generated through different methods (Alvarez et al., 2012). Understanding and harnessing these different propensities of human pluripotent cell lines, properties most likely influenced by both genetic background and epigenetic parameters, is one of the major current challenges in the stem cell field (Tobin and Kim, 2012).