In vitro immunogenicity of undifferentiated pluripotent stem cells (PSC) and derived lineages.

The observation that embryonic stem cells (ESCs) expressed reduced levels of major histocompatibility (MHC) class I genes, no MHC class II or costimulatory molecules suggested early on that pluripotent stem cells (PSCs) could be "immune-privileged" and were unable to induce immune reactions. However, soon it became apparent that in some instances, ESCs were recognized by immune cells but still could reduce an active and strong immune response. Similar results were obtained with other PSCs. Almost 10 years later, the exact mechanisms are still not well understood and seem to differ between the different human and rodent PSC lines (even between different murine cell lines). These differences could be due to differing experimental approaches, different derivation protocols (to obtain the PSC lines), species specificity, or genetic background of the cells lines. A better understanding of the immune regulatory mechanisms deployed by PSCs and early derivates may inform us on immune regulation and could be exploitable for regenerative medicine using allogeneic cells. As PSCs grow robustly in culture and can easily be gene-modified, one could envision the generation of cell lines that maintain these immune suppressive properties through terminal differentiation, thus generating universal donor cells.