Nature Medicine will report today that the ALL of the human stem cell lines which are approved for federally funded research are contaminated with mouse material. The LA Times reports here.
Based on the LA Times report, it looks like what happened is the contamination came during the expansion of the stem cells, when the stem cells increase in numbers without losing their stem cell properties. During this phase, they do not live well in a petri dish, but need the support of "EMFs" or embryonic mouse fibroblasts, which secrete signals which keep the stem cells undifferentiated. There has been concern that the EMFs might contribute non-human features to the human stem cells. Indeed, the researchers report that the human stem cells incorporate a type of sugar which decorates the surfaces of mouse, but not human cells. Since the sugar is outside the cell's surface, it is "visible" to the immune system and could provoke an immune attack, were these cells to be put in to humans. For the moment, I don't quite understand if there has been a genetic transfer of a mouse glycosylase to the human cells, or if there are just mouse cells mixed in the prep.