Stem cell lines from human embryos, which are grown with animal-derived cells, have become contaminated with a product that human cells are genetically unable to make, according to a new report—meaning that these contaminated cells could be dangerous if used in people.

All human embryonic stem cell lines currently approved for study under federal funding in the US have been grown on or derived from a so-called feeder layer of mouse cells.

These stem cells grown on mouse feeder cells and in culture media containing animal products become contaminated through a “Trojan horse” type of mechanism, investigator Dr. Ajit Varki, of the University of California, San Diego, told AMN Health.

The cells take up a sialic acid called N-glycolylneuraminic acid from the animal cells (Neu5Gc), and Varki and his colleagues report in the journal Nature Medicine that approximately six to ten percent of total sialic acids in human stem cells are made up of Neu5Gc.

If these stem cells are used in humans, “there is a significant chance of a deleterious immune reaction and/or rejection of the transplanted cells,” Varki said.

Attempts at growing contaminated cells on animal-free feeder systems and in heat-inactivated human serum have failed to remove all traces of Neu5Gc.

“Either more needs to be done to figure out how to eliminate this problem or we have to start over with new stem cell lines” that are never exposed to animal-derived products - an option that “generates a lot of political and policy concerns” - the researcher said.

SOURCE: Nature Medicine, online January 23, 2005.