Human embryonic stem cell lines currently available have been contaminated with a non-human molecule that endangers their potential therapeutic use, investigators at the University of California, San Diego (UCSD) said Sunday.

In a study published in the on-line version of the journal Nature Medicine, the researchers found that human embryonic stem cells, including those currently approved for study under US federal funding, contain a non-human, cell-surface sialic acid called N-glycolylneuraminic acid (Neu5Gc).

The investigators led by UCSD professor Ajit Varki determined that the Neu5Gc is incorporated by the stem cells when they are grown or derived from laboratory cultures that contain animal sources of the non-human Neu5Gc molecule. All traditional culture methods used to grow human embryonic stem cells contain animal-derived materials, including connective tissue cells from mice and fetal calf serum.

The human embryonic stem cells remained contaminated even when grown in special culture conditions with commercially available serum replacements, apparently because these are also derived from animal products, Varki said.

In the paper, the scientists said the human embryonic stem cells contaminated with Neu5Gc could easily be attacked by human antibodies, and thus rendering them useless as a potential therapeutic tool in humans.

The researchers used recently developed probes to detect the presence of Neu5Gc on the cell surface of human embryonic stem cells that had been grown in traditional culture conditions. They further confirmed the presence of Neu5Gc with a process called electrospray mass spectrometry.

They found the percentage of total sialic acids present as Neu5Gc in the embryonic stem cells varied from 2.5 to 10.5 percent.

In human embryonic stem cells that had been allowed to differentiate into embryoid bodies, which is the first step in preparing them for potential use in humans, the percentage of total sialic acids present still ranged from 5 to 17 percent, they said.

Varki and his team noted that many efforts have been made during the last few years to try to eliminate any animal-derived culture components in human stem cell culture. However, most specialized media used for growth and differentiation still contain materials from animal sources and are hence contaminated.

The researchers suggested the possibility of using cells from mice with a human-like defect in Neu5Gc production. Another possibility being attempted by groups in other parts of the world is to use human embryo-derived connective tissue cells in the culture.

A further solution might be a short-term culture in heat-inactivated serum from the actual patient who is going to receive the therapy, the scientists said.

However, it may still prove difficult to completely eliminate the Neu5Gc, because it has metabolically incorporated into the currently available human embryonic stem cell lines.

"Such issues will, of course, become irrelevant if complete elimination of Neu5Gc can be achieved by deriving new human embryonic stem cells that have never been exposed to Neu5Gc-containing animal products of any kind," the researchers said in the paper, noting that none of the suggested approaches guarantees the complete elimination of Neu5Gc from existing cultures.

Therefore, it seems best to start over again with newly derived human embryonic stem cells that have never been exposed to any animal products. But such an approach cannot be pursued under existing rules for US federal funding, they said.