Concerns for Existing Human Embryonic Stem Cells
by Susan Garfinkel, Ph.D., and Suzanne Kadereit, Ph.D.

Human embryonic stem cells (hESCs) are of great interest, largely for their potential to treat diseases. In trying to develop future therapies with hESCs, safety is a major issue. Thus, a number of scientists are now studying the cells to determine their precise properties and behaviors.

Many of the hESC lines that have been isolated to date have been grown in laboratories under defined conditions. These conditions include the use of a layer of mouse cells, called feeder layers, in the culture dishes with the hESCs. A major problem for the development of therapies has been the debate centered on the possible risk of contamination from these mouse cells. Recently, a report published by Martin, et.al. in the journal Nature Medicine (1) demonstrated that when hESCs were cultured in the presence of the animal cells, or serum from animals, they could incorporate foreign sugars into the proteins that are part of their cell surface. This makes the hESCs susceptible to immune attack.

The sugars, called sialic acids, are important components of many biological processes. In their report, the research group showed that a specific form of sialic acid, called Neu5Ac, was released by the mouse feeder cells and was also found in serum that was used in these cultures. Neu5Ac is produced only in animal cells and not in human cells. The group showed that the Neu5Ac in the culture could be taken up by the hESCs and could incorporate into the “glycoproteins” (compounds composed of both a protein and a carbohydrate) found on the cell surface.

Most healthy humans carry pre-existing natural antibodies that react with Neu5Ac to cause a rejection reaction. The effect is that upon transplantation of hESCs with Neu5Ac incorporated into their cell surface, the human immune system would see the hESCs as similar to animal cells, or as foreign, and ultimately cause the death of the cells. While it is not yet clear where the antibodies in humans come from, Martin et. al. showed that an immune reaction could occur following exposure of the cells to serum from adults with high levels of the antibody. The group also showed that with removal of the mouse feeder cells and replacement of the animal serum with normal human serum, the presence of the Neu5Ac gradually declined to very low levels.

These data re-ignite an ongoing debate about whether the current human embryonic stem cell lines approved for federal funding in the U.S. are suitable for clinical applications. These “NIH-approved” cells lines are all grown in the presence of mouse feeder layers. Thus, the findings in this report suggest that in order to avoid rejection, the hESC lines would have to be processed free of Neu5Ac in order to be suitable for clinical transplantation. More to the point, new human embryonic stem cell lines derived in the total absence of animal products would alleviate such problems and may speed up bringing hESC research into the clinical setting. The derivation of new cell lines would not only avoid the risks of contamination from animal pathogens, but would also be more immune compatible as they would expectedly not carry animal contaminants capable of inducing an immune response.

(1) Martin, et. al., Nature Medicine, 11(2):228-32, 2005.

Updated: February 17, 2005