ALL the embryonic stem (ES) cells grown to date to treat human diseases have been contaminated with animal products and cannot be used.

Research shows that existing cell “lines” contain a foreign molecule, so tissue derived from them would be unsuitable for transplants because it might provoke a serious immune reaction.

The research indicates that new methods of creating ES cells, which have the potential to form any kind of tissue, will have to be developed before they can be used to treat conditions such as diabetes and Parkinson’s disease.

It also puts pressure on the Bush Administration to ease its restrictive rules on ES cell work. At present it allows federal funds to support research only on lines created before 2001.

ES cells are taken from human embryos in the early stages of development, and hold great promise for medicine as replacements for the tissues that go wrong in many diseases because they have not yet differentiated into a specialised cell type.

Several dozen lines of ES cells have been generated around the world, including some by teams in Britain, but producing them involves the use of animal tissue. The ES cells are grown on a scaffold of “feeder cells” derived from mouse embryos, which secrete unidentified proteins that stop them from differentiating. They are also treated with a serum from foetal calves, which promotes their development.The new research, from scientists at the University of California, San Diego (UCSD), and the Salk Institute in La Jolla, California, has shown that either or both of these processes leave an indelible mark on ES cells.

All current lines contain a molecule called Neu5Gc, which is produced by animal cells, rendering them useless for transplant because humans have antibodies to it that would attack any grafted tissue.

Professor Ajit Varki, of UCSD, said: “The human embryonic stem cells remained contaminated by Neu5Gc even when grown in special culture conditions with commercially available serum replacements, apparently because these are also derived from animal products.”

The results, published today in the journal Nature Medicine, add to concern that ES cells grown using animal products will not be safe for transplantation. Many experts were already worried that this procedure might transmit animal diseases to people.

Developing methods to grow ES cells without animal tissue, however, will create problems in the US because of the 2001 cell-line restriction — imposed to appease religious conservatives who object to the destruction of human embryos.

Given that the existing lines are contaminated, Professor Varki said: “It would seem best to start over again with newly derived human embryonic stem cells that have never been exposed to any animal products, and ideally only ever exposed to serum from the intended transplant recipient.”

Stephen Minger, of King’s College London, who created Britain’s first ES cell line, said: “My perspective has always been that, when we are serious about getting ES cell therapies into the clinic, we will have to derive new lines completely free from animal conditioning.”

LINE OF HOPE

1998: James Thompson of the University of Wisconsin and John Gearhart of Johns Hopkins University create first cell lines from human embryonic stem cells

2001: the US limits research into embryonic stem cells to lines that already exist. Britain gives clearance for research, subject to defined rules

2002: insulin-producing cells grown from adult stem cells

2003: scientists at King’s College London produce their own stem cells line from embryos left over after fertility treatment

2004: South Korean scientists produce cloned human embryos and extract embryonic stem cells from them

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