BioEd Online

Just 150 different strains of stem cell would be enough to create a source of donor cells for a significant proportion of the UK population, say British researchers. Their calculation, made by studying how kidney donors match transplant recipients, shows that this relatively modest number of different strains could cater for some 40% of people.

Stem cells, which have the potential to turn into any other cell type in the body, could one day be used to regenerate diseased tissue or organs. They are often harvested from embryos discarded in IVF treatments. Although stem cells can be created using a patient's own DNA, at present these cells tend to develop abnormally.

A quicker route to stem-cell therapy would be to pick cells from laboratory cultures, or lines, that match the patient's own cells. This minimizes the chance of rejection by the recipient's immune system, and reduces the need for drugs.

Kidney comparison

To assess how many such lines a stem-cell bank would need to offer matching cells to a significant proportion of the population, the team studied the matches between organ donors and people on a kidney transplant waiting list. Transplanted stem cells would face the same biochemical hurdles as a kidney, such as ensuring the right blood and tissue types.

The researchers used these two groups to create an imaginary stem-cell bank, with the organ donors used as a substitute for stem-cell lines, and the people on the kidney waiting list playing the role of the general population. Donors were added at random to the bank, and their tissue types and blood group were used to see how many people in the 6,557 people on the waiting list they matched with.

As more donors were added, the number of matches increased sharply. But only up to a point. "When you get to 150 or so you get diminishing returns," says Andrew Bradley, a transplant surgeon at Addenbrookes hospital in Cambridge, who took part in the work. "New donors after that don't provide any extra benefits." Stopping at 150 provided an excellent match for just under 20% of people, and a reasonable match for 40%. The team publishes the findings in The Lancet¹.

Although 40% of the population might sound modest, increasing the stem-cell bank's potential coverage above this figure will require a very large number of new donors, the authors explain. The stocking of 150 stem-cell lines therefore seems to represent an optimum return on the investment, they calculate.

Super-donors

"This is an excellent advance," says Justin St John, who works on reproductive genetics at the University of Birmingham, UK. "This work provides a starting point to stock our stem-cell banks for future clinical use."

The UK already has a stem-cell bank with 24 stem-cell lines for research. St John adds that this research gives doctors the information they need to offer stem-cell therapy to the greatest number of people in the future, and that a surprisingly low number of donors is needed.

Some kidney donors were compatible with a broad range of recipients, the team found - presumably the same goes for some stem-cell lines. Their compatibility is governed by proteins, called human leukocyte antigens (HLAs), found on the outside of their cells and used by the immune system to identify foreign bodies.

Choosing just ten donors whose cells bear the commonest HLAs would provide a beneficial match for the same number of people as 150 random donors, the scientists found. But because stem cells can only be obtained from discarded IVF embryos, scientists must effectively work with what they are given, meaning that they cannot selectively screen for these common variants.

Roger Pedersen, a stem-cell researcher at University of Cambridge, and a member of the study team, calls for an international effort to ensure that all ethnic groups can find stem-cell matches from such banks. "This study should be repeated elsewhere, using this one as a template," he says.