By Merritt McKinney

NEW YORK, Mar 15 (Reuters Health) - Normally, patients undergoing heart transplantation must receive an organ from a donor with a compatible blood type. Otherwise, antibodies in the recipient's body mount a rapid and devastating attack on the transplanted organ.

But researchers in Canada report that this may not apply to newborns. Infants who have not yet developed these antibodies can safely receive hearts taken from donors whose blood type is incompatible.

Besides making hearts available to infants who might otherwise die on the waiting list, this approach, known as ABO-incompatible heart transplantation, will hopefully reduce the number of donated hearts that must be discarded because an appropriately matched donor cannot be found in time, according to the study's lead author.

In an interview with Reuters Health, Dr. Lori J. West, of the Hospital for Sick Children and the University of Toronto, explained that if the organ donor's blood type is not compatible with the recipient's, then antibodies called isohemagglutinins immediately latch on to the heart, leading to very rapid destruction, a process called hyperacute rejection.

Unfortunately, since donor hearts for infants have to be matched not only by blood type but also by size, from 30% to 50% of infants on heart-transplant waiting lists die before receiving a transplant, she said.

But since young infants have immature immune systems that have not yet produced antibodies to other blood types, West and her colleagues suspected that transplanting donor hearts that were ABO-incompatible might be safe. They reasoned that without these antibodies, the immune system would not begin hyperacute rejection.

The researchers' hunch was right, according to a report in the March 15th issue of The New England Journal of Medicine. None of 10 infants who underwent ABO-incompatible heart transplantation developed hyperacute rejection. And eight of the infants were still alive up to 4.6 years after surgery.

The researchers believe that the two deaths were due to causes unrelated to the ABO-incompatible transplant. However, one of the surviving infants did undergo a second heart transplant with an ABO-compatible organ.

The average age of the infants who had an ABO-incompatible transplant was 2 months, compared to 5.5 months in a comparison group of 10 infants who had a conventional transplant.

Since transplantation, the surviving children have developed normally, and they have been no more likely than children who received an ABO-compatible heart to develop episodes of ordinary rejection, which frequently occurs after a transplant. And while transplant recipients must take immune-suppressing drugs to prevent the rejection of the transplanted organ, none of the children who received an ABO-incompatible transplant has needed higher-than-usual doses of the medications, according to West.

So far, only two children have developed antibodies for the blood type of their donated hearts. But these antibodies have not caused any apparent damage to the children's hearts.

In the interview, West said that the death rate of infants younger than 6 months on the waiting list at the Hospital for Sick Children has dropped substantially, mostly as a result of the new transplant approach. Before the hospital started performing ABO-incompatible transplants, 58% of infants died before receiving a transplant, but the rate is just 7% now.

Not only does ABO-negative transplantation allow more infants to receive hearts, but it has the potential to reduce the number of donated hearts that cannot be used because a suitably-matched recipient cannot be found in time, West said.

What is also exciting about the finding, according to West, is that it suggests that there is a window of opportunity in early infancy when babies have a tolerance to accept unmatched organs. This window had been thought to close by the time of birth, she said.

The finding "opens up the avenues for us to continue to develop tools for the next round of children," West said.

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