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injury posttransplant and vascular and sinusoidal deposition of antibody was also seen on later biopsies. At autopsy the baboon livers showed extensive steatosis and biliary tract changes, which resembled the pathological lesions observed in ABO-incompatible liver transplants. Both livers failed to show evidence of severe cellular rejection. Thus, antibody-dependent mechanisms of graft rejection appear to play a significant role in the baboon-to-human species combination.

Despite the promising results with nonhuman primate-to-human xeno-transplantation, species such as the chimpanzee and baboon are unlikely to be widely used as donors. First, chimpanzees and other anthropoid apes, the species which are most closely related to humans, are endangered. The few that are made available for biomedical research are used in AIDS or hepatitis research. Old world monkeys (baboons, cynomologous monkeys or macaques, and vervet monkeys) still exist in the wild in fairly large numbers so are now most common in xenotransplant research. However, a growing segment of society is outwardly opposed to the use of primates in medical research. It is unclear whether an increase in the use of old world monkeys-an increase sufficient to solve the current organ shortage-would be acceptable to society.

Secondly, there is a theoretical risk that nonhuman primate viruses will be transmitted when their organs are transplanted into humans. Some of these viruses, though harmless to nonhuman primates, can be deadly to humans. The risk involved is unclear, but the development of virus-free primate colonies necessary to address these concerns would be an arduous task.

Thirdly, the limited size range of primate species such as the baboon may pose problems for appropriate donor-recipient size matching for certain organs, such as the heart and lungs.

In light of the many real and potential obstacles to use of nonhuman primate organs, focus has shifted to the use of more phylogenetically disparate species as donors.

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