Organ Transplantation - Biotechnology and Your Health: Pharmaceutical Applications - page 128

the risk of infection. However, researchers have recently discov-
ered that pigs harbor in their genome the sequence for several
RNA viruses, which, when activated, may infect humans and
cause disease.
Another problem with using pigs as organ donors is the very
vigorous immune attack that would have to be blunted. Human
blood normally contains antibodies to sugar molecules present on
the surfaces of pigs' cells. If the antibodies latched onto the cells that
line the blood vessels of the pig organ, hyperacute rejection would
occur. In addition, several types of destructive lymphocytes are
poised to attack organs and tissues from species that are as different
from us as pigs are. Some researchers have tried to genetically engi-
neer pigs to reduce the antibody problem. Techniques have included
putting human proteins on the surface of the pig cells that prevent
activation of complement proteins or disabling the pig gene for the
enzyme that puts the antibody-targeted sugar on the cell surface. To
blunt the cell-based attack, several researchers have developed
methods to replace some of the immune system cells in the poten-
tial organ recipient with pig immune-system cells in an effort to
make the recipient better tolerate the pig tissue. This approach has
succeeded in experiments with pig-to-monkey transplants.
Whether these maneuvers will prevent hyperacute or quick cell-
based rejection in humans is unknown, though experiments with
pig cells have been encouraging.
A number of small biotechnology companies and academic
research teams are working on these problems. However, worries
about the pig RNA viruses have cooled enthusiasm for xenotrans-
plantation. Although some laboratory work continues, there appear
to be no immediate human xenotransplant studies on the horizon.
Some researchers have, however, proposed using pig liver cells in a
device kept outside the body to help remove toxins from the blood
of patients whose liver has failed, to help the patients survive while
they wait for a human liver transplant.