Download Xenotransplantation

Xenotransplantation
Georg Kallert
February 14, 2002
Xenotransplantation
• Transplanting animal organs and tissues into
human
• “Excitement”
– Provide organs to all the patients who need
them
• “Controversy”
– Infectious agents from the donor animals
might be passed to the organ recipients and
the population at large
Transplant Organs
• More than 75,000 Americans are waiting to
receive human organs (est. 180,000 globally),
but less than 1 in 3 will get one
• Some countries have allowed the sale of organs
from executed prisoners
• Scientists have been experimenting for many
years with animal transplants, especially from
primates
• Globally, the market for transplant organs is
estimated at $6 billion
Transplant Organs from Pigs
• Scientists are now focused on using pigs as a
source for transplant organs
• Why pigs?
– available in large numbers
– their organs are similar in size and nature to
those of humans (especially miniature pigs)
Recent News
• Two firms that are researching organ transplants
from pigs have each announced that they cloned
female miniature pigs without a specific gene
that causes immune rejection in humans
• Important milestone: first pigs to be cloned with
a removed gene, as cloning pigs with added
genes has been done in the past
• The pigs are missing one of the two genes
needed for making an enzyme for sugar
production
Knockout Pigs
• Piglets without one gene for -1,3-galactosyl transferase,
yet since they have the other gene, they are still
producing this enzyme
-1,3-galactosyl transferase
• This enzyme enzyme catalyses the synthesis
the sugar, -1,3-galactosyl, or alpha-gal
• This sugar is on the pig cell’s surface and is
easily detected as foreign by human antibodies
• If the pig cells with alpha-gal were inserted in the
human body they would be killed immediately
How was the Gene removed?
• Used a "gene trap" vector, a piece of DNA
containing snippets complementary to the target
gene
• Vector was moved to Nucleus using electricity
• Odds of a successful insert were 1 in 5 million
• The modified fetal cells were fused with oocytes
whose chromosomes had been removed
• Implanted the embryos into sows that had just
come into heat
Remaining Obstacles
• Produce a male litter with one gene and mate
them to with the female litter to get piglets with
no genes for -1,3-galactosyl transferase
– Takes up to 18 months, not guaranteed
– Pigs without any gene for the enzyme may
die, but mice with the double knockout did not
• Treat any further immunes responses
• And prevent the spread of viruses across
species from transplants, xenozoonosis
Immune Responses
• Hyperacute rejection (in minutes)
– Destroys blood vessels in organ, cuts off oxygen
– Alpha-gal is the main cause, but other sugars exist
• Delayed xenograft rejection (in days)
– Build-up of antibodies and killer cells in the organ
– Little data to determine effect of absence of alpha-gal
• T-cell-mediated chronic rejection (months-years)
– Presence of alpha-gal does not effect this barrier
– Medicine used in human transplants would not work
– Inject donor pig’s thymus cells develop tolerance
PERV
• Porcine endogenous retroviruses are present in the
genome of all pigs
• They could spread to donors and then all humans
• Researchers state their cloned pigs consistently test
negative for transmission of PERV to human cells in vitro
• Cultured human cells can be infected by PERVs
released from cultured pig cells
• But, humans receiving pig tissues or blood plasma have
never been affected
• Three possible explanations for this protection
Defense #1
• PERVs lack the “equipment” to infect human
cells in vivo
• However, immunodeficient mice have been
infected by PERVs in experiments with
transplanted islet cells
– Believe that rodent viruses and retroviral
elements may have combined with PERV to
allow infection
– This could happen with humans
Defense #2
• Human cells lack the proper receptor for binding
or the machinery to allow PERVs to replicate
– If this is the case, then PERVs will pose little
or no threat
Defense #3
• The body’s immune system, T-4 cells and
antibodies
• Even when immune system is suppressed for
transplant, PERVs could still be eliminated
• Experiment to try: do PERVs infect mice with
functional immune systems
Role of the
Antibodies
• As the virus envelope is
made of cell membrane it
would also contain the
alpha-gal sugar
• Human antibodies would
attack it just as they
attack a pig cell
• However, altered pigs
could yield the virus
without alpha-gal, and
the antibodies may not
detect it
Ethics
• Should we be tampering with animals?
• Should we be putting animal parts in humans?
• Do the researcher’s cloned pigs really have no
PERV DNA?
• Is it worth exposing the entire human population
to dangerous viruses?
• Should xenotransplantation be a “last resort” or
the “best option?”
• What are humans with animal parts?
Polling Data
• In a 1998 study, more than 75% of Americans
would consider a xenotransplant for a loved one
if no human organs were available
• However 75% of Americans admit to knowing
very little about xenotransplantation
• Opposition is greatest among the most informed
• Most worried about compatibility, not viruses
• Preferred primate donors, as opposed to pigs
• Spiritual leader approval was important
• Europe: only 36% saw it as morally acceptable
Conclusions
• By removing the gene for the enzyme an
important hurdle has been crossed
• Immunological barriers still remain
• Virus risks are not yet fully understood
• Even though there is ban on non-human primate
transplants, pig clinical trials may be allowed
• So human trials may be just four years away
Bibliography
Butler, Declan. “Poll reveals backing for xenotransplants.” Nature 391,
315 (1998).
-----. “Xenotransplant experts express caution over knockout piglets.”
Nature 415, 103 - 104 (2002).
Kaiser, Jocelyn. “Cloned Pigs May Help Overcome Rejection.” Science
295, Issue 5552, 25 (2002)
L. Lai et al. “Production of -1,3-Galactosyltransferase Knockout Pigs
by Nuclear Transfer Cloning.” Science 295, Issue 5557, 1089-1092
(2002)
Platt, Jeffrey L. “Xenotransplantation: New risks, new gains.” Nature
407, 27 - 30 (2000).
Stolberg, Sheryl Gay. “Breakthrough in Pig Cloning Could Aid Organ
Transplants.” New York Times, Jan. 4 (2002)
Q&A