Replacement hearts, lungs, kidneys and livers may some daybe as available as pork chops, ham hocks and bacon. Thatsurreal scenario was examined this week by 355 transplantsurgeons, immunologists and biotechnologists at the SecondInternational Congress on Xenotransplantation. The event,which is sponsored by The Transplantation Society, openedMonday at Cambridge University in England.
"Xeno" in Greek means "strange, foreign, different,"xenotransplantation aptly describes grafting of replacementorgans from non-human animals into human recipients. Most ofthe speakers at the three-day conference lamented the criticalshortage of donor organs, which renders the quest for graftsfrom lower life forms urgent. Their prime candidate is thedomesticated hog, Sus scrofa.
Speaking Monday evening and Tuesday, three molecularbiologists from DNX BioTherapeutics Inc. of Princeton, N.J.,reported on their development of transgenic mice and pigs tomake Sus scrofa a viable candidate organ donor for Homosapiens. David Kooyman, Lisa Diamond and Guerard Byrnereported achieving erythroid expression of the human CD59mini-gene and transfer in vascular endothelial cells of therodents and swine.
CD59 is the final facet of the strategy being pursued by DNXand other leading xenograft researchers to overcome theswine's strategy for rendering its organs ungraftable. This isthe human complement system, first-line enforcer of immunedefenses against foreign invaders.
A porcine heart or liver transplanted into a human recipient,for example, is totally destroyed within minutes or hours afterbeing hooked up to the patient's blood supply. Its cells areperforated and killed by a molecular membrane-attackcomplex mounted by the human complement system and calledinto action by the body's pre-formed anti-pig naturalantibodies. This instant destruction is called the hyperacuterejection to set it apart from the longer-term and less lethal Tcell-mediated rejection of human-to-human donor organs. It iscustomarily controlled by life-long immunosuppressivemedication.
"Complement is the least appreciated arm of the immunesystem," said DNX's Holtzman. "You and I have complement inus; why aren't we attacking ourselves?" He answers, "Becausewe have present on the endothelial cell surfaces inhibitionfactors that normally keep the complement cascade in check."
Hence the strategy of creating transgenic pigs carrying humancomplement inhibition factors to stop the human recipient'scomplement system from punching the porcine organ full ofholes. In practical terms, scientists in the U.S., Europe andAustralia are seeking to transfer into transgenic swine thegenes for a number of complement inhibition factors to bluntthe anti-organ attack at several intervals, early and late.
This transgenic game plan faces two hurdles: (1) integration ofthe human genes into the pig's genome; (2) expression of thecomplement blocking inhibitor proteins at a high enough leveland wall to wall across the surface of the porcine donor organ'sendothelial-cell vasculature.
The three DNX papers presented at the conference reportedthat transgenic mice, bearing genes encoding both early (CD55)and late (CD59) complement suppressers, did just that in invitro tests. "To our knowledge," Holtzman said, "that is the firsttime the hypothesis has been proven."
They demonstrated both erythroid expression and directfunctional transfer of both the early and late inhibitors ontothe endothelial cells themselves.
DNX also reported having made transgenic pigs with similargene constructs. "But we haven't done the test that showsinhibition," Holtzman added, "because we don't want to take theorgans from the pigs. First, you raise a single founder animal tosexual maturity, then breed the heck out of him and gethundreds of progeny." He pointed out that the FDA will requirethat for all transgenic pigs in commerce, all the organs shouldideally trace back to a single founder.
DNX is far advanced in dealing with the FDA over one pig donororgan, namely blood. It has a transgenic herd of whiteYorkshire/Landrace cross-breeds producing cell-free humanhemoglobin for use in transfusion and is progressing towardclinical trials.
"We already have 25 percent expression in our pigs," DNX's vicepresident of research and preclinical development, John Logan,told BioWorld, "and expect to file for an IND late in 1994."Meanwhile, he added, "DNX is looking for a corporate partner tohelp fund and develop this extremely expensive and complexundertaking."
DNX also holds a broad basic DNA microinjection patent. "Theonly way proven successful in making transgenic livestock isby DNX microinjection," said Holtzman. The company haslicensed 20-odd pharmaceutical and biotechnology companiesunder this patent but expressly retains its exclusive use forpigs in particular and xenotransplantation in general.
PIGGYBACKING ORGAN TRANSPLANTS
As of the end of July, 31,868 critically ill men, women andchildren were on the U.S. national waiting list for organtransplantation. Of these, 23,948 awaited a kidney, 2,827 aheart, 2,735 a liver, 1,124 a lung, 856 a kidney plus a pancreas,149 a pancreas alone, 198 heart and lung, and 31 an intestine.
Of all of these organ applicants, just over half -- 16,676 -- werein the productive prime of life, age 19 to 45.
Round numbers cited by Steven Holtzman, president of DNXBioTherapeutics Inc., show that actual organ transplantsperformed in 1992, reflecting the number of organs available,totaled 16,050 -- helping only half of the patients in waiting.
"But the important thing to understand about the nationalwaiting list is that it's held artificially low," Holtzman toldBioWorld.
"Since you are rate-limited by the number of organs available,"he said, "there's no point in putting any more than twice thatnumber on the list. That would just raise unreasonable andfalse expectations."
Holtzman estimated that if organs were accessible on an as-needed basis, the number of people who might benefit fromtransplants would total from 100,000 to 400,000 a year.
The more so, he added, "as we have an increasing agingpopulation; we're going to need another source of organs. Andthat leads to animals -- xenotransplants. The first source thatcomes to mind is the higher primates, baboons andchimpanzees."
Although these are close to humans in size and genetic makeup,they breed poorly, slowly and expensively in captivity. Also,their very similarity to Homo sapiens means that they andtheir donor organs are host to a range of pathogenic virusesthat also affect humans.
"Finally, people have a number of ethical concerns about theuse of primates," Holzman said. "So you ask: What animal is,one, plentiful; two, genetically distinct, (to avoid the host-virusproblem); and lastly, not subject to the same degree of ethicalconcern? Answer: the pig." -- David N. Leff
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.