In a well-stocked zoo, the monkey house exhibits cage after cage ofOld World primates _ ranging from baboons, chimpanzees,orangutans, and gorillas to rhesus monkeys _ plus New Worldmarmosets, tamarins, howlers, wooly, spider and squirrel monkeys.
In the section housing Old World primates, one cage is missing,because it's rightful occupant, Homo sapiens, is on the outsidelooking in.
Humans share with the lesser Old World primates one specific trait:Their immune systems react with extreme prejudice to cells from allother mammals on earth. That is why attempts to transplant the heartsof pigs into people fail so dramatically. The donor organ turns black,and dies within minutes. (See BioWorld Today, Oct. 3, 1995, p. 1.)
The immediate culprit is a glycoprotein sugar molecule, which studsthe surface of all mammalian cells. This carbohydrate, Gala-1,3-Gal,fatally attracts the hair-trigger attention of antibodies and killercomplement-system proteins in human and subhuman Old Worldprimates. Their attack on the alien Gala epitopes in a porcine donorheart spells hyper-acute rejection of the graft.
It also spells frustration for the growing army of immunologists andtransplant surgeons who see in pig organs a tempting solution to thecruelly constricted supply of donor organs for people dying of failedhearts, livers, kidneys and other organs.
Another target in their efforts to de-fang the alpha-gal epitope is theretrovirus. This large family of Retroviridae (RV) is a favored vectorfor transferring DNA into human cells for gene therapy, but RV alsosports the immunogenic alpha-gal residue on its envelope.
"The virus picks up this epitope from mouse cells," observedimmunologist Russell Rother, of Alexion Pharmaceuticals Inc., inNew Haven, Conn., "and it's responsible for RV being immediatelydestroyed in human serum by naturally occurring antibodies. It's kindof like a graft-rejection mechanism."
Rother, a senior scientist at Alexion, focused on gene therapy,observed that this RV finding by company scientists "was surprisingfrom the standpoint of the 20-year-old dogma stating that murineRVs were destroyed by a direct mechanism, whereby the firstcomponent of complement, C1q, bound directly to the virus andactivated the killer complement cascade."
Rother is first author of a paper in the current issue of Cell, datedJuly 29, 1996, aptly titled: "The a-galactosyl epitope: A sugar coatingthat makes viruses and cells unpalatable."
This discovery's practical significance, Rother told BioWorld Today,"is that most of the RV-based gene therapy has been done ex vivo. Astechnology progresses, it will allow direct in vivo RV-mediated genetransfer into patients."
Manipulating Retroviruses Keeps Them Alive
Pioneer gene therapist French Anderson showed, Rother pointed out,"that if you directly inject murine-derived RVs into Old Worldprimates, they are immediately inactivated. It now turns out," headded, "that if we manipulate the viruses so they no longer displaythis epitope on their surface _ by some enzymatic remodeling of theviral carbohydrate content _ then we can prevent the virus frombeing killed in human serum."
Scientists at Alexion and in Australia jointly reported their RV-re-engineering feat late last year in Nature Medicine for December1995. Its title: "Enzymatic remodeling of the carbohydrate surface ofa xenogeneic cell substantially reduces human antibody binding andcomplement-mediated cytolysis."
This paper, Rother explained, "came out when Alexion was in aquiet period connected with our public offering, so we never got out apress release on the viral remodeling, which Cell has now picked upon."
Alexion's strategy to inhibit expression of a-galactosyl involvesupping expression of a competing human enzyme, a-1,2-fucosyltransferase (a.k.a. H-transferase), which occurs in the human blood-group pathway. This enzyme puts the human "universal O blood-group" antigen on the cell surface instead of the a-gal epitope.
From Gene Transfer To Alien Organ Transfer
Rother observed, "We're now looking into producing the RV in cellsthat are devoid of the a-gal epitope altogether." He concluded: "TheRV gene-transfer field is hot."
So is the field of porcine organ grafting.
Molecular biologist and geneticist William Fodor directs Alexion'sxenotransplantation program. He intends to put that universal humanblood-group epitope into pig cells, in place of the highly antigenic,rejection-triggering, a-gal.
"We're now in the process of developing transgenic pigs," Fodor toldBioWorld Today, that express the H-transferase, in an attempt todown-regulate expression of the terminal glycosyl residue."
So far, he related, "we've expressed it in porcine kidney cells in vitroand transgenic mice in vivo." Those rodents, none the worsephysically for their gene replacement, yield cells that "are completelyresistant to hyper-immune rejection."
At this point, Fodor and his colleagues "have identified transgenicfounder pigs, but they do not express. So we will generate additionalanimals, obtain the transgene, then screen for the pigs that give us thebest expression, as we have seen in our transgenic mice."
Alexion is not alone in this pig-to-patient donor-organ race. A paperby a group at Nextran, a division of Baxter Healthcare Corp., inDeerfield, Ill., came out last month in the Proceedings of the NationalAcademy of Sciences (PNAS) dated July 9, 1996. Its title: Reductionin the level of Gala-1,3-Gal in transgenic mice and pigs by theexpression of an a-1,2-fucosyl transferase," _ thus seeing and raisingAlexion. (See BioWorld Today, Aug. 4, 1995, p. 1.)
Alexion's vice president of research, Stephen Squinto, observed:"Nextran are our competitors, and it's clear that they feel ourapproach is useful and important. They obviously heard about thiswork," he added, "back in June of 1994, and may have appreciatedthe significance of our finding at that time."
Squinto concluded: "We filed for patents immediately after makingthe discovery, so we feel we're in a pretty good position in terms ofintellectual property." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.