Emma Dorey
 

Two pieces of news in August revived the debate about whether the technical hurdle of preventing transmission of porcine endogenous retrovirus (PERV) during xenotranplantation can be overcome. In mid-August, a team from the Scripps Research Institute (La Jolla, CA) announced what they claim is the first evidence of cross species PERV infection. Then two weeks later, at the 18^th International Congress of the Transplantation Society in Rome, BioTransplant (Boston, MA), a biotechnology company focused on transplant tolerance, presented data indicating that it may have developed a breed of pig that doesn't produce PERV capable of infecting human cells. However, neither piece of news is definitive, and investors perceptions of xenotransplantaion safety remain unchanged.

The Scripps work seems to show cross-species transmission of PERV from pig cells to mice. Two months after transplanting pig pancreatic islets into immunodeficient mice, the islets continued to make infectious virus particles, according to one of the authors Daniel Salomon, associate professor at the Scripps department of molecular and experimental medicine. In addition, some mouse tissues in non-transplant sites like the liver and spleen were found to be infected. The group also showed PERV can infect isolated human cells in vitro (Nature, 407, 501–504, 2000). "If you transplant pig tissues—at least as pig tissues exist today—there's a real possibility that you're going to move PERV. . .from these pig tissue into humans," concludes Salomon.

However, the results do not convince all observers. Jay Fishman, clinical director, transplant infectious diseases, Massachusetts General Hospital and a consultant to BioTransplant and various pharmaceutical companies, points out that detection of infection is not clearcut. Limits on technology available at present mean "we can't tell directly whether or not there is infection by PERV of host tissue when there are pig cells floating around," he says. "We impute the fact that there is infection based on a variety of ratios of PCR signals."

Alan Colman, director of worldwide research at PPL Therapeutics (Roslin, UK) agrees with Fishman. "It is a model, but it's a very difficult model to extrapolate from," says Colman. He explains that it's not clear that the infectivity had spread to mouse cells and was not just in the pig cells that had entered various organs. "There's a possibility that what they were assaying was the remnants of the cells that they originally put in those animals," he says, explaining that completely immunocompromised mice wouldn't reject foreign tissue in the way a normal mouse would. However, although several companies are working on knocking out the -1,3-galactosyl transferase gene so that pig organs don't prompt acute rejection in humans, patients would still need to be put on immunosuppressives to avoid chronic rejection—something that puts the patient at an increased risk for the development of retroviruses.

Nevertheless, Salomon's findings may not be as discouraging as they first seem because, as Salomon himself points out, even though the mice are severe combined immunodeficient (SCID) they are not sick and there was no evidence for any kind of active viral spread. "All our data suggests is that by the 2 month time point, the virus had already become latent."

In any case, transmission from pigs to mice is a bit of a non-issue, points out Fishman. "The fact that we may or may not be able to infect humans or non-human primates is much closer to the area of concern."

One of the proposed solutions to cross-species PERV transmission is that cloning be used to generate PERV-free animals after first knocking out or disrupting crucial PERV genes. BioTransplant's news that peripheral blood cells from one of its line of in-bred mini-swine don't transmit PERV A or B—2 variants that infect human cells in culture—seems to add credence to this, even though the swine were produced by traditional breeding methods. Although the primary data has yet to be published in a peer-reviewed journal, Salomon is enthusiastic about what they suggest. "The scientific impact is way, way beyond the fact that the company accidentally has this pig," he says.

BioTransplant's chief scientific officer Julia Greenstein says that rationally knocking out PERVs is a high technical hurdle because there are around 50 copies of PERV in the pig genome, and the majority are not full length or absolutely correct in their sequence—they've been modified or are missing chunks of information. "It's not clear at this point whether [knocking that out genetically] is really achievable," she says.

Salomon says BioTransplant's results suggest that the target is actually much smaller. He agrees it would be impossible to knock out or clone out 50 alleles by selective breeding or by accident. Thus, he proposes, another way to explain the BioTransplant data is if only 2 or 3 of the 50 genes are transcriptionally active for PERV A and B. "If this is correct, then it's relatively easy to do," he says, "First, they'll figure out which of the copies are transcriptionally active, they'll mark them, they'll go back to the cloning technology—in 5 years this [PERV transmission] should be a dead issue."

Meanwhile, it is difficult to assess the commercial impact of the two pieces of news because most xenotransplantation research is being conducted by subsidiaries of large companies—Novartis's Imutran (Cambridge, UK) and Baxters' Nextran (Princeton, NJ), for instance. BioTransplant's share price did rise nearly 8% to $12 the day of its announcement. "In a sense you could say that that proves that people are concerned and anything that's done to reduce the potential risk [of PERV transmission] is viewed well," says PPL's Colman, "But it's too early to say how much it's affecting investor enthusiasm."

However, there are some signs from the investment world that the PERV issue is already moribund. Paul Haycock, director responsible for healthcare and biotechnology at Apax Partners (London), says neither announcement will change Apax's investment approach. "There's one little hurdle that's come along, there have been others before, there'll be others in the future," he says. Apax has been an investors in PPL since before the days of Dolly. "Xenotransplantation is a fascinating area of science which could bring terrific benefits obviously to treatment of disease," he says, "but its risks are not yet fully evaluated and won't be for some time so one must proceed with caution."